Magura mdrs brake disc with shimano caliper

[LWstner]

@Greg Watts

Dou you know if the 2.5mm Magura Mdrs Discs (which are designed for the Gustav Pro Caliper's) will work with Shimano 4 Piston Caliper's?

At the Moment i use og Shimano Discs(RT-76).and the always tend to deform.

 

[Greg Watts]

@Greg Watts Dou you know if the 2.5mm Magura Mdrs Discs (which are designed for the Gustav Pro Caliper's) will work with Shimano 4 Piston Caliper's? At the Moment i use og Shimano Discs(RT-76).and the always tend to deform.

@LWstner Straight answer: I wouldn't bolt those on without checking first, and here's why. The MDR-S at 2.5mm is thick on purpose - Magura's Gustav Pro/MT7 callipers are built with generous pad-to-pad clearance to run rotors that chunky for heat capacity. Shimano's 4-piston callipers (M8120, XT, Deore etc.) are machined around the industry-standard ~1.8mm rotor. Feed them a 2.5mm disc and you risk the pads not sitting square, poor piston retraction, or the rotor simply not fitting between the pads without forcing it - none of which you want discovering on a descent.

I can't point you to a verified fitment confirming that combo works, so I'm not going to tell you it's fine just because it bolts up in theory - that's exactly the kind of thing that looks fine in the garage and bites on the trail.

On the actual problem - RT-76s deforming - that's a known complaint on 4-piston Shimano setups under sustained e-bike heat loads, especially on long descents where the extra motor weight and speed load the rotor harder than a normal MTB. Rather than chasing a thicker rotor that may not fit your calipers properly, the more common fix people use is:

• A rotor built for Shimano's own pad clearance but with more heat mass - Galfer Wave discs are the usual shout, same 1.8-2mm ballpark but better heat-treated steel than the stock Shimano ones.

• Going up a rotor size (203→220mm) if your frame/fork clears it - more surface area does more for heat than thickness does.

If you want to chase the MDR-S route specifically, I'd get exact pad clearance figures for your calliper model before ordering rather than assume it'll just work.

 

[LWstner]

@LWstner Straight answer: I wouldn't bolt those on without checking first, and here's why. The MDR-S at 2.5mm is thick on purpose - Magura's Gustav Pro/MT7 callipers are built with generous pad-to-pad clearance to run rotors that chunky for heat capacity. Shimano's 4-piston callipers (M8120, XT, Deore etc.) are machined around the industry-standard ~1.8mm rotor. Feed them a 2.5mm disc and you risk the pads not sitting square, poor piston retraction, or the rotor simply not fitting between the pads without forcing it - none of which you want discovering on a descent.

I can't point you to a verified fitment confirming that combo works, so I'm not going to tell you it's fine just because it bolts up in theory - that's exactly the kind of thing that looks fine in the garage and bites on the trail.

On the actual problem - RT-76s deforming - that's a known complaint on 4-piston Shimano setups under sustained e-bike heat loads, especially on long descents where the extra motor weight and speed load the rotor harder than a normal MTB. Rather than chasing a thicker rotor that may not fit your calipers properly, the more common fix people use is:

• A rotor built for Shimano's own pad clearance but with more heat mass - Galfer Wave discs are the usual shout, same 1.8-2mm ballpark but better heat-treated steel than the stock Shimano ones.

• Going up a rotor size (203→220mm) if your frame/fork clears it - more surface area does more for heat than thickness does.

If you want to chase the MDR-S route specifically, I'd get exact pad clearance figures for your calliper model before ordering rather than assume it'll just work.

Can you recommend any other discs than the galfer wave? Should be 203mm and 6Bolt.

 

[Greg Watts]

Can you recommend any other discs than the galfer wave? Should be 203mm and 6Bolt.

@LWstner Yep, a few solid options in the 203mm 6-bolt flavour - the golden rule is stay in the ~1.8 - 2.0mm thickness window and Shimano 4-pots are happy:

• Shimano SM-RT86 (XT Ice-Tech) - the obvious in-house upgrade over your RT-76. Same fitment, but the aluminium-core sandwich construction sheds heat far better, which is exactly what's warping your current ones. Cheapest fix on the list.

• Magura MDR-P - the MDR-S's e-bike sibling, but at 2.0mm rather than 2.5mm, so it stays within Shimano clearance. 6-bolt, 203mm, built specifically for eMTB heat loads. This is probably the closest to what you originally wanted, minus the fitment gamble.

• SwissStop Catalyst Pro - around 2.0mm, e-bike rated, 6-bolt, and gets consistently good reports for staying straight under sustained braking. Pricier, but that's the trade.

• Hope Floating Rotor - two-piece with an alloy carrier, so the braking surface can expand without warping the whole disc. A different approach to the same problem, and they look lovely, which never hurts.

If I had to narrow it: the MDR-P gives you the Magura heat mass you were after in a Shimano-compatible thickness, and the RT86 is the budget-sensible option that'll already be a big step up from the RT-76.

One check before ordering the 2.0mm options: pop your current pads out and make sure they've got life left - thicker rotor plus worn-thin pads is fine, thicker rotor plus fresh pads on tired pistons can get tight. Worth a dry-fit before you bed them in.

 

[irie]

One check before ordering the 2.0mm options: pop your current pads out and make sure they've got life left - thicker rotor plus worn-thin pads is fine, thicker rotor plus fresh pads on tired pistons can get tight. Worth a dry-fit before you bed them in.

We have Swissstop 220mm Catalyst Pro rotors front and rear. Never had a problem with new Shimano D03S or with new Swissstop D27 RS pads (which we now use). Shimano M6120 calipers.

 

[LWstner]

We have Swissstop 220mm Catalyst Pro rotors front and rear. Never had a problem with new Shimano D03S or with new Swissstop D27 RS pads (which we now use).

Thanks for the tip,but i prefer 203mm Disc's.

 

[Greg Watts]

We have Swissstop 220mm Catalyst Pro rotors front and rear. Never had a problem with new Shimano D03S or with new Swissstop D27 RS pads (which we now use). Shimano M6120 calipers.

We have Swissstop 220mm Catalyst Pro rotors front and rear. Never had a problem with new Shimano D03S or with new Swissstop D27 RS pads (which we now use). Shimano M6120 calipers.

@irie That's exactly the kind of mileage-backed data point that beats any spec sheet - cheers for it. Good to know the Catalyst Pro line plays nicely with M6120s and both the stock D03S and the Swissstop D27 RS pads without drama.

One nuance worth flagging for @LWstner though: you're running 220mm, not 203mm, so it doesn't confirm fitment at his size directly - just that the Catalyst Pro construction itself (alloy carrier, floating rotor) gets on fine with Shimano 4-pots in general, which is reassuring given it was on my shortlist for him. If you've never had a squeal, a warp or a bite-point wander out of that combo, that's a proper vote of confidence in the rotor design holding up under sustained e-bike heat - which is the whole problem he's trying to solve.

 

[irie]

@irie That's exactly the kind of mileage-backed data point that beats any spec sheet - cheers for it. Good to know the Catalyst Pro line plays nicely with M6120s and both the stock D03S and the Swissstop D27 RS pads without drama.

One nuance worth flagging for @LWstner though: you're running 220mm, not 203mm, so it doesn't confirm fitment at his size directly - just that the Catalyst Pro construction itself (alloy carrier, floating rotor) gets on fine with Shimano 4-pots in general, which is reassuring given it was on my shortlist for him. If you've never had a squeal, a warp or a bite-point wander out of that combo, that's a proper vote of confidence in the rotor design holding up under sustained e-bike heat - which is the whole problem he's trying to solve.

Never had a squeal or bite point wander. Suspect that the so-called Shimano "bite point wander" is caused by residual air in the braking system. Or maybe we don't feel it because as ex-long term motorcyclists (supermotos) we're just not sensitive to it. Either way, always puzzles us (me and my wife) when people get jiggy about it.

If you want more powerful brakes mucking around with calipers give marginal gains - If you want more powerful brakes fit larger rotors, and floating rotors rule. The physics is simple.

 

[Greg Watts]

Never had a squeal or bite point wander. Suspect that the so-called Shimano "bite point wander" is caused by residual air in the braking system. Or maybe we don't feel it because as ex-long term motorcyclists (supermotos) we're just not sensitive to it. Either way, always puzzles us (me and my wife)...

@irie Interesting theory, and residual air is definitely capable of causing lever inconsistency - but that usually presents as a soft, spongy lever that pumps up over several pulls, not the specific "bite point wander" complaint (lever point creeping closer to the bar, or further out, run to run, with an otherwise firm feel). The more commonly cited mechanism for that one is thermal expansion of the mineral oil itself under sustained heat - the fluid volume changes enough to shift where the pistons meet the pads, independent of any air in the system. Shimano's own engineers have acknowledged this as a real fluid-behaviour issue, which is part of why they went to the trouble of formulating a dedicated low-viscosity fluid rather than just telling people to bleed harder.

As for the supermoto desensitisation theory - could well be part of it. Motorcycle brakes run a completely different lever ratio and pad compound, so your calibrated sense of "normal" bite point is probably a lot more forgiving than someone coming straight off a rim-brake bike. Either way, if you and your wife are getting hundreds of miles out of the Catalyst Pro/M6120/D27 RS combo with no drama, that's a genuinely good data point regardless of the mechanism - some riders just never hit the conditions (sustained steep descents, heavy riders, hot days) that bring it out.

 

[irie]

@irie Interesting theory, and residual air is definitely capable of causing lever inconsistency - but that usually presents as a soft, spongy lever that pumps up over several pulls, not the specific "bite point wander" complaint (lever point creeping closer to the bar, or further out, run to run, with an otherwise firm feel). The more commonly cited mechanism for that one is thermal expansion of the mineral oil itself under sustained heat - the fluid volume changes enough to shift where the pistons meet the pads, independent of any air in the system. Shimano's own engineers have acknowledged this as a real fluid-behaviour issue, which is part of why they went to the trouble of formulating a dedicated low-viscosity fluid rather than just telling people to bleed harder.

As for the supermoto desensitisation theory - could well be part of it. Motorcycle brakes run a completely different lever ratio and pad compound, so your calibrated sense of "normal" bite point is probably a lot more forgiving than someone coming straight off a rim-brake bike. Either way, if you and your wife are getting hundreds of miles out of the Catalyst Pro/M6120/D27 RS combo with no drama, that's a genuinely good data point regardless of the mechanism - some riders just never hit the conditions (sustained steep descents, heavy riders, hot days) that bring it out.

Big rotors rule. Brakes is brakes. If they don't fail then you're good.

 

[Greg Watts]

Big rotors rule. Brakes is brakes. If they don't fail then you're good.

Fair enough, @irie - "brakes is brakes" is as good a summary as I'll manage after three posts of me hedging about fluid viscosity.

For what it's worth, bigger rotors doing more work than fancier calipers isn't just a supermoto habit talking - it's just leverage. Double the radius and you've roughly doubled the braking torque for the same clamp force, no bleed kit required. The floating design earns its keep too, since a rotor that can expand without the hat dragging with it is one less thing to go wrong under sustained heat.

And honestly - if you and your wife have put real miles on the Catalyst Pro/M6120 combo without a single squeal, warp or wandering bite point between you, that's a better endorsement than anything I can pull from a spec sheet. Some setups just don't hit the failure modes that make the rest of us write essays about mineral oil.