Bicycle Geometry Terms

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.

Wheelbase is important if you're using a bike for transportation. A longer wheelbase makes for more stability and helps with carrying groceries, children, or other cargo. It does make the bike feel less nimble so may not be optimal for purely recreational riders.

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

Everyone should think about this comment the next time they’re wow’ed by some link here.

When you’re knowledgeable about something and read an article/blog/whatever about that topic, it’s shocking how much is either somewhat off-base or just plain wrong.

The articles that aren’t about your areas of expertise are no different.

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.

Thanks for the feedback. We tried to keep these succinct and approachable, and have probably over-simplified some things as a result. I agree that we've left out some important details, and are probably too focused on the frame geometry without clearly explaining how it relates to the contact points on a complete build. We'll try to make some improvements here. If you feel inclined to point out more specifics where you feel we missed the mark I'd love to hear so we can improve the site. You can reach me at bryan@99spokes.com, or use the feedback form on the site.

Too many risers or an unorthodox stem can adversely affect the handling, aerodynamics, and structural integrity of a bike. Not to mention they can adversely affect the aesthetics.

While other measurements (wheel base, trail, rake, bottom bracket drop, etc.) may more significantly affect the performance and handling characteristics of a bike, no measurements are more important to the correct sizing of a bike than stack and reach.

Dan Empfield has written extensively about this at Slowtwitch.com if anyone out there wants to read more about why stack and reach are essential measurements when correctly sizing a bike.

There is a slow movement to using stack and reach for frame sizing, which are the two most critical measurements for bike fit.

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.

> Much more important is reach and standover height, since those can’t easily be changed.

I try, buy and build quite a lot of bikes (6 this year). Reach has never been very helpful for me. Seat tube angles differ quite a lot and the position of the bottom bracket on the x axis does not affect me that much. Effective top tube and stack is what I look for.

Being a long legged person, standover has never been an issue. I can straddle most any bike. Stack is important because that gives me a hint if I can get the saddle and handlebars on the same height without crazy stem solutions. I ride bikes from the horizontal top tube era that favor the "fistful of seatpost" rule, so C-T seat tube length is also very important to me (for purely aesthetic reasons.)

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.

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?

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

Thanks for finally posting!

> I'd be interested in any feedback.

I'll start with pointing out that there seems to be no instructions or hints on the landing page.

Also https://bikeinsights.com/

Nice! https://www.bikecad.ca/ Costs CAD500 is a one time fee. Runs on Linux, Mac and Windows.

A Free online version is also available. It takes >5min to load, just be patient.

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

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.

Very useful site, the geometry comparison amazing!

Anecdote:

My friends and I are avid mountain bikers, and semi-frequent road bikers and most of us have a pretty decent grasp on the effect of the main factors that will affect how a bike rides and handles and discussions are often had on the validity and suitability of different bikes geo choices for different kinds of riding. None of us are experts and we couldn’t tell you exactly how a bike would feel just by looking at the geometry, but you get a decent mental model after a while.

yes. most people probably haven't read a guide like this, but if you've ridden a good variety of bikes you get a sense for how the bb height affects stability, or how the head tube angle and the trail affect your steering.

some bikes even offer adjustable geometries by moving a bolt to a different position, which really makes it easy to understand how a change in geometry affects the handling characteristics.

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 be adjusted to account for body size (limb lengths), mobility etc, in order to level the hips, get the knees correctly over the pedals, aero position, correct weight on the hands, no chafing in the butt etc. It's really an art to adjust everything at once, as fixing one thing affects the others.

So while a bikefit can cost about $300, it's money well spent if one is serious about the biking (and one has then probably spent ~20x that on bike+gear anyways.. ). I'm about to head out for a ~120km ride today, so I've found a bike fit invaluable.

An example: if you want to carry saddlebags (panniers) attached to a rack over the rear wheel, it helps to have a bike with long chain stays so that your heels do not bump into your saddlebags when you pedal.

In my experience knowing and caring about bike geometry to this level of detail is definitely for the advanced enthusiast. Just knowing it doesn't mean you can apply it either. I knew the terms in the article, I even knew what I personally prefer and how a bike might feel by reading the numbers, but I won't be offering to do anyone's bike fit any time soon!

For your second bike, yes. ;-)

Granted, different people are enthusiasts for different reasons, for instance I rely on my bike for getting around town, but will never be a competitive rider. So, the interest in geometry ranges from just being safe and comfortable to ride, up to performance aspects of activities such as mountain biking, racing, or touring.

One hassle is that different makers sometimes supply a different set of measurements, especially inexpensive or older bikes, so you have to figure things out a bit. Also, a short test ride might not tell you how happy you will be after 100 miles. If there are things that you are happy or unhappy about on your existing bike, I suggest measuring it and try to figure some things out that way. When shopping for a bike (or hunting through piles of used bikes, as I've done), bring a tape measure.

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.

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.

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.

I wrote a lengthy comment here about bike fitters. They can also help with this. It's however often more complicated than just a few measurements. They often have a special bike where literally everything can be adjusted (google retül bike), and then when the fit is done (~1h) they got lots of parameters that can be put into a DB to spit out bikeframes+gear that will work for the rider. However, my experience is that the system still needs a human expert to verify and tweak the results.

However, for a basic fit your method works. For instance when buying a Canyon they ask for your measurements for some things to recommend frame size. But this of course if very rudimentary and already assumes you want that model.

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.

This also the first thing I checked. For many it does, actually, e.g.: https://99spokes.com/bikes/trek/2021/fx-sport-carbon-4

But I’m in no way a bike specialist -- maybe the weight of a bike is a feature that can be overlooked for gear of this level/price (expensive, I mean). Maybe a {1,2,...},000$ non-electric bike will always weight less than 10 kg (carbon and stuff) ? It depends on the bike purpose, of course.

I’m currently looking for a bike, but more in the 200$ range... ;)

This is the cheapest bike listed: https://99spokes.com/bikes/trek/2021/fx-sport-carbon-4

We rely on spec data published by manufacturers, and unfortunately many do not specify the weight. It's also possible we're just failing to match it. If you find a bike without a weight that the manufacturer does have on their site you can use the feedback form & we'll get it fixed up.

Weight can vary quite a bit for the same frame based on easily changed components like (1) wheels, (2) groupset (gears, pedals/cranks, brake/shift levers, that kind of thing), (3) handlebar/stem/seatpost material.

I recently bought a medium-spec build of a lightweight frame bike, and could shave 2+ lbs with upgrades, if I wanted to. Not paying for those upgrades out-of-box saved me ~4800 euro up front, and the bike is 17.9 lbs instead of 16.2 lbs. Meanwhile, I could stand to lose 15 lbs myself. :-)

Are you able to elaborate on what you'd hope for from a "Musical Geometry" site?

I have a side project on the backburner that tries to understand the building blocks of music in order to answer questions like "What could I call this chord?" and "What notes are in the key of C?". Hence my interest!