Geometry Comparison

Discuss light weight issues concerning road bikes & parts.
Hexsense
Posts: 933
Joined: Wed Dec 30, 2015 12:41 am
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.

dgasmd
Posts: 1744
Joined: Sun Apr 15, 2007 5:10 am
Location: South Florida
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???

Hexsense
Posts: 933
Joined: Wed Dec 30, 2015 12:41 am
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.

dgasmd
Posts: 1744
Joined: Sun Apr 15, 2007 5:10 am
Location: South Florida
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

Karvalo
Posts: 736
Joined: Fri Aug 10, 2018 6:40 pm
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.
Last edited by Karvalo on Mon Apr 22, 2019 10:52 pm, edited 1 time in total.

Karvalo
Posts: 736
Joined: Fri Aug 10, 2018 6:40 pm
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 A half degree difference on a 100mm stem comes out to less than 1mm of handlebar height.

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