have bb heights increased-and how low can they go?

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papacojones
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have bb heights increased-and how low can they go?

Postby papacojones » 1 Oct 2008, 9:16pm

Has the mountain bike explosion caused the height of bottom brackets to increase across the board?
I have felt for some time that bbs are higher now than in the olde days. Recently I came across the theory that manufacturers had raised them on road and touring bikes because people got used to being able to pedal while cornering on mtbs.Fear of US litigation was cited as primary motivation.As a kid in the seventies I remember pedal grounding at pretty small angles of lean.

Any opinions. Anybody got an old bike handy to measure clearance?

I have a related theory that the mtb popularity has put kids off cycling.-Heavy cheap steel bikes-knobby tyres-having seat too low for efficient pedalling so as to be able to put a foot down(because of high bb) Any opinions.?

I am designing a small audax bike at the moment (see my other post)and am considering a lower bb than is found on modern road bikes. Anybody got any figures on this.?
Would like some input from anybody using extra long cranks if possible.May start new topic for this
Last edited by papacojones on 2 Oct 2008, 7:22am, edited 2 times in total.

glueman
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Re: have bb heights increased

Postby glueman » 1 Oct 2008, 9:20pm

papacojones wrote:I have a related theory that the mtb popularity has put kids off cycling.-

I agree with some of your other points but not this one. Cheap MTBs are heavy but mid-priced ones are the kind of thing I'd have loved as kid. Bags of gears, the ability to ride up steps and over logs - fantastic - and I say that as mainly a road cyclist.

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Mick F
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Postby Mick F » 1 Oct 2008, 9:21pm

The BB height on my Mercian is 11" (or 280mm).
The frame was built for 700c wheels in 1986 (22 years ago).

Does that help?
Mick F. Cornwall

papacojones
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Postby papacojones » 1 Oct 2008, 9:33pm

Mick F wrote:The BB height on my Mercian is 11" (or 280mm).
The frame was built for 700c wheels in 1986 (22 years ago).

Does that help?

Could you reply a little more quickly next time :D .This seem about standard for this era. What I am trying to decide is if I can get away with much less than this on my small bike.This would be useful for a little person as they have smaller feet and hence find i difficult to put a toe down when saddle is high enough for efficient pedalling. Thanks for reply.

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cranky
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Postby cranky » 1 Oct 2008, 9:40pm

My LHT (2008) is 280mm with 40c tyres, my Ridgeback (2003) is 265mm with 23c tyres.
Iain

Ridgeback Genesis Day 2
Surly Long Haul Trucker

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Phil_Lee
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Postby Phil_Lee » 1 Oct 2008, 9:53pm

I measured 11" (280mm) on my '87 Raleigh Royal (28mm tyres).

Bear in mind that you can reduce the BB height by the same amount as you reduce the crank length, without any effect on cornering clearance at all.

papacojones
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Postby papacojones » 1 Oct 2008, 10:29pm

Phil_Lee wrote:I measured 11" (280mm) on my '87 Raleigh Royal (28mm tyres).

Bear in mind that you can reduce the BB height by the same amount as you reduce the crank length, without any effect on cornering clearance at all.

I have borne this in mind. I have worked on the assumption that full size road bikes are designed with up to 175mm cranks in mind(largest size made by major manufacturers) My bike will use maximum 150mm cranks. I think I should be safe at 250mm bb height going on this. However the aforementined designers also have to assume standard pedals with toeclips and straps and ,say, size 11 feet. My rider has size 3 feet,very narrow,and will also use clipless pedals at all times. I have also read that bbs were kept high on larger frames for stiffness of frame. Some taller riders are now using cranks up to and over 200mm using the theory that cranks shoulld be about 21% of inseam. (Campy are rumoured to have secretly made 195mm cranksets for Indurain. They kept it quiet as it does not suit them to have to make a larger range of cranks commercially) Taking all this into account I think I could drop my bb another 15-20mm to allow Mrs Papa to more comfortably get a toe down.Any opinions??

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cranky
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Postby cranky » 1 Oct 2008, 10:51pm

150mm cranks and 250mm BB height gives you 5mm more pedal clearance than my Ridgeback. I don't think chainring clearance when dropping off a kerb would be an issue either but it would be best to check it on the drawing board before committing it to steel.
Iain



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papacojones
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Postby papacojones » 1 Oct 2008, 11:15pm

cranky wrote:150mm cranks and 250mm BB height gives you 5mm more pedal clearance than my Ridgeback. I don't think chainring clearance when dropping off a kerb would be an issue either but it would be best to check it on the drawing board before committing it to steel.


Iknew this height should be perfectly safe with 42/32/22 triple. Was toying with the idea of droppin an inch. Will be doing some experiments with actual bikes next weekend= raising ground under pedals and checking angles of lean with various heights and pedal widths etc.Thanks for everybodys input. Drawing of my bike is herehttp://www.box.net/shared/u6fvka5c4s%20if%20anyone%20cares%20to%20look/comment.%20Cheers%20Papa

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squeaker
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Postby squeaker » 1 Oct 2008, 11:43pm

My 70's Carlton Corsa on 32-630 tyres => 265mm
Wifey's 80's Rayleigh Royal on similar wheels => 280mm

So you might well have a point, which also explains the childish delight I have in pedaling around corners at large lean angles on my SWB recumbent - if that grounds a pedal then you definitely are in trouble :roll:
"42"

aesmith
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Postby aesmith » 2 Oct 2008, 8:50am

The height will of course depend on the wheel and tyre size. Some makers publish "BB drop" which is the vertical distance that the bottom bracket is below the wheel spindles.

The published figure my Thorn is 72mm, which I thought came out the same as my old Dawes. Current Dawes road and Audax models have the BB higher with a drop of 65mm (published figures)

Coincidently I scraped a foot the other day, which I can't remember ever doing before. Different pedals of course, and 700C instead of 27" wheels.

Tony S

glueman
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Postby glueman » 2 Oct 2008, 9:12am

On a 70s road bike with 27" wheels and a freewheel block I would regularly ground a pedal, especially on roundabouts. On a 90s equivalent with a fixed (converted from gears, not a fixed special) I can't recall hitting a pedal, so yes, MTB BB heights probably have had an influence on road design.
It seemed to be easier to stay in the saddle and get a toe down at lights back then but that could be just anno domini.

papacojones
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Postby papacojones » 3 Oct 2008, 1:25pm

I think I have a touch of the anno domini myself. :)