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Saturday, January 9, 2010

My best shifting recumbent drivetrain yet: Sram XX rear derailleur, SRAM TT-500 shifters, KMC DX10SC chain, Shimano cassette

Over 30 years of riding with derailleur drivetrains, I have used mostly Shimano systems. From Altus in the 80s to many years on Ultegra and Dura Ace STI 8spd on the road and Deore XT off road. I did spend a few years on Sachs 8spd Ergopower. Since getting bent (riding recumbents) in 2003 I have been on Shimano 9 and 10 speed systems, the only non Shimano parts being the occasional stock Bacchetta SRAM 9spd Rocket shifters, SRAM chains, and SRAM and Interloc Racing cassettes. Most of my shifters have been Shimano 9 or 10 speed Dura Ace Bar-Cons.

When I originally built my new Bacchetta CA2.0 in September of 2009, it was with a full Shimano drivetrain: Dura Ace 10s Bar Cons, Dura Ace RD-7703 rear derailleur, Ultegra triple front derailleur, Dura Ace 11-28 cassette, and Dura Ace chain. About 1000Km of randoneurring later, I needed a new chain. My brother John had been praising the KMC DX10SC, as the best bang for the buck option for 10s drivetrains, and the most durable for cyclocross. I ordered 3 in silver. You can also save ~20g per chain with the KMC X10SL.

Shimano rear derailleurs have upper jockey pulleys with over a millimeter of lateral play. This this is supposed to make them self centering, and less sensitive to cable adjustment. The shifters also have a certain amount of overshift built in to them to work with the upper pulley. This works well with Shimano chains. Unfortunately, the KMC chain is a bit shiftier. Unless you consciously try to minimize the amount that you over shift with the Dura Ace shifters, it will occasionally try to jump 2 cogs when you try to shift one. That is annoying, especially when you are going hard and have to deal with 2 jolts to your legs rather than one per shift.

After 2 weeks of this, I got lucky. Dana offered to let me demo a SRAM XX rear derailleur and SRAM TT-500 shifters. Cool! This was my first experience with a SRAM rear derailleur. Both pulleys use ceramic sealed bearings. They feel just as good as the old Suntour sealed pulleys I used to use on MTBs, and have no lateral play in either pulley. The RD is about 10 grams lighter than a Dura Ace long cage. Most of that is probably due to the carbon fiber cage plates. The shifters are about 30g lighter than the Dura Ace ones, and don't have plastic covers that can move or slip off. They don't have a friction mode though. You can save another 15g with the SRAM TT-900 shifters. Keep in mind though that they are carbon, and bar end shifters are one of the first things to hit the ground if you happen to knock your bike over, or worse. There are even SRAM bar end shifters with a return to center feature, no matter what gear you are in the lever is always at the same angle after the shift. I have not tried these but worry about not having any tactile or visual indicator of what cog I am in, and they have skinny levers, and they are heavier.

The shifting of this combination is the best I have ever experienced on a bent. It works very well with both a Dura Ace 11-28 or the much more affordable Ultegra 11-28 cassette. It just feels like there is better direct control of the what gear you are in. No build in play or overshift needed. I am still using a Ultegra triple front derailleur, as SRAM has nothing to compete with it, yet.

There are a couple of issues that I have had to deal within making this change:
  • Although the RD will handle up to a 36t cog, more than any current top end Shimano unit, and it has a long cage, it does not handle as much chain wrap. It was designed for those wide range cassettes and a double crank, not the triple that most bent riders use. Using the best current tactic for setting chain length with my 54/39/26 chainrings (make the chain just long enough that you can ride in the big chainring and biggest rear cog combo) I could only use the biggest 3 cogs on the cassette when in the small chainring without having the chain go slack. Besides the lack of a B pivot spring, the issue is the stop on the outer cage plate. It prevents the unit from taking unit more chain slack. I have run for weeks with that stop removed. However this means that if you have a flat and pull out the rear wheel, the cage will wrap a lot more and make it more challenging to out the wheel back on. I am going to put the stock stop back on and just live with more frequent chainring shifts on climbs. If you have less than the 28 tooth difference between largest and smallest chainring that I have, this will affect you less.
  • The rear shifter puts the shift  lever position in a different range than the Dura Ace shifter. it does not go as far down and forward to get to the small cog, but it goes farther upward to get to the big cog. With my shifters flared out at about 30 degrees from horizontal, this causes not issues with my hand when in the big cog. However, I have learned to adopt a new shifting technique when using the biggest 2 cogs: grasp my grip with my finger and palm facing upwards, and use my palm to move the lever. This works well. More varied hand movement to shift may be a good thing for those of us doing stupid long rides and races. 
  • Unlike the rear shifter, the front one goes farther forward when in the small ring. I have left a touch of slack in the cable to mitigate this. Even if I did not, its not really a problem.
  • There is no cable adjusting barrel on the RD. I have added a Jagwire Adjuster at the front end of the last piece of cable housing, which is easy to reach when riding. Easier than reaching the RD itself actually. 
  • You need Torx wrenches to work on SRAM components. I rarely need to do that on the road, plus in a pinch you can fake it with allen wrenches and screwdrivers. You can also replace the torx pieces with something else if you prefer. I am fine with them as they are.
Bottom line: In spite of the fact that I have plenty of Shimano parts around, I will be keeping this set up for far longer than any demo period. 

Update 011310: It has been brought to my attention that only being able to use the 3 biggest cogs on the cassette when in the granny ring might be an issue for some. I have done some more experiments. But here is why is does not bother me too much:

  • Chain bracing still affects how stiff a frame feels under pedalling forces, even on a CA 2.0. The bigger the chainring you are in, the stiffer the boom is. So, I tend to only use the granny ring for the gears it adds relative to the middle ring, i.e. the 2 smallest cogs. Once I get to the 21t cog I tend to shift to the middle ring, On flat rides, I ride almost exclusively in the big ring. 
  • A Shimano rear derailleur has some adverse behaviors when pushed too far beyond its total teeth capacity too. Instead of a slack chain, the spring loaded B pivot moves the derailleur so far back that the chain passing under the upper jockey pulley hits the chain heading forward. This causes some more drag and noise than a slack chain.
  • The slack chain you get with the SRAM rear derailleur does not seem to allow the chain to slap laterally too much, I have not been able to get it to hit the front fork or brake even in 26t chainring and 13t cog combo.
So what have I done to the bike? I took 1 link (1") out of the chain. This is now the absolute minimum chain length. I also experimented with the adjusting screw on the B pivot to take up a bit more slack. I did only enough so as to not adversely affect the shifting quality in the big ring. Result: I can now get 5 cogs down on the cassette without slack (down to the 17t cog). Again, this is using an 11-28t cassette. With a 11-32 or 36, the cassettes that this unit was designed for, you would adjust the B pivot even more which would help reduce chain slack too. I will say this though, not having a spring in the B pivot, the XX is less adaptable to different sized cassettes. If you want to run a 23 or 25 big cog on your cassette, look elsewhere. As a guy who prefers small gaps between cogs, like most racers, I am OK with a 10 speed 11-28 cassette, as long as its got a 14t cog!! ;-)

Update 01312010: As I have been asked a few more questions, time for an update. There is one more downside to a B-pivot that is not spring loaded. On a bike with a Shimano rear derailleur, you can put it in the big ring and small cog (as you should do every time you do a rear wheel swap) undo the brake quick release, then the quick release skewer on the wheel, and give the wheel a solid whack down and forward. You can have the wheel out in under 2 seconds if the bike is already in the right gear. That does not work with the XX rear derailleur. In fact if you do it hard enough you might damage the derailleur or hangar. With the spring loaded B pivot on a Shimano rear derailleur, as you move the wheel down and forward, it reduces chain tension, causing the B-pivot to move the upper jockey pulley rearward and downward and out of the was of the cassette as the whee exits. So, with my current B pivot adjustment, the upper pulley stays where it is until I manually move it back. Not what most experienced race mechanics are used to, and more time consuming and awkward without someone or something else to hold the bike. So I thought to myself, perhaps if I had the B pivot on the XX adjusted to where it needed to be for a 11-32 or 11-36, the cassettes that it was designed for. Well I cranked the B pivot adjuster all the way out, and its still an issue. On such a big cassette I would also be using a longer chain, but on this derailleur, that would make no difference in the position of the upper jockey pulley, so I did not try it.

I still love the way this set up shifts thought.

Also, you may notice some product links to specific vendors in my posts here. Those vendors give me a small commission if you click on the link and buy the product. So, if you cannot buy these parts at your local bike shop, please use my links to get them from Bent Up Cycles, or Amazon.  Product links with pictures:





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