More 308 brake madness

In my never ending quest to drive myself to drinking...

For those who are planning really big brake kits for 308's because of these new motors being put together ie stroker, supercharged etc, I wanted to know the actual pedal mechanical ratio. Boosters don't work too well on low vacuum motors anyways. It becomes very important to know this if you are planning to eliminate the booster and run dual masters. Important I guess if you plan on engineerig the system as opposed to throwing stuff together.

The pedal box on the 308 is a nightmare of levels and contraptions designed to turn the booster and MC 90 degrees to the pedal centerline. Previously we eyeball measured and calculated because we never had one out of a car. I found this little gem on the shelf over at Ferrari service of Costa Mesa (now of Irvine). Micheal was nice enough to let me borrow it.

Here is what I did. I disassembled the booster from the pedal box and put two dial indicators on it to measure actual input distant vs. output distance.

Turns out it is EXACTLY 4 to 1. So there.

Please paypal all beer fund donations to sales@************.......

Martin
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This was a fluke to be sure... it was measuring out to one or two thou...

 

Yes, but, don't forget your Bozo the Clown shoes push your contact point with the pedal farther up, effectively giving you a 3 to 1 ratio...

mk e, you can put anything on anywhere, you just need the master cylinder volume and pedal ratio to make it right. The stock master will not support the piston sizing in those calipers, they have very large total piston area. On the bright side, you'll never have to worry about the rears locking up prematurely ;-)

Martin

 

May this post go down in history!!!!!!!!

Read very carefully. There is no mystery to a prop valve. In it's simplest form it is a spring in a block, that when a certain pressure is reached, the spring is overcome and fluid pressure gets reduced at a constant rate compared to the front. Adjustable ones change the spring pressure. It DOES NOT balance poorly selected components. You are not reducing pressure to the rear until that pressure is reached.

Imagine, if you will, a car with brake calipers/rotors sized to provide just the right amount of torque to avoid lock up at full forward weight transfer under track conditions. This is what real race cars do, the prop valve, and frequently the balance bar, is for tuning WHILE racing to adjust for fuel load, changing track conditions ie rain, dirt etc. One catch, a race car is rarely using brakes less than at maximum(if so, you're getting passed!). A street car set up this way would never have to replace rear brakes because the front is always doing all the work. We want the rears to work as hard as the fronts(given the weight on the wheels) to have maximum brake capability. Tires stop the car, if 2 tires aren't working you're not stopping as well as you could be.

The old school way of doing this in production cars was the prop valve. Engineers would test the system, finding the point the rears would lock(from weight transfer) and set the knee point of the prop valve to somewhere before that, attempting to have the correct pressure show up at max braking/max weight transfer. The knee point is the pressure that overcomes the spring and causes rear line pressure to be reduced, typically at a 3 to 1 ratio. I am not aware of any valves that adjust the rate, only the knee point. As these systems got more sophisticated, they put levers on them that were attached to the rear chassis somewhere and would further tell the prop valve how high the rear of the car was getting. A little cam inside the valve was actuated by this lever further tailoring the system to the given conditions. I have heard mention of some that acted like a level, and when the level was tilted forward it changed the settings as well. Finally, all of this was done away with when Electronic Brake Distribution (EBD) came about and all pressure reduction is handled by wheel speed sensors, ABS and stability control algorythms.

Where does that leave us? It is entirely possible to design a system with rear components so large a prop valve won't even keep the rears from locking before the fronts. This leads to spectacular spins. Not cool. Production cars are always designed to be slightly more forward biased than optimum, so as to never induce this condition. Bumps in the road, new tires in front with worn out ones in the rear, different pad compound front to rear all have to be accounted for by the factory. If you are setting up your street car into a race car, there are typically gains to be found in bumping up rear brake torque. The amount can only be found through testing different components in the conditons the car will see.

It is possible to tune brake torques with pad compounds, in fact these have the biggest (and easiest) effect on brake torque. You just better pick compounds that have similar heat/friction curves or you will have some surprises! The best is to always start with the factory bias and the same pads front to rear. Straying from this is OK, you just have to be willing to dedicate time to experimenting and testing.

So what can I do with this box of Ebay stuff I bought that now doesn't look too good? Dual masters and a balance bar. The different masters put different line pressures front to rear(or however you have them hooked up). The balance bar fines tunes this further. You still want your prop valve, you still have weight transfer issues, we didn't change the laws of physics! Make sure to remove your stock valve if you are using an adjustable one. Ultimately we are trying to maximize component life and provide line pressures that keep all wheels braking to their fullest capacity for the weight on them.

I have left out alot, and hopefully not confused you. There are so many scenarios to consider, towing a trailer going down a mountain pass, emergency braking in a hairpin, straight line max decel, manufacturing and engineering expense... EBD is really a quantum leap from the lowly prop valve.

Now change your ride height, move the battery or fuel tank, shift the engine location, anything that moves the location of the CG and return to step one.


FWIW

Martin