magohn

Hi all,

I ride a Specialized Roubaix Comp (2010) and also a Trek Fx7.3 (2010). While I love the Roubaix, the Trek is so much easier to use for hilly routes. Though the Trek (with panniers etc) is 2x the weight of the Roubaix, it is so much easier to ride uphill in its lowest gear (Front "1", and rear "1"). Obviously this is due to the gearing ratios and so I am wondering if it is possible to switch out the cassette on the Roubaix so that its easiest climbing gear is comparable to the Trek - and still just have a double on the front of the Roubaix. Would the Roubaix's Sram Rival shifters still shift the same? Here are the gearing specs for the two bikes:

Roubaix
======
FRONT CHAINRINGS 50 x 34T
CASSETTE Shimano 105, 10-speed, 12-27t

Trek Fx 7.3
=========
Front Crank Shimano M361 48/38/28
Cassette Shimano HG31 11-32, 8 speed

What I want is the same (or easier) climbing ability as when in the easiest gear on the Trek.

Any help appreciated.

canam73

With out going to a triple I don't think you get to that gearing. But to get lower you will want to get an Apex RD and cassette (11-32) and you will need a new chain as your current one is most likely too short.

rogerstg

A little simple math says you'd need a 39 on your cassette for this. I don't think there is such a mass produced item. But since the bike is lighter, maybe you can get away with a 36 max ring cassette.

Steve530

I can confirm that a medium cage Rival will work with a 12x32 cassette and you that will need a new chain. Your current shifters will be fine.

irwin7638

I've been looking at shimano's 12-36 cassette. The only problem is a derailler which will handle the granny cog, several will.

Marc

magohn

Im a heavier guy (but Im working on that) and so really notice the difference on the hills.

Thanks all - it looks like its more involved than just switching out he cassette. Whats the max cassette (low end) I could go with without switching out the deraiileur or the chain? Would I feel a difference?

Thanks!

Steve530

Your rear derailleur probably limits the maximum cog you can use. If it's a standard short cage Rival, yhe maximum cog is 28t. The chain may work with a 28t cog, but I doubt that difference would be worth the price of the cassette.

Al1943

But he's stuck with a SRAM rear derailleur. A Shimano rear derailleur is not compatible with the Rival shifters due to the SRAM cable pull and actuation ratio. Or he could replace the shifters and derailleurs with Shimano.

LongIslandTom

to the OP:

Your Specialized Roubaix Comp is the one equipped with all Shimano 105 components, correct?

If you want a lower gear ratio, the most common modification I've seen with 105 and Ultegra setups is a 32T cassette. While that will not give you a gear ratio quite as low as your Trek's 28 / 32 combo (0.875 gear ratio), going down to 34 / 32 on your 105 bike will give you a 1.06 gear ratio, which is a significant drop from your current 34 / 27 combo, which has a gear ratio of 1.25.

34 / 32 will give you a gear ratio halfway between your Trek and your Roubaix's current setup, and I have a feeling that would be "good enough" since your Roubaix probably rolls more freely than the Trek.

The Shimano 105 medium-cage rear derailleur (5700GS) will work with a 32T cassette, and it's an easy modification to make. Just make sure the B-tension screw is properly set so the RD pulley will clear the biggest cog on the cassette, and you will need a longer chain to accommodate crosschaining the biggest chainring and the biggest cassette cog.

That's what I'm running and I have no problems. 105 shifters, 5700GS rear derailleur, and an SRAM PG1070 11-32T cassette.

Hope this helps.

MNRon

Another option, though it does require a RD change, is to use a 9spd Shimano Mtn bike RD (Deore, STX, etc..) with the 10spd shifters and then you could go with a 34 or even a 36 rear cog. I'm going to be "upgrading" my Synapse from it's Ultegra RD to something like that for a Colorado trip next spring/summer. Needed lower gears this year.

Al1943

As was said above, to use a Shimano rear derailleur would require replacing the shifters. A Shimano RD is not compatible with his SRAM shifters.

himespau 

So that apparently rules out a microshift mtb RD then too, huh? No one makes a mtb style RD (or maybe I should just say SGS) compatible with SRAM, or how about SRAM's own? Is it possible to take the long cage from a cheap SGS derailleur (I got my microshift for $35 shipped and I know they have two levels lower than that) and somehow swap it for what's on his derailleur now?

Al1943

The incompatibility is the actuation ratio designed into the derailleur's geometry, how far it moves for each increment of cable pull. A derailleur's cage length controls how much chain can be wrapped to avoid chain sag when on the smallest chainring and the smallest cassette cog. These are two totally separate variables. Some SRAM derailleurs have been designed to be compatible with Shimano shifters, but there are no Shimano derailleurs designed to work with SRAM road shifters.

himespau 

So, if cage length is completely independent of derailleur geo) that says that a mtb cage might work if swapped onto an SRAM derailleur body (assuming cage attachments look at all similar) or are these riveted together/built in such a way so that's not possible?

Or is the Apex already SGS (or whatever SRAM's equivalent of very long cage is) and there's no room to go longer?

ericm979

The OP can use an Sram XX mtb RD and cassette. The XX will handle something like a 36t cog.

Apex goes to 32t at least and may be able to handle more. I think that the newer X.0 derailleurs also use 1:1 actuation and would work with Sram road shifters, but double check that before you buy.

Al1943

As I said in post #14, cage length controls how much chain can be wrapped to avoid chain sag. The cage length does not control or limit the maximum cog size. The maximum cog size limit is built into the derailleur's geometry and is also partially controlled by the derailleur hanger length. The wrap capacity needed can be calculated as (big ring size - smallest ring size) + (largest cog size - smallest cog size). Every rear derailleur has a recommended wrap capacity and a recommended large cog size limit, although these limits are often exceeded a little. Every rear derailleur also has an actuation ratio (throw ratio). The distance the derailleur travels with each click of an indexed shifter is equal to the cable pull multiplied by the actuation ratio.

himespau 

yeah, I knew that geo determined the actuation which led to indexing, but I didn't catch from your earlier post that it had anything to do with maximum cog size (or that hanger length was involved - but that makes sense).

magohn

Thanks again guys. I had no idea it was so involved but your excellent explanations have cleared up a lot of the mystery for me. As my stock setup is a standard SRAM Rival groupset and runs over a Shimano cassette, it looks like Im stuck with making do with what I have or spend bigger bucks than I wanted to anywhere close to the Trek's easeability.

I really dont want to switch out the derailleur and certainly dont want to switch out the shifters. I was hoping it was a matter of switching out the cassette - you live and learn - thanks to you guys.

Camilo

I think all the apparent complexity and confusion had to do with people being confused about your shifters. You have Sram shifters. Any Sram 10 speed mountain bike derailleur will work with those shifters and will handle up to 36 t cassette.

The parts will cost you $125 though, but for me if it made the bike a lot more rideable, it would be worth it.

Parts:
cheapest Sram 10 speed MTB derailleur: $50
cheapest Sram 36t cassette: $50
Chain: $25

All of these are extremely simple DIY projects, or very cheap for a bike shop to do them for you. Changing and adjusting a rear derailleur is actually quite simple and doesn't require any special tools.

ultraman6970

Don't take me wrong but it would'nt be easier just lose some weight and train better???, you are already with a triple in both bikes man (the ct is almost a triple too) the next step is just buy a unicycle or a motor.

If the cassettes are both 10 speed and u have the same spineline in both wheels (shimano?) sure u can but if your climb abilities are related with age, weight and training, then to change the cassette with one that will have a 28 or a 32 wont make a lot of difference IMO. The other thing is climbing, shifting techniques and cadence, if you lack in those 3 then is clear that the approach won't work like expected, then u have another issues. Yes to change the cassette will help, but IMO you could just band-aid patching the real problem, to suck as a climber.

Again don't take me wrong ok?

Good luck with the fix.

magohn

Thanks Camilo - $125 does seem more reasonable. I will go hunting on eBay. My main concern was that things would "shift" as they do now.

ultraman6970 - yes I suck as a climber. But I do notice a real difference when riding the Trek versus the Roubaix. I just want to get the most bang for my energy-buck. You can count me as a "clyde" rider (proudly) but its not for want for trying. In the last 18 months I have dropped 30+lbs and ridden 3000+ miles (including multiple back to back century weekends). So I think my fitness is there its just when I hit the 10%+ grade hills, I notice how well-equipped the Trek is to tackle them.

Thanks again all.

Camilo

The shifting should be just like it is now. I've used MTB rear derailleurs and cassettes w/ road shifters on a couple of occassions, and I've never noticed any meaningful change in the shifting - except there will be larger gaps between the rear gears. I haven't done it with Sram, only Shimano 8 and 9 speed. But it's one of the "selling points" of Sram 10 speed vs. Shimano: all the Sram10 speed drive train parts, whether they are MTB or Road, will work with each other while Shimano 10 speed shifters won't work well with Shimano 10 speed MTB RD (or it takes a lot of fiddling to make them work OK).

Good luck. More lower gears are a good thing if you need them, and you're the one and only judge of that. But the conversation always seems to bring in the oh so helpful advice to get stronger as if it's somehow we (who are looking for lower gears) have never thought of it or are just lazy.

I say: make the bike work the way it will make riding more fun for you. It's fairly cheap and easy. I've done it to two bikes this summer and it really made the bikes more fun, and the rider stronger because of more and funner riding!

magohn

Exactly

Meanwhile, I have ridden more miles on this bike in the last 18 months than I ever did in the previous 48 yrs on all my bikes combined. I love the Roubaix and giving myself a little more wiggle room on the low gears would benefit me on the many more miles to come. Im a heavy guy (6'0" - 280lbs but down from 316lbs) so I know the weight is what i need to lose but why not make the weight-loss journey more enjoyable?

Thanks for your advice it was exactly what I was looking for. To the point and void of any "what if you just tried harder" remarks