Shimano RX100 Groupset cassette help
 

Last summer I bought an alum GT Strike with the RX100 groupset/short derailleur. The chain-rings are the SG 53/B39 and the chain is the CN-HG50 (Narrow) w/106 links. (I've replaced the 53T w/ an alum NOS and the chain). The rear cassette is a Shimano 12-25 - I somehow think it's not original as it's in nice condition still. I'm back to riding again after many years away, in my mid 60's and logged almost 5k miles so far. Although I think I still have some strength to gain it won't be all that much more so I'm thinking I'd like to change the rear cassette to one with a 28T cog. Can anyone help with learn the right cassette part # I need and tell me if it will work with my short rear derailleur? Will I have to add links to the chain? If so, how many? Or will I need to change to the long derailleur and again how many links will I need to add to the chain with this configuration?
Thanks in advance for any help.
Robbie
I failed to mention above that it's a 2 x 8 road setup w/brifters.

I am assuming your bike has a 7 speed cassette - If I recall correctly, RX100 only ever came in 7 speed.

Bikeman Dimension Cassette - 7 Speed, 12-28t, Silver
Here is one example.

Basically, any 7 speed Shimano or Sram compatible cassette will fit on your wheel.

Since cassettes and chains wear together, get a new chain when you get the cassette. Chains come a few links longer than you will need and you have to take a couple links out when installing. Generally you want to size the chain so that it can go around the big cog in the rear, the big chainring in the front, and through the derailleur, + one link. There are other methods for sizing a chain and these are easy to find.

Short cage derailleurs were generally listed as compatible with up to a 27 tooth cog, and in my experience this is not a hard limit, so a 28 should work without issue.

I've got a bike with almost exactly this setup. I think mine is 53-42 and 13-26. Maybe it could do 28t but there's not much wrap, it also only does 28 links. You also can't go 8+ speeds with the 7-speed version. It won't swing over the whole width. I recommend a different RD.

Bike Forums - View Single Post - RX100 Rear Derailleur

VeloBase.com - Component: Shimano RD-A550, RX100

That derailleur will easily handle a 28 large cog with a 53/39 chainring combo. Resize a new chain and enjoy.

I've got an RX100 rear derailleur on a bike on the stand, and with friction dt shifters, it happily spans an 8-speed cassette.

<edit> I'm pretty sure I took this rd off a 7-speed bike, but sure it could be an 8-speed rd.

I don't know the whole story, but while I consider RX100 a 7-speed group, I think the RD geometry got updated to span 8-speed by the end.* I've got one of these A551 RDs on my bike: VeloBase.com - Component: Shimano RD-A551-GS, RX100

I'll echo the advice above -- get a 12-28 cassette (discontinued 13-28 can still be found here and there, if you don't need a screaming 53/12 top gear), brand-new chain, and you should be in business. :thumb:


* If I had to speculate, it would be that since RX100 was basically 105 with a different finish, perhaps the RD had no choice but to become 8-speed when the 105SC group was updated to 8-speed. :)

Mine is on a 7 speed and definitely can't swing any further in. It could actually use a little more inward to do the overshift, it's not very happy staying in 1st. Check out how it’s trying to upshift. Not a limit screw or shifter issue!


https://cimg5.ibsrv.net/gimg/bikefor...9b27539cc.jpeg
https://cimg7.ibsrv.net/gimg/bikefor...baa075122.jpeg
https://cimg9.ibsrv.net/gimg/bikefor...c0984b59f.jpeg

Here's mine.

I kept the chain in the front ring, it'd certainly have a better chainline if it weren't so crossed up - but happily, even like this, it doesn't rub on anything and seems to turn smoothly (at least on the stand, we'll see how it is on the road once I finish the build).


https://cimg4.ibsrv.net/gimg/bikefor...cedbcb568b.jpg
https://cimg6.ibsrv.net/gimg/bikefor...faacc48aed.jpg
https://cimg7.ibsrv.net/gimg/bikefor...b59786fb05.jpg

Thanks for all the replies everyone. I should have mentioned I have a road bike with a 2 x 8 setup. I can understand how one would connect GT to mtn bikes.
I think I'm going to search for a cassette with a 28 cog then since it seems it will work with my short derailleur. From looking at Shelton Browns info it seems perhaps the one I want is a CS-HG50-8, an 11 - 28, 8 speed cassette.

IF you find the 11T cog is higher than you need, you can use the 5 smallest cogs from your current cassette with the 3 largest from the newest cassette.
That would give a little more even steps between the higher gears.
3rd gear on both is 21T.

You should also look up- Chain Wrap Capacity.
You'd be increasing yours from 27 to 31.
There's more to RDER's than just the largest cog size.

Ah, chain wrap, something I've never heard of that apparently further complicates my DIY desire. Thank you for the pointers Sir, it's back to school for another lesson it seems. Smoother steps just might be nicer.

I think I will swap the 11t on the new one out for the 12t of the old one. Otherwise they are the same up to 21t then the old is 23, 25 while the new is 24, 28. So nothing to soften that last step.

Well, I fussed with mine again and it seems to be doing ok in first now. I'm not sure what the hitch was.

12 miles at lunch :)

RSX was the 7sp group, when RX100 STI levers were introduced it was a full 8sp groupset, can't speak to rx100 downtube levers as that's older then my involvement in fixing bikes.
The rear derailleur wasn't really designed for it but I'm running a 28t with the same generation shimano 600 and it doesn't mind that much, just make sure you get the chain right, that 53/28 gets dangerous if you don't change the chain at the same time and size it to handle the largest combination. If you want a better range you can change the rear der to an acera or alivio 8 or 9 speed version and run a 12-32. Sram makes a pg830 with an 11-26 and shimano makes an hg50 with an 11-28.

I show-
12-13-15-17-19-21-23-25
and
11-12-14-16-18-21-24-28
You'd probably want to replace the 2 smallest.

Both the HG-50 & 51 I found have these same cogs: 11, 13, 15, 17, 19, 21, 24, 28.