Is proportioning valve very necessary ?

[siddharth_261991]

I am using tandem master cylinder, with X circuit. We are not going for balance bar and dual master cylinder. With calculations i m getting around 360N-m braking torque at front wheel and 227N-m at rear. In this case can I go without brake biasing through proportioning valve. Is it very necessary. If Going without installing proportioning valve, what will be the effects..

Guidance & suggestions welcome.

Siddharth
GSRacers

[MCoach]

"Is a proportioning valve very necessary?"

No.

What coefficient of friction are you using for the interaction between your tire and surface?

[Canuck Racing]

MCoach, I'm not sure if you are trolling or not, but either way it isn't nice to provide sarcastic answers to our language handicapped brethren.

Siddharth, while a proportioning valve or bias bar is not required by the rules, and no street car uses them (adjustable anyway,) the cost to benefit of running one is hard to beat - particularly the bias bar. It possibly has the best trade-off of any cost/complexity/weight/benefit ratio of anything you can add to your car.

There is a lot of difference between a wet, new asphalt surface such as MIS and a hot concrete surface such as Lincoln and you need to be able to adjust your bias accordingly.

Here's some further reading: http://www.stoptech.com/techni...proportioning-valves (http://www.stoptech.com/technical-support/technical-white-papers/proportioning-valves)

[Warpspeed]

Agree with Canuck...

Designing in a balance bar right at the very beginning is not such a big problem.
And it will give you the quick and easy adjustability you will later find very well worth having.

[Pete Marsh]

You can't use a balance bar properly in an "X" circuit. You Can get dual path valves, and would need one for a competitive race car.

BUT, why "X"? There are easier ways.

Pete

[MCoach]

Totally not trolling by the way.

Proportioning valve isn't necessary.

The problem I do see is that there is a calculated braking force at the front and rear but no correlation to why or why not he may need one.
I mean, with a 61/39 f/r bias, it seems about right, but would just like to get him to explore his own decisions.

Now, I would personally run a bias bar, just saying.

[Owen Thomas]

I am in agreement with MCoach. Siddharth does not need a proportioning valve to operate or be competitive. Will it help? Only if they understand what adjusting it does, and why, and how much.

Also, a balance bar is not an option for them since they are using a tandem MC.

Also also, 60/40 front/rear seems about right for road tires, and I would test without a proportioning valve and determine from there if it is required. Not hard to add one in.

[Adambomb]

Originally posted by Pete Marsh:
You can't use a balance bar properly in an "X" circuit. You Can get dual path valves, and would need one for a competitive race car.

BUT, why "X"? There are easier ways.

Pete

+1...you can't use a balance bar with a tandem master cylinder, and you can't use it with an X circuit.

Again, why use an X circuit? Why are they used on passenger cars? Do we have the same priorities?

You will want some way to adjust bias. Your calculations can never be spot on simply because your friction coefficient varies wildly with track conditions, and even your CG height varies with different drivers. If you are stuck with a tandem master, an adjustable proportioning valve would be the easiest way to do it.

[siddharth_261991]

Thanks all of you guys for your suggestion. Since, we have qualified for the first time, thus short of funds so i can't import wilwood or AP racing dual master cylinders. And if I go setting up dual master cylinder assembly using master cylinder of OEM in india, then it is unncessarily adding weight to our vehicle.

Thus, I have decided to for Tandem Master cylinder, with two ports and it is dammed style. Brake circuit is diagonal (i.e X circuit)

And regarding specs, I am giving little bit idea about my vehicle estimate.

[B]Laden Mass of vehicle - 350 kg and weight ratio of 35:65

Brake torque for front : 368 N-m
Rear : 230 N-m approx

So for above required specs, should i use proportioning valve OR i can install brake without it..?? Waiting for your replies

[siddharth_261991]

@ Mcoach - Friction coefficient i've assumed to reach till 1.3 (worst case limit) since we are using Hoosier slicks.

[siddharth_261991]

@ Owen Thomas -

Ya i can add proportioning valve later on, but if with some inputs and referring some of design reports, it was stated that "If brake biasing is not provided by balance bar or proportioning valve, then rear wheels get locked up early". So does it really affect? As my race car will be still in straight path. So what is the basic reason for biasing. Kindly guide me in same regard, and kindly be elaborative too.
-Siddharth

[Owen Thomas]

Originally posted by siddharth_261991:
@ Owen Thomas -

Ya i can add proportioning valve later on, but if with some inputs and referring some of design reports, it was stated that "If brake biasing is not provided by balance bar or proportioning valve, then rear wheels get locked up early". So does it really affect? As my race car will be still in straight path. So what is the basic reason for biasing. Kindly guide me in same regard, and kindly be elaborative too.
-Siddharth
The statement which you quoted is generally correct, and yes it will affect your performance.

The basic reason for having brake bias is to make the car decelerate as quickly as possible. I will leave it to you to figure out the "how". A good place to start is a free body diagram of your car in side-view. You can find all the information you need in any classical mechanics textbook; additionally you can try Wikipedia/Google or the "Find" function on this forum.

[MCoach]

Yay, critical thinking! http://fsae.com/groupee_common/emoticons/icon_smile.gif


Glad to hear back, siddharth.

With a friction coefficient of 1.3, It sounds like you are on the right track.

To give a little insight, the X circuit in production cars is for safety, so that if one circuit fails, the car will still have front and rear brakes to use. Unfortunately, this causes the the car to be impossible to use a proportioning valve on. A tandem master cylinder can be set up so that one port is front and the other is the rear. This will allow you to use a prop valve if you feel you need it.

As suggested by Adambomb, with multiple drivers, and track conditions, the best performance balance can vary A LOT. This is not an exaggeration.

However, don't let this deter you. If the wheels are designed to lock up all four together under 'perfect' conditions such as a warm, sunny, Indian day and the track is clear of debris then it'll always have a safe stopping condition. Every point of stopping under that will lead to understeer during a locked wheel condition. This is where the prop valve could benefit you, getting everywhere under perfect closer to the perfect brake balance for that surface.

As Owen Thomas noted, it is still important to understand the component of a prop valve/ bias bar itself and how it functions. I would suggest doing some reading on balancing the brake systems.

Points to look for:

Brake balance in relation to car properties (CG height, wheelbase, weight, etc)
Brake balance in relation to locking condition
Prop valve/ bias bar definitions and uses

[Adambomb]

Originally posted by MCoach:
Yay, critical thinking! http://fsae.com/groupee_common/emoticons/icon_smile.gif


+1!

Now here is another hint with regards to running non-adjustable bias and tandem master cylinders. The OEMs use fixed proportioning valves, and at least in the old days (according to my old vehicle dynamics professor) they typically weren't all that tuned to the application.

If you plot braking efficiency (ie at which point the fronts vs. rears lock up) vs. friction, you will find that it is possible to get a much broader response with a proportioning valve than with a bias bar. The reasons for this, unfortunately, go much deeper than I can (or honestly have time to) explain in a forum, but a good book on brake system design or even basic vehicle dynamics should cover it in detail. I believe this book does if I remember right...if nothing else it's a good primer to get you started if you don't have time to fully digest RCVD by Milliken (which is basically the bible on race car vehicle dynamics). If you buy either through SAE directly you can get a good discount.

Basic vehicle dynamics (http://www.amazon.com/Fundamentals-Vehicle-Dynamics-Thomas-Gillespie/dp/1560911999)

[Warpspeed]

That is all very true if you are designing a general purpose type of vehicle to be driven on wet roads, or dry roads, loaded, or unloaded.

But a racing car is different.
You go out there and do practice laps and set your balance bar up for how things are on that track, on that day, with those particular tires, and that driver.

You can very quickly change the bias to wherever the driver feels it should be, and that will probably get you a lot closer to optimum on the day.

[Gruntguru]

Originally posted by siddharth_261991:
Since, we have qualified for the first time, thus short of funds so i can't import wilwood or AP racing dual master cylinders. And if I go setting up dual master cylinder assembly using master cylinder of OEM in india, then it is unncessarily adding weight to our vehicle. There are lightweight master cylinders fittd to many production cars. Try looking at clutch master cylinders eg Hyundai.

[siddharth_261991]

thanks for your advises..
and also what if I don't use balance bar and not even proportioning valve.
From literature, I've got idea that rear wheels get lock up. So thats all the reason?
If that so, then I can go without biasing also..
or NOT??

[Warpspeed]

You can design a whole braking system to have any front/rear brake bias you decide to give it.
But once it is built, that is what you are going to be stuck with.

If you guessed wrong in your initial design, you will have a car that is not going to brake as well as another car that has a quickly adjustable brake bias system.

You don't need to buy very expensive parts.
Many production car calipers and master cylinders can be obtained at low cost, especially secondhand, and tested on the car.
And you can make your own pedal box and balance bar fairly easily.

[Z]

Sid,

Listen closely...

You DO NOT NEED a proportioning valve!

You DO NOT NEED a bias-bar!

You DO NEED to finish building a car that can travel 30kms under its own power at the comp.

You DO NEED that car to be capable of locking all four wheels during the brake test, WITHOUT ANYTHING BREAKING!

Z

(PS. Questions to those advising otherwise:
What is the cost to a first year Indian team of time spent "optimising" their brakes by fitting bias-bars, etc.?
(My guess = DNS.)
What is the benefit to a first year Indian team, in points-gained-by-faster-lap-times-due-to-more-optimal-braking-by-their-untrained-drivers, of bias-bars, etc.?
(My guess = 0 (because of untrained drivers).)

[Jay Lawrence]

Definitely agree, Z.

An untrained driver is probably not going to use all ~1.6g's of braking and is probably not going to have any use for biasing or proportioning.

Make it as simple as you can and get the most out of that simple package before you bother with adding tuning variables and their necessary parts.

[Adambomb]

Along those lines, could you source the fixed proportioning valve from wherever you got the master cylinder? It would be a good start anyway.

I've driven passenger cars without any proportioning valves (or bias bars)...generally rear lock-up is pretty wicked, and makes corner entry no fun (spin city!). You could design that out to some extent, although it would probably be cheaper and easier to slap on some stock proportioning valves.

Bonus: I've found that many times stock proportioning valves are serviceable. If stock is still too far off, you can pull it apart and slap some washers under the spring to adjust the cutoff pressure.