Shimano BCD weirdness - advice please!

[finbar]

I've just built up an Orange Evo 2 commuter for my girlfriend (she used to live in Halifax and her favourite colour is purple - aren't I sweet ), but the chainline is horrendous and I need Retrobike's advice please.

I've put a Shimano LX-M570 chainset on, running single ring. I have a 104BCD chainring (obviously), but I can only fit it in the outer ring position. The middle ring tabs definitely appear to have a slightly smaller BCD - hopefully you can see this in my photo below. There's simply no way the ring will fit in the middle position.

What's going on? I thought all Shimano chainsets of this era were 104/64 BCD, and Google is revealing nothing... any ideas?

 

[xerxes]

Re:

I had a similar issue with a 5 arm 110bcd chainset/chainring once, the shoulder of the spider was too big for the ring to fit over. It was only by a couple of millimetres and it was a cheap chainset and so that future, standard 5 hole chainrings would fit, I filed down the shoulders on the spider arms, rather than filing a couple of mm off the chainring.

Shimano have done some odd and non standard sized stuff over the years. I had an STX chainset, it had 5 hole 104bcd chainrings and the granny ring was attached to the middle ring, so 20 years later they are now rare and expensive. Rather than pay silly money for a new chainring, I replace the whole thing with a standard 5 arm 110bcd chainset, for which chainrings are cheap and easy to find.

 

[ishaw]

Re:

I have a Middleburn ring that will only fit on the outer spider position, the tabs on almost every chain set I've tried prevent me running it on the middle. This also includes middleburn spiders.

My theory is, the ring was designed for a 2x10 where whilst the bcd is the same, the way it fits is different.

Annoying for sure.

 

[fearfactoryüberalles]

This problem affects all square taper, fairly modern chainsets, as FC-M460/480/510/570 ones and the IMHO the main reason the big S did this because Hollowtech ones fitted with all alu rings and bolts would weigh consideably lighter than high end XT & XT stuff of the same era. :evil:

Hint: steel Shimano 32t 4x104 BCD rings would fit perfectly, they were destined there originally anyway.

I've built several ones which weighs 605 gr. or so using alu only components (bolts&rings), and using a 150 gr cheap titanium 68x107 BB with them, I think the overall weight is pretty good/competent given the weight/cost ratio, let alone you don't need to use Octalink/ISIS or any newfangled cynical marketing hoaxes, brrrrr.

I found only the TRUVATIV 32t alu black rings fit OK on these cranks, even Shimano alu ones would fail here, because the shoulder on the crank spider is too high. You can carefully file from the height of the shoulder on the spider but the sufficient amount of alu to be removed is very tiny so be very aware!

 

[finbar]

Thanks all - I knew Retrobike would have the answer .

My next idea was to buy a normal steel Shimano middle ring and see if that fits - good to know it will. It hadn't occurred to me that Shimano might have done this because these old square taper setups could be lighter than their successors...

 

[hamster]

The simplest solution is to measure the chainline, decide what you want and then buy a shorter square taper BB. I run a 103mm BB with my Middleburn RS7 for example.

Clearly this isn't possible with hollowtech or later chainsets.

 

[finbar]

Not a bad shout, thanks - though a steel Deore middle ring is cheaper than a new BB...

 

[hamster]

You can get UN26 BBs for under a tenner!

 

[ishaw]

You can get UN26 BBs for under a tenner!

Which can weigh more than most of a bike. If you do go down the BB route, get a nice one. It does a lot of work and can be a real weight saver.

 

[hamster]

Agreed, UN55 at £15 if you look. It's the same hollow axle as UN72 of old. The Token one is even lighter if a little more pricey.
And the Deore chainring is steel IIRC, so another boat anchor component...