Members with 4 wheel disc systems: Can you lock your wheels?

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Dale Davies said:
Without ABS you should be able to lock all four wheels on dry pavement in a "panic" stop. It should be difficult to lock the rear brakes. You do not want the rears to lock first as the back then wants to pass the front.
Brake pressure is a finction of the pedal ratio and booster multiplication. Frequently the balance or bias valve is to reduce rear pressure preventing rear lockup. Drum diameter and widyh or rotor diameter and pad diameter work in conjunction with piston bore diameters to create brake torque or force. The master cylinder diameter converts the pedal force to line pressure. Wheel cylinder area converts the pressure to pounds force.
Standard accepted on our old cars is that under braking with weight transfer, the fronts do 70% of the breaking, on average. The front wheel drive cars shift that to 80% to 95%. Rear engine cars with more weight at the rear shift this closer to 50% to 60%.
In your case you should be able to lock the fronts but not quite the rears. Modern tires have considerably more grip than the skinny bias plys when these cars were new, which combined with wider tires creates more force on the brake surface to lock them.
You need to compare front and rear line pressures out of the differential valve and multiply by the piston area of the caliper. This will give the force applied to the drum or disc. Once. You know the line pressures I would talk to the component supplier for advice.
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I could not have said this any better except to add that if you go buy all this mondo brake stuff and can't lock the fronts, you're wasting your time. If you can't lock the front brakes, you're leaving stopping ability on the table.
 
jos51700 said:
I could not have said this any better except to add that if you go buy all this mondo brake stuff and can't lock the fronts, you're wasting your time. If you can't lock the front brakes, you're leaving stopping ability on the table.
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True. It is just a question of force applied to the brake pedal, unless something is bottoming-out so you are no longer pressing on just the hydraulic fluid. Might need to use both feet if poor boost with disk brakes.
 
The pedal has full travel. I've ran hydroboost, manual and vacuum boosted setups which required bleeding each time. The pedal goes to the floor when the bleeders are cracked.
 
jos51700 said:
I could not have said this any better except to add that if you go buy all this mondo brake stuff and can't lock the fronts, you're wasting your time. If you can't lock the front brakes, you're leaving stopping ability on the table.
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For stability under braking, the fronts should lock a little before the rears. Once the tires are skidding adhesion is lost and you have less ability to stop.
The opposite effect is acceleration as in drag racing. If you break traction and smoke the tires, the other car wins.
Proper brake setup is a delicate balance to not quite allow the rear brakes to lock when the fronts lock. This affords the driver notice to ease off the brakes very slightly to get the fronts rolling and maintain the ability to steer. Better to drive around something or hit where damage is minimized.
 
I do not know what the problem was with the Hydroboost you tried. I have two friends, brothers, that installed Hydroboost on their '57 and '58 Chev p-ups and they worked great.
Kern Dog said:
The pedal has full travel. I've ran hydroboost, manual and vacuum boosted setups which required bleeding each time. The pedal goes to the floor when the bleeders are cracked.
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The one I had wore out 3 P/S pumps during the short period where I tried to bleed the system. I never could get the HB system to provide boost to the brakes.
 
I've never used anything other than manual on old cars. What's the benefit of boost?
 
I had to lock em up on a Dakota vs deer.
Deer survived, Dakota got $1400 damage.
The 4 skid marks were on the road for a month or so.
I put a grill guard on shortly afterward.
 
Might mention that on the cable show (since retired) Fantomworks, a test they did on every car was to see if all 4 tires left skid marks. They didn't go further and judge the front-rear bias. I've heard for that, such as adjusting a rear proportioning valve (aftermarket w/ knob), you set so the fronts skid just slightly before the rears skid, in a wet parking lot, though never done that (have such valve on 2 cars, but not driving them).
 
My 2007 Ram is anti-lock on the rear only. I've skidded the fronts before while feeling that ABS pulse in the brake pedal that is the system cycling pressure at the rear.
Every classic vehicle that I have except this Charger has the ability to skid the tires.
This car does stop well but it isn't to the point where I'm so impressed, I'd want to brag about it.
I think that it may come down to the brake booster. It is the single diaphragm 73-76 A body unit designed for front disc, rear drum.
I suspect that the stock 1966-70 B body dual diaphragm booster would be an improvement since it has double the diaphragm area and is spec'd for heavier B body cars.

PST booster.jpg
 
Definitely need a booster in a big RAM pickup. When the booster diaphragm ripped in my 1965 Newport, I noticed it when I rolled thru a stop sign, pushing hard on the brake pedal. Would have had to use both feet and press like on the gym machine to stop. That is with drum brakes which have a self-amplifying effect. Disk brakes would take even more force to stop.
 
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Kern Dog said:
My 2007 Ram is anti-lock on the rear only. I've skidded the fronts before while feeling that ABS pulse in the brake pedal that is the system cycling pressure at the rear.
Every classic vehicle that I have except this Charger has the ability to skid the tires.
This car does stop well but it isn't to the point where I'm so impressed, I'd want to brag about it.
I think that it may come down to the brake booster. It is the single diaphragm 73-76 A body unit designed for front disc, rear drum.
I suspect that the stock 1966-70 B body dual diaphragm booster would be an improvement since it has double the diaphragm area and is spec'd for heavier B body cars.

View attachment 1716112667
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My experience with the Early KB dual diaphragms, is not good.
Mine was lazy and late. It required more vacuum to work, and it generated less assist, than the later F-body single D.
I took it apart and checked it out, and there was nothing wrong with it, so I bagged and shelved it.
BTW
as you probably know, the booster is just an assist-device. It amplifies whatever pedal-force your brake-foot generates in direct proportion. The booster does not increase the ability of your brakes to create more speed reduction.
What I mean is this;
if your master cylinder is able to generate a maximum of 1000psi into the brake lines, by your maximum two-footed pedal-application without the booster; then that may be all it will generate With the booster..... except it will just need less pedal pressure to do it.
However
by varying the length of the pushrod, I have found that it is possible to change the timing of the assist relative to the pedal-stroke; AND this will or can, change he amount of assist, early in the pedal-stroke.
However,
if you make the pushrod too long, you run the risk of breaking the control valve in a panic situation when your Adrenalin is apt to increase your leg-power, beyond the usual. When the control valve breaks, in my experience, there is no longer any ability to modulate the amount of assist. In my case any pedal application, generated maximum assist.
In any case;
the pushrod length has to stay in a fairly narrow range, such that the compensating port always remains open with the pedal parked.
If your p-rod is not adjustable, You can fake it by spacing the M/C away from the firewall. This will delay the movement of the M/C piston, which will increase the control-valve opening inside the booster, causing it to increase assist. The downside is the extra pedal travel before you get that assist.
In my experience;
it should take about 1" of pedal-travel before assist begins. But assist should remain progressive and easy to modulate.
BTW
I don't know anything about your truck; but in some vehicles in the past, the pedal ratio was different between those with boosters as compared to those without.
and
Some boosters require more vacuum to work properly than others. If the threshold is low enough, you will get assist earlier.
When I tried the KH DD, I had the 292/292/108 cam in my 367, that idled around 10" vacuum, which provided zero assist. and it did not provide any serious assist until about 13 inches.
I have a manual trans.
Thus when coming to a stop, clutch pedal down, and engine returning to idle/vacuum dropping, I would lose assist at/during the stoplight wait. On hills this was very annoying, so I installed a hill-holder. As soon as the vacuum was restored,by driving away, it worked fine.
Eventually I was annoyed enough to try some other boosters I had kicking around. The F-body booster worked the best and it has remained on the car, during the following two cam-swaps, since IDK maybe year 2001. The hill-holder has also stayed, doing it's primary function of being a line-loc valve.

Hope this helps.
 
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Sorry, guys....I never closed this one out.
I did order a 1966-70 B body dual diaphragm booster from Dr Diff and swapped it in a couple weeks ago.
Holeeee crap, the car stops great now. I was finally able to skid the front tires on dry pavement. The pedal effort is slightly reduced but the brake grip is amazing. I have 275-40-18" Nitto NT 05 tires with a soft 200 treadwear rating. The tires are sticky.


NITTO 2.jpg
 
Kern Dog said:
Sorry, guys....I never closed this one out.
I did order a 1966-70 B body dual diaphragm booster from Dr Diff and swapped it in a couple weeks ago.
Holeeee crap, the car stops great now. I was finally able to skid the front tires on dry pavement. The pedal effort is slightly reduced but the brake grip is amazing. I have 275-40-18" Nitto NT 05 tires with a soft 200 treadwear rating. The tires are sticky.


View attachment 1716141878
Click to expand...
Pizza cutter tires.
 
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