The Fundamentals of Fit

By far the most important thing to get right when purchasing a bike is the fit. You might have all the finest carbon fiber or titanium frame and components but you’ll be going nowhere fast if your bike doesn’t fit you properly. By MICK ALLAN

In order to make life easy for the folk who manufacture bikes human beings have been fairly well standardised. It's possible to make quite sweeping generalisations about our proportions; the ratio of our limbs to our torso and the range of sizes we are available in doesn't vary much. It's the reason why 99% of the population can walk into a bike shop and find a bike to suit them right off the shop floor.

Why then are so many cyclists riding bikes which haven't been adjusted correctly or worse; entirely the wrong size? We have no trouble buying shoes to fit us why are bicycles so difficult?

Bikes are available in a range of sizes. The least expensive new bikes out there, the ones from mail-order catalogues and the supermarket, might only come in one or two sizes. At the other end of the scale are those from custom builders who will measure every major bone in your body before brazing together the cycling equivalent of a Saville Row suit. Regular bikes, the kind you'll find in your local bike store, might be available in three to five sizes. They are also a bit gender specific but we'll get to that later.

These frame sizes almost always refer to the nominal length of the seat tube (from the centre of the bottom bracket where the pedals go to the top of the seat tube where the seat goes in). You'll hear numbers like ‘seventeen and a half inches' or ‘48 centimeters'.

So far so good. But now it gets silly. As anyone who fits people to bikes for a living will tell you; the length of the seat tube is actually kind of irrelevant. What we are really interested in is what's known as reach. Reach is the distance from the seat to the handlebar (centre of the seat-post to the centre of the bar). You are probably aware that it's possible to raise or lower the seat on most bikes. Anything up to eight inches on some, but what you can't do is significantly alter the reach. Which means you have to get it right. In addition, and just to confuse us even more, the seat tube length to reach ratio varies between brands, models and even frame sizes. Which means that a 52cm Brand A is unlikely to have the same reach as a 52cm Brand B.

When we are sizing a bike for someone we start by establishing their Minimum Stand-Over Height. Standing with the bike between your legs it is essential that there is a decent gap between your nether regions and the top-tube. It's the difference between your inside-leg measurement and the height of the top tube from the ground.  To establish your Stand-over clearance simply stand astride the bike with the top tube under your crotch and with your feet flat on the floor, then grab the stem with one hand and the saddle with the other and lift the bike up as high as it will go. Try and keep it level and ask someone to measure how much air you have under your wheels. On a street bike, racer or touring bike this might be as little as an inch (25mm), on a mountain bike we aim for no less than four inches (or 100mm). (In the figure above represented by the line; 'SO', With thanks to Kona bikes for the diagram) This procedure establishes the upper limit to the frame size in that particular model. Establishing the lower limit is less straightforward, the seat-post will always have a minimum insertion line but it's rare for anyone to ride a frame so small that their post is maxed out.

At this stage we adjust the saddle to suit the inside leg dimension of the rider. Technically we are starting with the lower pedal as our datum and adjusting the saddle in relation to it. That's the height. It's a given (read Bike Culture's Saddle Height Adjustment article for a more in-depth analysis). Fore and aft adjustment of the saddle is determined by the length of the rider's femur. We aim to have the back of the rider's patella (knee cap) directly above the pedal spindle when the pedal is at its furthest forward position. Again, a given. So using a couple of tools and a piece of string, within a few minutes we can have an individual's saddle to within a very few millimeters of ideal. This establishes a pretty accurate benchmark from which the rider can make fine tune adjustments.

If you are in the market for a street bike and have got to this stage it's likely that when you reach forward for the bars they will be in pretty much the right position, nevertheless there is usually some adjustability available; up, down, fore, aft and rotate. If you feel the urge you may fine tune your position to the nth degree using just an Allen key or two. And it's worth doing, spend a little time getting the set-up just right, even adjusting the position of the brake and shift levers, and your cycling miles will be so much more comfortable. The trouble is most people dont. Because they don't know how? Because they haven't the tools? Can't be bothered? I don't know for sure. I suspect that most people just don't know what a well fitted bike is suppoesed to feel like. So they contort their bodies and spend their days riding with the saddle too low and the bars too far away and then they wonder why their bike is so uncomfortable.

On modern mountain and road bikes the almost universal adoption of the ‘threadless' headset system means that handlebar stem adjustment is very limited or even impossible. Relocating the bars is a matter of completely changing the stem for one with a different length and/or rise.

Women

Considering that women make up half of the population it's taken a shockingly long time for manufacturers to get their collective brain around the idea that women might like to ride decent bikes too.

Women's saddles are slightly shorter and slightly wider than men's saddles. This is because women have shallower pelvises and their Ishial Tuberosities are slightly further apart. It is quite impossible for most women to feel comfortable on a man saddle. For some the experience will put them off cycling forever because a skinny saddle sits up in between the sit bones and body weight is carried entirely by the soft tissue. Ouch.

‘Women's' bikes come in two forms; ‘Ladies' bikes, more correctly referred to as ‘open framed' or ‘loop framed' bikes (without a top tube and often seen sporting a fetching basket) and Women Specific, that is regular looking, top tube equipped performance bikes which have been designed to fit the unique proportions of the female form. You see (generalisation alert!) women have longer legs than would a man of the same height. They have smaller hands and shorter arms and, most significantly, those differently shaped pelvises. As a result women require shorter top tubes or less reach. This is the key difference between men's and women's bikes.

It is possible to fit your average woman on to your average bike. Before women's specific bikes were available it was simply a case of fitting a women's saddle, a slightly smaller frame and possibly a shorter stem to compensate but we would often have, particularly with small women, the problem of brake levers which were too long or bars too wide. Women's specific bikes are absolutely a step in the right direction but if you are not a particularly small woman the option of installing a women's saddle on a regular (man's) bike is perfectly acceptable and will give you more model options.

Loop frames on the other hand are ideal for anyone, male or female, with limited mobility or who just want the facility to get off really quick, in heavy traffic for example. Kids seat on the back? A ‘ladies' bike allows you to mount your bike much more gracefully and without kicking your progeny on the chin.

Racers and tourers

Dropped bars serve two purposes, they allow you to change hand positions which helps reduce fatigue from your hands all the way through to your lower back and they enable you to reduce your aerodynamic profile, allowing you to go faster for a given effort (at 20mph 80% of your effort is being used simply to overcome wind resistance). Before the introduction of the mountain bike they were almost universal unless you rode a three speed roadster. But dropped handlebars bring their own set of ergonomic issues. The most common mistake people make is having them set too low. Lots of folk are put off by the perception that their nose will be touching the front wheel when they are in the drops but unless you are a competitive cyclist or double jointed you simply don't need them very low. If a bike is sized correctly for you you'll be comfortable in the drops all day long. In other words, if you can't reach into the drops comfortably the bike doesn't fit you. Place them right and you'll reap the benefits of improved comfort over long distances and better aerodynamics.

Aero bars

When after 3000 miles Greg Lemond beat Laurent Fignon by just eight seconds in the Tour de France twenty years ago his Scott Aerobars were the talk of the industry. Modern equivalents are still used in time trial competition but most people don't know that they weren't originally designed to make you go faster, they were designed for competitors of the Race across America to take the weight off their numb wrists. The reduction in Cd was a happy bonus.

A few things to watch out for:

* Only a perfectly level saddle can support your weight effectively. Your weight should be fairly equally spread between the front of your pelvis and your sit bones  (Ishial Tuberosities). A saddle which slopes up at the nose applies excess pressure to soft tissue and forces the pelvis to rotate backwards in an effort to get some weight on the sit bones. It also makes the bars seem too far away. The rider cannot push hard on the pedals for fear of sliding off the back of the saddle. A saddle which slopes down causes the pelvis to constantly slip off the front. To compensate the rider must constantly push their body weight back off the bars and pedals. This set-up failure encourages the use of unduly high gears and, because so much weight has been transferred to the bars they feel way too low. It's really easy to mess up an otherwise perfectly good set-up just by having the saddle a couple of degrees out. As a general rule the more upright the riding position the wider the saddle should be.

* Bars, brake and gear levers are not set in stone. It all just nuts and bolts, go ahead and adjust them to suit the angle of your wrists and your own personal preference. Brake levers on flat or riser bars should be in line with your arms when you are sitting on the bike. You can always move them back...

* Contrary to expectations firm saddles are more comfortable than squishy ones. It's because a firm saddle is better able to support your body weight.

If your saddle is doing its job properly it should never enter your thoughts. The same can be said about the whole bike. For extreme long-distance cycling comfort is all important but the same ergonomic principles apply to whatever bike you ride. Pedals, bars and saddle and their relative positions; placed right can give you miles and miles of trouble free cycling. Or make your bike an instrument of torture.

Read More on Fit

Children's Cycling

Women's Cycling

Saddle Height Adjustment

Saddle Height (Blog Post)