# How Does This Happen: Uneven Pad Wear?



## Stealth_TDI (Nov 12, 2003)

Hello,
Yesterday I heard something a bit odd from my front right wheel. So I pulled the wheel today and saw that my pads were in severe need of replacing. This was a surprise because my Akebono pads appeared to be wearing well. So I pulled the right pads and found this:








The thinner pad was on the outboard side of the rotor. The wear is also crooked:








Here's the driver's side for comparison:








These have about 40,000-miles on them. *How did this happen?* Did I perhaps put them in crooked? I've never felt any weird pulling in one direction or anything. Everything seemed quite normal until Friday when the new noise started. The rotor is just barely scoared from the worn pad. I caught this really fast. The new pads will cover the scoring in no time I'm sure.








You'll notice the pads are beveled when new. I thought nothing of it before. But now that my Axxis pads are installed, I have to wonder if unbeveled pads bite better since there's more pad touching the rotor? I appreciate your opinion on that. For the record, Axxis was not my first choice of pad. But it was my best choice given my time constraints: I needed pads NOW since the car had to be back on the road just three hours after I discovered I had a problem; and I preferred a ceramic pad if I could get one.
Thoughts?
Scott


----------



## GTijoejoe (Oct 6, 2001)

*Re: How Does This Happen: Uneven Pad Wear? (Stealth_TDI)*

Scott,
The slot in the pad is for friction clean up, as the pad/rotor/corrosion wears off the slot takes up the debree insted of sliding across the entire pad. The bevel is really minimal compared to the overall area of the pad, more pad allows for less pad wear and less heat.
There are two directions of pad taper. Radial and axial. Radial of course being from the bottom of the pad out to the top, and axial being from leading edge of the pad to the trailing edge. You have can have a common case of both types of wear on the same pad, this is not so abnormal. 
Reason for radial pad wear can be from the caliper flex (low stiffness caliper body), the claw of the caliper will deform like you are opening a book with two hands. As the caliper bites the inboard pad stays fairly even (but still can taper in some cases) and the outboard pad will shift higher on the disk and have an uneven load distribution. The uneven load will increase the heat on outer edge of the disk causing an uneven wear or "taper". A misconception is most ppl believe the inboard pad will taper more although it is not always the case. 
Reason for axial pad wear is when the caliper bites the torque developed will make the caliper pitch in the direction of the disk's motion. From the caliper's pins and body being less stiff the caliper will be ****-eyed in the housing, causing uneven load distribution and heating the pad up non uniformly, this causes taper. 
So no, you did not put the pads in the calipers wrong. 
Hope this helps http://****************.com/smile/emthup.gif 
PS... I do think it is strange one corner side wore a lot differently than the other. Could be from many things from the caliper itself, to not tightening the bolts properlly, or to doing heavy braking while turning often.










_Modified by GTijoejoe at 2:53 AM 8-3-2008_


----------



## Stealth_TDI (Nov 12, 2003)

*Re: How Does This Happen: Uneven Pad Wear? (GTijoejoe)*

Thanks for that explanation. I guess the only question remaining for me: Why are the wear rates so dramatically different between the driver's side and the passenger side?


----------



## GTijoejoe (Oct 6, 2001)

*Re: How Does This Happen: Uneven Pad Wear? (Stealth_TDI)*


_Quote, originally posted by *Stealth_TDI* »_Thanks for that explanation. I guess the only question remaining for me: Why are the wear rates so dramatically different between the driver's side and the passenger side?


_Quote, originally posted by *GTijoejoe* »_I do think it is strange one corner side wore a lot differently than the other. Could be from many things from the caliper itself, to not tightening the bolts properlly, or to doing heavy braking while turning often.









_Modified by GTijoejoe at 2:53 AM 8-3-2008_

As stated I honestly don't know, I could only continue to speculate. Usually when we do testing analysis on calipers it is on a bench dyno to similate different senarios. It is much faster and often cheaper to do it under these conditions. I would put my money on the caliper itself. If such a wear condition continues to occur even with different friction and/or disks, I would replace the entire caliper. If that didn't fix the problem then you would have to investigate more and determine if your line pressure right to left are even.


----------



## Stealth_TDI (Nov 12, 2003)

*Re: How Does This Happen: Uneven Pad Wear? (GTijoejoe)*

Ahh,
Thanks again. I didn't see your PS. I went straight from what I read in my e-mail to submitting my reply. FWIW, I autocross the car once a month. It explains uneven wear between rotor sides; but not between car sides. I will keep an eye on this and see how these new pads wear in after a while. I'll replace the calipers if things look funny again.
Scott


----------



## GTijoejoe (Oct 6, 2001)

*Re: How Does This Happen: Uneven Pad Wear? (Stealth_TDI)*


_Quote, originally posted by *Stealth_TDI* »_Ahh,
Thanks again. I didn't see your PS. I went straight from what I read in my e-mail to submitting my reply. FWIW, I autocross the car once a month. It explains uneven wear between rotor sides; but not between car sides. I will keep an eye on this and see how these new pads wear in after a while. I'll replace the calipers if things look funny again.
Scott

As noted, if you are doing high G decels while you are turning, it will load on side higher than the other, giving you the ability to increase brake torque/decrease brake torque on the opp. side and load the caliper respectively from one side to the other... just a though, its certainly possible if you do it ALOT.


----------



## Old Guy GTI (Jun 16, 2008)

Perhaps making a lot more left hand turns will wear the right wheel pads a little faster from the increased distance the wheel travels?


----------



## 20vtVdubb (Feb 27, 2008)

*Re: How Does This Happen: Uneven Pad Wear? (GTijoejoe)*


_Quote, originally posted by *GTijoejoe* »_Scott,
The slot in the pad is for friction clean up, as the pad/rotor/corrosion wears off the slot takes up the debree insted of sliding across the entire pad. The bevel is really minimal compared to the overall area of the pad, more pad allows for less pad wear and less heat.
There are two directions of pad taper. Radial and axial. Radial of course being from the bottom of the pad out to the top, and axial being from leading edge of the pad to the trailing edge. You have can have a common case of both types of wear on the same pad, this is not so abnormal. 
Reason for radial pad wear can be from the caliper flex (low stiffness caliper body), the claw of the caliper will deform like you are opening a book with two hands. As the caliper bites the inboard pad stays fairly even (but still can taper in some cases) and the outboard pad will shift higher on the disk and have an uneven load distribution. The uneven load will increase the heat on outer edge of the disk causing an uneven wear or "taper". A misconception is most ppl believe the inboard pad will taper more although it is not always the case. 
Reason for axial pad wear is when the caliper bites the torque developed will make the caliper pitch in the direction of the disk's motion. From the caliper's pins and body being less stiff the caliper will be ****-eyed in the housing, causing uneven load distribution and heating the pad up non uniformly, this causes taper. 
So no, you did not put the pads in the calipers wrong. 
Hope this helps http://****************.com/smile/emthup.gif 
PS... I do think it is strange one corner side wore a lot differently than the other. Could be from many things from the caliper itself, to not tightening the bolts properlly, or to doing heavy braking while turning often.









_Modified by GTijoejoe at 2:53 AM 8-3-2008_
 http://****************.com/smile/emthup.gif http://****************.com/smile/emthup.gif


----------



## JazzGTI (Jul 23, 2003)

*Re: How Does This Happen: Uneven Pad Wear? (Stealth_TDI)*

Possible siezed caliper or sticking one.


----------



## Mortal_Wombat (Jan 29, 2004)

*Re: How Does This Happen: Uneven Pad Wear? (JazzGTI)*


_Quote, originally posted by *JazzGTI* »_Possible siezed caliper or sticking one.

i was about to say.
whats seen in the pictures is NOT normal pad wear.
could also be caused by binding slide pins.


----------



## inarisilver78 (Apr 6, 2008)

*Re: How Does This Happen: Uneven Pad Wear? (Mortal_Wombat)*


_Quote, originally posted by *Mortal_Wombat* »_
i was about to say.
whats seen in the pictures is NOT normal pad wear.
could also be caused by binding slide pins.


Ya, take off your calipers and check the slide pins, use only HIGH TEMP caliper grease to lube the sliders, make sure the slider seals (accordian looking things) are not cracked or dry rotted. lube well and slide the pins in and out a couple of times to lube the tube where the pin slides into. You will be good to go after that


----------



## germancarnut51 (Mar 23, 2008)

*Re: How Does This Happen: Uneven Pad Wear? (Stealth_TDI)*

Some uneveness is always evident on sliding calipers disc brake pads.
It comes from the fact that there is only one piston, and the slide provides the pressurefor braking, and that force is not evenly released.
The same type of wear is not seen on dual piston brake calipers.
More wear at the leading edge of the pads is also normal. In fact, it's desirable. Many brake manufacturer's purposely toe-in the brake pads to improve brake performance (toeing in the brake pads makes the leading edge of the pads make contact first, which helps increase braking friction by using the rotation of the rotors to pulls the pads in against the rotors.).

Don't worry about the slightly uneven wear, or the fact that the driver's
side wears faster than the passenger side. That is related to the length of the brake lines that lead to each wheel.


_Modified by germancarnut51 at 9:40 AM 8-11-2008_


----------



## GTijoejoe (Oct 6, 2001)

*Re: How Does This Happen: Uneven Pad Wear? (germancarnut51)*


_Quote, originally posted by *germancarnut51* »_1) Some uneveness is always evident on sliding calipers disc brake pads.
2) It comes from the fact that there is only one piston, and the slide provides the pressurefor braking, and that force is not evenly released.
3) The same type of wear is not seen on dual piston brake calipers.
4) More wear at the leading edge of the pads is also normal. In fact, it's desirable. Many brake manufacturer's purposely toe-in the brake pads to improve brake performance (toeing in the brake pads makes the leading edge of the pads make contact first, which helps increase braking friction by using the rotation of the rotors to pulls the pads in against the rotors.).

5) Don't worry about the slightly uneven wear, or the fact that the driver's
side wears faster than the passenger side. That is related to the length of the brake lines that lead to each wheel.

_Modified by germancarnut51 at 9:40 AM 8-11-2008_








Where have you got your information from??? Can you provide any supporting evidence for statements 1-5?
Thanks


----------



## Mortal_Wombat (Jan 29, 2004)

*Re: How Does This Happen: Uneven Pad Wear? (germancarnut51)*


_Quote, originally posted by *germancarnut51* »_Some uneveness is always evident on sliding calipers disc brake pads.
It comes from the fact that there is only one piston, and the slide provides the pressurefor braking, and that force is not evenly released.
The same type of wear is not seen on dual piston brake calipers.
More wear at the leading edge of the pads is also normal. In fact, it's desirable. Many brake manufacturer's purposely toe-in the brake pads to improve brake performance (toeing in the brake pads makes the leading edge of the pads make contact first, which helps increase braking friction by using the rotation of the rotors to pulls the pads in against the rotors.).

Don't worry about the slightly uneven wear, or the fact that the driver's
side wears faster than the passenger side. That is related to the length of the brake lines that lead to each wheel.

_Modified by germancarnut51 at 9:40 AM 8-11-2008_

that last statement is just completely incorrect.


----------



## germancarnut51 (Mar 23, 2008)

*Re: How Does This Happen: Uneven Pad Wear? (Mortal_Wombat)*

I have been fixing and driving cars with disc brakes for heding on 40 years now. I own cars with both dual piston and sliding piston brakes.
In my experience, the dual piston calipers wear the pads fairly evenly (inner and outer). This is not the case with single piston sliding caliper disc brakes, which are more economical to manufacture, and so are more common. Single piston sliding caliper brakes wear the pads unevenly, weigh more, have more parts, and require more maintenance than dual caliper designs (which are more expensive).
Many brake manufacturers build their brakes with toe-in. When your pads are toed-in, the leading edge strikes the rotor first. As the rotor turns, the leading edge of the pads are pulled, rolling/pulling the rest of the pad into contact with the rotor.
Don't understand how this works? The easiest way to see how this works would be a visit to a bicycle shop. Take a look at the new bikes that have been prepped and are ready for delivery. Look at the brake pads. You will see that they are toed-in. Try spinning a wheel slowly by hand, and pull the brake lever, watch how the brake pads react.
On my Porsches, the factory service workshop manuals give a fair explanation of this along with the instructions to make sure that the caliper pistons machined steps (which hold the brake pads toed-in) are properly positioned. I made a gauge to substitute for the factory tool that sets the piston angle.
I have not seen the same machined steps on the brake calipers of the Golfs and Jettas that I have worked on, but that does not mean that VW does not build the toe-in to the calipers when they are manufactured. This would save the steps that are required when rebuilding Porsche single piston sliding caliper brakes (like those used on 924, 924S, 944. The more expensive Porsche Models use dual piston or quad piston disc brake calipers (non-sliding). 
Brake wear is never going to be exactly even on all four wheels. Because of the way that brakes are designed, the fronts will always wear more than the rears (less so on cars with ABS because more braking occurs on the rear on cars with ABS). The wheel with the shortest brake line (on the same end of the car) will always show the most brake pad/shoe wear unless there is a mechanical problem.
Most all brake calipers flex, expensive ones as well as the inexpensive ones. The "M" alloy dual piston disc brake calipers desired by many early 911 owners are known to be flexy. Their main claim to fame is their light weight. Later production 911s went back to iron calipers, not only for the lower cost, but for the fact that they are more rigid.
I have never seen any sliding caliper single piston disc brake that wears the inside and outside pads at exactly the same rate. I have never seen any car (regardless of whether it has dual piston or single piston sliding caliper disc brakes) that wears the pads exactly the same on the left and right wheels.
So, unless your car is pulling to one side or the other because a brake caliper or shoe is dragging, don't worry about it. Check your brake pads and/or shoes on a regular basis (all four corners) and drive on.


----------



## germancarnut51 (Mar 23, 2008)

*Re: How Does This Happen: Uneven Pad Wear? (Mortal_Wombat)*

Are you saying that every brake system, regardless of type or design will wear the pads or shoes evenly, and every wheel should have exactly the same wear on each pad or shoe, regardless of where on the car it is located or the length of the brake fluid line?
Show me some of those cars.


----------



## germancarnut51 (Mar 23, 2008)

*Re: How Does This Happen: Uneven Pad Wear? (GTijoejoe)*

I never said that the wear on the pads shown by the OP was normal, just that unwear wear on the inside and outside pads, or comparing the left and right wheels on the same axle is normal.
Looking at the photos, the OP states that the outside pad from the right side is the one show the extreme wear. I agree that is not normal, and probably due to some mechanical defect or malfunction. The fact that the right side wheel brake pads confirms this. I would expect that the problem is due to bent or stuck slider pins, or a bent caliper or carrier that is sticking.
In my experience with single piston sliding caliper disc brakes, it is the inner pads that normally wear faster than the outers.


----------



## GTijoejoe (Oct 6, 2001)

*Re: How Does This Happen: Uneven Pad Wear? (germancarnut51)*

I was just curious of your sources because some of this information is false (in my professional experience).

_Quote, originally posted by *germancarnut51* »_1) Some uneveness is always evident on sliding calipers disc brake pads.
2) It comes from the fact that there is only one piston, and the slide provides the pressurefor braking, and that force is not evenly released.
3) The same type of wear is not seen on dual piston brake calipers.
4) More wear at the leading edge of the pads is also normal. In fact, it's desirable. Many brake manufacturer's purposely toe-in the brake pads to improve brake performance (toeing in the brake pads makes the leading edge of the pads make contact first, which helps increase braking friction by using the rotation of the rotors to pulls the pads in against the rotors.).
5) Don't worry about the slightly uneven wear, or the fact that the driver's
side wears faster than the passenger side. That is related to the length of the brake lines that lead to each wheel.

1) True, depending on the caliper, you can never achieve perfect wear, there is no such thing as perfect. Although this is does not only true for floating calipers, fixed calipers will also develop taper. Fixed calipers will usually be more proned to axial taper than radial because of the more equally opposed loading.
2) False The number of pots does not qualify the likely hood of taper. The caliper/brkt stiffness is the deciding factor. I will admit that a multi pot caliper will more likely have a stiffer body to endure higher loads from the larger piston area. I state this because if you really wanted to get into caliper design you could simply design a multi pot caliper to have more taper (of course you would never want to do this). My point is single pot caliper can be designed with a very rigid body.
3)False, depends on the caliper and the loading
4)Not 100%true, not 100% false. Do I understand the toe-in braking? Of course I do, I design brakes for a living. This typically is similar to drum brakes, causing a "self energizing" brake. This gives good inital bit with less effort. Why would it not be desired, it also causes drag. Is this done by "MANY" brake manufacturers? Not that I've seen today, there are other ways. Having a slightly tilted pad also gives other stresses on the body that are not desired, by having a more distributed flat pad leads to better opposing forces on fixed caliper designs. To even out the distribution as you may know is countermeasured by different diameter pistons and also different effective piston radii (the pistons are at different heights), multi pads or even two seperate caliper bodies in the same brkt (there may be others).
5) False, this would imply that the shorter line would have the highest line pressures. Not true, if so this would be countermeasured by line lengths. I have seen extremely similar pad wear from LH side to RH side, there are so many other factors that could result in uneveness.


_Quote, originally posted by *germancarnut51* »_I have been fixing and driving cars with disc brakes for heding on 40 years now. I own cars with both dual piston and sliding piston brakes.
In my experience, the dual piston calipers wear the pads fairly evenly (inner and outer). This is not the case with single piston sliding caliper disc brakes, which are more economical to manufacture, and so are more common. Single piston sliding caliper brakes wear the pads unevenly, weigh more, have more parts, and require more maintenance than dual caliper designs (which are more expensive).
Many brake manufacturers build their brakes with toe-in. When your pads are toed-in, the leading edge strikes the rotor first. As the rotor turns, the leading edge of the pads are pulled, rolling/pulling the rest of the pad into contact with the rotor.
Don't understand how this works? The easiest way to see how this works would be a visit to a bicycle shop. Take a look at the new bikes that have been prepped and are ready for delivery. Look at the brake pads. You will see that they are toed-in. Try spinning a wheel slowly by hand, and pull the brake lever, watch how the brake pads react.
On my Porsches, the factory service workshop manuals give a fair explanation of this along with the instructions to make sure that the caliper pistons machined steps (which hold the brake pads toed-in) are properly positioned. I made a gauge to substitute for the factory tool that sets the piston angle.
I have not seen the same machined steps on the brake calipers of the Golfs and Jettas that I have worked on, but that does not mean that VW does not build the toe-in to the calipers when they are manufactured. This would save the steps that are required when rebuilding Porsche single piston sliding caliper brakes (like those used on 924, 924S, 944. The more expensive Porsche Models use dual piston or quad piston disc brake calipers (non-sliding). 
Brake wear is never going to be exactly even on all four wheels. Because of the way that brakes are designed, the fronts will always wear more than the rears (less so on cars with ABS because more braking occurs on the rear on cars with ABS). The wheel with the shortest brake line (on the same end of the car) will always show the most brake pad/shoe wear unless there is a mechanical problem.
Most all brake calipers flex, expensive ones as well as the inexpensive ones. The "M" alloy dual piston disc brake calipers desired by many early 911 owners are known to be flexy. Their main claim to fame is their light weight. Later production 911s went back to iron calipers, not only for the lower cost, but for the fact that they are more rigid.
I have never seen any sliding caliper single piston disc brake that wears the inside and outside pads at exactly the same rate. I have never seen any car (regardless of whether it has dual piston or single piston sliding caliper disc brakes) that wears the pads exactly the same on the left and right wheels.
So, unless your car is pulling to one side or the other because a brake caliper or shoe is dragging, don't worry about it. Check your brake pads and/or shoes on a regular basis (all four corners) and drive on. 

All calipers "flex" nothing is completely rigid. Iron calipers are 30% more stiff than Aluminum. Certainly iron is less expensive, heavier, and more stiff on the SAME geometry, but aluminum calipers can be countermeasured with adding material to equal or exceed the stiffness. Why would one be choosen over the other, simply for OEM application, COST on low volume production.
Brakes are most always designed and sized for 0% grades. At very low decels 0.1-0.2G you can get your FR and RR pads to almost wear the same (this is because your brake bias is closer to 50/50). But no body decels like that, and the fact that the brakes also have to be designed for its curb weight braking and GVWR (all your cargo and passengers). Because these weight values can be so different especially with trucks, ABS is very useful to control the lock of FR or RR brakes depending on which weight senario you are closer too and which decel rate. So either FR or RR can wear faster, depending on loading and decel. Usually the fronts will wear quicker because we are not always lugging a load around wear ever we drive.
I have seen RH and LH wear to be similar, and I have seen inboard and outboard pads wear to be similar too. Over time with many different senarios in the market, there are many reasons why this does not happen. So I can't deny your experience, though these situations are not likely to be countermeasured by design. http://****************.com/smile/emthup.gif


----------



## Stealth_TDI (Nov 12, 2003)

Hello,
I really appreciate the dialog here. For now, I'm going to suspect the likely need to relube the sliders. If my new pads show the same wear pattern by next year, then I'll suspect a well-worn and flimsy caliper and replace them both. The car has over 270,000 miles and gets abused just a little:
























Take Care,
Scott


----------



## secondgen (Jan 9, 2004)

*Re: How Does This Happen: Uneven Pad Wear? (Stealth_TDI)*

If you find that the slides aren't tight/causing binding, replace the brake hose on that side. They can fail internally creating something like a check valve, building pressure on the caliper side of the failure and not letting the pressure release when you get off the brake. They (brake hoses) can also fail in he opposite direction, which would yield a stiff pedal and a pull in the direction of the functioning caliper. http://****************.com/smile/emthup.gif 
And if you have SS hoses, I've seen them fail just like a normal brake hose.


----------

