Brake pad area

[chris.richard]

Can some boffin tell me what the effect of the area of a brake pad has on the braking effort?
Vague memories of schoolboy physics are that friction is independent of area, but.... :confused:

[chris.richard]

Well, glad we cleared that up then!! :rolleyes:

[mudhut]

This is what I've always taught the kids in Physics, based on F=µR. :) However if that was really the case under all conditions and the only factor, we wouldn't use wide tyres, so the reality is more complicated (and no I'm not going into it cos I can't get to the bottom of it either).

What is clear though is that if the frictional force generated by the pad was independent of area, the braking effort would obviously be too and so would be the energy that the brake takes out of the system.

In the extreme case of a tiny pad this would have at least two repercussions:

1. Pads would wear really quickly.
2. Pad temperature would go through the roof as the small contact area would fail to provide a sufficiently free path for the heat produced to get away.

Big pads can just get the heat away better but then present their own problems in getting rid of gases produced on their surfaces.

That still doesn't leave things any the clearer to me regarding the relationship between area and braking effort though. :rolleyes:

[Martin K]

Does the following make sense?

Brakes.

If you have a larger pad and the same piston area, the result is more pad on the disc but at a lower pressure per square mm. Because of F=µR you would have less braking force at the disc for the same pressure on the pedal.

The pad would run cooler though so you could have a pad with a higher µ to restore the level of friction and therefore braking force.

Then if you then increase F, by having more piston area, you will have more braking power. The brakes will run hotter, so you will have to reduce µ to bring temperatures down. You could be back to square one!

The advantages of larger pad area as Mudhut says above is in reducing wear and fade by having more material to wear and allowing the pads to run cooler than smaller pads would.

However, a big increase in braking power comes with larger diameter discs with the pads running right on the outer edge. The larger the diameter of the disc and the further the pads are from the centre of the disc, the more mechanical advantage they have to reduce the speed of the rotating disc.

Now for tyres!

Because of F=µR, the only way to increase grip is 1) a higher µ - i.e. softer rubber or 2) more load on the tyre - up to a point.

Look at F1 - either softer tyres or more downforce increases cornering power.

Wider tyres can only offer more grip if they use softer rubber - which has a higher µ - as the load on the tyre remains the same per square mm of contact patch.

If wider tyres are made of the same rubber as the narrower tyre they are replacing they won't give any more grip - they will just wear slower than the narrower tyre.

If you are increasing F, the vertical load on the tyre, by cornering harder (more weight transfer) or by aerodynamic downforce, then you will get more grip up to the limit of the tyre, which is a function of the F/µ coefficient of the tyre. i.e. at some loading the tyre will offer less grip than the load on it and it will slide.

This coefficient is not linear, varies with load and also from rubber compound to compound and from one tyre construction to another.

For example, a tyre with a 250 Kg vertical load might have a capability of 350 Kg of lateral load - better than unity. With 500 kg vertical load it might have a capability of 500 Kg of lateral load - a 1:1 realtionship. Increasing the vertical load to 750Kg and the lateral load capability might increase to only 625 Kg - less than unity. Increasing vertical load on the tyre beyond this and the increase in the amount of grip becomes progressively less.

Note that on the same set of tyres a lighter car can corner at a higher G than a heavier car because there is less load on the tyre to start with - so more weight can be transfered when cornering before the tyre slides.

A better tyre will have increased side load capabilities (grip!) at higher vertical loads. This is evidenced by the fact that in the 1970's, without downforce, the best club racing tyres could corner at 1.1 - 1.2 G. Now 1.6 G and more is attainable. This increase has come from improvement to rubber compounds and tyre carcass design.

As I said at the top, I hope this makes sense.

[SUSIT]

Martin,
you beat me too this post by a whisker, it was almost the same as i was going to say

Stephen

[Martin K]

Martin,
you beat me too this post by a whisker, it was almost the same as i was going to say

Stephen

Does that mean we are both right - or both wrong! :D

[SUSIT]

Does that mean we are both right - or both wrong! :D



Both correct of course Martin :D

[chris.richard]

Wider tyres can only offer more grip if they use softer rubber - which has a higher µ - as the load on the tyre remains the same per square mm of contact patch.

Surely (may I call you that?) ;)
same load and larger area = less kg/square mm of contact patch ?
The kg/contact patch remains the same.

[BAS]

Surely (may I call you that?) ;)
same load and larger area = less kg/square mm of contact patch ?
The kg/contact patch remains the same.

yes you may :D

[Martin K]

Surely (may I call you that?) ;)
same load and larger area = less kg/square mm of contact patch ?
The kg/contact patch remains the same.

No, I don't think that's correct. The wider tyre will have the same contact patch area, for the same tyre pressure, but the shpe of the contact patch will be different - wider rather than elongated around the perimeter of the tyre as it would be on the narrower tyre.

This gets a bit complicated, but the elongation of the contact patch requires that the sidewalls are less stiff than is possible on a wider tyre with the same contact patch area, as the sidewalls of the narrow tyre have to deform, in a controlled manner, to allow the contact patch to elongate. The stiffer sidewalls on the wider tyres reduces the slip angle and so allows for greater cornering loads.

xx

Surely

[Chris J]

yes you may :D

The cockpit?,...what is it?

http://www.youtube.com/watch?v=midrADL_kHI

[Steve Strain]

Hi Guys

I have read the above comments and agree with what you say having had first hand experience with larger pads on larger disks. Unless you push harder or increase the pedal ratio, braking force can be reduced.

To overcome the original Lancia Beta brake setup issues I bought and fitted a full Wilwood kit thinking I would get relatively fade free brakes with improved balance and feel. In my case the result isn’t so (well not yet).

Since this thread was titled “Brake Pads” I thought I would ask the collective wisdom about pad compounds and what’s best for a Wilwood setup. What I would like to achieve is a good result for around town and motorway driving but also brakes that can last up to say 6 laps of our local 3k track.

Wisely or otherwise the Wilwood setup I fitted comprises
- Twin ¾” master cylinders
- Billet Dynalite 4 pot calipers with 1.75 pistons front and 1.38 on the rear.
- Polymatrix 7112D pads (a compound that I have since found was discontinued before I bought them).
- Pedal ratio (un-boosted) at approx 7:1
- Fluid is Dot 4.

When first driven the brake pedal felt spongy. Bleeding them resulted in no bubbles but when investigating creaking noises (when pushing the pedal hard) I found that by removing the pads and pumping the pistons out only one in each caliper moved. After freeing them up the creaking vanished and so did the spongy-ness. I guessed that being new some of the pistons were perhaps tight compared to others.

Anyway long story short it’s my impression that the car seems to take for ever to pull up from speed. On smooth surfaces I can feel the front wheels chirping over painted lines etc, so they are working and a hard stab will lock the front wheels. What I feel is that I need more rear bias but as I add it the brakes seem to get worse. (The balance bar is at the moment about 75% front 25% rear).
The more you I use the brakes the worse they seem to get.

I suspect the brake pads are the problem. They make lots of dust but seem to do little else. I wonder if in-fact that they are made of old cheese. Maybe gas or brake dust is affecting the surface of the pads and disk.

Does anyone any recommendations with something that works or is my expectation too high? Maybe there should be more metallic in the pads to make them bite more aggressively and provide more feel.
I don’t like having to experiment too much since every dealer has their own opinion.

Any advice?

Cheers

Steve

[chris.richard]

- Pedal ratio (un-boosted) at approx 7:1
Is that a Hawk setup? I thought it was 5:1.

What is your pedal travel Steve?

[Steve Strain]

Hi Chris

I modified the Hawk set up by lengthening the lower part of the balance bar connector (as arrowed in the picture).
I can’t remember exactly but I only had to add several mm's to its length to change the pedal ratio significantly. The push rod from the pedal and clevis now move through a slightly lower and longer arc.
The master cylinders only move about 10mm or so for full on brakes.

I will have to dig out my card board model of the setup for more exact measures.

The ratio was calculated assuming I would put my foot on the middle of the brake pedal pad. However I do tend to put my foot nearer the top of the pad and so reduce the leverage.

The recommended ratio was 6.25:1.

I will check again

cheers

Steve

[colin artus]

I would suggest something from Performance Friction, their Carbon Metallic would suit as a pad. Also I would get some grooves put in those discs (5mm ball nosed end mill) as it will help prevent pad glazing.

[Steve Strain]

Thanks Colin

Yes I suspect glazing could be part of the problem but I don't know how to recognise it. Also when I remove the pads there is always evidence of dust on the surface of the pad. The pads have a grove up the centre, this is also usually full of brake dust.
My guess is that grooves are better than drilling holes so as to ensure that the entire pad area is swept. I assume this might also give the brake dust somewhere to go.

Steve

[Arthur]

I changed from "standard road" Ford pads to Ferodo DS2500 - instant change for the better, on the otherwise bog standard Corse I set-up (Scorpio all round). I belive the "Green/red stuff" pads work OK too. But there's loads of compounds out there. I've no idea really, and take advice. Thing is, there's lots of overlap in pad performance as well, hence the often conflicting views - essentially the recommended pads are all the same performance.

The grooves in the disc "wipe the fireband" - that is, they give the burning dust from the pads (and god forbid the discs as well) somewhere to go. The heat during braking mechanically wears material off, but also releases a lot of very hot, and very low friction gas, which does nothing for braking performance.
Tapering the front egde of each pad at about 45 degrees for about half the material depth helps, as does the vertical groove in the pad material, as does another groove at 90 degrees to the first if you have real problems, which you shouldn't unless your'e a hell of a better driver than me (no big deal, actually), and you get and keep the brakes really hot. But then you'd be better off with a heat-tolerant compound; the downside being that they then need to be really hot to work at all.
Pad glazing - how does it feel? No brakes, no matter how hard you push the pedal, and doesn't recover when cool. Pedal high and hard.
Pad fade - brakes getting worse, pedal high and hard, . May or may not recover when cool, and pad surface wears a bit.
Disc glazing - poor braking, getting worse. Recovers a little when cool, and a little more when driven normally on the road, as the surfaces bed up again, but will never give 100% again. Pedal high and hard.
Fluid fade - no brakes at all, pedal on the floor. Recovers fully when cool, but may need bleeding again (the heat and vapourisation can release non-soluble gasses).
Seals gone - no brakes at all, pedal on floor, no fuid in reservoir, doesn't recover (predictably).

Arthur.

[chris.richard]

Can you tell glazing by looking at the pad/disc surface?

[Steve Strain]

Hi Arthur

Many thanks for your comments, I thought i had already taken good advice when buying pads. Yes the pedal is always high and hard so pads and the disk surface are the two variables.
I think I recognise the glazing by the feel and noise the pads make, but looking at them I wouldn't know how to tell.

http://brakepads.wilwood.com/02-graphs/index.html

The above link shows the performance difference between Wilwood pads.
The D compound I have aren't shown but I am sure they must have a very low Cf and temp range or in glazing them I have stuffed them.

Its a bit of a gamble but maybe compound H is the way to go. A high cold Cf and gets better the hotter they get.

Has anyone any experience?

cheers

Steve

[colin artus]

Glazing is a hard glassy surface on the pad giving a very low coefficient of friction - so lots of effort at pedal = zilch at disc (often accompanied by a squealing noise). You can remove glaze with medium grit paper on a flat surface. I have seen people using a file at racetracks which is likely to lead to a uneven pad surface - better than no brakes at all I suppose. When fitting new pads follow the manufacturers directions about bedding in as this can avoid problems later on - there are some formulations which claim not to require bedding and others that are available prebedded.

[CorseChris]

Steve, I was using D compound on my car but found the pedal way too hard for my liking. I switched to the A compound. Massive improvement for me, but they are really dusty and quite noisy as well - sounds like you're grinding the brake disc at low speed!

Pure road use for me.

An H looks like it would work well too, so if you do try them, let us know how they behave for noise and dust - I might switch. I opted for the A so I get instant cold bite. Having said that, first use of the day and they still feel pretty dead - soon come on song though.

[Martin K]

Steve

The choice of pad material is very much dependent on how you drive the car. If you don't drive the car hard, particularly not using the brakes hard, then I would suggest you use a road pad. I don't know about the D compound but the Wilwood site says that H compound is for racing only - so I would avoid fitting them them. I would think that unless you do track days frequently the choice would be between Polymatrix Q and Polymatrix E, using the latter only if you frequently brake hard from 100 mph as the E has a higher operating temperature than the Q.

I have more experience with EBC pads - both on road cars and on track. If you can get EBC pads to fit your calipers, I would suggest you try EBC Green Stuff first and only if you end up cooking them (lots of black brake dust and very high wear rate) move up to EBC Red Ceramic. I have never had fade on Green Stuff - only rapid wear as temperature rises - so they are great for both road and a little track use. Red Ceramic is used successfully, for example, on Subarus in tarmac rallying. These cars are quite heavy and repeatedly braking from 100mph + to 30 mph or less on rally stages and they are man enough for the job.

Another point is that glazing often occurs on racing compound pads if you don't use them hard enough - as in repeatedly touching the brakes to ease your speed down to a road junction. So you can 'bust' the glaze either as Colin suggets on a surface plate with an abrasive paper, or you can do a series of very had stops from high speed, which will sometimes also remove the glaze by a combination of heat and friction. But, once you have cleared the glazing by either method it will rapidly build up again if you don't use the brakes hard on a regular basis.

[Stratos]

I can verify Martin's recommendation for the EBC Greenstuff pads.

I have used them for several years on my Allora, and now on the Corse, and they are awesome - tremendous stopping power and no fade.

I previously tried RedStuff but wasn't getting them up to temperature fast enough, and even after part way through a stage I was still not completely happy with the performance, so I swapped to GreenStuff and the difference was astonishing.

I was worried at first that I would overheat them on longer stages, but I haven't had a problem with fade. Even at Abingdon, where the stages are 12 miles long, and there is lots of very heavy braking, they are fine - although it might just be that I am not driving fast enough.

I definitely wouldn't advise anyone to use EBC RedStuff for road use, but GreenStuff is perfect for either track use or road use.

[CorseChris]

I chose the A compound purely on what the graphs implied - reading the words suggests they are for hard-core race use. Not something I can be accused of! I find they work great on the road.

The Green version for the Wilwoods are pretty cheap though, so probably worth a go.

[CorseChris]

Bought some EBC greens for mine...went to fit them last night and discovered that the 'inner' pad fouls the caliper mount. Need to relieve them a wee bit to make them fit. Outer pad was fine. The Wilwood pads already have a slight curved cutout on the inner edge - the EBCs are simple rectangles.

I'll report back once I get them on & bedded.

One bonus.....the pads are painted green, which matches the car :)

[Stratos]

My car has just had new EBC GreenStuff pads fitted ready for Abingdon because the car had been sitting unused for 2 years, so I thought it prudent to fit new pads.

Took the car testing on Saturday to "bed them in" and they were great.

[Sando]

Took the car testing on Saturday to "bed them in" and they were great.

Ash said your car was very quick, not seen any pics of any over exuberance yet though ;):D

[Stratos]

Chris Newham has the evidence. He's sorting me out some copies of his photos.

I was taking it relatively easy to make sure I didn't damage the car before Abingdon.

I had to be careful cos I was "bedding-in" the brakes. :D