Up fix'n the brakes
07-07-2002, 01:08 AM
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Up fix'n the brakes
Well played with brakes all day yesterday. Stuck pistons and rusted pads to caliper frames. It is a wonder I ever stopped at all.
Put in new 19MM master cylinder Boy sure have a firm pedal now Don't know if it really helped the stopping distance. Well since all 8 pads are now doing the stopping it is much shorter.
Now I will get off my butt and rebuild those "M" calipers and put those on to see what happens. Maybe get some data together as to what upgrades are really worth. Need to find some Beemer calipers too. Also increased the pressure to the rear bias. Fronts are still locking before rear. Can't test this to many times as flat spotting tires sucks.
Put in new 19MM master cylinder Boy sure have a firm pedal now Don't know if it really helped the stopping distance. Well since all 8 pads are now doing the stopping it is much shorter.
Now I will get off my butt and rebuild those "M" calipers and put those on to see what happens. Maybe get some data together as to what upgrades are really worth. Need to find some Beemer calipers too. Also increased the pressure to the rear bias. Fronts are still locking before rear. Can't test this to many times as flat spotting tires sucks.
07-09-2002, 01:55 PM
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[quote]"Fronts are still locking before rear." <hr></blockquote>
This is as it should be. If the rears were to lock first, you might find that the car becomes a bit unstable and may even spin.
Edit: However, I can fully appreciate that you want to get the most bang for the buck from the rear brakes. I put in a new bias valve in my S2 to help get more rear braking and it made a big difference. :End Edit
Consider the following from <a href="http://www.connact.com/~kgross/FAQ/944faq.html" target="_blank">924/944/968 Frequently Asked Questions</a>, Chapter 14
[quote]Why is braking proportion important? You may remember from your high school physics courses that the frictive force generated by an object moving across a surface is directly proportional to the weight of the object, that is, the force with which it pushes down on the surface. So if you double the weight on a car's tire, it can grip the road twice a strongly.
For tires, the contact patch of rubber has a total limit to the force it can generate while still rolling. This force depends on the weight on the tire: the more weight pressing a tire down onto the road surface, the more force that tire can generate along the road surface. When this force is exceeded, the rubber loses grip with the road surface and the contact patch slides along the road surface instead of interlocking with it. The car's wheel stops turning and is said to have "locked."
Another important bit of physics takes place when you use your car's brakes: weight transfer. The harder you use your brakes, the more weight is transferred to the front wheels and tires, and the less weight is borne by the rears. This affects the braking force which can be generated: the less weight on the rear tires, the less ultimate braking force they can generate before locking up.
So, it turns out that as you brake harder, it's best that the rear wheels do proportionally less of the braking. Since it's hard to change caliper piston size on the fly, a proportioning valve does the trick by instead reducing line pressure in the rear circuits. The chart on the right shows how this would look ideally, rear pressure increase gradually rolling off as system pressure (pedal effort) increases.
A proportioning valve gives you an approximation to this ideal curve, using two straight-line segments. Up to the point of the change-over pressure, both the front and rear brake circuits receive full system pressure, i.e., the pressure within the brake master cylinder. Above the change-over pressure, only a fraction of additional system pressure is shared with the rear brake circuit.
There are two ways to affect the proportioning in your Porsche. By far the simplest, and perhaps safest, is to add or change the factory-style proportioning valve, which threads into the brake master cylinder's port for the rear circuit. The 33/5 designation on the valve currently used in 944-series cars is decoded as follows. The "33" refers to a 33 bar change-over pressure. One bar is 14.5 PSI, and so this valve starts reducing incremental pressure to the rear circuit when system pressure exceeds 478.5 PSI. The "5" indicates a reduction factor of 0.46, so that each additional bar of system pressure will add only 0.46 bar to the rear circuit's line pressure.
A second approach to modifying braking behavior is the installation of an adjustable proportioning valve. These are commonly used in professional race cars, whose drivers will fine tune the change-over point to cope with changes in outdoor temperature, tire compound, track surface, and wet/dry condition.
Before you rush off to implement any of these features, keep a few things in mind. The first is that it is perfectly normal for you car's front brakes to be doing most of the work. For this reason, you will always go through front brake pads and rotors faster than rears. The only way to change this is to reduce front-to-rear weight transfer under braking. And the only way to change weight transfer is to alter the car's center of gravity, its weight, or its wheelbase.
Another thing to keep in mind is the effect of locking up under braking. For a pure racecar, the ideal set-up is one such that the rears and fronts lock at the same pedal pressure. For a street car, and for most track enthusiasts, biasing the set-up in a way which causes the fronts to lock a little sooner that the rears is ideal. If your car locks its rears, it becomes extremely unstable in a hurry. Think of a pendulum being held with its weight above its axis. If you lock the fronts but keep the rear turning, you may flat-spot your tires and dampen your linens, but your car will remain controllable.
<hr></blockquote>
This is as it should be. If the rears were to lock first, you might find that the car becomes a bit unstable and may even spin.
Edit: However, I can fully appreciate that you want to get the most bang for the buck from the rear brakes. I put in a new bias valve in my S2 to help get more rear braking and it made a big difference. :End Edit
Consider the following from <a href="http://www.connact.com/~kgross/FAQ/944faq.html" target="_blank">924/944/968 Frequently Asked Questions</a>, Chapter 14
[quote]Why is braking proportion important? You may remember from your high school physics courses that the frictive force generated by an object moving across a surface is directly proportional to the weight of the object, that is, the force with which it pushes down on the surface. So if you double the weight on a car's tire, it can grip the road twice a strongly.
For tires, the contact patch of rubber has a total limit to the force it can generate while still rolling. This force depends on the weight on the tire: the more weight pressing a tire down onto the road surface, the more force that tire can generate along the road surface. When this force is exceeded, the rubber loses grip with the road surface and the contact patch slides along the road surface instead of interlocking with it. The car's wheel stops turning and is said to have "locked."
Another important bit of physics takes place when you use your car's brakes: weight transfer. The harder you use your brakes, the more weight is transferred to the front wheels and tires, and the less weight is borne by the rears. This affects the braking force which can be generated: the less weight on the rear tires, the less ultimate braking force they can generate before locking up.
So, it turns out that as you brake harder, it's best that the rear wheels do proportionally less of the braking. Since it's hard to change caliper piston size on the fly, a proportioning valve does the trick by instead reducing line pressure in the rear circuits. The chart on the right shows how this would look ideally, rear pressure increase gradually rolling off as system pressure (pedal effort) increases.
A proportioning valve gives you an approximation to this ideal curve, using two straight-line segments. Up to the point of the change-over pressure, both the front and rear brake circuits receive full system pressure, i.e., the pressure within the brake master cylinder. Above the change-over pressure, only a fraction of additional system pressure is shared with the rear brake circuit.
There are two ways to affect the proportioning in your Porsche. By far the simplest, and perhaps safest, is to add or change the factory-style proportioning valve, which threads into the brake master cylinder's port for the rear circuit. The 33/5 designation on the valve currently used in 944-series cars is decoded as follows. The "33" refers to a 33 bar change-over pressure. One bar is 14.5 PSI, and so this valve starts reducing incremental pressure to the rear circuit when system pressure exceeds 478.5 PSI. The "5" indicates a reduction factor of 0.46, so that each additional bar of system pressure will add only 0.46 bar to the rear circuit's line pressure.
A second approach to modifying braking behavior is the installation of an adjustable proportioning valve. These are commonly used in professional race cars, whose drivers will fine tune the change-over point to cope with changes in outdoor temperature, tire compound, track surface, and wet/dry condition.
Before you rush off to implement any of these features, keep a few things in mind. The first is that it is perfectly normal for you car's front brakes to be doing most of the work. For this reason, you will always go through front brake pads and rotors faster than rears. The only way to change this is to reduce front-to-rear weight transfer under braking. And the only way to change weight transfer is to alter the car's center of gravity, its weight, or its wheelbase.
Another thing to keep in mind is the effect of locking up under braking. For a pure racecar, the ideal set-up is one such that the rears and fronts lock at the same pedal pressure. For a street car, and for most track enthusiasts, biasing the set-up in a way which causes the fronts to lock a little sooner that the rears is ideal. If your car locks its rears, it becomes extremely unstable in a hurry. Think of a pendulum being held with its weight above its axis. If you lock the fronts but keep the rear turning, you may flat-spot your tires and dampen your linens, but your car will remain controllable.
<hr></blockquote>
07-10-2002, 02:33 PM
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A stock 914-6 has the 'M' calipers, and they are not great. Granted the car is a bit heavier, but the braking is not great. I bought an old set of 'M' struts and welded new mounts that allowed me to use some rx-7 turbo 4 piston calipers that I got at the junkyard for $30 ea.
The braking is better, but still not awesome. I am still using the stock rotor diameter. I am still using the 17mm master also. Going to the 19 would only increase required pedal effort for the same stopping force(that is why it feels firmer).
The rx-7 calipers have about 12.5% more piston area and are a pound lighter per wheel. The extra piston volume gives a little more pedal travel, but they work pretty well. This was a fun upgrade that did not cost much compared to Porsche prices.
Art
The braking is better, but still not awesome. I am still using the stock rotor diameter. I am still using the 17mm master also. Going to the 19 would only increase required pedal effort for the same stopping force(that is why it feels firmer).
The rx-7 calipers have about 12.5% more piston area and are a pound lighter per wheel. The extra piston volume gives a little more pedal travel, but they work pretty well. This was a fun upgrade that did not cost much compared to Porsche prices.
Art
07-11-2002, 12:32 AM
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That is what I intend to find out and document the effects. Yes pedal effort is more with the 19MM MC I expected that. But the decreased stopping distance was mostly because I repaired all the 914/4 calipers so now all the pads are moving. Now that I have a base line it so far is cheap as I have "M" calipers laying in my shop sucking up space. So I will try this and document results. I am always shopping for parts when BMW 320i calipers present themselves I will continue testing and posting results.
I also have a 78 928 single piston floating calipers that mount to the 914/6 struts. BIG pads capable of stopping my 3300lb beast. Just need to get rotors. next year
I also have a 78 928 single piston floating calipers that mount to the 914/6 struts. BIG pads capable of stopping my 3300lb beast. Just need to get rotors. next year