Changing chainrings on rx100 chainset

jamieaphex

Hi

I'm am considering building myself up a CX bike and wanted to use an 8-speed RX100 groupset that I have on my old GT Edge roadbike as it is in very good condition. However, the chainrings are 42-52 which is not exactly ideal for CX. Does anyone know what issues/limitations I would have with changing the chainrings to something a bit more hill/mud friendly? If I can't reuse my groupset, it'll end up cheaper buying a whole new bike.
Any advice would be much appreciated.

Thanks

mz__jo

Check the PCD of your chainset (the diameter of the circle for the fixing bolts). You will find lots of sites on the net which will give you the PCD from the distance between two consecutive bolts; I don't have the links to hand but someone else will undoubtedly post them. Searching the sites of Sheldon Brown, Highpath or TA will yield results and there is certainly a link on the site of SJS Cycles if you are in the section selling replacement rings.
If your chainset has a PCD of 130mm (the standard PCD for Shimano road chainsets) the smallest ring you can fit is 38T. I think the RX100 may be a compact chainset in which case you can fit a 34T (PCD of 110mm).
I don't know what rings 'crossers use these days. I am sure that Peugeot did a commercial cross bike with an RX100 chainset (although perhaps not the whole group) with 46/34 or 46/36 but that was a few years ago. Your rear mech should handle a 28T sprocket so that shouldn't pose a problem.
My cheapskate way to do this would be to use the 42 ring on the outside and a 34 on the inside. Rings are not particularly cheap; if you want to replace both of them you might want to check on the price of chainsets or even complete bikes!

mz__jo

Even more back in the past there was a time when crossers used a single 42T ring with guides bolted either side to stop the chain derailling. TA used to do them (40 years ago!) I have done touring mtb circuits with a single 38T ring and a 14-34 freewheel on a roadbike with cross tyres. Very low bb and 175 cranks were a bit inconvenient but the gearing wasn't so bad as all that.

jamieaphex

Thanks guys. It sounds like it will be relatively straightforward. I might well try using the existing 42 chainring and moving it to the outside.

An entry level cx bike is about 600-700 quid, so i have a budget to aim for. Planet X are doing a Guerciotti frame bundle for 399. Adding a new chain ring and some Avid shorty 4s, shouldn't add to much to the overall cost. A friend has some Khamsin wheels he'll sell me for not too much. I don't have the tools or expertise for all the work, but my LBS (Elmy Cycles in Ipswich) are good value. I spoke to the mechanic at the shop after posting my last message and he mentioned that they have some Alan frames new and second hand that they could do a good deal on, so I'll take a look at those too.

http://www.planet-x-bikes.co.uk/i/q/FTG ... set-bundle

As I said before, thanks for your help, it was really useful.