Parking Brake Clunk

[Luns]

I have a rear-end clunk that I haven't been able to diagnose.

I've noticed that some times if I lightly yank my hand brake while in motion, I get a loud clunk from the back. The yank can be light enough that I don't even feel any braking, and the motion can even be as slow as just a few mph. But the faster I'm moving, the lighter the handbrake pull can be for the same clunk.

Trying again immediately afterwards, there's no clunk. Coming to a full stop with just the foot brake and trying again there's no clunk. But if I fully engage the handbrake when stopped and release it before setting off again, there's a clunk the next time I try.

I don't know if this indicates something wrong in my parking brake, or if this is just how things are by design, but I don't have another Toyobaru (or even comparable Subaru) to drive for comparison. None of the other cars I have ready access to do the same thing, but none have been disc-over-drum in the back.

I'm not concerned with the immediate symptom - I'm not trying to start handbrake drifts , and it'd be no problem for me to always engage/release it only when fully stopped, in which case this clunk never happens. Again, I'm just wondering if it's a sign of something else going on.

The whole reason this came to my attention was in trying to track down a similar if not the same sounding clunk that I think happens when traction control kicks in on certain hard right turns. That clunk doesn't happen every time I take the same turn, which may correspond to my not always getting stuck at the last light before said turn and setting the parking brake while I wait that out. But I haven't made that particular turn hard in a while now, so haven't gotten to test that theory.

So I'm wondering, does this happen for anybody else?

[EndlessAzure]

Normal. Just play in the brake shoes.


When you set the park brake and let the weight of the car off on to them, it will load the shoes with a little offset depending on the direction of incline. You'll notice the clunk when pulling the brake while driving forward if you had previously set the parking brake to prevent the car from rolling backwards. The play in the brake assembly snaps forward the moment the shoes engage. The clunk is probably a good sign that the parking brake is actually disengaged and not bound up when the handle is down.

[Luns]

Originally Posted by EndlessAzure

Normal. Just play in the brake shoes.


When you set the park brake and let the weight of the car off on to them, it will load the shoes with a little offset depending on the direction of incline. You'll notice the clunk when pulling the brake while driving forward if you had previously set the parking brake to prevent the car from rolling backwards. The play in the brake assembly snaps forward the moment the shoes engage. The clunk is probably a good sign that the parking brake is actually disengaged and not bound up when the handle is down.

Thanks for confirming this!

None of the other drum brake cars I've driven have ever had this sort of clunk, so absent other details, I wouldn't have expected the clunk here either. I think I now understand what's different and why we have this clunk the other cars don't.

The other drum brakes I'm familiar with all have what's called a leading/trailing design. One shoe is self-energizing, but the other one being the mirror image works the other way and discounts this self-energizing effect. However, both shoes rest against their fixed abutment on one end, regardless of whether you're braking against forwards or reverse motion, and the brake cylinder (or parking brake lever) push between both shoes at their other end.

Our parking brakes appear to have what's called a double servo arrangement. The parking brake lever pushes the two shoes apart at the top, pushing BOTH away from the fixed abutment at the top. One shoe pushed by this applies braking force, and pushes in turn on the other shoe via the floating adjuster. The force of braking makes that push harder than the what's at the lever. The other shoe in turn also brakes, and wants to push even harder yet against the parking brake lever. The parking brake lever only pushes as hard as what's it's doing at the other shoe, so the rest of the force (all the actual braking) pushes that brake shoe to rest against its abutment.

Which brake shoe rests against the abutment, and which one gets pushed away from it, depends on the direction of braking. There's free motion between the two cases, and the clunk heard is that free motion being taken up.

I still don't completely understand why it clunks sometimes and not at others. I would expect the return position for the shoes when at rest to be the same regardless of how things got there, but presumably there's some stiction in the system that prevents things from always returning to the same place.

[humfrz]

Originally Posted by Luns

Thanks for confirming this!

None of the other drum brake cars I've driven have ever had this sort of clunk, so absent other details, I wouldn't have expected the clunk here either. I think I now understand what's different and why we have this clunk the other cars don't.

The other drum brakes I'm familiar with all have what's called a leading/trailing design. One shoe is self-energizing, but the other one being the mirror image works the other way and discounts this self-energizing effect. However, both shoes rest against their fixed abutment on one end, regardless of whether you're braking against forwards or reverse motion, and the brake cylinder (or parking brake lever) push between both shoes at their other end.

Our parking brakes appear to have what's called a double servo arrangement. The parking brake lever pushes the two shoes apart at the top, pushing BOTH away from the fixed abutment at the top. One shoe pushed by this applies braking force, and pushes in turn on the other shoe via the floating adjuster. The force of braking makes that push harder than the what's at the lever. The other shoe in turn also brakes, and wants to push even harder yet against the parking brake lever. The parking brake lever only pushes as hard as what's it's doing at the other shoe, so the rest of the force (all the actual braking) pushes that brake shoe to rest against its abutment.

Which brake shoe rests against the abutment, and which one gets pushed away from it, depends on the direction of braking. There's free motion between the two cases, and the clunk heard is that free motion being taken up.

I still don't completely understand why it clunks sometimes and not at others. I would expect the return position for the shoes when at rest to be the same regardless of how things got there, but presumably there's some stiction in the system that prevents things from always returning to the same place.

Luns you are waaaaaaay overthinking this -

[Ultramaroon]

Originally Posted by humfrz

Luns you are waaaaaaay overthinking this -

Is he? I like it.

[Sapphireho]

Originally Posted by Ultramaroon

Is he? I like it.

You can take things apart on the porch of knowledge.

[humfrz]

Originally Posted by Ultramaroon

Is he? I like it.

You would -

I have a whole basement full of stuff you can take apart and fix. Just send a semi tractor trailer up here and I will load it up for you.

[Ultramaroon]

Scandinavian flick is smoother anyhow