Uneven wear on brake discs.

[Macadoodle]

The uneven wear on my rear discs was getting really bad after 2 winters and 25k miles so I thought I'd try to figure out what was causing it.
Having stripped the brakes down, it was obvious that the sliding part of the caliper wasn't pulling the outer pad evenly against the disc, the inner disc surface where the piston simply pushes on the pad directly is completely shiny and evenly worn.
The root cause of this unevenness is the unusually strong anti- rattle spring as far as I can see, this in combination with the oddly large and squishy rubber inserts lining the slider pins.

As soon as the spring is installed, it pulls the sliding part of the caliper very noticeably rearwards, meaning the outer pad is now sitting at an angle and it will likely only sit flat against the disc if a fair amount of pressure is applied to overcome the anti- rattle springs' constant tension.
This would explain why the front discs are much less troubled by this, although it's still there on my car at least.

I ended up taking some of the tension out of the spring by gently bending its 'forks' making sure to leave enough to prevent causing a rattle, which would be more annoying than scabby discs ?

The tweaked spring was pulling the caliper rearward much less when I refitted it and in the 20 or so miles I've since travelled, the outer rusty section of the disc is showing definite signs of pad contact and wear.

Fingers crossed that this easy and cheap solution works but it's just another example of half baked design on this car, the calipers are SAIC branded , not from the established manufacturers.

 

[OriginalBigAl]

Nice info, how did you manage to release the auto handbrake and stop it reapplying itself? with the caliper stripped down. And also is the piston a wind back one or just a push back in model, cheers.

 

[siteguru]

Place the driver's seat belt into the holder, press down on the seat to turn on the car, then press down on the brake pedal to get to READY so you could then select Neutral?

 

[OriginalBigAl]

From the passenger side, I take it? That means the car is on whilst you do your work? Bit dodgy, should the car, for some reason, turn off, it might return to P and the handbrake re-engage.
Would disconnecting the 12v keep it in neutral after selecting it?

 

[Lovemyev]

Maybe disconnect the electrical loom plug connector behind the handbrake actuators?
Without any power supplied to the actuators they are unable to operate.

 

[Bricktop X PWR]

From the passenger side, I take it? That means the car is on whilst you do your work? Bit dodgy, should the car, for some reason, turn off, it might return to P and the handbrake re-engage.
Would disconnecting the 12v keep it in neutral after selecting it?

The car won't switch off.
It stays on for as long as you need.

How to Leave my MG4 parked but movable?

Here in Thailand almost every covered car park allows parking at 90 degrees to the marked parking bays, but vehicles must be left in neutral with the handbrake off so the attendants can push them to allow access to the blocked areas. So far I've managed to avoid using these spaces with my MG4...

www.mgevs.com

 

[Macadoodle]

Nice info, how did you manage to release the auto handbrake and stop it reapplying itself? with the caliper stripped down. And also is the piston a wind back one or just a push back in model, cheers.

Handbrake stays off once you select it, I make sure I keep my fingers well clear of any possible problems in the (highly unlikely) scenario of it activating though Piston is wind back.

As a further update - The tweaked spring side is lightly rattling over rough roads so I think I'll have to re-evaluate that.
On the other side I've left the spring as is and simply glued a 0.5mm shim strip made from some scrap steel covering about 1/3 of the back of the outer pad, idea being it tilts the pad slightly towards the unworn portion of the disc.

This seems to be working as I can see the previously untouched outer 50% of the disc now showing signs of contact and wear from the pad.
The rear brakes don't do much so it just needs to tilt the pad enough to fully 'wipe' the disc in normal operation rather than the half wipe they get as standard.
The outer part of my discs are already heavily corroded and pitted after less than 2 years and 25k, worried about it's first MOT next year

 

[OriginalBigAl]

Maybe disconnect the electrical loom plug connector behind the handbrake actuators?
Without any power supplied to the actuators they are unable to operate.

I think, but am not sure, that the default setting is handbrake on, and power is supplied to open them up and release them. Disconnecting the plug and adding a secondary 12v supply should work, but then you are relying on that supply to stay constant! Putting a disc thickness sized spacer in the gap and disconnecting the secondary supply should also in theory be a method.
I was composing and posting this at the same time as Macadoodles response!!!!!

 

[Lovemyev]

Agreed ?.

If the handbrake was released via the toggle switch manually (which will slightly wind back the actuator, this then releases the pressure on the fiction material to the disc) then by disconnecting the weatherproof plug, it will prevent any electrical signal being sent from the car to the actuators, this should prevent any signal being sent from the handbrake module to both of the motors surely ??.

As the rear callipers are of the single piston sliding type, the inside friction pad always contacts the steel disc / rotor first, the outside pad is always late to the party unfortunately ?.

This is indicative problem of that design of brake calliper and they are prone to sticking / binding if not cleaned and brake pad lubrication applied at the time of serving etc.

I have never seen brake lubrication applied to pads on any callipers fitted to cars straight from the factory, on any cars that I have worked on.
I guess it omitted to help create future work for service departments at the dealership ?.

The front callipers are of the twin piston acting design and therefore more efficient and the brake pad wear should be more equally balanced both inside and outside.

 

[Macadoodle]

The front callipers are of the twin piston acting design and therefore more efficient and the brake pad wear should be more equally balanced both inside and outside.

That would be nice as it's a far better solution but it's definitely not the case on my model

 

[Slipstream]

The uneven wear on my rear discs was getting really bad after 2 winters and 25k miles so I thought I'd try to figure out what was causing it.
Having stripped the brakes down, it was obvious that the sliding part of the caliper wasn't pulling the outer pad evenly against the disc, the inner disc surface where the piston simply pushes on the pad directly is completely shiny and evenly worn.
The root cause of this unevenness is the unusually strong anti- rattle spring as far as I can see, this in combination with the oddly large and squishy rubber inserts lining the slider pins.

As soon as the spring is installed, it pulls the sliding part of the caliper very noticeably rearwards, meaning the outer pad is now sitting at an angle and it will likely only sit flat against the disc if a fair amount of pressure is applied to overcome the anti- rattle springs' constant tension.
This would explain why the front discs are much less troubled by this, although it's still there on my car at least.

I ended up taking some of the tension out of the spring by gently bending its 'forks' making sure to leave enough to prevent causing a rattle, which would be more annoying than scabby discs ?

The tweaked spring was pulling the caliper rearward much less when I refitted it and in the 20 or so miles I've since travelled, the outer rusty section of the disc is showing definite signs of pad contact and wear.

Fingers crossed that this easy and cheap solution works but it's just another example of half baked design on this car, the calipers are SAIC branded , not from the established manufacturers.

Does this not invalidate the warranty if you have to take the vehicle to the dealer and he notices you've been tinkering with the brakes ?

 

[Lovemyev]

That would be nice as it's a far better solution but it's definitely not the case on my model

Are you running on single acting piston callipers on both the front and the rear ?.
What model do you have ?.

 

[siteguru]

Forum profile shows MG4 SE SR.

 

[OriginalBigAl]

Does this not invalidate the warranty if you have to take the vehicle to the dealer and he notices you've been tinkering with the brakes ?

The warranty details also state that not looking after the car and keeping it in a good state of repair might invalidate it ??????? Damned if you do, damned if you don't.

 

[Macadoodle]

Does this not invalidate the warranty if you have to take the vehicle to the dealer and he notices you've been tinkering with the brakes ?

I tinker with everything on it. ? it's just in my nature so I've pretty much given up on claiming on the warranty anyway, fingers crossed that I won't ever need to.

Are you running on single acting piston callipers on both the front and the rear ?.
What model do you have ?.

SE SR on a 72 plate. Plain Jane single pot sliding calipers all round on mine for sure.
Do any MG 4's have multi piston stoppers ? Thought they hadn't even fitted them to the X power

 

[bedsev]

I noticed the uneven wear on my rears like this too, so I’ve done a couple of hundred miles this week on regen 1 and braking late and hard whenever possible and it's pretty much cleaned up the outside of rear disc that was not getting touched.

 

[Macadoodle]

I noticed the uneven wear on my rears like this too, so I’ve done a couple of hundred miles this week on regen 1 and braking late and hard whenever possible and it's pretty much cleaned up the outside of rear disc that was not getting touched.

Yes it helps, but it shouldn't be necessary if the caliper designers had done their job correctly.
Single piston calipers are a compromise and are built to meet a price point but all the other cars I've owned including Ev's managed to apply fairly even pressure on their outer pads.

SAIC's design seems to have failed on this pretty fundamental aspect of caliper operation

 

[Lovemyev]

I do prefer the design used by VAG on a lot of the EV models.
They have opted for using enclosed brake drums on the rears, they are both effective enough to supply enough braking power and because the shoes are enclosed, they do not suffer from usual problems we are use to seeing on rear callipers braking systems ( pads sticking / disc pitting / corrosion / premature replacement ).
Of course, I think we all understand that the primary cause of this premature decline, is accelerated massively by the use of heavy Regen braking.
Roughly 70 - 75% of the braking efficiency is supplied by the fronts on most cars, leaving the rears with much less to do.
This is clearly evident when you compare the brake lining material on a set of new front pads, to a set of new rears.
The manufacturers understand that brake pad wear if significantly less on the rears.
So, if you think rear pads have a easy life on an ICE model, then factor in the use of max regen to slow the car down, then the rears brakes on an EV are almost on a permanent holiday ?.
When I remember, I turn down the KERS ( Regen ) down to its lowest setting once a week / fortnight.
Then I use the car all day in this mode, in order to give the brakes a bit of work to do.
Yes it slightly decreases any range that I could recover from it’s higher setting, but if it reduces the chances of being stung for an expensive repair bill, then it’s all good IMHO.

 

[tsedge]

I use adaptive regen always and I haven't had a problems so far with corroded discs.

 

[glend14]

Most newer cars with electronic parking brakes, require a special scan tool to activate the parking brake retraction/reset function. Missing around with the physical components will not change the operation nature of the electronic parking brake. You need the tool to wind back the pads and reset the system. In my experience.