Contour Enthusiasts Group Archives Forums Modifications Brakes 99% Front Brakes!

Actually it was no joke at all. Weight over the rears will most likely allow the rears to do more work and the car 'hunker down' a bit more as it won't nose dive. You might be surprised. Granted, it's not altering fluid pressure which seems to be your desire.

There is static bias and there is dynamic bias as well. You are chasing static. Try some stiff springs in the front or stiff shocks in the rear- these things will change dynamic bias as well.

Sure, you can gut the stock MC and valve bodies and run full line pressure out back. Better plan on the prop valve or two. And you'll be adjusting it when you have friends in the back seat too- it's that weight thing again.Or if it's raining, snowing, etc.

Want to really take a stab at this then lets fit prop valves in the front. That should prove interesting. You'll of course need two of them- one for each channel. Or would you prefer to Tee them? You can do that too if you like. Forget tha ABS working correctly however.

Other thoughts; source some larger bore calipers for the rear. Not sure what might fit. Buy a larger rear rotor kit (yea, I sell them) or some super sticky rear pads. All will effect static and dynamic bias.

Find some smaller front rotors. Smaller front calipers. Put in junk pads up front. All of this will do the same as the front prop valve in reducing front effectiveness.

Nobody has suggested driving backwards yet...lol (not that one's a joke)

you know, I'm really tired after a long and bad weekend of racing, so I'm not going to make a long-winded reply; but, I'm an OEM brake engineer, and Todd designs and sells high performance brake kits, and we both are heavily involved in various forms of racing; and basically, Todd is right, and you should listen to him.

hmm, maybe I should do a quick primer on how brakes and brake proportioning work . . .

The first and foremost thing to remember, is that maximum braking is always limited by how much grip on the road surface the tire has. A tire's grip is controlled by two main factors, the coefficient of friction, and the amount of vertical load on the tire, or basically, how much weight that tire is supporting.
Changing the coefficient of friction can only be done by changing the tire to something different, or driving on a different surface.

Changing the load here is what our discussion is about.
A stock contour is a typical fwd car where the vast majority of the weight is supported by the front tires, even while the car is just sitting there. Because I don't recall the contour #'s offhand, i'll use 70/30 as an example. So, 70% of the vehicle weight is on the front tires, and 30% is on the rear. This means, in the best case situation, the front brakes still can do more than TWICE what the rears can do, regardless of what hardware is being used. Then, when you consider weight transfer during braking the numbers get worse. The actual nose dive of the car isn't a big deal during braking, its the transfer of weight from the rear tires to the front that reduces the effectiveness of the rear brakes. And you should note that, while they are related together, the weight transfer still happens even if you make changes to limit nose dive under braking.

Depending on the initial wieght distribution, and the way the car is set up, you can get so far as to basically remove all load on the rear tires. I've seen photos of even cars like Porsche 911's where the rear tires have actually come off the ground because the weight transfer was so severe (that case was a rear heavy car to start with, but the deceleration rate was high enough to cause the "stoppie").

So it all comes down to what load you have on the rear of the car as to how effective the rear brakes are. If you increase the load at the back of the car, the rear brakes will do more before reaching lockup.

Now for proportioning. For a car that is safe for the public to drive, you need to have a brake system that will never have the rears lock up before the front. If you lockup the rear tires and not the fronts while going through a turn under braking, you will spin the car. To do that, brake designers design in some system to limit the brake torque generated at the rear tires. There are many different methods of setting this bias to the front tires, and some vehicles use several methods, and some use only one . . .

One method is pure hardware sizing. Where the rear brakes are just smaller in general, and capable of less brake torque. This method is pretty common, especially when used in conjunction w/ other methods.

Another is hydraulic system sizing. This is where the circuit controlling the rear brakes is smaller, and provides less pressure and less fluid volume than the circuit for the fronts. Don't confuse this w/ prop valves.

And the final, and most common (in conjunction w/ the hardware sizing) is the use of a proportioning system, that limits fluid pressure to the rears, relative to the front brake fluid pressure. There are hundreds of variations on this, from the simple line restriction like the aftermarket prop valves, to a complex electro-hydraulic unit (the ABS module used for EBD).

An additional prop valve for the rear brakes will never increase braking capability, it will only limit.

Also note that, most if not all fwd vehicles are split into diagonal brake circuits, rather than front/rear like most rwd vehicles.

Okay, I get how the system works. But from what you say, the system is not dynamic. It provides pressure to the rears that is linearly related to the pressure to the fronts. If you add weight to the rear of the car, both will squeeze harder because it will take more friction to stop the heavier vehicle. However, no tire is running out of traction in this case. I really do get your arguments, I'm just saying that weight will not make a difference in a normal stop. The rears cannot do a different percentage of the work with a different weight balance unless you change the braking system. If you do shift the weight rearward when designing/modding a car, it makes sense to let the rears do more by giving them more pressure, but I can't see how you could change the relative amount of work done by the rear brakes unless you put more pressure to them.

It depends on how the system is proportioned. Not all methods are linear; in fact most aren't, and have at least a knee point and then become linear after that. I don't recall if it was on the contours from the beginning or not, but the later cars have EBD, which uses the wheel speed sensors to decide when to limit line pressure. Also some OEM prop valves are sensitive to weight transfer, like the prop valve in my XR4Ti; and limit pressure as the vehicle pitches forward (a sign of weight transfer).


I guess one of the main points that you are missing is that in a car like the contour, you don't WANT the rear brakes to be doing more, unless you have drastically changed the balance of the vehicle or the brake system proportioning.

A good case in point is the Mustangs our team races. We run larger wilwood brakes up front, because that is what is allowed, but the rears are still stock parts, because the stock parts will provide all the brake torque we can use, even with the race tires, and the modifications we have made to move weight to the back of the car.

awesome, thanks. I didn't see the dynamics.

Bravo.
And well stated entirely. I didn't get back soon enough and probably could not have put it so well anyhow.

One thing that seems to be a misconception is how most prop valves work, so this might help you better visualize the pressure thing. Most of them work off total pressure. Thus if you put in 600lbs of pressure you get out 600 right? Wrong. This is the kneee Jim is speaking of.

Suppose you have a knee of 450. Now when you put in 400 all four wheels get 400, but when you put in 600 the rears might only get 300. How's that? Well, as the pressure input goes up; the pressure output to the rear goes down. Intentionally. Most factory valves have a preset knee and reduction, they generally just shut down the rears for safe measure. (*the Impala goes from 65/35 to 90/10 under hard braking! And with the weight and wheelbase of this you KNOW there's room for improvements!!)

So what's a manual valve do? It allows you to change the point which reduction occurs. BUT it does it differently than most 'cut off valves' in that it they go up to about 55% reduction total based on the set knee and input pressure. You change where you want it to cut out. Seeing your'e not going this way we'll leave it at that.

Ok. Now you sitll wonder what the hell all of this has to do with those sand bags in the trunk don't you?? With the weight over the rear wheels there is less dive and weight shift is not so quick. So what? Well, as you put in 400lbs of pressure all four wheels brake, as you increase this to 500 all four do some with the rears doing only slightly less...600lbs and they are begining to shut down. Fair enough. Now consider the stock 'nose dive' where the 400-600 input goes up real fast as there is no weight out back to hold down the rear LONGER thus immediate shift places nearly all the work up front.

To this end both shocks, springs, tires, ride height, weight bias and alignment can play on this. That's dynamic braking. What's the magic set up? The one that works best for your parts and car in those conditions operating in.

I'm done, fingers tired. I'll let Jim put in more data and correct any comments he feels I have incorrect. He's the engineer, I just try to improve on what he's done! \\

*excuse my spelling...