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Rear Drum Brakes For Track Use

4956 Views 52 Replies 10 Participants Last post by  silverblueBP
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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Most all vintage cars that don’t have cheater brakes uses these with good success.

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… 🤔
How much fuel do you keep in the tank?

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.
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.
Traction bars and a panhard bar? Nope!

I’m not sure what your “nope” is referring to? They act independently and each has their specific purpose… 😐
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!
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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You don’t need those traction bars. Disconnect them next time you go to the track and see the difference.

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!
You have problems with your rear. A 65-66 does not need traction bars of any type if you have good springs, shocks and a locating device. Not one single person that I’ve raced with has traction bars.

The more responses I read the more it appears this may be the case. I guess I will have to dive deep into the rear suspension bits and give everything a thorough once over.
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I don’t have lowering blocks per se but I do have small angle wedges installed to take care of a pinion angle issue. I’ve wondered if that extra bit of distance from the springs (~1/2”) is adding to the rotational forces the springs need to resist as the axle winds up. Might have to think about this relationship in more detail.

I’ve got a panhard bar already yes, and it’s addition was very noticeable and very positive to the driving experience. I’ve got poly bushings in the front of the spring eye, and they are VERY hard… not aluminum hard, but stiff nonetheless.

Based on everyone’s input thus far, I am questioning the integrity of my 20 year old leaf springs and couple year old shocks. One thing I noticed recently is when I jack up the rear of the car, by placing the jack under the center diff, the right rear tire lifts off the pavement about an inch before the left rear tire starts coming up. When it’s on the ground, the fender wells are the same distance from the ground, but when the rear end is in the air, the right rear tire is tucked further up in the well (with the jack under the differential). I’d be curious if anyone else sees this phenomenon with their own vehicle…. It could be nothing, but I won’t know until I rip everything apart.
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I have angle shims and a 1” drop block. No problems. What type rear springs do you have?

Good to know about the spacers/wedges, thanks!

Current rear leafs are “heavy duty” 4 leaf standard eye springs, and are 20 years old with at least 75k of not so kind miles on them. 😇
A quick update… I went ahead and replaced the old leaf springs with 5 leaf units. My shocks were still good, so those stayed. It can brake hard now with no more wheel hop, which is great… but the rears still lock up way before the fronts. My proportioning valve knob is cranked all the way decreased, which I presume is least pressure to rear brakes? I will double check this just in case my logic is incorrect.

If I assume my brake bias knob is cranked the correct direction, then my next step is to look at the front brakes. I’m wondering if the front brakes aren’t creating enough friction; cooked pads, glazed rotors? Gonna have to dive into the front brakes and check them out. If somebody knows of a place to get better brake pads for Granada brakes, please let me know!

Also, I didn’t find any cracked leafs or anything major, but I did notice that the drivers side leaf was de-arched more than the passenger side, see pic.
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Thank you all for your input! I just finished swapping in some bigger front brakes onto my Granada spindles (a project I have been working on for some time). So instead of going with a different pad compound I am now running 12.5” rotors and dual piston calipers from a 2011-2014 V6 Mustang (2005-2010 Mustang GT brakes). I have lots of front brake grab now, yet under very hard braking, the rears still lock up first.

I have also confirmed that my adjustable proportioning valve is cranked all the way to reduce rear brake pressure… but I found something out. My adjustable valve can only reduce rear brake pressure a max of 35% (of total system pressure). I looked up the Wilwood valve and it can reduce all the way down to 57%. I am going to order the Wilwood valve and swap it in to see if does the trick. If not, like others have suggested, I might need to play with different rear wheel cylinder sizes to dial back rear brake force pushing on the shoes.

Thanks for the ideas and I’ll let ya’ll know how it goes!
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Quick question - Have you ever had good brake balance?
This is actually a very good question… with a not so easy answer.

I have been running the Granada setup for over 20 years, with three different variations of rear ends (meaning three different rear brake setups). I have never been able to lock up all four tires on clean dry pavement with sticky rubber tires as far as I can remember… but it was also not something I really tried doing on the street either! There have been a few occasions where I’ve had to stop rapidly on the street and the rears would always lock first, so I would regulate brake pressure accordingly using that as my threshold limit.

My few recent track days have really brought this issue front and center… my braking distance is WAY worse than any other car on the track to the point I feel like I may cause an accident as I have to start braking on Tuesday for my track day on Friday.

Short answer is… for the street it was fine/tolerable, for the track, nope!
I fear that if the new proportioning valve does not solve your problem, you need stop looking at the rear and concentrate on the front. I have never heard anyone at the track say..."man these Granada front brakes are the bomb".
Ha! And that is why they are no longer on my Mustang! The only Granada part left is the spindle… all the other stop quick bits on the front end are now late-ish model Mustang parts, which are MUCH more capable than grandmas Sunday cruiser brakes. 😂
just some thoughts - locking up brakes at medium to low speed on the street may not be a good indicator of track performance. most any car should be able to lock up things a bit at lower speeds. on a front-heavy mustang, weight shifts forward under braking, and locking up the rear brakes is always easy to do if the brake balance is set too far rearward. as an example, i have to set my balance bar far to the front to get my rears not to lock up first. i have strong wilwoods up front and large drums in the rear. front brake performance is related to pressure on the front rotors - a large single piston caliper should be able to generate a lot of pressure if used with a booster. sticky tires may resist lock-up better. real brake performance is probably best measured with stopping distances from higher speed, and under repeat efforts, where brake fade is a bigger issue. brake fade depends on the system's ability to absorb and manage heat.... so many confusing issues when it comes to brakes.

PS - i hate the pedal feel and performance of the old fashioned sliding single piston calipers. modern calipers and systems do feel so much better although it is not always clear to me why that is the case
The braking performance was fine for street driving, problems arose when I took it to the track and drove the thing like it has never been driven before! I’m not normally the type of driver to go from 100mph down to 30mph every minute or so (with a lot of intermediate braking in between)… the track really brings to light what is and isn’t going to work at that duty cycle!

Currently, with the larger front brakes, the rear tires start to drag first but if I apply just a smidge more brake pedal pressure the fronts will lock up as well. I feel like I need just a few more percent of brake bias towards the front and it will be good. These recent tests were done from 60-70mph on a deserted back country road, on warm pavement, with warm tires, and a clean/smooth road surface. The track conditions are much less forgiving, so my hope is if I can dial it in for my street test I should be at a much better starting point to adjust it for track use.

And I totally agree with you regarding the modern brake systems. I am wondering if the drum brakes are the leading cause of the slightly mushy, not 100% repeatable pedal feel compared to the modern systems. Drums will grow in diameter as they heat up causing an increase in pedal travel, and they will have the tendency to flex and distort under heavy forces… all things a rotor won’t do.
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I’m not sure what proportioning valve you are using now or the one you are changing to. However, the Tilton valve I use has a automatic diversion circuit for threshold braking. Essentially what happens is the valve allows pressure to the rears under normal braking. During threshold braking a larger portion of that pressure is diverted to the front.
I thought that is essentially how the adjustable valves worked... taking a chart from the model Mark is using...

Brake pressure is equally distributed in the system up to a point, the "knee" if you will, and then outlet pressure only increases a small percent over inlet pressure past this "knee".

I first grabbed this Summit special one...

And now have this one on the way...

I had no idea how much pressure reduction I would need to dial it in, but apparently its more than the 37% bias the Summit one allows. That Tilton one looks like it can reduce bias by ~58% of total pressure, just like the Wilwood one. That Summit one would only reduce pressure to half way between position 3 and 4 on the Tilton model, I need those few extra positions to dial mine in.
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I have this in my car.

It takes all the guesswork out of, where the “knob” is. My version is 20 years older without the fancy wording

For curiosity sake, what position is your lever in?

And hopefully your mind isn't in the gutter like mine is otherwise your answer might not be very helpful... :LOL:
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