Oostal

I just can't get my head around the seat tube angle. Suppose I have 2 frames with identical top and down tube lengths. Bike A has seat tube angle 72.5 and bike B has 73.5.

As I understand I would be shifted more forward on bike B. Hence I would need a 10 mm longer stem to reach the handlebars. On the other hand my knee would also be positioned 10 mm forward relative to the pedal axis. Hence I should move the saddle 10 mm back to maintain the knee position and thus can go back to the original stem. So nothing changes? Confused

redtires

Well, it's not quite like that...but I see why your getting there. The seat tube angle on a bike does a couple of things, one, in production bikes it is set off of the AVERAGE femur to lower-leg ratio for a neutral position and also, it is only one part of the overall design to have a favorable weight distribution between the front and rear wheels. This is why when you have a custom bike built, the builder will actually measure your thigh length and your lower-leg length. But seat tube angle is only part of the equation. Depending on what type of riding/racing you do and what type of bike your getting, it's part of the larger picture including the head tube angle, rake of the fork, and the chainstay lengths. In your scenario of simply moving the saddle back a centimeter, you've just moved a good portion of your weight over the rear wheel and taken it from the front wheel which may alter the handling.

As said, most production frames are built around a set of averages so they can fit the most people ok.

Bacciagalupe

That would still depend on other factors, such as the setback on the seatpost, and the final saddle position.

Time trial bikes will have STAs closer to 77º. This puts the rider much more forward and facilitates the more horizontal back position used by TT positions. It also can be used to leverage muscle groups typically used in running.

My guess is that if the STA is around 73º, a 1º difference isn't that significant. The designer might be facilitating a slightly more relaxed or aggressive position, or they might want to adjust the triange sizes.

If you are switching from a bike with 72.5º to one with 73.5º, and this moves the top of the seatpost 10mm forward, and you wanted to maintain the exact same position, you'd want to move your saddle back by 10mm. I.e. you'd want to keep the saddle in the same position relative to the BB, despite the change in STA.

SteveV

Actually, you're leaving out the most important thing which is that your saddle should have a specific position relative to your bottom bracket and that should not change with seat tube angle. You'll just have to move your seat. For example, let's say you have a professional fit and it was determined that the nose of your saddle should be 7cm behind your bottom bracket. No matter the seat tube angle, this 7mm should not change. Typically (and this will vary by seat tube length), 1 degree of seat tube angle equates to approximately 1cm of front to back at the top of the seat tube. This is for an approx 58cm seat tube - a shorter seat tube will have less of an effect. So let's say that your 7cm was determined on bike A that has a 72.5 seat tube. If you transfer that saddle and seat post as is to the 73.5 ST bike B, your saddle nose will now be only 6cm back from the bottom bracket. So to get back to that 7cm setback, you need to move your saddle back 1cm. So now, your head tube is actually 1cm farther away from your seat because you had to move your seat back.
Take a look at this image (hopefully it will enlarge if you click on it) and you will see that Bike B would need a 10mm shorter stem to put the bars in the same place as bike A.

DaveSSS 

If you're comparing bikes with different STAs, you can't compare the TT lengths directly. You need to add 8-10mm per degree to the TT length of the frame with the steeper STA, then compare the lengths.

Frame comparisons always assume the the saddle remains in the same position, relative to the BB.

Campag4life

A suggestion since you are getting too much information from posters above...is...post a real life example with respective geometry charts...perhaps different frames you are considering with different sta's. It isn't too bad really. But if look at stack and reach charts from several mfrs' where sta's shrink with increased frame size, you will notice there isn't much reach different from frame to frame. This is because decrease in sta negates some of the increase in top tube length.
Post an example if you want a concrete answer.

ancker

What did you use to generate that pdf?

DaveSSS 

You can't compare the reach of different sized frames without making a correction that gives a reach value for each frame, at the SAME stack height. The correction is about 3mm for each 10mm of stack height difference (cosine 73 degrees).

Take a look at the Cervelo geometry chart for the R3. Although the frames are listed with the same STA, the reach increases from 360mm to 405mm, 45mm is NOT the real difference in reach. There real amount should have have about 37mm added to it, so the true difference is 82mm, which is a lot.

As a more practical example, if you're trying to decide between the 54 and 56cm sizes, the reach difference is not 387-378 = 9mm, it's really about 16mm, due to the 25mm difference in stack height.

https://www.cervelo.com/en_us/bikes/2012/R3/geometry/

deepakvrao

Are you saying that you need to ADD 10mm to effective TT length because with a steeper STA, the saddle would be further forward with regard to BB and you need to account for that by pushing it back?

SteveV

You are correct about pushing the saddle back. I guess adding 10mm to the top tube would be one way of looking at it. If you use the same TT length and the same saddle setback from the bottom bracket, then yes the handlebars will be farther away from you on the steeper seat tube. The steeper the seat tube, the farther it pushes the head tube and the bars in front of the bottom bracket. Think drastic and it may make more sense - What if you had a truly vertical seat tube (that would be 90 degrees) with a 57cm TT? Just imagine where the front of the bike would be. The bars would be awfully far away.

SteveV

I actually drew out the geometry in AutoCAD and then printed it as a pdf.

Bacciagalupe

You have way too much time on your hands.

Oostal

Thanks a lot! This is all very helpful.

I do not have very specific bikes in mind. I just try to educate myself on bike geometry and it's role on fit. I was thinking that most online fit calculations give top tube length. However, the rest of the geometry might have an important role on whether going for a one size smaller or larger frame.

So assuming that bike B (73.5 STA) has a longer effective reach in comparison to bike A (72.5 STA) - can this be reversed by using longer crank arms, thereby shifting knees forward? What about seat post setback? Can it be generalized that less setback increases distance to handlebars (when maintaining saddle position in respect to the bottom bracket)?

Campag4life

Yup good point. Reach is affected by stack.

Campag4life

You have the autocad program. Draw two bikes in totality..including handlebars and analyze the distances you want to compare. Pretty simple. The permutations of different relatiionships you may want to consider are many but the trig isn't difficult to follow. If you read Dave's point...reach is affecedt by change in stack height between frame sizes. This is because as a head tube grows in height the handlebar moves back with cosine of hta. If you have plotted a frame with autocad, it is a very simple task to analyze distances of different top tube and sta scenarios.

PS: A question. Can you suggest a good cost effective home PC Autocad software? Freeware would even be better if available.
Thanks

RollCNY

I have used BlueCAD, if you can still find it, which is free, and very reminiscent of the early CAD programs from the early 90's. Many others are trial versions that don't let you save. Don't know if QCAD is available for platforms other than Linux, but the little bit I have played with it it seems much more modern than Blue, and its free (not a trial).

Campag4life

2-D would be OK. If you go back aways, I was trained on IBM CADAM...lol...back in the stone age.
Didn't know if any hacked Autocad has made it onto the web that was free.
I do have some Rhino CAD software I believe I could dig up.
Thanks Roll...maybe a winter project. Uh oh...I said a bad bad word...winter.

ancker

This probably exists but I just don't know about it. But it would be nice if there was a website where you could plug in the geometry and measurements of Bike A, then the geometry of Bike B and have it spit out the correct measurements to make the overall geometry/fit be the same between the two.

So if the geometry of your two bikes are different and you've been fit and are happy on bike A, it will tell you that you need to place your saddle at a certain height/fore/aft, add 10mm to your stem, etc to make Bike B match in terms of fit.

Bacciagalupe

Crank length usually influences saddle height, not reach.

However, because the seat tube is at an angle, changing your saddle height also requires you to adjust saddle fore/aft position. IIRC moving the saddle up pushes it away from the stem, so you'd need to move the saddle forward.


Seatposts have different amounts of setback. E.g. most will have ~20mm, sometimes they're zero setback, TT'ers sometimes use negative setback. Usually you'd use a different setback to compensate for a fit parameter, or if you want to use a road bike for TT's.




Can it be generalized that less setback increases distance to handlebars (when maintaining saddle position in respect to the bottom bracket)?[/QUOTE]