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How much can seat tube angle matter?(10 posts)

How much can seat tube angle matter?GMS
Sep 10, 2002 5:13 AM
A lot of people think that seat tube angle is a vital part of a frame's geometry, and I'm one of them. That said, I can't think of a specific reason why seat tube angle can't be corrected for by using a seat post with a large setback/set forward.

What is the difference between having a 72.5 angle and a post with 0 setback, and having a 75 angle and a post with a lot of setback to compensate?

Similarly, how is turning a seat post around so that it is set forward on a 72.5 angle frame any worse than having the seat in the same position (relative to BB) on a 75 angle frame with a no-setback seatpost?

Something tells me there must be a disadvantage to correcting STA problems with seatpost setback, but I don't know what it is. If it was just as good, then we'd all correct our geometries with seatposts instead of frames. Seat posts are a hec of a lot cheaper. And then we could choose any frame we wanted, too.
Seat angle is only one variableFez
Sep 10, 2002 5:32 AM
For the sake of your argument, it appears that you assume the geometries of bikes are otherwise identical with a variance in seat tube angle.

But there are plenty of other factors that frame designers use in bike design (top tube length, head tube angle, chain stay length, etc.) so the resulting product is the best possible combo of ride and handling. The 2 bikes you mention may also be designed for different purposes as well, and the seatpost setback/setfwd may be just to accomodate the individual rider to that frame.
Seat angle is only one variableGMS
Sep 10, 2002 5:47 AM
I agree with both of you. As for difference in other things such as top tube length, yes, the STA inherently affects tob tube length because if you make the STA more laid back, the top tube needs to be longer to reach the seat tube.

I guess the question is: Is it reasonable to eleminate a frame based on its STA? I'm not sure. It seems like it could be corrected for in almost all cases. This assumes that the other properties of the frame are desirable, of course.
Weight Distribution?Ray Sachs
Sep 10, 2002 5:37 AM
I would assume that frame designers assume something close to a mid-rail saddle position when designing the rest of the frame for proper weight distribution. As with stems, there's probably some adjustability before this becomes a problem, but radically adjusting either the stem length or the saddle position could throw off the ideal weight distribution and, hence, bike handling. Then again, every rider is built differently and I can't imagine the amount of setback would have as much effect as whether the rider carries a lot of weight in their upper body vs. carrying most in their lower body. So, it could be something of a crapshoot regardless of the seat tube angle.

Perhaps why a full custom is a nice option, even for people who don't technically "need" one to get a good fit.

Of course, if you can't get the saddle back far enough with ANY seatpost, you need a slacker seat tube and the reverse is true if you can't get far enough forward.

-Ray
effective top tube length...C-40
Sep 10, 2002 8:40 AM
What you have not mentioned is the effect of saddle movement on the reach to the handlebars.

On a frame with a 74 degree seat tube angle, the saddle must be moved further back than a frame with a 73 degree seat tube angle, to achieve the same position relative to the bottom bracket. This movement of the saddle must be accounted for. Since the TT length is the standard reference value for comparing the reach to the handlebars, the difference in SADDLE POSITION is added or subtracted from the actual TT length to produce an "effective" TT length. The effective TT length will accurately predict the difference in reach to the handlebars between any two frames and allow you to determine the difference in stem length that would be needed to maintain the same reach. When comparing frames with different STA, add TT length to the frame with the steeper (74) angle or subtract length from the frame with the shallower (73) angle, using one of the following formulas: 1.32 x (cosA-cosB) x frame size, or an alternate formula, saddle height x (cosA-cosB). An average amount is 1.2cm per degree for a midsize frame.

Some frames (LOOK for example) may combine a slack 72.5 STA with a relatively short TT length. If your optimum STA happens to be 74 degrees (like mine), this combination might require a stem that is as much as 2cm longer that some other brands (like Colnago) to achieve the same reach to the bars. I'll probably never buy a LOOK frame, because I'm not wild about using a 130mm stem on a 52-53cm (c-c) frame.
AgreedGMS
Sep 10, 2002 9:10 AM
LOOK frames were actually what got me thinking about this. Not only is their STA slack, but it is constant throughout all frame sizes. This is rare.

But your point is important. It almost means that STA has more of an effect on stem length and reach than it does on position relative to the BB. This is contrary to the common line of thinking, but I think it might actually be more accurate. Stem length has a more significant impact than seat setback, so considering the implications of certain STAs on stem length seems more important than relation to the BB.
another exampletarwheel
Sep 10, 2002 9:30 AM
I ride a Gios Compact Pro, which has 74 seat tube angles and short top tubes in all sizes. The only real problem I've had with this frame is finding a saddle that I can move far enough back on the rails to get the right knee over pedal position. I was about to buy a setback post when I tried a Koobi saddle, which has the rails positioned just perfect for my frame. The saddle sits midway on the rails with the correct position.

The important thing to keep in mind is that a steep seat angle effectively lengthens the top tube, assuming your are keeping your knees in the same position in relation to the BB or pedals. A slack seat angle shortens the top tube. So, the funny thing is that a LOOK or Merckx frame with slack angles and longer top tubes fit about the same as a Gios with steep angles and short top tubes. For comparison, the newer Bianchis have steep seat angles and long top tubes, so they fit really long across the top -- which is why I could never get comfortable on the Bianchi frame I sold a year ago.
Gios web site?GMS
Sep 10, 2002 9:42 AM
Yes, I've been meaning to look into Gios geometry, but I haven't found their web page. I forget if I tried the obvious or not. Where do you purchase them?

I'm just now having the opposite problem with a new Koobi saddle. It really likes to be further back than my previous saddle, and the rails are short, and Koobi claims the saddle will explode and destroy your bike if you mount it more than 1 cm away from the center of the rails ;)
Gios web site?tarwheel
Sep 10, 2002 11:58 AM
Excel is the sole US distributor for Gios, which is why their prices are so good because there is no middleman. Their web address is www.excelsports.com, and there is a link to the Gios webs site from there. However, the address for the gios site is www.gios.it

I bought the Koobi saddle on a lark because of the 30-day return policy, figuring I would be mailing it back in a week or so. But so far it is working out better than any other saddle I have tried, which means it might not fit too well on a frame with a lax seat tube angle, but you never know ...
A bit or a lotKerry
Sep 10, 2002 4:58 PM
If the rest of the dimensions of the bike remain the same and the saddle stays in the same place relative to the BB, a steeper or slacker STA only changes the feel of road shock transmission. Steeper angles are a bit rougher. However, the reality is that steeper STAs often mean shorter chain stays, less BB drop, and a host of other changes to make the bike handle "faster." Rarely will you get to see the effect of only a change in STA - builders change a number of other things simultaneously.