MOT - warped brake discs
 

last MOT i had 1 advisory for warped front discs and under reasonable braking i had a slight pulse effect from the front brakes as they are warped, MOT due next month so i gave my discs to my brother to skim. I told him that as soon as they got to 4mm he should stop and if they are still warped then I'll get some new ones.

anyway gave them to him and he did them yesterday, one disc was 4.5 and other was 4.7- something, he put them in a jig and did the sums or what ever, found out one was worse than the other and said there's no point because to get them true he would have to go bellow 4mm so i said leave it.

put them back on this morning as a temp until i get new ones, just did the customary "up and down the road to check everything is gravy" and the pulsing has stopped :confused: and the brakes are brilliant, i didn't label what side the discs came off so they could be round the other way than i had them

question is how do they check for them being warped for the MOT because if it will get through again i sure as hell am not bothering paying for new discs :lol:

tar

Re: MOT - warped brake discs
 

Sounds like he's managed to strighten them but they will now be below the 4mm min thickness won't they?
I'm guessing this guide line is for heat etc as when they get thinner they will heat up more under breaking?
TBH not sure what they say on MOT's regarding discs

Re: MOT - warped brake discs
 

no he didn't skim them, unless having them in the jig managed to flatten them :confused:

i took them off last Thursday and had them laid on a flat surface until monday evening but i cant see how they could straighten themselves out :confused:

Re: MOT - warped brake discs
 

MOT will test if they work with no obvious faults.It appears they now do?

Re: MOT - warped brake discs
 

Its your brakes. If you're happy they will work fine. Personally I'd bin them and get a new pair.

Money != Life

Re: MOT - warped brake discs
 

could have been a rusted up floating pivot that the jig managed to free up?

Had this happen on my 6-pot mountain bike calipers that have floating discs!

Re: MOT - warped brake discs
 

Your fecking about with brakes boy. Stop being a tight wadd!

Re: MOT - warped brake discs
 

a slightly warped disc is hardly going to kill me :smt108 most is going to do is cause my pads to wear un-evenly

i thought they had some sort of way of checking them for trueness

Re: MOT - warped brake discs
 

Are they not likely to fail the MOT this time?

Re: MOT - warped brake discs
 

well i duno, it was just an advisory last year and the MOT bloke said not to bother with it until next MOT

its not major pulsing on the brakes, just a slight one

Re: MOT - warped brake discs
 

Your call then...

Re: MOT - warped brake discs
 

the tester will not even consider warped discs unless there is some evidence of this in the way the brakes feel. If they apply smoothly with no pulsing you are good to go, no point in replacing them if they are ok.

If you really want to measure runout properly you can use an engineers dial indicator. It gets positioned onto the disc surface and then the wheel is rotated, you will get an accurate measure of the runout. Unless its excessive it will pass just fine.

If you can no longer feel it then you have nothing to worry about. Check them again though after your next bout of enthusiastic braking and see if the problem returns.

Re: MOT - warped brake discs
 

Bear in mind that even new discs may not be totally flat!

0.3mm IIRC is the limit which is also what one of the brand new GSXR1000K2 discs[1] was, didn't make a difference though as it cured the warped disc feel.

Druid

[1] Bought a pair of new ones as I was fed up with buying warped ones off ebay, worked a treat, no more pulsing! :-)

Re: MOT - warped brake discs
 

cheers peeps, done 30 miles and they still seem fine.

Re: MOT - warped brake discs
 

This Ship is unsinkable.
Comes to mind. But true what you say

I'd be thinking the same.

Re: MOT - warped brake discs
 

Maybe not, but warped disks will push the pistons further back up into the calipers and may depending on the extent of the problem limit the maximum braking effort that can be applied.

Re: MOT - warped brake discs
 

Had my MoT last week and got similar advice. The tester had the bike on a brake tester device (funny that) you know like the two roller type things in the floor that sit in front and behind each wheel. He applied constant pressure on the brake lever/pedal and the force generated showed up on the analogue dial. The dial was allowed to move a certain amount whilst under this constant pressure which indicates the pulsing. If the dial is fluctuating too much it's a fail.

http://www.boston-ge.com/images/r100-bike_l.jpg

One of them.

Re: MOT - warped brake discs
 

Yup, thats the way its done.

Mine had its first MOT last month and the tester (who I know) said to me "Did you know you had warped discs?"

Obviously my answer was no (second hand eGay GSXR discs). He asked if I had any pulsing through the lever when applying the brakes and I said no.

He said..."Don't worry about it then. They aren't that bad"

Having been told that my discs are warped, I am now more aware (aka paranoid) of any subtle difference in feel or brake response, but there still isn't any pulsing at all.

Re: MOT - warped brake discs
 

LOL my MOT tester just sits on the bike pushes it forwards and makes sure they work?!?! Will not state who they are tho......

Re: MOT - warped brake discs
 

just what i was looking for, very informative thanks :notworthy:



my last MOT'er was the same, just checked they worked and that was fine but he got shut down lmao :lol:, new bloke i use has a very high-tec testing station aswell as garage so he nit picks on any little thing :rolleyes:

Re: MOT - warped brake discs
 

tis my first bike and first mot but apparently my tester is the same. nice to know that the bike can still pass despite having been crashed and fixed by my own fair bodging hands!

Re: MOT - warped brake discs
 

you said youre not sure if the discs went back on the same sides they came off?
discs are rotational arent they?

Re: MOT - warped brake discs
 

no markings on them so guess not :smt102

Re: MOT - warped brake discs
 

Some are, some aren't, Looking at the fiche, it appears SV discs aren't, both sides identical.

Druid

Re: MOT - warped brake discs
 

Be aware that anything resembling variation in braking as the wheels rotate is almost always put down to "warped discs", which it isn't half the time.

Pad material gets transferred to the disc surface, and any uneven patches will give a variable feel. Often this can be retrieved with abrasive cleaning of the disc and re-bedding them. Just for info.

A few years ago a problem cropped up with a lot of aftermarket car discs, which turned out to be the foundry where they were sourced had changed the casting process slightly (and IIRC trace element content of the iron) and the metallurgical structure was variable round the disc, which gave variable friction coefficient and the feeling of "warped discs". Because it was in the material itself, skimming had virtually no effect and replacement was the only solution.

Re: MOT - warped brake discs
 

brother just came round and he reckons they where quarter of a mill out, he said it in thou tho but i cant remember what he said

Re: MOT - warped brake discs
 

10

Re: MOT - warped brake discs
 

I'm pretty sure the limit is 0.3mm in which case they're fine.

Druid