sometimes when ht my brakes they seem to shake side to side, but not all the time. any one have any idea as to why

 

Usually this is due to warped rotors. A warpled rotor makes the disc brake caliper vibrate side-to-side as the rotor spins through it. Most newer car rotors are thin and warp easily. You can have them resurfaced ("turned"), but the problem is the thin-ness is aggravated by the loss of surface material, and the chances are even greater for a recurrance. Also, many newer cars specify that the rotors must be resurfaced while still on the car, which takes special equipment, although many people still have their rotors turned off the car, and usually get away with it. I have given up doing so, having turned (off the vehicle) warped rotors on two Mazda cars and one Explorer, only to have the vibration persist. New rotors solved the issue. Replacement rotors are available at parts stores for not too much, usually in the range of $40-$75 each.

You can diagnose this problem with a "run-out gauge", but most shade tree mechanics don't have one. You can buy one for $25 or so (it will be cheap and not professional grade, but it will work and you won't be using it much). Also, many parts stores will loan you one, or check the run-out if you bring the rotors in. (But this check is not as accurate as doing it with the rotor on the hub). In general, a rotor should have not more than 3-5 thousands of an inch of run-out (surface wobble side to side as it spins).

Unless the shaking is severe, there is no emergency to fix warped rotors. But you will never love your brakes until you do.

 

I forgot to mention the pedal itself shakes and the rotors were turned out not long ago. also i thought if it were wraped rotors i would feel it in the braking to a stop. whereas this shaking seems to happen most when im going fast "highway' and i have not used them in a while.

 

My money is still on slightly warped rotors, especially if you had them turned while off the vehicle, or if the shaking was gone for a week or two after repair, and is now back. The pulsating brake pedal almost cinches it. If the warping is minor, the rotors have to be spinning pretty fast to feel it in the pedal, so sometimes it is more noticable at speed then seems to fade away when coming to a stop.

Other possibilities are an improperly seated caliper pad, a pad whose friction material is defective and separating, or caliper mounting bolts being left loose at the last repair. Less likely but possible is a loose wheel bearing, or a sticking caliper piston that doesn't release pressure totally on its pad(s) when the brake pedal is released. And I suppose it is conceivable that the hydraulic unit of the ABS system might be sticking and keeping pressure on a caliper (the equivalent of a sticking caliper piston). But this would be very unusual and costly to diagnose and fix. Simple things first.

I would pull the wheels off and look things over. If everything is tight and ship-shape (including being able to re-seat the caliper pistons manually with only the expected amount of pressure), I would just replace the rotors. You can spend a lot of money having a mechanic take things on and off, and the rotors are always going to be suspect.

 

i will give new rotors a swing

 

Please don't laugh, but check the lug nuts for tightness. If slightly loose, you'll really feel a shaking like you describe, especially at higher speeds.

 

Good point! I should have remembered it in my earlier list. Anything that allows the tire/wheel or wheel/rotor to wobble around its axis of rotation can cause brake pedal pulsation.

 

i agree with yall on everthing except that it happens so infreqenct (once per ride or so) and so ramdum. that it is confusing, when the pedal shakes it is such an odd sensiation for it litterly seems to virbrate side to side, when it happens i let off the brake and then hit them again and it stops. nothing like an warped rotor ive ever expeinced but i am by no mean an expert having had only two warp rotors in my life. anyhow new rotors are on the wy and i will see if that helps

 

If putting the brakes on shakes the vehicle from side to side, rather than just making the pedal pulsate, you need to check the wheel bearings, the tie rod ends, steering rack inboard joints, ball joints, and suspension arm bushings too. (I have been assuming all this was checked with the brake work that led to your rotors being turned. Maybe it wasn't). This is all checked by vigorously trying to rock the wheel when off the ground (but still on the car) back and forth from top to bottom and side to side. You should have no play at all. You need to apply as much force as you can muster, so if you do this yourself, make certain the car is on a jack stand, and you have a jack in place too, for backup safety.

Rarely, an unbalanced/out of round tire or bent wheel can cause more vibration when the brakes are applied, but usually they vibrate all the time, brakes on or not.

 

no no putting on the brakes doesn't shake the vehicle just the brake pedal, it feels like your putting your foot on a small jack hammer.

 

One thing you might do is pull the fuse to the ABS system and see if it goes away. ABS systems are designed to "fail soft" - that is, just quit working rather than interfere with braking. But it would be worth trying. You may have some weird off-ball problem, but it is still true that common things occur commonly. You most likely have simple rotor problems.

 

Rotors were the case with me with that same scenerio!