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I have Ultegra BR-RS805 flat mount hydraulic brakes on my gravel bike, and Shimano RT86 6-bolt Ice Tech 160mm rotors.

There's a shim between the brakes and the frame to fit the 160mm rotors. When the guy at my LBS looked at my bike, he held the brakes and rocked it back and forth and said there was a lot of movement on the back wheel due to the shim and recommended that I use a 140mm rotor instead so I could remove the shim. As I have another set of wheels I'm building out for the bike, I thought it made sense to get a pair of 140mm rotors and then use those on the two rear wheels, with the 160mm rotors on the front.

As the RT86 rotor does not come in a 140mm size, would a 6-bolt 140mm SRAM Centerline rotor be compatible and a good option?

  • You don't say if your bike is originally post or IS mount. – RoboKaren Feb 9 '17 at 16:16
  • It was never converted, the frame supports flat mount. – Ben W Feb 9 '17 at 17:54
  • Often pads can have a little fore/aft play when sitting in the caliper body. You will only notice this when the brakes are applied lightly. When you squeeze harder it should disappear as the pad can't slide on the caliper piston. Try squeezing the brake hard and seeing the small movement goes away. If so, find another shop as they are making up problems. – Rider_X Feb 9 '17 at 23:10
  • Be careful though if you want to use the 140mm rotor. They are recommended for light riders and for flat terrain. Shimano say that you should use 160mm rotors for hilly roads with longer descents. – Carel Feb 12 '17 at 9:42
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This doesn't answer your question, but your problem before the question.

If your brake body is moving about, the problem isn't the existence of the shim (which is used by all brakes when they upsize) but either:

  • the bolts holding the shim/adapter to the bicycle or the brake to the adapter were not properly torqued and loctited.

  • the bolts holding either the shim or the brakes are too long and are bottoming out so they can't lock down properly

  • the shim was poorly made and has a defect that is causing it to move

  • your bike store is either incompetent or trying to upsell you something

Note that brakes are supposed to move when the bolts are slightly loose so that you can properly center the pads against the rotors. Then you clamp them down to the proper torque tolerances.

Since most people want to go up in rotor size to have more effective braking, you'd be bucking the trend in going down. Are you 100% confident in the mechanical skills of your LBS?

  • I'm new to disc brakes. Is it normal when the rear brake is held and the bike is rocked forward and backward that the rear wheel will move the slightest bit forward and backward with it? This doesn't happen on my bikes with caliper or v-brakes, the bike won't budge at all if the brake is held. – Ben W Feb 9 '17 at 18:02
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    No, it's not normal. If anything, discs are even more solid than V-brakes as there is no rubber brake pad. Something is wrong with either your brake installation or brake pads. Getting rid of the shim is a bad idea as it downgrades your brake in search of the real problem. – RoboKaren Feb 9 '17 at 18:18
  • @benw this will happen sometimes if the brake pads shift inside the caliper slightly (normal, often times will stop if you squeeze brake as hard as possible), the rotor moves against the hub (not OK, tighten the bolts), or as flex from the hub to the rim via the spokes (normal, wheels are only so stiff) – Paul Feb 9 '17 at 18:45
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    When fully on the brakes, all that should move is the rear tire skidding on the ground. Everything else should be solid. You may not have something tightened all the way, all good points, i answered the question specifically but these are all very valid things to consider. – Nate W Feb 9 '17 at 19:17
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    I had a different LBS take a look and they tightened up the shim and all is good now. Thanks for all the help everyone! – Ben W Feb 14 '17 at 2:24
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Rotors are pretty much across the board compatible, so yes you would be fine using a Sram rotor with Shimano calipers. Running a smaller rear caliper is also pretty common.

Be sure to follow the proper burn in procedure when you do set up the new rotors and pads though.

  • Thanks! I'm a little nervous that it won't be enough stopping power when bombing down steep gravel roads and trails when I'll be relying most on my rear brakes, but I guess we'll see. Is burn-in procedure basically just making sure to use the brakes a bunch in a controlled environment? – Ben W Feb 9 '17 at 15:57
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    @benw just a side note: most of your braking power comes from the front, hence the bigger rotor is common on the front. if the conditions your describing require you to use your rear to stay in control, that's another thing. otherwise, learning to use your front brake effectively will unlock way more stopping power – Paul Feb 9 '17 at 18:48
  • Thanks! I do normally rely on the front, but going down steep roads with lots of loose rocks I tend to use the back more. – Ben W Feb 9 '17 at 19:06
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    With a little practice you should be able to get about the same stopping power as using just the rear, while using the rear and a larger front in conjunction if that makes sense. You may also find a brand with a bit more modulation in the lever feel could help as well. I personally like Sram disc brakes for this reason, they feel like there is more in between while to me, Shimano feel either on or off, but that's just personal preference. As for the burn in procedure there are a lot of good topics on it if you search this site. It's slightly more technical than just using them a lot. – Nate W Feb 9 '17 at 19:15
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    Stack exchange won't let me upvote more than one answer, but what Nate said is also true and answered my question as well. – Ben W Feb 14 '17 at 2:26

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