CX Geometry: Head Tube/Seat Tube Angles

Replies to This Discussion

Permalink Reply by Cyclocross Magazine on October 9, 2010 at 9:59pm

hey there. so 1 degree will affect handling for sure, but whether you will mind the change is the big question. I'm wondering why you want to? it will give you less toe overlap, assuming the fork rake and axle to crown stay the same. It'd make steering feel a little slower. if you have fast crit-like races that are very turny, maybe you won't like that. if you have big feet, ride singletrack and bomb down descents, you might. that's an over simplification of course. you could play with other variables (fork rake, stem, axle to crown) to reverse/offset the effect if you didn't like the one degree change as well.

definitely make sure you know which fork you'll use when you plan all these things, as they are not all the same. the seven fork, for example, has a 390mm axle to crown, and some ritchey and specialized forks had a longer rake. good luck, and please post pics of the building process. want to document it for the mag/website? what kind of tubing? brazed or welded? fun!

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Permalink Reply by sup on October 10, 2010 at 8:22am

Thanks for the quick reply. I haven't really thought about the fork that much. I'd definitely get a steel fork to keep the cost down. What's readilly available at the local shops is a 410 mm axle to crown aluminum fork, but that will be my last resort. If I get the aluminum fork, the result will be a slacker geometry. Straight blade or raked - I haven't decided yet. Anyway, I'm still in the planning stage so any input is greatly appreciated.

I'm going for Deddaciai Tre Zero tubing, but the frame builder is still waiting for supply. If I can't control my itch, I'll be ordering Columbus tubing from Nova Cycles. It will be brazed. As far as documenting, there are some local articles about the frame builder. Contributing for the mag would be cool! I'll stick to posting here first 'coz I'm new to cross.

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Permalink Reply by c-note on October 10, 2010 at 12:28pm

The slacker HT angle of 71 will slow handling compared to a steeper angle given the same fork rake. It will have the effect of increasing the trail, making the bike more stable at speed, but perhaps producing wheel flop at slow speeds. You can increased the fork rake with the slacker HT angle to reduce the trail. The steerering will still be slower than the steeper HT despite the reduced trail. Old school mountain bikes often had 71 degree HT angles with lots of rake.

If your frame is small and you need pedal/toe clearance, the slackened HT is the solution most often used. Recall, this was the case for small framed 700C wheeled bikes, and it produced adequate but somewhat quirky handling. The change to 650 C wheels made all the difference in the small frames, as the geometry could be "normalized". Some builders used only the 650 front wheel, but that created some issues with 2 tube and tire sizes.

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Permalink Reply by sup on October 10, 2010 at 7:35pm

Helpful tips, c-note! Now this may be the reason why size 49's from some manufacturers have a slacker head tube angle.

I dug up the geometry of my road bike and the HT and ST angles are 71.5 and 73.5 respectively. The fixed gear I'm using is a 72/74. The thing I noticed between both is that the fixed gear has no toe overlap. I didn't take into account the fork rake because I don't know what they are. They seem to be the same. Anyway, I can't really compare both because of the BB drop which would also affect toe overlap (I think).

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Permalink Reply by Kevin J. Rusch on October 12, 2010 at 9:43pm

Are you having someone build the frame for you or are you building it yourself? I think that you're best off leaving geometry to the pros - buy a frame from someone who has made a bunch of CX bikes and leave the design to them. Also, as was noted previously, the frame and fork have to operate as a unit so the builder will be able to make a good recommendation. Just my two cents...

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Permalink Reply by sup on October 12, 2010 at 11:53pm

Yes, I'm having a frame builder do it for me. Thing is, he's used to making road bikes. By the way, I'm not from the U.S. and cyclocross is relatively unknown here.

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Permalink Reply by Cyclocross Magazine on October 13, 2010 at 1:45pm

i'd avoid that 410mm fork - most forks are 395 so if you don't like it it'll mess with the handling if you switch to another. i agree with kevin that i wouldn't have him experiment too much with geo if not a big cx builder - maybe base on a established builder - there are many advertising in the pages of cxm.

straight blade or raked has no effect on handling in terms of geometry, fyi - many folks get confused by that, maybe you knew that but don't let aesthetics confuse you. of course, a raked forked might absorb more vibration, etc. if built a certain way, but that's a lot more subtle.

ALL THIS BEING SAID, i've swapped all kinds of forks on my bikes and ridden and race them and have never felt I lost a race or place because of my fork. but if doing custom you might as well try to do the homework beforehand.

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Permalink Reply by Katsu Tanda on October 13, 2010 at 2:18pm

Build around a 395 fork with 45mm rake. Pretty standard CX fork.

A 1 degree angle change will be noticeable.

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Permalink Reply by sup on October 14, 2010 at 4:07am

Alright guys, research it is for me! One thing's for sure, I would base my decision around the fork.

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Permalink Reply by sup on October 19, 2010 at 5:50pm

I've decided on the HT/ST angles. Thanks for the inputs on the fork guys. Read up about it thru articles I found thru Google as well. Now to choose tubes and a fork. I'll still document the build in this topic, but it would take a while because of sourcing issues.

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Permalink Reply by sup on November 4, 2010 at 2:09am

Well, problem now is the supply of tubes. I'll go for an aluminum frame for now.

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