Bicycle Mechanics - Switching Shimano Tiagra 12-30t Cassette to SRAM 11-32t on Trek 4.5
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02-01-12, 08:05 AM
I'm switching my Shimano Tiagra 12-30t cassette to an SRAM 1070 11-32t for the broader range. My bike is a 2012 Trek 4.5 Madone.
Having reviewed a couple of youtube videos on the replacement, I think I understand what I need to do. However, some of the videos include a spacer in the switch while others do not. Can anyone tell me whether a spacer will be required for my switch and, if so, which one I need to buy? How can I determine whether I need to add or remove a spacer?
Finally, will adjustment to the RD be required, or will the switch be seamless between the two? I would like the keep the old cassette on another wheelset (set up differently for a more bombproof ride), but I would want to be able to switch out the wheelsets without adjustment, or it's just more trouble than it's worth to me.
02-01-12, 08:22 AM
I'd doubt you're going to get seamless switching between the two wheels even if the cassettes were the same unless the wheels are identical in every aspect as the freehub will probably be positioned a little differently. I could be very wrong though. Or you might just get lucky. Might need a slightly longer chain too.
02-01-12, 08:41 AM
The Shimano cassette will have a very thin spacer on the backside of it when you take it off, you will not have to use this with the new Sram cassette, which requires no spacer.
As far as derailleur adjustment, its always a good idea to check when swapping cassettes, but it will more than likely be a minor turn of the barrell adjuster.
02-01-12, 09:53 AM
I assume you have the 5700A RDER, since it appears to be the only one rated to handle a cog as large as 30T.
It specifies 30T largest cog & 39T chain wrap capacity.
Now you didn't specify if you have the triple or double chain rings.
IF the triple, you would be increasing your chain wrap to 41 from 38.
Things MIGHT be a bit iffy.
Usually, you can go a couple teeth more on the largest cog over the rated size.
Chain wrap might be loose on the small-small, but you shouldn't be using the smallest cogs anyway.
Basically, what I'm saying, is shifting performance might suffer a bit.
02-01-12, 09:56 AM
I am using a triple crank and the 5700A RDER. Sounds like it may be iffy then?
02-01-12, 11:38 AM
If you have a 53-28, and the 11-32, your chain wrap will be 25 + 21 = 46. The derailleur is rated for 39. So the derailleur is not rated for the chain slack, and could become damaged if you accidentally shift into the big ring on the front a big cog on the back. You can try the combo, but if it doesn't work you'll have to use a mountain bike rear derailleur. I have a 46-34 double and SRAM 11-32 cassette on my cyclocross-commuter bike, and use a XTR rear derailleur (only $15 at a bike swap meet!), instead of the 5700 105 RD it came with. Shifts much better than my 105 RD ever did and has plenty of clearance for the 32t big cog on the back.
As said above, you won't need the spacer with the SRAM cassette. I've forgotten that spacer on Shimano cassettes before and figured it out easily as there was still vertical play in the cassette after I tightened down the lockring to the torque spec.
Why not just go with an 11-30? If you already have a triple I can't see needing an 11-32 unless you plan to do a whole lot of really steep climbing.
02-01-12, 02:46 PM
Thanks to everyone.
My triple is a 50/39/30.
I also have a double crank Trek 2.3 50/34. Perhaps I could use the 11-32T SRAM on that set-up (although it has a 28T largest sprocket in the 10-speed cassette now)?
02-01-12, 04:40 PM
Your chain wrap would be (50 - 30) + (32 - 11) = 41. Better, but still just outside of the derailleur range. You could probably run it like that, if you were careful never to shift into the big-big combination, which could damage the derailleur.
Your 50-34 double with the 11-32 SRAM cassette would actually have a higher lowest gear (34-32) than your current 50-30 and 12-30 (30-30), so you would actually have higher gearing on the low end than you currently have, and just one more tooth on the high end (you wouldn't notice it unless your regularly pedal at 30+ mph)
If you really want to run the 11-32, either keep your current derailleur and never shift into the big-big (30-32), or upgrade to a mountain bike rear derailleur. If you choose the latter, be sure to get a Shimano 9-speed mountain bike rear derailleur. The 10-speed mountain stuff won't work with your STI shifters, but the 9 speed will (what I'm running on my bike). Also make sure you don't get a reverse-normal derailleur, which will reverse your shift levers, but work fine otherwise.
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