Wednesday, March 25, 2009

Geometry: Fit by the numbers.

In my last geek-tastic post about bike geometry, I talked about the trend toward moving the rider back on XC bikes with respect to the center of the bikes' wheelbases. And mused about the usefulness / comfort of exploiting the lengthened wheelbase to move a rider more forward on a bike with respect to the back wheel, and by association, the pedals.

After spending some time at my LBS riding bikes, I'm convinced that comfort and riding style are largely independent of seat angle, provided there is ample cockpit to acommodate one's wingspan at the required seating position. This means that the cockpit can be shifted forward and back along the wheelbase freely to affect other characteristics of the bike.

What I did find to have a big effect on my comfort with a bike, however, was the relationship of my position (in two dimensions) to the front and rear contact patches of the bike. Being an unusually tall guy, it's easy to get myself going head over heels, both forward and backward if I'm not careful about shifting my weight properly, so every little bit of stability in either direction helps.

Though there were many things I disliked about it, one of the best climbing and descending bikes I rode was the Specialized Epic Comp. As compared to my current bike, the rider sits quite a bit more forward of the rear wheel, but because the BB height is higher, it's only a little bit more forward in terms of stability. In any case, with 63mm more wheelbase, the rider not only sits a little more forward of the rear wheel, but also a lot more behind the front, lending the bike to more confidence on both climbs and descents. Normalizing all of the other bikes for the same climbing position of the Epic (because we've now decided that this is ok--imagine simply shifting the saddle and lengthening or shortening the stem), the rest of the bikes from the previous analysis stack up like this (click for big):

(higher bars indicate a higher ratio of [horiz. distance to front wheel]:[vert. distance to ground] from the rider's Center of Mass. All cockpits have been shifted so ratio for rear wheel is standardized)

The take home message here is that my poor old ride (Superlight '06 modified), despite being awesome in a lot of ways, is really tippy on both ends compared to everything else out there right now. Just about anything I could buy would descend more confidently without a decrease in climbing stability. Maybe this is a long path to a short conclusion regarding my own ride (I already knew it was too short), but it does yield a good quantitative comparison tool the extrapolate one handling aspect of a whole bunch of bikes I'll never get to ride from the few that I can find at my local shops. As you can see, there is a lot of choice among new bikes in simply this one measure. Deciding where I want to be on this spectrum immediately narrows the options enough that it's possible to optimize the path for getting there: Are lower pedals a reasonable compromise for stability in a shorter bike (new superlight), or does it make sense to go bigger and sacrifice some nimbleness for decreased pedal strikes? Is it better to have a steeper head angle and longer TT or the other way around? These are all questions that still go unanswered... for now.

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