# BB Height. Handling vs Clearance



## modifier (May 11, 2007)

I haven't bought an assembled bike since around 1995 except the $350 29er commuter I bought a month ago and I'm always changing things around with different forks, shocks, custom links, odd size tires etc and I prefer longer travel. So the outcome of all that is I have very little experience with "normal" bicycles with current BB heights. The three main bikes I ride are at 14" to 15 1/2" static and I don't have any problems with that.

Yesterday I took a test ride on a demo Cannondale Scalpel to see what it felt like to ride a really light carbon 29er. Overall the bike was fun to ride. The weight was amazing. For me I would have preferred 2 extra inches of travel but it was ok. But what really got me was the 13.2" bb height. Or at least that is what the specs say. My shock may have had a bit too low pressure and I don't know if it was on the high or low BB setting. But what I do know is I hit my pedals at least 15 times during a 1 1/2 hr ride on rocky trails. Anywhere from grazing to 'I wonder if that broke my pedal' to stopping the bike and causing a high side launch into a the rocks on a high speed corner. Ouch. That has never happened to me and I generally don't hit pedals at all. 

This was all in the rocks. On smooth or dirt trail it was not an issue. After the crash I started being really cautious and coasting around corners making sure the inside pedal was up. In places where I had to pedal to clear bigger stuff I still hit pedal to rock at times.

I've been riding for 30 years so I can do it. Avoid pedal strikes that is. But it's not fun to have to think about it so much and not be able to just power through the gnar or have to go slower. The bike did rail around smooth dirt corners really well and the front tire stuck, and I suppose eventually I would adjust, but I'm not sure the trade off in handling vs having to mind the pedals all the time is worth it. It's not like the higher BB bikes handle really bad. You just have to lean a bit farther. 

I checked a few other high end 4" bikes and they were all about the same place. About 13". I read that Jeff Jones Space frame is a 10 1/2 inches. That just seems crazy to me.

What are your thoughts on the subject?


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## Jon Richard (Dec 20, 2011)

I'm not experienced enough to have a definite opinion, but my current 155mm travel trail ride has a static BB height of 12.75" and the handling is so much more intuitive for me than the others I test rode before I purchased.

I've only just started riding again after having hung it up around 96'. I seem to recall BB heights ranging between 12" and 13" was the norm from my hardtail days.

I love the handling, but perhaps I may see it differently if I start riding rockier more technical jagged terrain.


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## shoo (Nov 11, 2008)

I am in total agreement with you. I have had many long discussion with engineers and product managers from several big companies. Each of them defends the low BB height as being faster. One company insists that everytime they design a new bike they do blind testing and the low BB always gets picked. I asked them to come to were I ride and do the blind test to see if the results would be the same. No luck there. Not sure how they do blind testing, I can feel a low BB as soon as I get underway. On a full suspension I want 13.5 at sag. One of the reasons I am going to build my own frame is to get the BB height I want. I once spoke to a custom builder and he did not think the height I wanted was acceptable. 

I cannot speak for others but I am faster with a high BB.

Cheers,
Steven


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## kpaynter (Jun 18, 2008)

There are two ways to look at the position of the BB: Ground clearance(height) or drop from wheel center(I prefer this one, from a design perspective). BMX's (as well as most DH bikes) have a rise above the wheel center making them nimble. Touring frames tend to be of the lowest, making them very stable at high speeds. All that mass (you, on the pedals) above the wheel center feels tippy where as having the mass below wheel center helps keep the bike upright... hope that's not too vague.


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## Jon Richard (Dec 20, 2011)

kpaynter said:


> There are two ways to look at the position of the BB: Ground clearance(height) or drop from wheel center(I prefer this one, from a design perspective). BMX's (as well as most DH bikes) have a rise above the wheel center making them nimble. Touring frames tend to be of the lowest, making them very stable at high speeds. All that mass (you, on the pedals) above the wheel center feels tippy where as having the mass below wheel center helps keep the bike upright... hope that's not too vague.


Not vague at all, in fact a very valid perspective and perhaps an important factor to consider that would influence BB height choice depending upon wheel diameter preference.


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## TrailMaker (Sep 16, 2007)

Hey;

Designers have taken the opportunity of the tall wheels to drop the BB for lower CG. From my perspective, I think they have over done this. I've had less supple lower travel XC bikes that never bashed pedals and cleared every log, and I've got my Heckler and RIP9 that bash them a LOT. Especially the Niner. I bash peddles constantly, and frankly it is the only thing that I don't like about it. These bikes are also far more supple, which adds to it. While some of the grounding out is due to incorrect damping curves for my size, I would gladly trade some "stability" for the extra clearance I could really use in the rocky, rooty, loggy, creeky conditions I ride in.

I too am doing a frame one of these days, a Fatbike in my case, and plan on a 13.5" BB/H with a 2.5" drop. I will only have 1-2" of "sag" so I feel this will give me a nice rollover height... I hope.

Faster, schmaster. I don't ride bike paths, and pranging peddles aint what I call faster!


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## team_wee (Mar 26, 2006)

I agree, bb's now seem to be getting way to low. For me, on a hardtail 12.5" is the sweet spot for a 120mm travel fork. 10.5" seems nuts! How long are the crank arms 150mm???


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## dr.welby (Jan 6, 2004)

TrailMaker said:


> Designers have taken the opportunity of the tall wheels to drop the BB for lower CG.


You might want to sketch that out.


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## GMF (Jan 10, 2004)

dr.welby said:


> You might want to sketch that out.


Thank you...

The axle height doesn't have anything to do with bottom bracket height. You don't lean the bike about the axle, you lean it around where the tire touches the ground.

But yeah, bottom bracket height is a regional thing. Typically "east coast" bikes had a high bottom bracket for the tighter rootier trails, while "west coast" bikes had a lower bottom bracket for the buffer higher speed trails. Sweeping generalizations, of course.


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## Jon Richard (Dec 20, 2011)

GMF said:


> The axle height doesn't have anything to do with bottom bracket height. You don't lean the bike about the axle, you lean it around where the tire touches the ground.


Well, when you say it like that I feel silly for having followed that train of thought. I know I am a novice but indulge me, is it true that the height of the mass being carried is only relevent in relation to that of the contact patch?

I guess I thought where the riders weight was being carried in relation to the centroid axis of the wheels centrifical force would have some influence, at least in terms of feedback or percieved stability.


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## shandcycles (Jan 15, 2008)

But if you keep the bb height the same and increase the wheel size, then you need to increase the bb drop which will lower the CG.


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## dr.welby (Jan 6, 2004)

shandcycles said:


> But if you keep the bb height the same and increase the wheel size, then you need to increase the bb drop which will lower the CG.


You might want to sketch that out too.


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## shiggy (Dec 19, 1998)

kpaynter said:


> ...Touring frames tend to be of the lowest, making them very stable at high speeds...


The best reason I have heard for touring bikes having low BBs is it makes it easier to stop/start/support the bike when you can get your foot on the ground from the saddle.


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## dr.welby (Jan 6, 2004)

shiggy said:


> The best reason I have heard for touring bikes having low BBs is it makes it easier to stop/start/support the bike when you can get your foot on the ground from the saddle.


And your panniers will lower your CG more than the difference in BB drop between a tourer and a racer.

Tourists don't spend much time "at high speed" so I don't think that's a design goal. They are less likely to be pedalling through sharp corners, so I think the low BB is more tolerable and as Shiggy says makes it easier to stop/start as well as puts the bars a little higher relatively.


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## modifier (May 11, 2007)

GMF said:


> Thank you...
> 
> But yeah, bottom bracket height is a regional thing. Typically "east coast" bikes had a high bottom bracket for the tighter rootier trails, while "west coast" bikes had a lower bottom bracket for the buffer higher speed trails. Sweeping generalizations, of course.


This may have some validity and lot's of bike companies are out west driving the lower bb.

I'm talking about the Midwest which is very similar to east coast terrain. Short up, short down and lots of rocks, if the trail is to my liking. I learned to ride off road in the east and have never ridden out west.

So if you're riding up relatively smooth long trails and using gravity to go down then a low bb height would not be much of an issue and perhaps a benefit. But around here or out east it's a hindrance, as far as I see it. So maybe it's not about the overall standard being wrong but about the need for diversity. And I'm sure there is a happy medium for the all location, ride anywhere, AM bike.

My FS fat bike is at 14 inches after sag. That's a bit extreme but you just have to use a lot more body english and going over rough stuff is a dream.

Walt. You care to share your wealth or wisdom with us on this subject? :smilewinkgrin:


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## modifier (May 11, 2007)

dr.welby said:


> And your panniers will lower your CG more than the difference in BB drop between a tourer and a racer.
> 
> Tourists don't spend much time "at high speed" so I don't think that's a design goal. They are less likely to be pedalling through sharp corners, so I think the low BB is more tolerable and as Shiggy says makes it easier to stop/start as well as puts the bars a little higher relatively.


Touring is a whole outer criteria. You are riding on pavement 99% of the time and coasting down hills to conserve energy plus with all the weight keeping the CoG as low as possible is paramount.


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## Walt (Jan 23, 2004)

*Shockingly...*

...I am awake and relatively coherent. The Bean only woke me up 5 times last night, a new record!

I don't think there's much controversy that the ideal bb height:
A: Depends on the rider's preferences and terrain.
B: Is always pretty much as low as you can go until pedal strikes become intolerable (which is a very subjective thing).

This was discussed to some extent last year, here was one of my posts about it (the context was a thread about short chainstays): http://forums.mtbr.com/7970524-post46.html

If you think about what happens when you lean to steer your bike, a 10 degree lean will be a very different thing on, say, a bike with a 10" bb vs a bike with a 50" BB (tall bike). The tall bike rider has to *move* sideways relative to the tire contact patch (the pivot point) a LONG way - so much so that lean steering at lower speeds can be completely terrifying and it can be impossible to recover from the lean (then you keep turning, then you fall over, it's bad).

-Walt



modifier said:


> This may have some validity and lot's of bike companies are out west driving the lower bb.
> 
> I'm talking about the Midwest which is very similar to east coast terrain. Short up, short down and lots of rocks, if the trail is to my liking. I learned to ride off road in the east and have never ridden out west.
> 
> ...


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## modifier (May 11, 2007)

Thanks Walt. The low speed stability comment makes sense. I remember that thread but didn't keep up with the whole thing. Opinions on short chain stays is a good topic as well so I'll check it out in full.


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## GMF (Jan 10, 2004)

Jon Richard said:


> I guess I thought where the riders weight was being carried in relation to the centroid axis of the wheels centrifical force would have some influence, at least in terms of feedback or percieved stability.


Well, i can't say anything from experience here, but you are going to theory territory with words like "centroid" ;-). Theoretically, if you get a heavy 26"er wheel and a light 29"er wheel with the same moment of inertia, you should have a very similar response to changes in direction. But you are talking the effects of wheel diameter, not bottom bracket height, now.

Bike geometry is a crazy, interesting thing. LOTS of factors go into how a bike rides.


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## Jon Richard (Dec 20, 2011)

GMF said:


> Well, I can't say anything from experience here, but you are going to theory territory with words like "centroid" ;-). Theoretically, if you get a heavy 26"er wheel and a light 29"er wheel with the same moment of inertia, you should have a very similar response to changes in direction. But you are talking the effects of wheel diameter, not bottom bracket height, now.


So, will seat tube angle have an effect on the moment directed at one's "hemorrhoid"? theoretically ofcourse  please refrain from saying anything from experience.

Seriously though just to clearify, I take this as an emphatic no, there is no correlation between BB height and axle height that induces a discernable effect on handling.



GMF said:


> Bike geometry is a crazy, interesting thing. LOTS of factors go into how a bike rides.


I don't doubt that at all. I can kinda rap my head around car handling and suspension kinematics, but two wheels are a new concept for me.



modifier said:


> This may have some validity and lot's of bike companies are out west driving the lower bb.


Perhaps 150mm length crank arms are the next trend to complement our 10" tall bottom brackets and 11 speed cassettes


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## modifier (May 11, 2007)

Jon Richard;9209454
Perhaps 150mm length crank arms are the next trend to complement our 10" tall bottom brackets and 11 speed cassettes :([/QUOTE said:


> Well maybe not 150 but I was thinking if I end up with a bike that has a low BB (from my perspective) I'll be rocking some 165s at least :thumbsup:
> 
> And from what I have read on crank arm length affecting efficiency that won't matter.


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## TrailMaker (Sep 16, 2007)

dr.welby said:


> You might want to sketch that out.


OK;

Perhaps I did not phrase that properly - as a relational thing - but I think everyone knew what I meant. Are axles not higher on niners than sixers, and are not designers increasing BB drop on niners relative to sixers in many cases?


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## Jon Richard (Dec 20, 2011)

modifier said:


> ...from what I have read on crank arm length affecting efficiency that won't matter.


Agreed, I was just facetiously remarking about manufacturers marketing stuff to us we don't necessarily want. My concern, if any, in that regard is having a feeling similar to that of my 34" inseam powering my little brothers 16"er.



modifier said:


> Well maybe not 150 but I was thinking if I end up with a bike that has a low BB (from my perspective) I'll be rocking some 165s at least :thumbsup:


I heard that. I run 170's at a static BB height of 12.75", have know idea where it measures under sag. I'm in the process of changing from 1x9 to 2x6 with a single speed hub and am considering 165's myself. I'm even thinking about a 650B up front to help clearance as well as slacken the headtube angle, hence my above questions about axle height.


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## dr.welby (Jan 6, 2004)

TrailMaker said:


> OK;
> 
> Perhaps I did not phrase that properly - as a relational thing - but I think everyone knew what I meant. Are axles not higher on niners than sixers, and are not designers increasing BB drop on niners relative to sixers in many cases?


But that doesn't create a "lower CG".


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## DWF (Jan 12, 2004)

dr.welby said:


> But that doesn't create a "lower CG".


Where the BB goes, the saddle follows. So yes, relative to the ground, dropping the BB drops the center of mass whether the rider is aboard or not.


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## Mr.Magura (Aug 11, 2010)

dr.welby said:


> But that doesn't create a "lower CG".


Nope, you are absolutely right.

Actually a 29" wheeled bike with a 12" BB height, will have higher center of gravity, than a 26" with 12" BB height, due to the taller wheels. 
All else equal.

To sketch it up was not such a bad suggestion.

Magura


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## DWF (Jan 12, 2004)

I think to answer the bigger question, mfr's are trying to replicate 26er handling in 29ers. The higher the CG of a bike/rider the slower the bike is going to respond to input (from ground or rider) and the higher the rotating inertia which slows down the handling and ~can~ increase stability. The negative side is that it can make the bike feel lethargic and less responsive to directional or attitudinal changes which can be both a blessing and a curse. Personally, I'd worry about pedal clearance first.


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## Eric Malcolm (Dec 18, 2011)

Love the one liners Dr Welby.

C of G unfortunately is governed by our unique physical bodies. If you're short, you are blessed, I'm tall, and always at a disadvantage in this area. 

Ultimately, you just have to find whats right for you and then stay with what works for you. The journey will always be one of compromise.

Perhaps, as Dr Welby suggests, sketch it out. That is, firstly, place yourself where you believe you want to be relative to pedal height, then put the wheels around you. Regardless of wheel size, there will be only 1 result, the BB height fixes your C of G.

Eric


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## dr.welby (Jan 6, 2004)

DWF said:


> Where the BB goes, the saddle follows. So yes, relative to the ground, dropping the BB drops the center of mass whether the rider is aboard or not.


No argument there, but we're not talking about dropping the BB relative to the ground. We're talking increasing BB drop to compensate for an increase in distance to an imaginary axle line.

Where's PVD when you need him?


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## DWF (Jan 12, 2004)

dr.welby said:


> No argument there, but we're not talking about dropping the BB relative to the ground. We're talking increasing BB drop to compensate for an increase in distance to an imaginary axle line.
> 
> Where's PVD when you need him?


The result is the same to the ground or the axle line, i.e., increasing drop/decreasing BB height are two methods of determining the same thing. Both result in lowering the center of mass relative to their opposites.


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## dr.welby (Jan 6, 2004)

DWF said:


> The result is the same to the ground or the axle line, i.e., increasing drop/decreasing BB height are two methods of determining the same thing. Both result in lowering the center of mass relative to their opposites.


This is true if the axle height is a fixed value.

I think you missed the claims of several earlier posts, where 29'ers had "more stability" because the "designers" could add "more drop" since the *axles were higher*.


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## febikes (Jan 28, 2011)

Like Walt says it depends a lot on your style of riding and where you ride.

Like most things BB height is a trade. My feeling is your are trading high speed handling for better low speed handling as you increase height. Bikes built for low speed handling especially on technical trails will be higher but if you are looking for improvement for fast fire roads you will want to move to a lower position.

Things are also very personal. I like 320mm (around 12.6"). This works well for me and is high enough to make pedal strikes rare and low enough for a stable and fast bike. As an east coast rider and think the trails I cut my chainring teeth on influence my preference. I also think that for a single speed the all out speeds are slower and personally on SS I enjoy more slow technical stuff with a higher BB. 

Typical geo for hardtails tends to be in the range between 11.2 and 13.0 in terms of production bikes. Like anything extremes are rarely the best approach. 13.5 seems very high and might be near far side of what most people would prefer. The 1995 Cannondale "beast of the east" model was 13.0" while other dales from that year were 11.6". I would think twice before going higher then 13" or lower then 11.2"


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## DWF (Jan 12, 2004)

dr.welby said:


> This is true if the axle height is a fixed value.
> 
> I think you missed the claims of several earlier posts, where 29'ers had "more stability" because the "designers" could add "more drop" since the *axles were higher*.


Laterally no, longitudinally, yes.


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## shoo (Nov 11, 2008)

Riding style is a huge part of the equation.

I want a higher BB for high speed riding (30- 45 mph), not slow speed. I want a higher BB on a fully rigid bike as well as a full suspension bike. A higher BB allows me to corner harder and I can start pedaling sooner as I come out of the turn. If it is full suspension and it is lower than 13.5 at sag then it is too low for me. If it is fully rigid then I need 12.5 mininum and if it has front suspension I want at least 13" static. I can ride lower it is just slower. I have been riding a rigid Fatback ( primary bike) for well over a year, it is 12" and a great handling bike, just not as fast as it could be. For me...

The original Cannondale MTB had a 13" BB in 1985, it was years later after lowering it down to mid 11's that they came out with the Beast Of The East to satisfy the people that did not want to adapt to the low BB height on there standard bikes. I had a Gemini 10 years ago that had over 14.5 BB height and 167.5 cranks and it was one of the fastest bikes I have been on.



febikes said:


> Like Walt says it depends a lot on your style of riding and where you ride.
> 
> Like most things BB height is a trade. My feeling is your are trading high speed handling for better low speed handling as you increase height. Bikes built for low speed handling especially on technical trails will be higher but if you are looking for improvement for fast fire roads you will want to move to a lower position.
> 
> ...


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## kpaynter (Jun 18, 2008)

modifier said:


> Yesterday I took a test ride on a demo Cannondale Scalpel to see what it felt like to ride a really light carbon 29er. Overall the bike was fun to ride. The weight was amazing. For me I would have preferred 2 extra inches of travel but it was ok. But what really got me was the 13.2" bb height.


I should have paid closer attention the the original post. I think 30mm of extra wheel base has more to do with the handling than 30mm of bb drop.* In the case of the Scalple, it doesn't really make sense to me to put such a low bb on a bike meant for off road.* Maybe they don't expect these bikes to be used for technical trail riding; more of an XC Marathon type thing. They could improve things by making that head angle a bit steeper too(thus, shortening the wheel base, tightening turning radius. keep the bb in the same place, so it goes up as the fork comes in..)
Someone also mentioned chainstay length earlier: this has a lot to do with stability(fore/aft rider placement and wheel base)...wrt. to my comment about touring bikes, again, the chainstay length plays a bigger roll in handling than bb drop(or height), in my opinion. I know touring bikes aren't all-mountain 29ers, it was just an example.


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## TrailMaker (Sep 16, 2007)

Well...

All battles of semantics and definitions aside, and in reality, the CG of a bike & rider could likely not even be found in any fashion, and is such a variable enterprise as to surely be rather pointless. An academic exercise, at best. Given that, no one is right or wrong, and in all honesty I was simply parroting every test of every 29r I have ever read. I can only assume that either the testers have done the math for themselves, or are taking the word of the designers. In any event, there have been people far smarter than I - and that is not that difficult - that have been offering this advantage up from the get go where 29rs are concerned.

A simple drawing shows that the BB height is obviously the same if the ground clearance is the same, but the BB drop is obviously different. Perhaps what is creating the "extra stability" in a 29r is the moment arm created by this added BB drop compared to the same 26r? I am assuming that if this moment arm is correct, that it is a function of the axle height relative the BB, and not a function of the increased radius of the wheel. Again, someone far smarter than I will have to carry this ball, as my head is starting to hurt.

Perhaps I have simply jumped on the "Lower 29r CG Bus to Know Where" without knowing who is driving? Well, I wish some folks would get off this bus, as it is rather crowded.


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## Eric Malcolm (Dec 18, 2011)

Thanks Trailmaker, a sketch. Yes, this thread has moved a little off course. It is amazing to note how a 80kg rider is so influenced by a 10-12kg bike.

The original context of the thread was regarding mass market bikes being lowish in the BB area compared to the writers own bike with a higher BB.

Basically, if you like the high BB, get one custom made, if 1 builder didn't want to make one, try elsewhere. 

Remember that the mass market dictates ease of use, and 1 of those design tenants is that a rider can touch the ground with their foot while seated on a bike with a correct saddle height. Manufacturers sell as their priority, to the masses, and most of what they make do not see the trails, let alone are riden by skilled riders, so if they hit a pedal, they simply compensate by freewheeling that piece, enjoy the adventure and go home full-filled. I see many riders on the trails this way......and await the opportunity to pass.

If you want to upgrade from 26 to 29", look deeper into the custom option with a good builder who can sketch it out and explain the trade off's. Its well worth the effort.

Eric


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## dr.welby (Jan 6, 2004)

TrailMaker said:


> I am assuming that if this moment arm is correct...


Do your tires scrub side to side? How does this imaginary line connecting your axles stay fixed in space?


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## shoo (Nov 11, 2008)

I am in agreement with the OP, my coments were in regard to his statement. I think people are just trying to understand the rational. I prefer to allow my 80kg's to influence the bike and the lower BB limits my influence. I find it odd and a little backward that manufactures tout low BB to improve handling. For me, and it seems the OP that is not the case, in my case it hinders handling. Further to the point is that Trek builds a full suspension bike with a lower BB than a fully rigid bike from 25 years ago.

I am not trying to force my style on anyone, I just know what works for me. I am curious as to what really drives the lower BB.



Eric Malcolm said:


> Yes, this thread has moved a little off course. It is amazing to note how a 80kg rider is so influenced by a 10-12kg bike.
> 
> The original context of the thread was regarding mass market bikes being lowish in the BB area compared to the writers own bike with a higher BB.


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## modifier (May 11, 2007)

I'm trying to gather educated opinions on this topic because it's likely I'm going to be building my own bikes at some point and if I'm going to go with the flow or against it I want to do so with as much information as possible. I have little interest in building a traditional double triangle bicycle that hits all the right buttons concerning what is status quo in the industry and what the masses fawn over. Even though that would be more profitable.

Another thing I'm interested in is test data on Q factor because to make room for really wide tires and still keep everything centered a 100mm bb shell and a 170mm rear hub is best. This creates a wide Q factor. Biomechanically is this bad? Is it going to screw up my hips and my knees over time? It seems more natural unless you have really narrow hips. 

I basically want to build a better bicycle for riding rough terrain, and solve some of the problems that persist because of an industry concentrating on marketability rather than a desire for perfection. Not that progress isn't happening. Bicycles get better every year. It's just happening in baby steps.

I road my long travel fat bike tonight on the same trails I road the Cannondale on. If you want to go fast through rocky terrain or climb up a trail that you have never been able to get up before and all with the least wear and tear on the rider, ride the bike I built. It is a technological advancement in some areas. Even if it has a monster truck BB height. 

I do tend to lean forward and low on this bike to put more weight on the front tire in corners and maybe this is partially a product of that high BB. Chain stays are at 17 3/8 which isn't super long for a bike with 29" diameter tires so I don't think it's because my weight is way back. It's not that slack either.


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## TrailMaker (Sep 16, 2007)

Modifier;

You should build the bike that makes you happy. That sounds like a bike with a high BB. It fits the stuff you like to ride. Sounds like a bike I would enjoy myself, riding what sounds like similar favored terrain. I am juggling the same sort of balls myself with my own design. At the moment, I've settled on a 47" WB, 12.8 BB height with a 2" BB drop for my proposed Fatbike. My only question is whether that long wheelbase will make we wish I raised the BB even higher.

Build, ride, study, revise, Build II, repeat.


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## Mr.Magura (Aug 11, 2010)

TrailMaker said:


> Well...
> 
> All battles of semantics and definitions aside, and in reality, the CG of a bike & rider could likely not even be found in any fashion, and is such a variable enterprise as to surely be rather pointless. An academic exercise, at best. Given that, no one is right or wrong, and in all honesty I was simply parroting every test of every 29r I have ever read. I can only assume that either the testers have done the math for themselves, or are taking the word of the designers. In any event, there have been people far smarter than I - and that is not that difficult - that have been offering this advantage up from the get go where 29rs are concerned.
> 
> ...


Ok, so now we have the sketch, that's a good start 

Have you ever tried riding a HPV of the two wheeled kind ?
They have the lowest COG I am aware exists.
Now that low a bike, turns real agile.
The COG versus the axle height on one of those, is like if you had no BB drop, but rather a BB raise.

So take another look at your sketch, and think about which axis the bike is tilting over when turning? That would be the ground. So anything above the ground, could not possibly be the reference point for the COG, as that is the ground. 
The only factor you can play with, is to move weight of the bike or the rider, closer to the axis, AKA the ground plane.

Magura


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## TrailMaker (Sep 16, 2007)

Well...

I thought I had thrown out the COG idea, as it is "impossible" to solve for? The bike/rider is the moment arm working against the contact patch of the tires/ground, and this "lever" is then articulated in many places beyond that (pedals/foot interface, flexibility in the rider's joints, etc). The only difference in the drawing is the BB drop, and perhaps the gyro effect of the bigger wheels (different issue). The ground is the ultimate foundation for any forces here. Given. Moving on....

Question; Why is BB drop spoken about as a parameter of frame building?

(Remember, I said it would take someone smarter than me, and that finding this person would not be hard :madman


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## dr.welby (Jan 6, 2004)

TrailMaker said:


> Question; Why is BB drop spoken about as a parameter of frame building?


1) It's relevant to setting up most jigs, which don't have a ground reference.

2) Back in the day when "custom frames" only meant 700c road bikes with a narrow range of tire sizes, it was a useful number for comparison.

3) It's independent from wheel size, tire size and suspension sag, so you can use it to calculate bottom bracket height given a range in those parameters.

It's a useful parameter in *frame building*, which comes after *frame design*.


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## TrailMaker (Sep 16, 2007)

Aha!

The King of Kryptic finds his voice! 

And so, BB Drop has no effect on handling?


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## Blaster1200 (Feb 20, 2004)

As a BMX/4X/DS/DH racer, I'm an advocate for low bottom brackets for all types of technical riding. For low-speed stuff, a hub with a high points of engagement make it easy to prevent pedal strikes. But this is a skill that's learned.

Turning skills are something else as well. Most riders don't use proper turning technique. Even a lot of Cat 1 XC racers don't really know how to turn.

Those who do understand proper turning technique understand where you want your weight to be in turns (outside, by the way), and so this image may make more sense. (Sorry not to scale.) As you may be able to see, the lower BB allows you to get your weight further to the outside of the contact patch.


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## dr.welby (Jan 6, 2004)

TrailMaker said:


> Aha!
> 
> The King of Kryptic finds his voice!
> 
> And so, BB Drop has no effect on handling?


I prefer to be referred to as the Sultan of Socratic Dialog.

BB drop has no meaning without knowing wheel and tire size, suspension sag, etc. It's a number internal to the frame, not descriptive of the whole bicycle.


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## dr.welby (Jan 6, 2004)

Blaster1200 said:


> Those who do understand proper turning technique understand where you want your weight to be in turns (outside, by the way), and so this image may make more sense. (Sorry not to scale.) As you may be able to see, the lower BB allows you to get your weight further to the outside of the contact patch.


Oh dear. That's a physics trainwreck.


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## modifier (May 11, 2007)

Blaster, Yes that makes sense on smooth tracks (BMX) or gravity assisted (all the others) but when you are on a rocky trail, which is my choice of fun, pedaling or you stop, things are different. I can time pedal misses many feet ahead like someone mentioned as well as the next guy, but if you want to pedal around a corner with rocks and your pedals are closer to the ground that's just not going to happen without strikes. Or the example I gave where you're riding along the rocky trail and there is a big rock or a log coming up and it's level or up hill so there is no gravity to keep you going without pedaling; you approach the obstacle then just before you get to it you need to pedal to get the front wheel up and gain some momentum to lift the rear tire up after the front clears. If there are big enough rocks on that approach a pedal will hit if it's low. Or you slow down and negotiate the situation slow enough to avoid the strike. Something I really don't want to do.

Last night riding the high bb bike taking the same corner where I fell, I saw where I screwed up. You come around the corner at the bottom of a DH with the outside pedal down then it flattens out with some rocks and a big root up ahead that you want to hit at speed and just as you pedal and the inside pedal hits bottom stroke there are 2 imbedded rocks right there where my Scalpel pedal hit and launched me. On the high BB bike it was not an issue and I powered through. Sure on a trail that you know really well, if you are on a low BB bike, you would be aware of this situation and avoid it, but you don't know every inch of every trail that well. 

I think for true AM the BB should be higher. For other specific applications a lower BB may be better.


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## modifier (May 11, 2007)

dr.welby said:


> Oh dear. That's a physics trainwreck.


Is it really? If your foot/pedal is closer to the ground your body will be too and therefore lower your CoG. Right?

I just read an article in MTB Action about the use of drop posts, which I have yet to adopt. Still too heavy. Carbon baby:thumbsup:

They make a comment about how your CoG will be lower if the saddle is down and increase cornering speed. I haven't experimented with a lower saddle yet as I want it up 99% of the time but it's worth considering. Dropping your body down 3 or 4 inches is going to do a lot more than dropping your BB down 1 inch. Maybe a d-post is more reasonable. Idk.

I know if I want to go around a corner really fast on my dirtbike I want to be up on the tank with my inside leg out and to the front to get as much weight as possible low and on that front tire.


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## Drew Diller (Jan 4, 2010)

modifier - don't want to hijack too much, but a dropper post is one of those items where I can't go back to a regular post once I've had a taste of the alternative. Total control going down technical stuff.


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## G-reg (Jan 12, 2004)

Folks seem to be talking about pedal strikes like they are some kind of apocalyptic catastrophe. To jack up the BB in order to avoid all pedal strikes is a bit "the nail telling the hammer what to do." 

I'm in the low as practical BB crowd. Why compromise handling 99% of the time just to avoid that 1% where the hammer has remind the nail of it's place.


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## modifier (May 11, 2007)

Drew Diller said:


> modifier - don't want to hijack too much, but a dropper post is one of those items where I can't go back to a regular post once I've had a taste of the alternative. Total control going down technical stuff.


It's not too much of a hijack since CoG is key to the discussion and the OP brought it up first anyhow. Me 

Most of my bikes are pretty burly as it is so adding another pound is something I've avoided. If it was a 1/2lb penalty I would be more inclined. 100g even better. If they would ramp it up a bit and use Ti or Carbon I think they could get it down lower. Maybe I need to buy one and take it apart to see where weight could come off. I've thought about designing something that is a lot simpler and lighter but haven't invested any real time in it so far.

I should try one. I'm also in process to build a 29er DH bike and wanting to get the bars a lot lower to compensate for the big tires. A dropper post might find it's way onto that bike.


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## dr.welby (Jan 6, 2004)

modifier said:


> Is it really? If your foot/pedal is closer to the ground your body will be too and therefore lower your CoG. Right?


Of course. No one is claiming that lowering your CG does anything other than make your CG lower.

The problem is with the idea that you should have your weight outside the contact patch. Leaned vehicles don't work like that.










We haven't seen any trends towards wider pedals (or motorcycle pegs) to get your outside foot further outside, either. Not to say it's not a useful techique - since your body is the dominant mass moving it "out" really is moving the bike "in", and maybe a flatter bicycle angle loads the tires or suspension better.


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## Blaster1200 (Feb 20, 2004)

Aggressive bike racers don't makes fast turns sitting on the seat as shown above. That's why I said that proper turning technique gets lost among most people. The style above does work for some applications, but not so good if traction is limited, or you're on the verge of exceeding available traction. The purpose of loading the outside of the bicycle or offroad bike applies more downward force to the tires to the ground. Talk to any expert level or higher DH or offroad racer (modifier gets it by putting your nuts over the tank, but you also weight the outside peg, even if your inside foot is down. Wider pegs or pedals won't help.

It would be helpful if more frame builders took high level riding courses to get a better understanding of how to properly handle offroad bicycles so they don't spread bad advice based on poor or average riding technique.

1st photo: From leelikebikes.com
Lee Likes Bikes

2nd photo: Note Jimmy Lewis is shifting his weight to the outside of the bike in this turn. This photo is from one of his riding classes. One of the first things he teaches in his classes is low-speed turning, whether you're on a YZ125 or GS1200, and he has you weighting the outside peg to better balance the bike. In this case, it's not for traction, but for balance. The lower CG is grealy helpful when making 2 mph super-tight radius turns on a GS1200 and wieghting the outside is where the balance comes from - the point is that there are several reasons to weight the outside peg/pedal.

3rd and 4th photo: And then below Jimmy is Sam Hill and then Brian Lopes.

Noting a pattern here of when they lean the bike that they're weighting the outside pedal? They do not sit inline with the bike like the static riding position shown in the illustration. You're looking at some of the best riders in the world. Perhaps you want to tell them that your physics books say that they're doing it wrong?


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## dr.welby (Jan 6, 2004)

Blaster1200 said:


> The purpose of loading the outside of the bicycle or offroad bike applies more downward force to the tires to the ground.


How do you get more downward force to the tires? Your mass doesn't change, and gravity is a constant.


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## dr.welby (Jan 6, 2004)

Blaster1200 said:


> Perhaps you want to tell them that your physics books say that they're doing it wrong?


Read what I wrote again, and try looking up Tony Foale.

And here''s a hint - their CG is generally around their solar plexus. You can draw the line downwards on any of your pictures to see if it's inside or outside the contact patch.


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## Blaster1200 (Feb 20, 2004)

Tony may know chassis design, but I don't see anything that would indicate that he explains riding technique. 

So why is it that people are weighting the outside of the bike instead of staying neutral on the centerline of the bike as your earlier image suggests? Again, do you know something they don't? 

Note that I said regarding weighting to get your weight further to the outside of the contact patch - this doesn't necessarily mean outside of the contact patch - but getting your weight further out is better than getting it too far in.

Try this, go into a turn leaning way to the inside of the bike where your instincts are insisting that you put a foot down, but don't, and let me know how it works out for you.


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## dr.welby (Jan 6, 2004)

Blaster1200 said:


> So why is it that people are weighting the outside of the bike instead of staying neutral on the centerline of the bike as your earlier image suggests? Again, do you know something they don't?
> .


*Fercrissake, it's a simple illustration of the physics, not a technique manual!*

The image shows that the the forces (gravitational and centripetal) on the center of gravity sum to aim through the contact patch. You can run the bike low, you can run it high, but if you're cornering your center of gravity is inside the contact patch and the resultant force aims right through it.

You don't weight outside the bike. You ARE the weight. You can put the bike inside or outside of your personal centerline, and as I already said, running the bike inside (or low) probably works better because of the way the tires and suspension are loaded. Evidently you missed this. Twice.


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## Tkul (Mar 1, 2007)

The question is: does 10 or even 15 mm difference makes that transformation on handling?
I see bb height like many here. Depends where you ride, and how much travel you got.
For me, it's more important the HA and chainstay length than the actual bb height that will vary a lot on travel and sag you normally use.
Static measurements are only for reference. Bikes should always be measure with recommended sag (taking into account suspension efficiency and cogs that normally are used).

Also, rider weight is more than 80% of the bike+rider weight. CoG is in belly, and if drop your heal and lye on the bike, your CoG will very low.
Having so low value son bb height, and using this technic on rocky terrAin, I hope you use combat boots!


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