Wednesday, November 30, 2005

Geometry Class: The Historical Perspective

Today I want to start to take a look at some historical perspectives relating to bicycle geometry. Perhaps we can see where we came from, where we are at, and why. I also want to insert the notion that perhaps we've been "painted into a corner" geometry-wise.

The history of mountainbike geometry is heavily influenced, at least in the beginning, by what happened to road bike geometry from the 30's to the post war studies of geometry by the French in the 40's and 50's. After the Marin pioneers took mountain biking to the next level by building purpose built frames in the late 70's, mountain bike geometry was in a state of constant flux. This experimenting reached it's end during the heigth of the "pre-suspension" hard tail days of the early nineties. It seemed as though that the rule of thumb for off road bicycles had become engraved in stone at the following numbers: 73 degree seat tube angle, 71 degree head tube angle, and 1.5 inches (or 38mm.) of fork offset. If you varied your build much from these numbers, then you were likely to be crucified by the press that reviewed the latest XC machines. Then something happened that began to change this up a little bit. Suspension!

Now it's not uncommon for angles and measurements to be all over the place depending upon the purpose of the machine. More downhill oriented machines get "laid back" angles for "stability" while the XC machines are relatively still stuck in the early nineties mode without much deviation. Some of this can be blamed by the needs of manufacturing. It's obvious that suspension manufacturers cannot sell several different offsets for forks without making astronomical increases in production costs. Therefore, the fork offset is a figure cemented in place at 1.5 inches, or 38mm. This handcuffs designers into getting different handling characteristics by varying the head angle to affect the trail measurement, and thus everyones attention gets focused on head angle to determine steering qualities of bicycles.

Another thing that gets extreme amounts of attention is the chainstay length, as previously mentioned in another class article. It seems as though the industry has arrived at a figure to please all the hard tail folks out there, and any deviation raises eyebrows. The results of all of these factors: fashionable component choices, manufacturing constraints, and percieved notions on geometry, have all conspired to bring us in a very narrow place concerning current geometry of trail bikes. Any deviations are seen as detrimental to handling(by the riders) or to manufacturing costs by the companies.

Are the current philosophies on geometry too constrictive? Are their other ways that might be an improvement? These are some questions we will look at next time when we take a look at the 29 inch wheeled movement and what's happening with geometry there.

Class dismissed!

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