Monday, July 21, 2014

Bike Comparison and Sizing

On 07/17/2014 03:56 PM, David Bloxom [tcsd] wrote:
Hi all,

I'm looking at a few used Tri bikes. I ride a 56cm Scott carbon frame road bike, which I have fitted with aero bars, and have been fitted for that set up. I've been told the sizing is right for me.

My question is, in general, should I be looking at 56cm Tri bikes or smaller / larger. I realize the "right answer" is to fit the bike to me and not the other way around. But when looking to but used, that doesn't appear to be the best option.

Hi Dave,

The number "56" is a completely bogus way to compare bikes, even different models from the same manufacturer, much less from different manufacturers.  This number fails to take any of the rest of the bike geometry into account.

The only way I'm aware of to compare any two bike frames is by "stack & reach", so you first need to find the stack and reach for each bike you want to compare.  Here's the Slowtwitch stack&reach database:  Some bike shops and manufacturer websites have additional information, so be sure to check.

But that's only part of the story: There's more to comparing bikes than just the stack and reach: You also need to take cockpit differences into account (unless the same stem, spacers and bars are on both bikes), as well as the relative position of the saddle.

Finally, (well, actually **FIRST**), is to have numeric data for your bike fit, so you can see if any given bike is compatible with your "fit numbers".  After my fit at Moment I was given a printout with not only my fit numbers, but also a drawing showing their geometric relationship to the bike.

I keep my fit numbers in my phone.  When I was demoing bikes, I brought along a small a tape measure, and would adjust each bike to my fit in a few minutes, which permitted me to demo different bikes with the SAME FIT!  Apples to apples.  It would also be immediately clear if a bike couldn't be adjusted to match my fit.  Sometimes, a quick stem and spacer change could turn an "almost but not quite" bike into one that could match my fit.

There are some additional details to take into account before making a final choice, but the above should make it easy (well, straightforward) to eliminate the "no way" bikes.


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