National Grooved Discs Failure (Read if you have them)

[Darren]

I'm not sure whether I did something wrong or there was a defect but I've had a serious failure with one of these discs.
Yesterday I fitted some new pads. My calipers are seizing and I've been reading the threads regarding them so I'd scheduled that job for next week. However I needed to get some grease for the job so just stuck in the new pads to get me to work.
On my testing run the pads were a little tight but freed up after a few pumps and everything was going as expected. I got up to about 35 mph not far from home, applied the brakes and there was a pop and the ABS went crazy on the drivers side. Initially I thought it was just the pads springing free so I turned around and headed back home. The braking was poor and there was a horrible grinding noise, so obviosly I was pretty concerned. When I got back I whipped off the wheel and didn't, at first, see a problem. Then I noticed that the disc was moving freely. However the hub wasn't.
I've managed to completely separate the disc from the bell / hub in a nice clean and uniform shear. The pads aren't touching the disc and it will happily spin so the seize isn't that bad.
I've never heard of this happening to another member and I'm not a track day warrior so there really isn't that much stress on the car.
I'm mightily perturbed by this. I just thought you guys should know since they a popular. Imagine losing all that braking power at 70? It was scary enough at 35.
You're thoughts would be appreciated.

Darren

[WildCards]

hmm, bummer. Glad your ok. I haven't got any suggestions as to why it happened but I do know the National Grooved discs are no longer available for the VR4. Out of interest, how many miles do you estimate the discs had done?

[White Lightning]

Blimey, that sounds nasty. As you say, good job you weren't doing 70 !!

[BraindG]

hmmm, that is abit worrying Darren!..

Im gonna check on mine, look for fractures..

Any of the trackers still got nationals?

[Darren]

with reference to the age of the discs I would recon about 16000 miles. fitted 2 years ago. I'll try and get some pics up so you can see what happened.

[I-S]

Pictures would be interesting.

I have a theory.

If your caliper (assuming it's a stock caliper) is seizing, what can happen (and happened to me before) is that because the caliper can't move properly then only one pad will hit the disc. If this one pad was dragging then one side of the disc will be heated and the other won't. This thermal gradient would produce quite significant stress in the disc material so then it wouldn't take much to get it to break. Applying the brakes caused mechanical stress in addition to further asymmetric heating at which point the cast iron (which is well-known for being brittle) broke as described.

I could be talking cobblers of course.

[richy rich]

Pictures would be interesting.

I have a theory.

If your caliper (assuming it's a stock caliper) is seizing, what can happen (and happened to me before) is that because the caliper can't move properly then only one pad will hit the disc. If this one pad was dragging then one side of the disc will be heated and the other won't. This thermal gradient would produce quite significant stress in the disc material so then it wouldn't take much to get it to break. Applying the brakes caused mechanical stress in addition to further asymmetric heating at which point the cast iron (which is well-known for being brittle) broke as described.

I could be talking cobblers of course.

this is the same as i was thinking.

[Kieran]

this is the same as i was thinking.

Yep, I was thinking that Isaac was talking cobblers too!


Nah, seriously, sounds about right!

[HJM]

Don't think its fair to judge National discs on one failure, especially when running faulty calipers

Many folks here have used, abused and been perfectly happy with them

H

[zentac]

Sorry to say this but its down to you not the disks. If your callipers have siezed then you should not have been driving the car. A siezed calliper will snap a brake disk every time!!! Thats the problem when people start messing with cars when they dont know what they are doing!

I did the same thing when I was 18 and didnt know better, but with no abs on my astra GTE it just dragged me across the road as one brake worked and the other disk snapped. But I learnt the hard way, and so have you.

[zentac]

If your caliper (assuming it's a stock caliper) is seizing, what can happen (and happened to me before) is that because the caliper can't move properly then only one pad will hit the disc. If this one pad was dragging then one side of the disc will be heated and the other won't. This thermal gradient would produce quite significant stress in the disc material so then it wouldn't take much to get it to break. Applying the brakes caused mechanical stress in addition to further asymmetric heating at which point the cast iron (which is well-known for being brittle) broke as described.

I could be talking cobblers of course.

Thats exactly what happens, although the force alone from one pad pushing one side of the disk is enough to snap it, without the heat buildup. (remember theres enough force to stop you quite quickly from 100+ mph)

[amsoil]

I have seen this before; No special heat is necessary. What you have knowingly done is continued to use a siezed caliper and compounded matters by fitting new pads which normally would require thecaliper to move to a balanced positionwith pistons back in. Just wondering how you managed to get the pads in. As you went along and applied the brakes you applied tons of force to one side (indeed one spot) of the disc. The caliper being seized applied all this force on one side. If you think about it the caliper wasn't gripping the disc on both sides with the caliper seating in a positon so that there was no lateral deflection of the disc it was just pushing on one side and then trying to grip the disc in a positon offset to where it actually was. This extremely high localised side thrust then moves around the disc with each wheel rotation; time and time again Bit like someone trying to wiggle a very tight cap or wheel off. Well the disc couldn't come off as it was clamped in place by the wheel so when the fractures caused by side stress had grown enough because of the flexing .......bang catastrophic failure.
Not something to do chaps.

[Spirit]

Don't think its fair to judge National discs on one failure, especially when running faulty calipers

Many folks here have used, abused and been perfectly happy with them

H

Well said that man

Of course Heath and I have KAD's now, but that was not because of the National's....they did a perfectly acceptable job for their cost and abuse !

[BraindG]

Having read through the other posts now, my mind is a bit more at ease.