Rear Drum Brakes For Track Use

4922 Views 52 Replies 10 Participants Last post by silverblueBP, Sep 6, 2021

daveoxide

Discussion Starter · Jul 1, 2021

My last HPDE in my 67 involved a lot of excessively long braking zones with me fighting rear brake lockup. On heavy high speed braking zones with less than smooth pavement, the rear axle would get unsettled and start hopping. A quick release of brake pedal pressure would calm it down but another bump while I reapplied brakes and it would start all over again.

I do have an adjustable pressure reducing valve installed, and after every session I gave it another turn or two to reduce rear brake pressure. Unfortunately it never completely cured the problem, and I think I only have a few turns left on the knob before it’s at max pressure reduction. I was able to work around the issue by starting my braking early, ease up over the bumpy stuff, then hard on the brakes again when the pavement smoothed out. I could also just brake less aggressively, which worked too, but my braking zones were twice as long as everybody else out there with modern vehicles

Anyway, my questions are:
1. Is anyone using rear drums successfully for track use? Successfully… as in no excitement high speed braking (100+ down to 30mph).
2. Is the self energizing nature of how drum brakes work part of the problem with modulating brake pressure during heavy braking events?
3. Could my issue be the lack of axle location devices to control axle wrap while slowing down? I have traction bars that work great during acceleration, but have zero effect when slowing down.

I am curious what others are using to control rear brake lockup and axle hop during braking. Comments and input is appreciated!

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patrickstapler

· #2 · Jul 1, 2021

Most all vintage cars that don’t have cheater brakes uses these with good success.

opentrackerracing.com

11 X 2.25" Drum Brakes for Small Bearing 8" or 9" Ford Rear - Opentracker Racing

New to the market, 11 X 2.25″ drum brakes for the small bearing Ford 8″ and 9″ rear ends with the 5 X 4.5″ Ford bolt pattern, 28 spline axles and 2.43″ hub. Two drums and two loaded backing plates complete with self adjusters and emergency brake hardware. All new parts made to meet or […]

opentrackerracing.com

Quick Performance also sells them.

You will want a good proportioning valve, such as a Tilton. Mount It where it’s accessible in the car while on track.

Explain axle hop during braking. I have never experienced this. Do you mean axle hop during down shifting? If so, rev matching will cure that.

Reactions: 1

silverblueBP

· #3 · Jul 1, 2021

How much fuel do you keep in the tank?

Mark

daveoxide

Discussion Starter · #4 · Jul 1, 2021

patrickstapler said:
Most all vintage cars that don’t have cheater brakes uses these with good success.

11 X 2.25" Drum Brakes for Small Bearing 8" or 9" Ford Rear - Opentracker Racing

New to the market, 11 X 2.25″ drum brakes for the small bearing Ford 8″ and 9″ rear ends with the 5 X 4.5″ Ford bolt pattern, 28 spline axles and 2.43″ hub. Two drums and two loaded backing plates complete with self adjusters and emergency brake hardware. All new parts made to meet or […]

opentrackerracing.com

Quick Performance also sells them.

I currently have an 8.8 rear end in it, and it has 10x2.5” shoes. I think the last thing I need is a more powerful drum brake! Assuming that’s the intent of those drum kits…

You will want a good proportioning valve, such as a Tilton. Mount It where it’s accessible in the car while on track.

I forget the brand, but the one I have works fine. I will try adjusting it more to decrease rear brake force… I just don’t know what will happen first… if I will reach equilibrium or run out of adjustment.

Explain axle hop during braking. I have never experienced this. Do you mean axle hop during down shifting? If so, rev matching will cure that.

Just like axle hop while accelerating, only it happens while pressing the other foot pedal! If pavement is smooth and my downshifts well rev matched, it’s fine. Hit a slight bump in the braking zone to upset the rear end or miss the rev match by a few RPM on a down shift and it starts to hop. And the faster I’m going the more exciting that hop gets!

The more I think about it, I’m wondering if I should be looking at the rear suspension bits (shocks, springs) for issues and not blaming the drum brakes… 🤔

daveoxide

Discussion Starter · #5 · Jul 1, 2021

silverblueBP said:
How much fuel do you keep in the tank?

Mark

All of it 😁. I start having fuel feed/sloshing/starvation issues at around 1/4 tank level, so nothing lower than that. It will do this hop thing with a full tank of gas as well though.

patrickstapler

· #6 · Jul 1, 2021

At a minimum you should have Del-A-Lum bushings and a quality set of leaf springs. A PH bar would be better.

CJM68GT390

· #7 · Jul 1, 2021

patrickstapler said:
At a minimum you should have Del-A-Lum bushings and a quality set of leaf springs.

^^^ +++
There was a recent thread on here about leaf springs with multiple leaves on the front half to control axle hop.

silverblueBP

· #8 · Jul 1, 2021

And don’t discount quality shocks. They help control the rebound. Lots of parts working together. Sorting these cars out is a process. Sometimes it takes awhile, but once done the car can be a dream to drive in anger

Mark

Reactions: 4

StevenOrlando

· #9 · Jul 1, 2021

My long-time experience in short-track oval racing has taught me quite a bit about weight transfer and how shocks/springs affect handling.
I have a 1965 Fastback with 425hp, 360 torque that I vintage road race with SCCA, SVRA and HSR here in FL, GA, and AL.

I have the same problem, here's how I've begun to solve it:
I've tightened ( set to fully 'firm' ) my front shocks for compression. This will slow the weight transfer from rear to front end upon heavy braking.
I don't have dual-adjustable rear shocks but if I did, I'd adjust - tighten/increase the rebound on them - to further slow the transition away from the rears when braking.
The reason your rears are locking is because the weight transfer away from the rear end is too fast and this allows the rear brakes to lose traction and hop.

I have a set of dual-adjustable rear shocks so I can complete the modification.
Remember that SPRINGS adjust how much weight is transferred to/from that corner while SHOCKS adjust how FAST that amount of weight is being transferred.

Weight is always transferred diagonally, not longitudinally. Since springs and shocks are generally changed in pairs, the result of changing an axle set is weight being transferred from one end of the car to the other end.

Since you want those rears to do their fair share of braking effort, you can also increase the spring rate at the front end to reduce the AMOUNT of weight that is removed from the rears upon braking. This will help keep the rears from chattering upon heavy braking.

If no weight is being transferred to a corner of the car because the shock or spring won't allow it (being too stiff), no traction is gained when it occurs. Weight management is your friend.

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Reactions: 3

daveoxide

Discussion Starter · #10 · Jul 1, 2021

StevenOrlando, that is good feedback and info, thank you.

I replaced all the springs over 20 years ago shortly after I purchased the car. The fronts are the old Mustangs Unlimited 620 springs (I think that refers to the spring wire diameter) and the rear is the “heavy duty” leaf springs, whatever that means. Also on the rear I have poly front leaf bushings, rubber rear, panhard bar, and traction bars. Front shocks are the stiff KYB’s, rears are the GR2. Balance at street speeds is great, and this problem just reared it’s ugly head after taking it to the track.

I have a sneaky suspicion that my leafs might be getting tired. I need to check the rear shocks too, even though they were replaced not too long ago. I need to rule out what’s still good and what isn’t before diving into this further, it appears.

silverblueBP

· #11 · Jul 1, 2021

Traction bars and a panhard bar? Nope!

Mark

daveoxide

Discussion Starter · #12 · Jul 2, 2021

silverblueBP said:
Traction bars and a panhard bar? Nope!

Mark

I’m not sure what your “nope” is referring to? They act independently and each has their specific purpose… 😐

jkg2101

· #13 · Jul 2, 2021

I have a grand total of one track day on my car. 11” drums with porter field shoes. Maier leafs with more leafs up front. Hoosier Tdr race tires. Dual mastrr cylinders with balance bar. I spent some time getting the balance bar right so the fronts lock before rear. So far while braking from 100 mph I have not had rear lock or axle hop. I really like the balance bar setup but it is tough to fit. It doesn’t limit pressure to the rear like a proportioning valve. It just truly proportions

daveoxide

Discussion Starter · #14 · Jul 2, 2021

jkg2101 said:
I have a grand total of one track day on my car. 11” drums with porter field shoes. Maier leafs with more leafs up front. Hoosier Tdr race tires. Dual mastrr cylinders with balance bar. I spent some time getting the balance bar right so the fronts lock before rear. So far while braking from 100 mph I have not had rear lock or axle hop. I really like the balance bar setup but it is tough to fit. It doesn’t limit pressure to the rear like a proportioning valve. It just truly proportions

Thanks for that info. Any chance you have some track data, specifically, measured G-force values during braking? I was just going over my track data and on average, saw I was only able to achieve a max sustained reading of 0.5-0.6g during braking, with short peaks to 0.8g (probably those damn bumps!).

If anybody else has this data please share!

patrickstapler

· #15 · Jul 2, 2021

Down the front stretch entering T1 at Hallett on my fastest lap. .83 on hard braking. Almost every lap was between .77 and .82.

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Reactions: 1

daveoxide

Discussion Starter · #16 · Jul 2, 2021

patrickstapler said:
Down the front stretch entering T1 at Hallett on my fastest lap. .83 on hard braking. Almost every lap was between .77 and .82.

Awesome!!! Thank you for that screen shot too! I just went to my app and did the same graph as you (attached).

Please tell me the details of your rear brake, axle type, and suspension setup.

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patrickstapler

· #17 · Jul 2, 2021

Everything I suggested above is what I have with the exception of the PH bar. My rear brakes are the QP flavor as they came with the QP rear. Front suspension and rear leafs are all ORP. Shocks are Bilstein. Front brakes are CA. All vintage legal. Nothing fancy.

Reactions: 2

silverblueBP

· #18 · Jul 2, 2021

daveoxide said:
I’m not sure what your “nope” is referring to? They act independently and each has their specific purpose…

You don’t need those traction bars. Disconnect them next time you go to the track and see the difference.

Mark

daveoxide

Discussion Starter · #19 · Jul 2, 2021

silverblueBP said:
You don’t need those traction bars. Disconnect them next time you go to the track and see the difference.

Mark

I’ve run without them on the street and it isn’t pretty, I don’t envision the track will be any kinder! These aren’t the Shelby under riders, they are very similar to the slide-a-links in how they operate; no resistance to axle movement up and down nor at an angle. And they most definitely do the intended job during acceleration… braking not so much, but they weren’t designed for that. Without them, it sounds like I have a 200 lb basketball on the loose back there!

Russstang

· #20 · Jul 2, 2021

Sounds like you need a torque arm.

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