So imagine a disc that is machined perfectly flat both sides with an even thickness everywhere, but is mounted ever so slightly askew. Put a dial indicator on it and it will tell you that there is disc 'runout' and the diagnosis that is commonly leapt to would typically be; 'warped' disc. But it isn't warped, and the askew disc won't cause braking force variation or pedal pulsation. If these symptoms exist then there will be an additional problem of thickness variation on that disc, or one or more of the other discs.
The slightly askew disc won't cause a braking force variation or pedal pulsation because the caliper will accomodate a slightly 'off square' disc. A 'fixed' multi piston caliper will do this by allowing the pistons on one side of the disc to move inward while the pistons on the other side simultaneously and equally move outward (as the wobbly disc rotates). As the pistons move the pressures inside the system don't change to any significant degree because there is no net change in the fluid volume inside the caliper (just fluid transferring from the pistons on one side to the pistons on the other side). The only resistances to the free movement of the pistons will be piston seal flexure and the inertia and viscousness of the brake fluid (as well as inertia of the pistons and pads), which are extremely minor. A sliding caliper will do a somewhat similar thing, but instead of the piston moving the pins will slide, which is a very minor resistance with a miniscule affect on braking force variation and no affect on pedal pulsation. For a pedal to pulsate there must be a significant rise and fall in overall sytem pressure (which can be felt at the pedal), and this doesn't occur if all the pistons etc move fairly freely in equally positive and negative ways.
So, a slightly askew disc will cause the pads, pistons and the fluid to move ever so slightly from side to side in the caliper, but this won't result in a significant change in the force pushing the pads against the disc or the friction strength at the pad / disc interface. There will be some minor friction where the pads bear against the pad carrier that will try and inhibit the free lateral movement of the pads, but again it won't have much affect.
The symptom that leads to the common diagnosis of 'warped discs' is in reality a symptom of thickness variation of the disc. That is, an uneven build up of pad material on the disc faces. This starts off as a spot of build up on one side of the disc (i.e. one section of the disc at which the disc is effectively ever so slightly thicker than the rest of it), which causes the pads to 'pinch' the disc at that spot with each revolution of the disc. This in turn causes a localised hot spot on the disc (both sides), at which hotter location more pad material is further deposited (than on the cooler parts of the disc). Now we start to have an increasing thickness variation on both sides of the disc at the same location on the disc.
Each time the slightly thicker part of the disc passes between the pads the thick spot forces the pads on both sides to be simultaneously pushed into the caliper body, which forcefully expels some fluid from the caliper (up the brake hose), momentarily increasing the pressure in the system (felt as pedal pulsation), and momentarily increases the friction between the pads and the disc (manifested as braking force variation). The problem just gets worse because it is self generating, and the only cure is to machine the discs or to replace them. The person who made the diagnosis of 'warped discs' then says, "see, told you that would fix it"...
I know from personal experience that an 'askew' disc doesn't cause pedal pulsation or force variation. Racing karts usually have only one brake (on the rear axle), and I've more than once whacked the disc hard on a ripple strip (or something, other drivers helmet perhaps...), and continued racing neither having nor suspecting any brake problems whatsoever, only to discover later in the pits a huge disc wobble because the aluminium disc carrier / hub had been bent. I'm talking a serious out of true here, easily visible with the naked eye, yet no problems at all in use, no symptoms whatsoever.
Last edited by johnlear; 1 Week Ago at 01:00.