Brake Rotor Thickness and Caliper Displacement

[anjello91]

Hi guys,

I've been assigned to be in charge of brakes and have been doing quite a bit of calculations and research on brakes and I seem to be stuck with a few things and would appreciate some help.

First, decided on the thickness for the brake rotors, is it ok to choose any thickness as long as it fits between the pads, or must it be a recommended width from the caliper manufacturer?

My problem right now is that I was trying to figure out the displacement of the master cylinder pushrod when the pads meet the rotor, I realized I needed to know the displacement of the calipers pistons first. And considering I do not have the materials yet, I cannot just measure it and I'm sure its all small amount.

And also, before deciding on thickness, rotor material has also crossed my mind. Our team has been using 1018 steel for years, but wouldnt Cast Iron be better for multiple reasons that it is lighter, better heat dissipation and higher coefficient of friction between the pad and rotor?

Thanks.

G. Angelo Lopez
Cal Poly Pomona FSAE

[Jonny Rochester]

To get an idea of displacement, go to a bike shop and look at mountain bike (Hydraulic disc) brakes. With the brake off and the wheel free to spin, you can just see a gap between the pads and the disc. More or less, that is the displacement distance. Or take the front wheel off your car, spin the front hub with the brake off. Get someone to put their foot on the brake, watch the pads more. That is the distance. You can measure lots of disc systems to get a general idea. Then do your calculations and you will know if it doesn;t look right. With the brake off, the pads move back to their position due to the square section O-ring that is slightly deformed when the brakes are on.

For disc material, I don't know. But think about this. Shimano XT bicycle discs are made of a sandwich of 3 layers. The middle part is aluminium, and the outer layers of the disc is stainless steel (I don't know the grades). The 3 layers are bonded together. I think the lower grade Shimano SLX discs are just stainless steel. Something to think about.

[apalrd]

We use a low pressure check-valve sort of thing, which basically keeps the pressure in the lines above ~2psi all the time. This is just enough force to keep the pad touching the rotor, so the displacement at the caliper is 0.

If there was no compliance in the brake system, then the displacement would be 0. This isn't entirely the case, but we shoot for 0 displacement.

The forces are what you care about when sizing the system, check where the forces go and how they are multiplied at each stage, then check back on the intermediate variables (such as line pressure and displacement) to make sure they aren't crazy.

In general, we select rotor thickness for thermal reasons, thinner is lighter, as thin as you can get away with is better. We use steel for rotors for cost reasons, and haven't had any brake performance issues.

[anjello91]

To get an idea of displacement, go to a bike shop and look at mountain bike (Hydraulic disc) brakes. With the brake off and the wheel free to spin, you can just see a gap between the pads and the disc. More or less, that is the displacement distance. Or take the front wheel off your car, spin the front hub with the brake off. Get someone to put their foot on the brake, watch the pads more. That is the distance. You can measure lots of disc systems to get a general idea. Then do your calculations and you will know if it doesn;t look right. With the brake off, the pads move back to their position due to the square section O-ring that is slightly deformed when the brakes are on.

Well I actually have data from the previous car that has a displacement of about 0.030" per caliper I believe? But considering we might use new calipers and new rotors, I assumed using the same value wouldn't be good enough. But maybe it is.


We use a low pressure check-valve sort of thing, which basically keeps the pressure in the lines above ~2psi all the time. This is just enough force to keep the pad touching the rotor, so the displacement at the caliper is 0.

This is where I see alot of inconsistency reading around the forums. Some people say there should be minute contact between your pad and rotor to minimize your displacement, to essentially zero, is what you're saying. Then however, there are many others who say a gap SHOULD exist between the pad and rotor to eliminate any drag whatsoever. But I suppose its all preference between zero deadplay with minimal drag, or zero drag and slight deadlplay?

[apalrd]

This is where I see alot of inconsistency reading around the forums. Some people say there should be minute contact between your pad and rotor to minimize your displacement, to essentially zero, is what you're saying. Then however, there are many others who say a gap SHOULD exist between the pad and rotor to eliminate any drag whatsoever. But I suppose its all preference between zero deadplay with minimal drag, or zero drag and slight deadlplay?

How much drag torque do you get from a 2psi line pressure?

We have won 3 efficiency events in 2 years with a 2psi check valve on both circuits, we found far bigger gains in rolling resistance due to a redesign of our hub/bearing/upright assembly this past year.