# The great bottom bracket drop thread



## Walt (Jan 23, 2004)

I've split this off from the "Big Wheeled Bikes are Built Wrong" thread. Be nice, guys. 

Basically, all y'all framebuilders, what do you know about bottom bracket drop? How much do you like to build your mountain bikes with? How do you think it affects the handling of the bike? Is there an alternate geometry number you prefer to use instead? How does it interact with the other variables that go into a design?

-Walt


----------



## pvd (Jan 4, 2006)

Bottom bracket drop has no meaning when describing bicycle handling. It is an imaginary property.

Bottom bracket height, chainstay length, and wheel diameter play critical roles in bicycle handling and are thus primary dimentions.

Bottom bracket drop only has use for measuring frame geometry on surface tables or in jigs. It is then converted to bottom bracket height, the important number.


----------



## gabe (Mar 25, 2004)

My take on BB drop.......bikes like to have the center of gravity below the hubs, clearance to the ground is the limiting factor. A dissertation could be done on this subject but then when would we have time to ride and build stuff. :thumbsup:


----------



## DeeEight (Jan 13, 2004)

pvd said:


> Bottom bracket drop has no meaning when describing bicycle handling. It is an imaginary property.


Then why is this made...

http://www.anvilbikes.com/gallery/d...10&sessionid=408f888f313886a141ea40f25665c21a

If it has no meaning ?


----------



## pvd (Jan 4, 2006)

gabe said:


> bikes like to have the center of gravity below the hubs, clearance to the ground is the limiting factor.


1. The height of the bb defines the ground clearance.
2. The center of gravity on a standard bicycle system is always a few feet above the hubs.


----------



## pvd (Jan 4, 2006)

DeeEight said:


> Then why is this made...If it has no meaning ?


That is not a bicycle. That is a frame jig. We are talking about bicycles.


----------



## gabe (Mar 25, 2004)

Time to learn.............this is a good thread. R. Talbot ("Designing and Building Your Own Frameset") says...."Raising the BB makes for a slightly less stable ride by elevating the combined center of gravity. Lowering it slightly improves stability and has a subtle effect on the feel of the bicycle under sprinting conditions". He goes on about specific ride types..... MTB, Track, Touring.....but I think the premise is that an understanding of this (like rake and trail) is of paramount importance when designing a custom built frame.


----------



## TortugaTonta (Jun 14, 2004)

pvd said:


> 1. The height of the bb defines the ground clearance.
> 2. The center of gravity on a standard bicycle system is always a few feet above the hubs.


The center of gravity is never "allways" in a specific location when one is riding a bike. Well maybe if you are riding a trainer but that is really not "riding" a bike.

Just being a stickler.


----------



## pvd (Jan 4, 2006)

TortugaTonta said:


> The center of gravity is never "allways" in a specific location...


That's why I was not specific about where the CG was, just the neighborhood.


----------



## VT Mike (Jan 12, 2004)

Walt said:


> Basically, all y'all framebuilders, what do you know about bottom bracket drop? How much do you like to build your mountain bikes with? How do you think it affects the handling of the bike? Is there an alternate geometry number you prefer to use instead? How does it interact with the other variables that go into a design?
> 
> -Walt


I think it's important to keep crank arm length in mind here too. The center of gravity is really influenced mostly by the saddle height (handlebar height and the length from the saddle to the bar play a role too), which is going to be determined by the bottom bracket height (or drop, however you want to define it) and the crank arm length. An example would be two similar bikes, one with a 12.5" bottom bracket height and 175mm crank arms, the other with a 12" bottom bracket height and 165mm crank arms. On both bikes the pedals will be roughly the same distance from the ground at their lowest point. So both bikes should have the same saddle height relative to the ground, giving them the same center of gravity.


----------



## G-reg (Jan 12, 2004)

I think PVD is right here, in his own way. BB drop exists, but it is a means to achieve a desired BB height. Point being you don't build a bike around a specific BB drop, you use BB drop after determining a lot of other factors to set your BB height. 

That being said, I think I prefer a slightly taller BB. What height is considered optimal, I used to assume as low as you can get away with. But a friend had a custom IF Deluxe built with a relatively low bb, and it felt really strange to ride. It was like you were sitting between the wheels not above them, and it really messed with my head as far as cornering and using the pedals leverage.


----------



## TortugaTonta (Jun 14, 2004)

I have thought about this and I think everyone is a little right and a little wrong. I am also not trying to say that I am totally right. 

Just some things to think about.

Every one seems to be thinking of center of gravity as a fixed point, it isn't.

Lets say you are riding a pump track or a bmx track and you are manualing a set of rollers. You would move your center of gravity (some where around your belly button) behind the rear axle and below the seat to smoothly hold the front wheel at a consistent elevation while forcing the rear wheel to follow the contours of the ground. Now at this point if you compared a 20" bmx to a 24" bmx with the same bb height the 24" would obviously have more drop. Now if you were to draw lines from your grips to pedals to belly button in relation to the bb and rear axle you would see a difference in the bb height based on drop. Sure it would be tiny but it would be there.

Not trying to start a argument about math because I really don't care. Just giving an example of how the seemingly irrelevent things can add up to a total "feel". Bikes with the same bb height but different drop because of wheel size will have a different feel for sure.

The other thing to keep in mind is optimal bike geometry is a matter of opinion. Someone who rides in delaware isn't concerned about hitting pedals on rocks, someone who rides in western Pennsylvania is.

How many great bikes have been ruined by poor setup? I see people riding a frame that is clearly to small and they put a 120mm stem on it and wonder why they go over the bars.

I agree that most bikes are built wrong but for different reasons.

I am suprised nobody seems interested in asymetrical chainstays for a zero dish 135mm rear wheel. I personally hate the banjo string tight drive side spokes and dull loose nondrive spokes. 

Equal spoke tension all the way!!!! Woooo!!!


----------



## ferday (Jan 15, 2004)

i personally believe in BB drop rather than BB height.

BB drop is a fixed number (assuming no rear suspension of course). BB height will change with fork swap, tire swap, wheel change. also BB drop is _directly_ related to tube length / chainstay length, therefore a more useful number IMO. particularly in the 29er world, people are buying bikes based on chainstay length and TT length, which makes the BB drop a far more useful number. i've personally never held BB height very important when buying a new bike.

since BB drop and BB height are directly related, it will affect handling exactly the same way BB height will just a little more calculation...so kind of silly to pretend that one is "imaginary".


----------



## Walt (Jan 23, 2004)

*Don't get me started!*

I've offered this option (rear end offset 5mm to the driveside) for years, and a grand total of _2 people_ have ever taken me up on it. I get comments like "it'll look weird" and "but I'll need a proprietary wheel!" (or alternately "I can't use a crossmax!") and "how do I build a wheel like that?". When I explain, they just shake their heads and tell me to build a standard symmetrical setup. Crazy.

FWIW, on geared mountain bikes with 135mm rear spacing, I think all rear ends should be offset to allow a near-dishless wheel. Heck, it's even a way to get a better chainline with the newer 50mm chainline cranks!

Maybe we should start a "pet peeves" thread? Someone want to do that? Or should I?

-Walt



TortugaTonta said:


> I am suprised nobody seems interested in asymetrical chainstays for a zero dish 135mm rear wheel. I personally hate the banjo string tight drive side spokes and dull loose nondrive spokes.
> 
> Equal spoke tension all the way!!!! Woooo!!!


----------



## CBaron (May 7, 2004)

Walt said:


> Maybe we should start a "pet peeves" thread? Someone want to do that? Or should I?
> 
> -Walt


Honestly one of my biggest pet peeves is pontificating about "theory". I'm much more of an experience kinda guy.

Therefore in the same spirit as the adjustable rake fork...we've had begun putting together an adjustable BB height frame (HTA too).


----------



## greyspoke (Apr 30, 2007)

Corblimeystrikealightguvnor I'm with you on the offset bit Walt. Got it on my Demo8 (Big Hits also had it) and the rear wheel holds up well on that (though this may have more to do with my mincing DH style and advanced years). No problem building a wheel for it. In fact I asked on the 29er forum what peeps thought of the idea on this thread - I guess you missed it. Looks like you're not alone!


----------



## pvd (Jan 4, 2006)

Walt said:


> FWIW, on geared mountain bikes with 135mm rear spacing, I think all rear ends should be offset to allow a near-dishless wheel.


off topic - Walt - While I belive that a dishless rear wheel is a nice thing, and good chainline to be even more important, offseting the rear wheel off plane from the head is really really bad. In autosport, it's called crabbing. Think of a crab walking. It screws every thing up. left turns differnt than right, increased squirm, unstable tracking. I would really reconsider this concept.

on topic - I strive for the lowest BB possible on every build. PROVIDED the pedals do not hit the ground or rocks 'TOO OFTEN'. This involves crank lenth, width and pedal geometry as well. Wherever my CG is in this case, it is at it's lowest. This improves climbing, decending, and cornering. What do I gain by having it any higher?


----------



## Walt (Jan 23, 2004)

*Not sure you quite got it...*

Pete-

The wheel gets dished 5mm back to the nondrive side, so it stays in plane with the head tube. I've done several of these for myself and they ride great. All you're doing is moving the hub and cassette - the rim and tire stay in exactly the same place they would be on any "normal" bike - in the plane of the front wheel/head tube/seat tube.

-Walt



pvd said:


> off topic - Walt - While I belive that a dishless rear wheel is a nice thing, and good chainline to be even more important, offseting the rear wheel off plane from the head is really really bad. In autosport, it's called crabbing. Think of a crab walking. It screws every thing up. left turns differnt than right, increased squirm, unstable tracking. I would really reconsider this concept.
> 
> on topic - I strive for the lowest BB possible on every build. PROVIDED the pedals do not hit the ground or rocks 'TOO OFTEN'. This involves crank lenth, width and pedal geometry as well. Wherever my CG is in this case, it is at it's lowest. This improves climbing, decending, and cornering. What do I gain by having it any higher?


----------



## Walt (Jan 23, 2004)

*Fugly enough, but I still win*

I believe that my fork has you beat for general amateurishness and terrible looks. Plus, I can use it as a rain gauge. 

-W



CBaron said:


> Honestly one of my biggest pet peeves is pontificating about "theory". I'm much more of an experience kinda guy.
> 
> Therefore in the same spirit as the adjustable rake fork...we've had begun putting together an adjustable BB height frame (HTA too).


----------



## CBaron (May 7, 2004)

Walt said:


> I believe that my fork has you beat for general amateurishness and terrible looks. Plus, I can use it as a rain gauge.
> -W


Ok, I'll see your _"fugly_" and raise you....*DANGEROUS!!!*

How about a HT junction that is BOLTED into place?

....pic of our adjustable HTA concept.


----------



## Walt (Jan 23, 2004)

*Didn't Cannondale do something similar?*

I think it was the Jekyl. It had 2 or 3 different HTA settings. Maybe just 2.

You know if Cannondale does it, it must be a really good idea!

Seriously, post more pictures of this beast as you go, eh? I think it's pretty neat! Like a fit-cycle for actually riding (you should make it have an adjustable TT length, too!)

-W



CBaron said:


> Ok, I'll see your _"fugly_" and raise you....*DANGEROUS!!!*
> 
> How about a HT junction that is BOLTED into place?
> 
> ....pic of our adjustable HTA concept.


----------



## NEPMTBA (Apr 7, 2007)

To all:

One thing though is the fact the pedals can be at different places on the clock A pedal at 6 o'clock yields a different feel as well as the pedals level at 3 & 9 o'clock. This is the neat thing about bicycles that can't be imitated on a motorcycle due to the fixed foot pegs.
The user can actually change the handling of the bicycle by doing so. Dropping a pedal to the lowest point on the inside of turn gives the feeling of rocketing around the corned where as keeping the pedal level or higher gives a more upright slower turning feel. Of course if you strike a pedal then you know the result! Hopefully it was facing back when it hit!... LOL!

Thanks for listening... Just rambling...


----------



## TortugaTonta (Jun 14, 2004)

Walt said:


> I've offered this option (rear end offset 5mm to the driveside) for years, and a grand total of _2 people_ have ever taken me up on it. I get comments like "it'll look weird" and "but I'll need a proprietary wheel!" (or alternately "I can't use a crossmax!") and "how do I build a wheel like that?". When I explain, they just shake their heads and tell me to build a standard symmetrical setup. Crazy.
> 
> FWIW, on geared mountain bikes with 135mm rear spacing, I think all rear ends should be offset to allow a near-dishless wheel. Heck, it's even a way to get a better chainline with the newer 50mm chainline cranks!
> 
> ...


Not so quick story..

Specialized offers this on some of their p bikes, big hits and the demo 7. So a guy I know gets a p bike, he is ripping on it and another guy tells him the frame is bent. A few other guys look at it and say "dam how did you bend it so much" he takes it back to the bike shop and they are so stupid that they send it back to specialized for a "warranty issue" Now by the time the story gets to me the "mechanic" at the shop tells me that "specialized screwed up the whole run of frames so they had to build special wheels for them" I was speachless. I really don't know how some people get high school diplomas. The best part of the story is about a year after this Bicycling magazine puts the genius on the cover with the quote "he can fix anything"

From what I have read above there seems to be a "genius" here as well.

I actually just came back from the lbs, I had to drop off my race x lite, it hada broken flange on the drive side. Hmmm... I wonder why, oh by the way it broke while I was riding ROLLERS in the basement.


----------



## CBaron (May 7, 2004)

Walt said:


> Seriously, post more pictures of this beast as you go, eh? I think it's pretty neat! Like a fit-cycle for actually riding (you should make it have an adjustable TT length, too!)
> 
> -W


This frame/concept is something we began a while ago and just never got around to completing. (Un)Fortunately, we got busy building actual frames.... I think during the holidays I'm going to get back in and try to complete this project. [largely inspired by the discussion in these threads]

Will all that said, I can still make changes to the design. Do you have any idea on how to make an adjustable TT? (and also still be a safe(?) rideable frame) I'm planning to weld in track end rear DO's build with a wider spaced rear axle so that I can then BOLT ON another set of track end DO's. This will effectively give me an adjustable slider configuration which will result in variable wheelbase (up to 2-3").

Off to lunch...
CJB


----------



## pvd (Jan 4, 2006)

Walt said:


> The wheel gets dished 5mm back to the nondrive side, so it stays in plane with the head tube.


Got it. That's fine. But why not just use a 140mm hub? Dishless is nice, but wider is better.


----------



## Walt (Jan 23, 2004)

*Because there aren't any?*

I guess I could use a 145mm tandem hub (expensive, heavy, not many out there), but I've never seen a 140mm hub. Besides, a 140mm hub would only get you 2.5mm less dish, not 5 (which is close to zero dish on a typical hub).

I tend to think that wider hubs would be great, but the industry isn't there yet. The offset rear end is a good workaround in the meantime.

-Walt



pvd said:


> Got it. That's fine. But why not just use a 140mm hub? Dishless is nice, but wider is better.


----------



## pvd (Jan 4, 2006)

Saint 150:

http://bike.shimano.com/media/techd...I_3C40A/SI_3C40A_En_v1_m56577569830612455.pdf

CK: 140, 145, 160mm:

http://www.chrisking.com/hubs/hbs_tandem.html

phil 140:

http://www.philwood.com/Touring Hubs.htm


----------



## Walt (Jan 23, 2004)

*Those are nice!*

That King hub is sick! <400 grams for a rear tandem hub, including the bolts? Wow!

I'll keep those in mind for the next customer who asks about super strong wheels. Thanks!

-Walt



pvd said:


> Saint 150:
> 
> http://bike.shimano.com/media/techd...I_3C40A/SI_3C40A_En_v1_m56577569830612455.pdf
> 
> ...


----------



## DeeEight (Jan 13, 2004)

Walt said:


> I guess I could use a 145mm tandem hub (expensive, heavy, not many out there), but I've never seen a 140mm hub. Besides, a 140mm hub would only get you 2.5mm less dish, not 5 (which is close to zero dish on a typical hub).


Shimano offers both 140mm and 145mm tandem hubs. I'm sure CK offers something in both also. 
[/QUOTE]


----------



## pvd (Jan 4, 2006)

Walt said:


> That King hub is sick!


I'm not the biggest King fan, but I just noticed this new hub: ISO150

https://www.chrisking.com/hubs/hbs_150mm.html










336g, 63.5mm flange spacing, 9.5 more than the ISO135's.

I'm acutally tempted if I could get one with a 10mm TA instead of 12mm.


----------



## TortugaTonta (Jun 14, 2004)

Walt said:


> I guess I could use a 145mm tandem hub (expensive, heavy, not many out there), but I've never seen a 140mm hub. Besides, a 140mm hub would only get you 2.5mm less dish, not 5 (which is close to zero dish on a typical hub).
> 
> I tend to think that wider hubs would be great, but the industry isn't there yet. The offset rear end is a good workaround in the meantime.
> 
> -Walt


Basically every dh bike has a 150mm hub. They also have at least a 83mm bb shell some have a 100mm bb shell. The spindle length is to long to really pedal comfortably all day long. 135 zero dish is ok for a xc type bike.

On a side note Mavic spends quite a bit on r&d to try to come up with a dished wheel with equal spoke tension, combo of radial and cross spoke patterns and what not. To me it seems like a band aid to fix the real problem. But then again I would like to see 5 speed with 8 speed spacing so we could move the flanges a little further apart, but that is just my crazy talk.


----------



## eMcK (Aug 22, 2007)

If you put one of these:









https://www.interlocracing.com/freewheels_steel.html

On one of these:










You get a 5 speed with 8 speed spacing and a dishless wheel.

Or one of these:










https://www.profileracing.com/products_home.php?productid=52


----------



## D.F.L. (Jan 3, 2004)

A 145 or 150 rear hub is just as incompatible with your buddy's bike as an asymetrical 135 is. Weighs more, costs more, and has uneven spoke tensions. And price that King tandem hub. I seem to think that it wholesaled for $375. There are very few disk tandem alternatives that don't weigh a ton. 

And with an asym 135mm rear you'll only rub your heel on one stay, rather than both! My 29er is set up asym and it's a noticable improvement over standard. 

What helps mask the bad chainline on 29ers is the additional chainstay length.

BB height is the term that customers understand. Drop doesn't translate easily into log clearance and pedal strikes: the stuff riders care about. Drop is best used for setting up the fixture.

Now, if you really want fightin' words, then start a thread about actual vs effective chainstay length. Yikes.


----------



## Walt (Jan 23, 2004)

*Naw, I still rub 'em both*

I walk pretty much like Donald Duck, so even on asym bikes, there are two bright shiny spots where my heels hit the stays. I once actually BROKE a frame (admittedly, a carbon one) by wearing through both chainstays with my heels.

-Walt



D.F.L. said:


> And with an asym 135mm rear you'll only rub your heel on one stay, rather than both! My 29er is set up asym and it's a noticable improvement over standard.


----------



## pvd (Jan 4, 2006)

eMcK said:


> Or one of these:


https://www.pvdwiki.com/index.php?title=Six_Speed_MTB



















135mm Hadley SS with 6 nine speed spaced cogs. Dishless! 10mm TA or 9mm QR. DOPE!


----------



## pvd (Jan 4, 2006)

D.F.L. said:


> Now, if you really want fightin' words, then start a thread about actual vs effective chainstay length. Yikes.


Everyone knows that chainstay length is just a radius.


----------



## eMcK (Aug 22, 2007)

pvd said:


> Everyone knows that chainstay length is just a radius.


It is flamebait statements like this that ruin threads.


----------



## D.F.L. (Jan 3, 2004)

Walt said:


> I walk pretty much like Donald Duck, so even on asym bikes, there are two bright shiny spots where my heels hit the stays. I once actually BROKE a frame (admittedly, a carbon one) by wearing through both chainstays with my heels.
> 
> -Walt


I've had one person ask me if I rub my heels on the drive side like he did when he borrowed my bike(giant feet), but I never have (manly feet, but ..). I know what you're saying; I put a little extra s-bend on the drive side to help keep it away from the heels, but it's probably still a little closer than normal.

P, radius, yeah, but mine was just a feeble attempt at humour referring to the oft silly things that framebuilders will debate to death.


----------



## pvd (Jan 4, 2006)

eMcK said:


> It is flamebait statements like this that ruin threads.


Yeah. The last time I suggested that 2+2=4 on a forum the thread went on to 4 pages of flames.

It seems that irrefutable facts are always the most hated concepts on message boards. Rumor is regarded as the gold standard of knowlage.

I figure that I would sound really stupid if I said, "2+2=4, IMHO", but people do this all day long. Crazy.


----------



## netpyro (Nov 26, 2007)

D.F.L. said:


> Now, if you really want fightin' words, then start a thread about actual vs effective chainstay length. Yikes.


effective is more important. actual is just a number for the brochure.


----------

