Skimming brake discs on milling machine

[Johnnybravo]

Well now, I know you've eagerly been waiting for an update on this...

The runout I have on the discs are from 0.06 to 0.11 mm so within reasonable tolerance. Visually they all look fine - no gouging or uneven wear pattern.

However, I do have a thickness discrepancy of 0.06 mm on one disc. I'm not entirely convinced this could cause the significant juddering I experience while braking so I guess I'm going to have to investigate further...

Strange as it may sound, I have had similar issues in the past. Discs ,wheel bearings all changed and it still persisted. Changed the tyres even though they were like new,recently balancedI had no wheel wobble on the motorway….problem solved. I do have a vague recollection of reading something on here about certain performance type cars being very fussy when it comes to tyres…

 

[jordhandson]

I did my Nissan 350 discs on my milling machine but I ground them rather than cut them made a nice job to, will see if a got some pics or a vid when I can get on the other pc.

I couldnt find the 350 disc but i found a pic of another disc this is the same setup i used when grinding the fly wheels on trhe milling machine, works for me

 

[Mr Fro]

Cheers @jordhandson, that's what I would end up doing if I don't stump up for new ones.

Interesting what you say @Johnnybravo - the F10/F11 are renowned for tramlining. I had the rear tyres replaced with Michelin pilot sport 4s a few months back. They made no difference to the tramlining. I'm waiting for the fronts to wear a bit more before I do them too.

 

[Munkul]

Wide tyres and toe-out front tracking are the main causes of tramlining. Both of which BMWs are known for

 

[Justme]

Those on car brake grinders were very popular in Urmerika, never seen one over here.

Bob

Used to get disks skimmed on the car all the time in the 80's & 90's.

 

[knighty]

good mechanic I trust will only fit original / main dealer disks on anything quick or anything heavy, had too many warp

I warped 2 sets of pagid disks on the front of my XJR before I talked to him... pagig are supposed to be decent and I very rarely pushed the car hard - and even when I did it was nothing like the car is built for etc.

can't remember what the main dealer disks set me back... I really like that car so didn't really care

 

[Ubique]

Came across this today:-
Very new brake pads (11mm) on very old brake discs, done by an independent who told the driver that the minimum thickness was just a scare tactic used by dealers....
The minimum thickness is stamped into the hub of the disc.

These were well below - typically there is 2mm of wear on new brake discs, roughly two sets of brake pads worth of wear.

Now, What you want to do with your own brake discs is one thing, and if it goes wrong then you have to bear the cost and possibly account for your actions, to not change or strongly recommend changing the brake discs if they are below discard thickness or within 0.7mm (Merc. wear limit for replacing pads only) of it is just plain wrong. The vehicle was in for an unrelated issue (autostop not functional due to reduced starter battery capacity) and the issue with the discs was picked up on a visual inspection, replacement front brake discs & pads was £310, cheaper than the diag & replacement battery!

 

[Bornfree]

The problem with going below the minimum disc thickness is that if you wear the new pads down to the metal they can slip through the gap between disc and the carrier.

 

[Lewis_RX8]

Came across this today:-
Very new brake pads (11mm) on very old brake discs, done by an independent who told the driver that the minimum thickness was just a scare tactic used by dealers....
The minimum thickness is stamped into the hub of the disc.

Thought you dodgy car mechanics charge for brakes when they don't need to be done not the other way round

 

[Morrisman]

The average set of discs and pads for an average car is chump change. The manufacturers don't stand to make any profit from it, so I'm not sure why this is being played as some sort of a scam.

Thinner discs means less heat dissipation, easier to overheat, lose brake efficiency etc. Plus, you have to draw the line somewhere, every part that wears down needs a tolerance. How far out can the piston push before the seal clears the bore it sits in?

 

[Ubique]

The average set of discs and pads for an average car is chump change. The manufacturers don't stand to make any profit from it, so I'm not sure why this is being played as some sort of a scam.

Thinner discs means less heat dissipation, easier to overheat, lose brake efficiency etc. Plus, you have to draw the line somewhere, every part that wears down needs a tolerance. How far out can the piston push before the seal clears the bore it sits in?

The pistons can go until the pads are metal-to-metal, it's actually the other way round - the pad wear sensors will be cut prematurely by heavily lipped discs, meaning that the dash warning will come on at 4mm friction material remaining, rather than the 2mm they're designed to trigger at. Most manufacturers recommend minimum friction material thickness of 1.5mm for heat dissipation.

 

[Ubique]

Thought you dodgy car mechanics charge for brakes when they don't need to be done not the other way round

It's because of those 'historical practices' that visual health checks recorded on video and emailed to the driver were brought in, difficult to argue with measurements done in front of you, some still do though.... usually the ones that just manage to get into a <insert car badge of envy here> without thinking of how they'll keep it on the road.

 

[Fastdruid]

Not all replacement brake discs are equal, in most cases you get what you pay for, personally I wouldn't trust £20 aftermarket ones to last any time at all or trust their ability to bring me to a hard stop if required. - you'll also need replacement brake pads if you replace or skim the discs.

That really depends on the car. I bought a set for a Mondeo mk2 once, £15 each... They were (genuine) Brembos. Even now they're only £17.10ea (although it doesn't give you the choice between Brake Engineering, BREM TECH or Brembo).

 

[PhillipM]

The pistons can go until the pads are metal-to-metal, it's actually the other way round - the pad wear sensors will be cut prematurely by heavily lipped discs, meaning that the dash warning will come on at 4mm friction material remaining, rather than the 2mm they're designed to trigger at. Most manufacturers recommend minimum friction material thickness of 1.5mm for heat dissipation.

The wear sensors are usually attached to the pads so they wouldn't be any different with them coming on, and metal-to-metal would be different distance depending on the disc thickness. If you're getting down to 4mm+ thinner than a new disc when it's metal to metal you can definately have the pistons pop out - I've seen it happen!

 

[AVI_8]

Apologies if this has been mentioned before as I haven’t read all of the comments but juddering brakes are often caused not by warped disks but by friction material from the pad getting deposited on the disks.
I had this once on my 911 after a track day.
It‘s caused by coming to a stop when the disks are extremely warm whereby the area of the brake disk against the pads gets friction material transfer, it can be visually noticeable as a dark patch on the disk.

 

[Ubique]

The wear sensors are usually attached to the pads so they wouldn't be any different with them coming on ...

They do come on earlier due to the lip on the disc cutting into the wear sensor rather than them being worn through on the working face of the disc.

If you're getting down to 4mm+ thinner than a new disc when it's metal to metal you can definately have the pistons pop out - I've seen it happen!

Well I've stripped a fair few brake calipers down and the piston has to be extended maybe 2/3-3/4 the width of the yoke before it'll come out, maybe some specific combination of pads, discs & calipers will do as you suggest, I've never seen it happen on production vehicle brakes though.

 

[Ubique]

Apologies if this has been mentioned before as I haven’t read all of the comments but juddering brakes are often caused not by warped disks but by friction material from the pad getting deposited on the disks.
I had this once on my 911 after a track day.
It‘s caused by coming to a stop when the disks are extremely warm whereby the area of the brake disk against the pads gets friction material transfer, it can be visually noticeable as a dark patch on the disk.

Also overheated discs becoming polished & hardened on the working face can cause brake judder, agree about the friction material deposits though, quite common.

 

[Edward Teach]

Brake pad wear sensors,
I need a newer car and van

 

[Fastdruid]

Brake pad wear sensors,
I need a newer car and van

Probably better rather than newer...

Although some makes fit pad wear "squealers", small tabs of metal that are meant to contact the disc when the pads are worn low and make a horrible squeal. Not in my experience that they work consistently however so I wouldn't want to rely on them.

 

[Ubique]

Probably better rather than newer...

Although some makes fit pad wear "squealers", small tabs of metal that are meant to contact the disc when the pads are worn low and make a horrible squeal. Not in my experience that they work consistently however so I wouldn't want to rely on them.

Not seen those for years! I've a feeling they are used on some of the older PSA and Ford stuff - I remember going out to a 'breakdown' where the blokes brakes were metal-to-metal on his front discs and he was moaning that it 'just happened, no warning', I said to him that someone had cut the squealers off the brake pads, his response was along the lines of 'oh yer, that was me - didn't want them damaging the brake discs.....'