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Ideal bottom bracket height if you never had to pedal?

D

Dh builder

Chimp
Sep 30, 2013
54
2
Doing some research here on a project and was hoping some of you could give me advice.

Assume 8" travel front, 10" travel rear downhill geometry bike. 36T or 32T chain ring with 165mm cranks and pedals but you never pedal. Assume gravity pulled you fast enough where you never had to pedal, even uphill.

Assuming the above, what would be the ideal static (unweighted) bottom bracket height?

I think most people like around a 14" bottom bracket height, so would you want to go lower if you never had to worry about pedal strike? If so how much lower can you go? Or possibly you would still want the 14" for ground clearance issues over rocks?
Or optimally without ever having to pedal, you would want the chainring up high enough so that when you bottom out you would not hit it?
 
Nick

Nick

My name is Nick
Sep 21, 2001
24,311
15,224
where the trails are
14" on a DH bike? My trail bike isn't that high!

12.5" would be nice. But I pedal, as does everybody else, so I'm not sure this helps you.
 
- seb

- seb

Turbo Monkey
Apr 10, 2002
2,924
1
UK
Nick said:
14" on a DH bike? My trail bike isn't that high!

12.5" would be nice. But I pedal, as does everybody else, so I'm not sure this helps you.
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No, it isn't. But it doesn't have ~9" of travel, which would put your BB axle only 3.5" from the ground on a 12.5" BB.

And if you're on uneven ground, or slightly leant over, that's pedal-dragging time, pedalling or not.
 
M

marshalolson

Turbo Monkey
May 25, 2006
1,772
523
- seb said:
No, it isn't. But it doesn't have ~9" of travel, which would put your BB axle only 3.5" from the ground on a 12.5" BB.

And if you're on uneven ground, or slightly leant over, that's pedal-dragging time, pedalling or not.
Click to expand...
9" of wheel travel =\= 9" of bb drop
 
HardtailHack

HardtailHack

used an iron once
Jan 20, 2009
6,989
6,041
I want to see frames that can run low Q factor cranks to get the pedals in a bit.

The last setup on my bike was 12.9"(8" travel) with 155mm cranks and it was fine if I knew the tracks well but if I was somewhere new it sucked.

If it were a 10" frame yeah I'd go 14" or maybe a bit higher depending on the sag you need to run.
 
4130biker

4130biker

PM me about Tantrum Cycles!
May 24, 2007
3,884
450
HAB said:
He is, but marshalolson is still right.

I had a DH bike with a 12.9" BB. That was a little too low. My current 13.4" is much better.
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Ahhh yes, I misinterpreted his response.
 
Lelandjt

Lelandjt

adorbs
Apr 4, 2008
2,556
897
Breckenridge, CO/Lahaina,HI
My V10 has 10" travel and claims a 14.7" BB. I use a MRP taco made for a 40t ring. When landing big drops I've felt it hit small rocks and roots so I'd say it's as low as possible with that taco if your landings aren't butter smooth. If you use a taco for a 36t ring you buy yourself maybe 1/4" more clearance and another 1/4" with a 32t taco (do those exist?). That still puts you over 14" BB height with 10" travel.
 
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maxyedor

maxyedor

<b>TOOL PRO</b>
Oct 20, 2005
5,496
3,141
In the bathroom, fighting a battle
In the hypothetical scenario, no pedaling means you don't have to actually have a BB do you? Why would you have cranks on a bike you don't pedal? Does that make the correct answer zero or infinity?

On a bike you have to pedal, I've found it doesn't matter all that much. anything over 12" works, you just adapt your riding style. my old Kona had like a 20" BB, that was too high, but at least my bash-guard never got bashed up.
 
Udi

Udi

RM Chief Ornithologist
Mar 14, 2005
4,917
1,211
4130biker said:
There's always this little gem:
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This thing is actually spot on.
It doesn't matter if you don't need to pedal, good tracks tend to have ruts and rocks on the fastest lines that will catch pedals even when cranks are kept level. By all means if there's a track where you can get away with lower, go for it (Barel @ Livigno anyone?), but in my experience most tracks worth riding can be ridden faster with a BB in the 13.75-14.25 region.

I'm currently at 13.9-14 and can pick quicker lines than at 13.5. Obviously sag affects this too, but that's with ~28-29%.
 
norbar

norbar

KESSLER PROBLEM. Just cause
Jun 7, 2007
11,472
1,684
Warsaw :/
Udi said:
This thing is actually spot on.
It doesn't matter if you don't need to pedal, good tracks tend to have ruts and rocks on the fastest lines that will catch pedals even when cranks are kept level. By all means if there's a track where you can get away with lower, go for it (Barel @ Livigno anyone?), but in my experience most tracks worth riding can be ridden faster with a BB in the 13.75-14.25 region.

I'm currently at 13.9-14 and can pick quicker lines than at 13.5. Obviously sag affects this too, but that's with ~28-29%.
Click to expand...
If someone only rides bike parky flowy trails that I'm all for a sub 13'' bb but then he should go for something with less travel. The rest is true. I'm at 13.7'' and if you want to ride real dh tracks you will grind your bash and pedals from time to time with that height. I wouldn't go much higher through. 14''+ bikes I tried weren't much better in the same situations.
 
V

vikingboy

Monkey
Dec 15, 2009
212
2
kidwoo said:
That's so it makes sense in europe.
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Makes sense to me (in UK) anyway....I'd have called him on it had it been a imperial double fraction though. :)

We also run different heights each side over here to optimise for our right hand biased trails, left crank at 13.25" and right at 12.75".
 
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