wbeaver

Can I swap 12-30 8-speed cassette for a 12-25 8-speed cassette without swapping my derailleur? Hoping to get some lower low gears with minimal time and investment. I have a Shimano 2300 rear derailleur on a Trek 1.1 with 52/34 at the crank. I expect to have to lengthen chain, of course.

MKahrl

The easiest way to get lower gears is to swap out the crankset for a 50-30 or 48-30. The chain would be shortened a little.

A 48-12 combo for your highest gear will yield 29 mph at 90 rpm which is high enough for most people.

When you go to a cassette with a 30t large gear the derailleur must move in relation to the cassette at a different angle. Usually you go to a mountain rear derailleur for 30t and greater.

ptle

I think your derailleur should be able to handle the 12-25. Derailleurs are limited by their max tooth, which is usually 28. But since your derailleur can handle 30, then it will be fine.

52/34 gearing on the front? I thought most front derailleurs can have a max of a 16 tooth jump... which would be 50/34 or 52/36.

And yeah you would probably have to take out a few links in the chain.

banerjek

Short answer is yes if you have a long cage, but expect to adjust the B screw since you're probably exceeding the maximum cog size for your RD. If you have a short cage, forget it.

Longer answer is it's probably a good idea to get another RD.

You need to know both the maximum size cog your RD can take as well as the chain wrap ((52 + 25) - (34 + 12) = 31 in your case) teeth maximum.

If you have a long cage RD, most likely you're covered on the chain wrap, but on maximum cog size, you may have a problem as most of the stuff tops out at 28. I've seen people go to 32T with a long cage with the B screw adjusted all the way out so you can probably make it work.

wbeaver

Argh! Sorry. Front Chainrings are 50/34, not 52/34.

ptie, what makes you say my rear derailleur has max tooth of 30? Is it signified by the Shimano model number (2300)?

Does anyone know how to look up the max tooth any given rear derailleur will handle? Sounds like moving from a cassette with 25 max teeth to 28 is not much problem, either way.

Downsizing the chainrings could be in the picture, too, I suppose....

david_594

I think people are misreading this. Pretty sure the OP has a 12-25 setup currently and wants to replace it with a 12-30. Not the other way around.

banerjek

The only way to know the capacity of your RD is to get that from the company (or experimentally verify it). Before you got your compact crank, did you have a triple? If so, you'll have a long cage RD that you can probably make work with a bit of futzing. However, if you just had a standard double, you most likely have a short cage RD which I definitely would not try with the 30, but it would probably be fine with the 28.

You won't be able to downsize your rings if you want a lower gear without replacing your crank. The reason you have a 34 on right now is that's the smallest ring that will fit with 110 BCD. If all you want is super low gears on your road bike, it would be considerably cheaper to get an MTB RD (XT would work great for your application) and then you'd be able to run cassettes with up to 34T no problem.

wbeaver

david_594, is right - I have a 12-25 setup currently am considering replacing it with a 12-30.

Mfg, info found elsewhere on-line say max rear cog of 25T for my current RD. I trust the spec is conservative, so banerjek is probably right to say 28T. Thanks, banerjek for the specific RD reference - a 34T rear would be really nice. I know that most "real" roadies would never want anything that low, but I am new to road and need some help until I develop better climbing prowess. I can always keep the old cassette and RD for when I improve.

knobster

Most RD should handle a 30 tooth cog just fine. Most say 27, but 30 will work. You'll probably need a new chain though. 34T, you'd need a long cage RD.