geometry: seat angle and top tube

[tch]

I've searched and can't seem to find this info exactly....
I seem to remember that the general agreement by the fit gurus is that at the 54/56 cm size, each degree of angle roughly equals one cm in top tube length, right? For instance, a 55 bike with a STA of 72 and a top tube of 55 roughly equals reach for a 55 with a STA of 73 and a top tube of 54.

Is this right? Is it one degree? I can't remember the figure..... And does changing the angle on the head tube have the very same effect? Or more/less?

Alternatively -- or in addition -- does anyone else have a quick-and-dirty fitting guide that can relate different bikes to one another? I'm trying to figure what bikes might be in the ballpark to fit me from home, and all I know is that I have two bikes with very different geos which I've set up to fit about the same reach. A Riv with slack angles and longer top tube and a Serotta with steeper angles and shorter top tube. \
BTW, I know I can adjust reach with stem length and height; I'm after the frame effect.
Someone got insight?

[old_school]

BikeCad (http://www.xcontario.com/bikeCAD.html) is a handy tool that might answer some of your questions.

[Orin]

I've searched and can't seem to find this info exactly....
I seem to remember that the general agreement by the fit gurus is that at the 54/56 cm size, each degree of angle roughly equals one cm in top tube length, right? For instance, a 55 bike with a STA of 72 and a top tube of 55 roughly equals reach for a 55 with a STA of 73 and a top tube of 54.

Is this right? Is it one degree? I can't remember the figure..... And does changing the angle on the head tube have the very same effect? Or more/less?



Yes, it's very close to 1 cm for a 1 degree change in seat angle.

It's actually (cos(72) - cos(73))*size where size is center to center (or center to virtual center if a sloping top tube).

You are better off comparing what some manufacturers call reach which removes the interdependence of top tube length and seat tube angle. It would be: (top tube length - (cos(seat tube angle) * size)) assuming a horizontal top tube and size being center to center again.

Head tube angle doesn't make much difference to bar position - you'd have to have your bars a lot higher than the top tube to make more than a couple of mm difference in reach.

Orin.

[gt6267a]

One thing to point out that never seems to get pointed out ... or maybe I missing the picture, someone will correct me. It is nice to know the frame size difference, but that still leaves open certain problems. Specifically that at your saddle, the difference is greater than at the frame by nature of the arc being longer at a greater distance from the center of the circle.

For example, I have two bikes. One with a 74’ STA and another with a 73’ STA. At the frame this causes a .9cm difference in TT length, BUT out where my seat is, the difference is actually 1.5cm.

So … while sizing up and comparing frames it appears like there will be a .9CM change in reach but in practice it is 1.5CM.

In the image, I set the math up so one might step through it ...

[Orin]

One thing to point out that never seems to get pointed out ... or maybe I missing the picture, someone will correct me. It is nice to know the frame size difference, but that still leaves open certain problems. Specifically that at your saddle, the difference is greater than at the frame by nature of the arc being longer at a greater distance from the center of the circle.

For example, I have two bikes. One with a 74’ STA and another with a 73’ STA. At the frame this causes a .9cm difference in TT length, BUT out where my seat is, the difference is actually 1.5cm.

So … while sizing up and comparing frames it appears like there will be a .9CM change in reach but in practice it is 1.5CM.

In the image, I set the math up so one might step through it ...

Compact frame or are you measuring the seat height from the pedal at the bottom of the stroke?

The thing is, that 5 mm merely changes the saddle position on the rails, or perhaps requires a different setback seatpost. You shouldn't be changing your saddle position relative to the BB due to a different seat tube angle.

Your total reach is determined by the setback of the saddle behind the BB plus the length of the top tube in front of the BB. Seat tube angle doesn't come into it as long as the saddle rails and seatpost allow the correct saddle setback.

There is too much emphasis put on seat tube angle - it should depend on the saddle setback, not the other way around.

Orin.

[RPS]

You both make compelling arguments for what is important.

gt6267a, have you thought about using the BBKT-to-saddle instead of pedal-to-saddle dimension in your spreadsheet?

[gt6267a]

You both make compelling arguments for what is important.

gt6267a, have you thought about using the BBKT-to-saddle instead of pedal-to-saddle dimension in your spreadsheet?

you guys are right, for these purposes it should be the BB height. I just received a used Legend and have been farting around with numbers and this and that and whatever. for this post i should have done it from the bb. my bad. even so the concept remains the same. there are two triangles to consider. the frame and riding position. one is bigger than the other. therefore, just dorking up the math on the frame doesn't really mean a whole lot.

no doubt, any one measurement whether it be STA, TT, ST ... don't matter ... but all together they do. one thing that is true, with at 73' STA, I am right between needing setback and not so i end up closer to the end of the rails. with the 74' STA i am closer to the middle of the rails. not a huge deal, but a nice to have.

[Orin]

no doubt, any one measurement whether it be STA, TT, ST ... don't matter ... but all together they do. one thing that is true, with at 73' STA, I am right between needing setback and not so i end up closer to the end of the rails. with the 74' STA i am closer to the middle of the rails. not a huge deal, but a nice to have.

I just put a zero-setback Thompson seatpost on my Rambouillet for a couple of reasons - the Rambouillet has a slack seat tube angle and I wanted to move the saddle back relative to the seatpost clamp in order to make room on the saddle rails for the clamp for a saddlebag support...

I wish setback data for seatposts was easier to find - they are either zero setback or something else...

Orin.

[Dave]

One thing to point out that never seems to get pointed out ... or maybe I missing the picture, someone will correct me. It is nice to know the frame size difference, but that still leaves open certain problems. Specifically that at your saddle, the difference is greater than at the frame by nature of the arc being longer at a greater distance from the center of the circle.

For example, I have two bikes. One with a 74’ STA and another with a 73’ STA. At the frame this causes a .9cm difference in TT length, BUT out where my seat is, the difference is actually 1.5cm.

So … while sizing up and comparing frames it appears like there will be a .9CM change in reach but in practice it is 1.5CM.

In the image, I set the math up so one might step through it ...

Although it might seem like figuring the difference at saddle height is correct, it is NOT. I used to post the advice to figure the difference based on saddle height, but as I said, it's easy to prove that this is WRONG.

Basically the reason is that angular effects stop at the ST/TT intersection and horizontal movement of the saddle corrects for this amount of error.

What you're you're figuring is the difference in the position of the saddle on the seatpost. That's good to know too, but it does not predict the difference in the stem length required.

The proper formula to figure the difference in reach is (cosA-cosB) x c-c frame size. As other's noted, this comparison can only be made between frame of the SAME size. If you try to compare frames that are 2cm different in size, then you have to pick one as the baseline size and do a little more complicated correction to the other one, so the reach is calculated at the same distance from the BB. You'll be plenty accurate enough just using the 1cm per degree rule of thumb.

For example, I ride a relatively small 51cm (c-c) frame, for my 73cm saddle height. Figuring the change in the reach between a 73 and 74 degree STA, you get .85mm. At at height of 73cm, you get 1.22mm.

[gt6267a]

...
Basically the reason is that angular effects stop at the ST/TT intersection and horizontal movement of the saddle corrects for this amount of error. ...


huh ? angular effect stops at ST / TT junction but then you account for it with the saddle setback ... those statements contradict each other.

the point still stands that there are two triangles in question. the frame and your contact points. a good portion of figuring out if bike A or B will fit or C will fit better is the ability to negotiate the contact points triangle onto the frame triangle via seat post / stem ...

[Dave]

huh ? angular effect stops at ST / TT junction but then you account for it with the saddle setback ... those statements contradict each other.

the point still stands that there are two triangles in question. the frame and your contact points. a good portion of figuring out if bike A or B will fit or C will fit better is the ability to negotiate the contact points triangle onto the frame triangle via seat post / stem ...

I know it's tough to understand, but the only thing that using the saddle height in the equation tells you is the amount saddle movement required to produce the same position relative to the BB. It also tells you how much change in the seat post offset is required to keep a saddle perfectly centered on the post. It's good information, but it tells you nothing about the change in fit.

To figure the difference in reach and the stem length required, the c-c frame size is the amount to use in the equation. By definition, reach is the horizontal distance between a vertical line through the BB and the TT/HT centerline intersection point. If the reach of a frame is shorter (or longer), then a longer (or shorter) stem is required to correct that, not a change to saddle position.

[RPS]

You are both saying almost the same thing except that one is focusing on the saddle and the other on the stem.

If the goal is to replicate the contact points given a different frame (not the original question), then the issue for most of us is knowing which stem and seat post to buy. And for precision, the angles of both the seat and head tubes must be taken into account above the top tube until differences in angles no longer matter (like the seat post clamp pivot point or top of bars).

Personally I don’t need that much precision to set my bikes up since they are all different; but know that many want exactness.