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PostPosted: Thu Feb 26, 2009 10:19 am 
Pumpy's Bear
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Location: Hereford
I've always had a fairly cavalier attitude to b/b taper compatability and never experienced problems and I've used Super Record cranks happily with a non-asymmetric b/b without any ridiculous chainline/Q factor issues.

Is this just due to luck or are JIS and ISO really incompatible and I need to mend the error of my ways?


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PostPosted: Thu Feb 26, 2009 12:32 pm 
Old School Grand Master
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Joined: Wed Feb 20, 2008 12:21 pm
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Location: Lost in Translation
It can work - I ran my Superbe Pro cranks for years on a JIS bottom bracket before I discovered they were actually intended to be ISO.

In some cases it may even be better. Crank tapers enlarge with time as they're repeatedly removed and reinstalled. After many cycles, an ISO axle may bottom out in an ISO crank, and a JIS axle may be a better fit.

But in general it pays to pay attention. A JIS axle in an ISO crank concentrates more stress around the base of the taper, which can lead to cracked or rounded cranks. An ISO axle in a JIS crank may bottom out before the correct press fit is reached, letting the crank squirm on the axle and leading to a mangled interface.

Chainline doesn't have to be spot on for derailleur bikes, but it pays to know what you're doing if you're pushing the limits of derailleur capacity or chainring difference. A more practical concern is having enough clearance between the chainrings and chainstay. Symmetry isn't such a big deal as long as the chainline is in the ballpark, and neither crank is whacking the chainstay.


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PostPosted: Thu Feb 26, 2009 5:49 pm 
Old School Grand Master

Joined: Tue Oct 09, 2007 1:55 pm
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Location: New Forest, UK
I wrecked a Campag crank by fitting it onto a Dura-Ace BB. On your head be it!


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PostPosted: Mon Mar 02, 2009 5:41 pm 
Retro Guru

Joined: Sun Aug 03, 2008 12:31 am
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Location: London
Yes it matters, simply because eg a Campag crank doesn't fit on far enough on a Shimano/JIS bb axle, nothing to do with chainline.


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PostPosted: Mon Mar 02, 2009 5:54 pm 
MacModerator
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Location: Sol Kitts
Theres a good article on sheldon's site.


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PostPosted: Mon Mar 02, 2009 10:09 pm 
Retro Guru
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Ran a C record chainset on a shimano BB for 2000 miles commuting without a problem. Just doesn't go on quite so far.


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PostPosted: Mon Mar 02, 2009 11:26 pm 
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Joined: Wed Aug 16, 2006 12:36 am
Posts: 586
In engineering tearms, using the correct b/b and crank combination taper ensures maximum contact area of the two components. If you use the wrong taper then all the drive stresses are concentrated though a small area of the taper usually causing the soft alloy of the crank to deform on the harder BB spindle material.
The taper no longer holds the crank in place and the tension on the bolt is released as the crank is no longer 'wedged' in place.. crank falls off!


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PostPosted: Mon Mar 02, 2009 11:38 pm 
Pumpy's Bear
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So the concensus is don't mix and match (and I totally understand why not) but others (me included) have had no problems at all.

Hmm, not really sure what conclusions to draw....


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PostPosted: Mon Mar 02, 2009 11:59 pm 
Retro Guru

Joined: Wed Aug 16, 2006 12:36 am
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In some cases the combination you go with will work as the taper on the crank will deform as you torque the crank down and sit snugly on the BB spindle, lots of surface area in contact. The the crank morphs itself to the 'wrong' BB taper ... i.e. it work with no problems. The issues will arise when you take the crank and try to use it afterwards on the 'correct BB'.


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PostPosted: Tue Mar 03, 2009 1:04 pm 
Old School Grand Master

Joined: Tue Oct 09, 2007 1:55 pm
Posts: 8222
Location: New Forest, UK
ededwards wrote:
So the concensus is don't mix and match (and I totally understand why not) but others (me included) have had no problems at all.

Hmm, not really sure what conclusions to draw....


It's like smoking 80 a day and living to 80... nice if you can get away with it.

The chances are you won't !


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