Brake lining glue

Why wrong? The self servo action simply pulls the lining against the drum allowing the shoe to go with it. Under your theory, you could release the brakes and the shoe would come back but the lining would stay where it was until the drum stopped revolving.

Rob

Coyoteboy mumbled incoherently to the rest of uk.rec.cars.maintenance:

Looking at the material datasheets you'll want to cure it at around 100-150C for 10-20 minutes then let it cool for a day, and make damn sure you have it well pressed on. This should give you the best shear strength under hot load when in use.

But if the mating surfaces - in this case the lining and shoe - are rough then there is going to be more friction than if they are smooth like between the drum and lining. There's more pressure per square inch of contact if the contact area is small than if it's large. Plus the effect of mountains on one side digging into valleys on the other.

Rob

Rob graham mumbled incoherently to the rest of uk.rec.cars.maintenance:

Surface science is not a light subject, certainly something i try to avoid at all costs. but... although the larger area of contact between two surfaces would create a larger source of frictional contact, it also reduces the pressure between the two surfacesfor the same braking force. Since pressure equals force divided by the area of contact, it works calcs out that any increase in friction-generating area is exactly offset by the reduction in pressure.

The forces then, are dependent only on the frictional coefficient of the materials and the FORCE holding them together.

HTH

Rob graham mumbled incoherently to the rest of uk.rec.cars.maintenance:

I'm trying to think back to my drum brake system. IIRC the slave cyl pushed the tops outward, this meant that one shoe has its trailing edge contacting first and one its leading edge first. The one with leading edge first would tend to catch and be pulled against the drum. However the force involved would be miniscule in comparison with the return-spring force or you'd end up with chatter and seizures.

Coyoteboy mumbled incoherently to the rest of uk.rec.cars.maintenance:

I might be wrong but i *think* this is the reason drums are generally considered to lock up easier than discs when used in the same application, if the drum and shoe get hot they expand and jam the pad against the drum until you let off again.

When the drum gets hot it expands away from the lining, meaning you need more pedal travel. This is so-called "drum fade", noticeable on cars with front drum brakes.

The flip side is that as the drum cools it shrinks onto the linings, and the car stays where you parked it instead of rolling off down the hill.

This has been a problem with handbrakes operating on disks, which shrink away from the pads as they cool.

The self-servo action is a different effect, not related to heating. The cylinder pushes the leading edge of the lining against the drum first. Once that leading edge is in contact, the rest of the lining is pulled onto the drum by its rotation. I don't know if the design of the pivot the other end of the shoe is on is involved in this. This does cause the brake to snatch a bit, especially if it's applied suddenly.

The temperature does sound like it might be a problem. In my experience of bodging things with Araldite it turns into marmite at somewhere in the region of 200C. How hot do brake linings get? Could be more than that on a bad day.

Generally, and certainly in my case, the rear brakes are as you describe. The front brakes, however, have two slaves. One pushes one shoe from one end and the other pushes the other shoe from the other end. This means that both shoes are leading shoes when travelling forwards and trailing when going backwards.

This set-up means that when going forwards the fronts act more than the backs. This takes advantage of the weight transfer to the front wheels under braking and also reduces the chance of the rear wheels locking and inviting a skid. Furthermore, the rear brakes have some leading action in reverse.

Rob

Agreed!

certainly something i try to avoid

I think this is more or less what I've said in a previous posting.

True. But I believe there must be an additional effect of the hills and hollows (very small hills and hollows) on two mating surfaces locking into one another.

Rob

No, not really. The fade is due to the heating up of the lining which can reduce its coefficient of friction. Years ago Ferodo used to advertise themselves as the anti-fade brake lining because they'd got to the bottom of the problem with developing their materials.

The same thing haqppens with clutch plates. If they are worn and prone to slipping, letting them slip heats them up and they slip even more. But stop at the side of the road and let them cool down and they'll probably work again.

SNIP

In some cases maybe. But I've never ever had brake snatch and I've driven a lot of drum brake cars over the years.

Rob

It all begins to go a tad pear shaped when you consider the shoes when they're rattling around & not being used.

You're assuming the forces are perpendicular to the brake shoe surface, they're not , that's why it self servos.

Must admit im experienced at this and its definitely true - give it some beans and my clutch slips in 5th, let it cool and it stops slipping. Could be the spring constant of the pressure plate changing due to heat though. Interesting.

Rob graham mumbled incoherently to the rest of uk.rec.cars.maintenance:

That would be the surface science bit, and its considered negligible on a macro scale in comparison with the normal frictional forces until you get to a certain roughness, i forget what it is but is a defined roughness. As I say I try to avoid that side of things at all cost! :)

There are two different things here. The "drum fade" Ben C mentions causes increased pedal travel; unless this reaches the point where the pedal is on the floor then braking effort is unaffected.

The more traditional brake fade, caused by the friction material's effectiveness being reduced by overheating causes an instant reduction in braking effort.

Chris

You get that too.

The effect I used to get in my Mini was that the early stages of "fade" were easily solved by pumping the pedal a bit (there wasn't enough travel on the pedal to just press it further down-- it hit the floor). This is consistent with drum expansion. At higher temperatures the next effect was that you needed harder and harder pedal pressure (but not much more travel) to keep the car slowing down. This is consistent with the coefficient of friction on the linings going down as they overheated.

The usual way to drive was to avoid getting into the pad fade zone.

A third kind of "fade" is "fluid fade" where the fluid boils and the pedal gets a distinctive spongy feel. I've never had that in a car with recently-replaced brake fluid.

The problem on Minis was that the front drums were manually adjusted. If one was adjusted a bit "tighter" than the other, it could easily hit the drum first, exacerbated by the self-wrap servo thing. It's OK if you start braking gently before coming up to full force, which is a good practice anyway.

I rarely had problems with the brakes locking easily.

Indeed, but the forces can be resolved into components perpendicular to the shoe surface, and those components must be in equilibrium or the shoe wouldn't be sitting against the drum, but moving towards it or away from it.

Yes, but the shoe isn't parallel to the drum. And th shoes curved. So you've got to allow for the bending moment.