Steve’s Bike Tech

The Art & Science of Bike Fitting

By Steve Bowen

This subject is fresh on my mind, as I have just returned from a three day advanced fitting course at the headquarters of Specialized Bicycle, in Morgan Hill California. Last year I took the standard fitting course there, and the year before I had four days of training in bike fitting at a seminar sponsored by Waterford Bicycles and the makers of the “Fit Kit.”

Old methods of fitting bikes

Ideas about how a bike should fit have changed greatly over time, but unfortunately a lot of myth, misinformation and old ideas are still promulgated. In the 1970’s, when there was a great boom in the popularity of cycling in the US, a book on bike fitting was published in Italy. The methods contained in that book were widely used for the next 20 years, and indeed are still popular today. The problem is that this system was primarily aimed toward road racing. If you’re a fit twenty-something rider with very good flexibility, and your riding goal is to go as fast as possible every time you ride, that’s OK. But even for racers there are problems with those methods, because they don’t take into account differences in body proportions, distribution of body weight, flexibility or riding style.

So for at least twenty years, many road bike riders were set up with very low handlebars and a long reach between the saddle and the bars. This turned many people off from riding, and made others (myself included) very reluctant to get a true road bike because they were sure it would be uncomfortable. The problem with low handlebars was further exaggerated in the mid-to-late 90’s with the change to threadless stems; the headsets used had lower stack height, and the handlebar height was no longer adjustable (except in a limited way by changing the stem to one with a different angle.)

Determining correct bike frame size

There are two key considerations for bike fit: what size bike to buy and how it should be set up for an individual rider. The old method of finding the right bike size was simple – measure your true inseam, from the floor to the bottom of the pubic bone, then multiply that by .65 or .67 depending on where the bike was measured. There was some wisdom to this when all bikes had level top tubes, as it wasn’t safe to ride a bike  if you couldn’t stand over the top tube with a little bit of clearance. Unfortunately, some manufacturers measure frame size by measuring from the center of the crank to the center of the top tube, some by measuring from the center of the crank to the top of the top tube, and some by measuring from the center of the crank to the top of the seat tube. Even with that, there was no allowance for differing top tube lengths, so riders were at the mercy of each manufacturer’s idea of how an “average” rider for each height would be proportioned.

How, then, does a cyclist answer the two questions above (frame size and setup)? The good news is that many of the bikes made today are made to fit a wider range of riders. Most bikes no longer have a level top tube, rather they use one that slopes down from the front of the bike to the intersection of the top tube and seat tube, so standover height is no longer the determining factor in bike size (in fact it is one of the least important considerations.) And sloping top tubes have led to even greater confusion in the stated size of bike frames.

The only accurate way to compare one bike’s size with another is to measure the true reach of the bike. This is done by finding the center of the intersection of the junction of the head tube and the top tube, and measuring back on a line exactly parallel with the floor to the point where that line intersects the seat post (or seat tube.), as if the bike had a horizontal top tube. This virtual top tube measurement is really the best way to compare bikes and find the right size. Seat posts on most modern bikes have a much greater range of extension than older bikes, so the seat post can be raised or lowered to match your leg length once you have determined the correct reach. As long as your weight distribution is within normal bounds, you are OK. It takes an experienced bike fitter to make these determinations.

Handlebars and stems

Once you have found the correct size frame, you will need to consider stem length, handlebar reach (the distance from the center of the bar to where the brake/shift levers are mounted) and handlebar drop (the distance from the top part of the bars to the lowest part.) You also need to make sure that the bike allows you to get the handlebars high enough to be comfortable. Some bikes have taller headtubes that allow the handlebar and stem to go higher. Some come with reversible stems that have a rise in one position and a drop in the other, and some come with longer steerer tubes on the fork, and a stack of spacers between the top of the headset and the bottom of the stem.

Goals in fitting a bike

When I do bike fittings at the shop, I have three goals: injury prevention, comfort, and greater efficiency, in that order. Usually if the first two are achieved, the third comes naturally. There is no mechanical or aerodynamic advantage in setting up a bike in a way that puts you in an uncomfortable position. Also, in choosing a bike make sure you have clearly defined goals for what type of riding you intend to do, and make sure the bike is designed to be able to meet those goals.

Let me give one concrete example of why formulas simply don’t work in determining bike size and fit. If you looked at and measured the bike I ride most often, you would probably guess that I’m close to 6 feet tall, and that I ride with a very low back and shoulder position. But I am actually under 5’ 11”, and I ride fairly upright. The reasons are these: I have a very long inseam for my height (compared to the average), have a short torso but very long arms. So I need a bike that is fairly tall, but one where the top tube is on the short side, and I have a long handlebar drop because my arms are so long. I prefer doing long rides with a lot of climbing so I don’t like to be too stretched out. I also use a seatpost with no setback, because my femur (thigh) bones are relatively short compared to my tibia (shin) bones.

Take your time, do lots of research, and above all find someone qualified when choosing a new bike or having the fit of your current one adjusted. If you make the right choices, you will be able to ride longer, faster and with greater comfort. Isn’t that what most of us want from our bikes?