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what diff. does of degree seat tube actually make?(6 posts)

what diff. does of degree seat tube actually make?secourir
Mar 2, 2002 7:59 PM
looking at a new frame with a 73.3 degree seat tube (53cm C-C) and a 54.3 cm top tube with a 10 cm stem.
setback is quoted as 15cm. (merckx)

currently ride a 74 degree seat tube (53cm C-C) and a 54.3cm top tube and a 11.5 cm stem. setback is quoted as 14.6 cm. (colnago)

from where do they get the figures for setback?

and will i seem to fit ok considering that i am shortening the stem length by 1.5 cm in allowance for 2/3 of a degree slacker seat tube?

i have read a lot of comments to the effect that merckx geometry is much more relaxed than that of colnago and that the slacker seat tube will have me sitting further back rather than upright and that the two are very different in terms of fit. is this just garble or an accurate perception.

looking at the data that is applicable to the frame size i am interested in i feel the difference is relatively small and not that different. i cant find any data with reference to bottom bracket height for either and am wondering if they are different enough to affect how i will fit?

any opinions?
re: what diff. does of degree seat tube actually make?CT1
Mar 2, 2002 8:23 PM
These two frames are not really that different in terms of setback. Mostly a VERY minor difference that CAN be accomodated via different saddle positions and stem lengths.

*** Note: You've got the stem change wrong. You will need a longer stem on the Merckx as compared with the Colnago. Since the numerical TT's are the same and the setback is 0.4 cm greater you would need a 0.4 cm longer stem on the new frame. Sounds like a 12cm stem would put you in the same position on the new frame...... assuming you want the same position. ???

Look at the geo chart on the trialtir site to get a sense of what setback is.

JohnG
what CT1 saidgtx
Mar 2, 2002 9:21 PM
basically on the Merckx you'll have your seat about .7cm farther forward on the seat post if you want to keep the same positon relative to the cranks, and you'll probably want a 12 or 12.5 stem. I believe the Merckx has a slightly lower bb and longer c-stays. The two bikes will feel a bit different in terms on handling/geomety but they will fit pretty close. The Merckx will not be a good choice if you are currently having problems getting your seat forward enough.
btwgtx
Mar 2, 2002 9:36 PM
two measurements I don't trust are "setback"--mostly because most builders don't include this measurement and so it's hard to compare between bikes (thus I like STA better) and "bb height"--since it depends on the tires (thus I think "bb drop" is better).
Maybe noneKerry Irons
Mar 3, 2002 9:04 AM
Assuming you have your position right, switching to a bike with a 0.7 degree smaller seat tube angle means that you have to move the saddle forward about a cm. If the top tube lengths are the same (as in this case) then you need a longer stem to keep your h-bars in the same relationship to the saddle. Unless you are going to change your position on the bike, this change means only that you'll need a longer stem. Any changes in handling or performance are due to something other than the slacker STA.
an accurate assessment....C-40
Mar 3, 2002 2:16 PM
The others who have posted on the effective top tube difference have not taken into account the saddle height. The acutal difference in effective top tube length between the the two frames is .85cm.

If you move the saddle forward on the Merckx to get the same KOP that you have on the Colnago, you will want a 1cm longer stem (that's as close as you can get).

Setback is the horizontal distance between a vertical line through the bottom bracket and the intersection point of the centerlines of the top tube with the seat tube. It is quite common for frame builders to use linear dimensions that are much easier to measure than angular dimensions.

In the case of the Merckx, dividing 15 by 53 gives the cosine of the seat tube angle. The STA would be 73.55 degrees. Can't explain the small discrepancy. The Colnago's figures are right on the money. The inverse cosine of 14.6/53 equals 74 degrees.