Brake Bleeding - Pedal soft then hard - Page 2 -

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post #16 of 18 Old 10-10-2013, 11:21 PM
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Originally Posted by paulkeith View Post
Spieg8 - Once an axle locks, directional control of that axle is lost. Locked tires have less traction than non-locked tires. If the rear locked first, you'd have no control over where it goes and therefore lose directional control of the vehicle, rear trying to pass the front. The proportioning valve is designed to reduce incremental pressure applied to the rear brakes in order to PREVENT the rears from locking up before the fronts. Ideally you lock all 4 at the same time, but if 2 have to go first, you want the fronts first, and then the rears act like the feathers you mentioned.
You are so WRONG, on so many levels......
Really makes me wonder how long you have been driving.
Where do you get this bad information?

Have'n you along, is like loose'n 2 good men
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post #17 of 18 Old 10-11-2013, 12:33 AM Thread Starter
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I guess we’ve pretty much left the thread topic behind but since we’ve levied the personal attacks, might as well bring some fact to the picture:

I will concede that the "you want the fronts first" is an inaccurate way to characterize the point as that too would indicate an unbalanced system; it is really just not rears first.

For starters, the reason race cars use non-ABS is to enable the drivers to threshold brake and get the maximum amount of friction out of the tire. This is not a full (0 rotation) skid or "lock up." Probably about 15% slip but I forget the exact number. Feel free to refer to any number of articles on threshold braking and details on full lock tire friction versus some slip tire friction.

Moving reference to my “bad information”:

Truth be told, it's a pretty simple concept to grasp: for vehicle stability under braking, it is required that the rear brakes do NOT lock before the front brakes.

In the not-so-desirable situation where the rear tires are the first to lock we say that the car is “rear biased”, but the driver would probably have a few more choice adjectives to add. In either case, however, one end of the car has given up before the other, limiting the ultimate deceleration capability of the car.
a car which is severely rear biased will be a scary, twitchy ride resulting in a bad case of the white-knuckle syndrome.

A stock vehicle, in most cases has more of the weight on the front wheels. When you combine this with the fact that there is an approximate weight shift of another 10% of the vehicle weight to the front during braking, you can see the rear brake will lock up first if this condition is not corrected. When the rear brakes lock up on a vehicle, the rear wheels are now traveling faster than the front wheels, which are still in full contact with the road. The result is a car that has the back end pass the front end, better known as "spinning out".

Because braking force should be applied to each wheel in proportion to the weight on it (more weight - more braking force should be applied), there is a requirement to “balance” the braking forces to front and rear wheels. Failure to do so will result in premature lock-up of lightly loaded rear brakes and resulting skid and loss of control. In fact, in hard braking the front brakes perform up to 85% of the braking! In properly balanced brakes, neither the front nor rear brakes will lock up first – braking force is “proportioned” so that they lock up together. A proportioning valve is installed inline between the master cylinder and the rear brakes (be they disc or drum) in order to reduce the pressure increase to the rear as the brakes are applied. The pressure to the rear is not prevented from increasing – the valve just ensures that, after a certain point (the “changeover pressure”) it rises at a lower rate than it does to the front brakes. As the brakes are applied, full pressure is allowed to the rear up to a certain point. Beyond that point, the pressure to the rear increases at a reduced rate, preventing rear brake lock up.

The proportioning valve modulates the pressure to the rear brakes. The modulation is necessary to minimize rear wheel lock up found in heavy braking and to compensate for the differences in braking conditions in front disc / rear drum systems. As pressure is applied to the system full pressure is allowed to the rear drums up to a certain point. Beyond that point the pressure to the rear is reduced preventing rear brake lock up.
It is ultimately the same reason that understeer is safer than oversteer. Not sure how else to explain it.
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post #18 of 18 Old 10-12-2013, 10:07 AM
John Strenk
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Thank you Paul...

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