I've got a question for you guys.

I have a seat that is 76cm (measured at rails [top of saddle measurement would be greater]) from my BB spindle. If I went from a 72 degree seat tube to a 73.5 seat tube angle how far forward would that move my saddle if I kept it on the same position on the rails.

Does anybody have a scientific calculator handy? (simply a matter of plugging in cosine right?)

It may very well be a 72.5 STA so could somebody do the math for that too. :p

almost 2cm for 73.5*

2/3 cm for 72.5*

I've figured out my prefered cockpit length= center of seatpost measured to brifters. That length includes a 16cm drop to the bars.

Now if I go to a bike with a steeper seat tube angle I need to take into consideration how much more rearward I've have to move my seat back in comparison. I can easily reduce stem length from 130mm to compensate , but how much length are we talking about?

you want a more than 6" drop from saddle to bars? Whatcha gonna do with your testicles. (Oh boy, that joke never gets funny)

2 cm going from 72 STA to 73.5 STA.

1.3 cm for 72.5 STA to 73.5 STA, right?

Thanks that really helps me figure out the math.

That means I can probably use a 110 mm stem with the same 57cm top tube if all I do is change the STA 1.5 degrees. (assuming I keep the same reach handlebars).

Or I can use a 100mm stem with a 58cm top tube and 73.5 STA.

I just like it that way.

I measured from the center of the BB to the saddle rails (not the top of the saddle)

Yeah the saddle height would be exactly the same.

All I would do with the new frame is use a 2 cm setback seat post.

The BB to saddle height and the fore-aft relationship would be the same.

I just don't like the aesthetics of having my saddle moved slightly forward on my straight clamp which is bound to happen since I have it jacked up pretty high. The higher I have it jacked up the more rearward the saddle moves, thus the forward compensation.

what about at the back part of your pedal stroke? By pushing the seat forward, you're now further away from the pedal spindle at that point. Crank length should be about your leg length, not your seat position.

It's not really that forward though. (seat is actually in the middle of the straight clamp)

The seat tube angle is slack and it's 76cm ( probably more like 80+ cm if you measure from the top of the saddle) from the BB. So if you draw a line from that point you can see that it's pretty far rearward without forward compensation.

My 180mm spindle is also a little more forward in the 3'oclock position also.

This set-up makes the most power yet for me (subjective feeling)

16 cm to the hoods or the drops?

Somewhere around there. I have a quill stem so I'm always playying with it. That's how I rode today (seemed nice)

Is this another one of your hypothetical exercises or are you actually trying to maximize? If you want more power, perhaps get on a fit cycle hooked up to power meter and computer? I'm doing that in eight days :)

I suggest you measure how far up and back your saddle is using your current bike. You can do with by droping a plumb-bob off the nose of the saddle and measuring how far back it is from the centerline of the crank. Measureing how high the saddle is can be done with either a tape or ruler.

Calculations are a good way to compare different frames, but no substuite to set up the bike. Remember, "To measure is to know".

Good luck.

Ed

That really is the best way. It just feels fast this way.


Who offers those services? I would mind it if they could test crank length too. When you change the crank length it screws up all the adjustemnts though.

I really can't afford that kind of test though, but it's neat know that it can be done.



I'll do that too. Nothing better than real world measurements.

Hi 53...yup that's right,
...for posterity here is the simple trig:
sin (change in angle) X 76cm = change in setback
sin (1.5 deg) X 76 = 1.99 which is ~2.0 cm
sin (1.0 deg) X 76 = 1.33 cm
for what its worth the net setback difference for small angles are proportional to the angle change, i.e. 1 deg/1.5 deg = 1.33cm/2.0 cm

As to your comments about set back and seat clamp attachment...its worth noting
that it is more than aesthetics in terms of clamp attachment without a set back seat post...its best to use a setback post to attach the seat in the middle of its rails for best load distribution for the seat itself as well...more critical with heavy riders.
Also since you and I are close to the same size...I of course would like to see you in a bigger frame with shorter stem...but I am likely older ;)

HTH,
George

I like the seat to be in the middle of the clamp for those reasons.

I think I want about a 100mm stem too. Just for weight balance.


Thanks for the info (everybody)

I am running a 100mm stem on my 61 c-t-t frame with 586mm virtual top tube and think for a tall guy with long legs running a bit bigger frame with a nominal stem size like you are targeting is a good ticket especially if doing some touring.
Good Luck...may be moving my campy brifters up a tad on my Deda bars per your tip the other day. Does anybody know how to remove the rubber hood covers on a Campy brifter (Veloce)? They may just peel off. Need to get at the bar tape which runs under each hood cover. There is a little button on the inboard wall of each brifter hood cover and not sure if this is a pin to hold the rubber hood cover in place or not.
Thanks,
George

i hate to be a math nazi, but your trig is wrong.

you've kind of, but not really, calculated the change in height of his saddle given a change in seat tube angle. even if you were trying to do that you did it wrong.

the proper formula is
76*(cos(72)-cos(73.5)) = 1.9cm forward
76*(cos(72)-cos(72.5)) = 0.63cm

if you were trying to caluclate the change in height:
76*(sin(73.5)-sin(72)) = 0.59 cm higher

Crank length is about your seat position. (not saying that's ideal, but that's how the cycling industry deals with a lot of different leg lengths and token adjustment in crank length)

Basically all cyclists ride one length crank (170mm to 175mm is a token 3% spread). All the adjustment for the huge spread in leg length is done by altering seat tube angles. That's why you have guys with 29" inseams riding 172.5 cranks and a tall guy with 37" + inseam riding 175mm cranks.

That's why 50cm frames have 75-76 STA's and 62-64 cm frames have 72 STA's.

Sorry for the late response I just reread your post.

FYI , I just looked at my seat again--it is approx in the middle of my straight clamp. On my new bike I want less stem and more top tube for better balance.

Biker7's posted formula is correct for very small changes on an isoceles triangle. Since sin(89) is almost = 1. I'm not saying your's is wrong, just that there are different ways to get there.

Sin(a)/A = Sin(b)/B and all that :)

I've noticed that it occassionally pisses off female forumites, however. It apparently indicates that we men believe that only men cycle.

Thank you Lord Opie. I guess baxtefer missed class that day and why the Nazi's lost the war.
George