I am a former bike shop mechanic, salesman, fitter and manager (These days I’m a financial advisor at Smith Barney).
I still build or fit the occasional bicycle for friends, and have never had a problem when I stuck to the rules I’ll
outline in this article. They are pretty much what you’ll find in the “Fit Kit” product, but most of my hard numbers
come from sections of “Greg Lemond’s Complete Book of Bicycling” (17 years old, but still a good read), where he
outlines his coach’s method for fitting. It is easy to scoff at information found in a book by a racer, but racers
are the people who spend the most time on a bicycle, and comfort and protection from injury is their primary concern.
On top of that, most every “road bike” sold today is much closer to racing geometry than any other type of bicycle.
So I feel confident that using the information generated by over a century of racing experience applies even to
those of us with leg hair and rear view mirrors.
The first thing to understand about bicycle fit is that it is an attempt to get your body to properly interact with off-the-shelf shoes, saddles, handlebars, etc. The manufacturers of those goods designed their products to interact with the human body in a particular way, and made certain assumptions about how they expect those products to be employed. In other words, a saddle maker expects riders to sit on their saddles in the “traditional” manner, so riders will get the most out of that saddle by employing it with the same rules of thumb that the maker used to design it. Even “custom” frame geometries are likely to fall within a fairly normal range, or correct the frame to normalize fit to a somewhat non-average body.
So you’ve decided to buy the newest Unobtanium bicycle frame from Joe’s House O’Tig;
but what size do you buy? Frame size is important because frames are proportional: They get
longer (reach from seat to handlebar) as they get taller. A tall person can’t just use a long
seatpost with a small frame because the handlebar reach will be way too short. So let’s talk about
“standard” bodies first: That is, people who have a fairly average ratio of inseam to height and arm
length. Most people who don’t fit in this category know it because shopping for clothes is a pain for them.
Also keep in mind that women are often “leggier” than men, but I’ve seen so many exceptions to this
rule that we can ignore it for the moment.
The first measurement you need to make is your actual inseam. No; not the one on the back of your Levi’s. To take this measurement, use a large book (encyclopedia, for instance) and stand in front of a wall in stocking feet. Place the spine of the book between your legs, firm against your crotch (as if you were sitting on it) and the top edge against the wall, to keep the spine perfectly horizontal. Make a mark on the wall where the top corner of the book touches. Do it three times to make sure you’re doing it right with consistent results. Now measure from the floor to the mark. That distance will likely be an inch or so longer than your pant inseam, and is the inseam measurement we’ll be using.
For frame size, take that inseam measurement and multiply it times 2/3 (or .666). For example, my inseam is 30” (yeah, I’m not too tall). 2/3 of that is 20”, or 50.8 cm. So I fit a bicycle frame that is 50cm or 51cm from bottom bracket center to top of top tube, measured parallel to the seat tube. (Beware; some people use center-to-center measurement, which is usually 1 to 2 centimeters smaller than center-to-top. I’d subtract 1.5 cm from our calculation result if buying a center-to-center frame.)
For those with shortish legs, the best frame size is best gauged from height: You would use the same frame size that someone your height with a more average inseam would choose. This doesn’t always work, but is a good starting point. If you keep the rest of this article in mind, you can get an idea by trying bikes around this starting point to see if those sizes will adapt, or not. The new compact style frames make fitting shorter legged people easier since they have a lower standover height for a given reach to the handlebar.
For the long leg people, height can also be used as a starting point, but so can upper body length. If you know that your legs are two inches longer than would be considered typical for your torso, use that shorter inseam number. Why? Because seat height is easy to adjust, but reach to the handlebars is more limited. Again, this is a method of guesstimating your “ideal” frame size, and needs to be approached with some care. For example: I recently fit a lady friend who is 5’7” and has very long legs. Instead of the 52 or 53cm frame that would be more typical for someone 5’7”, I found her a 50cm that fits her beautifully for height and length. That’s because the 50cm frame had a top tube length appropriate to her upper body length.
A note on compacts
Since the seat tube length is not a useful measurement on compacts, most are just sized like underpants: S, M, L, XL. The measurement that is the most useful in selecting a size is the virtual or horizontal top tube length. This is what the top tube length would be if it were horizontal and not sloped down to meet the shortened seat tube, and is usually listed in the geometry charts for compact frames. If your traditional bicycle frame size is a 54cm, consult a geometry chart for a traditional frame style and obtain the top tube length for that size – then match that number to the virtual top tube of the range of compact sizes. As stated earlier, compacts are great for people with short legs for their heights. They were created to make bikes lighter, though, so their inherent geometry is no different than those of traditional looking bikes. Otherwise, you can take ‘em or leave ‘em (I prefer the look of traditional frames).
Now that we have a frame of (hopefully) correct size, the next choice to make is crank size. Longer cranks give you more leverage, but that’s not a good reason to buy them. Different crank lengths should serve to keep some proportionality to the leg length of the person using them. The other article on this site has a formula you can try, but I just use a rule of thumb: 170mm cranks for 49 to 53cm frames, 172.5 up to 57cm and 175s larger than that. There are larger and shorter cranks than those, but they usually aren’t necessary. If you want to diverge from that, just have a reason for doing it. If you just want to stick with the old standard of 170mm, you’ll also be just fine. I would just caution against going too long for your leg length as this may put unnecessary strain on your knees.
Once you have the bike together it’s time to set seat height and level. For the most part, seats
should be roughly level. The seat makers understand anatomy and have designed a seat that should
support that pair of pelvic bones you actually are sitting on while creating no undue pressure on the
rest of you. Nose up, and you’re likely putting pressure on soft tissues. Nose down, and you’re
sliding forward, putting unnecessary pressure on your arms. If you need to make a one or two
degree change later on, that’s fine, but start with level. (I’m assuming you’ve purchased an
appropriate seat for your anatomy.)
Once the seat is level, take that inseam number again, but multiply it times .883. (Mine is 30” X .883 = 26.5”, for instance.) Use this number to measure from the center of the bottom bracket spindle up to the average top of the seat along the same angle as the seat tube. I believe this measurement is critical, because it establishes a fairly precise leg bend, which translates into the correct torque on your knees. I have corrected more knee and hip problems by either raising or lowering seats with this formula. The old straight leg method can yield similar results, but I feel this is more precise. You can fudge this number up or down if your shoe/cleat set-up is especially high or low, but most systems are close enough that it isn’t a factor.
Shoes and Cleats
Next, get your shoes and cleats seat up. The cleats are attached to match the natural angle of
your foot when standing or walking – most of us have our heals slightly turned in and toes slightly
turned out. Also, position the cleat so the widest outside portion of your foot will be directly
over the pedal spindle. Get this as close as possible, keeping in mind that your pedals have some
float and that modifying this adjustment later on isn’t going to impact the rest of your fit down
Setting saddle fore and aft position usually involves a plumb line, a friend, bike stand, mirrors, or whatever else allows you to do the following: Sit on the saddle and level the pedals. Using the forward pointing leg and crank, place the plumb line (a string with a rock on the end), on the soft part of the knee just next to the kneecap. Move the seat forward or aft as necessary to make the string intersect the pedal axle. This will give you the correct seat location, which translates into the correct average leg to hip angle throughout the pedal stroke.
Shouldn’t different length shins and thighs make this measurement useless? Nope. Given two identical inseams, but different thigh lengths, you’ll still get approximately the same results. The shorter thigh will end up with a higher knee, but that knee will still be located right above the pedal axle as the long thigh would be. If you don’t believe me, try it yourself by measuring off a fixed hip, ankle and pedal axis points and dividing a given leg length a couple different ways. It works out pretty well, and more importantly, sets your hip to leg angle optimally. Again, we’re looking for efficiency and decreased risk of injury. If this method helps, it’s worth it.
Handlebars are coming in more varieties these days, but let’s stick to a very traditional road handlebar pattern. The ends of the bar should roughly match the widest part of your shoulder bones. The easiest way to determine this is to simply pick up the bar and touch the ends to both shoulders. Women ‘tend’ to use a 38cm or 40cm width, men go 42cm, 44cm or 46cm. If you’re shopping online, take a measurement and make sure to ask if the bars are measured center-to-center or outside-to-outside. Some bar companies have been using outside-to-outside lately – if so, go up one size to get the correct size. When in doubt, too wide is probably better than too narrow. The goal is to accommodate a natural arm position and to not constrict your breathing.
Last stop – finding a stem length. This step is all about positioning your handlebars so that they are under your upper body in a spot that is natural and easy for your arms to get too. The correct stem length doesn’t “stretch you out”. Issues of back comfort are best accommodated by stem height, not horizontal distance.
To accomplish this, get on your bicycle with a correctly adjusted seat position (both for height and fore-aft position) and lean into a comfortable ‘drop’ position with the handlebar. You may need some help to support the bar in the air (since you don’t have a stem yet), but we’re looking for the point where the center of the bar tends to obscure the front hub when you glance down. When you find that point, have your helper make a horizontal measurement back to the center of the steerer tube from the center of the bar – that’s your stem length. Talking about a traditional frame and fairly average body plan; if the frame is the correct size you should end up needing somewhere between a 90mm to 130mm stem and have a gentle bend at the elbow. If you are well outside that, something else went wrong somewhere along the line with your frame size or seat position.
The stem length formula does assume your arm length is reasonably proportional to the rest of your body. But most people with short arms tend to have short torsos and/or legs, so this usually works out okay. When in doubt, try to duplicate the arm angle you see on other cyclists.
As I stated at the beginning; suspect or not – this system seems to work. Spending three hours on a bicycle that is misadjusted to your critical body measurements is an invitation to some permanent repetitive stress injuries – so if you are diverging from these guidelines, know why you’re doing it and what the expected results should be. I won’t speak for any particular area bike shop, but all of them should have fitters that are going to be using most of these methods to achieve results that are usually very close to each other. When in doubt, consult an expert. But I did my first fitting at seventeen with directions from a book. Nearly twenty years later, that bike still fits me and my other road bikes are set up the same way. Fitting needs to be done with care, but it isn’t rocket science.
What you’ve just read doesn’t apply 100% to all bicycle disciplines. While the seat height formula generally does, triathalon bikes (for instance) have a different methodology when it comes to seat and bar position – so find an authority for that kind of bike fitting. But I would use the exact same method for a touring or cyclocross bike – but with a higher bar position.
If you don’t feel that you can be made comfortable on a racing style bicycle using the methods outlined here, maybe you should be looking at some other kind of bicycle altogether – like a recumbent.
I’m not reinventing the wheel – this information is out there from a variety of sources. My aim is just to make it more available and explain the rationale behind some of it. Fitting is art and science, so there is plenty of room for opinion – you’ve just read mine.
If you have questions, send me an email at firstname.lastname@example.org or call me at (608) 283-2330.
Thanks for reading and happy bicycling!