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Rear Brake Pad Wisdom

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#1
Sphinx

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Other than using a softer rear brake pad, is there any magic here that we should be looking for and do you guys mix/match brands for fronts and rears?  For example, I have some 1/2 used Cobalt XR5 and Hawk blacks rears - any reason I can't use them up with Carbotech XP10 fronts?

 

Back when I raced SpecRX7, the class gurus would say that it didn't matter what you ran on the rear, even stock pads were ok (though back then preferred Blues Front/Blacks Rears).

 

We're always talking about front pads, thought it might be useful to talk about rears for once. 



#2
Steve Scheifler

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Why softer on the rear? Because your experience dictates it or that’s what you were told?

We gave up on that approach with our 1.6s LONG ago. All it did was transfer more weight to the front more quickly, exacerbating rather than preventing rear lockup. If you do a lot of serious trail braking and find that difficult with the same compound F&R then perhaps it makes sense, or maybe you should try a different brand of pad. But one of the things I liked best about our 1.6s was the way they just squatted down under hard braking with much less nose-dive than most around us. Obviously, if you can keep more weight on the rears then they can contribute more G’s to slowing the car. Why use two tires to stop rather than four??
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#3
ChrisA

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Agree with Steve. I used to run Carbotech XP10/8 combo. I ditched them because of pad deposit issues and went with PFCs, same front & rear. Car braking feels more balanced with less pitching/nose-dive. 


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#4
OrangeCrush86

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I don't see why it would matter if you mixed brands, it's the compound characteristics that matter. Seems like most people run slightly lower temp compound on the rears simply because the rear wheels can't generate as much heat due to weight transfer. I'm in the process of changing to G-Loc and I'll probably try running R10s front and rear to see how it feels.


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#5
Steve Scheifler

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Mixed brands can work, or be a mess. It isn’t about heat as such, particularly on the rears. It’s about initial bite and the rate at which the friction builds with pressure. A lot of time and effort goes into designing those properties and just randomly mixing them can give you peculiar results.

One other factor can be the F/R bias valve, whether it is working smoothly and whether it is the correct one for the car. There are a number of different versions and a lot of those were swapped around back in the day. Hopefully that is being checked at big events (is it?) I’ve certainly mentioned it more than once, but I wouldn’t be surprised if there are still quite a few older cars with the wrong one. IMO the original part in the 1.6 is excellent if working and the correct pads are used, but that didn’t stop people from creating problems with their pad choice then trying to fix it by cheating.
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#6
Danny Steyn

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I can tell you that I ran Carbotechs and I most certainly did not run XP10/8 - not even close. But then not everyone has the same approach to braking


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#7
Ron Alan

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I like Steve's approach when he talks about the car squatting in the rear. But I think there is a line between 4 wheel braking and not getting the most out of the fronts(because the rears want to lock up first). 

 

Initial bite and pedal pressure is a feel/preference and everyone is different. Obvious Danny doesn't like a lower initial bite pad(10's/8's)...but these will slow down your car every bit as much as a higher number...just takes more leg! 

 

If you understand the characteristics of the used pads you have then you'd be safe mixing similar with similar...Don't mix high initial bit compound with a low bit!

 

Yussef...stop talking about your car and get out and drive it! :)


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#8
Steve Scheifler

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I need a better simple one-word term than squat. It implies the tail dropping more than the front, when what I really mean is that front and rear hunch down more evenly rather than taking a nose-down attitude. But hunch isn’t right either.

Anyway, back when Memphis was still open there was a section with very long hard braking into a low 2nd gear corner and a lot of people locked up rears there, including us. As recommended by someone we switched from 1 compound less aggressive in the rear to 2 less and it got worse. I suspected the problem so I got video of my car from someone and it was clear that too much front bias caused excessive nose-dive. That took all the weight off the rears so even with less grippy pads they locked more easily. I switched to the same pads at both ends and was able to brake later with minimal locking.
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#9
Sphinx

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Why softer on the rear? Because your experience dictates it or that’s what you were told?

We gave up on that approach with our 1.6s LONG ago. All it did was transfer more weight to the front more quickly, exacerbating rather than preventing rear lockup. If you do a lot of serious trail braking and find that difficult with the same compound F&R then perhaps it makes sense, or maybe you should try a different brand of pad. But one of the things I liked best about our 1.6s was the way they just squatted down under hard braking with much less nose-dive than most around us. Obviously, if you can keep more weight on the rears then they can contribute more G’s to slowing the car. Why use two tires to stop rather than four??

 

Good question, a little of both.  I started with Hawk Blue/Blue and after too many spins at 10A at Road Atlanta, an experienced racing engineer said it was too much brake bias.  So, I started using blacks in the rear, which I had previously used that combo (blue/black) in my SRX7.  As a back marker, I could put wood blocks for pads and I suspect it wouldn't make a difference.

 

 

I like Steve's approach when he talks about the car squatting in the rear. But I think there is a line between 4 wheel braking and not getting the most out of the fronts(because the rears want to lock up first). 

 

Initial bite and pedal pressure is a feel/preference and everyone is different. Obvious Danny doesn't like a lower initial bite pad(10's/8's)...but these will slow down your car every bit as much as a higher number...just takes more leg! 

 

If you understand the characteristics of the used pads you have then you'd be safe mixing similar with similar...Don't mix high initial bit compound with a low bit!

 

Yussef...stop talking about your car and get out and drive it! :)

 

True!!  Actually, the question came up as I was finishing up prepping the car for the next event.



#10
OrangeCrush86

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I need a better simple one-word term than squat. It implies the tail dropping more than the front, when what I really mean is that front and rear hunch down more evenly rather than taking a nose-down attitude. But hunch isn’t right either.

Anyway, back when Memphis was still open there was a section with very long hard braking into a low 2nd gear corner and a lot of people locked up rears there, including us. As recommended by someone we switched from 1 compound less aggressive in the rear to 2 less and it got worse. I suspected the problem so I got video of my car from someone and it was clear that too much front bias caused excessive nose-dive. That took all the weight off the rears so even with less grippy pads they locked more easily. I switched to the same pads at both ends and was able to brake later with minimal locking.

 

Based on that it sounds more like you are changing compounds to achieve bias (since we can't actually adjust F/R bias). I do think that if you can lock the fronts way before locking the rears, then you are probably have some unused stopping power at the rear. From my limited knowledge, in a maximum braking scenario the fronts should lock just before the backs do.


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#11
Steve Scheifler

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Semantics perhaps, but yes, using different compounds to effectively change bias. But that’s what many OTHER people are doing, not us. With everything else working properly we almost always run the same compound at all corners and let the original design deal with bias.
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#12
SaulSpeedwell

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As a guy who knows some hominids who know a rescue dog that is a brake engineer, I'm confused by most of this thread.

Some thoughts from my rescue dog friend:

 

0.  We have 125 HP engines and ~450-800 HP of brakes.  Clearly a lot of passing happens under braking.

 

1.  We have OEM proportioning valves.  Curves are published (Flyin' Miata, etc.), but none of us really know where we are on those curves, except to say we can safely assume we are often, except for initial transients, and "brushing" the brakes, and light trailbraking, past the "knee" of the curve.  My rescue dog friend instrumented and proved this, once upon a time.  According to my rescue dog friend, if you are going to modify your proportioning valve for legal use in a class other than SM, you should probably change the spring rather than alter the spring preload with shims.

 

2.  The grabbier the pads, the LESS likely you are to be past or well past the knee of the prop valve curve.  The proportioning valve doesn't know your decel rate, it only knows front line pressure.  Depending on how grabby your brakes are, you may or may not be traversing the "knee" of the prop valve curve depending on whether you are brushing the brakes into a Kink, trailbraking into a sweeper, or divebombing into your best friend's driver's door in hopes of winning 13th place on the practice day of the Smitty McSmitterson Memorial Double Regional.

 

3.  The driver is a "Flatspotting Prevention Controller" when it comes to braking (and also an "Anti-Loss-Of-Control Controller" in the event of rear brakes locking early and often). Thus, the earliest locking brake is what trains the driver with respect to acceptable pedal pressure, and no road racer prefers a car whose rear brakes lock early and often.  Therefore, your braking performance shown in data and your GoPro vids while playing "divebomb chicken" is most likely determined by your most *overbraked* wheel, not your most *underbraked* wheel.

 

4.  Camber and suspension matters.  When the nose of the car slams onto the bumpstops and has -5 (-7? -45? what are we up to now?) degrees of camber, it affects the braking potential.  Same for the rear.  Induced rear toe is a thing, too.  You can play around on the alignment rack to determine what happens "statically" ("kinematically", for the twitchy engineers) when the nose drops and the ass raises, but the compliance in cornering matters even more, in my opinion.  This is ancient history, but the stock bushings are/were softer ahead of the rear knuckle than behind, but the "Comp" bushings were the same stiffness ahead and behind.  Presumably the goal was to reduce the induced toe-in under cornering (and the induced toe-out under braking).

5.  Pad "grabbiness" (coefficient of friction) is a function of temps.  Are your pad temps the same at Gingerman as they are entering T5 at Road America?  Of course not.

My rescue dog friend's opinion:  71.3% of brake problems can be fixed by simply giving the car what it is clearly asking for.  If YOUR car locks the rear early, and it is holding you back on any given weekend and track, just fix it.  If YOUR car locks one corner of your 52.3% crossweight car as you enter the sidehill braking zone of the Mid Ohio backstraight, and it is holding you back, just fix it. Different pad compounds is an easy and reversible way to do this. As I've told many who go to Grattan, if your RR wheel locks early into the backstraight hairpin passing zone, FIX IT.  Put on a lesser pad, a stock pad, flip an outboard pad metal to metal.  If the laptimer is happy, it is the right decision.

Whatever you do, don't listen to the Internet.  At least not without testing it first.


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#13
Steve Scheifler

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Always glad when you chime in, and even more so when the topic is brakes, Mr. Saul Bendix Speedwell. But after panning most of the discussion thus far it would be nice to get more specific. Most of us are adults and can take it, and the rest can play elsewhere if yhey don’t like it.

Camber certainly plays a role in this and if one person is running significantly different relative F/R camber than another person then your point is particularly valid. But I’m not sure it in any way alters my fundamental recommendation, which is to START with what Madza designed in terms of hardware, and use the same compound on both ends. Then as you say, see what the car “wants” but be aware that there may be a hardware, setup or driver induced problem rather than a F/R compound issue. But perhaps the point I most wanted to make is to not assume that locking up rears under level straight-line braking necessarily means you need a less aggressive rear pad, particularly if you are already using a “softer” one back there.

Do you take issue with any of that?
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#14
Ron Alan

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The Tony Stark of MR has made his monthly visit :)

 

I think I can summarize his 5 point thesis on braking in 5 words...do what works for you! 


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#15
davew

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I think I can summarize his 5 point thesis on braking in 5 words...do what works for you! 

 

EXACTLTY

 

Every car is different, every drivers is different and every track is different. The only way to know what works best for YOU on YOUR car at YOUR track is to test different things. True testing is boring and expensive. And deciding what brake combo you prefer is time consuming.

 

Very experienced drivers, who have made soooo many changes that they can evaluate something in a few laps are great to work with. But most of us are not that sensative. I sell several companies brake pads. Some have multiple compounds. I get the question often. Pick whatever pad combination you think will work for you, go to your home track, use new rotors any time you change compounds (even within the same brand). Run that combination for an entire test day. You need to retrain your brain to a different feel, and for most of use that takes a day. If you picked the right combo, use it. If you don't like it, go back to the drawing board and repeat the process until you find the combo that works for YOU.

 

I have fast cars in my shop running PFC, Hawk Blue and Hawk DTC, as well as Cobalts. Some use the same compound front and rear. Some are different compounds and some are different brands. Go back to what works for YOU. Take most of what you read on the Interblabble as worthless knowledge

 

Dave


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#16
Steve Scheifler

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Or we can summarize even better by saying “Ignore everyone; always start from scratch and reinvent the wheel no matter how little track time you get; and stop coming here looking for answers because any advice found on the internet is completely worthless, you are unique!” Got it. Very sage.
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#17
davew

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Ease up Steve.

 

What I said is that most of what you read here or anywhere else is from people who really have no clue what they are talking about. I consider you to be one of the few people who do know what they are talking about. But even the experts do not know all of every questioners issues. Nor their setups, nor their driving styles, nor the general condition of their car, nor their track.

 

There is no easy answer. There is no wrong answer. It all boils down to personal preference and personal situations. What works for you may not work for me. Therefore every choice is a right choice for someone.

 

Dave


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#18
Danica Davison

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Or we can summarize even better by saying “Ignore everyone; always start from scratch and reinvent the wheel no matter how little track time you get; and stop coming here looking for answers because any advice found on the internet is completely worthless, you are unique!” Got it. Very sage.

 

I can relate to this. Everyone thinks what I post is Bullshit and nonsense, but I am being serious, and it saddens me that nobody takes me seriously :'(


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#19
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I can relate to this. Everyone thinks what I post is Bullshit and nonsense, but I am being serious, and it saddens me that nobody takes me seriously :'(

:crying2: 


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#20
Ron Alan

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I can relate to this. Everyone thinks what I post is Bullshit and nonsense, but I am being serious, and it saddens me that nobody takes me seriously :'(

We will ignore this...


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