Proportional Valves

[Gerard87]

Hey, I'm currently building a single seater racer as a college project. We're using a 600cc motorbike engine and it weighs approx 300kgs. I am currently designing the Braking System of the car. I am restricted in using a single caliper mounted to the differential at the rear.

I am unsure of the benefits of using a proportional valve in the rear circuit and
is it necessary to use one.
Any information or help would be great fully appreciated.

Thanks, Gerard.

[WhiteBlue]

A directional valve works in binary fashion and will not modulate the brake pressure applied by the braking pedal.

Only a proportional or servo valve will give you brake assist.

Proportional valves are medium sophisticated valves which turn an analogue (often electrical) signal into an analogue flow or pressure. Lately voice coil proportional valves (Parker) have broken into the performance territory of servo valves.

Servo valves are higher sophisticated analogue valves which will use multiple stages to achieve higher amplification, accuracy or frequency response. They use mainly close loop control.

I'm not familiar with the use of valves in race cars but it appears that you will need a proportional valve if you want simple fly by wire. If you are looking for amplification a servo valve would be right.

[autogyro]

Maybe a simple answer will do.
Find a leyland mini brake balance valve as fitted to the rear subframe.
Drill and tap a 12mm thread in the end cap, fit a 2cm bolt with lock nut.
Connect the valve between the front and rear brake lines as in the car.
Screw in the bolt to reduce rear brake pressure, screw out to increase.
If needed change the spring.
On one mini racer I had a knurled knob welded to the bolt and the valve fitted next to the driving seat.

[xpensive]

Interesting to learn that you are into hydraulics WB, so am I actually.

Even if my xperience with servo- and proportional valves are basically from position control of double-acting cylinders, ie linear hydraulic motors, I have also built systems for pressure-control, using unly one port of the valve.

But I think that would be bit of an overkill for a brake cylinder, don't you?

[Gerard87]

No time for dick measuring lads..

Thanks for the simple advice I'll have a look into the mini idea, but is if after my calculations with the braking system components, I feel it is under strength. It may not be able to lock up the rear wheels. Does the proportional valve reduce the working pressure?

[autogyro]

No time for dick measuring lads..

Thanks for the simple advice I'll have a look into the mini idea, but is if after my calculations with the braking system components, I feel it is under strength. It may not be able to lock up the rear wheels. Does the proportional valve reduce the working pressure?

Yes it does reduce the pressure to the rear apply cylinders or in your case calliper.
Not a problem with a mini with a handbrake for locking the rear wheels.
Why do you need to?
Most of the mass and the braking is on the front with a racing mini.
I would not know the bias needed or the range of adjustment when fitted to your vehicle, it would definitely be less front brake bias than a mini of course.

[Gerard87]

The car is for a college project, Formula Student. Each car has to complete certain tests , one being locking up all four wheels under braking. The competition focuses more on the Engineering involved in producing the car, where actual racing is a much smaller part of the competition. I understand that for efficient braking you require less pressure in the rear (weight transfer).

Here are some of my calculations, any comments, advice or criticism are welcome.

I have worked out the Pedal Ratio is 3.454
The two master cylinders Front + Rear bore diameter 0.442 and 0.75 inches.
Two pot calipers all round 1.6141 inch Piston , Two on the front and one on the back axle. (Calipers tested to 1000 Psi)
The car weighs 300 Kg (120kg front, 180kg rear)

Ive made an excel spreadsheet with some of the major calculations:

I calculated the torque needed ( front 1103 Nm , rear 393 Nm)

With a 50/50 split in the balance bar + 30Kg leg input,
i'm getting a line pressure of

( front: 744.66 Psi , Rear : 258.63 Psi )

I have calculated a disk torque achievable
( front 1,407 Nm , rear 244 Nm )
...................................

How would you go about setting up your own brakes, any tips on: initial set up of the bias bar, correctly installing calipers and brake discs.. etc.

Again any other information is very helpful, thanks

[Gatecrasher]

"I am restricted in using a single caliper mounted to the differential at the rear."

How are you going to lock all 4 wheels with only one caliper ? I take it this was a typo.

[Tim.Wright]

I am unsure of the benefits of using a proportional valve in the rear circuit and
is it necessary to use one.
Any information or help would be great fully appreciated.

Thanks, Gerard.

I think there is a terminology mix up here. What you are talking about Gerard is a proportioning valve. This drops the pressure to the rear brakes usually by a fixed ratio. Obviously, the rear brakes have less tractive capbility for braking with the longitudinal load transfer. The proportioning valve is one way to alter the balance away from the rear brakes.

In my opinion however (Im just a suspension guy though) is that since you are designing your system from scratch, you can get the balance you require with the right combination of MC and piston diameters, disc sizes, pad selection and bias bar geometry without needing a proportioning valve.

Our FSAE vehicle a few years ago had the same radius for braking on the front as the rear, but the balance was achived by using different piston diameters in the calipers fron to rear.

A proportional valve on the other hand is indeed what WB was talking about. I belive many F1 teams use the sickeningly expensive Moog servo valves on their gearboxes and other hydraulic system. However, they are completely NOT what you want in your brake system.

"I am restricted in using a single caliper mounted to the differential at the rear."

How are you going to lock all 4 wheels with only one caliper ? I take it this was a typo.

If your brakes are applied to a disc on an LSD you can lock up both wheels with one caliper. Quite a common solution in FSAE.

Tim

[Gerard87]

The single caliper at the rear is mounted directly onto the limited slip differential differential, when the brake is applied it prevents the differential from rotating, with the weight distributed equally on the rear wheels, the two rear wheel will lock up.

Thanks for clearing that up Tim, yea terminology mix up, my bad...
We only work on the F.S. in final year as our Thesis, so we have no input into the work carried out in previous years. Its our college first year entering properly into the competition, so we're all learning.

The sickening thing is, all the braking system components have been bought from previous years, over eager lecturers biting off more than they can chew, without understanding anything about braking. Our budget doesn't cover any new purchases in Braking System.

They just bought the most common parts from AP Racing. All three calipers are the same, 2 pot front/rear.. All three brake discs are the same.the two master cylinders are different 0.442 and 0.75 inches bore.

My first job was to justify the selection of these parts, not easy.

[gator]

Proportioning valves have no place on a properly designed braking system of a race car. If your team showed up to competiton with one, the judges will certainly wonder why the engineer chose to use it.
If you are forced to use it, make sure to include it in your pre-design report and talk about to the judge at the design event when you touch on why you chose components. Let them know you are operating under a tight budget and re-using old parts. There is no shame in that; what is a shame is someone trying to justify the use of a prop. valve on a race car. (Again, they have no place on a race car, they are a band-aid for a screw-up somewhere in the system.)

My suggestion is that you make getting a new properly sized master cylinder(s), or a new properly sized caliper(s), or a new properly sized rotor(s) one of the TOP priorities for your team. Try to put it to them in a way they might understand, some things on the car simply take precidence (brakes, suspension, steering, etc.) Why does the braking system have to take the budget hit? I am sure there is money in there somewhere if you look hard enough. Safety before speed.

At the end of the day, what will a judge think of a team with a few go-fast-gadgets but an improperly designed braking system?

Good luck

[Gatecrasher]

The single caliper at the rear is mounted directly onto the limited slip differential differential, when the brake is applied it prevents the differential from rotating, with the weight distributed equally on the rear wheels, the two rear wheel will lock up.

I understood that part, what I did not get is how this would lock up ALL 4 wheels as stated in the original post.

You should design the system based on what the overall requirements of the system are, if the only success criteria is to lock up all wheels at once then put in the biggest available. If however you care more about stopping power + rotating mass + un-sprung weight +..... the you have more to chew on.

The best braking system that I currently own is setup such that with hot tires, suspension loaded and maximum braking force applied it never locks up.

If you really want to lock-up then choose skinny hard compound tires, over inflate a few PSI, make sure the tires are cold, setup the front suspension to be soft and if all that is not enough then there is a nice product from ArmorAll that will make your tires shine like new and wont do any harm to your grip levels.

[autogyro]

Proportioning valves have no place on a properly designed braking system of a race car. If your team showed up to competiton with one, the judges will certainly wonder why the engineer chose to use it.
If you are forced to use it, make sure to include it in your pre-design report and talk about to the judge at the design event when you touch on why you chose components. Let them know you are operating under a tight budget and re-using old parts. There is no shame in that; what is a shame is someone trying to justify the use of a prop. valve on a race car. (Again, they have no place on a race car, they are a band-aid for a screw-up somewhere in the system.)

My suggestion is that you make getting a new properly sized master cylinder(s), or a new properly sized caliper(s), or a new properly sized rotor(s) one of the TOP priorities for your team. Try to put it to them in a way they might understand, some things on the car simply take precidence (brakes, suspension, steering, etc.) Why does the braking system have to take the budget hit? I am sure there is money in there somewhere if you look hard enough. Safety before speed.

At the end of the day, what will a judge think of a team with a few go-fast-gadgets but an improperly designed braking system?

Good luck

How would you design a method of adjusting the front to rear braking bias?

[Tim.Wright]

How would you design a method of adjusting the front to rear braking bias?

Bias bar on the pedal and two master cylinders.

The sickening thing is, all the braking system components have been bought from previous years, over eager lecturers biting off more than they can chew, without understanding anything about braking. Our budget doesn't cover any new purchases in Braking System.

Very frustrating... seems like they've taken all of the actual engineering out of the project already.

Tim

[autogyro]

How would you design a method of adjusting the front to rear braking bias?

Bias bar on the pedal and two master cylinders.

The sickening thing is, all the braking system components have been bought from previous years, over eager lecturers biting off more than they can chew, without understanding anything about braking. Our budget doesn't cover any new purchases in Braking System.

Very frustrating... seems like they've taken all of the actual engineering out of the project already.

Tim

Sorry Tim but a master cylinder bias bar will not give a method of adjusting brake bias on the move.