from Rookie’s keyboard
Hello, friends
Nice afternoon, right?
Anyway, today’s session at the bike shop came with a dilemma once again. I had to replace a worn Magura rotor. All I had lying around were SRAM models. (We are expecting a delivery next as we are low on supplies).
To be honest, if it was my bike, I would’ve just put the SRAM rotor on and called it a day. But you can’t do that with a customer’s bike. You must be absolutely certain that your work is by the book.
Hence I did some research. And the conclusion was clear – you can use a SRAM rotor, but it’s not ideal due to the dissimilar thicknesses.
For that reason, I didn’t do the swap and informed my boss of the issue. He told me that we will have the needed rotors in stock next week.
So, what’s the problem?
SRAM rotors can fit on a Magura brake as long as they’re of the right diameter. But SRAM rotors are 1.85mm thick whereas original Magura rotors are 2.0mm and have a wear limit of 1.8mm.
In other words, SRAM rotors start very close to the minimum thickness that Magura brakes are designed for. To preserve the modulation of the brakes for as long as possible, it’s recommended to stick with the original rotors.
Compatibility Requirements
A rotor has to meet the following criteria to operate with a set of brakes:
Size/Diameter
Before all, the diameter of the new rotor should be compatible with the brakes.
SRAM and Magura rotors are available in the following sizes – 160mm, 180mm, 203mm, and 220mm.
The new rotors should match the size of the original ones.
If you purchase a rotor that’s too large, you won’t be able to install the wheel because the rotor will push against the caliper.
That said, it’s possible to switch to bigger rotors by using an adapter.
The main function of adapters is to increase the distance between the brake mount and the caliper.
The adapter that you will need depends on the original mounts of the frame and fork.
There are three main disc brake mounts – International Standard, Post Mount, and Flat Mount
You will find IS mounts on older bikes whereas PMs are common for newer models.
Flat mounts are a new standard used for road bikes. This mount is limited to 160mm rotors.
Thickness
Ideally, the thickness of the replacement rotor will match that of the original one.
In this case, this doesn’t happen because SRAM’s rotors are 1.85mm thick whereas Magura’s are 2.00mm.
Also, some Magura rotors have a marking on them indicating a wear limit of 1.8mm. In other words, the rotors should be replaced when their thickness has fallen to 1.8mm.
Thus, it won’t take long before a SRAM rotor is at the limit specified by the brake’s manufacturer.
Thinner rotors increase the distance that the brake pads have to travel to grab the rotor. As a result, the rider has to press the lever harder to initiate braking. The effect is similar to using a brake with worn pads.
This outcome can be mitigated to a degree by pushing the pads closer to the rotor via the available settings on the caliper. However, this adjustment reduces the modulation of the brakes.
In conclusion
Due to the notable difference in thickness, it’s recommended to stick with the recommended rotors so that the brakes perform as intended.
Hub Compatibility
The attachment system that the rotors use should also be compatible with the hub on which they have to be installed.
There are two main options – center lock and 6-bolt.
Center lock hubs rely on a Shimano-patented technology. The rotor is mounted and secured to the hub via a lock ring similar to that of a cassette.
If you have bolt-on hubs, you will need a bolt-on rotor.
If you have a center lock hub, you have two options:
- Install center lock rotors
- Install bolt-on rotors by using an adaptor
Brake Pad Overhang (a dangerous phenomenon)
When using rotors that aren’t originally designed for the brakes, a phenomenon known as brake pad overhang might take place.
If the new rotors have a brake track narrower than the original, a small section of the pads will remain untouched and will therefore “hang”.
Subsequently, that section will never get thinner due to the absence of friction while the remaining body slims down.
As the pads get smaller, there will be a point when the non-damaged sections of the pads touch each other and effectively prevent the pads from grabbing the rotor. When that happens, the brakes will fail to do their job.
Normally, this problem doesn’t occur when the new rotors match the size of the old ones. For 100% certainty, however, it’s recommended to examine the brakes and make sure that the pads are in full contact with the rotors.
If that’s not the case, the rotors are not appropriately shaped for the particular system and have to be replaced.
Sometimes brake pad overhang occurs because the brakes use an adapter that’s too large for the rotors in question.
Until next time,
– Rookie
Leave a Reply