Members with 4 wheel disc systems.....Can you get the tires to skid?

[4406bbl]

It would be interesting to see if it would lock either end on wet pavement. Rear drums do work well because they self energize, needing less pressure, but they like to lock up in a panic stop, disc's make it easier to modulate the brakes. It is a bad feeling not to be able to lockup the brakes when stopping, even though you are probably stopping faster. I swapped over to a 1" aluminum master and single diaphram booster on my 4x4 with 35x12.50 tires, it could not lock a wheel unless it was in the snow. I finally found a correct one ton dual booster and 1-1/8" iron master cylinder for it and now it will lock the fronts on dry pavement, the rears drums do not but they are small 7/8" wheel cylinders. I am happy with it but should get rid of the factory proportioning valve, get bigger rear wheel cylinders and adjustable prop valve, but doubt I ever will.

 

[4406bbl]

This doesn't help the dilemma, but some info on braking. When a tire is skidding, it is sliding on melted rubber, greasy, which is not slow. When going through EVOC (Emergency Vehicle Operations Course) and Bondurant n Bragg Smith school of driving, the most effective braking is "threshold" braking. At the point of impending lockup but not locked. Threshold braking is shorter than ABS, ABS just allows you to steer in panic brake situations.

Correct, just got back from Radford school. They taught us how to brake just on the edge of abs with redeye hellcats. One other thing is transfering the weight to the front before you turn. I will say this, the hellcats have incredible brakes in every way. The brakes are what make that car fast, I doubt a guy could design anything that good without engineering and a track.

 

[Don Frelier]

I don't see any mention of different pads?
I've seen noticeable difference in braking on cars and mini-vans just from changing pads from semi-mettalic to ceramic to cheapies.

 

[Sixpactogo]

I'm no expert on brakes by anyone's standards but I would look at the A body booster as not being up to the task. Is it a single diaphragm or dual?
I had the same symptoms on my 68 Charger using the stock drum brake booster. They don't provide enough boost for discs no matter how much vacuum you apply. I think I read somewhere that Disc brakes need about 2.5 times as much boost as drums need. For me, I would swap out that A body booster for a B body dual diaphragm unit.

 

[Kern Dog]

That is a consideration as well. Thank you.

 

[4406bbl]

I bet your car is stopping better than it was, if you end up leaving it alone at least test it in a wet parking lot....the last thing you ever want to have happen is the rears to lock before the fronts in the rain on the road.

 

[ckessel]

Who gets to ride the creeper to check pressures when braking?

 

[68 Sport Satellite]

Kern - I'm interested to have you measure front caliper fluid pressure like you did for the rears.

 

[Kern Dog]

I have the car at the alignment shop now. I’ll measure pressures and report. A guy at FABO posted that Dr Diff suggested to GUT the distribution block. I have some spares that I can use. Of course, I’ll test pressures before and after swapping the blocks.
Having the “Motive” one man bleeder tank makes it easier to bleed the brakes when a helper isn’t around.

 

[Sixpactogo]

I'm no expert on brakes by anyone's standards but I would look at the A body booster as not being up to the task. Is it a single diaphragm or dual?
I had the same symptoms on my 68 Charger using the stock drum brake booster. They don't provide enough boost for discs no matter how much vacuum you apply. I think I read somewhere that Disc brakes need about 2.5 times as much boost as drums need. For me, I would swap out that A body booster for a B body dual diaphragm unit.

I just googled the question of how much pressure is required for disc brakes as opposed to drum brakes. The answer was drum brakes require about 400 lbs while disc brakes need closer to 900 lbs.
There is a formula for brake boosters and it uses a single 8" booster for reference. The math works like this......4" X 4" X 3.14=50.24 square inches X 8=401.9 pounds of force. A dual diaphragm booster doubles it to 803.8 pounds. It also said 4 wheel discs require a dual diaphragm booster.
That said, a dual diaphragm 9" booster provides 1144.5 lbs.
I don't think you want the same pressure going to the rear wheels as you have to the front. You will be locking the rears up way too early since when you dynamite the brakes, the weight goes to the front and will put you in a tail spin.

 

[Nxcoupe]

This doesn't help the dilemma, but some info on braking. When a tire is skidding, it is sliding on melted rubber, greasy, which is not slow. When going through EVOC (Emergency Vehicle Operations Course) and Bondurant n Bragg Smith school of driving, the most effective braking is "threshold" braking. At the point of impending lockup but not locked. Threshold braking is shorter than ABS, ABS just allows you to steer in panic brake situations.

Exactly. Abs keeps it on this threshold, it involves going along the line between static and dynamic friction.

 

[Nxcoupe]

I just googled the question of how much pressure is required for disc brakes as opposed to drum brakes. The answer was drum brakes require about 400 lbs while disc brakes need closer to 900 lbs.
There is a formula for brake boosters and it uses a single 8" booster for reference. The math works like this......4" X 4" X 3.14=50.24 square inches X 8=401.9 pounds of force. A dual diaphragm booster doubles it to 803.8 pounds. It also said 4 wheel discs require a dual diaphragm booster.
That said, a dual diaphragm 9" booster provides 1144.5 lbs.
I don't think you want the same pressure going to the rear wheels as you have to the front. You will be locking the rears up way too early since when you dynamite the brakes, the weight goes to the front and will put you in a tail spin.

Yep, agree that an adj proportioning valve is in his future but for now he needs to see what's happening in the system and then any changes he makes can be quantified rather than being a subjective appraisal.
I think the booster is probably the culprit. Shame the hydroboost didn't pan out. I have hydroboost in a drum disc car and it throws me theough the windshield. I'm swapping to rear discs and a larger bore master cylinder. I really hope you get this to your liking.

 

[Runner 68]

Here are a few combinations that I've tried:
11" front disc 2.6" caliper, 10" rear drum. 1975 A body brake booster. Disc/drum proportioning valve.
12" front rotor, 2.6" caliper, 10" drum. 1975 A body brake booster. Disc/drum proportioning valve.
12" front rotor, 2.6" caliper, 11.7" Dr Diff rear discs with 1.5" single piston caliper. 1975 A body brake booster. Disc/drum proportioning valve.
12" front rotor, 2.75" caliper, carbon metallic pads. 11.7" rootr with 1.5" single piston caliper. 1975 A body brake booster. Disc/drum proportioning valve.
Same as above without booster but with 15/16" iron, 15/16" aluminum, 1 1/16" and 1 1/8" manual master cylinder. Disc/drum proportioning valve.
12" front rotor, 2.75" caliper, 11.7" rear rotor, 1.5" single piston caliper, drum drum distribution block, 1975 A body brake booster, modified pedal for increased leverage.
13" front rotor, twin 1.58" piston calipers, 11.7" rear rotor, single 1.5" caliper, drum-drum distribution block. Hydroboost unit with 1 1/8" master cylinder.
13" front rotor, twin 1.58" piston calipers, 11.7" rear rotor, single 1.5" caliper, drum-drum distribution block. 1 1/8" manual MC.
13" front rotor, twin 1.58" piston calipers, 11.7" rear rotor, single 1.5" caliper, drum-drum distribution block. 15/16" manual MC.
13" front rotor, twin 1.58" piston calipers, 11.7" rear rotor, single 1.5" caliper, drum-drum distribution block. 1975 A body booster, 1 1/8" MC.
13" front rotor, twin 1.58" piston calipers, 11.7" rear rotor, single 1.5" caliper, drum-drum distribution block. 1975 A body booster, 15/16" MC.
13" front rotor, twin 1.58" piston calipers, 11.7" rear rotor, single 1.5" caliper, drum-drum distribution block. 1975 A body booster, 15/16" MC, electric vacuum pump.
Now the current setup is:
13" front rotor, twin 1.58" piston calipers, 11.7" rear rotor, single 1.5" caliper, drum-drum distribution block. 1975 A body booster, 15/16" MC, electric vacuum pump and storage tank.

Yeah....I have tried LOTS of combinations.

Since you've done so many combos Kern, do you see an issue with 13" rotors all the way around?
I know that less rotor is needed in the rear. My reasoning for 13s all the way around is simply aesthetic. 17" wheels up front, 18" in the rear.
I think a 12" inch rotor in the rear with a one inch larger rim would look a little lopsided when compared to a 17 inch rim with a 13" rotor.
Thoughts everyone?

 

[Kern Dog]

If it is an appearance issue, I can understand the desire to have equal sized rotors. You could dial back the rear bias with an adjustable proportioning valve so the car doesn't lock the rear brakes too easily.

 

[6872n73]

Diameter of the rotor
Friction material- coefficient of friction
Area of friction material
Area of piston calipers
Area of master cylinder
Pedal ratio.

 

[66 Sat]

You've got the other Charger to compare.

Could you find a long quiet stretch and do some actual measurements of stopping distance between the 2 cars?
Admiittedly difficult to get to the exact same speed in both if your speedos are as inaccurate as mine (but could use GPS if you have a smart phone or watch, or maybe calculate off the rpm/gearing), but would be an interesting comparison nevertheless.

Just pace out the distance and see how much difference between the two cars.

Completely unrelated to wanting to be able to lock up the wheels, but would give you some peace of mind if the red car stops considerably shorter.

 

[Tony Tee]

Did you try a bendix dual diaphragm brake booster yet?

 

[Cranky]

What happened with the failed hydroboost system? And weren't there any vehicles that used a 4 wheel system without a booster? I know pedal ratio is important too. Also know that there were plenty of front wheel disc systems that didn't use one......

 

[68 Sport Satellite]

I just googled the question of how much pressure is required for disc brakes as opposed to drum brakes. The answer was drum brakes require about 400 lbs while disc brakes need closer to 900 lbs.
There is a formula for brake boosters and it uses a single 8" booster for reference. The math works like this......4" X 4" X 3.14=50.24 square inches X 8=401.9 pounds of force. A dual diaphragm booster doubles it to 803.8 pounds. It also said 4 wheel discs require a dual diaphragm booster.
That said, a dual diaphragm 9" booster provides 1144.5 lbs.
I don't think you want the same pressure going to the rear wheels as you have to the front. You will be locking the rears up way too early since when you dynamite the brakes, the weight goes to the front and will put you in a tail spin.

Not saying I agree or disagree, but you may want to double-check those pressure requirement norms. I watched a few brake videos where someone had a similar issue. He claimed he needed at least 1200 psi at the front and 900 rear based on his research. When he measured he had 900 psi at the front and he was having poor braking. He was running a big cam and after measuring the low pressure slapped on a Wilwood hydroboost and BAMM - good brakes. Remember the gauge is in kPa so you need a multiplier to convert to psi.

 

[4406bbl]

I agree it will need more pressure, a front brake hold off valve as way to get the rears on slightly ahead of the fronts to transfer weight, and once it will skid either an adjustable proportioning valve. How to get the pressure up is the hard part. I would not gut or eliminate the warning light permanently on a street driven car for legal reasons if anything ever would happen.