Saturday, October 2, 2010

Lets Build A New Road Frame

I'm going to design and build a new road frame and fork. I've been riding this Raleigh for some time now. It's a little tall for me, but this is old school geometry.
Over the next week I'm going to decide on what changes to make. I'm supposed to ride a 54cm or 55cm frame Measured from the center of the bottom Bracket, to the center of the seat cluster. This frame measure 58.5. So it's almost 2" too tall.
It also has a 57cm Top tube. At first most people will say that too long for me. But you have to do the math. Top tube length and reach are not the same. This frame has a . . .

. . . 72 degree seat tube angle. (This digital angle finder is accurate to 1/10 of a degree) A typical 55cm frame might have a 73.5 degree seat tube angle and a 55.5cm top tube. These number come out to be about equal. The slacker 72 degree angle brings the head tube back towards the bottom bracket about 1.5cm. So the reach on this bike work well for me, I just can't get the bars down to where I like them to race. For commuting this height is actually pretty nice. The top of the bars are about 2" below the saddle, about where I like them on my MTB. However I'm going to build this frame to race, so I'm going to build a frame where I'm going to ride in the drops of the handle bars, as opposed to a touring/commuting bike where you ride on the top of the bars.

By the marking on the lugm we might assume it's a 72 degree head tube, but we all know it could be off. The head tube is too short to use my digital angle finder, even removing the fork doesn't really help because I've still got the head tube badge to navigate. What I did was bolt on a couple tubing blocks from my jig to the head tube, then measure it with a combination of different levels and campass, and my digital angle finder. I found using 3 different methods the angle is between 72.0 and 72.1 . . . not bad work. The seat lug is also labled 72 and I suspected it was 72/72 since it's a 70's frame from England.

Now the fork . . . got rake?

62mm of it. I've still got to do the math to find the trail. Most frames today have a head tube angle around 72.5-73.5 degrees. Almost all forks have a rake of 43mm to 45mm.
I guess I'm old school, or maybe it's just what I'm used to. but I like lots a rake. I've ridden a few frames with a 73.5 head angle, and 43mm of rake, and while I would describe it a twitchy. I will say it's like steeering a marble. or like one of those Dyson ball vaccum cleaners, it'll turn quick, but I'm not sure to what end. I mean your going to spend a great proportion of the race fighting the bicycle to keep it straight, so you might take advantage of quicker handling for a moment.
Don't get me wrong. I know the over years bicycle manufactures have gravitated to the geomtries they have through trial and error, and these geometrys work good for the masses. If your not buying custom, your buying one size fits all so to speak. I will say it's a lot more open now than it used to be where almost every manufacture used about the same geometry give or take a 1/2 degree or centimeter here or there.
I'll have to work this trail thing out tomorrow, should be interesting. 62mm of rake Not to mention the 395mm axle to crown, most forks these days are 370mm-372mm. It's a commuter's dream, just not a racer's.

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