Brake Pad Question

I need to replace the pads on my car (Mazdog Protege 4WD), which made me think a bit (now I've got a headache!).

Do Rallyists tend to use different compound brake pads in the winter vs. the summer?

I guess in winter the ambient temperature tends to be colder by 80° or more, but on the scale of brake temperatures, is that really even so much as a blip?

--sarge

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---** To be in compliance with the Anarchy **---
Jorden R. Kleier
Baraboo, Wisconsin, USA
1990 Mazdog Protege 4WD
1973

SgtRauksauff Wrote:
-------------------------------------------------------
> I need to replace the pads on my car (Mazdog
> Protege 4WD), which made me think a bit (now I've
> got a headache!).
>
> Do Rallyists tend to use different compound brake
> pads in the winter vs. the summer?
>
> I guess in winter the ambient temperature tends to
> be colder by 80° or more, but on the scale of
> brake temperatures, is that really even so much as
> a blip?
>
> --sarge
>
> ---** To be in compliance with the Anarchy **---
> Jorden R. Kleier
> Baraboo, Wisconsin, USA
> 1990 Mazdog Protege 4WD
> 1973

Nope. You change compounds based on Mu values and what works for your app, not based on ambient temps.

Different pads will have different coefficients of friction, though, and tend to operate best within a certain temperature range.

How much effect does the ambient temperature affect getting to this effective range (I suppose this could be termed "initial bite" ) ?

Especially, say, on a really long straight with a bunch'o'twisties at the end of it? Do you need to get the pads warmed up on the straightaway so that you actually HAVE some bite to your pads in the corner, and don't just shoot off the other side?

Or in other words, does the pad that works perfectly on your setup in 90° hot sunny weather give you the exact same performance when it's -20 with windchill?

--sarge

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---** To be in compliance with the Anarchy **---
Jorden R. Kleier
Baraboo, Wisconsin, USA
1990 Mazdog Protege 4WD
1973

I'm no rallyist but I use Porterfield r4e on the so-called rallycar and when they wear out I'll put another set on and I don't give a good goddamn what the ambient temp is...

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Jay Woodward
Snohomish, WA
'90 Mazdog Frankenprotege
Chronologically, 46...

SgtRauksauff Wrote:
-------------------------------------------------------
> Different pads will have different coefficients of
> friction, though, and tend to operate best within
> a certain temperature range.
>
> How much effect does the ambient temperature
> affect getting to this effective range (I suppose
> this could be termed "initial bite" ) ?
>
> Especially, say, on a really long straight with a
> bunch'o'twisties at the end of it? Do you need to
> get the pads warmed up on the straightaway so that
> you actually HAVE some bite to your pads in the
> corner, and don't just shoot off the other side?
>
> Or in other words, does the pad that works
> perfectly on your setup in 90° hot sunny weather
> give you the exact same performance when it's -20
> with windchill?
>
> --sarge
>
>
>
> ---** To be in compliance with the Anarchy **---
> Jorden R. Kleier
> Baraboo, Wisconsin, USA
> 1990 Mazdog Protege 4WD
> 1973

Like I mentioned before, the ambient temps are not a concern cause it wont have an effect on getting the brake temps up to op range.

The coeficient of friction is the Mu value I refered to, and that again doesnt mean that the pad wont have cold bite even if it has a high coef of friction.

The initial bite term you use has nothing to do with temperature, it has to do with the friction properties of the pad, some pads have higher initial bite with the same overall TQ output of a pad with less initial bite for example. These are the properties that Pro race teams test in order to fine tune suspension setups, a less initial bite pad would work great for the rear axle while maintaing high brake torque.

Anyways, back to your original question - NO, ambient temps even if they differ 80F from summer to winter will not make much of a difference when your disc temps will run anywhere from 900-1200F. Now, if you have a ton of cooling, and you have a half mile portion with no brake input, the temps will come down, if the pad has bad low temp friction it will be a puckering situation!

Brakes work by converting the kinetic energy of the rotor into thermal energy (heat). That range is the range that the pad will work efficiently (convert energy the best) and then the limit before it starts to die. I assume it gets glossy and then cracks? I've never seen a pad blow up, so I'm not really sure how they die.

Like cosworth said, the ambient temp is just the ambient temp... but as tires are the contact patch to the ground and actually slow you down, if you WANTED to be technical, lol, it would influence your tire compound choices to get efficient traction to actually stop well. But for the brake pads, no.


Oh yeah, heres a random fun fact, When something is sliding, say, a locked tire on tarmac, it has kinetic friction. When that tire is just sitting on the surface, again, we'll use tarmac, and you go to push it (with the brakes on so the wheels dont roll) it's got static friction between the tire and the surface. The static friction is actually STRONGER then the kinetic friction.

This is why when you brake, (not in special instances where you want to lock anything up) you usually dont want to lock up the wheels, because a rolling wheel has static friction to the ground. The small bit of sliding that actually causes normal tire wear is negligible. So you have MORE friction, thus more traction.

But if you look at brakes, if you stomp on the pedal, and lock them up, you've stopped the pad on the rotor. Now you have static friction between the pad and the rotor, as the rotor is trying to break free. Once it does, its back to kinetic friction. But the static friction will have a much higher Mu, although you can't really use it because you'll be dragging the tires along with their kinetic friction... increasing stop distances.

Funny how that works out? That's why brake pads give you the Mu of their Kinetic friction.

(go physics midterms, lol)

(fixed a typo)

---

Welcome to the cult of JVL drink the koolaid or be banned.



Edited 1 time(s). Last edit at 12/16/2009 03:57PM by Dazed_Driver.

Dazed_Driver Wrote:
-------------------------------------------------------
> Brakes work by converting the kinetic energy of
> the rotor into thermal energy (heat). That range
> is the range that the pad will work efficiently
> (convert energy the best) and then the limit
> before it starts to die. I assume it gets glossy
> and then cracks? I've never seen a pad blow up, so
> I'm not really sure how they die.
>
> Like cosworth said, the ambient temp is just the
> ambient temp... but as tires are the contact patch
> to the ground and actually slow you down, if you
> WANTED to be technical, lol, it would influence
> your tire compound choices to get efficient
> traction to actually stop well. But for the brake
> pads, no.
>
>
> Oh yeah, heres a random fun fact, When something
> is sliding, say, a locked tire on tarmac, it has
> kinetic friction. When that tire is just sitting
> on the surface, again, we'll use tarmac, and you
> go to push it (with the brakes on so the wheels
> dont roll) it's got static friction between the
> tire and the surface. The static friction is
> actually STRONGER then the kinetic friction.
>
> This is why when you brake, (not in special
> instances where you want to lock anything up) you
> usually dont want to lock up the wheels, because a
> rolling wheel has static friction to the ground.
> The small bit of sliding that actually causes
> normal tire wear is negligible. So you have MORE
> friction, thus more traction.
>
> But if you look at brakes, if you stomp on the
> pedal, and lock them up, you've stopped the pad on
> the rotor. Now you have static friction between
> the pad and the rotor, as the rotor is trying to
> break free. Once it does, its back to kinetic
> friction. But the static friction will have a much
> higher Mu, although you can't really use it
> because you'll be dragging the tires along with
> their kinetic friction... increasing stop
> distances.
>
> Funny how that works out? That's why brake pads
> give you the Mu of their Kinetic friction.
>
> (go physics midterms, lol)
>
> (fixed a typo)
>
> Feisty Peacock?
>
>
>
>
>
> Edited 1 times. Last edit at Dec 16, 2009 by
> Dazed_Driver.

WTF did you just planted on the intranetz? lol Geebus, the Mu is not kinetic friction, its the unit measure of the friction coeficient at a particular temp, disc speed, and clamp load, and, and, and... What kinetic energy is really is in this case is the momentum of the vehicle.

^^ no, he's right.....he just worded it funny.

Kinetic = dynamic

Force of 'Dynamnic' Friction = Mu(D) X Force

Force of 'Static' Friction = Mu(S) x Force

Brake companies give you the Mu(D) of their brake pads, which, yes...is a constant for the particular material, and assumes a given temperature and rotor material/surface, etc.


FYI, clamp load has nothing to do with Mu. Clamp load = Force in the above equation.


and yes, brakes essentially xfer kinetic energy/momentum into heat energy.

again, Tim likes to word things funny to keep us on our toes.

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Jon Burke - KI6LSW
Blog: http://psgrallywrx.blogspot.com/

Which part did I word funny? I thought i only mentioned Mu as the coefficient of friction (as it should be). I probably just typed the wrong word. It happens, lol.

---

Welcome to the cult of JVL drink the koolaid or be banned.

Jon Burke Wrote:
-------------------------------------------------------

> FYI, clamp load has nothing to do with Mu. Clamp
> load = Force in the above equation.

No man, I'm talking real world not text book, clamp load has nothing to do with Mu true, but clamp load will affect Mu. Many reasons, but one of them being the compressiblity and integrity of the friction material under different pressures.

Regards.

See now, all them guys are rallyists! All I know is one pedal makes it go, one better make it stop and I care not how as long as it does. ambient temp shmambient temp.. What gets my full attention is how they behave when you're most of the way through a stage and you come into that !!!hairpin R exposure don't cut oh crap you better not have brakefade here...

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Jay Woodward
Snohomish, WA
'90 Mazdog Frankenprotege
Chronologically, 46...

I kind of already understood most of that, regarding Mu (although I dislike using a random greek letter that sounds like a cow to mean "Coefficient of Friction". "Mu" can have any value you choose to give it, as it's a variable in whatever equation you want to use. The greek letter Mu is used IN NOTATION to indicate "Coefficient of Friction" for shortening of said notation. In my own opinion, they (greek letters) should'nt be pronounced when speaking or referring to said notation. Like I said, this one sounds like a cow. Others sound like food (Pi) or fish (beta).. etc....

The maths (new or old) were not really the questions, I was just wondering how super-cold weather affected the aforementioned sphinctorial clench when late-braking into the '!!!Hairpin R exposure don't cut'.

But to add, what does snow and ice and moisture do when they're brought into the equation? a little snow, fine, it melts, now you've got moisture sitting around your brakes.

Lately, with the two or so feet of snow around here, I've been taking the back roads in my Mazdog. Kinda like Jay's, but the testicles haven't dropped yet, it's all stock where it counts.. Vaguely plowed roads, but not salted, only lightly sanded. Playing around with bits of ditch-hooking at a few corners, being sideways as much as possible and generally being a hooligan on unused roads.

Consistently, I've found that if I don't get the brakes heated up before I actually have to slow down, I've gotta give it a LOT of pressure, with nearly zero effect. It's like the pads are completely glazed, then suddenly I've got friction and it slows down.

So, it's either the pad itself not being in its operating temperature, or it's something blocking the friction surfaces from meeting properly.

continue!

---

---** To be in compliance with the Anarchy **---
Jorden R. Kleier
Baraboo, Wisconsin, USA
1990 Mazdog Protege 4WD
1973

Not having a bizillion dollars to go do testing of pads etc with I have found that Porterfield R4's work for me in all applications and all temps, since they work I have not looked at any other option. Never could figure out why Jay uses the R4e version witch technically stands for endurance, likely since he found they worked for him and thus why would he chance changing to the R4. I used the Porterfields on my 323GTX and Golf.

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In the long run reality always wins.

derek Wrote:
-------------------------------------------------------
> Not having a bizillion dollars to go do testing of
> pads etc with I have found that Porterfield R4's
> work for me in all applications and all temps,
> since they work I have not looked at any other
> option.

That seems to be the final say in just about anyone who's ever used Porterfield's that I've ever talked to.

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Grant Hughes

Jordan what I've seen on cars doing double duty---ie street and occasionally "more vigorous play" often have a glaze on the rotors deposited even in light use from the binding agents in the pad, the glues and shit (see there's an excellent word that coveys exactly the idea, sorta like "junk-n-stuff"---so we could say the glaze is from the pad binding crap and junk-n-stuff).
Sometimes it seems that varishy shit needs to be roasted off.
Or lightly sanded off.

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John Vanlandingham
Sleezattle, WA, USA

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