Small frames = steep angles. Why?

It probaly has 700c wheels, if it was 650s, there would be lots more clearance between the rear tyre and the seat tube, so a slacker angle. Also for chainstays of less than 400mm, gear changing can have some issues..

76 degrees may bring her cadecne up a little, if the cranks are short say 165-160mm. Its all compromise..

gclark8 said:

It probaly has 700c wheels, if it was 650s, there would be lots more clearance between the rear tyre and the seat tube, so a slacker angle. Also for chainstays of less than 400mm, gear changing can have some issues..

76 degrees may bring her cadecne up a little, if the cranks are short say 165-160mm. Its all compromise..

So ... a very small frame (49) should have equally short cranks ... 160 - 165. It currently has 172.5, which I feel are too long in any case.

B

Yes, my GF is 4'11" with short lower leg, 160mm cranks, 650 wheels, loves it!! (cycling too) Another friend is 5'4", normal proportions, 650 wheels, 165 cranks, spins like a Chipmunks record (90).. 😎

I am 5'7", old and shrinking, OK on 170 cranks, but wanting to try 167.5s, and looking for a 650c wheel bike in 52cm with seat tube 76deg, Felt B2 if I win Lotto 🙂

As has been said, it's so the wheels will fit the frame. The head tube angle will also be fairly relaxed to provide enough toe clearance. Don't go out and buy 650c wheels, the frame needs to be designed around smaller wheels in the first place for that to work.

Jay.

bing181 said:

My wife just got herself a nice old Colnago bike/frame. Catch is, at 49 cm c-c, the seat tube is so steep (75 degrees from what I can see), that to get anything approaching the right setback, the seat is pushed back to the end of its rails.

Why not a more "normal" seat angle ... 73 degrees or whatever?

B

The reason could be a combination of things, but in general smaller frames have steeper seat tubes becuase smaller people "typically" have a shorter femur length - make it harder for them to get positioned over the bottom bracket correctly.

I've never owned a Colnago, and have never really looked at their geometry. Most frame builders/designers tend to stick with a similar geometry through models range. Perhaps this is a criterium style frame where you typically see steep seat angles. I know that Gios typically uses steep seat angles and very short chain stays (read: tight).

On all of my custom bikes I have a 75 seat angle. That seems to work best for me, but I do have relatively short femur length. If it is a big problem you might be able to find a seat post that offers more rear offset.

Good luck!

bing181 said:

My wife just got herself a nice old Colnago bike/frame. Catch is, at 49 cm c-c, the seat tube is so steep (75 degrees from what I can see), that to get anything approaching the right setback, the seat is pushed back to the end of its rails.

Why not a more "normal" seat angle ... 73 degrees or whatever?

B

Mostly because of cost cutting I think. I don't know of any research that has validated any of the morphological excuses given for the bad geometry on the small frames. I think they want to skimp and use the same chainstay length and BB height as the other size frames to save time and money and let that make the decision on seat angle. My wife is proportionally quite similar to Bernard Hinault and trying to find a frame that would fit her was almost impossible.

We are having similar discussions on another forum... http://www.triwa.org.au/discus//messages/7/2403.html?1127462446
http://www.triwa.org.au/discus//messages/8/3248.html?1127462917