More on using a Shimano 203mm rotor on a Code RSC 200mm brake

[sdcoffeeroaster]

Conventional wisdom says if you used the larger Shimano rotor on a Sram brake then you need to add a washer equivalent to the difference in diameter/2. That would be a 1.5mm washer to go from a 200mm rotor to the Shimano 203mm rotor using the existing Sram 20mm mount (for an existing 180mm frame mount). However I've found that the 203mm mount made by Shimano to go from 180mm to 203mm is a bit different. Here are the measurements of the Shimano 203mm VS the Sram 20mm (for 200mm) mounting brackets. I am going from the Sram centerline rotors to Shimano 203mm rotors because I've found them to be silent compared the Sram rotors that tend to squeal a lot on the trail. Hope this helps others doing the same. Just buy the 203mm post mount since it's only about $10. and get it right.

 

[Gary]

Conventional wisdom says if you used the larger Shimano rotor on a Sram brake then you need to add a washer equivalent to the difference in diameter/2.

Your "conventional wisdom" seems to have missed quite a lot here.
Firstly the fact that post mounts are not positioned symetrically to the wheel axle meaning the brake calipers are mounted at an offset plane from the wheel axle.
This is why you'll find the theory of simply adding a couple of 1.5mm spacers to use a 3mm larger diameter rotor isn't actually correct.
secondly if you were to mount a Shimano caliper next to a SRAM caliper you'd also see that the centreline of each of the pistons from each caliper will not be in exactly the same position (or indeed plane if using 4 piston calipers). And this is why using a Shimano mount with a SRAM brake is also (technically) incorrect.
If you take your verniers out again and measure the leading and trailing edge thickness of each of the adapters where the caliper sits against the mount and you'll clearly see they're angled rather than flush as hinted at on your piece of paper .

In practice, mixing and matching mounts from different manufacturers or adding washers can work just fine though. If you're super anal about lining up your caliper so the pads follow as correct an angle on the rotors braking track of the rotor as possible. Trial and error with washers can be more accurate than your advice to. "Just buy the 203mm mount".

and get it right.

You haven't
But I'm sure it'll work out just fine for you anyway.

 

[sdcoffeeroaster]

Your "conventional wisdom" seems to have missed quite a lot here.
Firstly the fact that post mounts are not positioned symetrically to the wheel axle meaning the brake calipers are mounted at an offset plane from the wheel axle.
This is why you'll find the theory of simply adding a couple of 1.5mm spacers to use a 3mm larger diameter rotor isn't actually correct.
secondly if you were to mount a Shimano caliper next to a SRAM caliper you'd also see that the centreline of each of the pistons from each caliper will not be in exactly the same position (or indeed plane if using 4 piston calipers). And this is why using a Shimano mount with a SRAM brake is also (technically) incorrect.
If you take your verniers out again and measure the leading and trailing edge thickness of each of the adapters where the caliper sits against the mount and you'll clearly see they're angled rather than flush as hinted at on your piece of paper .

In practice, mixing and matching mounts from different manufacturers or adding washers can work just fine though. If you're super anal about lining up your caliper so the pads follow as correct an angle on the rotors braking track of the rotor as possible. Trial and error with washers can be more accurate than your advice to. "Just buy the 203mm mount".
You haven't
But I'm sure it'll work out just fine for you anyway.

When I mentioned "conventional wisdom" I was not endorsing it and only pointing out that I've seen more than a few people just use the washers. And yes I can see the angled sections of the mounts as well and understand why one end might have to be different from the other end. And I'm not really sure it would be that easy to tweak this a lot closer with various washers or probably shims in this case because washers might be limiting in adjustability. And I appreciate your comment about the Shimano mount and that it might not be optimal as well but is more likely more accurate than just adding two 1.5mm washers. And of course it depends on how much difference there is between the SRAM calipers and Shimano calipers as to piston locations relative to the mounting surfaces. I don't have that data and wonder if anyone does.

Your comments did prompt me to look at the pad to rotor interface now and what little that can be seen from outside the caliper and above it looks OK without any other data or guidance. There looks to be pretty good alignment with full contact and no obvious misalignment or pad not contacting the rotor. I think this might be the best I can do without turning this into a research project looking for data that might not be available to users. Maybe if I had lots of extra time and a Shimano set of calipers I would take some measurements and trig all of this out in Solidworks and while I'm probably that anal, not this time due to lack of data and resources, lol. I don't feel any compromise in braking either or noise. Optimal...maybe or maybe not but the Shimano rotors are dead silent compared to the constantly singing Centerline rotors that are very annoying out on the trail. One last comment is that when I did have a bike with Shimano brakes (mt520 4 piston), the pads did not contact the rotors completely and there was a thin strip at the top of the pad that didn't really wear. I guess I could have filed the mount a bit to get full contact and will also look for this as these pads wear.

I didn't jump to this drastic step without trying multiple bleed routines (picked up a lot variations on that online), freeing up stuck pistons and lubing with DOT fluid, repeated alignments and various pad materials. I even swapped out my Code R brakes with Code RSC brakes given to me. I finally zero'd in on the resonant behavior of the centerline rotors which I did suspect from the beginning and found that to be the source of my noise. So should the average person just use the Shimano mounts or try to shim to some unknown goal? I'm thinking now that if there isn't any gross misalignment that obvious then either way might be close enough without any other expert guidance on this. I know I hated the obnoxious and unpredictable squealing the centerline rotors made and that was not even wet. I've heard they are much worse in the wet.

 

[Gary]

but is more likely more accurate than just adding two 1.5mm washers.

it's not actually. Neither are accurate. That was my point really. Pedantic as it is.

I don't have that data and wonder if anyone does.

it'd be pretty easily measured. But ultimately fairly pointless information to anyone other than SRAM or Shimano.
I have both Shimano and SRAM calipers here (not fitted to bikes) but in honesty don't think I could ever be bothered to measure them. I'll get back to you if that day ever does arise though.

it looks OK
There looks to be pretty good alignment with full contact and no obvious misalignment or pad not contacting the rotor.

And once you're happy with that there's not a really lot of point in overthinking it any further.

As for the noisy brakes. Cool that a nice easy rotor swap fixed it for you. But not sure if you're aware. Certain frame designs, rotor diameters, rotor mounting/manufacture, pad and caliper mount combinations can be the cause of noisy braking too.

FWIW I have centreline rotors on 4 different bikes and 6 wheelsets (out of 10 bikes and probably 20 wheelsets in total) and none of the centrelines have given me any issues with noise unless it's the sort of wet conditions where every disc brake makes noise a little. Living in Scotland I only use sintered pads. Which are generally regarded as the noisiest option. But work well and last well in our aggressive wet environment)

 

[sdcoffeeroaster]

it's not actually. Neither are accurate. That was my point really. Pedantic as it is.

it'd be pretty easily measured. But ultimately fairly pointless information to anyone other than SRAM or Shimano.
I have both Shimano and SRAM calipers here (not fitted to bikes) but in honesty don't think I could ever be bothered to measure them. I'll get back to you if that day ever does arise though.

And once you're happy with that there's not a really lot of point in overthinking it any further.

As for the noisy brakes. Cool that a nice easy rotor swap fixed it for you. But not sure if you're aware. Certain frame designs, rotor diameters, rotor mounting/manufacture, pad and caliper mount combinations can be the cause of noisy braking too.

FWIW I have centreline rotors on 4 different bikes and 6 wheelsets (out of 10 bikes and probably 20 wheelsets in total) and none of the centrelines have given me any issues with noise unless it's the sort of wet conditions where every disc brake makes noise a little. Living in Scotland I only use sintered pads. Which are generally regarded as the noisiest option. But work well and last well in our aggressive wet environment)

Well after looking at this further I decided that for now the 1.5mm washers will at least preserve the angle established by the sram mount and move the pad slightly closer to the center of the rotor from what I can see at the top of the rotor/pad interface. I still have the Shimano mounting bracket on the front and once I wear these out I can compare the two. I'm using rt66 rotors now and found that I had one from Japan and one from China. The later has obvious punch distortions where the Japanese one is wearing well and contacting evenly. I've heard the rt66 has now been moved to China and the RT76 while slight more expense is still made in Japan. I'm not sure if it's laser cut or punched but once suppliers have that one (most are out of any 203mm rotors here) I'll probably move to it but will wait until I need a brake change.

My bike is a 2021-2 turbo SL expert carbon. I know there are people out there using sram brakes that don't squeal but this bike (bought it slightly used, 150 miles) has squealed and been noisy from day one. And there seems to be a divide about SRAM brakes where a large number of people just HATE them intensely and will spend lots of money to convert to something else. I like how the SRAMS feel but hate the fact that they use DOT fluid which really complicates things a lot. So this bike squealed with the stock wheels and with the Spec control carbons I put on (DT Swiss 350 straightpull hub). After freeing pistons, bleeding and multiple tries at aligning it did squealed less. I went from sintered to resin to ceramic (Gorilla brand) pads. It mostly made noise when allowed to cool down on the trail but was sort of unpredictable. I did notice that just taking a clean cloth and moving the rotor between your fingers cause the rotors to sing like a tuning fork. The rotors lack much material near the mounting bolts and if you hold that area while rubbing the pad it damps out this noise. Shimanos have a bit more material bridging this area and are not resonant like this and have been dead quiet so far.

 

[Gary]

Shimano since entering the disc brake market have done an amazing job of over complicating rotors with various "new" features and options of which many in reality haven't really offered any great improvement over the good old basic steel one piece 6 bolt rotor.

Shimano and SRAMs brakes both have their flaws. (Being 100% honest every bicycle brake does).
I wouldn't say Dot. Being a long proven hydraulic brake fliud is really a flaw and I'm not sure why you'd feel the fluid type complicates things.

Interesting that you mention you think the resonance was coming from the rotors thinner spurs. It does make some sense for that to be the case. I've always been of the opinion that bicycle rotors should have been designed both thicker and stiffer than the 1.8-2.0mm thickness they've been from introduction. Especially as rotor diametewr has increased. But this would have added rather a lot to the weight of a bicycle disc brake system and low weight is a big part of what sells high end bicycle parts.

 

[sdcoffeeroaster]

Shimano since entering the disc brake market have done an amazing job of over complicating rotors with various "new" features and options of which many in reality haven't really offered any great improvement over the good old basic steel one piece 6 bolt rotor.

Shimano and SRAMs brakes both have their flaws. (Being 100% honest every bicycle brake does).
I wouldn't say Dot. Being a long proven hydraulic brake fliud is really a flaw and I'm not sure why you'd feel the fluid type complicates things.

Interesting that you mention you think the resonance was coming from the rotors thinner spurs. It does make some sense for that to be the case. I've always been of the opinion that bicycle rotors should have been designed both thicker and stiffer than the 1.8-2.0mm thickness they've been from introduction. Especially as rotor diametewr has increased. But this would have added rather a lot to the weight of a bicycle disc brake system and low weight is a big part of what sells high end bicycle parts.

With DOT there is the issue of getting all the dissolved air out. The affinity for water absorption and of course the handling issues with DOT(corrosive to hands and many painted surfaces) in general. Mineral oil seems to be easier to deal with and maybe it's just the Shimano bleed process is much more straightforward. It took me about a half hour first vacuuming out the air before injecting the DOT and then vacuuming more air out once again from the system until I finally gave up getting it all. It does payoff to get as much dissolved air out as possible. I made a slotted tube out of a PVC tube to hold the vacuum on the syringe because my hands eventually were hurting, lol. I did buy the SRAM bleed kit and that made it easier and the process is well documented with some people adding in a variety of extra steps.

Yes all brakes have their issues of course but they sure do beat rim brakes of old. The sram code rsc brake lever has a bite point adjuster that really works unlike the Shimano one I had which didn't appear to do much of anything.

 

[Gary]

I think you might have misunderstood the degassing procedure slightly. (not too surprising considering all the BS surrounding the procedure to be found online)
The fact that penty other manufacturers brakes use Dot and do not require the procedure at all should tell you something. All you're actually achieving by trying to pull a "vacuum" with a cheap plastic syringe is aerating the fluid through the poor sealing of the syringe. If you do it. do it very gently and just the once or twice and tap/flick the syringe to allow any air to accumulate together in the top portion of the syringe.
Yes. Dot 5.1 is corrosive to paint. But not as bad as many folk report. It also happens to be water soluble so having brake cleaner (or simply water) to hand it should be no more difficult to work with and clean up than mineral oil. (ie. using gloves/towels etc.)

BTW.
Shimanos bite point adjustment screw on their higher end brake levers does actually work. Just not in any sort of useful way as it LENGTHENS lever throw rather than shortening it and it doesn't make a huge difference anyway.
Also SRAM brakes can be bled using the shimano cup lever bleed method if you prefer that procedure. By simply fitting the lever syringe with the plunger removed. at the end of the process simply use the caliper syringe to suck out most of the Dot fluid from the lever syringe before removing it.

 

[sdcoffeeroaster]

I think you might have misunderstood the degassing procedure slightly. (not too surprising considering all the BS surrounding the procedure to be found online)
The fact that penty other manufacturers brakes use Dot and do not require the procedure at all should tell you something. All you're actually achieving by trying to pull a "vacuum" with a cheap plastic syringe is aerating the fluid through the poor sealing of the syringe. If you do it. do it very gently and just the once or twice and tap/flick the syringe to allow any air to accumulate together in the top portion of the syringe.
Yes. Dot 5.1 is corrosive to paint. But not as bad as many folk report. It also happens to be water soluble so having brake cleaner (or simply water) to hand it should be no more difficult to work with and clean up than mineral oil. (ie. using gloves/towels etc.)

BTW.
Shimanos bite point adjustment screw on their higher end brake levers does actually work. Just not in any sort of useful way as it LENGTHENS lever throw rather than shortening it and it doesn't make a huge difference anyway.
Also SRAM brakes can be bled using the shimano cup lever bleed method if you prefer that procedure. By simply fitting the lever syringe with the plunger removed. at the end of the process simply use the caliper syringe to suck out most of the Dot fluid from the lever syringe before removing it.

Not sure that the syringes are leaking as you have stated. I did buy the sram kit and not a knock off. I do have some other medical grade syringes to test this statement for for myself. A vacuum oven if I had one would be a better method. One used to degass epoxy for example. Once you get the syringes connected I think the actual pushing air out of the system is very effective. And vacuuming out potential dissolved air does lead to an eventual small amount coming out. If the syringes were leaking wouldn’t they just continue to yield large amounts of bubbles in size and quantity….that is not the case.

Checked with my friend that works on hundreds of cars month in the shop he owns and they don't vacuum out DOT fluid so you may have a point.