Warped discs

cantdecide

I just picked up my new car yesterday- a '00 legacy B4 blitzen auto;)

There are a couple of things to take care of including what feels like warped brake discs. You can feel that distinctive grabby feeling as it slows down. My dad had a '00 GS300 and it too suffered from this but it's a known soft point of those cars.

At the time, I read a lot online about having to make absolutely sure the Lex calipers were torqued up exactly to factory specs and this kind of thing. With that in mind, how do discs generally get warped? Is it sheer abuse (it's that kind of car anyway)? Is it a known fault with Legacys? What can you do to avoid warping discs, generally? Is there anything else it might be?? Will I be insulting my local mechanic if I suggest he pays extra care to torque setting etc?

I'm very worried about this because my Dad's Lex brake issue seemed to go on and on and on with several sets of new discs and pads being fitted and refitted.

E39MSport

Hi,

My experience of warped dics is a pulsing from the brake pedal.

It's usually caused by holding the car at a stand still after heavy braking that caused the discs to heat up. The pressure of the pad can warpp the disc in these circumstances (I think). Can be exacerbated if one side is under performing.

You can also get the same feedback from the brake pedal if theres something amiss with the suspension (worn joints) or wheel itself (bearings etc).

You can test for a warped disc with a gauge fixed to a point on the chassis with a magnet and spin the disc (can't remember the name of the gauge).

CJhaughey

E39MSport wrote: »

Hi,


You can test for a warped disc with a gauge fixed to a point on the chassis with a magnet and spin the disc (can't remember the name of the gauge).

A dial gauge.

Mr.David

Warped discs are somewhat of an industry myth. They are extremely extremely uncommon.

What actually happens (and is mistaken for warping) is DTV (disc thickness variation). Every disc will be machined to a tolerance in terms of flatness - this will be measured in microns e.g. 10 microns runout as a max tolerance for the disc. Couple this slight runout in the disc with the fact that the material properties are never going to be 100% homogeneous on either disc or pad and you end up with high spots and low spots.

As the pad contacts the disc, for the reasons mentioned above the local temperatures on the disc will vary giving non-uniform temperature distribution. As it heats, this effect multiplies and highspots become higher and so the runout increases. The result is that the thickness of the disc will vary and so the pressure applied by the pad will vary too. Hence you get a pulsing feeling through the pedal, often braking from motorway speeds and under low to medium braking application.

Hope that helps!

mullingar

cantdecide wrote: »

My dad had a '00 GS300 and it too suffered from this but it's a known soft point of those cars.

I have a 98 GS300 for the last 4 years, its not a soft point on these cars, they are probably just cheap discs.
I know as I put a set of cheap discs on her 3 years ago, lasted 2 months and warped. I then put a good set in and its been perfect ever since!

mullingar

These are the brakes I put on

EBC 'turbo groove' front
EBC standard rear
Green stuff pads all round (Dont use red stuff pads, makes sh1te of discs)

It cost around €500 for the lot , but large increase in braking power! and still >75% life left! IMO Worth every penny.

Also never ever cheap out on tyres or brakes!

E39MSport

CJhaughey wrote: »

A dial gauge.

that's the one - ta. Have one aswell. Should have known. Blame the hangover.

E39MSport

Mr.David wrote: »

Warped discs are somewhat of an industry myth. They are extremely extremely uncommon.

What actually happens (and is mistaken for warping) is DTV (disc thickness variation). Every disc will be machined to a tolerance in terms of flatness - this will be measured in microns e.g. 10 microns runout as a max tolerance for the disc. Couple this slight runout in the disc with the fact that the material properties are never going to be 100% homogeneous on either disc or pad and you end up with high spots and low spots.

As the pad contacts the disc, for the reasons mentioned above the local temperatures on the disc will vary giving non-uniform temperature distribution. As it heats, this effect multiplies and highspots become higher and so the runout increases. The result is that the thickness of the disc will vary and so the pressure applied by the pad will vary too. Hence you get a pulsing feeling through the pedal, often braking from motorway speeds and under low to medium braking application.

Hope that helps!

Good explanation. Cheers.

bryaner

Car was lying up for ages id say bring it for a good burn (when the weather picks up) and stand on the brakes many times id be fairly sure that will sort it