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Re: Geometry Comparison

Posted: Mon Apr 22, 2019 7:28 pm
by Hexsense
Come to think about it, My preferred bike handling are from bike with bb drop of 72mm > 69mm > 68mm. I also tried a bike with 74mm bbdrop briefly and i like it a lot.
I see that i prefer bb to be as low to the ground as possible. So sure, there are something to look into.

Re: Geometry Comparison

Posted: Mon Apr 22, 2019 8:14 pm
by dgasmd
So assuming 2 bikes with equal reach and stack (within 2-5 mm), how do you go about finding the right stem size and angle? Can you assume they both need the very same stem dimensions?? Or does the head tube length factor in here? Assuming pedals, saddle, and bars all equal, will it get to the same exact numbers???

Re: Geometry Comparison

Posted: Mon Apr 22, 2019 8:14 pm
by Weenie

Re: Geometry Comparison

Posted: Mon Apr 22, 2019 8:37 pm
by Hexsense
Providing that stack and reach are both equal, head tube length doesn't matter. Only the headtube angle does.
I usually use http://yojimg.net/bike/web_tools/stem.php and try to plot bar to be at the same co-ordination despite different head tube angle.
I know it isn't ideal, there are better tools for this job though.

Geometry Comparison

Posted: Mon Apr 22, 2019 8:40 pm
by dgasmd
dgasmd wrote:So assuming 2 bikes with equal reach and stack (within 2-5 mm), how do you go about finding the right stem size and angle? Can you assume they both need the very same stem dimensions??
Same question stands!
Bike A has a 72 degree head tube angle and the Bike B is 71.5

Re: Geometry Comparison

Posted: Mon Apr 22, 2019 10:44 pm
by Karvalo
dgasmd wrote:
Mon Apr 22, 2019 8:14 pm
So assuming 2 bikes with equal reach and stack (within 2-5 mm), how do you go about finding the right stem size and angle? Can you assume they both need the very same stem dimensions?? Or does the head tube length factor in here? Assuming pedals, saddle, and bars all equal, will it get to the same exact numbers???
Head tube length is utterly irrelevant if you know stack.

Basically, to get precise positioning from headtube length, you need to know fork axle/crown, HTA, Fork rake, BB drop, and the trigonometry to figure out the actual vertical distance covered by the angled HT and fork. Some of these factors can vary massively in modern bikes, making HT length in isolation an extremely unreliable guide. You don't need to do any of that with stack, because stack is simply the measured vertical distance from BB to top of headtube, which is exactly what you want to know.

The one thing that makes a slight difference is HTA, however except for in very small or large frames HTA almost never varies enough to make a real difference.

Re: Geometry Comparison

Posted: Mon Apr 22, 2019 10:51 pm
by Karvalo
dgasmd wrote:
Mon Apr 22, 2019 8:40 pm
dgasmd wrote:So assuming 2 bikes with equal reach and stack (within 2-5 mm), how do you go about finding the right stem size and angle? Can you assume they both need the very same stem dimensions??
Same question stands!
Bike A has a 72 degree head tube angle and the Bike B is 71.5
Well, stems don't come in half degree increments, right? That should be a clue :wink: A half degree difference on a 100mm stem comes out to less than 1mm of handlebar height.