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Discussion Starter · #1 ·
Hey fellow PW'rs,

I have an issue where the rear brake pads are wearing out prematurely.. I just took my Variant in for its 40k tune, and they said the rear pads had only mm's of life yet, but that the fronts were fine.. Have any of you experienced this in your cars? I would expect the opposite from a normal functioning brake system.. I do use compression baking a lot and heel-toe my shifts on almost every stop, would the added braking of the engine lighten the load on the front and the rears wind up over compensating? Or is this some simple brake bias adjustment that was never properly set at the factory?

Thanks in advance,

DJ

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YES! I was wondering about this, because the Passat is the only car that came to mind that I've heard of with rear bias. Never figured out why...
 

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A rear brake bias is normal for Passats and I imagine quite a few other cars. I think most passat owners have experienced there rears wearing out far sooner than the fronts. One effect of the rear bias is that if your abs ever goes out the rears are more likely to lock up the rears and cause a spin. As to the reason why I am not positive but can think of a few plausible explanations
#1 Because the ABS prevents lock up having the rear brake more makes the tail of the car stay in line.
#2 it also evens out tire wear front to back (fronts wear out on accel rears on braking) probably just a side effect
#3 Increases percentage of front tire grip for steering and perhaps lessens nose dive

Ok so those were mostly spectulation hopefully someone with a real understanding will chime in
 

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Discussion Starter · #4 ·
Just knowing that I am not the only one experiencing this is almost comfort enough.. Your reasons seem to have merit, thanks for offering some insight.

DJ
 

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you should rather have the fronts lock up first under crazy braking.

you'll see race car drivers dial bias just enough to make the fronts lock up first. Under hard braking and while releasing the brakes to enter a turn... rear lock up equals SPIN! Most of the time we pound on them in an emergency we aren't facings straight on on a level surface.
 

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My V6 4Motion got 40k out of the rears, which the dealer said was about average.
I think Passat (and other VAG cars) is rear biased to prevent "nose dive" during braking as our cars have a 60/40 front to rear weight distribution.
BMW has a near perfect 50/50.
 

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You got 40K out of the rears? You're not doing too badly. Some people here have changed out the rears at 25,000 miles.
 

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i doubt increased wear is due to the brake bias. most likely its due to dragging pads...due to the parking brake ratcheting mechanism and sticky carrier sliders.
 

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A rear brake bias is normal for Passats and I imagine quite a few other cars. I think most passat owners have experienced there rears wearing out far sooner than the fronts. One effect of the rear bias is that if your abs ever goes out the rears are more likely to lock up the rears and cause a spin. As to the reason why I am not positive but can think of a few plausible explanations
#1 Because the ABS prevents lock up having the rear brake more makes the tail of the car stay in line.
#2 it also evens out tire wear front to back (fronts wear out on accel rears on braking) probably just a side effect
#3 Increases percentage of front tire grip for steering and perhaps lessens nose dive

Ok so those were mostly spectulation hopefully someone with a real understanding will chime in
1. No. The friction coefficient of a tire that is stick-slipping under the control of ABS is still lower than the friction coefficient of a tire that isn't slipping at all -- even if the ABS-controlled stick-slip is only for fractions of a second at a time. As such, if the back end skids first, the back end will take longer to slow down than the front. If you're going through a corner, you know what happens when the front slows down faster than the back.

2. This is probably true.

3. Doubtful. The front brakes are enormous compared to the rear brakes, and so they will generate more stopping force than the rears under any conditions.

Most likely, the reason why the Passat has a rear brake bias is because that allows the rear brakes (which are much smaller than the fronts) to generate more brakeforce proportional to their size than the fronts do, which is still much less total brakeforce than the fronts generate, but at least it means VW didn't have to spring for bigger and more expensive rear brakes to get the same effect.
 

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Discussion Starter · #10 ·
OK, so it seems this is normal and there is no need to worry about adjusting the bias more toward the front, if this would have even been possible.. I did replace the pads with Mintex Reds and thoroughly cleaned the caliper bodies when doing it, if there is drag now it is due more so to poor deisgn than anything else.. The pads do throw off a ton of overheated-brake type stink when pushed, I am guessing this will go away when broken in, just seems odd other than the explanation to overcome the weight distribution, but I would think spring and damping rates would solve for this.

Thanks for all the input,

DJ
 

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The bias mentioned is not a static bias. The rear are only given more juice under light braking.

So when your shaving a little speed off on the freeway or any other light braking instance, it's heavy rears... once you step in it, the front's do most of the work.

Any car with ABS is going to have this feature. It's there to keep the squatting to a minimum... that's it.

If you're going thru rears quickly, you are most likely a gentle driver that spends alot of time on the freeway or in traffic.
 

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if you have a couple of mm's left o fuseable thickness, then that's plenty of life. the difference between good and bad rotors are only a couple of mm to begin with.
 

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Discussion Starter · #13 ·
Thanks Pass-Variant,

Undedrstood, is there any influence from heel-toe down shifts when braking that would compound the issue you noted above? I don't consider myself a light driver, smooth maybe, but light I think would not apply.

Perhaps having the engine handle some of the front wheel braking is making the rears function in light mode?

Much appreciated,


DJ
 

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The bias mentioned is not a static bias. The rear are only given more juice under light braking.

So when your shaving a little speed off on the freeway or any other light braking instance, it's heavy rears... once you step in it, the front's do most of the work.

Any car with ABS is going to have this feature. It's there to keep the squatting to a minimum... that's it.

If you're going thru rears quickly, you are most likely a gentle driver that spends alot of time on the freeway or in traffic.
Wow. That's really useful information.
 

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Thanks Pass-Variant,

Undedrstood, is there any influence from heel-toe down shifts when braking that would compound the issue you noted above? I don't consider myself a light driver, smooth maybe, but light I think would not apply.

Perhaps having the engine handle some of the front wheel braking is making the rears function in light mode?

Much appreciated,


DJ
There's no perhaps about it. If you're using the engine to slow down, then you don't need to step on the brakes as hard, and the pressure-sensitive bias that pass-variant mentioned is going to make your rear rotors do most of the work and wear out faster.
 

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I learned about this on a 1995 Accord I had. I brought it in for a check and they said the rear rotors and pads needed replacement (25k miles). I couldn' believe it. I said that it bust be a stuck caliper, and he said, no, all 4 pads are worn evenly, and explained why.

I made him save the old pads so I could see, because I didn't believe him. The machanic, said "Sure, but you owe me a 6-pack if I'm right". So I picked one up on the way.

He about fell over when I gave it to him.
 

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There was some thought into making the Passat handle like a rear drive car.
Notice you do not get twitchiness in the steering when you apply the brakes.
A rear brake bias may improve this feel.
But I agree with poster that said more total braking is done in the front; the rears may be undersized.
 

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... the rears may be undersized.
They are. The pads are smaller (surface area) than the fronts, and brand new are thinner (pad thickness) than the fronts. You would have thought to make the pads wear more evenly (front and rear) this would have been corrected early on.

But since pads are not covered under warranty, VW probably did not care.
 

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1. No. The friction coefficient of a tire that is stick-slipping under the control of ABS is still lower than the friction coefficient of a tire that isn't slipping at all -- even if the ABS-controlled stick-slip is only for fractions of a second at a time. As such, if the back end skids first, the back end will take longer to slow down than the front. If you're going through a corner, you know what happens when the front slows down faster than the back.

2. This is probably true.

3. Doubtful. The front brakes are enormous compared to the rear brakes, and so they will generate more stopping force than the rears under any conditions.

Most likely, the reason why the Passat has a rear brake bias is because that allows the rear brakes (which are much smaller than the fronts) to generate more brakeforce proportional to their size than the fronts do, which is still much less total brakeforce than the fronts generate, but at least it means VW didn't have to spring for bigger and more expensive rear brakes to get the same effect.

Ok the points are moot if what pass-variant says is correct but I don't think your points are valid.

The size of a brake doesn't deteremine the the stopping force it is capable of applying. larger brakes are needed to dissipate heat and reduce wear. If you have to make one stop a tiny brake will be just fine. Just about any size brake is capable of locking up your wheels. A larger brake just lets you do it over and over again.
Also while it is true that a wheel using ABS cannot apply as much braking force as a wheel ideally linearly braked the point is that it still applies a great deal more braking force than a locked wheel and because it turns 99% of the time it will substaintailly reduce that possibilty of fishtailing when the abs is activated for the rears and not the fronts (is that even possible?). basically what I am saying is that even if the rears provide less braking force than the ideal situation it doesn't mean they will cause a spin.
 

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My Passat used to be rear biased until I did a DYI brake job (rears lasted 50K mi), now the bias has shifted to the front. I would love to equalize it or even shift it backward a little. Anyone know how???
 
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