queer brake problem111

[AussieJohn]

I rebuilt my front calipers last year and have not been happy with them. I find that with a dial indicator on the disc it flexs 3 thou when pedal is pressed; I have dismantled the caliper and the pistons and seals are free and in good shape, I checked the wheel bearings in case the whole disc was moving and I blew out the drillings in the caliper; any clues? I have run out of ideas, cheers, John.

[alaric]

Hi. Are you saying that the disc run out is three thou? I don't think you are. I would have thought the pistons should come back slightly when pressure is released, as the seals are designed to do that for you, so is it poss one of the seals is in backwards.

Sean.

[AussieJohn]

Hi, the disc runout is 1.5 thou but the disc moves 3 thou, I thought that it could flex a little as the first piston reaches the disc but should be back to its original position when the other piston pushes on the other side, most puzzling!! cheers, John.

[andyelan]

Hi John

Interesting as you say.

In a caliper the hydraulic pressure must be equal on both pistons and as of course they're both the same size, they can't do anything other than push equally on both sides of the disc.

I would suggest that the most likly explanation is that the pads aren't parallel and so are trying to twist the disc. This would be especially true if the pads have been used and have not been replaced in the same position they came from. But I doubt even new pads are presion made items in this respect

Another alturnative is that if the calipers are worn, the pistons could be tipping slightly in the cylinders and not pressing equally on the pads

Just a couple of suggestions

Regards
Andy

[AussieJohn]

Thanks Andy, I am heading for the garage now to measure the pads, thanks, John.

[AussieJohn]

Hi Andy, I think you may have solved the problem, I have just measured the pads ( as new but s/h, ) there is about 10 to 15 thou runoff across them, new pads will be on order tomorrow, it doesnt pay to be a cheapskate!! I will post up the results, thanks, John.

[rgh0]

Pads dont wear evenly in use they develop a taper due to caliper flex from the inside to the outside.
They also wear unevenly from fropnt to rear

New pads are generally square to a few thousandths of an inch but wear makes them uneven after a few miles resulting in the uneven twist on the disk

cheers
Rohan

[john.p.clegg]

Rohan,John

Seeing as

"New pads are generally square to a few thousandths of an inch but wear makes them uneven after a few miles resulting in the uneven twist on the disk "

Couldn't you keep the eneven pads and let the first few miles bed them in?

John

[AussieJohn]

Hi John and Rohan, thanks for the info; I might have let the old pads more time to bed in but they were a very hard grade and I dont do many miles so I think new softer pads are worth a try, cheers, John.

[paddy]

3 thou isn't a lot, especially considering that the usual end float (when cold) is a lot more than that, and (as I recall) the permitted runout on the disc is 5 thou. Given even the slight resistance of the pistons in the calipers, I'd fully expect the hub/disc to move by a few thou. With the weight of the car on it it would be different.

Paddy

[Lincoln62]

It doesn't sound like a problem to me.

It isn't surprising that one piston would take up before the other with the rotor in a static condition and cause it to flex. The reservoir of the piston next to the hose would pressurise slightly before the other one. If the rotor is spinning it would be much more resistant to flexing and the pressure in both pistons would equalize in a fraction of a second.

Peter
66S2

[crannyr]

you might want to chech that your calipers are dead on center to the disks. even a few thousands off center maybe part of the problem.

[CBUEB1771]

3 thou isn't a lot

I have to agree. You have the accumulation of tolerances of quite a few parts that can contribute to the centerline of the caliper piston bores not being perpendicular to the surface of the disc. Disc, hub, axle, upright, caliper mounting bracket and caliper. I have not measured this myself but thinking about it I am surprised that the disc moves as little as 0.003" when the caliper is pressurized.

[freddy22112211]

John,
Does the 3 thou cause any problems at all? - you don't mention any effects, or are u just looking for mathmatical perfection (ie to match the rest of the car, ha, ha)
Gordon

[AussieJohn]

Hi guys, have just fitted new pads ( which had 2.5 thou runout across the faces ) and still have 3 thou flexing of the disc; it appears to be the disc at the caliper which is flexing rather than the disc moving on the stub axle; its too bl**dy cold to work in the garage but I now suspect the caliper bracket or stub axle. Thanks for all the help, what would we do without this great little Lotus community, cheers, John.