Due to having a 130/130 BCD SRM I can not change to compact cranks without buying another SRM. I have heard (but not seen) of people that put a mountain bike rear derailleur on the TT &/or road bike to get the same gear ration that a compact crank would provide. I am looking for any suggestions/tips on making this conversion, specifically: what shimano RD model work work best, do I need a new chain (or at least add links) and anything else that would be helpful (pics would be great too).
a 50/34 w/ 11-23 & a 53/39 w/ a 12-27 are pretty close & you don’t need a long cage to move on a 12-27.
What exactly kind of cassette were you planning going to ?
Don’t laugh, but I actually have 11-28T now but, like to climb at a higher cadence (and staying below FTP) than I can with my current set-up. Of course, raising my FTP is in the works as well but, would like to have the wider range of gears, especially for long course races.
sram makes long cage in mtb, short/mid in road, but only on rival or apex.
however, have a look at apex. it sounds like it might be perfect for what you are looking to accomplish.
gonna be a big difference in rpms as you shift, but you’ll get the range you want.
if you play around with srams site, they should tell you the max chain the rd can takeup, and that will tell you if you need long or short for the gear combo.
What rear der are you currently running? Shimano makes lots of road long cage ders. And any MTB der will work, just a matter of matching it to the quality of your road der. (eg XTR=Dura Ace)
You’ll have to add some links to the chain. It depends on what size cassette you end up with.
It’s a pretty simple conversion. New cassette, correct chain length and then adjust the der.
I’ve seen the short cage RDs work for up to 28 teeth (12-27 or 13-28). For the long cage, any 10spd shimano long cage rear will work (RDs don’t have special teeth profiles or anything else that would make one shift better). If you get a low end RD (ex. 105) swap out the pulleys for some with bearings instead of bushings.
The long cage will work fine; as noted you will need a longer chain. The shifts won’t be quite as fast/crisp and there are minor aero and weight penalties, but the big thing is (drum roll) the dork factor amongst us image conscious racers. Long cage is not bike porn.
sram makes long cage in mtb, short/mid in road, but only on rival or apex.
however, have a look at apex. it sounds like it might be perfect for what you are looking to accomplish.
gonna be a big difference in rpms as you shift, but you’ll get the range you want.
if you play around with srams site, they should tell you the max chain the rd can takeup, and that will tell you if you need long or short for the gear combo.
Sram XX RD is compatible with all Sram road shifters too. I have put several people on the 11-36 XX cassette using that deraileur with force shifters with no problem.
You didn’t specify if you are on 9 or 10 speed, Shimano or Campy. (Edit - you did say Shimano…)
Shimano 9 speed road and mountain rear dérailleurs are compatible; same cable pull per gear. This IS NOT true for 10 speed.
So if you are on 9 speed Shimano, and it sounds like you want to go larger than 28 teeth on the cassette, you need a 9 speed mountain RD. Note that 9 speed mountain RDs come in both “normal” spring (RD returns to the largest cassette cog) and the newer version where the RD naturally returns to the smallest cassette cog. You need the former type.
I run this setup on two Shimano 9 speed CX bikes, that I use for commuting and riding in the mountains on dirt trails. Works great. I use an 11-34 cassette on each.
You will likely need to buy a new chain, as your current chain will likely not be long enough.
If you are on Shimano 10 speed, you are out of luck. The only option I am aware of, as mentioned earlier in this thread, would be to change over the shifters, rear dérailleur, and cassette to the SRAM Apex.
sram makes long cage in mtb, short/mid in road, but only on rival or apex.
however, have a look at apex. it sounds like it might be perfect for what you are looking to accomplish.
gonna be a big difference in rpms as you shift, but you’ll get the range you want.
if you play around with srams site, they should tell you the max chain the rd can takeup, and that will tell you if you need long or short for the gear combo.
Sram XX RD is compatible with all Sram road shifters too. I have put several people on the 11-36 XX cassette using that deraileur with force shifters with no problem.
dave
X2 Now even x-9 and x-0 are available. Get a medium cage and a 11-32 or a long and go 11-36. Gears for years.
I put a Shimano MTB rear derailleur on my wife’s 10 speed 7900 Dura Ace Cervelo R3 for our trip to Europe. We one of the new Shimano 10 speed MTB cassette’s (11-34 I think) and it worked great. The downside is that the gear jumps are pretty big. But it made extended sections of 10+% doable.
Frankly though I’d look into selling the SRM and buying a compact SRM or maybe even a Quarq.
You are right. The long cage road RD is for a triple.
The problem is that the road rear dérailleur does not have the spacing to allow for a large cassette to not interfere with the pulley wheels. The mountain RD has extra space.
So with the road RD you do get extra (chain) capacity for the triple, but not the clearance you need for a mountain cassette (I.E. larger than 28 teeth.)
You actually don’t need a triple rear derailleur, though if you go that route, an old Dura Ace 7703 is your best bet; bigger pulleys and the cage isn’t too long.
I don’t think you need to go full compact, either. Consider going to a 50x38 up front and an 11-28 in the back. That’ll put everything well within the capacity of the derailleurs (rather than at the limit), it’ll shifter better (front and back) and a 38x28 should really be enough gear for pretty much any situation.
You actually don’t need a triple rear derailleur, though if you go that route, an old Dura Ace 7703 is your best bet; bigger pulleys and the cage isn’t too long.
I don’t think you need to go full compact, either. Consider going to a 50x38 up front and an 11-28 in the back. That’ll put everything well within the capacity of the derailleurs (rather than at the limit), it’ll shifter better (front and back) and a 38x28 should really be enough gear for pretty much any situation.
Thanks for everyone’s input. I currently have 52 tooth big ring, but have not seen a 50 tooth big ring 130 mm BCD? I was under the impression that 52/38 was the smallest available for 130/130 mm BCD?
lengthen the chain to be long enough for the large-large combination. In the small-small combination the der won’t be able to take up all the slack, but you shouldn’t be riding in small-small anyway… and it’s unlikely to break anything. It’s easy to get into the large-large in the heat of racing, if the chain is too short this will be very painful.
I went to a 32 on the rear for one race, and changed to a long-cage der only because I was simultaneously going to 9sp from 8sp. The old 6sp der that was on there could handle 8sp chains, but was not happy with 9sp. The plain old Shimano 105 long-cage road der is what I got, works just fine. It looks a bit odd but then so do I…
I suspect there is some aero penalty from the longer cage too. My OC-disorder leads me to contemplate replacing it with a short cage again…