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Seat Tube Angle(4 posts)

Seat Tube AngleSuzie-Q
Aug 3, 2001 8:04 AM
Here's a question I need help with. I ride road bikes but also do triathlons. I recently bought a Litespeed Blade. I bought a 55cm frame because that is my road bike size is 56cm. The shop I bought the bike had a 57cm frame as well but said a lot of triathletes opt for 1 size smaller to help get their aero position better. I tested both sizes and bought the 55cm. I found to be really comfortable on it, I had to slide the seat all the way forward on its rails (Thomson Elite straight seatpost/Selle Italia Ti seat). The seat tube angle is already 78 degrees, so I felt I must have been at 80 or so by the time I slid the seat forward. I asked my LBS about this and they said by getting a smaller size and exposing more seatpost length, the effective seatube angle (relative to the bottom bracket)lessens. I tried drawing a vertical line on a wall and then lining the bb on the line and comparing the saddle nose position to the line when sliding the seat post to its lowest length and then to its highest length. What my LBS said seemed to be true. The difference at the lowest position compared to the highest was about 3 inches. The only thing I can't figure out is why then do compact frames (like Giants) work. They have seatposts that range from about 8 inches all the way to about 15. seems to me if the handlebar height remained equal, the difference in effective seat tube angle would be huge. Sorry this was so long. Any ideas?
is the seat tube curved?club
Aug 3, 2001 9:00 AM
If it is curved, rather than straight, or otherwise offset so that it does not merge at the center of the BB shell as on most bikes, then what the LBS said would be true. If the seat tube is "normal" then the amount of seatpost showing would not change the seat setback relative to the bottom bracket when compared to another size frame of the same seat tube angle. It's true that the higher you raise the seatpost, the more setback you get, and maybe that's what they're talking about. But whether the distance from the BB to the seat is achieved by a long seat tube and a short seat post, or a shorter seat tube and a long seat post does not matter, the setback would be the same, because your BB to seat dimension is governed by your fit on the bike. So unless the two frame sizes have different seat tube angles, which is possible, the seat setback relative to the bottom bracket would be the same with the smaller frame and longer post, as with the reverse combination.
from an engineer...C-40
Aug 3, 2001 9:01 AM
Your LBS guys never took geometry lessons. A 78 degree seat tube angle produces the same rider position relative to the bottom bracket (BB) regardless of frame size. The amount of exposed seat tube has nothing to do with rider position.

Saddle height does affect your position on the bike, but you don't raise or lower the saddle relative to the center of the BB because of a change in frame size.

You need to realize that changing from a road bike with a 73 to 74 degree seat tube angle (STA), to a Tri bike with a 78 degree STA moves the knee-over-pedal(KOP)position forward by about 1.2cm for each degree. This could be as much as 6cm (2-3/8"). By using a straight seatpost like the Thomson, and sliding the saddle all the way forward, you've probably moved yourself 10cm or nearly 4 inches further forward than the position that you use on your road bike.

This is huge transition to make. Triathletes like this type of forward position because it makes better us of the muscles developed by running. Road riders often do not change their STA significantly for a time trial bike. Road cyclists usually train to produce optimum power with a particular KOP position, and would be disadvantaged by making such a drastic change.

The discomfort that you have experience could be merely the result of too long a reach to the aero bars, or it could be due to your position being much further forward from the center of the BB. Unfortunately you can't correct your position relative to the BB without drastically lengthening the effective top tube. If you move the saddle back several centimeters to be more like your road bike, you would have to shorten the stem by an equal amount.
well since Blade's seat tube isn't bentcyclopathic
Aug 3, 2001 10:11 AM
rising seat will not change seat angle, it will change seat offset and that's what you refer to.

My guess you feel more comfortable with seat forward because of the reach you needed shorter effective top tube length. You could get similar effect by getting shorter stem.