Geometry question. How much difference does 1º make in the seat tube angle?
 

I am looking at the Ritchey Break-Away and its seat tube angle in my size is 75º. My current bike has a 74º seat tube angle and I was wondering how much of a difference it will make.

Head tube angle in my current bike is 71.5º. The Break Away has 72.5 in size 50 and 73º in size 52

enough to slide your saddle forward a by 1/8" or 4mm if the frame is large enough.

If the headtube angles of the two bikes are the same, the steeper (75) seat angle will shorten the top tube by approximately 7-8mm. To get the to fit the same, you have to push your seat back a bit on the rails and/or get a longer stem.

head tubes angles are bot the same. Head tube angle in my current bike is 71.5º. The Break Away has 72.5 in size 50 and 73º in size 52 I would probably choose the 50, because it has a 53 top tube, which fits me better.

Whats the seat tube angle on the 53 cm frame. If you assess it carefully I think that you will find next to no difference in size between the 50 cm and the 53 cm except that the steeper seat tube angle on the 50 cm frame makes the top tube length look shorter on paper which is the EXACT reason why they do such a thing. By the time you fit a rear set seatpost to the smaller frame to position yourself back at KOPS there will be NO difference anymore.

When you get down to such small frame sizes it highly likely that you will be better off looking at 650c frames and I know there's a lot of prejudice against 650c wheels but that's the way it is. If you think to yourself, I don't want to ride a 650c bike then you know EXACTLY the reason why manufacturers fiddle with the geometry of 700c bikes to make it look right on paper.

It won't be a comfortable ride with such a steep seat tube angle of 75º but your more likely to buy it which is why manufacturers are more likely to make it.

Regards, Anthony

Exactly, if you use KOPS and a constant distance center of the BB to the top of the saddle, then you're going to end up with the same "effective" seat tube angle on any bike that you set up this way. However, the actual seat tube angle will determine how easy it is to get a KOPS fit (if that's your goal).

Dru

If the seat tube angle is 1 degree different. You will have to move your saddle either foward or back to get the same set back from the center of the BB.

You may or may not need a setback seat post or zero set back post depending where the saddle is clamped on it's rails.

what will change is the front center. The steeper the seat tube angle, then the longer the front center will be. You have to consider the tube length and angles of any frame to see how it can be set up.

If 2 bikes have the same top tube length, then the one with the steeper seat tube angle (asuming the distance behind BB is equal) will need a shorter stem as the reach will be longer.

Yes what I left out is that if you have 2 frames with the SAME nominal top tube length but DIFFERENT seat tube angles then the frame with the steeper seat tube angle has a LONGER Bottom bracket to head tube distance. I would say that the frame with the more relaxed seat tube angle but the same top tube length is actually a SMALLER frame with a shorter BB to HT distance. When you correct the steeper seat tube angle for KOPS then suddenly you have a longer effective top tube length.

Regards, Anthony

Angle and effect is dependent on the length of the seat post used... (exposed). 1* @ 8" = .1" difference.

Here is a question I posed back in December…

"How much further back would my saddle be with a seatpost angle change of 1 degree (74 to 73) at a height of 73.5 cm..."

Here is the answer from durk onion…

“Assuming that the 73.5 cm is the length of the seat tube:

73.5cm[cos(73)-cos(74)] = 1.23cm farther back”

I find this formula very handy…

Even if you don't use KOPS, if you tilt the seat tube forward, to keep your butt/knee/BB relationship the way your body likes it, you'll have to move the seat back on the rails. The problem will come in if your saddle is already back pretty far on the more laid-back frame - there is such a thing as not being able to get enough setback.

Road Fan

most saddles have 4 in., 10 cm., of adjustability, it won't matter a whit.

Except if you are on the limit of the saddle rails (I mean physical limit, not the stupid "stop" markings on the rails)

Actually those stop marks aren't really so stupid. I used to ride with a saddle as far back as it could physically go on a rear set seat post in an effort to get it back far enough and I ended up breaking the saddle rails. Modern lightweight saddles just aren't strong enough for such abuse and they need to be positioned within limits in order to distribute the weight better.

That also raises the other point. With a 75º seat tube angle it is quite possible to not be able to position the saddle far enough back, even WITH a rear set seatpost to achieve KOPS. With a 75º seat tube angle + rear set seatpost + saddle ALL the way back I could only achieve 50 mm of saddle set back which isn't much at all. I think that I read that TT bikes (or was it road bikes?) need to have at LEAST 50 mm of saddle setback to be legal. I needed about 70 mm of setback to get to KOPS which has me at an effective seat tube angle of 69º to achieve.

Regards, Anthony

Legal for UCI rules, which don't concern 99% of US races.