Cassette change query

gidysport

Regular
I've a Merida Race Lite road bike that has only covered 650 miles and I'd like to change the rear cassette from a 12-25 to an 11-34. My question is will I need a new (longer) chain and/or rear mech? It is currently fitted with Shimano Sora rear mech and KMC Z7 8s chain.
 
You will certainly need to a longer chain, but you may also need to look at your rear derailleur as well. It may not be able to handle a 34 tooth sprocket. Bikes with 12-25 cassettes are often restricted to max 28 tooth by the rear mech.
 
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gidysport

gidysport

Regular
the front is 50-34. Would it be cheaper to go triple on the front rather than change cassette, chain and rear dérailleur?
 

Broadside

Guru
Location
Fleet, Hants
the front is 50-34. Would it be cheaper to go triple on the front rather than change cassette, chain and rear dérailleur?
I would leave the front as is; a triple will only go down to 30 teeth, giving you approx 12% lower gear, but you will need to buy a triple crankset, new shifters and possibly a new bottom bracket; lots of faffing about and expense.

Going to a 28 tooth cassette will give you approx 11% lower gear for the cost of a new cassette and chain. Going for a 32 tooth cassette will give you a 22% lower gear for the additional cost of £15 on a new rear mech. This is what I would do if you want the max benefit.
 

the snail

Guru
Location
Chippenham
Unless you're struggling really badly, I wouldn't go for 34T cassette, apart from needing a new RD, it gives too large spacing between the gears. If you can get by without the super low gear, I'd go for 11-28 or 11-30.
 

jack smith

Veteran
Location
Durham
Shouldnt you always have the chain as long as possible anyway so youll need to remove a couple of links making it shorter to get it to work with an 11t?
 
Shouldnt you always have the chain as long as possible anyway so youll need to remove a couple of links making it shorter to get it to work with an 11t?
That's what the derailleur spring is for. Chain tension.

Chain length is guided by biggest front chainring, biggest sprocket plus 2 extra, so put the chain over the 2 largest rings without the derailleur involved, pull the ends together, include 2 extract lengths and that is how long your chain should be. Hence why when you increase the size of the largest chainring or largest sprocket you add chain length.
 

jack smith

Veteran
Location
Durham
Ahh i think i do it the opposite way so i always have the chain as long as possible just so it dosent drag on the rear mech, i put it in the two smallest cogs and pull tight until the chain is just under enough tension not to flop about, then thr chain is as long as possible for the larger cassettes
 

shadow master

Well-Known Member
If you go 34t you'll need a chain and an MTB rear mech,as your only running 8 speed won't cost much,definetly don't go triple its expensive,ie levers,front mech etc.. and the cheap triple road stuff is awful.
 

andrew_s

Guru
Location
Gloucester
Ahh i think i do it the opposite way so i always have the chain as long as possible just so it dosent drag on the rear mech, i put it in the two smallest cogs and pull tight until the chain is just under enough tension not to flop about, then thr chain is as long as possible for the larger cassettes
The trouble with that approach is that you may end up with a chain that's too short to change onto big/big. If you ride in the dark with brake/shift levers, it's not impossible that you may lose track of what gear you are in and change into big/big by mistake, which could end up breaking the bike.
 

jack smith

Veteran
Location
Durham
I can ride in the 32 50 okay with the sram wifli rear mech, but shimano may he different but the issue with running any longer is it will scrape if your on small small which is usually more common than big big
 

shadow master

Well-Known Member
I can ride in the 32 50 okay with the sram wifli rear mech, but shimano may he different but the issue with running any longer is it will scrape if your on small small which is usually more common than big big
I've been fitting chains all day for 20 years on the small small ratio so the chain just clears the relaxed derailleur,never been a problem yet
 
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