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Bicycle Geometry Terms

99spokes.com

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Re: Bicycle Geometry Terms

#31

Post by barrkel »

seesawtron wrote: post #3

Do you think people who fix bikes or are bike enthusiasts know which of these components affect which aspect of riding? I can imagine knowing all the specific names for the parts is difficult but their affect on biking experience seems very relevant. Something that was not so obvious to me.

Many of these have equivalents on motorcycles, and even if you don't know the terms, you feel the effects, and if you have a bit of knowledge, you'll know that adjusting suspension preload (or, in extremis, raising or lowering the forks in the clamps) affects rake and trail which then affects steering.

http://cycle-ergo.com/ is a site some motorcyclists use to figure out which bike suits their body and preferred riding position.

Re: Bicycle Geometry Terms

#33

Post by usrusr »

matsemann wrote: post #9
seesawtron wrote: post #3

Do you think people who fix bikes or are bike enthusiasts know which of these components affect which aspect of riding? I can imagine knowing all the specific names for the parts is difficult but their affect on biking experience seems very relevant. Something that was not so obvious to me.

There are professional "bike fitters" that know how to adjust all these things (those that can be adjusted) and more in such a way you get everything set up the best given your body and goals. For instance, given a bike and a rider, one can adjust seat height, seat position, stem length, handlebarhheight, crank arm length and possible more things to get the correct reach and stem height. However, all that also has to…

Money well spent if the fitter is actually good. I've seen some rather unconvincing results.

Re: Bicycle Geometry Terms

#34

Post by radlad »

bill1306an wrote: post #32

Although I'm an avid reader, this is my first ever comment. I built this site https://comparemtb.com/ to give people a way to visually compare the geometries of many different mountain bikes. I'd be interested in any feedback. Thanks.

As a motorcycle rider, this site was invaluable to me when considering new bikes to buy: https://cycle-ergo.com/

Now that I'm getting into bicycles, I'd love something like that. Have you considered adding bikes other than MTB?

Re: Bicycle Geometry Terms

#35

Post by SeanLuke »

The description of trail could be a lot better, given how important it is. It's effectively the degree to which the front wheel acts as a caster. When you push a shopping cart the caster wheels automatically straighten out and trail behind the steering axis in a straight line. The front wheel is doing the same: when moved forward it resists being steered and wants to go straight. The degree of trail effects how much it wants to go straight.

Racing bikes etc. are high-trail bikes because by going straight the steering is stabler at high speeds. High-trail bikes can also be ridden no-hands because the wheel is effectively on autopilot. Any time you need more nimbleness and precision in steering -- perhaps a mountain bike? -- you want to lower the trail. Porteur bikes -- bikes that carry big boxes of flowers over the front wheel say -- also need very low trail because they're already hard to steer due to all that inertia. And with small wheeled bikes it is very difficult to achieve significant trail without a very shallow head tube (steering) angle, so many folding bikes by necessity have a very "live" steering. One of the dumber bike decisions I can think of is Brompton's inclusion of a rake on the front wheel, thus making its steering worse.

Re: Bicycle Geometry Terms

#36

Post by loeg »

As a bicyclist, I'm amused this made it on to the HN front page.

I'm not sure I agree with the description of stack. Cockpit height can be adjusted with riser stems and different handlebar shapes; it's not solely a function of axle-crown and headtube length.

Without getting into it too much, some other descriptions are not great either. The general overview is ok but they make some overly strong claims.

I'm not sure I've seen wheelbase used much in a bicycle context, although it makes sense. You see it used for cars a lot.

Edit: The go-to source for bicycle information I'd point others at is Sheldon Brown's webpage, https://www.sheldonbrown.com/ .

Re: Bicycle Geometry Terms

#37

Post by loeg »

SeanLuke wrote: post #35

The description of trail could be a lot better, given how important it is. It's effectively the degree to which the front wheel acts as a caster. When you push a shopping cart the caster wheels automatically straighten out and trail behind the steering axis in a straight line. The front wheel is doing the same: when moved forward it resists being steered and wants to go straight. The degree of trail effects how much…

Trail on road bikes, racing and not, seems to be universally in the range of 55-60mm (across frame sizes, too). I suppose it depends on what kind of racing you're doing, but some kinds of bicycle racing involve cornering agility / nimbleness, and you want a shorter trail for that.

Re: Bicycle Geometry Terms

#38

Post by loeg »

bill1306an wrote: post #32

Although I'm an avid reader, this is my first ever comment. I built this site https://comparemtb.com/ to give people a way to visually compare the geometries of many different mountain bikes. I'd be interested in any feedback. Thanks.

Recently I've used https://geometrygeeks.bike/ to compare geometry numerically. Having a database like that is pretty useful.

Re: Bicycle Geometry Terms

#39

Post by loeg »

cranekam wrote: post #7

Great explanations! It’s really silly that seat tube length is used as the single measure of frame size. Much more important is reach and standover height, since those can’t easily be changed. A longer or shorter stem will adjust reach at the expense of changing how responsive the steering is, whereas a short seat tube can be fixed with a longer seat post. Seat tube length is also measured differently by different ma…

Seat tube length only really makes sense for sizing for classic flat top-tube road bikes. It definitely doesn't make sense for aggressively sloped top tubes on carbon MTB and road bikes.

Most vendors these days seem to do Small/Medium/Large (etc) sizing, rather than seat tube measurement. It isn't portable across bikes, but neither would seat-tube length be.

I disagree that standover height has much relationship to fit. There is some practical aspect, especially for newer riders. But you can have a very high standover with a high bottom bracket with similar fit to a lower standover with lower bottom bracket.

Re: Bicycle Geometry Terms

#40

Post by loeg »

audiometry wrote: post #8

If you are interested in messing around with these settings and many more, look at the great parametric bicycle cad system “Bikecad”. Brett curry maintains and updates it for eons now. And crazy licensing — one time payment gets lifetime of updates.

If you want to compare a bunch of bikes, https://geometrygeeks.bike/ has a solid database of geometry data.
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