# Basic frame geometry explained



## Cormac (Aug 6, 2011)

Did a search and came up empty.

Can someone please explain or link to a site that explains what all the angles and such in frame geometry do? Advantages over a bigger angle or smaller angle etc...

I look at geo specs and all I come up with is wtf? I don't even know what I'm looking at to know of a bike has good geometry or not.


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## febikes (Jan 28, 2011)

Geometry is about giving the bicycle a personality that works for certain goals. A bike built for twisty single track climbs is going to be quite a bit different from one built for fast fire road descents. General xc trail riding is going to look different from beach cruiser.

In a sense you can make a bike fit a rider with a longer or shorter seat post and stem. You can think about fit independent of geometry. A design for a frame will effect the size of stem/post the rider will need and how the bike rides.

For the basics start by looking at how the big brands lay out various bikes and then make small tweaks to blend aspects of various bikes. For the most part geometry is about making trade offs so good and bad need to be viewed in the context of what you intend to do with the bike. 

Seat tube angle is mostly about fit for the rider and comfort. There is some impact to potential chain stay length but for the most part it is about putting the seat in the proper place for a given style of riding. 

Top tube length changes how a bike handles since for a given position you can use a long TT and a short stem or a short TT and a long stem. The length of the stem is going to have a big impact on how the bike rides in terms of steering and the amount of weight on the front vs. rear wheel.

Head tube length and seat tube length will impact how stiff the bike rides (short seat tube and long post is less stiff then long seat tube with shorter post). 

Chain stays length will impact how much weight is on front and rear wheels and how long the wheel base of the bike is. Longer stays will not respond as quick to steering inputs and at the extreme you can get a bike that rides like a bus or one where the front wheel will pop off the ground too often.

To narrow things down you should let us know what type of riding you want a bike. With an idea of the type of riding and the changes you want to make in the way your bike rides I am sure many people will have ideas of what is "good".


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## jgerhardt (Aug 31, 2009)

I do not profess to have a complete understanding of frame geometry, and all of its subtle nuances...

Sheldon Brown however DID, here is a great article of his regarding it...


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## pvd (Jan 4, 2006)

Designing a bike is about robbing from Peter (ha!) to pay Paul. You compromise on everything in the hopes that you can approximate just a few things. Here's a quick metal spazm.

* Top tube length doesn't exist. It's not a number of any importance with regard to frame design. The same can be said for 'drop'. Silly garbage repeated over and over.

* Head angle is the steering rate. It's important. Don't mess with it to change trail, that's what rake is for.

* Bottom bracket height for every single bike in exisitance should be as low as freaking possible without causing honest issue. Mainly this has to do with pedal drag or strike.

* Seat tube angle means nothing. Seatpost setback means nothing. The two together mean something, kinda like sodium and clorine. Each is deadly, together is pimp. You are trying to postition the saddle with these two parameters. That's the goal. Good frame design, typically, will give you the slackest seat tube angle possibe with adequate clearance of the rear tire. The rest is made up with offset. Negative offset is very poor form.

* Mechainical trail means something. Ground trail means nothing.

* We move weight over the tires when designing the handling. 50/50 is nice for crit. 65/35 is nice for super d. The shape of the rider can have some influence here. KOPS approximates a nice road/peddaling fit if you have a very light euro body. Fit with scales for best results.

* Current trends are for a mid-sole cleat position. The cleat is fairly far back. Look more into this to see what I'm talking about. I love this trend as my road cleats are in the same position as my DH cleats. Sweet!

* Front Center is one of the most important parameters to focus on. This and chainstay length will define how your bike rides. Longer front centers are great for high speed offroad setups. Road will have the minimum generally except for very tall riders. Chainstay length will grow for taller riders too.

* Seat tube length and head tube length mean nothing. You just need to have clearance for the riding you plan. As a general rule, a mountain bike should be able to swing the front end 180 degrees without the bars or levers striking the top tube. Low top tubes are very nice if you ride technical terrain.

* For pedaling, you generally want 150 degrees of knee bend plus or minus 5 degrees.

* KOPS is a nice starting point but it's not a rule. You can move things forward or rearward as the design goals require. TT bike = forward. Super D = rearward. DH, as high as possible without snagging the seat jumping or in gnar.

* Never design with pencil and paper. AutoCAD is pretty bad too. Both are very booleen. Parametric CAD is the only serious choice. BikeCAD is excelent for basic novice builds. The key here is seeing what changes really do and feeling free to make them. Paper and pen is the sure way to a lame bike.

* Cheap tubes are rad. True Temper is sick. Start with thicker butted tubing. I don't use fancy tubes and my bikes are pretty nice.

* Tube diameter is super important. All of them. Seat stays are often overlooked.

* Braze-on type and position can make a sweet frame lame and visa-versa. Pay attention. Most people do it all wrong.

* Deore SLX and 105 are sick parts when mounted to a killer frame. XTR and Dura-ace suck on a lame frame.

* The people you think know what they are talking about, don't.

* Tires. Tires. Tires. That's where it starts.

* Good headsets hold frames together. Use Cane Creek. Simple.

* Tapered forks are used on every decent bike these days other than DH bikes.

* Think of why the bends of the rear end are shaped the way they are. Some are bent for marketing. Some are bent for a reason. See the differernce.

* The right dropout and headtube and dropouts can make a frame. The wrong ones can ruin one. It really is important. My current head tubes take longer to make than the entire rest of the bike. Why? Because anything else wouldn't be right. Simple.

I'm tired. We'll see home much flame this post gets tomorrow....


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## pvd (Jan 4, 2006)

jgerhardt said:


> Sheldon Brown however DID, here is a great article of his regarding it...


That is the worst artical ever. I'm sorry. I love Sheldon Brown, but he didn't know anything about bike design. Just ignore that entire artical.


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## Cormac (Aug 6, 2011)

good food for thought guys! Will give me something to chew on for awhile. Thanks!

f.y.i. I do really only xc/trails. It's all that's anywhere around me. So good a great climbing geometry coupled with a solid downhill geo would be nice! Climbing is my weakness for sure. But gotta do that to get to the fun part! Also techy...eh, I'm so, so with that.


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## Blaster1200 (Feb 20, 2004)

PVD, I think that was one of your best and most informative posts ever. 

Although you have way more experience at this than I do, I'm actually surprised how much of that I was already thinking (agreeing with).


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## j-ro (Feb 21, 2009)

Cormac said:


> good food for thought guys! Will give me something to chew on for awhile. Thanks!
> 
> f.y.i. I do really only xc/trails. It's all that's anywhere around me. So good a great climbing geometry coupled with a solid downhill geo would be nice! Climbing is my weakness for sure. But gotta do that to get to the fun part! Also techy...eh, I'm so, so with that.


 Thas right there is the dilemma for the ages.

It all depends on how you look at it- if you have a hill that is a five mile climb that takes you 45 minutes to go up and 15 minutes to go down, which percentage of time would you prefer to be comfortable for? 75% going up or 25% going down?

Personally I prefer to enjoy my time going up and the geometry of my bike reflects that.
It's not that it sucks going down but it's no downhiller that's for sure.

Most box-stock mtb geometry is 'safe' but what do you expect from a stock bike.

Are you just looking at frame geometry for help in making a purchase or are you trying to design a new frame to build for yourself?

What are you riding now and what is the geometry of it?


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## Cormac (Aug 6, 2011)

j-ro said:


> Thas right there is the dilemma for the ages.
> 
> It all depends on how you look at it- if you have a hill that is a five mile climb that takes you 45 minutes to go up and 15 minutes to go down, which percentage of time would you prefer to be comfortable for? 75% going up or 15% going down?
> 
> ...


Looking to make an informed purchase, and learn. Just don't know what all the different measurements and angles represent when it comes to different aspects of riding. I'd like to be able to understand what I'm looking at when I look at the chart for a given bike.

Right now I ride some off brand wal-mart FS. Big no-no I know, but I rushed into the whole thing with no research/knowledge at all and a very limited budget. It's probably high school geometry compared to what it should be. Anyway, planning on buying a 29er first of the year or so. And I'm spending the next few months studying so to speak.


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## j-ro (Feb 21, 2009)

Cool, gotcha.

If you are just getting into it and working up to your first 'real' bike then just get a Niner.

You will either love it or have a little more information to choose something else.

At this point, comparing numbers without practical knowledge of how they relate to a bike is meaningless.

The good news is that the upcoming hands on reasearch portion of your quest is funner than heck!

good luck and have fun!


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