Quote from Sheldon Brown that doesn't make sense
02-25-16, 10:36 AM
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Quote from Sheldon Brown that doesn't make sense
"Comparing two frames mainly in terms of top-tube length is only valid if both have similar seat tube angles. Generally, each degree of difference corresponds to about a centimeter of top tube length. Thus, frame "A" with a 58 cm top tube and a 72 ° seat angle can give the same riding position as frame "B" with a 57 cm top tube and a 73 ° seat angle. This presumes that the saddle would be slid 1 cm farther forward on the seatpost of the frame "A."
I don't understand why there needs to be a presumption the saddle on A was slid 1cm further.
I don't understand why there needs to be a presumption the saddle on A was slid 1cm further.
02-25-16, 10:40 AM
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Saddle position is set relative to the bottom bracket.
02-25-16, 10:41 AM
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Because your position relative to the bottom bracket changes with seat tube angle....you have to account for this by moving the saddle. Stick with reach and drop measurements.
02-25-16, 11:21 AM
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It's called "reach", combines ETT and STA to understand fit between two different frames. If two frames have the same ETT but one has a 72* STA and the other has a 73* STA, then the saddle must be positioned in the seat post differently by ~1cm (if the rider wants the same fit). And the stem length will differ by ~1cm. Because the "reach" differs by ~1cm.
It takes a little high school trig to work this out, and the ~1cm rule-of-thumb varies with frame size.
It takes a little high school trig to work this out, and the ~1cm rule-of-thumb varies with frame size.
02-25-16, 11:22 AM
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Because the distance between my elbow and fingertips doesn't change when the frame size does. 
02-25-16, 11:54 AM
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02-25-16, 01:19 PM
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02-25-16, 01:19 PM
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02-25-16, 01:27 PM
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Yeah I'm thinking about banning him. 
The gall.
The gall.
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02-25-16, 01:40 PM
#13
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It does seem a little confusing. Obviously, there's a 1 cm difference to begin with but the effective top tube length on the frame with the slacker ST angle will increase at a faster rate for any given increase in seat height, which will require more exposed seatpost on the slacker frame... but, the angle of the head tube also needs to be taken into consideration as that will effect the location of the handlebars, assuming the stem is raised in conjunction with a change in seat height.
02-25-16, 01:51 PM
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02-25-16, 03:05 PM
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Shaken, but not stirred...
02-25-16, 04:00 PM
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It's called "reach", combines ETT and STA to understand fit between two different frames. If two frames have the same ETT but one has a 72* STA and the other has a 73* STA, then the saddle must be positioned in the seat post differently by ~1cm (if the rider wants the same fit). And the stem length will differ by ~1cm. Because the "reach" differs by ~1cm.
It takes a little high school trig to work this out, and the ~1cm rule-of-thumb varies with frame size.
It takes a little high school trig to work this out, and the ~1cm rule-of-thumb varies with frame size.
To the original poster:
Looking at this figure, you can see that if the seat tube angle was slacker (meaning it made a smaller angle with respect to the horizontal) and you left the saddle where it is, your distance from the tip of the saddle to the handlebar would increase by some amount. The rule of thumb is 1 cm for 1 degree.
Last edited by fa63; 02-25-16 at 04:04 PM.
02-25-16, 04:13 PM
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I remember when a post like this on BF would get a visit from the Man Himself. It would be like arguing on Physics | Physics and all of a sudden Richard Feynman would appear.
02-25-16, 04:47 PM
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True, but not all that much. Just about all the calculations using different frame sizes round off to 1 cm.
02-25-16, 04:51 PM
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Good point. Maybe you instead.
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02-25-16, 04:54 PM
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I'm too late to this party. The guys who say that seat tube angle affects the effect of top tube length on reach are correct. For a certain function type bicycle the position aft of the BB should not change. When you slide the saddle fore or aft to account for a difference in STA, you are changing the distance to the bars. When you use effective top tube length to estimate reach fit, you therefore need to account for the STA too.
02-25-16, 06:00 PM
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02-25-16, 09:27 PM
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Not frame reach, but actual reach from the saddle to the bars on the finished bike. Two bikes with the same nominal effective top tube but different seat tube angles will give finished bikes with different reach from the saddle to the bars when the saddle setback from the BB is the same and all else is equal. That is beyond dispute.
