Geometry question - road V cyclocross
 

Dear All,

bit of a technical question now that I am researching my next build.

For most of the road frames I have built in the middle-sized 54-58cm range I have found little difference between the head tube and seat tube angles. I've used 73deg for both on my bikes and have noticed little variation in the geometry charts of most major bike makers.

However, I'm planning a commuter bike (basically a cyclocross style bike, disks, rack mounts, fender clearance etc) and all of the geometry charts I've seen are slightly more upright on the seat post (74deg) and slightly slacker on the head tube (71deg) or a slight variation in that direction.

I've managed to confuse myself as to why. Steering changes (more responsive?), shortening the top tube/lengthening the wheelbase?

Could someone with a better understanding of geometry and how changes effect things make a comment?

The whole thing about custom is that it is custom. You are starting out wanting a comuter, then switching to a cross bike, fake or real as your model, (not to mention assuming ad copy geometry is actually truthful) then wondering what those numbers are there for.

If you have done a few bikes for yourself, you should know what cockpit works for you, is comfortable, and allows you to generate power, and stay healthy. That is your seat tube angle, unless you need something weird like a triathlon bike, or whatever. From that you figure out where you need the bars to be for your reach, and the stance you will have on the bike, and so forth. And just keep going design from first principles.

slacker head tube and steeper seat tube sounds like the usual way of making a small frame. I have no idea why anyone would make a steeper seat tube, and certainly wouldn't do that on a cross bike for a person a person in the 56cm range.

I can see why some people would go for a slacker head tube on a CX bike, it normally gives more trail.

My CX bike has horrible geometry and very few people like it. LBS had to clearance them to get rid of that model

Thank you.

It sounds like I should just stay with 73deg for both, or perhaps reduce the head tube a little to alter the steering (less responsive). A basic copy of my road frame, but perhaps with a slightly taller head tube, is sorta what I thought I needed.

I was quite surprised to see so many companies doing a similar thing...

Trail is only Partially HTA fork offset and wheel size are 2 other factors .

You just buying a Fork, so no choice in rake distance?


There is branches of what is called Cyclo cross .. & 1 is a drop bar jack of all bike taking racks and Bottle cages

and the other Full Competition Cyclocross race that takes none since the races are an Hour long.

and the courses are Varied but to win you hammer between tight technical corners, and Run rather than gear down.

Homework; Look up specs of the type of bike you need .






./.

I will be buying an off the shelf fork, so probably limited to 45mm rake.

Bike will be 700c wheels (unsaid above, but I though that would be presumed).

For a true 'cross bike, toe overlap might be an issue and could be a driver for the slacker HTA. More trail isn't really that attractive for a 'cross race bike, though.