Oops…rear disc cracking..

[B21]

Whilst doing an emergency rear camber alignment check noticed this through the spokes of the LH rear wheel…

First thought was that should teach me to buy drilled discs…

Subsequently having taken both calipers, pads and discs off the answer looks more interesting.

The LH pads were asymmetrically worn, the outboard worn much more than the inboard..the RH pads were symmetrical.

Also the outboard pad was tapered in its wear profile…

The best guess I have is that one of the spring clips at either end of the caliper wasn’t mounted straight and therefore was not allowing the pad and caliper to float properly..putting more pressure on the outboard edge of the disc..

 

[Mr Tidy]

Eek.

Just a thought but is your boosted power making the DSC/TC work hard a factor?

I know my MC gets the warning light flashing far more often than my 3.0Sis ever did!

 

[B21]

Eek.

Just a thought but is your boosted power making the DSC/TC work hard a factor?

I know my MC gets the warning light flashing far more often than my 3.0Sis ever did!

Yes that was an initial thought..but…the inboard part of the disc and both sides of the other disc are all ok.

Actually with the Quaife LSD and 275 mm of super sticky Conti Sports 7 I don’t get much DSC flashing…of course if I prodded it hard enough especially in the wet then maybe…

Having said that the rear discs are used quite a lot by the DSC system….

Overall I’m tempted to suspect a seemingly modest twist in the stainless steel shim as the culprit…

It’s an expensive crack…two new discs and a full set of pads…

I’ll post some images up shortly of the culprits

 

[Busterboo]

... but … the inboard part of the disc and both sides of the other disc are all ok ...
... I don’t get much DSC flashing ...
... a seemingly modest twist in the stainless steel shim ...

No worries there, then.

 

[B21]

Sometimes things just unravel…decided to do an emergency adjustment to the rear cambers as they were badly out…upto 5% negative camber..in that process discovered a crack in the LH rear disc..initially thought it was a classical ‘well what do you expect buying drilled discs’ syndrome.

Turned out that it was one set of pads that the outer one was binding resulting in accelerated wear plus more stress on the outer edge…

So..thought no problem..however the Foxwell NT520 lied..RH EFB motored back into service position to allow new pads to be fitted.

However the LH unit did not move back…

Tried various things to no avail.

In an attempt I made the fatal mistake of pressing the handbrake lever.

So I now know what happens…pistons pushed all the way forward..till they fell out!

I read up on how this could be fixed by removing the electric motor drive from the back of the caliper and manually intervening in things..

One torx screw came out albeit reluctantly..however the other torx screw just wouldn’t come out..enter a couple of hours of sequentially drilling the screw out…completely corroded in position…

I now know in great detail how the EFB works…

Manually screwed back the EFB carriage that pushes the brake cylinder from inside the unit..and plopped the slave cylinder back in…having taken the protective rubber bellows of first and then reassembled.

Now waiting for some replacement screws to reassemble the motor to the caliper..and whilst I’m there some stainless bleed screws…they were rusted up badly too..

Heh ho..

Still new work bench in my ‘clean room’ and my ‘dirty room’,elevates my tinkering

 

[Smartbear]

Out of curiosity when was your car mot tested Pete?
Rob

 

[Pondrew]

You learned how the electric parking brake works the hard way, then! :wink:

They are very simple. A helical screw pushes or pulls the piston via a bi-directional motor, which is controlled by a current switch. Simple but clever.
The ones with auto apply and release are probably a lot more complex. They must have wheel sensors and pressure sensors attached.

 

[B21]

Out of curiosity when was your car mot tested Pete?
Rob

End of July this year Rob ..pour quoi?

 

[B21]

You learned how the electric parking brake works the hard way, then! :wink:

They are very simple. A helical screw pushes or pulls the piston via a bi-directional motor, which is controlled by a current switch. Simple but clever.
The ones with auto apply and release are probably a lot more complex. They must have wheel sensors and pressure sensors attached.

The EPB is very simple in practice..there are only 3 wires running to it and no onboard ECU or sensors...the DSC unit must do all the maths and most of the logic, the EPB ECU does a few things at the comand of the PB switch or DSC....compared to a mechanical handbrake via an auxillary drum its got a lot going for it..just need to improve my brain engagement...but quite why the LH cylinder didnt come back..I've got a nasty feeling that the internal scew had moved but the cylinder hadn't and just needed some manual pushing back..when I reassemble them I'll try again..but not this time without some pads in the way..

 

[Pondrew]

when I reassemble them I'll try again..but not this time without some pads in the way..

Probably best! I've got this picture in my mind of your motor whirring away, pushing the piston further and further out until it fell out. It does sound quite funny TBH. Not so much for you, obviously.

 

[B21]

when I reassemble them I'll try again..but not this time without some pads in the way..

Probably best! I've got this picture in my mind of your motor whirring away, pushing the piston further and further out until it fell out. It does sound quite funny TBH. Not so much for you, obviously.

You need to marry the soundtrack of the motors whirring to the dawning of the the inevitability of my actions on my face.. :tumbleweed: :rofl:

 

[flybobbie]

I would replace the motors, they are not expensive.

 

[Smartbear]

Out of curiosity when was your car mot tested Pete?
Rob

End of July this year Rob ..pour quoi?

I wondered if they’d picked up on the uneven wear/unbalanced operation of the rear brakes on their machine?
Rob

 

[B21]

Out of curiosity when was your car mot tested Pete?
Rob

End of July this year Rob ..pour quoi?

I wondered if they’d picked up on the uneven wear/unbalanced operation of the rear brakes on their machine?
Rob

I’ve just looked and the two sides were pretty close…brakes seemed strong enough to exceed the requirements by some distance…

I never at any time felt any effect so I guess the pain was suffered by the pad..and then the disc..

 

[flybobbie]

Get the Mtec slotted and drilled discs, had them years on my car, reduces the scoring a lot being grooved.
At least once a year i take the calipers apart, clean and lube the slider pins.
Small price to pay rather than crawling under car adjusting manual cable hand brake.

 

[B21]

Get the Mtec slotted and drilled discs, had them years on my car, reduces the scoring a lot being grooved.
At least once a year i take the calipers apart, clean and lube the slider pins.
Small price to pay rather than crawling under car adjusting manual cable hand brake.

Noted, I’ll see how the replacement Zimmerman drilled rears do this time around..the front BMW Performance Brake is a Brembo design with slotted and drilled discs..