Changing Crankset - compatibility issues

Paddigol

Would appreciate any feedback on this if anyone has the time...

I bought a 2nd hand Trek for pops a couple of months ago from LBS. Was in great nick, fully serviced and for a good price. It was an 8 speed so I thought it would suit him nicely.

However, I didn't check the gearing up front at the time and it transpires that it's a 53-39. Which explains why the old man is struggling up any hills!

So I want to replace it with a compact to make life easier. The LBS have a new one in stock, incl new BB, for just over €100. That's 25% of the value of the bike so I was hoping to find something 2nd hand for half that price.

My problem is compatibility - what are the critical issues I need to be looking out for if I'm essentially replacing the cranks and chainrings only and keeping the current BB?

WillyFXP

Paddigol wrote: »

Would appreciate any feedback on this if anyone has the time...

I bought a 2nd hand Trek for pops a couple of months ago from LBS. Was in great nick, fully serviced and for a good price. It was an 8 speed so I thought it would suit him nicely.

However, I didn't check the gearing up front at the time and it transpires that it's a 53-39. Which explains why the old man is struggling up any hills!

So I want to replace it with a compact to make life easier. The LBS have a new one in stock, incl new BB, for just over €100. That's 25% of the value of the bike so I was hoping to find something 2nd hand for half that price.

My problem is compatibility - what are the critical issues I need to be looking out for if I'm essentially replacing the cranks and chainrings only and keeping the current BB?

Do you know what type the current crankset or BB is? Is it a triple or double chainset? Edit: oops just noticed it's a double!

macnab

What is the current crankset, ie make and model.

Paddigol

I'll have to check this - I know it's a Bontrager crankset, not sure of the model or the current BB (it's a Trek 1000 SL if that helps). Thanks for the replies though!

Will check later when I get home.

WillyFXP

http://www.chainreactioncycles.com/ie/en/truvativ-elita-2-1-compact-9sp-double-chainset/rp-prod105889
This should work fine with an 8 speed setup. From what I can find the Trek will have a 68mm BSA threaded BB, this is included with that crank.

mirv

If there's much chain noise from the narrower crankset, then buy him a new 9/10spd chain and a cheap 8spd cassette while you're at it. It'll be worth it in the long run, as the current chain and cassette is probably quite a bit worn, and it's only 23e extra on top of the 37e crank. You will however need to buy the appropriate bottom bracket tools for removing the current square taper bottom bracket, and installing the new hollowtech style bottom bracket.

60e for drivetrain parts, and 15-20e for tools isn't so bad for an entirely new drivetrain. Also, I bought that particular crankset and it comes with a bottom bracket and is 200g lighter than my previous setup!

http://www.chainreactioncycles.com/ie/en/shimano-acera-hg41-8-speed-mtb-cassette/rp-prod68154

http://www.chainreactioncycles.com/ie/en/sram-pc971-9-speed-chain/rp-prod9873

07Lapierre

mirv wrote: »

If there's much chain noise from the narrower crankset, thn buy him a new 9/10spd chain and a cheap 8spd cassette while you at it. rl]

8spd cassette and a 9/10spd chain? Will that work?

mirv

Works fine, although 9spd chain on 8spd is more compatible than 10 on 8. As long as the chain isn't so narrow so that it will fall in between the gaps on the chainring and cassette, you'll get less noise and scraping when cross chaining - like on the small ring and the smallest cog.

Lawr

I don't think it matters that it is an 8 speed. That is not the issue. The important thing is how the front dérailleur mounted. Is it on a mount that is riveted into the frame or is it mounted on a band that is wrapped around the seat tube and cinched on with a screw? If the latter, you can move the dérailleur mount down so that the dérailleur works with 50/34 or 50/36 chain rings.

You may have to replace the bottom bracket also, depending on the crank set you buy to replace the Bontrager one you have. I believe it is a squared axle that fits into the crank arms, rather than a rounded, ribbed axle end like the FSA (10 ribs) or some of the Shimano bottom bracket axles (8 ribs) (if memory serves). I think you'll be fine with the chain you have, but you will have to shorten it (remove some links). Personally, I'd change the chain and cassette, probably the cables too if they are very old. You shouldn't have to change the dérailleurs or shifters at all.

Paddigol

Thanks for the replies folks. I've ordered the crankset incl bb posted by WillyFXP above. Hopefully it'll work. If not, I haven't spent a fortune and I'm sure I'll find use for it somewhere. Seems like amazing value too, for what it is.

Thanks again. I'll let you all know when I've butchered my attempt at installing it!

browsing

I think it should work fine. I think 9speed Chainrings and cassette are the same thinkness as 8 speed. On an 8 speed cassette, the spacers in between are just thinner to fit a larger number of cogs on the back. I *think* they also make the chainrings on the front a bit closer due to the skinner chain.
Sheldon browne (as usual!) has covered compatibility, he has all the info there(and hopefully none of the above is wrong!).

That looks like a really nice crankset, I'm half thinking about replacing my current one even thought there is absolutely no need! For 40 euro it's hard to say no, damn you boards! Enjoy it, it looks pretty sweet.