Desired front/rear brake balance - per GM early 1960s

[Tmoose]

The attached pdf is a page from some SAE paper from the very early 1960s.
Author is the Corvair's chief designer,. K. H. Hansen.

I don't believe I've ever seen another recommendation that rear wheels should lock up before the fronts.

I wonder if it really was a 1960s thing, or if K.H. Hansen had at most a curiously limited willingness to cater to average drivers' skills.

 

[BrianPetersen]

Interesting. This would have been back in the days before dual-circuit brakes and, I think, before the common use of proportioning valves. It's also before the days of Ralph Nader, and society was much less lawsuit-prone at the time. Expectations of vehicle safety and handling were lower.

The brake force distribution is pretty close to the effective weight distribution during a 1/2 g stop, which appears to be their design criteria. The situation was probably that when the rear wheels locked up first, the front wheels were on the verge of doing so. On slippery surfaces there would be less weight transfer, so then the front wheels would have locked first.

I had a 1970s-era Honda Civic, and that one locked the rear before the front (personal experience). It went sideways pretty easily. This is one situation where a rear engine is an advantage over front wheel drive. There was so little weight in the back of the Civic that in the absence of proportioning valves, the rear was almost sure to lock first in a hard stop. I don't recall off-hand but I don't think it had proportioning valves. My next vehicle, a 1980s Toyota pickup, had a load-sensing proportioning valve operated by a linkage from the rear axle.

 

[DAVIDSTECKER]

I have found that my 1999 Suburban front brakes will lock up first on snow and ice if I leave it in drive.
If I put it in neutral, I have more control.
I have a 1993 manual pick up which is where I got the idea to put the automatic in neutral.
It takes the extra force off the brake shoes.

 

[Heaviside1925]

I am wondering if that article is more of an official engineering justification for the weight imbalance of the rear mounted engine. There seems to be an implied assumption that the car will handle better with the majority of weight over the rear and with the rear brakes locked, the majority of the mass of in the rear, that handling will not be an issue even though it's admitted a sliding wheel can take no direction.

 

[GregLocock]

Even in the absence of proportioning valves you can still alter the cylinder diameters to get a more reasonable brake balance. The justification he gives for locking the rears first is directly opposite to the way we are taught to drive, but back then they did a lot of daft things. (Shuffle steering etc)

Cheers

Greg Locock


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[TugboatEng]

I'm thinking if you're going to lock one wheel you want it to be a rear. Locking one front will steer the car hard to the opposite side.

Keep in mind that this is in the days of drum brakes and the contribution by each wheel could be highly variable due to differences in adjustment.

To contort the words of Charles Kettering, you're looking at a drum brake problem with a disk brake mentality.

 

[BrianPetersen]

Yeah, when you are talking about 4 wheel drum brakes, whatever braking you got was probably plus or minus 50 percent anyhow. I recall reading a review of one particular car with 4 wheel drums, and the complaint was that the two wheels on the (I think) left side would lock first in their braking tests, leading to a pretty unstable situation.

 

[TugboatEng]

Ok, I dug up the Kettering quote. This is in reference to the development of the EMD 567 series engine.

Many years before, in the 201A days, that is in the early days of the 2 cycle engine, pressed in bronze bushings were tried for piston pins. A considerable number of designs were played with and, if I remember correctly, a pin of 4 Inch diameter was tried with the most elaborate grooving imaginable. None of the Plain bearing variations was made to run with any success until along about 1940 when somebody who did not know that bronze bearings would not work tried them again in a 567 engine. When this attempt was made the person used two cycle brains instead of four cycle. He simply made the bushing so that it would not be pressed in the rod, but would float in the rod and he gave the bushing a great deal of clearance on both the inside diameter and the outside diameter. This is possible because there is no piston pin reversal in a 2 cycle engine.

 

[waross]

The initial proportion of front-rear braking force is determined by the relative diameters and numbers of hydraulic cylinders, both in the dual master cylinder and in the wheel cylinders.

Combining Strategies – The Misnamed Proportioning Valve

Conventional proportioning valves should really be referred to as “braking force regulators” or “brake pressure regulating valves.” While their name might imply true proportional control, in reality they provide a combination of the control found in Figures A and B.

Up to certain pressures, these valves allow equal pressure to both the front and rear brakes (à la Figure A). However, once a preset pressure point is reached (600 psi in the example), the rear brake pressure continues to build, but at a slower rate (or slope) than the front brake pressure. Figure C displays this for us quite clearly.

You will find a true proportioning valve on semi trailer tractors.
It is called "Bob-Tail Valve" rather than a proportioning valve.
When a semi tractor is running without a trailer the back end is very light and prone to locking up.
Hence the Bob-Tail valve.
This is a true proportioning valve and directs a much higher proportion of the braking force to the front wheels when a trailer is not connected.
The driver cannot move a trailer before he applies "Service Air"to the trailer brake system.
Applying service air disables the Bob-Tail valve automatically.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[LionelHutz]

Bill - a dual master cylinder wouldn't have been used in 1960. The 4 wheels all get the same brake pressure.

 

[waross]

Was here any requirement for the front and rear wheel cylinders to be the same diameter or for the front and rear brake drums to be the same diameter or width?
The master cylinder wasn't the only cylinder.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[BrianPetersen]

It says in the article that the wheel cylinders were different diameters. GM knew a thing or two about brake balance, even back then.

I don't think proportioning valves were in use in that era, so it would have had fixed front-rear brake balance, picked at a certain representative deceleration (looks like 0.5 g).

 

[waross]

Many so called proportional valves are not proportional valves.
There is a valve that is common on domestic cars that. as I understand it, does not allow pressure to the back wheels until a small pressure has been applied to the front wheels.
There are few true proportional brake valves in use. Most of the valves that are called proportional valves to not perform a proportioning function.
Some so called proportioning valves retard the onset of rear braking slightly, but not proportionally.
Some so called proportioning valves retard the maximum pressure of rear braking slightly, but possibly not proportionally.
There may be a need for a true proportioning valve on a truck that is either heavily loaded or unloaded and very light on the back.
Unfortunately this need is seldom addressed.
There are available adjustable, true proportioning valves but though I have seen a catalogue listing I have never seen one in the wild.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[BrianPetersen]

Nowadays, ABS mostly renders all this obsolete. There will be a front-rear balance applied just like in the old days (determined by relative piston sizes front and rear, with the same hydraulic pressure applied throughout) but if ABS or ESP or EDL (electronic differential lock!) determine that something should be done differently, the ABS hardware is used to either reduce applied hydraulic pressure or apply extra hydraulic pressure to individual wheels.

Then there's regenerative braking ... My Chevy Bolt has a normal master cylinder and ABS module but it has an electric servo actuator in place of the usual vacuum (or hydraulic) power-brake servo. If you apply brake pedal lightly in normal driving, it uses regenerative braking via the drive motor to do the braking and it uses that electric actuator to simulate the normal brake pedal feel, and the hydraulic brakes are not applied until you actually come to a stop. If you stomp on the brake pedal (panic stop), your applied force on the pedal powers through whatever that actuator does, and the hydraulic brakes take over. If ABS activates, it de-activates regenerative braking and goes into normal hydraulic braking with ABS activation. If there's a big failure of on-board electrical control systems, you stomping on the brake pedal will power through the actuator and apply hydraulic brakes as normal. This all works quite well, and the brakes feel like those in a normal car.

The de-activation of regenerative braking and use of the normal friction braking when ABS activates, is probably because with the car being front wheel drive, regenerative braking can't be applied to the rear wheels. Doesn't make a whole lot of sense to have front braking limited by ABS while the rear brakes aren't contributing anything.

 

[GregLocock]

These things tend to have a 2 stage proportioning effect, backing off the rear brake at high decels to account for weight transfer

Cheers

Greg Locock


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