brake drum / disc runout

[Rich_s]

Hi All,

The brakes on my spit1500 are doing my nut in, this does not happen all the time, more so when the car has been drivin for 15miles or so, but sometimes all the time.

If at high speed and hit the brakes the car vibrates, sometimes very slightly, other times quite badly, the pedal feels ok though. at slow speeds when braking sometimes the pedal seems ok but the car is slowing down like i am pumping the brakes.

All cylinders and calipers are freely moving, front pads are new ish as are the discs, the rear shoes are a bit old and the drums don't look that old.

i have tried to test the run out on them using a digital DTI and the front discs are 0.00051 inch and 0.00063 inch so i think they are fine. The rear drums are quite the opposite, to test them i took the drum off and fitted back to front, not sure if this will give true answers though as they came out as 0.0131 inch and 0.0177 inch, not had chance to check another cars drums yet but anyone know if this is way too much and if i have tested them correctly?

thanks
rich

[Velocita Rosso]

Test the drums by....from about 20/30mph, start to slow down ( in a quiet lane somewhere) and just use the handbrake ,pulling up with ratchet off and with light pressure.
You should/or not feel the shoes biting the drums either smoothly or like a pump action . The hand brake going up and down very slightly in your hand
That may prove or not the perfect circle or whether you have oval drums
Did this with mine and had to have them machined concentric

[Rich_s]

tried this, initially there is a little notch  about half way through the handbrake lever movement but only once if that makes sense then nothing, just slows down smoothly to a halt.

as the fronts are ok and the backs seem ok using the hand brake, what else could this be, the bearings are all ok with no play so i am stumped,

not sure where to go from here, only got one brake pipe clamp so may get another and then take it out on a quiet road with both back brakes shut off then both front, (only on a very quiet road and slow speeds though, it is noticeable at 15 mph) other than this anyone got any ideas?

rich

[Rich_s]

Been doing some more searching on the net and a possible solution is just uneven deposits on the drums or discs. Anyone ever come across this, or how i could fix this at home?

rich

[RedRooster]

Do you feel the vibration through the wheel, your foot or the seat? Before you go blaming the brakes it could be suspension bushes, TRE's, gearbox mounts, etc...
RR

[Rich_s]

it feels more like the vibrations are through the seat, the brake pedal and steering wheel seem ok, not been able to find any bushes that have a problem but planning on going over the whole car soon to try and find the problem.

rich

[piman]

Hello Rich,

just looking at your figures, have you one too many zero after the decimal point, i.e "0.00051" should be 0.0051"?
I would expect the discs to be true within 5 thou or 0.005". You are quoting to the hundredth of a thou and I've never seen a DTI more accurate than 1 thou. Given that, your drums are very off true assuming all faces were clean and the drum sat snugly on the hub register? That amount of runout will easily be visible by eye, so it does look if your drums need replacing.

Alec

[Rich_s]

Hi Alec,

Its a digital dti i used and i measured in mm, then converted to inches and thats the figure it gave, 0.00051.

the drums seemed to fit pretty snug and looked clean enough, i will give them another go when get a chance and make sure the drums fit ok, i will try and see if i can measure the hub as well, but i think they are ok.

If i presume it is the drums, is it worth getting them skimmed or best to get some new ones?

regards
rich

just another thought, if the drums were as far out as my gauge says, wouldn't i feel this in the handbrake if i apply it while moving, which i dont, if i use the handbrake it pulls up smoothly. weird!

[piman]

Hello Rich,

in other words the discs are true to half a thou, that is exceptional I would say?

As the shoes are free to float it's conceivable that you won't feel anything in the handbrake lever at slow speed? If re checking your drums gives a similar reading I would have them skimmed?

Alec

[rhodes]

The problem of vibration from the brake drums is an old chestnut.  It was very bad on one of the Spitfires I had in the 1970s.  When I refurbished my current 1500 I got the drums skimmed and they are perfectly OK now.

However, my drums were very badly made by BL, or at least whoever did the casting.  The strengthening rim around the outside of the drum very clearly varies in thickness.  This results in a badly balanced drum which is compensated for by a strtegically placed weight on the wheel.

[Rich_s]

Finally had a weekend where i have fixed more problems on the spit than i have found woohoo!!!!!!!!!!!!!!!!!!!!!!

Finally cracked the brake problem, i found a car park with smooth tarmac and clamped off the front brakes as i still thought the problem was with them, then drove around the car park getting the rear brakes hot and seeing if still had the problem, turns out the problem went away so confirmed the front discs as the problem.

I checked the run out on the discs again and same result, half a thou out on both discs both sides, so i though i would go over them with some sandpaper anyway as is worst comes to the worst i will just get some new discs. I go the drill out and had someone spin the hub then i touched the disc with the sandpaper, on both the outsides of the discs all was fine and smooth running, on the inside of the nearside disc it kept snagging on something, couldn't feel it with my finger but the sandpaper picked up, so after a light sanding all seem ok so moved to the offside disc, on the inside was a large area the size of the pad that was hard rust and the sandpaper snagged alot on it, but the dti hardly picked it up, after a bit of harsh sandpapering then working my way up to a 600 grade i got the disc feeling alot better, i have now drive it back home, about 23 miles and the brakes feel sooooooo much better, no vibration or pulsing, finally!!!!!!!!!!!!!!!!!!!!!!

Hopefully all will be ok now, if not i know i only need to get new discs.

I have also got the door window seal to finally fit on the clips and sorted out the tappets so the engine is less like something from massey fergusson, so it has been a good day for once.

[junkuser]

If you have a mobile, on car, disk machining service in your part of the World it may be worth considering if the disks are not down to minimum thickness.  
A good operator can get a good result if the wheel bearings are in good condition.

Had my disks machined, off car, by a "Brake Specialist" many years ago and they managed to machine run-out into both of them so have put up with rattling pads at low speeds ever since.