Rear brakes - Partial Pad Contact on Discs?

[voxmagna]

I just finished servicing MY12 Yeti Tdi 4X4. I measure the pads and discs as routine to determine if I need to do anything this year or next as we are low mileage drivers.

 

When I look at both discs there's a heavily encrusted rust ring about 10-15mm wide where I would have expected pad contact. On my other vags the non contact area on the disc forms a ridge which is only a couple of mm wide.  From what I can see looking into the caliper, the pads should be making contact on most of the disc surface. Before I consider tearing the calipers apart to look further, does anybody else have these wide rust bands of non-contact which you can see through the rear wheel spokes? I already know there are different size discs and pad combinations used on the Yeti rear and discs can be solid or less common vented types.

 

One suggestion put to me is the caliper slide may have seized, but I find it strange to see the same thing on both rear discs. I'd appreciate a comment before I consider pulling the calipers apart - is it normal? Thanks

[FlintstoneR1]

Our 2010 MY Yeti developed the same thing, despite plenty of use and mileage (over 150,000 now). Heavy corrosion towards the outer radius of the rear discs, on the outer face. Despite plenty of friction material left on the (outer) pads and ostensibly still sweeping the corroded area of the disc. (In theory you might think the corroded area would give more friction, but.......)

 

On disassembly of the caliper  (easy peasy - just the two slider bolts and it lifts away) I discovered the inner, piston side pads were down to the thinnest sliver of friction material I've ever encountered, short of metal to metal contact! But still had not become noisy!  Clearly then a pad sticking in the carriers phenomenon, as opposed to the sliders, but could be elements of both I guess.  A new set of Pagid disks and pads mandated and were to hand already!  Interestingly, Euro Car Parts also do new slider kits for Yeti of this era and rear disk design, which my local depot suggested was "quite a common sale item we find". So may also be a wise precaution when ordering the discs and pads. I found the sliders on mine were fine and dandy. So just cleaned them up and replaced. Returning the new sliders to ECP unused.

 

Hence I'd recommend as you suggest and drop the calipers out to check the pads are sliding properly on the carriers.  I plan to do this annually from now on, having fitted the new disks. A little drop of copper grease on the contact surface of the pad "ears", where they sit onto the carrier brackets, will hopefully do the trick?

 

A word of warning though - my Yeti - 2010 2.0 TDi CR 110PS 2WD utilises the smaller diameter solid rear disks, 254mm from memory. I was able to angle these enough to remove and replace the new discs without removing the pad carrier brackets. Which I just cleaned up carefully and left in situ on the hubs. If your Yeti uses the larger diameter (272mm??) discs, and I believe most 4x4s do, then the pad carrier brackets may have to come off to get the discs off?  If so beware the pair of bolts that hold the carrier to the hub are a well known "sod" to remove. They are loctited into the hub threads AND require a relatively unusual short length, 12-spline male, M14 socket to fit the female splined bolt heads. There is little room to access the bold heads past the drive shafts and suspension arms. A long extension and a universal joint may be necessary, as well as a good breaker bar. There's a very good description by the Plumber in the Yeti Technical section!

 

Hope it all works as well for you as it did for me.

Edited 27 June, 2016 by FlintstoneR1

[voxmagna]

Thanks, that's what I wanted to hear - somebody who has been there before. I'm picking up new pads tomorrow and am holding off on the discs until get a good look. I've seen a lot of disc and pad combinations in the parts list. it's almost as if they couldn't make their minds up. I'd rather have the vented discs, but putting those on would need a load of expensive parts. What I cannot understand yet is why the rusting and crusting is so bad  (like you normally get on the outer edge of the disc). The pad hasn't been running there or the pad surface isn't flat anymore and has a groove worn across it. Even then as the pad wears I would have expected some contact to wear off the rust.

 

Thanks for the heads up on the Loctited bolts and forewarning I might need to get another spline socket. I've met those before and you never cheap out on quality else you spend all day with an angle grinder removing the bolt. I've dealt with Loctite before, Usually it needs blow torch heat to around 170 deg. C if you can heat the part without destroying anything else.  I've also got a 12 volt impact driver with digital torque settings as backup. I guess those bolts come pre-coated and are recommended to replace, although I have plenty of Loctite. If the bolts are the type needing angular torque, they will have been stretched so I'll check that out and let you know what I find.

[wiilydog]

The rear spline carrier bolts will be stretch bolts as you tighten 90 nm + 90° so should replaced as you are only suppose to use them once only.

[FlintstoneR1]

There you go!  Thanks Wiilydog.  I'll add that to my personal Yeti knowledge base.

 

Hopefully I'll not need to replace the carriers on my car, as I can remove the discs OK with pad carriers still in place. Although if the discs I put on new in March last as long as the originals (140,000 miles and nearly 6 years), I'll not need to swap them again? 

 

I suspect the corrosion the rear discs seem to be a little susceptible to, toward the outer radius of the swept surface, has as much to do with rear disks getting very little of the braking effort overall, compared to any other cause.  So the rear pads never get full force applied, compared to the fronts, and so become prone to sticking on the carriers over time?  Unless anyone has a better theory?

[voxmagna]

I'm on the case now. Had frustration getting the correct pads and wasted trips to Eurocarparts. There are 4 variants and even the VIN number doesn't help. Had to take in an old pad. WEWhen I removed the pads they were down from 11mm to 6mm and not exactly worn out completely. The discs measured against the OE spec are not down much and replacing pads first, discs and pads next year. The inside pad was running flat with less evidence of rusting on the 10-15mm outer contact edge of the disc. The outside pad definitely had a wide wear groove corresponding to the rust band, but there were no signs it had been making contact due to the build up of rust.

 

I am at a loss for the cause and ideas. The single piston caliper design can centralise the inside pad in direct contact with the piston, but doesn't do a good job on the outside pad which is centralised by the fork. The design relies on the whole assembly being able to float. At the moment it looks like there is uneven contact on the outer pad with the fork because it wasn't making contact at the outer 10-15mm of the disc.

 

Knowing your disc diameter is very important to identify the correct pad parts. Run a length of string around the disc and measure its circumference. Work out the disc diameter  by dividing the measured circumference  by 3.143 (pi). Less prone to error than measuring diameter across the disc.

[SuperbTWM]

I've had various cars with this issue and not just the rear brakes either. No obvious issues it just seems to happen sometimes.

[muddyboots]

The original rears I took off mine were terrible, never seen discs like it. The rear faces (hard to see) were the worst. The inner pads had worn down to the metal backing while the outers still had a few mm.

 

The replacements I fitted seem to have worn nice and evenly; as it happens I ordered a complete set of front & rear discs & pads last night as they're due for changing again.

Am also changing sliders front & rear - not sure it's strictly necessary but might as well while I'm at it.

 

I've seen a number of recommendations not to use good old copper grease - but to use something like Ceratec ceramic grease, apparently gets less sticky/congealed than copper grease ?

It's cheap enough, I ordered some last night.

 

The rear carriers are a pain to get off. IIRC I had to lift the rear of the car quite high off the ground to get a long enough handle on the bolts. The first spline bit I bought was too long and I couldn't get at the bolts, with any combination of 1/2" extensions and universal joints.

I also fell foul of getting the wrong size discs and pads for the rears first time. Turns out mine has the 272mm Bosch rears.

 

In fact - if anyone needs some 285(?)mm rear discs and EBC Yellowstuff pads, they're still sitting in my garage....

Edited 29 June, 2016 by muddyboots

[voxmagna]

I haven't owned MY12 Yeti long enough to prove the rears are wearing faster than the fronts. Just a gut feeling (is it due to 4WD?). I was therefore surprised not to find pad wear indicators and I will be keeping a close eye. But as you said, I think the inner pad wears faster and is the hardest to see. I've found those spline carrier bolts but I'll leave those for another day (or year).

Edited 29 June, 2016 by voxmagna

[Urrell]

I haven't owned MY12 Yeti long enough to prove the rears are wearing faster than the fronts. Just a gut feeling (is it due to 4WD?).

It's because they don't get as much use and so suffer more from corrosion.

[Ryeman]

The weight of the rear drive components (.Haldex etc) should mean more effective rear braking grip I'd have thought.

[voxmagna]

There's another curious thing about their pads. You normally aim to get as much friction surface from the pad on to the disc as possible.  These pads are shaped with huge wedges at each end of the friction surface. Initially it looks like only 2/3rds the pad area is in contact with the disc, which drops when pad wear is about half way down from 11mm. Why would they sacrifice such a large section of the available pad surface and shorten pad life? Surely not just for stopping squeaks. Or with pressure distributed over the full area of the available pad surface, are they worried the hand brake might not hold??

[Urrell]

The fronts suffer too.
This was off my Fabia, the outer looked fine and pad only about half worn but the inner was a different story.
The sliders were in excellent condition!!!

[voxmagna]

On your right photo I'm seeing the same kind of rust band around your worn disc as on my rears. The pads cannot be making contact over the whole disc surface, otherwise there would be no banding, just a shiny disc with wear grooves. On your left photo showing the new disc and pads, those pads have the same taper depth reduction as mine.  Modern discs have been criticised for their poor wear properties due to their metallurgy when actually I think their high wear rate is due to manufacturers switching to hard pads. On a diesel vehicle heavier on brakes they have design choices. More expensive larger heavier ventilated discs and pads running cool, or cheaper smaller discs and pads adding less weight and fit harder pads.

 

When I looked at my rear calipers I could see how they are pulled in and out, but not how pads is are centralized.  I had zero play on the pins. If there was wear , maybe there would be some wiggle room for each pad surface to run true to the disc? I think the problems start with this design of single piston caliper, which is cheaper to make.

[muddyboots]

My original rears:

 

 

Didn't look too bad from the outside:

 

[Ryeman]

Did things change around the time asbestos was banned?........a long time ago now

Did the discs last longer back then?.

I'd recon voxmagna is right about the handbrake grip issue determining the pad design.

[voxmagna]

You don't get that kind of rusting from just water. The deep pitted rusting comes from water and heat cycling plus all the salt we put on UK roads.  The harder pads and relatively small size discs create a lot of friction = heat when used on heavier vehicles. When the vehicle weight and braking duty (for diesels) goes up, manufacturers can either replace solid discs with larger ventilated or change pad formulation (harder) to achieve the required braking effort and reduced fade. But the heat output on standard solid discs goes up. The cheapest option will always be fit the harder pads and not change the basic design.  According to the parts lists there are some Yetis that have ventilated discs on the rear I would like to have, but I don't know which models they are. What I do know is doing a conversion requires the entire caliper kaboodle to be replaced with compatible parts.

 

Asbestos and asbestos free pads aren't the issue as many older cars had to swap over and I read no reports of them being much different. The problem is the pad material and quality of steel for the disc. The old standard pad used to be called Metal Fren and were impregnated with soft brass. Todays pads are a much harder material composition and brass is no longer used - I suspect iron or steel impregnation. Disc steel can be from castings, forged and contain various amounts of expensive chromium the more chromium you add, the steel tends towards stainless steel. Car discs will rust very quickly so I suspect they are cheap casted with very little chromium in the metal.  Some motorcyclist interested in Sport riding switch their pads to HH hard grade for the anti fade benefits. But their disc life is a lot shorter. I looked into changing pads on one of my vags for a softer pad. It's hard to find them because any aftermarket product has to link to the OE spec. I think Greenstuff can do something softer but I didn't go further on the research. I only fit Kevlar pads on my bike. They are soft and the discs don't wear or score.

[Ryeman]

I remember 'sintered' pads which seemed like metal filings added to the pad material .

[trundlenut]

The pads don't move properly in the sliders due to a build up of crap and corrosion (even with the metal shims) and so when the piston pushes the pad onto the disc it twists and doesn't hit the disc straight. Hence the rust on the outer edge of the disc.

You need to clean up everything carefully and preferably replace the shims. Watch out though and not all pads come with the shims.

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[voxmagna]

There are no shims fitted on my Yeti rear brakes! How could they fit a shim between the pad backing and the fork on the outer side? The fork makes direct contact with the pad back and there is no shim on the other side - or are they missing. I thought the purpose of shims was more for anti sqeel than alignment? The rears have stainless spring clips which I expect are there to allow some float. I understand what you mean on normal 2 pot calipers, but not for these. You say the build up of crap. That is true of a 2 pot caliper where the pads are pushed through slots in the caliper assembly which get gunged up.  But on these the pad movement is from the single piston on the inside and equalizing force from the fork on the outer side. The pins holding the caliper to the bracket only allow the caliper to centralize and don't fix any twist there may be between each pad and its contact with the disc. I can't see how they can get each pad to run and stay true to the disc if there is no float. I think they rely on making each half of the sliding parts in perfect alignment which works when all parts start off new, but doesn't seem to stay true when there's some wear use.

 

If others see a similar rust band effect then I'll put it down to the design. I think if you replace both pads and discs everything will start off looking ok, until the pads start wearing in.  I have an idea for next time I'm in there to replace the discs. I kept the old pads and I can strip back the friction material, true up the metal surface then replace the friction material with 10mm tool steel or plate. With these dummy pads fitted I can blue them up or use feelers to see how surface contact is made on a new disc.  

[muddyboots]

If others see a similar rust band effect then I'll put it down to the design. I think if you replace both pads and discs everything will start off looking ok, until the pads start wearing in.  I have an idea for next time I'm in there to replace the discs. I kept the old pads and I can strip back the friction material, true up the metal surface then replace the friction material with 10mm tool steel or plate. With these dummy pads fitted I can blue them up or use feelers to see how surface contact is made on a new disc.

Rusty rear discs is a fairly common complaint, I've seen a number of discussions about it and noticed it on many cars that I've walked past (I'm a bit sad like that...).

My Yeti - which I bought at 3yrs old / ~30k miles) had corroded rear discs (see photos above).

I replaced the rear discs and pads; I've done 30k since changing them and the rear pads are now worn out, but there is no sign of any corrosion on the disc whatsoever.

To me that suggests it's not a fundamental design issue of the pads/caliper, but either due to materials used, or how the brakes are used (or both).

[countryboy]

Mine is like that, it seems to be a common complaint om the Bosch rear brakes used on the 4x4 Yeti's. There is a good how to guide in the Yeti technical section on how to solve this by replacing the sliders posted by The Plumber a few years ago. Have a look at this. http://www.briskoda.net/forums/topic/286945-yeti-4x4-bosch-rear-brakes-from-wk4509-fitting-a-slider-kit-repair-set/

Ian

Edited 3 July, 2016 by countryboy

[trundlenut]

There are no shims fitted on my Yeti rear brakes! How could they fit a shim between the pad backing and the fork on the outer side? The fork makes direct contact with the pad back and there is no shim on the other side - or are they missing. I thought the purpose of shims was more for anti sqeel than alignment? The rears have stainless spring clips which I expect are there to allow some float. I understand what you mean on normal 2 pot calipers, but not for these. You say the build up of crap. That is true of a 2 pot caliper where the pads are pushed through slots in the caliper assembly which get gunged up. But on these the pad movement is from the single piston on the inside and equalizing force from the fork on the outer side. The pins holding the caliper to the bracket only allow the caliper to centralize and don't fix any twist there may be between each pad and its contact with the disc. I can't see how they can get each pad to run and stay true to the disc if there is no float. I think they rely on making each half of the sliding parts in perfect alignment which works when all parts start off new, but doesn't seem to stay true when there's some wear use.

If others see a similar rust band effect then I'll put it down to the design. I think if you replace both pads and discs everything will start off looking ok, until the pads start wearing in. I have an idea for next time I'm in there to replace the discs. I kept the old pads and I can strip back the friction material, true up the metal surface then replace the friction material with 10mm tool steel or plate. With these dummy pads fitted I can blue them up or use feelers to see how surface contact is made on a new disc.

All the vag rear disc brakes I have seen have the shims, they're stainless and are there to stop the pad catching against the cast carrier, which isn't particularly smooth.

Like I said not all pads come with the shims.

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[voxmagna]

O.K I'll see if I can see anything on the parts list. I've just read through The Plumbers write up on the slider pins. Mine were a bit stiff. I'm curious though about a rubber sleeve that fits on the end of each pin. Does that provide some 'compliance' which allows the fork to centre on the pad? I'm only seeing partial contact on the outside pads i.e those centered by the fork which suggests the fork is where I should be looking further.

Edited 3 July, 2016 by voxmagna

[trundlenut]

O.K I'll see if I can see anything on the parts list. I've just read through The Plumbers write up on the slider pins. Mine were a bit stiff. I'm curious though about a rubber sleeve that fits on the end of each pin. Does that provide some 'compliance' which allows the fork to centre on the pad? I'm only seeing partial contact on the outside pads i.e those centered by the fork which suggests the fork is where I should be looking further.

IIRC the rubber sleeve covers the pin, that's all it does.

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