Rear brake pad friction coefficient

[rj]

Would anyone happen to know the friction coefficient of the standard rear brake pads (for non uprated front brakes that is) or alternative the type used (not the Caterham part No)?

Still trying to re-gain the brake balance from before the front brakes were uprated due to overheating of my right knee - not the brakes - on trackdays...

[Wrightpayne]

There is a post in the archives concerning uprating the rear pad material to increase retardation without locking up the rears first (which is very bad).

The author was basically saying the rears do a lot of work stopping the car and the standard pads didnt do enough braking IIRC.

Ian

[Jonathan Kay]

Bounce.

Jonathan

[Wrightpayne]

https://www.lotus7.club/forum/techtalk/brake-pad-advice-please-1

[rj]

Thanks,

Not quite what I'm after.

Previously I had Mintex 1144 on Triumph calipers at the front and Sierra calipers with standard pads on the rear.

I had absolute faith in the braking balance although someone might have thought that the rears locked up to early. My local trackday organisers often thought I'd lost it at the end of the long straight and even sometimes I saw the yellow flag in the rear view mirror. This in spite of never having been in the gravel trap there - yet.

Unfortunately my knee could not cope with the force needed on the pedal, so I uprated the front brakes to the AP four pot brakes offered by CC. At the same time I got and fitted the "blue dot" rear pads.

Now the fronts lock up way to early, and I have probably moved my braking point some 35 meters because I no longer have faith in the brakes.

I know the friction coefficient of Mintex 1144 and I can get brake pads for both AP fronts and Sierra rears where I know the coefficient. As the area of the front pistons has gone from 1810sqmm to 2268sqmm I know how to calculate the needed friction once I know how the balance was before the upgrade, but not knowing this it becomes difficult.

[Blue C7]

rj when I search on line for Sierra rear brake pad coefficient infomation comes up but I’m not sure if that is what you are looking for

[rj]

Blue C7,

It is, but there are various pands, hence I need to know the exact type used as "standard" pads.

The Mintex 1144 should be 0.46

[Richard Price]

RJ,

you're experiencing what I've been suggesting for quite some time....

its more important to have suitable brake bias than simply having powerful brakes at one end of the car.

Unfortunately, it's never going to be simple. There are multiple factors to be considered.

If using a pad with a constant coefficient of friction, weight distribution, tyre size and compound, and camber angles also need to be considered.

If using "uprated" pads with a variable coefficient of friction, then, ideally, you need both axles to generate heat at the same rate.

I would suggest, that most folks who get a soft pedal on track are suffering fluid boil in the rear brakes, so, sticking big brakes on the front won't cure heat in the rear brakes if the balance is still optimum.... (yes, I know you're not complaining of a soft pedal, but, having fitted big brakes on the front, you're finding the car does not stop as well as it did)

For reference, my sons car (k series, ZZS tyres, 185s and 215s) with AP's from and standard Sierra rears is running 1144's all around, and does not seem over braked at the back.

[rj]

Hi Richard

I would expect 1144s all around and the bigger fronts would result in overbraking front. The piston area of the fronts is 1.56 times bigger than on the rears. Having a weight distribution on my car with me in of 49:51 front to rear and the relative low centre of gravity, which I'd expect to be around 50cm compared to a wheel base of 222.5cm, which means that the COG would not move very much forward during 1G braking force.

There has never ever had issues with overheating brakes, not with Triumph fronts nor has the pedal gone spongy.

I am going to use CL brake pads; their friction vs tepmerature coefficient is quite flat.

A friend has borrowed the car from me and is on his way to Spa, where  he's going to hoon it on Monday. Next Sunday I have a local trackday at Ring Knutstorp (if the car is still in one piece that is :-o) and when I'm there I'd like to have the bias as it was before changing the fronts. I more or less hate the brakes as they are now (!) so something has to be done.

First shot is extreme - I have a set of RC8 pads which should be disc killers if not warmed up, but thats not what I care about.

Those have a friction coefficient of 0.6 - but still I don't know where I started which frustrates me.

 

 

[Richard Price]

RJ,

you should have blatmail

[MADMALC]

A bit old school and thinking outside the box, if you get a pad that is too aggressive and slowly reduce the pad area till you get the balance you require.

This was used on the old minis, admittedly it was the brake lining that was removed but may be a similar principal could be used to solve your  problem.

[rj]

Richard,

YHM back.

Madmalc,

In theory reducing the pad size should not affect the braking power. This alone is given from the force applied to the pad and to the friction between disc and pad. Of course a bigger diameter disc and thus moving outwards the caliper will add to the arm and therefor result in a bigger moment.

[Wrightpayne]

RJ,

I dont understand your logic - a bigger surface area of pad is going to create more friction surely?

Ian

[rj]

Nope. the force required to move an object where two surfaces meet is "clamping" force (newton) times friction coefficient (numeric number between 0 and 1) and does not mention surface area.

If you think of it, the force per square mm when you apply a total of 100 Newton at an area of 100sqmm will be one Newton.

If you double the area the force per sqmm will be 0.5 Newton..

It's only the distance from center of pad to wheel center, piston size, friction coefficient and how much pressure you apply to the caliper that counts.

In practice there may be a marginal difference but it's only marginal.

[Jonathan Kay]

1. Regin is describing the Coulomb model of frictional forces. I have no idea how closely it is followed by real pads and discs. (And those of you who have just spent a fortune on wider tyres needn't worry: their grip is nowhere near independent of contact area.)
2. The description above also assumes that coefficient of friction is constant. In the real world it changes with temperature, and that change might depend on the size of the pad.

Shirley

... thus moving outwards the caliper will add to the arm and therefor result in a bigger momentum.

"Moment" rather than "momentum".

[rj]

JK,

Have edited post and corrected my broken English.  - The way to learn.

 

"CL Brakes RC8 / RC8R Brake Pad Compound

CL Brakes produce two versions of the RC8 pad compound. The RC8 has a smooth backing plate, the RC8-R a special grooved backing plate. The CL Brakes RC8 brake pad compound has the highest friction level in the CL Brakes range.(μ=0.6). The CL Brakes RC8 pad compound is used in top level motorsport such as WTCC, WRC and NASCAR and is suited to heavier racing cars or short sprint racing where a no compromise high performing brake pad is required and discs are regularly inspected / replaced. The -R version has a grooved packing plate to increase the surface area of the brake pad , giving extra cooling to the braking system. This brake pad compound is best for high powered vehicles that put high demands on their brakes, giving shorter braking distances, no brake fade and again a flat torque curve from cold to extreme temperatures."

The torque curve is, however, not flat, but I think is's as good as it get.

Maybe RC5+ at the front would be a good match, but one step at a time. Otherwise I have no clue of where I am or where I was.

[Jonathan Kay]

How many of us have a second language half as good as your "broken English"?

:-)

Jonathan

[elie boone]

There is not a figure mentioned on the TRW aftermarket website, so best is to send the pad manufactor  a mail and ask the friction coefficient.

[rj]

You only have a chance to get better if someone bother to tell you when it actually is broken :-p

[rj]

Elie

This still requires knowledge of whom was the manufacturer of the original non-blue-dot pad and the part No. Neither of which I have.

[MADMALC]

Hi RJ,

So for a set pressure on the brake pedal you can lock the front brakes but not the rears, can you then reduce the pad size on the rear and so get more clamping force and eventually get enough to lock that axle? I am ignoring heat and assuming pad performance is linear.

I am full of logic but little knowledge but always willing to learn.

M.

[rj]

No. The braking force will be the same regardless the area of the pad.

[DJ.]

I have to admit to being mystified by this thread. Early on in the thread,  I understood that the brake balance had changed, after a swap to uprated front brakes which have increased the pad area and over braked the front.

However, later on it would appear that science tells us that pad area is not relevant to braking friction. Surely if this is true, the uprated fronts wouldn't have affected the balance. I suppose that the uprated fronts have different callipers, so the balance may be altered more by increased clamping force rather than pad area. 

I am interested to hear how you get the problem sorted. It's a shame you couldn't just have added a servo to reduce pedal pressure with the old set up

[7 wonders of the world]

The brake balance has been altered due to the difference in piston area and therefore the force generated at the front brakes.

Pad area only affects wear rate not the overall braking perfomance, a small pad area will give more responsive feel though - a kind of stiletto heel effect.

[Mike Biddle]

Hi RJ, if you do a lot of track days and place high importance on well balanced brakes, then surely a dual master cylinder with balance bar set up would be ideal, then you can set the balance according to to your needs, track, road etc.