I am trying to replace a rusted crankset for a friend's hybrid and I was wondering if I get the same ratio set would I have to change the mechanism as that's fine.
Some photos would help, and an indication of the bike's age. A good modern bike is unlikely to rust so I assume this bike is 1980s or older, or its possibly a BSO made from cheap steel parts.– Criggie ♦Apr 18, 2016 at 22:49
1No it's about 5 or so years old bit it looks as if it had a replacement crankset before but a cheep one so I said I would replace it for them but not sure if it would be worth it if it would need a whole new mechanism also– Paul naylorApr 18, 2016 at 23:02
Can't get pics as they are at work all week and I won't get down again till the weekend– Paul naylorApr 18, 2016 at 23:04
Another thought - is this change for asthetics only? You might get away with a scratch then coat of rust converter, sand and paint. I've had good results using phosphoric-based rust converter, then a coat of car exhaust/header paint, cook it with a hot air gun to set the paint, and a coat or two of semigloss enamel spray paint. A day's drying time between coats and a light sand produces good results.– Criggie ♦Apr 18, 2016 at 23:07
Assumptions: the bike is a geared bike with a rear shifting mechanism that works okay. You're looking to replace the chainring(s) on the front because they're rusted.
If you keep the same number of teeth, then everything else will continue.
However if the chainring is worn, there's a good chance you'll need a new chain and new rear cassette as well. Old and new parts don't play nicely on bikes, because they bed-into each other.
Also, if the cranks have rusted you might find the bottom bracket is in similar condition. If you want it going and don't care about originality, just replace the whole bottom bracket with a cartridge. So much nicer and easier, and often more water-resistant too.
The thing that will decide whether you want the same or slightly different sized chain rings is the front derailleur mechanism, in how far it can move and what differences it can cope with. If you intend riding the bike on hills, a smaller inner ring might be a good change. Likewise, if you top-out then a bigger big ring might help.
The rear mech doesn't care so much about the front, it only cares about chainline.
Yeah the chainring is fine apart from being rusty the teeth are not worn I was going to replace the bottom bracket as it looks a little rusty and also a new chain but the rear mechanism and sprocket looks in good condition. So from what I gather I should get away with keeping the front mechanism? Apr 18, 2016 at 23:12
Agreed - if its not worn out then its worth cleaning up and reusing. Take the chainrings off, clean the oil off with petrol, clean them with a brass scratching brush, and paint with phosphoric rust converter. Once dried, sand lightly and paint with several layers of good paint, in black or grey or silver. Then reassemble.– Criggie ♦Apr 19, 2016 at 6:46