Rear Drum Brakes For Track Use

4956 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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41 - 53 of 53 Posts

TXBoss302

· #41 · Aug 26, 2021

Now need to know M/C bore size, pedal ratio and rear wheel cylinder bore size

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daveoxide

Discussion Starter · #42 · Sep 5, 2021

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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daveoxide

Discussion Starter · #43 · Sep 5, 2021

TXBoss302 said:
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!

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patrickstapler

· #44 · Sep 5, 2021

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".

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daveoxide

Discussion Starter · #45 · Sep 5, 2021

patrickstapler said:
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. 😂

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jkg2101

· #46 · Sep 5, 2021

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

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daveoxide

Discussion Starter · #47 · Sep 6, 2021

jkg2101 said:
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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patrickstapler

· #48 · Sep 6, 2021

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.

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silverblueBP

· #49 · Sep 6, 2021

I have this in my car.

Tilton Lever Style Brake Proportioning Valve - Pegasus Auto Racing Supplies

The compact Tilton Brake Proportioning Valve allows a driver to rapidly select 7 distinct and repeatable settings for brake bias. Easy installation.

www.pegasusautoracing.com

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

Mark

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daveoxide

Discussion Starter · #50 · Sep 6, 2021

patrickstapler said:
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...

Summit Racing SUM-760254 Summit Racing™ Adjustable Brake Proportioning Valves | Summit Racing

Free Shipping - Summit Racing™ Adjustable Brake Proportioning Valves with qualifying orders of $99. Shop Brake Proportioning Valves and Distribution Blocks at Summit Racing.

www.summitracing.com

And now have this one on the way...

Wilwood Disc Brakes 260-8419 Wilwood Brake Proportioning Valves | Summit Racing

Free Shipping - Wilwood Brake Proportioning Valves with qualifying orders of $99. Shop Brake Proportioning Valves and Distribution Blocks at Summit Racing.

www.summitracing.com

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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daveoxide

Discussion Starter · #51 · Sep 6, 2021

silverblueBP said:
I have this in my car.

Tilton Lever Style Brake Proportioning Valve - Pegasus Auto Racing Supplies

The compact Tilton Brake Proportioning Valve allows a driver to rapidly select 7 distinct and repeatable settings for brake bias. Easy installation.

www.pegasusautoracing.com

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

Mark

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...

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silverblueBP

· #52 · Sep 6, 2021

daveoxide said:
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...

When I had stock size rear drums it was all to rear (full rear pressure) or up one. depending on fuel level and grip. With the current 11”x2 1/4” drums it’s the least rear brake pressure or one up. It will vary with how your weight is distributed. My car likes the full size battery over the RR and full fuel load. That is where it drives best for me on Blue Streaks. Once I got it sorted out, I rarely need to change it.

** Correction, my lever is Wilwood not Tilton. Stupid memory.

Mark