Effect of bottom-bracket height

[Samuel D]

My Dawes Clubman has a bottom-bracket drop of 65 mm, which gives a bottom-bracket height of around 270 mm (maybe slightly more) with 25 mm tyres.

I see many older frames have a lower bottom bracket. Why was that? Or, if you prefer, why do many frames now have a higher bottom bracket than in the past?

I can think of effects on pedal strike in corners, standover height, maybe aerodynamics and frame stiffness … but presumably those fundamentals haven’t changed much over the years.

I notice the Thorn Audax (PDF) tends to have a slightly lower bottom bracket than the steel Spa Audax. Does that matter or is it just a consequence of prioritising different aspects of the geometry that perhaps matter more?

[karlt]

Mountain bikes. Then people got use to it. It's a bit of a pain, actually, for new riders who find that when the saddle is properly adjusted they can just about tap the ground with toe-point, which is a bit unnerving.

[Mick F]

I had a thread about horizontal chainstays a while back.
viewtopic.php?f=5&t=90169

You see them all the time these days, and they aren't all on MTBs.

[Samuel D]

I had a thread about horizontal chainstays a while back.
viewtopic.php?f=5&t=90169

Read it. Thanks. But it’s still short on answers!

I think a higher bottom bracket and thus higher centre of gravity would make the bike feel more stable (the opposite of a car), much like a broom but not a pencil can be easily balanced on a finger. Similarly, more steering input would be required to initiate a turn. But that seems to contradict the trend for racing bicycles – presumably needing agility – to have high-ish bottom brackets.

Perhaps a difference of an inch or so is not practically detectable, or perhaps pedal clearance in corners is more important than greater agility to racers.

Of course lower is better for getting on and off the bike and putting a foot down at traffic lights.

[Jeff31]

High bottom brackets are useful for off road riding as you are less likely to catch pedals or even bottom brackets on obstacles and for criterium racing so you can pedal whilst racing round a corner without grounding the pedal or less likely to catch it on a curb. This is also useful for inexperienced cyclists and perhaps manufacturers produce bikes with high bottom brackets to avoid law suits from such individuals coming a cropper when pedaling off the pavement or pedaling round a bend too close to the curb. Strangely, if this is the case, those same manufacturers don't seem to worry about toe clip overlap.

I also don't understand why small frames have the same bottom bracket drop as the same frame in a large size. Surely it should be in proportion to the frame (and crank) size.

Personally I do not come close to grounding pedals in normal use, know to keep the pedals clear of obstacles and appreciate being able to put my foot down comfortably whilst still in the saddle. To get a suitably low bottom bracket for mainly road riding would mean having to go custom or use some other devious means so I have to suffer balancing precariously on my toe at junctions with the 27- 29cm bottom bracket heights that cycle manufacturers deem necessary.
I suspect that the lower bottom bracket gives a slight aero advantage and a lower centre of gravity giving better overall handling and braking. You do not want your bike to be too tall or it will handle like a penny farthing and emergency stops would entail sailing over the handlebars.

In years gone by experienced cyclists didn't appear to have issues with bottom bracket heights just over 10 inches (26cm) for road riding and although tyres may be a bit grippier allowing greater lean the advent of clipless pedals has given a bit more cornering clearance to compensate.

[Mick F]

I had a thread about horizontal chainstays a while back.
viewtopic.php?f=5&t=90169

Read it. Thanks. But it’s still short on answers!

I agree!
I had hoped it would've gone on to a long involved thread.

It still puzzles me why "normal" bikes have high BBs.
Fashion, I guess.

[531colin]

............I notice the Thorn Audax (PDF) tends to have a slightly lower bottom bracket than the steel Spa Audax. Does that matter or is it just a consequence of prioritising different aspects of the geometry that perhaps matter more?

A few mm difference in BB height doesn't make any practical difference to riding the thing.

If you are going anywhere that isn't smooth tarmac, then a high-ish BB is useful to avoid pedal strike. Off-road, I certainly feel more in control with my feet clipped into the pedals and my weight on my feet rather than on the saddle. I think there is a real, practical difference between having your body weight acting through the pedals rather than through the saddle.....I simply don't believe there is much of a difference in "stability" if the BB and saddle are 10mm higher on one bike than another.

Just to climb on one of my soapboxes for a while, I set my saddle height so there is a bit of bend left in my knee with my foot horizontal on the pedal at the bottom.....this means I can pedal smoothly through the bottom with very little weight on the saddle, which is very useful off road. When I want "maximum power" for a bit, I can slide back in the saddle, which is similar to being higher. If you follow the current fashion for setting your saddle so high that you have to point your toes to reach the pedal at the bottom, then you will have difficulty pedalling through the bottom on a rough track, and you will only be able to get the tips of your toes on the ground when you stop.

[Sweep]

Just to climb on one of my soapboxes for a while, I set my saddle height so there is a bit of bend left in my knee with my foot horizontal on the pedal at the bottom.....this means I can pedal smoothly through the bottom with very little weight on the saddle, which is very useful off road. When I want "maximum power" for a bit, I can slide back in the saddle, which is similar to being higher. .

Thanks for this colin - you have reassured me that i'm not mad, or burdened with an odd twisted body.

I went for a bike fitting (as part of buying a bike) while ago and the nice person doing it had the idea that my leg should be somewhat more bent at the bottom of the stroke. Since he knows way more about bikes than me i went with it but it never felt right. In the end i had stuff changed to my "normal" position where my leg is just slightly bent at the bottom and now all is well. Feels fine.

I also do the sliding back in the saddle thing sometimes

[531colin]

Thats interesting.....the fashion seems to me to have too high a saddle....In the videos here https://www.stevehoggbikefitting.com/bikefit/2011/05/addendum-to-seat-height-how-hard-can-it-be-2/ you can see a bloke with his seat miles too high, pointing his toes and dipping his hips just to keep his knee bent at the bottom......the result of a "professional" bike fit.

[keepontriking]

High bottom brackets - at least 11" are essential for fixed wheel bikes to avoid pedal strike.

[Mick F]

Aren't all traditional BBs 11" high?

Mine is, but most bikes these days seem to have them somewhat higher.

[james-o]

I think a higher bottom bracket and thus higher centre of gravity would make the bike feel more stable (the opposite of a car), much like a broom but not a pencil can be easily balanced on a finger.

This is the idea behind the 14" plus BB on Geoff App's Cleland bikes. An amazingly agile bike at low speeds. But for fast corners and good handling in general I like a BB that's as low as possible without getting pedal strikes too often. I think 10mm change there can make a subtle but worthwhile difference in cornering feel and control but only if you're pushing your cornering speeds a bit. Road bikes can be a fair bit lower than the 68-70mm that most race bikes use if you don't want/need to pedal through corners. The European safety standards require a 25 degree lean angle with the pedal dropped to the side of the lean so that tends to limit BB drop.

[Mick F]

25deg from the vertical?
My bike will go to approximately 45deg .................... and my BB is 11" (28cm) high which is low for these days.

[fastpedaller]

25 Deg from the horizontal like Rossi and his mates - now that WOULD be fast cornering!

[james-o]

Yes 25 deg lean angle from the vertical, as the minimum lean angle. Should've clarified the 'minimum' bit : )