Brake Pad Wear

[dtype38]

Got some new four pot front brake callipers and grooved rotors a few months ago. I noticed that the rotors had quite prominent machining marks on them and weren't ground smooth, but assusmed they would wear smooth quite quickly. I did three race meetings and sure enough the discs are now smooth but when I checked the pads, they were heavily tapered, being some 2mm thinner at the leading edge than the trailing edge.

The callipers have equal sized pistons, but the pistons are offset towards the trailing edge to cut down on tapering. When I checked with the supplier they said that that much taper was way too much and could give probelms over about 0.5mm on each pad. He said to check I'd fitted the callipers the right way up!! Yes I had, but I put the wear down to the machining on the rotors, so bought new pads and went to a race meeting at Mallory.

I did two 15min practices and two 15min races and I've just checked the pads. They are heavily tapered again (about 1.6mm on each pad). I'm using Mintex 1144 pads which I'm told should be plenty hard enough for my car (990kg) so I'm at a bit of a loss. Any ideas why this should be happening anyone?

[Tim Falce]

I don't know but hurry up and find out as I need to renew my callipers asap

[dtype38]

That isn't helpful Tim

[1200Datto27]

Dtype,

Have you checked that the pads are fully releasing from the rotors when no pedal pressure is present?

What brand of calipers are you using?

Normally, multi piston race calpers use a differential bore pistion, so that the rear set of pistons can provide greater pressure to the rear of the pad, to counter the drag affect coming from the leading edge of the pad, that tends to transfer the contact patch forward. They only use equal bore calipers (for racing use) when the pads are small enough that the designers don't think that it would be an issue.

Have you tried a different pad material?

[dtype38]

1200Datto27... The pistons in the callipers are all free and smooth in operation, so they should be releasing ok. The callipers are HiSpec (I would have got AP ones, but I don't have a fairy godmother).

Differential piston size was available, but that would have meant changing the master cyl, so I stuck with the same size pistons. They're the same size as were on my original cast iron 4-pot callipers. They didn't cause unduely tapered pads, but were very heavy.

Could it be that grooved discs just wear the leading edge quicker and that I should have taken that into account and used differential size pistons?

[1200Datto27]

It might be that the grooves are just catching the leading edge of the pads, and chipping away at them. Try swapping he rotors over so that the left one is on the right, and vice versa, if this changes the tapering issue, it may be that the angle of the groove was slightly wrong for your pad/rotor/calipper combination.

[dtype38]

The grooves are angled towards the outside. I assumed this is so that they act like tyre grooves when clearing water from the disc, so making them "handed in rotation" ie. that I have a left hand one and a right hand one. Would you still try swapping them?

[1200Datto27]

You should have the grooves set so that they sweep into the center of the rotor, ie, the outside part of the groove should hit the pads first. Make sure that if you have directional rotors that you have the internal cooling vanes rotating from the inside out.

Do you have any tempreture paint available? if so, put some on the outside edge of the rotor, in between the swept part of the rotor and the hub, on the inside vanes of the rotor and on the backing plates of the pads and check what tempretures you get. It maybe that the pads are getting to hot hot at the leading edges, and failing.

[dtype38]

Ahhhhhhhhh! Err, ok so the rotors don't have arrows on them and the cooling vanes are exactly radial (ie no sweep to them). Sooo... I reasoned that the groves would want to sweep (fling) the water outwards so fitted them with the inner edge of the grove arriving at the pad first. Will swap the rotors and try that (and the temp paint) next time out.

PS race season over now so will have to wait till feb track day to let you know if it works. Many thanks.

[Tim Falce]

Originally posted by dtype38
Ahhhhhhhhh! Err, ok so the rotors don't have arrows on them and the cooling vanes are exactly radial (ie no sweep to them). Sooo... I reasoned that the groves would want to sweep (fling) the water outwards so fitted them with the inner edge of the grove arriving at the pad first. Will swap the rotors and try that (and the temp paint) next time out.

PS race season over now so will have to wait till feb track day to let you know if it works. Many thanks.

I thought hte grooves in the rotors were to dispel gasses to stop the pads and rotors glazing, didnt think about water being expelled

[Al Weyman]

They have nothing to do with water, water would immediately turn to steam on a hot brake disc, it has been used in truck racing to cool the discs (water) but it can cause discs to crack. The grooves and drilled discs are purely for expelling the gases that build up.

[greenamex2]

I have got Hi-spec on my kit car and the equivelent AP's on my race car (all equal bore). The race cars pads taper a bit, the kit car hasn't done enough mileage yet but I am not expecting the degree of problems you are having.

Hi-spec said that the pistons were offset but they also said a lot of other things which were a bit economical with the truth.

What disk size are you running and what is your cooling arrangement?

It sounds like the pad is getting far hotter than is good for them and exaggerating the inherent problems that equal size pistons will pose.

Personally I would have that that Mintex 1144 were a bit soft. I run a 830Kg car and don't use anything less than a 1155 or, preferably, a Ferrodo DS3000.

[dtype38]

Rotors are Hi-Spec 300/22mm and the calipers are Monster 4 with twin 41.2mm pistons. Coolin is not ducted, but there are large grilled vents in the bonnet (front skirt area) right in front of the inner part of the wheels. The bonnet is heavily ventilated and there is lots of airflow past the wheels.

The pads themselves are only slightly glazed but have no signs of major overheating. They have only small amounts of degradation around the edges with the leading edges having no more pitting or crumbling than the other edges of the pads. They are probably in better condition, face wise, than I used to get on my previous 1144s in my standard Jaguar calipers and discs.

Talking of which, that brings me to the subject of grooves and gassing from pads. I've only ever seen one think written on this subject and... well to be honest it sounded like one of those sales pitches for a cure for replacing lead in petrol by putting a gizmo in the fuel line... ie unsupported by any real science. I'm happy to be corrected if I'm wrong on this, but I drove motorbikes for years with smooth discs without "gassing" pads, and factory Jaguars have always raced on smooth discs. What I do know is that is that every biker knew about "disc lag" in the rain, ie that when the discs were wet the pads momentarily aquaplaned on the discs when the brakes were applied. Then grooved and cross drilled discs were invented and cured the problem immedidately. The film of water on the discs was instantly cleared into the grooves or holes and the brakes bit immediately. The downside was that in dry conditions, at any paticular instant there was less pad to disc surface contact area, so brake effeciency was actually reduced. Dunno if I'd been brainwashed, but the first time I got vented discs for my bike I noticed the slight drop in performance in the dry, but happily sacrificed that for instant stopping power in the rain!!

Don't be afraid to tell me if you know better

[1200Datto27]

Most modern pad materials do not produce the excess gas that older pads did, really rendering drilled rotors unnessecary (unless you are chasing weight reduction) but you can still get a gas/material buildup between the rotor face and pads, especially with longer pads, that will affect the redartation of the braking system. Slotted rotors will assist in the removal of this material whilst the brakes are applied, as well as slightly abrading the pad face to assist in preventing glazing of the pads.

[Al Weyman]

You are right and I was looking at a brand new M3 yesterday and it had drilled rotors, what on earth is the point on a road car, purely boy racer stuff and of no practical use whatsoever on a road car apart from maybe promoting cracking of the rotor and being able to charge the customer a huge amount of money for a replacement. I have used groved drilled and plain rotors on my heavy old Camaros and the last one I built, the 3rd gen, I just left plain as I could see no difference whatsoever.

PS (Edit) Apart from cracking of the drilled ones.

[boyracer]

Be not so quick to laugh at the Germans.
I heard froma friend that Porsche developed true cross drilling technology with some way of actually countersinking the holes in between the two halves of the discs (which is where most of the others fall down) and they do that for some pretty quick machinery.

I posted a thread asking about disc slotting ages ago, response I got was mostly be-wilderment but there were some good arguments, but at the end of the day I don't think it matters. Rotors I saw on WR Rally cars about 2 years ago had the groove crossing the pad on the inside first then head towards the outside. The rally car I service for has them the other way (as DBA {australian manufacturer} had instructed at the time but have now changed their mind) and we experience no pad taper problems at all, nor problems in wet weather (which most of our rallies are due to fire problems in the summer months).
I think more grooves helps cut the pad, for a fresher "bite" each time (while shortening pad life) and probably increases the disc area (marginally, but hey this is motorsport) which will of course help cooling. And I know the local V8 supercar teams play with hatching patterns on the discs even in the dry.
Apart from that, I'd be looking at caliper problems (could be flex ? )

[1200Datto27]

Porsche cast their rotors with the holes in place, so that it reduces the chance of a stress risers being formed by the drilling process. They also heat treat the rotors before fitment, to ensure a more consistent matrix through the metal.

[Dave Brand]

Originally posted by dtype38
The downside was that in dry conditions, at any paticular instant there was less pad to disc surface contact area, so brake effeciency was actually reduced.

Reducing the area of the contact area won't affect braking efficiency, as friction is independent of area. The braking force generated is a function of the coefficient of friction between the two components of the 'friction couple' & the clamping force aplied. Of course, there are many good reasons why 'bigger is better', mainly thermal, but also such criteria as friction material shear strenth, wear rate, etc.

A few thoughts on tapered pad wear....

It seems to me that it's part of the price you pay for using long thin pads clamped by more than on piston. Such palliatives as ofsetting the pistons, differential piston sizes, etc., will mitigate the effect, but probably only totally prevent taper for one given set of conditions. The ultimate answer is probably, as is being done on a few bikes, to use one pair of pads per pair of pistons.

There's some useful information, particularly regarding slotted/drilled discs, at:

http://www.coximport.com/brembo/faq.html

[dtype38]

Many thanks for the input folks. I think I'm inclinded to agree with Dave that tapering is just one of those things to be expected on very long narrow pads. Guess I'll just have to turn them every race or two and treat it the same as topping up the fuel and checking the oil.

[a.bart]

most pad tapering is caused by the caliper mounting flexing under heavy braking

[dtype38]

Just had another chat to Hi-Spec who think one possibility is that when I put brand new pads in the caliper pistons are pretty much bottomed in their liners. Apparently this can cause less than full pressure to be applied to the piston until the pad has worn a bit. There is the possibility, therefore, that as I wear the leading edge slightly, more pressure is being applied to the leading edge piston and making the situation worse.

I'm surprised they would know about this possibility but not have changed the design of the oil ways to prevent it, but maybe I'm being a bit harsh. Anyway, its an easy theory to test... I'll just grind my pads back to flat and see if I still have the problem!

a.bart : the front pads all have identical amounts of taper from the same edge. Would flexing mounts not tend to cause opposite edges to wear on each pair of pads?

[Eric Falce]

Ken,have you ever thought that the problem may be with your driving technique.If you are not cornering correctly,breaking to early then turning in early,this will cause uneven wear so I think you are not paying proper attention to the correct cornering lines.
By the way,have you ever thought that the problem may be air in the system,you have a completely new braking system,very easy for air to be trapped in the lower part of the caliper,are you having any brake fade at all,try back bleeding the system,as well as driving technique!Also,why not swap the pads over in the caliper to even out the wear if all else fails but I would not advise reshaping the pad,fit new ones if they are that low!

[dtype38]

ecktic... interesting thoughts. Was completely unaware that driving style could affect how evenly the brake pads wear. My own particular style is indeed to brake a little early, but that's because my car is front heavy and I don't like to trail brake in case it swaps ends. But even so, I then go very deep into corners and turn hard with the power on to get a late apex and better speed on the straights. What is your reasoning behind this affecting pad wear?

Re air in the system and fade. I've very meticulous when it comes to getting air out of the system because I absolutely hate any sponginess, fade or change in brake feel when I'm racing and so far (four race meetings) I haven't had any indication of anything like than.

Re: turning the pads regularly, yes I had considered that and it's still my fall-back plan if nothing else works. I just wanted to see if I could get a handle on what's causing the problem first. If the Hi-Spec suggestion is correct, then turning them at this stage may have little effect. But as to reshaping them... they aren't worn down as such (the trailing edge isn't worn at all!) just worn at an angle. I have a big bench belt sander which will sort that out if I want it to. :-)

[1200Datto27]

I think that what Ecktic is getting at is that some race type brake pads are suited to being treated quite harshly ie deep and hard on them, and that if you do not do that to them, they have really bad wear rates.

I know that Hawk Blues have that issue, as the guy that I bought the car from used to have one set last for a season, where as I was going throught them every two meetings. It turns out that my braking style was to gentle on the pads, leading to them spending to long at a less than optimal temp under braking, leading to higher wear.

[dtype38]

OK, understand that, and yes it's an issue with my driving that I've had to work to correct in the past. When I first started racing I braked far too gently and also used the brakes to steady the car a lot. I got through a lot of pads and boiled a lot of brake fluid before I learned to hit them "hard and short" and also learned better car control. My current car is more forgiving on that front so I may have unknowingly slipped back into bad habits. I'll definitely take that into consideration as a possible cause and analyse my driving accordingly. Ta.