Two fit questions for the mathematically inclined

[Climb01742]

1. How much more drop does a -17d stem give you than a -6d stem? On a 110mm or 120mm stem?

2. For every 1/2 degree of ST angle change does setback change? Let's say on a 56 ST, how much setback difference is there between a 72.5, 73 or 73.5 ST angle?

The mathematically-challenged thank you!:D

[ultraman6970]

JUst go there and figure it out :p

http://yojimg.net/bike/web_tools/stem.php

[Climb01742]

JUst go there and figure it out :p

http://yojimg.net/bike/web_tools/stem.php

Awesome. Thanks! Rather than giving me a fish, you taught me to fish.:D;):beer:

[tommyrod74]

Awesome. Thanks! Rather than giving me a fish, you taught me to fish.:D;):beer:

Well, he at least brought you to a river with free public fishing poles :)

[pdmtong]

2. For every 1/2 degree of ST angle change does setback change? Let's say on a 56 ST, how much setback difference is there between a 72.5, 73 or 73.5 ST angle? The mathematically-challenged thank you!:D

The saddle position relative to the BB doesn't change when the STA changes. what changes is where the post clamps the rails. Some math folks figured out that for a 56 a one-half degree STA change is about a 5mm difference in clamp position but I have never seen the "proof" of this other than many folks myself included propagating this belief..

[OtayBW]

Awesome. Thanks! Rather than giving me a fish, you taught me to fish.:D;):beer:No, he just brought dynamite to the fishing hole!!

[seanile]

The saddle position relative to the BB doesn't change when the STA changes. what changes is where the post clamps the rails. Some math folks figured out that for a 56 a one-half degree STA change is about a 5mm difference in clamp position but I have never seen the "proof" of this other than many folks myself included propagating this belief..

for a general rule, it can only be given in percentages of a 360 circle centered around the bb. a fine tuned measurement/specific case, it would require the rail-to-bb measurement.

[Climb01742]

No, he just brought dynamite to the fishing hole!!

True dat.:rolleyes:

The change in ST angle also impacts where the HT/fork steerer is relative to different TT lengths (and hence where the handlebars are.) I know it's imprecise but I'm just trying to get an idea of how easy or hard replicating fit coordinates on two different frames might be. Such as a 73.5 ST with a 56.5 TT vs a 72.5 ST with a 58 TT. Math never was my strong suit.

[benb]

This one is better since it doesn't require integers, which is crazy since javascript would make you work to enforce integers.

http://www.slowtwitch.com/Fit_Calculator/stem_calc.php

[Mark McM]

The saddle position relative to the BB doesn't change when the STA changes. what changes is where the post clamps the rails. Some math folks figured out that for a 56 a one-half degree STA change is about a 5mm difference in clamp position but I have never seen the "proof" of this other than many folks myself included propagating this belief..

The distance of of the seat clamp behind the BB is equal to the distance from the BB to the seat clamp times the cosine of the seat tube angle (assuming both seat tube and seat post are straight and in-line). So, if the seat clamp is 680 mm from the BB, then for these common seat tube angles, the set back of the clamp will be:

74.0 deg.: 187.4 mm

73.5 deg.: 193.1 mm

73.0 deg.: 198.8 mm

72.5 deg.: 204.5 cm

72.0 deg.: 210.1 mm


So, yes, roughly 5 mm difference in seat clamp position for a 1/2 degree change in seat tube angle.

[chiasticon]

big fan of this one if you want to compare two bikes: http://gearinches.com/blog/misc/bike-geometry-comparator

definitely overkill for what you're after, but it's a useful tool to have...

[wallymann]

http://brown-snout.com/cycling/tech/_stem-calc.html