My booster-less brake recipe

[keith kuchler]

I was recently asked how I created my booster-less brake system on the race car and what parts I had used.

The first part that you will need to procure is the master cylinder.  I used ATE 34311120832 from a BMW 528I.  This part will need new reservoir grommets and adapters.

https://www.autohausaz.com/pn/BM-34321102282

https://www.autohausaz.com/pn/0004310935

 

You will need to obtain your pedal housing and remove all frame work and unnecessary hardware from it.  (See picture )  Measure the bolt spacing and diameter of the new master cylinder to help create the doubler that will get welded on to the back of the pedal housing.

 

Once the doubler is welded, attach the brake cylinder to the pedal housing and clock it correctly.  It must be turned so that the cylinder will clear suspension components.

 

 

 

 

 

I used a spherical hiem joint along with an adjustable push rod to actuate the brake master cylinder.

( See push rod )

 

You will need to relocate the hiem joint in a new location on the brake pedal arm.  I simply drew a straight line from the master cylinder plunger thru to the brake pedal arm and marked the location.

Once the hole is drilled attach the hiem joint, adjust the plunger push rod and test the actuation.  It should move smoothly with no binding.

 

The next part of engineering the brake system will consist of several parts.  I used the Volvo calipers for front braking and the 320I drum brakes for the rear.

The rear brakes used the 22mm actuators to apply pressure.

Because the master cylinder sits below the other brake components in the chassis, it is critical to not have bleed back.  To prevent brake fluid bleed back, residual valves need to be incorporated.

I used wildwood valves.

260-13784

260-13783

 

I installed mine per the picture below.

 

Attach the brake reservoir to the master.

 

 

Attach the brake lines and bleed appropriate.

A nice firm brake pedal is the result.

Edited July 29, 2019 by keith kuchler
delete photo

[zinz]

Thanks for the write up and good pictures, Keith.  Perhaps we can get Steve to promote this into the Technical Articles Section.

 

Ed Z

[jireland2002]

Beautiful work.  How does it work?  Is your leg strong enough to stop the car?

 

[keith kuchler]

Just for giggles....let's do the math for this system to find out what you can expect from it.

 

First we will compute the area of the master cylinder used.   Its bore is 23.81mm.

23.81mm divided by 25.4 gives us 0.937 diameter of the piston.

0.937 divided by 2 gives us the radius of .468

Now we square the radius..... .468 x .468 = .2196

.2196 times PI 3.14 equals .689

 

.689 is our area of the piston in the master cylinder.

 

Next, we have to calculate the area of the caliper pistons.  I am using the Volvo brakes that have 4 - 38mm pistons.

For this calculation we are only concerned with using two of the 38mm pistons in the caliper.

38mm divided by 25.4 gives us a decimal of 1.496 diameter of the caliper piston.

1.496 divided by 2 gives us the radius of .748

Now we square the radius......  .748 x .748 = .559

.559 times PI 3.14 equals 1.75 area of one piston.

1.75 times 2 equals 3.513 total area used in the caliper.

 

3.513 is our net bore area of the Volvo caliper.

 

Now we need to compute the leverage ratio between the master cylinder bore and the caliper bore.

 

Effective Caliper Piston Area (3.513) / Master Cylinder Bore Area (23.81mm which is .689) =

3.153 / .689 = 5.1 for an 5:1 ratio

 

 

Calculating pedal ratio

 

The driver leverage is determined by multiplying the Pedal Ratio X the ( Caliper Piston Bore to Master Cylinder ratio ).

I will use figure 4 for my calculation.

My pedal is 9.125 long, point A.

B is 1.5 long on my pedal.

9.125 divided by 1.5 is 6.08.  6 to 1 pedal ratio.

 

 

The driver leverage for the front brakes is (pedal ratio) 6.08 x 5.100 = 31.00  or 31 to 1 ratio.

 

Rear Drum Calculations

 

The rear drums use 22mm wheel cylinders.  We need to calculate the rear wheel cylinder area.

22mm / 25.4 = .866

.866 / 2 = .433 radius

.433.x .433 = .1875  Radius squared

.1875 x PI 3.14 = .588 area of rear wheel cylinder

 

Leverage ratio is .588 wheel cylinder area / .689 master cylinder area = .8534 leverage ratio

 

Driver ratio for the rear drums is 6.08 (pedal ratio) x .8534 (leverage Ratio) = 5.1 Driver ratio rear.

 

So we now have 31 to 1 in front and 5.1 to 1 in rear driver leverage ratio

 

By applying 70 pounds of leg force against a 6.08 to 1 pedal ratio we generate 425 pounds of force into the master cylinder.

By taking the leg pressure and dividing it by the master cylinder area .689 we get brake line pressure.

 

425 / .689 = 617 PSI

 

We now take the 617 PSI and multiply it by the area of the caliper pistons.

617 x 3.513 = 2170 pounds of clamping force on the caliper.

 

Rear pressure is 617 PSI x .588 wheel cylinder area = 362 pounds of force to the drums.

 

Because of the servo action of the rear drums, less force is required by the wheel cylinder, so smaller pistons can be used.  Finally, the contact area between the shoes and the drum surface is large, much more than that of the disc brakes. This requires less pressure against the shoes since the force is acting against a large area.

 

 

Edited July 31, 2019 by keith kuchler
typo fixed

[jireland2002]

I put a pressure gauge on my front Volvo calipers on my race car.  I was getting 1000 - 1100 psi.

 

[jimk]

Doesn't change the result, but there is a typo in the 4th line of the calcs.  0.486 should be 0.468.

Just for the record.

[jireland2002]

The math is absolutely correct (less the typo) but my feeling is 617psi will not adequately stop (slow down) a race car or a street car in panic stop mode.  Generally I think a brake system should be able to produce at least 1200psi or a bit more.

[keith kuchler]

Well, I have been racing this car in the BSL group with no issues in braking.  

 

When strapped into a race seat, the example of 70 pounds of pressure is almost like setting your foot on the brake.

 

I am applying far more pressure.  Also, you must use the correct compound of brake material.

 

Every driver is different.  By doing the math, one can customize the system to suit a drivers comfort level.  

Edited July 31, 2019 by keith kuchler

[Lorin]

Without doing any math at all, i feel like a nearly 24mm MC is on the large size for a manual brake system in regards to pedal feel and pressure. Something closer to 20mm might be worth trying.

 

 

 

[NickVyse]

Great write-up. Never understood people removing the servo on a LHD car and leaving in the heavy and space consuming frame work on the pedal box.

 

You've basically replicated the 1600-2 braking system, except they soon added a small remote servo. Here's an out of focus pic ?

 

 

Lorin: I use a 20mm MC on my servoless NK for that very reasoning, very happy with the result.

[cmiceli]

keith this setup looks awesome! to bad you don’t provide the service or parts to do it

[Dudeland]

 

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Edited July 6, 2022 by Dudeland
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