An admitted shoe geek waxes philosophical about running, triathlon, and life in general.
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Thursday, October 4, 2012

Bike geometry basics

The idea behind this post is to provide some background base-work understanding to build on for future posts that will delve farther into the reasons for particular bike geometries, and why a certain bike will work well for one person, while being an absolute demon for another. Relax, though. I'm not going to try to make bike designers out of every one, nor make you all into bible--thumping bike fit gurus. But maybe somewhere along the line more information will be a good thing if you're looking at buying a bike for the first time, or even a second time, or looking to check the local bike shops philosophy of doing a fit and finding out that "the best bike is the one we happen to sell".

There are several terms that will come up many times related to bike geometry and how they affect handling:

* Seat Tube Angle (STA)
* Head Tube Angle (HTA)
* Front Center (FC)
* Chainstay length (CS)
* Bottom Bracket Drop (BB)
* Rake
* Trail

The accompanying illustrations will show you graphically what these terms are relative to the bike itself, and I'll go more into them below.

Seat Tube Angle (STA) is quite basically the angle of the frame seat tube relative to horizontal. While this is a frame measurement, with things like extreme set-back or forward-facing seatposts, the effective STA of the rider on the bike can be a fair bit different than the frame STA.

Chainstay Length (CS) is the distance from the center of the bottom bracket to the rear wheel axle. The shorter this measurement, it would seem that there would be more of the rider's weight on the back wheel, and this is true, but how much is dependent on the rider's position on the bike. Shorter also means more road shock and vibrations is transmitted directly to the bike saddle. But there are practical limitations for this dimension.

Front Center (FC) is the distance from the center of the bottom bracket to the front wheel axle. As this dimension grows, the steering of the bike becomes more tractor-like -- stable, but with a larger turning radius.

Effective STA (of the rider on the bike, not the frame), CS, and FC determine for the most part the weight distribution on the wheels. This, along with a few more factors has a huge bearing on how a bike handles. I'll go more into this in a future post.

Head Tube Angle (HTA) is the angle of the frame head tube (the steering axis) relative to horizontal.

Bottom Bracket Drop (BB) is the distance from the center of the bottom bracket to a horizontal line through the wheel axles. This measurement has a lot to do with how stable a bike feels in a straight line, but there's a limit to how far one can go with BB -- as it increases, stability goes up, but so does the likelihood of scraping a pedal as you power through a turn (with potentially catastrophic results).

Rake is the offset of the front axle from the steering axis. More rake means a softer ride -- more compliant over bumps. Though, especially with the current generations of carbon forks, the compliance can be built into the fork regardless of the rake.

Trail is a resultant measurement of the HTA, Rake, and wheel size. It is the distance between the point on the ground directly below the front axle and an intersection of the steering axis to the ground. Think of it like the front wheels of a shopping cart. There is a fairly narrow range within which trail yields a stable-handling ride, which is one reason that you can't just swap any fork into a frame and get the same feel.

With this basic understanding, we can go further into some of the nuances that affect bike handling, and how this can change drastically when trying to change a road bike into a tri bike, why it's a very good idea to have a professional fit done before buying a bike, and why the 650 wheel size is a very good idea on even some larger size tri bikes. Look for these subjects in the near future!

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