Sunday, July 3, 2011

Here we go with Z Frame

Looks like after a small break I'm ready to start another frame. This time in Zeke Dimensions.

The demands are simple. Design and build a frame like the 62cm, Surly Pacer with a longer head tube, and more standover height.

When I looked at the geometry I couldn't believe Surly has it spec'd with a 74 degree head tube angle, and a fork with 45mm of rake. The result is a bicycle with a trail of 50.7mm. I like my bicycles with more trail, somewhere in the area of 60mm. However Zeke says it's never been an issue, I didn't want to send him into culture shock, so I'm slacking the head tube angle to 73.5 resulting in a trail of 53.8mm. I'm still thinking about 73.0.

Last October I did some homework on this subject in regards head tube angle, rake and trail and different frame sets.

Interestingly enough I have the Surly Pacer in 56cm listed, and they spec that size with a 73 degree head tube angle, looks like they steepen it as the sizes get bigger. I guess they want the bigger bikes to handle quicker or have those characteristics.

He mentioned he likes the look of the straight blade fork. This is a great idea because I could make a straight blade fork with 40mm of rake, resulting in a bike with more trail.

Now in regards to the rear, it seems like manufactures will give a whole line the same length chainstay, for instance they might give evey frame from a 50cm to a 64cm, 415mm chainstays. However as the frame gets bigger, the rider weight is shifter back toward the rear wheel because the seat tube gets longer, or rather the distance from the bottom bracket to the top of the saddle. Some of these larger frames make it look like the riders is sitting on top of the rear wheel. Since I've built a couple road frame with 350mm chainstays I know a little bit about short chainstays. I like the cornering the reduced wheelbase gives you, but over all I miss the longer chainstays. I mean what percentage of your ride do you spend cornering vs. riding in a straight line? So I'm making them 420mm long vs. 415. I could take a couple scales over to zeke, and find out the hard numbers on the weight distribution, but this is geometry he likes. I wonder how much money a big bicycle manufacture saves cutting all chainstays the same length, vs. different lengths for different sizes?

Like the monoliths in the movie, 2001 a space odyssey, in the last 30+ years I've seen and heard a lot of different theories on bicycle geometry that became the new paradigm. I remember when most road race bikes were 72 parallel, then 74 parallel, Now a lot of frames have a 71 degree head tube angle, and a 73 degree seat tube angle. It won't surprise me if they change again. In my experience, there is a basic foundation, but a lot has to do with personal preference.

To add to that, I used to think there was some magic formula that you could enter your body's dimensions in, and it would spit out the perfect bicycle dimensions. and there might be if we were just skeletons. In other words you could have two people with identical dimensions, but different flexibility's. One would be comfortable, while the other might not.

This is the world of custom bicycles. Not the world of cookie cutter bicycles. So follow here, I'm going to try to make bi-weekly updates. As you can imagine Zeke and I have a few more twists that are going to be thrown in the weeks to come.

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