Front mech problem with new chainset

dodgyknees

I've upgraded the bottom bracket on my wife's cheap, and increasingly nasty, 18 speed mountain bike from the cup and axle variety to a cartridge one. The original set up was making an irregular clanking noise suggesting that the crank on the chainset was slipping.

Anyway, a simple upgrade seemed the solution so I have now fitted a Shimano LX UN54 sqr tapr cartridge and Shimano M410 Alvio Chainset 7/8 speed. The problem I now have is that the front mech doesn't seem to have enough movement in it to allow the chain to move onto the largest chain ring.

Any ideas? I always find adjusting the gears a bit of a nightmare, particularly on this bike.

nicklouse

different length axel? get a bb with the same length axel.

dodgyknees

I removed the old axle and measured it before I bought the replacement cartridge so thought I had got the correct length.

The old axle has a threaded section that extends out from the end of the tapered part of the axle. As such the cranks were connected with a nut rather than a bolt. The 'new' Shimano cartridge has an internal threading inside the tapered section so the cranks secure with a bolt.

The new Shimano cartridge I got is 68-122mm and if anything the old axle is longer at approx 124mm. The way I measured the old axle was from the end of the tapers at each end - ie. not including the threaded section. Is there some other way I should have measured this?

Also, would changing the front mech make any difference or is it all really about the length of the axle?

nicklouse

sounds like you did correctly. maybe more force is need to tighten the allen bolt.

But i would sugest bike shop time.

Anonymous

It could be that the rings are inset by different amounts. Check if the hole for the square part was flush or offset from the smallest ring. if one is offset and the other flush, this could well be why.

Drfabulous0

Have you tried simply adjusting the stop screw on the derailleur? Loosenning the one marked H will allow the mech to move further to the right. You may also need to tighten the cable and if the new cranks have different sized chainrings you will need to reposition the mech.