How to test warped discs?

[DannyVTS]

Afternoon, a simple one today.

I have all the symptoms of warped discs, but is there a way of testing they are definitley warped without buying the laser tool?

Danny

[Blue Meanie]

Go fast, slam brakes... Wibble = warped.

[Greg_D]

is it too obvious to suggest a straight edge rule across the disc????

Greg

[fatboy b]

You need a Dial Test Indicator. Just slamming the brakes on is not the right test as you may have worm suspension bushes that can also feel like the discs are warped. You'll need to find out the max run-out allowable for the car.

[-DeaDLocK-]

I have it on good authority that it takes some serious braking to actually warp a brake disc; the kind of braking that most of us will never see this side of a few consecutive laps at a track day.

What is regarded as "warping" (brake judder) is 95% of the time actually uneven build-up of brake pad material on the surface of the disc due to incorrect bedding-in of the brakes or bad braking habits.

The cure is to even out the disc again, either by undergoing a re-bedding in procedure, or more reliably, a disc skim followed imemdiately by proper bedding in to prevent reoccurence.

This is, apparently, one of the biggest car myths. Most folks, and many mechanics, think actual metal warping is commonplace.

[fatboy b]

-DeaDLocK- said:

I have it on good authority that it takes some serious braking to actually warp a brake disc; the kind of braking that most of us will never see this side of a few consecutive laps at a track day.

You've not driven 80's & 90's Fords, have you?

[Zad]

A lot of what appeared to be warped discs on 80s/90s Fords was the degradation of the made-from-pure-recycled-cardboard bushes that Ford used. Swap them for uprated ones and not only did the "warped discs" go away, but the wheel wobble at 65-80 miraculously disappeared too.

[lance1a]

Take them to a BMW main dealer in Oxford, apparently they have a technician there who is so good he can see if they are warped by just looking at them. That's what my BMW X5 customer told me, must be true. This was just before they replaced his enire front suspension and discs and pads because they were to scared to tell him to stop driving like a knob and building depositation marks on the perfectly good discs.

[Ozzie Osmond]

fatboy b said:

You need a Dial Test Indicator. Just slamming the brakes on is not the right test as you may have worn suspension bushes that can also feel like the discs are warped. You'll need to find out the max run-out allowable for the car.

^^^^^ This.

Disc warping can happen but very rarely happens. Pad material deposits on the disc are the usual cause of judder. Slamming the brakes on again is likely to make this worse, not better. "Normal" driving should clean up the discs over time.

A huge amount of brake problems are caused by the use of non-standard pads and discs. IMO it's best to buy replacements from the dealer or original manufacturer wherever possible. If you must "upgrade" your brakes then use a complete kit from a decent company which has put it together for your car.

[Blue Meanie]

Well, I must say, brake wobble has never gone after time for me, and nor has changing pads, etc.

[Superhoop]

Blue Meanie said:

Go fast, slam brakes... Wibble = warped.

Wrong.....

As someone has already said, it takes extreme heat to actually warp a disc, however; I disagree with the pad material build up theory too (although it can be a possibilty)

What most people refer to as a warped disc is most commonly disc thickness variation - Caused by an out of true disc catching the brake pad twice per revolution (once each side of the disc)

This over time starts to wear away the area of the disc that is catching the pad, leading to a variation in disc thickness - This then causes the caliper to pulse as it it keeps the pad in contact with the disc

This results in a pulsing sensation through the brake pedal, and a shudder through the car during braking.

Skimming the disc on the car will not only get rid of the thickness variation, but should if done properly reduce run-out to an absolute minimum

[lance1a]

Run out on the hubs caused by not cleaning the face of the hubs once new discs are fitted.

[Blue Meanie]

Oooh, the wrong brigade are out today. The guy asked to know how to test for it. I gave him a solution to see if they are. I'm certainly not convinced by the whole 'warping is a myth' brigade, and not do I think that it is a pad issue. I've changed pads before, and still had the wobble. Warping IS an issue, and no-one is arguing about how it happens.

[Blue Meanie]

doogz said:

Blue Meanie said:

Oooh, the wrong brigade are out today. The guy asked to know how to test for it. I gave him a solution to see if they are. I'm certainly not convinced by the whole 'warping is a myth' brigade, and not do I think that it is a pad issue. I've changed pads before, and still had the wobble. Warping IS an issue, and no-one is arguing about how it happens.

Read the other replies. No-one is saying that warping doesn't exist, and a couple of users have suggested other reasons why the discs might not be true, other than them having actually warped.

There has to be a massive amount of heat involved to warp steel that thick.

Well, would it happen if you have very hot brakes, and then whoop through a puddle? Hot, then cold?

[Mars]

Blue Meanie said:

doogz said:

Blue Meanie said:

Oooh, the wrong brigade are out today. The guy asked to know how to test for it. I gave him a solution to see if they are. I'm certainly not convinced by the whole 'warping is a myth' brigade, and not do I think that it is a pad issue. I've changed pads before, and still had the wobble. Warping IS an issue, and no-one is arguing about how it happens.

Read the other replies. No-one is saying that warping doesn't exist, and a couple of users have suggested other reasons why the discs might not be true, other than them having actually warped.

There has to be a massive amount of heat involved to warp steel that thick.

Well, would it happen if you have very hot brakes, and then whoop through a puddle? Hot, then cold?

No it wouldn't, or this principle would have warped all sort of products during their manufacture.

Changing pads, as you have tried, rarely works to cure friction hotspots. You'd need to change to a very hard compound, one which abbraded the disc surface over a period of time. Such pads just don't exist outside of the racing arena, and I'm not even sure such a hard pad exists there. I'm sure some people have managed to cure very marginal friction hotspots with harder pads but I think it's unlikely.

I have had this problem on a few cars. I temporarily cured it with a set of discs on my old automatic Legacy by sanding them with an aluminium sandpaper, but the symptoms returned within the first drive (not straightaway but the fact that they returned so quickly made the idea of sanding them pointless). After a couple of replacements by Subaru dealers, I went to a 3rd party for servicing and new discs and those discs never exhibited the problem in 50K miles, until I sold the car.

On my latest Legacy I had this problem again, so I had a set of new discs fitted by Subaru and then after some 10K miles the juddering returned. I approached the Subaru dealer about new discs but when I collected the car they had lathed the discs and the problem disappeared completely, and only started to return after some 15K miles.

I eventually stopped going to Subaru for my servicing (cheaper elsewhere) and had the next new discs from a 3rd party. These ones have not exhibited the juddering in 30K miles, so I conclude that the Subaru supplied discs are not up to the job... although I can't help but wonder whether the discs supplied by the 3rd party are anything other than Subaru ones anyway.


Conclusion: Sanding could possibly work if you use the right abrasive agent and use it aggresively. Lathing is better and does work but if the discs or pads are of a material that is susceptable to the depositing of friction material in uneven "hot spots" then new discs and pads is the only real solution. Changing to a different manf is my recommendation.

Edited by Mars on Thursday 6th January 15:17

[zx10ben]

-DeaDLocK- said:

I have it on good authority that it takes some serious braking to actually warp a brake disc; the kind of braking that most of us will never see this side of a few consecutive laps at a track day.

I have it on my authority that two good laps of the Nurburgring will do the job!

Took my XFS in for its service and asked them to look at the brakes as they were juddering. They came back and took me out to the car to see them, had a lovely blue ring going through the centre of all four disks, turns out "ragging" is not covered by warranty! Would've thought?

To the OP, arent there various high street chains that do a free brake check? Go to a few and see what the general opinion is.

[eltax91]

fatboy b said:

-DeaDLocK- said:

I have it on good authority that it takes some serious braking to actually warp a brake disc; the kind of braking that most of us will never see this side of a few consecutive laps at a track day.

You've not driven 80's & 90's Fords, have you?

Or early 2000's VAG cars. My Octavia and the Mrs' A3 get through discs every 40k ish. Even the top quality ones. I treat them as a service item

[Dave Brand]

Mars said:

Blue Meanie said:

You'd need to change to a very hard compound, one which abbraded the disc surface over a period of time. Such pads just don't exist outside of the racing arena, and I'm not even sure such a hard pad exists there.

You're confusing "hard" with "abrasive". A hard compound is one with a low coefficient of friction; although friction material development has made things less clear-cut, as a generalisation hard pads have low friction & better fade resistance, soft pads have high friction & poor fade resistance.

Modern pad materials to tend to be more aggressive because of the fibres used in their manufacture since asbestos was banned, but aggressiveness & friction level are not directly linked. Some aftermarket manufacturers sell pads with an abrasive coating to aid bedding in - this may help in cleaning up uneven friction material deposits & reducing the impression that the discs are warped.

Returning to the original question, in the absence of a DTI a quick method of checking it to use feeler gauges between the disc & the pads - do both sides & you can get a rough indication of any thickness variations.

[XG332]

fatboy b said:

You need a Dial Test Indicator. Just slamming the brakes on is not the right test as you may have worm suspension bushes that can also feel like the discs are warped. You'll need to find out the max run-out allowable for the car.

I thought this aswell, but i was told its useless in high tolerence situations (race cars) as the run out of the hub/bearing would also have to be acounted for.

[DannyVTS]

Not only a bit of juddering when braking at 50+ but also when slowing slightly from about 5mph I'm pretty sure i can feel the brakes gaining and losing friction through the braking force and my foot being in a static position

My discs squeak a bit sometime aswell, but I've been told my master cylinder is perhaps a bit too small for my brakes causing the squeak. Could this cause the juddering too?

Danny