I’m probably going to end up bugging P20 about this tomorrow, but I thought I’d try to get an answer here first cos it’s his birthday and I think he’s getting fed up of my stupid bike-related questions… 😉 😳

I have an ancient (?1992) cheapo rigid steel MTB that I use as a commuter cos I can leave it outside the hospital without it getting nicked. However, the drivetrain has seen better days and there are now very few gears that actually work. So I need to replace lots of things.

It’s currently got a triple chainset (22,32,42), 7 speed cassette and square taper BB.

I have seen this crankset on CRC which fits my budget for this bike perfectly (and gives me a more usable range of gears for my commute). However, I have no idea which bottom bracket size I am looking for. The shell on the frame is 68mm wide, but I don’t know which spindle length will fit my bike/those cranks.

I’m looking at this Shimano BB. Can anyone help me size it correctly, or should I just buy all the sizes and send most of them back? 😉

I just bought cranks and a BB for a singlespeed I’ve built that were square taper. I just guessed the size, went for the middle size actually and lucked out.
Really though you need to measure the length of the actual BB spindle. This would be easiest with the cranks removed.

Those are 8 speed cranks, and you currently have seven. Better check that’ll work(I’ve no idea)

Personally, I’d get rid of the triple and go 9/10 speed with a single ring up front for simplicity.

If you take the bb out you may be lucky and still be able able to read the writing on it!! otherwise just measure it with the cranks off!! That chainset isnt going to last very long though!!

But what if I’m buying new cranks? Will that not alter what spindle length I need? If not, I’ll just go outside and take the bike apart now…

No, the taper is standard (I think)
The length is dependent on the frame design, not really the crank design.

Ah, ok. Off to the garage then…

Maybe a single ring up front is simpler…

larger chainrings MAY clash on the frame, which can be corrected by a longer BB spindle to push the rings outwards. That crankset wont last very long, and you cant swap out the chainrings. i guess you cant get they chainrings off your current chainset?

The current chainrings don’t come off the cranks. To be honest, I’m not that bothered if the chainset doesn’t last that long – I might buy two at that price!

Maybe a single ring up front is simpler.

Well it will be simpler but a 7 speed hub cannot take a 9/10 speed cassette so you will only have 7 gears.

+1 for that crank being pish – I have one of a supermarket bike you can have which will last just as long- probably cost about the same to post mind!

FWIW i used a 9 speed crank with 7 speed gears and chain without issue so that will not be a limiting factor

IMHO Get a crank you want – fit it. if you have issues get a bigger BB as you wont really know what size you want till you try it.

Cool, thanks for all the advice. I’ll take JY’s advice to buy the cranks and fit it to the old BB and get a new one if I have issues.

Last question – which is better quality?
1) FSA Metropolis
2) Shimano Acera
3) Shimano Alivio

I have a set of Alivio cranks running on a 110 spindle and 73mm shell width. Mine are I think 2011 or 2012 and quality is quite good. Not light thtough.

The required axle length can vary from crank type to crank type as while the taper of the hole is standard the position of the taper relative to the rings can vary. I had some older shimano cranks that needed 115 and some old xt ones at 107.

I’d just spend the extra & get another bike. 😕

It’s very tempting! The beauty of this bike is that I can leave it anywhere and no-one would even think of stealing it. I once left it unchained outside the hospital I was working in for a whole day because I forgot my lock and it survived unscathed.

Plus, it’s the first bike I ever bought (at the age of 10 or 11, with money left to me by my Great-grandmother when she died) so I love it dearly!

You need a friend to cast his/her eye over who has a fair pile of old but functional bits to throw on.
If the square BB has no wobble then leave in. You probably will find removal tough.

I’ve been running Alivio chainsets on un-26 bottom brackets off-road for years without issues. Chainrings are Deore quality, last well and shift fine too

mightily

What about this Chainset ,still pretty cheap and the rings are a lot closer to the ones you’re currently running

Thanks for all the advice folks. I’ve gone for components that match as closely as possible the stuff that’s already on the bike (including the alivio chainset). They’ve already been dispatched, so look forward to Monday’s threads entitled “help! my chainset won’t come off” and “aaargh! I hate front mechs”.

Give me a shout. I’m about this week

Are the teeth on the current chain set ground down to tiny stumps? Is there play in the bb or is the crank wobbling on the square taper? If not I would leave them and try new cables and cleaning/lubing the shifter and derailleur first

The middle ring is worn down to virtually a perfect circle! There’s no play in the BB though, so I might get away without replacing it.

Get some penetrating fluid on the bits you intend to remove now as it will give it plenty of time to do its job and hopefully make disassemble easier for you(or P20).

So this only required a vice, a big hammer, a new bottom bracket (helpfully plucked out of a spares box), 2 1/2 hours of P20’s time and a broken nail (not mine). Thanks everyone (especially jack 🙂 )

Oh, and the shifters are knackered, but that’s a job for another day…