BB mounted f/m, what length bb axle?

Mike Muz

Mike Muz

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Happy Bank Holiday, folks!


Working tomorrow, so naturally I'm taking in some old tat , so I can clean and polish it! :LOL:

Something I've not come across before. A bottom bracket mounted front mech. Would it affect the size of bb axle I'd need?
Or even shell size, 73 or 68? ( frame not arrived yet, I've just won bidding on it!) :oops: :oops: :facepalm:

Or is this just a dumbass question?

Thanks for any help folks.

I'll be on the lookout for an XT mech for this build! Around M737/8/9 type. Anyone got a spare, gathering dust?

Mike
 
Re:

Got an BB mounted front mech on my P7 (M739 coincidentally)

The BB axle length and shell size remains the same as it would have been if you were using a 'normal' BB - you just need a specific E-type BB, not that easy to find for square tapers but they are out there..

 
Re:

Cheers mate ;)

E type? I suppose with an altered cup to allow for the mech


Mike
 
Re:

I just used a bog stock standard BB of the correct length to suit the cranks when I built the C16, and that was ridden constantly for several years until Kt went all modern.
 
Re:

From what I've found so far, ie sod all, that's great news Neil! ;)

Mike
 
Re:

The bottom bracket act like a 2.5mm spacer, hence the e-type mechs are design to allow that 2.5mm
If you use a normal one then you chain line is 2.5mm further out and the other side in.
So at the least buy the innetw been axle length.
I.e. a 110 and not the 107
Will give you 50mm chain line and the longer none drive side of a 110 shiukd keep the otherside out enough.
That is if you cannot find the e-type and need to bodge it a bit.

Or use modern bottom brackets that cater for all sizes.
 
Re:

It's for a 96 frame, really want to keep the bb square taper.

I understand what you mean about the nds crank position, but wouldn't a longer bb leave the ds crank even further out? Making the chainline worse?

Oh I see, yes. 2.5mm worse! :facepalm: My mishtake! 50mm instead of 47.5mm :oops:

Mike
 
Re:

If you aim for 47.5 lenght then you get 50mm, if you get the longer length 113 instead of 107 for example then you'll get 52.5
All 'ish' as these things are with square taper.

And of course the none drive side is pulled towards the frame (hence trying the slightly longer 110 which is linger on none drive side only normally, compared to 107).

your Lengths may vary ;-)
 
Re:

Thanks again FC!

Wasn't aware that the extra length was nds for clearance

Makes things easier now. Just have to wait for frame to arrive! :facepalm:

It's going to be very nice! :cool:

Mike
 
Re:

Of course I forgot to mention you should be able to just use a 73mm shell version and use the e type mount as the drive side spacer (as you would if fitting a 73mm shell in a 68mm frame, e.g. like them modern fancy things).
I think you are best of with metal cups for this sort of thing ?

I will say I have never used an e-type mech myself though ;-)
 
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