Spacing difference between middle and outer chainrings/ BB width change to achieve outer in middle position?

[TreaderSteve]

Current project is a 1x8 pathcrusher (a 1989 DB Ascent). I had a 1x7 BITD and must have guessed BB width to bring the outer chainring more into the middle position.

The OE cranks are M450 Exage Mountain - came on a crusty 118mm BB which Sheldon Brown's list says is correct.
If I want to bring the RH crank inwards so the outer chainring is more in the middle position, what width BB should do this do you reckon? There seems to be lots of clearance for bringing the arm and ring in before any problems - the heel of the RH arm stuck out more than the LH arm anyway.

I'd measure myself but I've already taken the crankset apart - wally. I only wanna have to buy one new BB for it, hence probing you lot! Thanks

 

[FluffyChicken]

Need to move 7mm in I think (some vague memory ring to ring is 7mm? Anyone?)
but 5mm or 7.5mm is easier.

107/113/118 are all the 2.5mm chainline jumps (keeping each side even).

But I do need coffee so can double check later.
As in, do you have a 107 to test?
Or a 110 but the NDS will stay out a bit further.

 

[FluffyChicken]

The only thing I'll mention is the M450 was late 80s, and chainline was different back then and the length of cranks worked out a but different
For instance it would have been a 117.5 nut type axle and chainline aiming at 46.0mm as the cassette alignment was different to the centre of a 7 and then 8/9 speed setup.

 

[TreaderSteve]

New ally chainring is in the post (everything was steel on the Ascent - quite a lump!), but I do have a 107 BB in the shed I can try. I had thought that would have been too much less width. My only other BBs are a couple of 118s and a 122 otherwise I'd have given some test-fittings a go

 

[FluffyChicken]

107 is your shortest, so give it a go.
Shorter is in to road lengths.

It should be a 5mm chainline shift. (and arm shift on both sides)
Even better if it can be tweaked like a Hope or similar for chainline.

 

[TreaderSteve]

Yes, the BB was a nutted type just under 118mm. I'll give the 107 a go when I get the frame and new back wheel back from the LBS. Failing that it's likely to be a 113 going by the 107/113/118 spacings
Cheers!

 

[FluffyChicken]

Here you can go mad. (using an old book from back in the day)
What the spec wanted is D-3A (well actually 3A 'nut type marking') for the original crank and setup.
A move to the modern 118 (XL118 shimano cartridge) places the SER (that frame shell edge to end of axle) is negligible.

With an MM107 it moves the SER to 19, so should shift your ring in 6mm, which is about what you need.







(for anyone else reading or using these) It should be noted, some years back a few people on here measured the MM107/MM110 when fitted and found the SER stayed the same, just the drive side shifted. That could be just Shimano (newer) doing it differently or other manufacturers may be doing it this way, or vice versa. I cannot remember.)

 

[TreaderSteve]

Great, I actually understand that, thanks! I'll update when the bike's back, ring is on the arm and have tried the 107. Be great if it's a good fit as it will save me £15.

 

[benjabbi]

Apologies if I've misread something and got the wrong end of the stick, if you're going 1x8 why don't you just put the single chainring in the middle ring position and use the correct size bb?

 

[TreaderSteve]

A fair question Ben. 'cos it's ugly! Simple as that!
There's loads of clearance to bring the cranks in a bit, as I mentioned, the RH side can come in a bit anyway to match the LH arm. Turns out I need to buy a BB anyway as the 107 I thought I had in the shed turns out to be a 103mm.
I reckon the original 118 hadn't been lubed since it went in the thing - neither had the rear wheel - bearings shot - wobbling side to side, hence a new wheel build for the poor old thing. A £50 bargain buy is racking up the ££