Brake Bias Bar for Street Car
Brake Bias Bar for Street Car
(OP)
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?





RE: Brake Bias Bar for Street Car
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//:www.vancopbs.com
RE: Brake Bias Bar for Street Car
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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RE: Brake Bias Bar for Street Car
Mike Halloran
Pembroke Pines, FL, USA
RE: Brake Bias Bar for Street Car
RE: Brake Bias Bar for Street Car
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
RE: Brake Bias Bar for Street Car
RE: Brake Bias Bar for Street Car
It will help with the differnt brake size and requirements from front to rear.
Cheers
RE: Brake Bias Bar for Street Car
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
RE: Brake Bias Bar for Street Car
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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RE: Brake Bias Bar for Street Car
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.
RE: Brake Bias Bar for Street Car
they look somewhat like a proportioning valve, to vary pressure to the rear brakes depending on load??
Ken
RE: Brake Bias Bar for Street Car
RE: Brake Bias Bar for Street Car
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.
RE: Brake Bias Bar for Street Car
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
RE: Brake Bias Bar for Street Car
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
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Brake Bias Bar for Street Car
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Brake Bias Bar for Street Car
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
RE: Brake Bias Bar for Street Car
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
RE: Brake Bias Bar for Street Car
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
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Brake Bias Bar for Street Car
I wouldn't think that you would want or need abs in a kit car but it would be a fun project.
-Jon