Choosing a replacement cassette

[maccyb]

Hi, question from a newbie here. I recently bought a 2008 Specialized Sirrus second hand. I believe the bike is standard spec as previous owner didnt indicate otherwise.

The cassette is knackered at the back, it has a major dent across one sprocket that i cant bend back with plyers/hammer. Im looking for a replacement, the spec says its a Shimano HG-50 8speed 12-25 range. Before I bought a new one I did a quick check of how many teeth are on the smallest and largest sprocket (I believe this is what is represented by 12-25??) only to find that the largest definitely has 24 teeth. Is there something I'm missing out here in regards to the gear range? There are none in the HG-50 range that have this combination and I cannot find any visible markings on it.

Im also unsure about the compatibility between using different cassettes, can I use a different range under the same chain/deraileur etc. Also, since Im having to replace it anyway, would it be worth considering a different brand/model under the same set up. I would be willing to replace the chain but funds are pretty limited.. this is where I have no idea with compatability.


(full spec here: http://www.evanscycles.com/products/specialized/sirrus-2008-hybrid-bike-ec001400?style=86039)

Hope this wasnt too much to take in and thanks for any help!

Bruce

 

[downfader]

The cassette can be replaced with a standard Tiagra 12-25:

http://www.chainreactioncycles.com/Models.aspx?ModelID=1864

Mine lasted for a good 6000 or so miles in total. Yours might be a previous spec of the same cassette from a few years back, which might explain the difference.

 

[RecordAceFromNew]

Hi Bruce if the right shifter is 8 speed you must get an 8 speed cassette unless you change the shifter.

If the rear derailleur is by Shimano (e.g. Sora) then you must get a Shimano compatible cassette, Shimano cassettes are good and need not be expensive.

Whether you want to get a cassette with the same range depends on whether you are happy with the existing range of gears. If you get one with the largest cog that is larger than existing then you might have to get a longer chain. You can check whether your existing chain is of adequate length for different cassette size by measuring the chainstay length, the number of teeth on largest rear sprocket and large chain ring and using this calculator.

If your chain is badly worn (measure wear following this) and you change your cassette without changing the chain too then your new cassette will wear out quicker.

Hope it helps.

 

[HLaB]

I doubt you'll miss one tooth maccyb. My 2006 sirrus came with a 12-26 but it also has a 52t chainring the 2008 model has 48. You can replace yours with any shimano type 8speed cassette (that includes SRAM, which I've got fitted because its easier to get in a 12-26). Might not be the case now but when I replaced the cassette on my other bike SRAM was cheaper and IME it has been better.

 

[welshdragon]

+1 on the Sram 12-26 cassette

 

[maccyb]

Thanks for the help guys

, ill probably stick with a Shimano 12-25 theres one im waiting for on ebay. I'll probs replace my chain cos I think it looks worn and a replacement is only around £5

 

[downfader]

Thanks for the help guys

, ill probably stick with a Shimano 12-25 theres one im waiting for on ebay. I'll probs replace my chain cos I think it looks worn and a replacement is only around £5

You should always replace the chain and cassette together as the wear will affect a new component of either if you replace just one.

 

[JNR]

You should always replace the chain and cassette together as the wear will affect a new component of either if you replace just one.

I was at the LBS today and my chain is close to being replaced. Bike has done about 1800 miles and I've looked after it well. The fella at the shop said the same thing, if you replace the chain you need to replace the cassette...if I do a straight replacement with Shimano 105 stuff that is going to cost me at least £200 a year if I continue to average 100 miles a week (that is 5 days commute and a club ride, not counting training rides!).

£200 a year on chains and cassettes?! Really?

 

[RecordAceFromNew]

I was at the LBS today and my chain is close to being replaced. Bike has done about 1800 miles and I've looked after it well. The fella at the shop said the same thing, if you replace the chain you need to replace the cassette...if I do a straight replacement with Shimano 105 stuff that is going to cost me at least £200 a year if I continue to average 100 miles a week (that is 5 days commute and a club ride, not counting training rides!).

£200 a year on chains and cassettes?! Really?

As I mentioned earlier imho you should replace your cassette with the chain if the latter is badly worn. The reason being that a badly worn/elongated chain would have loaded the cassette's teeth in such a way that the whole driving force would have been taken on by a single tooth at a time (see Sheldon) and would have killed the cassette (or at least some sprockets) already. A school of thought on economising the replacement cycle, where cassettes are way more expensive than chains, is to rotate say 3 chains regularly while sticking with one cassette until they are all badly worn. Another thing is that taking care of the chain properly not only saves money on chains, but on cassettes too, and for the same reason 8 speed is more economical than 9 than 10 than 11 etc. simply because those chains are more robust inherently.

A discussion on the different and somewhat more esoteric relationship between chain and chain ring wear is here.