Disc runout

[Mister Fishfinger]

I had an MOT test on the bike recently and got an advisory for 'warped discs'.

Now I know the tester doesn't actually test the disc runout, he's just looking at the brake force indicator, which I guess is fluctuating when he puts the brakes on. I can certainly believe it, I've always thought my brakes were a bit grabby at low speeds.

Anyway, I put a dial gauge on both discs today and got the following results:

Left: 0.10mm (outer edge) 0.09mm (inner edge)
Right: 0.13mm (outer edge) 0.09mm (inner edge)

Looking at the workshop manual, the limit is 0.30mm so my discs look like they are well within spec.

So the question is: change them anyway, or look for another possible cause. Maybe the calipers need a strip and clean, or the pads are glazed? I've checked the pad thickness and they are well within limits as well.

[seth]

I had the same at my mot earlier in the year .
It turned out to be dirty/sticky buttons once cleaned much better now.

[Mister Fishfinger]

I had the same at my mot earlier in the year .
It turned out to be dirty/sticky buttons once cleaned much better now.

Buttons? Pistons, right?

[seth]

No the buttons between the centre and the outer allowing the outer to be full a floating discs it's a common problem on any bike with this kind of set up.
Use a bolt to spin them along with brake cleaner spray.

[Mister Fishfinger]

No the buttons between the centre and the outer allowing the outer to be full a floating discs it's a common problem on any bike with this kind of set up.
Use a bolt to spin them along with brake cleaner spray.

Oh, I get it. I've already done that though.

[KiwiCol]

Think they're referd to as Bobbins, not buttons.  Could be wrong & wouldn't be the first time either

[Speedy1959]

My discs were "warped" after I had my wheels powder coated.. The powder coating place did the whole wheel including where the discs sit.. I got that familiar surging when applying the brake. It felt as though I was pulsing the brakes with the brake lever..
Gloom and Doom was being spouted forth to me..
All I did was use my digital vernier gauge and measure where the run out was on the actual disc and then mark both disc and wheel with a black marker pen and note the readings..
With the discs removed I again used the verier gauge to measure across the disc mount faces.. Sure enough there was just one area where the paint was thicker (maybe at the lowest point when being coated)..
I then scraped (with a Stanley knife blade) the layers of clear coat / paint off the disc mount area till the vernier gauge readings were minus the excess amount on my initial disc run out reading..

I had no idea if it would work but I was damned if I was going to splash out on new discs..
Well it did work..
I have now done 5 - 6000 miles with them and I would say they are 90% correct.
If I REALLY am over critical I can just about detect the faintest of pulsing as I come to a stop from 10mph and less.
I can live with that..
2 weeks ago my bike flew through its MOT so they cant be that bad !

P.S Since having my initial Disc issue I have heard several reports of GSX's getting "Brake Surge" so I don't think its just the powder coating process that's an issue.. Discs seem to be a weak point on these bikes!

[Notty]

On my K3  10 years ago the left disc was badly warped - probably because I tried to ride off with my disc lock on - ended up with new discs and Harrison billet 6 pots cos I told Mrs Notty they were worn and dangerous and 4got to mention the disc lock 

[Speedy1959]

Nice one Notty...
your cunning when it comes to your bike...
But brakes are a safety issue Mrs Notty, after all !

[Mister Fishfinger]

Top tips guys, I will keep you posted on progress.

[seth]

It doesn't always work but if your buttons/bobbins are clean .
You can try putting them in the oven on the highest temp leave in all day 12 hours then turn off and leave in the oven to cool down .
Not guaranteed but much cheaper than a pair of new disc's