OK, i have just picked up (like 5mins ago) a free bike, its a Malvern Star Sprint 12, girls frame...

the gearing is shimano RS

my question is can i swap the rear cassette onto my road bike (unknown brand, but has 6 speed shimano 600 rear cluster at the moment) as my current gearing is stupid silly for going uphills (42-18 is my lowest gearing) the one on the new free bike looks like a much wider range....

hoping you can help!

Droid - I assume that the '12' in the name of the girls's bike denote 12-speed, i.e. 2x6.

If both bikes use a 6-spd Shimano cassette, you should have no problem swapping the cassette onto your freehub. Some 6-spd setups use freewheels though (cassette and freehub mechanism are one unit which screws onto threaded hub body), and you can't interchange a cassette onto a freewheel-type hub or vice-versa.

If they are both of the same type, there's no harm in taking the cassettes/freewheels off and giving it a go. Unless the lowest ratio on your intended new set is massive (some Shimano cassettes go to 30-odd teeth) you should have no big derailleur compatibility issues.

See the sticky post of shortcuts to Sheldon Brown's website at the top of this forum for links to specific technical aspects of this.

If it appears that there's nowhere to install a freewheel tool (puller), you've got a Uninglide cassette and you'll need 2 chain whips. If only one of your hubs looks like this you're outta luck.

Another approach to get some easier gearing would be to swap the 42 chainring for a 39, maybe you can steal a smaller chain-ring from the MalvernStar.

what sort of difference would this give me by going from 42 to say 39?

im not good with figures so how would it relate to the old 42 tooth?

i.e. 42 - 18 would = 39 - ?

2.3333 to 2.1667 or 14:13 or about 7%.
It would be a rear cog euivalent of 19.4 teeth. Not much of a change.

Recently I had an older bike with a 6 speed freewheel Shimano 600 (Ultegra) components. The crankset was 52/39, the freewheel was 12-26. It was a great riding bike. Any good Shimano compatible (130 BCD) 39t chainring would be a good replacement for the 42. Look for a replacement for the rear cluster, something like a 12-26, 13-28, or 14-28. As the others have pointed out, just make sure it's compatible.

Al

Thanks for the replies guys,

well it looks like the only option if i want to use cheap, free parts (my bike cost me $25Aus so the cheaper the better) would be to use the smaller chain ring as the rear cluster, even if compatible has a large cog of 32 odd teeth.... thats almost a compact chain ring!!! im assuming that would bring me massive rear deraileur hassles

so if i was to switch to a 39 tooth small chain ring (which is on the free junk bike) what sort of improvement would i notice? as someone stated before it would be like running a 19.4 tooth cog instead of a 18? so that would make a 13% incline feel like a 12.5%?? lol

It kind of depends. If you can barely pedal up a hill with the 18, a 19 will feel like magic. If you need a 20, you're still "hitting the wall" with a 19.

You have me a bit confused now-
You said you have a 42-18, but it also sounds like you have bigger cogs on the back???
Exactly what do you have for largest/smallest cogs?

Sorry if there is any confusion.... my bike is running a 53 -42 up front and a 13-18 rear cluster

im not barely making it up the hill, (of which there is only 1 on my daily commute) im mashing like a mad man and sitting on about 17-18kph, but i would like it a bit easier as by the time i get to the top my legs are gone and my hearts jumping out of my chest.....

my first choice would be to change the rear cassette but im not willing to spend much money on this bike as its a bit of a mongrel thing that i got cheap, every dollar spent on it is a dolar i could be putting towards a decent roadie.. so i guess i'll take a look at swapping the smaller chain rings

If your rear is a 13-18 my guess is that it's a freewheel and you can pick up 6 speed freewheels cheap - some on BF will send you one for the price of shipping. Pull yours to make sure, then check out the free items from BF members. If it's a Shimano freewheel, it may be stamped MF-####, or Z###.

I wonder if your current 6 speed is a Sun Tour Ultra 6?
13-18 is listed as Sun Tour here- whereas I don't msee any other brands with that combo,

https://sheldonbrown.com/harris/freewheels.html#6u

trying to picture your question here?

If your rear gearing is that small. You have a Road derailleur and not Short cage.

This will only allow you to go to 26index or 28 friction. It will grind into a 28t.
You can lower the front to a 39, but won't find much of a diff.

Won't low? Get a GT or Long cage older shimano 105 allows up to a 30t

Ok a Cluster isn't the same as a cassette. Yes a 6sp cassette only works with two chain whips to take the outer one off. A Cluster is also called a Freewheel and is screwed onto a hub and uses a Freewheel tool. If you are staying with INDEX it's very important to stay the same. "BUT IN FRICTION - you can use any combo you want!"