Brake Bias Bar for Street Car 2

[swall]

My younger brother is contemplating building a Cobra kit car and at the moment, he is stuck on the idea of using two master cylinders and a brake bias bar to handle the proportioning. After purusing Gillespie's book, "Fundamentals of Vehicle Dynamics", I have concluded that while bias bars may be fine for the track, where the braking conditions are fairly consistent, this would not be a good idea for a street vehicle, where there is a much more variable range of braking conditions that will be encountered. I'm thinking he should stick with a single master cylinder and use a hydraulic prop valve.Comments?

 

[Fabrico]

Why doesn't he go all the way and use drum brakes too?

Bias bars are not recommended for the street by the various manufacturers. If he insists on using one, he better make darn sure one M/C will work if the other doesn't. Since there is no power boost, he will be messing with M/C sizing for a while, not to mention standing on the pedal from time to time.

The hot setup now is to use an OEM style Hydro-boost unit. When setup right, they are a little more consistant and predicatble than a vacuum boost unit. Not quite as ugly either. One place for info is http//:

http://www.vancopbs.com

 

[patprimmer]

It is possible to set up power boost with two master cylinders.

A balance bar is pretty much useless as a day to day tuning tool for a daily driver as conditions do vary a lot in road use.

As mass produced cars are normally designed by a team of qualified engineers with extensive resources and subjected to extensive pre release testing, a home builder could not hope to improve the overall compromise re brake balance, HOWEVER, for a one off kit car, a balance bar might save extensive costly experimentation with various calliper bore sizes in order to get a good average compromise. Once an acceptable compromise was reached, I would not recommend changing the balance as it will change the characteristics and will catch the driver unawares.

Unless they have changed a lot recently, I do not like proportioning valves as they kick in at a specific line pressure which is only appropriate at one specific load and traction condition. A system properly balanced with appropriate calliper bore sizes will be much more tolerant of varying conditions.





Regards

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[MikeHalloran]

I'd try adapting an OEM ABS system.



Mike Halloran
Pembroke Pines, FL, USA

 

[swall]

Great input guys! Pretty much confirmed what I gleaned from Gillespie's book. BTW, I used to work for the brake company that invented Hydroboost, and had already suggested that option to my brother. I don't know why he is so fixated on brakes at this point--he hasn't even bought the Cobra frame yet. I also thought about ABS, although since I've been out of the brake business for about 15 years, I don't know which units are the hot set-up today. Back then, the Delco ABS, which used variable displacement pistons instead of solenoids was thought to be a good system by some of our engineers, even though it was a competitor's product.

 

[NormPeterson]

My younger brother is contemplating building a Cobra kit car and at the moment, he is stuck on the idea of using two master cylinders and a brake bias bar to handle the proportioning.

Is he envisioning the use of a race car dual m/c setup or cobbling one up on his own based on OE bits?

I suspect that he's heard horror stories involving replicas thrown together with mismatched components (and little or no regard for the engineering) that ended up with absolutely horrible brake balance. Even some of the one-off cars considered "good enough" to attract magazine attention have suffered in this department.

Loosely related to the issue raised by Fabrico is what happens if the piston travels are significantly different in normal operation - will the balance bar details accommodate this without binding?

While a home builder is unlikely to be able to improve the overall average compromise found in any mass-produced car that's left in reasonably stock configuration, it's entirely possible to make improvements if the individual's own usage, vehicle loading, brake pads, tires, etc., differ much from the OE assumptions. Certainly this is the case for at least one of the more popular Cobra replicas, which uses a late-model Mustang as the donor (and adds empty weight, CG height, and wheelbase to the above list of variables when rebalancing the system).

I wouldn't think that an all-manual disc system would be unmanageable in an under 2500 lb car. Certainly the OE nonpower disc/drum setup on one of my cars during the 70's was at least reasonable in terms of pedal effort. Unusually small drivers might have objected to the additional effort as would have likely resulted from a swap to a disc/disc setup in that car, but even that could be mitigated by detail design.

Norm

 

[swall]

MikeHalloran--What about just using an RWAL off a pickup instead of going to four wheel ABS?

 

[thundair]

How about a PROPORTIONING VALVE

It will help with the differnt brake size and requirements from front to rear.

Cheers

 

[MikeHalloran]

Yeah, I guess that would be simpler to do.

I was thinking that ABS makes more sense on a flyweight, big tire car like a Cobra than on almost anything else.

I'd go for the traction control, too.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

Proportioning valves come on at a set line pressure. This results in premature lock up of the backs on a slippery surface, but poor efficiency from the backs on a grippy surface.

To offer a tuning aid, the proportioning valve would need to be adjustable. I have never seen an adjustable one (other than replacing or shimming the spring) but it is not my field. They are a band aid, or a correction for load in a light truck.

Properly balanced brakes proportion fairly equally on all surfaces, except for changes due to different weight transfer generated on different surfaces.

ABS will work, but still works even better if it only needs to come in later because the brakes are fairly well balanced by calliper bore size.

Regards

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[GregLocock]

Has anybody hacked an ABS system together? That is, is there an aftermarket for a bolt on, tunable, ABS/traction control/electronic stability system?

Obviously you'd use the actuator tone wheels and pickups from a production vehicle, but programming the strategy would be fairly interesting (and probably uninsurable).







Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[kenre]

What are those valves seen sometimes on the rear of utes (pickups)?

they look somewhat like a proportioning valve, to vary pressure to the rear brakes depending on load??

Ken

 

[swall]

Kenre--that would be the RWAL I mentioned (Rear Wheel Anti Lock).

 

[Fabrico]

There are a few companies that make adjustable proportioning valves. Wilwood is just one of them. Assuming you are using a solid rear axle like the real Cobras did, a truck ABS might be good because it is simple, reliable, and designed to work with a solid axle.

Again, if you are using a solid rear axle you definately need traction control, but not the kind with wires and sensors. The new style Detroit locker will hook things up. Keep it simple.

 

[evelrod]

Yes, Pat. They do make adjustable proportioning valves (I use the one pictured on the left) and it 'prevents' the rears on my Mini race car from locking up, even when the fronts lock, while allowing them to maintain as 'near' lockup as possible!---Obviously, this is an impossible thing to accomplish 100% of the time...it still works well in almost all conditions save the rears completely off the ground (it happens).

One other comment about "brake systems designed by properly credentialed engineers"...I designed and built a dual master cylinder brake system on my Oldsmobile V8/automatic trans '46 Ford coupe in 1960 (while I was still an engineering student)...no balance bar, dual in line servos and modified Lockheed proportioning valve (I threaded the end for a bolt to make it adjustable) that worked quite well I must say (as I pat myself on the back);-)

Rod

 

[patprimmer]

My distaste for proportioning valves arose out of a company car I acquired in 1988.

It was the GM "J" car which was a Holden Camira here, A Vauxhall Cavalier in the UK and an Opel Askona ? in Germany.

At least the Australian model had rear wheel cylinders with over twice the projected area required for balanced braking on a good flat dry surface with just the driver.

They used a proportioning valve to correct this poor design and I presume to get the car to pass a specific test under specific conditions.

The result was that in conditions where the rears locked before the valve kicked in, you could lock the backs with only about 30% efficiency from the fronts.

This resulted in my only time in over 40 years that I ever had a car accidental going backward, and accidentally getting all 4 wheels of the desired road surface.

Next week I reduced the CSA of the wheel cylinders by 60% (that being the smallest size I could get pistons and seals for). Without a proportioning valve, it still locked rears first in all reasonable circumstances. I refitted the valve, and the system performed reasonably well, at least well enough to get reasonable stopping power without needing to get it full on sideways top wash of speed.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

I would guess that some smarty commonised the wagon and sedan brake system, to get the volume up.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[evelrod]

I'd say that individual more than 'commonised' the system in Pat's car. I was only 20 years old and did a better job of it than that. 60%? S***...I don't think I could have done it that badly on purpose, not even as a student.

The valves used in racing are for MINOR adjustments, some of which can be made on track. I have said it before, forgive me, Stupid is irreversable and not correctable by further education. Ignorance is 'fixable'...stupid is not.

Rod

 

[MikeHalloran]

<<< Obviously you'd use the actuator tone wheels and pickups from a production vehicle, but programming the strategy would be fairly interesting (and probably uninsurable). >>>

Should I infer that OEM ABS programming is 'insurable', i.e., that insurance companies now have some input in system behavior?

Hacking ABS sounds rather fun. Difficult, as I'm not aware of any port maps, schematics or source code having leaked out anywhere, but difficulty just adds to the excitement.



Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

No, they don't intervene in our jobs... but I'm quite sure they'd join in the legal fun and games if a manufacturer released an ABS system that was thought to contribute to accidents.

What I meant was that if I were an insurer then I would not be prepared to insure a car fitted with a home-brewed ABS, for use on the road.

The basic strategy for ABS is reasonably well known, the trick is in making sure that the degradation in outright dry braking performance is minimised while maximising the benefits. I think for a circuit car the programming task is a bit easier, in some respects.



Cheers

Greg Locock

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