Seat tube angle...

[oldguy00]

I'm sure this has been discussed in the past....but I'll ask anyway...

On a typical 'medium' frameset, what difference is 1 degree going to make? So 73 vs 74 degrees.

Is the general rule 1 degree = 1cm? So if you go from a 74 degree to a 73 degree, using same post and saddle, in theory you'd need to push the saddle forward 1cm to get same position relative to BB? How is handling affected?

I've always seemed to fit well on 74 degree SA's (Calfee (54), Colnago (50S), Ridley (small)), and I mostly ride my tri bike which of course is much steeper (like 80). I'd like to build a new road bike, but so many of the bikes have 73 SA's, wondering if it is a mistake for me to get one...

Oh, assuming it matters, my saddle height from BB to top of saddle is about 73cm.


Thx!

[David Tollefson]

Assuming that your bars are in the same place but the (effective) top tube length is therefore shortened by 1cm, and also assuming the HTA and rake is the same (and therefore 1cm closer to the BB), then you'll (1) have proportionally more weight on the front wheel, and (2) likely experience a slight increase in "responsiveness" of the bike.

Whether that's positive or negative, or even felt by you, is up to you.

[dddd]

As David mentioned, getting the fit right is really what matters, i.e. the contact points in the right places. relative to the bb and pedals.

And, I would add, not having to use a stem length that doesn't get along with the bike's front end geometry and weight distribution.

As far as achieving that, with components/adjustments, I myself prefer using an inline no-offset post on bikes having a relatively slacker seat tube angle.
This way, the saddle isn't so much cantilevered behind the post and the rider is less likely to run out of adjustment along the length of the rails.

As for the reach vs. the STA, I recall that for a frame in the 56-58cm size range that the reach changes to the tune of 0.9cm per degree.

[NHAero]

My saddle height is about 71.8 cm and one degree is 1.25 cm at the saddle

[ultraman6970]

That means that u run like 69 to 70 cm saddle height if the frame has a seat tube with less angle??

[oldguy00]

Originally Posted by ultraman6970

That means that u run like 69 to 70 cm saddle height if the frame has a seat tube with less angle??

Think he is talking for-aft difference, not height

[ultraman6970]

Oh, thats why i was asking because I have like a similar fit issue with a bike, set tube has less angle, set it up the aft/fwr position with the center of the BB and still dont fell 100% right. TOday lowered the stem and looks like is better but the neck strain will be an issue, hands numb is gone, looks like have to go longer stem, at the same handlebar drop and will be ok. The slacker the angle the shorter the reach. Have no longer stems moving around to test. Besides some bikes like to seat forward other ones far back.

[Dave]

Pay attention to the stack and reach. If those are correct, the STA will not affect the saddle position, relative to the BB. A steeper STA will require more setback on the post or if there's room the saddle will be further back on the post, to get the same position, relative to the BB.

The saddle should always be set relative to the BB first, then the distance to the bars set as desired, with the required stem length and handlebar reach.

[tomato coupe]

Originally Posted by oldguy00

Is the general rule 1 degree = 1cm?

For rough purposes, yes. (Exact? No.)

[benb]

Yah.. start with stack and reach.

Then as long as it's not a frame with an integrated seat mast with weird min/max seat height you're pretty much good.

What you are dialing in with the STA is your position relative to the wheels and whether you end up on a goofy seatpost or with the saddle slammed to one in end in the rails.

This is not universally true but bikes with longer wheelbases are more likely to have a shallower STA and vise versa. Picking a bike with a short wheelbase with short chainstays + steep STA and then putting a heavily setback seatpost on it and the saddle slid back in the rails could end up putting you too far back, etc.. A bike with an excessively slack STA might stick you into a straight seatpost and/or feel like a bike that doesn't want to turn as nimbly. Again.. not always true, especially with the crazy stuff that can be done with carbon frames such as a slack STA with a cutout for the rear wheel.

This usually isn't that hard though... to screw it up you really need to be on a bike that is totally the wrong size. Usually IME the only thing you can really screw up badly buying bike A vs B in the same size is if they have wildly different stack. It's a lot harder to compensate for too much or too little stack.

We're all different.. I'm all legs.. 72.5-73.3 is the range I've lived in almost my entire bike experience, always with a setback seatpost + 175mm cranks to take care of the length of my femurs. I generally look first towards the 72.5 degree end of that range first too.

[dannymd]

Nice thread , thanks for this

[oldguy00]

Thanks for all the replies.

Yup, very familiar with stack and reach measurements, and understand that I can essentially get the exact same contact points on the bike using those numbers, and same saddle with same distance from saddle nose to BB, etc.

Guess I was just wondering how much 1 degree would shift my weight forward on said frameset when setup for same contact points.

And of course the more important thing, will it look stupid with saddle pushed so far forward lol.

[Gummee]

Originally Posted by oldguy00

And of course the more important thing, will it look stupid with saddle pushed so far forward lol.

Yes, hence the 0 offset post recommendation above.

AMHIK

M