Rear Derailleur Swap from road to MTB

[Alembicbassman]

I bought a older road bike with a 42/52 chainset and 13-24 8 speed cassette with Sora RD-3300 mech. It's a bit hard on the hills.

I already have a 12-28 MTB cassette, but the Sora mech will only take a 27t max.

I can get an Altus 8 speed MTB mech for £10.

This seems to me to be the best/cheapest way of getting easier gears.

Chainsets run at £35-50 even a chainring is £20

 

[TheDoctor]

I'd try the cassette with the Sora mech first. It may well be OK.
Also get a 38T inner ring from Spa for a tenner. That'll drop your lowest gear too.

 

[Alembicbassman]

That's the cheapest I've seen the chainrings. 38 is the smallest I can get with 130 BCD

I'll give the 12-28 a try, may just work OK

 

[RecordAceFromNew]

If the top pulley of the Sora mech is* more than 8mm away from the largest cog of the 24T when you are on it, the 28 cassette will fit. Your chance is excellent imho. You probably need a longer chain however.

It is cheaper/simpler/slicker to change the cassette to get lower gear usually. You should keep the chain links in case 28T at the back is not enough and you wish to go to e.g. 34T, when you will most likely need a mtb long cage rear mech.

* or can be made to be, through adjust the rear mech's B screw

 

[TheDoctor]

I was suggesting the 38T ring as well, not instead.
It is theoretically possible to make a 36T inner ring for 130 BCD - I've seen one.
Well, I say ring, it was more of a pentagon really...

 

[Alembicbassman]

I've put the 12-28t cassette on and the clearance seems OK, shifts OK too. I have ordered the 38T ring from Spa Cycles which should arrive tomorrow. The ratios are now almost the same as my other road bike that has a 34/50 12-25t 175mm cranks