Brake pads/rotors/drums - contact question. 1

[3DDave]

So, here's the question - just watched what I've experienced - the difficulty of removing pads or a drum due to the rust and uneven wear of the drum/rotor because the pads are inset from the edge of them.

So - I understand the force on the pad or shoe needs to be centered on the width of the pad but why cannot the pad/shoe extend off the edge of the rotor/drum? That way it can wear evenly and remove the rust ridge and everyone is happy? Plus, on pads it should/could leave a sliver of the original thickness so that a glance will give the owner a better idea of how much life he has used and how much is left.

Has this been tried? I don't mean recenter the pistons on the pad so it's off the edge, I mean just make the pad a bit wider so it goes off the edge a little bit.

The example is on an Escalade where they did not include any access to the parking brake adjuster and the rotor was destroyed trying to get by that ridge. Seems like on a parking brake even with some clearance that it would keep the edge of the drum smooth just like the rest where the brake normally contacts.

 

[TugboatEng]

Ford Explorer two rotor options on one of their SUV's a whole back, about 1/2 inch diameter difference. It was quite common for DIY mechanics to put the cheaper rotors on which also happened to be smaller. The pads did overlap the edge and wore into an L shape. I don't know if there were issues.

 

[BrianPetersen]

If the projecting unworn part of the pads contact each other rather than clamping the disk, you are going to have a bad day. It might not be an issue for a thick vented rotor, but it might be for an unvented one.

 

[3DDave]

I see that could happen, but then the pad would rock and wear faster at an angle and put a little side force on the pistons. I think it would still stop - but is that worse than the owners who use the steel-on-steel method of brake rotor removal for stopping power? Certainly in the case of the Escalade shoes that would not be the case.

 

[geesaman.d]

I would think anything that allows the pad lining to get thin and chunk off would be dangerous, as it can jam the mechanisms. Then if you make the overage significant enough to not be prone to breaking off, the cost increase gets to be problem. Random thought.

 

[cibachrome]

Pad force should not be centered on the pad. It needs to be at the neutral axis of the circumferential forces. Otherwise there is a net moment. Been there, measured, validated and verified.

 

[3DDave]

It was a generalization, not a specification of geometry - however, what happens? I've taken them apart, but don't know the industry names for the parts I did not buy - but the guides for the ends of the pads are seeing a large force already, so what happens when it's not properly aligned?

 

[gruntguru]

Pad force should not be centered on the pad. It needs to be at the neutral axis of the circumferential forces. Otherwise there is a net moment. Been there, measured, validated and verified.

Does that location also result in even wear across the face of the pad?

je suis charlie

 

[cibachrome]

Yes, even wear. Also needed for "pinch-out" to retract pads after a brake release so friction forces won't cause drag for rolling resistance/fuel economy penalties. This takes an extra booster process to quickly get the gap taken up during brake apply. If not done properly (pad geometry), piston(s) can/will get cocked, stuck, and damaged as the pad gets cambered during wear/ablation. Excess heat, wheel bearing failure, and asymmetrical pad wear (inside/outside) are the usual tell-tales. Broken/cracked rotors can accompany the stuck piston situation as well as lead/pull characteristics during a brake stop.

This is 25 y.o. technology BTW.

 

[cibachrome]

In the late '70's I worked with a Team at GM to stress, strain & thermal gauge pads and rotors put on a brake test dyno to show graphically how all this works out. We saw that the innermost area of some pads contributed very little to net braking force. So, redesign of calipers and pads was also called for, including consideration for substantial caliper flexing during a stop segment. I recall the use of mainframe SAS graphics procedures to 'see' the buildup and distribution of forces (radial and compressive) from these tests. Initial problems from brake component suppliers could cost you 15 feet in a Minimum Stopping Distance certification requirement validation test. The variation among OEM and aftermarket pad suppliers was huge. This was also the time period where asbestos, organic and metallic pad materials were being offered (and debated), some with ridiculous, unscientific, and "urban legend" property claims.

BTW, this is why I now never have my rotors machined when my cars are inspected. I prefer the extra rotor surface area from the grooves as well as the thermal mass being retained.

 

[EdStainless]

With drums the shoes are inset for good reason.
Applying stress and heat right at the edge is asking for thermal cracking issues.
It is in effect a bit of reinforcement.
You reach in and crank the adjuster if they don't slide right off.
BTDT

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[3DDave]

Ah - not on the Escalade. Which is what got me thinking of that. It's on the South Main Auto channel on YouTube. I have rarely seen that guy as frustrated in a task as that one and that includes looking for failed electrical wiring.

But how does it prevent cracking? I think the worse case is to have the center trying to expand and the edge in tension. On drums there's a reinforcing ring on the outside anyway. Even so, the metal is conductive enough that a half inch won't make much difference.

 

[gruntguru]

BTW, this is why I now never have my rotors machined when my cars are inspected. I prefer the extra rotor surface area from the grooves as well as the thermal mass being retained.

I have always been the same, but mainly to be a tightass.

je suis charlie