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.
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.
Long meandering post on trail.. after rim brakes vs disc brakes and wheel size, trail is probably the next most contested subject in bikenerdom.
SeanLuke provides a good example with the caster. I also recommend reading Dave Molton’s blog (who’s built frames longer than most people here have been writing bloated JS SPAs) for more:
Now porteur bikes and typical randonneuring bikes often have lower trail. People are often focused on the trail numbers, but the simple reason is to position the front load (handlebar bag or stack of newspapers) on or right behind the front axle. Framebuilders do this by increasing the rake (fork offset) to stretch out the front axle, which results in decreased trail. The reason is that you want the weight at the fulcrum to reduce any "inertia" when steering--so the bike remains easy to steer with the additional weight.
If you’re not intending on riding your bike with a front load most of the time, then you probably do not want a low trail bike as they are more sensitive to rider input on the handlebars when there is no weight. Also, randonneuring is just an activity, so you can ride any bike with any geometry you want. But in America, especially on the West Coast, there is a bit of bike-cosplay in the randonnerding scene, so folks like to ride bikes built in the old French Constructeur style.
Lastly, SeanLuke is probably not familiar with mountain bikes, but modern geometry for XC and enduro bikes is towards slacker headtube angles (resulting in higher trail) for stability when descending.
> Lastly, SeanLuke is probably not familiar with mountain bikes, but modern geometry for XC and enduro bikes is towards slacker headtube angles (resulting in higher trail) for stability when descending.
I'm certainly not familiar with modern mountain bikes. Very interesting that some have slacker headtubes for downhill, but it definitely makes sense.
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/ .
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.
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.
I agree with your general point but I think OP is being a bit picky about this particular page. The descriptions are pretty good and wheelbase is indeed a common bicycling term (I see it all the time in reviews and it’s listed on the product pages for the last three road bikes I bought—I just checked).
This website overall is a great resource for bike nerds, specifically their bike database; there’s nothing else like it AFAIK.
> I think OP is being a bit picky about this particular page.
That's fair. I didn't look at the other pages on the site, and in general the descriptions are solid.
To be clear, I was amused that it made the front page not because of inaccuracy, but because I don't think of HN as a community very interested in cycling.
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).
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.
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.
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.
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 manufacturers — sometimes it’s center to center (of the tubes), sometimes center to top, etc. Seat tube length is convenient but it’s not what people should focus on when trying to find a bike that fits.
> 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.
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.
This is a sad topic for me. It just doesn't exist a premium model all mountain bike, that I have even 1mm of clearance between me and the top tube. Even when standing on my toes.
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).
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.
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.
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.
There are so many opportunities for new businesses that want to provide people with high quality well maintained bikes. In my hometown Oslo, Norway, you can see super expensive e-bikes and carbon frame roadbikes everywhere. If your bike needs repairment, you have to wait at least one month.
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 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.
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.
I agree with you a 100%. Your bike should help you maintain good riding posture even when you are becoming road tired. Just be careful about who you pay to fit the bike. Most shops claim to be experts, but I have had bad experiences with some shops. It doesn't show until you've ridden a great many miles.
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.
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.
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.
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.
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.
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.
I got a rapid lesson in how changing the length of the seat stay to slacken the head tube angle leads to a reduced bottom bracket height. A rock I would normally clear snagged a pedal and I went careening over the bars. A good example of how all of these variables influence each other and how bike fit and geometry is always a balancing act.
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!
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.
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.
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.
If you are choosing a bike based on stack and reach then note that they are related. The reach and stack are measured from the same point (join of top tube and head tube). So if you see a bike that has 20mm more stack and 10mm more reach than another then it actually may have 16mm more reach at the same stack height from the bottom bracket as the smaller bike, (depending on head tube angle) so it's longer than you think.
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. :-)
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... ;)
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.
It’s been an interesting few years in relation to MTB geometry. Low bottom brackets with slack head angles and steep seat tube angles have really taken off. My last MTB purchase was in 2018 and it’s geometry seems pretty conservative to newer bikes.
I dont ecen know where to begin. So much of this is alightly incorrect its maddening. As someone who has setup race motorcycles please look elsewhere for correct terms and physics.
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!
I've seen lots of 'music explained' types of sites, but if someone were to use their mad graphics skills to explain the geometric/arithmetic relationship between various components of music, I'd pay for it. There are sites that show you the mathematical differences in scales and harmonic series - to represent this with visual geometry would be divine.
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.