IS250 rear brake discs minimum thickness

[james250]

Anyone know what the official minimum thickness is for the rear discs vented Vs non vented before they need changing? 

[KenMavor]

Would suggest that you will get either a lip in the disc, worn down brake pads, or a problem with a caliper, piston, caliper spring or bolt before the thickness causes of a problem.  I always change the pads and discs together as well.....discs are pretty cheap (Mintex for £70 for 2 rears) as I have found that new pads on old discs just cause more problems.  Do the pins as well at the same time.  Just my opinion.

[johnatg]

Solid - 8.5mm

Vented - 16.5mm

More to the point, check the friction surface on the Inner face of the disc. A crusty rust deposit builds up from the centre outwards. Quite why this happens I don't know - you would think that as the piston pushes directly onto the pad there it would keep the disc clean and the outside might suffer if the pins are sticking. That can happen but the inside face always seems to rust worse than the outer.

It's not easy to see.

If the rust deposit is significant, the discs need changing.

+ Ken's points are all good.

[james250]

3 hours ago, KenMavor said:

Would suggest that you will get either a lip in the disc, worn down brake pads, or a problem with a caliper, piston, caliper spring or bolt before the thickness causes of a problem.  I always change the pads and discs together as well.....discs are pretty cheap (Mintex for £70 for 2 rears) as I have found that new pads on old discs just cause more problems.  Do the pins as well at the same time.  Just my opinion.

But when I used to have car serviced at dealers I used to get through more pads than brake discs. Always seemed to be that the pads wear out more than the discs.

A question about lip in disc. Can someone kindly explain what exactly a lip on the disc is and what it means? I hear this term a lot where people talk about lipped discs. Is it possible to have plenty of thickness left on disc but have lipped disc that needs changing?

[Marleez]

Where the top/edge of disk forms a lip. And the inner disk is very worn in.  Causing squeaky brakes and pads to wear alot faster. 

[Marleez]

You also will get a grinding noise when you have a very thick lip on the edge of the disk when braking

[james250]

1 hour ago, Marleez said:

Where the top/edge of disk forms a lip. And the inner disk is very worn in.  Causing squeaky brakes and pads to wear alot faster. 

Thank you. So if I'm understanding correctly if the friction surface of the discs wear out will it not automatically cause a lip between the edge and the friction surface? Isn't that why we use a gauge to measure the thickness of the disc? Maybe I'm not understanding correctly 🥺 because a brand new disc for example will be flush on the edge with the friction surface right? And as the friction surface wears out the edge will remain the same thickness but the friction surface will get thinner over time and this is what we measure in terms of minimum thickness. But what I'm confused about is that if the wearing out of the disc creates a lip then the minute a bit of lip is seen any do people say to change the discs? Isn't there supposed to be a certain amount of lip to allow for discs to wear to the minimum thickness?

Like I said I'm not sure how this all is supposed to be just trying to understand so please forgive me if it's a dumb question 🥺

[KenMavor]

The brake pads rarely (possibly never) actually cover the entire brake disc on any car.

Hence you will have a nice shiny middle section of the disk where the brake pad actually comes into contact with the brake disc while you apply the brakes.  Whilst the outer part of the disc.....usually about 2mm to 3mm....is never actually touched by the brake pads and hence never gets worn.  Hence you will get a lip of unworn brake disc.  The internal part of the disc never gets touched by the pad either and it just rusts.

Garages can, and will, fail MOT where the lip is quite deep.

Also check that both brake pads are wearing the same as if not you may find one of the brake pads is stuck in the carrier and not actually moving when the brake caliper is applied. Have a look on Youtube and you will see some helpful videos

I usually check all my brakes once a year.....a garage can do the same.

[GS300h2014]

Regarding how deep can the lip on a disc get? On my GS 300h hybrid, the front discs are 30mm thick when new & the specs say the min thickness is 27mm i.e. a max lip of 3mm.

At 54k miles I measured my front discs to have worn 40% (i.e. 60% left) giving a calculated lip of 1.2mm.  The pads were worn down by 37%. This I guess means that my brake usage will cause me replace both pads & discs at the same time at about 120k miles. It's similar story for the rear brakes (24% & 27% respectively). However the reduced wear on the rear brakes may be due to the pads binding slightly in the carriers until I de-rusted them recently.

So I suppose an experienced person could evaluate the brake life left on a car just by feeling the lip on the discs.

[johnatg]

30mm to 27mm gives a lip of 1.5mm each side. You can file it/them off, which answers your last question.