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Compact frame geometry question(8 posts)

Compact frame geometry questionpedalAZ
Aug 12, 2003 6:32 PM
I'm finalizing the design of a custom frame and the two experts I am working with agree on just about everything but the top tube slope. One has it at 10 degrees, which will leave about 9 inches of seat post exposed above the clamp. He says this geometry results in a nice, firm frame that will not feel too flexy. The other guy says I need to ease off the top tube angle, to 7 or 8 degrees, extending the seat tube and seat stays a bit more to take some harshness out of the ride.

I don't give a hoot about the aesthetics; I want good handling and comfort for long road rides, and I want it as light as I can reasonably get it, albeit with Ti rather than aluminum.

The size I need equates to a 60cm frame with a 56.5 top tube (assuming a traditional horizontal top tube).
They are both full of it...jtolleson
Aug 12, 2003 6:50 PM
if they really want you to believe that the 2-3 degree difference in TT slope is going to affect ride quality. What a bunch of hooey! There's a huge debate whether compact geometry changes ride quality over traditional geometry at ALL, and to then evaluate based on incremental differences in TT slope... well...

Dat's my opinion and I'm sticking to it. You say you don't care about aesthetics but I'd advise you to go with the one you like best aesthetically. The rest is Princess and the Pea stuff.
nonexpert advice???C-40
Aug 12, 2003 6:53 PM
So you're going to ignore the advice of expert frame builders in favor of someone with no qualifications on the internet? Better stick with the opinions of the builders.

FWIW, most compact TTs slope 5-6cm, regardless of frame size. This is only about 5-6 degrees. Fondriest frames for example, slope 5cm and it's not radical at all. Personally, I think the seat tube is too short if you can't get the saddle up to height with a 250mm road seatpost.
nonexpert advice???jtolleson
Aug 12, 2003 7:12 PM
Yes, yes, we are all non-experts. I grant you all of that, but experts say a lot of things about bikes, particularly in attempting to indistinguish the indistinguishable, that cause unnecessary anguish in borrowers.

Don't anguish because you won't be able to tell the difference. That's my two cents.

My custom Seven has 6 degrees TT slope. It is much less dramatic than that on most of the production compact bikes (Giant, Specialized, etc.) which I had understood were around 10. Dunno.
experts???Rusty Coggs
Aug 12, 2003 7:31 PM
Even they have their own spin on things, and it often is not based on any real science.
go traditionalDrone 5200
Aug 12, 2003 9:20 PM
I once heard an expert call a sloping TT a girl's bike!
Spirito calls them 'mixte'! 60cm? Go slightly sloping/trad nmSpunout
Aug 13, 2003 3:39 AM
Do you like to use your legs?Mel Erickson
Aug 13, 2003 5:55 AM
I like to use my legs to clamp down on the top tube when descending. A 10 degree sloping top tube would make that difficult, although a 56.5cm equivalent top tube on a 60cm frame is pretty short and might mitigate this somewhat.