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Tuesday, November 30, 2010

The bike geometry problem

I know I've mentioned this one before, but it continues to cause issues every week, with at least one client that ends up on my fitting table that I need to bring it up again.

Carbon bikes can be a very good thing.  They can be comfortable (although often this is not usually a consideration of the stock bike company), they can be light, they can be stiff, and they can look cool.  The problem is that now they are cheap.  It costs so little to manufacture a carbon fiber bike that often the uber cheap and uber pricey stock frames come off the same lines, from the same hands, lay-up schedule and materials.

Okay, so that is another problem I can get into later.  The problem I want to talk about right now is molds.  Not like the fungus, but the casts that they create the frames and tubes from.  I don't have as big a problem with the frames as the forks.  If you look at the geometry charts of any major brand, and if they list the fork dimensions on the website (which more and more often they do not), you'll see that for a particular model the fork axle to crown and rake measurements are the same for every size.  Put simply, they use the same fork for every bike from 48 cm up to 62 cm.  The head and seat angles (and sometimes the bottom bracket heights) are altered between the sizes.  This doesn't seem like such a big deal until you look at the effect some of these angles, coupled with the fork rake measurements affects the handling of the bikes.

In general, the fork and frame angles are great for the "middle" size bikes -- the 54s, 55s, and 56s, or there-abouts.  But the small and the large bikes are sacrificed.  I've seen 76 degree seat angles on small bikes, and some steep head angles on the bigger bikes.

Not sure if you're bike is affected this way?  How does it handle?  Are you comfortable descending on the bike?  Do you have a "speed wobble"?  Even on the flats, do you have trouble holding a straight line?

Unfortunately, this problem is more and more common now with all the carbon manufacturing out there.  A well balanced bike fit is one way to lessen the problem, but even this has it's limitations.

Ride well.

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