CCrew

Ok, here's the scenario.. If I'm reading the Shimano tech specs right I should be using a SGS cage RD, but it just aesthetically looks odd to me. Shifts fine, crisp and precise.

Front Crank: 50/34 compact
Rear Cassette: 11-32 Shimano XT 10 speed
RD (Current): Shimano RD-M772. Top-Normal Shadow SGS.
Shifters: 5700 series 105

Shimano specs the RD at 45 tooth SGS and 33 tooth GS. MY math says 16 tooth at the crank and 21 teeth at the cassette equal 37 teeth and I should be using SGS. It just looks to me like the SGS cassette is boatload wrong. Again no issues with performance, it's all aesthetics.. Looks like a wedge from the side view with the chain in the 34 front. Part of that is a 50mm drop in BB due to the frame design and long stays - Frame is a Salsa Vaya.. But I just wanted to be sure I wasn't totally missing something.

I know, it's shifting fine HTFU and ride it Just need a warm and fuzzy that I didn't miss something in the spec.

Thx.

HillRider

You could have probably gotten away with the GS derailleur since Shimano is typically very conservative with their tooth wrap ratings and, at worst, you wouldn't be able to use the 34T chainring with the smallest one or two cogs. That is no sacrifice since you shouldn't cross chain anyway.

So, if the aesthetics are that bothersome and the expense of a new derailleur isn't a problem, go for the GS.

In my own case I'm running a 10-speed Campy Chorus triple group on one road bike with a 13x29 cassette and I changed the 30T granny ring for a 26T. Campy assured me it could not possibly work but it does. I just can't use the 26T ring with the two smallest cogs.

CCrew

Thanks Hillrider. You've pretty much confirmed/addressed where I was with it.

Salsa sells the "built" version of the bike with compact front and 11-32 rear (9 speed) with a long cage LX RD, so I didn't think I was too far off the mark. I wanted the capability to go 11-34 or even 11-36 if I desired so I think I may just suck it up and live with it. It's being built as a tourer and I live in the mountains - albeit little east coast ones so I wanted at least some wiggle room if I found the 32 not big enough.

Thanks again for the response.
Roger

HillRider

I live in the "East Coast Mountains" too and certainly appreciate the ability to have LOW gears. As you noted, the hills here aren't that long but they can dwarf the western mountain ranges with their steepness. Here in Pittsburgh, there are over a dozen hills with grades exceeding 20% and one is 37%!

Barrettscv 

I agree and have been using an Ultegra GS RD with a 50 & 34t compact Crankset with a 11-32 Sram Apex ten speed cassette. I'm able to use all 20 speeds without chain wrap or cog capacity issues. Using a MTB cassette with a 34 or greater cog size will not work (ever) with Ultegra or any other Shimano road derailleur, so using a MTB rear derailler provides the option of other MTB cassettes if you ever make that change.

See: https://www.bikeforums.net/showthread...-fear-no-hill?

CCrew

Thanks! Informative link. More I look at the bike the more I believe it'll finish up with a GS cage. I can always use the one on there now on one of the MTB's in the stable.

DaveSSS 

What's usually thought of as conservative wrap capacity rating is actually the result of a neccessary minimum 3T of wrap capacity that some bikes will just happen to get from a coincidentally perfect chainstay length for the gear combintaion that they are using.

If you plug in the 440mm chanstay length, 32T large cog and 50T big ring into the rigorous chain length formula at the Park Tool website, you'll get a chain lenght of 57.6 inches, which means that a 58 inch chain is required to wrap the big/big. That also tells you that .4 inch or almost 2T of wrap is lost due to unused, but necessary chain length.

If the 33T wrap capacity RD was used, the chain would probably hang loose in the smallest two cogs, but there is a chance that it might have some tension in the 13T cog.

Barrettscv 

Hi C. C.,

Just be sure the RD floats a little when on the 32 cog if using a GS RD. If, by chance, your rear hanger is shorter than the one on my bike, you run the risk of the hanger/RD combination not having enough clearance. Spin the drivetrain to check to see if run-out at the cog is an issue. The RD should always float, you should be able to rock the RD forward and back and no binding should exist while turning the crank. If no extra clearance exists between the big cog and the idler wheel, you run the risk of bending the hanger and/or damaging the RD.

I've been keeping track of several cyclist who are using a GS RD with an 11-32 ten speed cassette. None have had any issues and all seem to be happy with the drivetrain.

dkmatdrum

Hello
I have been working on doing the same thing with the ultegra and Sram but have a question about the derailleur pivot center to rear axle center distance. Can you tell me what it is on your bike? When I try it it seems like the jockey wheel and the largest rear cog have only enough clearance for the chain to go between them-----it seems to work without any noise or excess friction but it seems awfully tight. I backed out the limit screw that restricts the derailleur rotation around the pivot to get it to work but I don't fully understand that geometry. Would a mountain bike derailleur like the xt772 have a larger distance from the pivot axis to the jockey wheel or is that drop only determined by the hanger?

Barrettscv 

Yes, the MTB derailleur have a geometry that provides the clearance for larger cogs. The clearance on my bike is enough, but it also appears that the cog/chain/idler are stacked on each other. That's ok, IF the idler is still able to float. The float is needed only when shifting gears or if something goes out of alignment.

If the idler wheel is pinching the chain, without any additional clearance gained from fore-aft rotation of the RD, the clearance is insufficient. I would use a MTB RD in that case.

DaveSSS 

It would be normal to turn the B screw all the way in (CW) to increase the clearance between the upper pulley and the largest cog. Some people even instal a longer screw to increase clearance.

The RD pivot point can vary from 24-28mm below the center of the axle, but it's common for it to be on the larger side of that dimension, at least on road bike. Shimano does not show the desired dimensions in their RD instructions.

https://bike.shimano.com/media/techdo...830655487.pdft

Watchdog

37%??!!!?? I didn't think they built roads with such steep grades. I would think it's impossible to get up that in a car if there's snow on the road.