Road Bike, Cycling Forums banner
1 - 5 of 5 Posts

·
Registered
Joined
·
1,513 Posts
Discussion Starter · #1 ·
Okay all you fitting freaks out there I've got a problem I hope you can help me with..

I have a bike that has a 57.5 TT 73.5 head angle, 73 seat angle.. I'm looking at a frame that has a 57 TT 73 head angle, 72.3 seat angle..

Here's the issue.. The bike I currently have is a little too long in the TT. I've always ridden 57 and thought a 57.5 wouldn't be a big deal. What I didn't take into affect, would be that the steeper seat angle on the bike would make the tt seem longer than just the .5 cm difference. So now, I'm running a 1cm shorter stem than I normally did on the 57 and I still feel stretched out. I feel if I go to any shorter stem, it'll affect the way the bike rides..

I'm thinking of going back to a bike that has a 57TT, and with it's not as steep angles, the 57TT will actually seem much shorter than the 57.5 I"m currently on.. Problem is, I can't ride the new frame I'm looking at so I can't set it up to tell..

Can you help.. If you need more info, let me know, but I think this is all that is needed..
 

·
waterproof*
Joined
·
41,745 Posts
First you need to figure out what the correct "reach" for you really is. Swap stems till you find that (remember that seat to bar drop also affects this, significantly, as to handlebar bend/setup).

Then, worry about your foot/knee/hip relationship to the pedals /bb. Get your fore-aft setting nailed down.

Some fitters prefer to do step 2 before step 1, which is fine, but those are both critical.

Then and only then should you worry about frame geometry, mainly in the context of understanding how / what you'll have to change in order to make the bike fit you.

With a littl bit of math (trig) you can figure out the exact impacts of geometry changes.
 

·
chamois creme addict
Joined
·
1,479 Posts
Numbers

Mosovich said:
Okay all you fitting freaks out there I've got a problem I hope you can help me with..

I have a bike that has a 57.5 TT 73.5 head angle, 73 seat angle.. I'm looking at a frame that has a 57 TT 73 head angle, 72.3 seat angle..

Here's the issue.. The bike I currently have is a little too long in the TT. I've always ridden 57 and thought a 57.5 wouldn't be a big deal. What I didn't take into affect, would be that the steeper seat angle on the bike would make the tt seem longer than just the .5 cm difference. So now, I'm running a 1cm shorter stem than I normally did on the 57 and I still feel stretched out. I feel if I go to any shorter stem, it'll affect the way the bike rides..

I'm thinking of going back to a bike that has a 57TT, and with it's not as steep angles, the 57TT will actually seem much shorter than the 57.5 I"m currently on.. Problem is, I can't ride the new frame I'm looking at so I can't set it up to tell..

Can you help.. If you need more info, let me know, but I think this is all that is needed..
At your size (and mine since these numbers are close to what I ride), each degree of seat tube angle change is ~1 cm of top tube change provided the saddle stays in the same place relative to BB. There is also a little effect of the head tube angle change.

So, if you are riding 57.5 top tube with 73 seat tube and switch to a 57 top tube and 72.3 seat tube your total reach will drop by ~ 0.5+0.7=1.2 cm. That is for the same stem length your reach will drop by 1.2 cm due to the combined effects of top tube change plus seat tube angle change. The head tube angle will have a small effect as well, the new frame has a shallower angle that will shorten your reach but the effect is much less drastic than seat tube angle because the distance above fork is much less than distance from BB to the top of the saddle. At most it will be maybe 2 mm less depending on how high your bars are. The slacker head tube angle will also effectively raise the rise of your stem, for a 120 mm stem, 1 degree of angle change is about 2 mm difference in rise, so in your example the bars will rise by roughly 1 mm.

The handlebar height between frames is also influenced by BB height. If the old frame has a higher BB and you keep the total fork length the same but the new frame BB is 0.5 cm lower then your bars will effectively be 0.6 cm higher. This is because your saddle dropped relative to the bars that actually went up 1 mm due the head tube angle change.

Hope this helps. Remember all of the above assumes you set the saddle in the same place relative to the BB between the bikes.
 
1 - 5 of 5 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top