An admitted shoe geek waxes philosophical about running, triathlon, and life in general.
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Sunday, October 14, 2012

The case for the 650 wheel size

Much has been said for the use of 650 wheels on small frames -- often with the words "the only place they're useful". And I think that's just wrong. Usually the next few utterings will be of the many misconceptions regarding 650 wheels.

The fact is that the smaller wheels usually are found to only be useful on small-sized ROAD bikes, where toe overlap is a very real issue, but for tri bikes, the case can be made quite convincingly that 650's are useful, and maybe even necessary, for creating a stable-steering tri bike well up the frame size range.

There's a myth flying around that a tri bike is less stable than a road bike. I submit that this isn't true, IF the tri bike is properly fitted to your riding position. [Note: this is the way a bike should be fit - to the rider's position, and not the position to the bike.]

Part of the reason a road bike has stable handling is due to the weight distribution between the front and rear wheels -- approximately 55% or more of the rider and bike combined weight will be on the rear wheel if it's fitted properly. But a tri bike position can be loosely interpreted as a road bike position that's been rotated forward around the bottom bracket. What happens when you move forward? More weight on the front wheel. This is why a tri bike is often unstable-feeling compared to a road bike.

Back in the day, when all we had were road bikes, and triathlons were in their infancy, we started to experiment with body position: long and low stems, arms sitting praying-mantis-style over the front wheel, saddles pushed as far forward as the seatpost would allow, or in extreme cases a seatpost that was bent forward. And the bikes handled like crap. It took some pretty relaxed arms (and a zen-like calm) to keep the bike in a straight line, or at least to not panic when it decided to grow a mind of its own.

Dan Empfield and Ralph Ray simultaneously came up with similar ideas to remedy this problem, and while their philosophies were different (move the wheels forward vs move the bottom bracket back), they were arguing two sides of the same coin -- in order to tame the bike-handling beast of tri bikes, the rider's weight needed to be shifted onto the rear wheel.

Here's the deal: When you start moving the wheels forward to compensate for the forward movement of the rider/bike center of gravity, there's only so far that rear wheel can move before it starts interfering with the bottom bracket. Sure, you could just move the front wheel farther out... But USAT has this pesky rule that says the front center can only go to 65cm, or 7/8 of the measured distance from the bottom bracket to the top of the saddle (measured 14cm behind the saddle nose), whichever is greater. So if you ride steep, and "steep" really starts at anything close to an effective seat tube angle of 80 degrees, you're gonna bump into that front center rule before you get to 55% of your weight on the back wheel. UNLESS... you use 650 wheels to allow that rear wheel to get just a little bit closer to the bottom bracket.

Many of the tri bikes of yore sported the 650 wheel size, and in fact for a while it became THE identifying factor of a tri bike -- 650's were used across the size range for many makers, including some of the big swingers: Cannondale, Trek, Fuji, Klein, Kestrel... The list goes on.

Manufacturers have gone away from using 650's for all but the tiniest of frame sizes, and have for quite some time, and I think that's a shame -- physics hasn't changed, and the resultant bikes they make aren't any more stable because they're made from ultra-bling carbon fiber.

And this gets me to the search I went through several years ago when I was making my come-back into the tri world after several years away. Namely, finding a bike that would be stable given my preferred riding position.

I'd been a triathlete before I became a bike racer (much like another famous guy who seems to like the color yellow), but I've never been anywhere near as fast. After racing for several years, settling into riding on a custom TiCycles Softride (which was never accepted for international bike racing, and sadly is no longer in production). I took a few years off to put my time into a business venture, and some life changes led me back to triathlon. I pulled out my other Softride (made by Paul Barkley) in my tri configuration, gave it a few rides, and realized that it just wouldn't do -- the handling was horrid.

I read a lot. I checked all the dimensions and weight distribution on my bike, and realized its short-comings -- the geometry of the frame and beam, while awesome for road racing, and allowed for a wide range of seat positions, wasn't going to accommodate that wide range when it came to weight distribution. Too much weight on the front wheel. I determined the position of my center of gravity relative to the bottom bracket, and then overlayed that on the dimensions of every production frame I could find (it's great to have some multi-thousand dollar 3-D CAD at your disposal), and found that the SAME problem existed for every frame made at the time. And mostly it was because of chainstay lengths determined by 700c wheels coupled with the USAT limit on front-center.

I got pretty discouraged.

But I stumbled on a frame on the SlowTwitch classifieds using 650 wheels, and set up with a long-ish front center... and at a great price. Rumor was that it had been custom made for Scott Tinley some time back. It handles pretty well, even though it's still not quite where I'd like it to be.

I'm quickly coming to the conclusion that the next frame I ride on will be one I make myself, with no regard for the USAT limit on front-center, sporting 650 wheels, and it'll handle like a dream.


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