What crank do I need ?

[BADGER.BRAD]

Hello all,

I am looking to change my well worn Chain wheel ( 44 teeth 4 hole fixing Sr Suntour PFF crank) to a two or three speed system The problem I have is two road cranks I have tried fowl the frame and as far as I can see the 4 whole chain wheel only goes upto 44 teeth, if I change the whole assembly to a MTB crank the higher ratio is still way way to low, I'm looking for 52 high 42 low. How do I know which crank will kick he chain wheels away from the frame enough yet still allow me to use my 52/42 chain rings. There seems to be no way to tell ?

Many thank all.

Brad

 

[ASC1951]

That's not a crank issue. If you are going from a single chainwheel to a double or triple, you need a different bottom bracket with a longer axle.

 

[Amanda P]

He's right. You'll almost certainly need alonger bottom bracket axle. Loose-bearing bottom brackets and cartridge ones come with axles of various different lengths.

In theory, you can look up or calculate which axle is required to fit a given chainset. In practice, especially on older frames, it's a matter of trial and error.

A starting point might be to measure how much further out the cranks or rings need to be to clear the frame. You'll need at least that much extra length on your new bottom bracket axle.

 

[BADGER.BRAD]

Thanks very much, It all makes sense now. I have looked at the price for a new 44 tooth chain wheel and this come to more than the crank and chain wheel ??? so I thought I may as wheel go for something with a little more range of gears , the present setup for 90% of the time is stuck either in top or bottom gear so doesn't do the chain a lot of good being at extreme angles. It may well be more cost affective to change the bottom bracket and use the 42/52 chain set I all ready have

Again many thanks all.

Brad

 

[ASC1951]

It may well be more cost affective to change the bottom bracket and use the 42/52 chain set I all ready have

In theory the longer axle may give you an incorrect chain line, but TBH I wouldn't fret too much about that.

You do say the chainset is worn, Brad, and these things don't last forever. One thing to watch out for is that old chain + old chainset + old block all works fine - but replace just one of them and you may get dumped on the road with skinned shins because the chain slips.

 

[guitarpete247]

You might want to read Sheldon Brown's article on chainline to help you work out the BB length you need. You might also want to read his Bottom Bracket size database. These might help you make up your mind what do look for in your setup.