Weight re­duc­tions lead to brake pul­sa­tions

Winnipeg Free Press - Section F - - TECHNOLOGY - JIM KERR

BRAKE pul­sa­tion has be­come a com­mon con­cern on many newer ve­hi­cles. The ve­hi­cle stops smoothly when new but, af­ter a few thou­sand kilo­me­tres, the brake pedal starts to pul­sate when stop­ping, the brakes feel like they’re grab­bing and re­leas­ing, and smooth stops are im­pos­si­ble. Some­times the prob­lem starts af­ter the wheels have been re­moved. This was sel­dom a con­cern on ve­hi­cles in the ’70s and ’80s, so what has changed to make it a prob­lem to­day?

The an­swer is weight. To in­crease fuel ef­fi­ciency of new cars and re­duce man­u­fac­tur­ing costs, the ac­coun­tants en­sure that the weight of ev­ery part is re­duced as much as pos­si­ble.

This works fine for most parts, but ask a brake-sys­tem en­gi­neer about brake ro­tors and the an­swer will be that heav­ier is bet­ter. Heav­ier brake ro­tors can ab­sorb more heat, so they fade less and give bet­ter brak­ing per­for­mance. Heav­ier ro­tors also tend to be more sta­ble, so they sel­dom warp, which is one of the causes of brake pul­sa­tion.

Some­times the brake en­gi­neers get their way; other times the bean-coun­ters win and the brake ro­tors are built lighter. That’s when prob­lems may oc­cur. Sure, they could build brake ro­tors that are light, pow­er­ful and sta­ble — car­bon fi­bre and ce­ramic ro­tors are avail­able and used on rac­ing cars but the cost would be pro­hib­i­tive for most pas­sen­ger ve­hi­cles.

Porsche does of­fer a high-per­for­mance brake op­tion on the 911 Turbo model, which stops in amaz­ingly short dis­tances with the stock brake sys­tem. I can’t imag­ine how much bet­ter the $5,000 brake op­tion would be!

So most of us are stuck with con­ven­tional cast brake ro­tors which, if they’re not han­dled cor­rectly, can warp. That’s the be­gin­ning of brake pul­sa­tion.

A warped ro­tor wears un­evenly from side to side as it rubs against the brake pads, and soon the sides of the ro­tor are no longer par­al­lel to each other. Tol­er­ances for par­al­lel­ism are of­ten less than .0005 of an inch max­i­mum, or about onequar­ter the thick­ness of a hair. When the ro­tor sides are not par­al­lel, the pis­tons in the brake calipers that ap­ply the brake pads are pushed in and out rapidly. This rapid mo­tion is trans­mit­ted through the brake sys­tem and into the pedal. We feel it as a pul­sa­tion.

Chang­ing a tire is one com­mon cause of ro­tor warpage. If there is any dirt, rust or cor­ro­sion be­tween the wheel and the brake ro­tor, it will be clamped un­evenly when the wheel is in­stalled and the ro­tor will warp.

In­cor­rect tight­en­ing of the wheel nuts also can warp a ro­tor. When in­stalling a wheel, snug up the wheel nuts and then tighten them in two stages us­ing an al­ter­nat­ing criss-cross pat­tern. Us­ing a torque wrench is crit­i­cal on mod­ern ve­hi­cles. Some shops tighten wheel nuts with air im­pacts. Oth­ers use “torque sticks” de­signed to limit the torque on the nuts. Nei­ther is ac­cu­rate enough for to­day’s cars. Make sure they use a torque wrench

If you ex­pe­ri­ence brake pul­sa­tions af­ter chang­ing a wheel, loosen the wheel nuts and re­torque them. If this is done as soon as pos­si­ble, the ro­tor will usu­ally cor­rect it­self. Leave it too long (more than 1,000 km) and it re­mains warped.

An­other cause of brake pul­sa­tion is brake ro­tor run-out. Many man­u­fac­tur­ers al­low the ro­tor and hub to wob­ble up to .003 of an inch be­cause of ma­chin­ing tol­er­ances. More than that and the ro­tor wob­bles too much – it acts like it is warped. And some cars are es­pe­cially sen­si­tive to ro­tor runout — if it’s more than .001 of an inch, then brake pul­sa­tions can oc­cur.

The an­swer is to cor­rect the run-out to less than .001 inch. One way to do this is to ma­chine the ro­tors in place on the car in­stead of us­ing off-car brake lathes, as most shops do. This will cor­rect for run-out in both the ro­tor and the hub. The trou­ble with this is that on–car brake lathe ma­chines tend to be more frag­ile and cost three to four times as much as an off-car brake lathe, so very few re­pair shops own one.

An­other method of cor­rect­ing ro­tor run-out has been in­tro­duced to the mar­ket. Brake Align has de­signed spe­cial ta­pered shims that are placed be­tween a re­mov­able ro­tor and its hub to com­pen­sate for ro­tor run-out. Run-out is mea­sured on the ro­tor and one of three dif­fer­ent-sized shims is in­stalled. Run-out can be cor­rected to within .001 of an inch.

Brake Align shims have been tested for hun­dreds of thou­sands of kilo­me­tres, and re­search by the com­pany shows that if ro­tor run-out can be kept un­der .001 inch, then brake pul­sa­tions are no longer a prob­lem.

Cor­rect­ing brake pul­sa­tions, or pre­vent­ing them, is just a mat­ter of de­tails. Keep mount­ing sur­faces clean, ma­chine ro­tors so the sides are par­al­lel, re­duce ro­tor run-out to a min­i­mum, and use the proper pro­ce­dure to torque wheel nuts. Brake pul­sa­tions should no longer be a prob­lem on your ve­hi­cle. Jim Kerr is an ex­pe­ri­enced me­chanic, in­struc­tor of au­to­mo­tive tech­nol­ogy, free­lance jour­nal­ist and mem­ber of the Au­to­mo­bile Jour­nal­ists’ As­so­ci­a­tion

of Canada.

Heav­ier brake ro­tors can ab­sorb more heat, so they fade less and give bet­ter brak­ing

per­for­mance.

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