North by Northeast

So You Want to Buy a Nice Bike… part 2 – Fit
29 May, 2007, 10:13 pm
Filed under: bike, buyer’s guide to bikes

Above all else, fit is the most important thing when buying a bike. If you have a $2,000 bike that’s too small and a $500 bike that fits right, you’d never ride the expensive one. It’s that important.

This is a major reason you want to go to a real bike shop. They’ll know how to size you properly. Some bike shop employees actually get certified in bike fitting. Some riders have certain issues with their individual bodies that making fitting a bike more challenging than for other people. My body is pretty average, so I’m fairly easy to fit on a small or medium bike. However, you might have a longer torso than other people, or perhaps a shorter torso and longer legs than other people your height. None of these are deal breakers, because bike fit is very adjustable.

The very first thing people think of when thinking bike fit is seat height. However, you can adjust seat height so much that you could easily hop on and ride bikes that are too big or too small for you, so if you throw a leg over and can sit down with your foot on the pedals, that really means nothing.

Since we’re talking about seat height, let’s go on a tangent for a moment. When you have a bike that fits, it’s so important that you figure out proper seat height. Sadly, though, seat posts slip, so you’ll need know what your proper height is, how to find that and how to re-secure your post at that height. Seat height is critically important to the health of your knees. Most people ride their bike with the seat too low. You won’t be riding for long before your knees start to ache.

If your knees hurt, you need to adjust your seat. The most common misconception about seat height is that you should be able to reach the ground with your feet when you’re seated. This is too low. Your knees should be 95% extended when your leg is at the bottom of the pedal stroke, with the ball of your foot directly over the spindle (center) of the pedal. This is not only the healthiest height, but the one that allows you to produce the most power with the least effort from your legs.

This height is usually a little uncomfortable for people who’ve had their seats too low. They feel they’re not as safe. But you’re fine, because whenever you’re in a position that would require you stopping or catching yourself, you shouldn’t be on your saddle anyway. Once you stand on your pedals, you can easily bend one knee, and put the other foot on the ground. No problem.

One more seat position thing, your seat can more fore and aft (and some seat posts allow seat angle adjustments, as well). You should move your seat to the position horizontally that allows you to have your knee directly over the pedal spindle when your crank is at the 3 o’clock position in the downstroke, while the ball of your foot is again centered on the pedal spindle. Again, maximum health, maximum power. In regards to seat angle, some people prefer a little tilt, but this is mostly personal preference, often based on if you have some saddle numbness.

Back to bike fit, the most basic requirement of a bike frame is called stand-over height. This is how tall the bike is at the “top-tube.” This is measured at the point where you would be standing, when on the bike, but not going anywhere. Many bikes now have sloped top-tubes (the tube that goes from the headset (above the front fork that holds your front wheel) to your seat tube (the tube that holds your seat post and seat). This measurement is usually from the center of that top tube. Obviously, this height has to be less than the height of your crotch. This measurement is slightly more important for men.

How much space is enough? There is a minimum amount, but after that, it’s personal preference, within reason. My inseam is 30″ and the stand-over height of my bike is 28″ on a Cannondale size small. Personally I’d like another inch there, but next size smaller was too small for my other dimensions. Such is what you live with when you can’t afford full-custom bike frames! Lucky for me, 2″ is enough for mountain biking. I don’t do crazy jumping or “trials” style riding – that would require much more space below your inseam so you can maneuver and contort.

If you’re riding on the road, 2″ is also a good minimum for comfort riding, but if you’re looking to get a racing-style bike (think Tour de France) there’s a good chance you’ll have less stand-over height, usually about an inch. This is less of a concern for this style riding, because unless you’re in stop and go traffic, you rarely have to get off the saddle, and rarely in a hurry to get off, as opposed to when you’re riding with obstacles.

A note for women: you may remember women’s bike frames looking very different than men’s, traditionally. Usually the “top-tube” we’ve been talking about here, was bent way, way low, down near the down-tube (the part from the headset to the bottom bracket, where the pedals’ cranks meet the frame). This wasn’t for clearance of the body, but so women could ride a bike while wearing a full length dress. Since it’s pretty rare to see a woman cycling in a dress today, this style has mostly gone away, and is totally unseen on any bike used for fitness. So now, the only really visible difference of a woman’s specific frame is the top tube is often a little lower due to the average woman’s height being less than that of your average man. Having the top tube up top like on a men’s bike is useful because it makes the frame stiffer so that you can transfer more energy to the wheel and steer with greater control.

Stem length is the next topic we’ll talk about, but it’s clear to me now that the topic of fit is going to take at least a couple of posts! Tune in tomorrow!


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