Bicycle Geometry Terms • Hacker News Board

Re: Bicycle Geometry Terms

#51

Post by petre » Sat, Jun 13, 2020, 5:32 PM UTC

loeg wrote: post #37

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.

Randonneur bikes (also road but with fatter 650Bx42 tires) typically have longer trail (typically in the seventies) because they carry front loads.

Re: Bicycle Geometry Terms

#52

Post by zbtaylor1 » Sat, Jun 13, 2020, 5:36 PM UTC

loeg wrote: post #36

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…

Agreed. These are frame geometry measurements, not contact point measurements as they are sometimes described in the article. A small but important distinction.

The sentence: "Stack gives an indication of how tall or upright a frame’s cockpit is."

Should be more along the lines of: "Stack gives an indication of how tall or upright a frame’s cockpit can be."

Frame geo is very useful info but it's best to think of it as an indicator of a range of points in space wherein you can reasonably position your contact points.

Re: Bicycle Geometry Terms

#53

Post by petre » Sat, Jun 13, 2020, 5:38 PM UTC

orthoxerox wrote: post #14

What is missing is a way to convert your body's geometry to the bike's geometry. Like, you give it your inseam length (sole to taint) and the highest pull-up bar you can grab without jumping and the site tells you what bike geometry parameters determine the correct bike size for you.

Go see a bike fitter before you buy if possible. Totally worth it. Saves you time and money and potential injuries due to improper fit.

Re: Bicycle Geometry Terms

#54

Post by loeg » Sat, Jun 13, 2020, 5:46 PM UTC

petre wrote: post #51

loeg wrote: post #37

Earlier quoted context omitted.

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.

Randonneur bikes (also road but with fatter 650Bx42 tires) typically have longer trail (typically in the seventies) because they carry front loads.

Right — I was commenting specifically on "Racing bikes etc. are high-trail bikes because by going straight the steering is stabler at high speeds."

Re: Bicycle Geometry Terms

#55

Post by loeg » Sat, Jun 13, 2020, 5:55 PM UTC

WD-42 wrote: post #46

loeg wrote: post #36

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…

Wheelbase is one of the most important measurements for mountain bikes. It determines stability at the cost of the ability to turn quickly.

It's definitely also important for road / track bikes! My observation is just that it usually isn't described by that name (or even necessarily that specific measurement). (In my experience, YMMV, etc.)

For example, Look publishes Chainstay (sometimes called "Rear Center") and "Front Center" (BB to front axle) measurements, separately, for their frames, but does not publish axle-to-axle / wheelbase measurement.

Sometimes it is described sort of qualitatively as "cramped" (on the short side).

Re: Bicycle Geometry Terms

#56

Post by zbtaylor1 » Sat, Jun 13, 2020, 5:55 PM UTC

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.

Definitely! I've been riding road bikes for ~5 years now and have "built" the last 3 bikes I've owned. I've become increasingly interested in frame geometry as it relates to bike fit (as opposed to handling characteristics). After riding a few frames and comparing their geometries I've narrowed down an ideal range of measurements for myself based on how I got along with each frame. I can browse a geo chart and be pretty certain whether or not a frame will work for me by checking a few measurements.

Re: Bicycle Geometry Terms

#57

Post by loeg » Sat, Jun 13, 2020, 5:59 PM UTC

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.

Broadly, yes. Mechanics almost certainly. Enthusiasts can get away with not knowing some things to a point, but I think you either lose the enthusiasm or pick it up eventually.

Re: Bicycle Geometry Terms

#58

Post by loeg » Sat, Jun 13, 2020, 6:01 PM UTC

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…

Professional bike fitting is sort of a sub-discipline of physical therapy, at least in the US. (That might not be obvious to people who haven't had a professional bike fit; it wasn't to me before I did.) To the extent your health insurance covers PT, it covers PT bike fits.

Re: Bicycle Geometry Terms

#59

Post by zbtaylor1 » Sat, Jun 13, 2020, 6:04 PM UTC

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.

For anyone interested in tracking their own bike fit at home, I've been building a web app based on the park tools road positioning chart (https://www.parktool.com/blog/repair-help/road-positioning-c...).

This is different than frame geometry, which is what is being discussed in this 99spokes article, but it's quite useful for anyone that likes to play with their fit or wants to transfer their current fit to a new frame.

It's a work in progress, but the app can be found here: http://app.mybike.fit.

Re: Bicycle Geometry Terms

#60

Post by loeg » Sat, Jun 13, 2020, 6:04 PM UTC

monsieurgaufre wrote: post #44

jsinai wrote: post #13

Earlier quoted context omitted.

My thoughts exactly. Being on the shorter side what matters is the stand over height - too high and it greatly limits on/off/standing manoeuvrability and agility. I was surprised to learn that the smallest adult bikes tend to have a stand over height of 68-70cm. For some bikes, it’s even higher. For the lower tail of the adult height distribution, this is too tall to use comfortably.

Look into bikes with 650b wheels for this.

Pay attention to BB height, specifically. That'll dictate ground clearance and comfortable saddle height. 650B wheels are often fitted with bigger tires, and can have the same effective diameter as 700C wheels with smaller tires (depending on specific tire size).