Z4M Brake Bias and New Pads

[The HACK]

Originally Posted by Shipkiller

I went back and reread my first post.

I am still trying to figure out how anyone thought I was going to change the brake system bias??? I made no mention of it...

I only talked about pads and going to a grippier pad on the rear.

The OEM system is designed to take pad compounds with similar coefficient of friction and bite characteristics. By changing to two different pad compounds front and rear you are in essence changing the factory brake bias.

And what you propose, while may work, in essence is artificially adjusting brake bias to the REAR by increasing the relative brake torque level to the REAR.

[onebluemcm]

Originally Posted by The HACK

The OEM system is designed to take pad compounds with similar coefficient of friction and bite characteristics. By changing to two different pad compounds front and rear you are in essence changing the factory brake bias.

And what you propose, while may work, in essence is artificially adjusting brake bias to the REAR by increasing the relative brake torque level to the REAR.

This is exactly what has been recommended by the owner of the Z4MC Grand Am car to settle the rear down and remove said squirmyness. However, if you are running one compound at all 4 corners and don't have it, it's hard to say exactly what the issue may be. And beyond all this brake pad conversation, people are also running all sort of different street tire and I think we would be naieve to think it couldn't be the result of a combination of factors.... It's a tough one. On the race car, using a rear pad with more initial bite seemed to help. After initial bite, I don't know if the pads continued to function with rear bias or if they were more similar under brake modulation.

[lucid]

Originally Posted by The HACK

My 2 cents.

I used to run Cobalt XR3s up front and Cool Carbon R/Ts in the rear out of necessity, since the XR3 rears were too thick to fit in the rear (another long story that I'm not going to bother you with). Used to be, on California Speedway's high speed oval, I would get some serious rear end instability especially when we run with the chicane on the front "straight" to turn 2 transition, since you have to brake moderately hard, and if you're stupid like me, trying to hit 140+mph before braking, you would have to be braking REALLY hard to make the artificial chicane.

With the XR3 front, Cool Carbon R/T rear combo the rear end comes "loose" a few times. I chalk it up to the fact that I was trying to brake very hard on a slight bank going straight, but from discussion with fellow instructors it appears NONE of them have that problem. So I sort of ignored it and basically extended my braking zone and not try to be the 140+mph hero thinking the shorter wheelbase must be to blame.

Last year I swapped out the Cobalt for Hawk DTC-60s when the front Cobalts were less than 40% left. Luckily the DTC-60 rears fit the caliper housing just fine so I've been running the same compound front and rear. And surprisingly...None of the squirminess on the high speed bank returned. Brake as hard as I want at speed up on the bank and the rear end feels just as planted as ever. In fact, a pleasant side benefit is that all my tire pressure were EVEN front and rear. When I was running the XR3/CoolCarbon combo the front tires almost always are 2-3 PSI higher no matter how I played with the tire pressure. With the DTC-60 they were SPOT ON.

My theory? The XR3s had much higher brake torque and therefore were forcing the front tires to do most of the work when braking, and far more weight transfered up front than necessary thus resulting in the rear becoming somewhat "squirmy" when braking at a higher threshold.

In my opinion, it is best NOT to mess with the factory "bias" unless you know exactly what you're doing. While shifting the brake bias up front with my combination of pads works for a certain style of driving (it worked GREAT for autocross), same pads all around yield the best result in my experience.

Hack, I had a similar experience with increased front bias due to different pads F/R (the shop did not follow my instructions regarding instaling the same compound F/R), so I agree with you there. When I got rid of that setup, the car (my E30 M3) became more stable under braking.

However, increasing front bias does not increase weight transfer on a street car during braking. If anything, if decreases it. Weight transfer is directly proportional to the rate of deceleration. Street cars are set up for more front bias than optimal from a maximum grip/deceleration perspective mainly for legal reasons (they leave rear grip on the table). So, on a street car, by shifting bias even more forward, you would most likely decrease the rate of decelaration (as in increasing stopping distances), and hence, decrease weight transfer.

[The HACK]

Originally Posted by lucid

Hack, I had a similar experience with increased front bias due to different pads F/R (the shop did not follow my instructions regarding instaling the same compound F/R), so I agree with you there. When I got rid of that setup, the car (my E30 M3) became more stable under braking.

However, increasing front bias does not increase weight transfer on a street car during braking. If anything, if decreases it. Weight transfer is directly proportional to the rate of deceleration. Street cars are set up for more front bias than optimal from a maximum grip/deceleration perspective mainly for legal reasons (they leave rear grip on the table). So, on a street car, by shifting bias even more forward, you would most likely decrease the rate of decelaration (as in increasing stopping distances), and hence, decrease weight transfer.

I have no experience setting up race cars (or club racing for that matter) so I defer that expertise to you. And in a way, my experience with equal pads front and rear seems to suggest "braking" is far more efficient with the DTC-60s all around vs. Cobalt XR3 front and Cool Carbon rear. I think the "sensation" that there's more weight transfer comes from the fact that there's more brake torque applied quicker up front, thus resulting in a "dive" sensation...But I'm sure if I had hooked up a data acquisition unit that I can show the balanced braking was far more effective.

The other caveat about putting different compound in the rear vs. the front, is that you have to be very careful what compounds you pick. Different compounds heat up at different rate and also has different bite characteristics, different torque ramp-up curve, and different maximum operating temperature, thus the behavior can be unpredictable, especially if the "better" compound is put in the rear. Say, if you were to leave a street compound in the front and put in a track compound in the rear, you may end up getting into ABS far quicker than intended and result in actually decreasing available brake/grip.

The LAST thing I will leave y'all with in this discussion, is you will never know until you try it. But if you're going to try the grippier compound in the rear, test it in a safe environment first, understand the effect of the change, and then when you are comfortable operating with the new "bias," ramp up your speed slowly on track until you know for sure this combo works. All I know is this whole thing about driving and driving fast is counter-intuitive, and I have seen some people run different compound front to rear to some degree of success.

[lucid]

Originally Posted by The HACK

I have no experience setting up race cars (or club racing for that matter) so I defer that expertise to you. And in a way, my experience with equal pads front and rear seems to suggest "braking" is far more efficient with the DTC-60s all around vs. Cobalt XR3 front and Cool Carbon rear. I think the "sensation" that there's more weight transfer comes from the fact that there's more brake torque applied quicker up front, thus resulting in a "dive" sensation...But I'm sure if I had hooked up a data acquisition unit that I can show the balanced braking was far more effective.

The other caveat about putting different compound in the rear vs. the front, is that you have to be very careful what compounds you pick. Different compounds heat up at different rate and also has different bite characteristics, different torque ramp-up curve, and different maximum operating temperature, thus the behavior can be unpredictable, especially if the "better" compound is put in the rear. Say, if you were to leave a street compound in the front and put in a track compound in the rear, you may end up getting into ABS far quicker than intended and result in actually decreasing available brake/grip.

The LAST thing I will leave y'all with in this discussion, is you will never know until you try it. But if you're going to try the grippier compound in the rear, test it in a safe environment first, understand the effect of the change, and then when you are comfortable operating with the new "bias," ramp up your speed slowly on track until you know for sure this combo works. All I know is this whole thing about driving and driving fast is counter-intuitive, and I have seen some people run different compound front to rear to some degree of success.

To clarify, I have a competition license and have done time trials with it, but I start CR this year, so no w-w experience yet. Anyway, what I said has nothing to do with that or seat time though. It's physics. The faster you decelerate the more weight transfer the car will experience. The following is how I make sense of it. Let’s see if folks agree.

Braking force is generated at the 4 tire/road interfaces, and you can represent them as a net force and a torque which acts on the mass of the car at its cg. Assume the lateral direction (width of the car) is the z-axis, vertical direction is the y-axis (height of the car), and the longitudinal direction (length of the car) is the x-axis.

The net braking force is the sum of the forces generated at each tire/road interface.

The net torque through the z-axis results from the fact that the car's cg is above ground (higher than the tire/road interfaces at each wheel). That distance (along the y-axis) is the lever arm at each wheel. The lever arm is pretty much the same at each wheel, so it doesn't really matter which axle the braking force is applied (F vs. R) for the net torque calculation as long as the net braking force is the same.
The car dives (rotates around its cg through the z-axis and compresses the front suspension) because of the net torque through the z-axis (not referring to braking torque generated at the rotors). This is commonly referred to as "weight transfer" because the applied torque results in increased vertical force on the front wheels and decreased vertical force on the rear wheels since the car is not entirely free to rotate around its cg (the ground keeps it from doing so and applies a reaction force).

A similar consideration is squat due to acceleration. If you have two cars that are somehow identical in every way (including mass distribution and engine torque output) which are on a drag strip, but one is FWD and the other RWD, both going WOT, assuming traction is not an issue for the FWD car, they will squat the same amount.

Now, the braking forces also result in a torque at the cg through the y-axis. You can visualize this if you stare down at the car directly from above.

In an ideal world, when the car is going straight with the steering wheel straight, the cg of the car would be exactly in the middle of the width of the car, and the braking forces applied at each wheel along the x-axis would be exactly the same lateral distance from the cg (same lever arm), which would mean that there would be no net torque at the cg through the y-axis (assuming same amount of braking force is generated at the R and L rotors of each axle). In reality, that is not case. The cg is not exactly in the middle, and braking forces generated at the rotors of an axle are not identical, which means there is a net torque that wants to rotate the car through the vertical axis (and steer it).

If all the braking force is applied at the front, the car will spin unless you constantly correct by counter steering, which would be almost impossible to do if the forces are large. Conversely, if all the braking force is applied at the rear, the car might yo-yo a little, but ultimately, that is a stable situation since the lever arms associated with the forces applied at the rear wheels will change magnititude (and possibly direction) as the car rotates in such a way that the car will want to center. It’s like trying to balance a pole upright with your palm vs. grabbing it with your index finger and thumb from the other end. Or, pin a wooden rectangular block in the middle so that it can swivel around it, and pull on one end of the block and then try to just push on the other end. Which one is more stable? So, having as much rear braking force as possible is a good thing.

The issue is since weight is “transferred” to the front, the rear wheels have lost their available traction. And if you apply too much force and exceed the available traction in the rear, the rear tires will start sliding, friction will go down, and the car will go into oversteer—especially if you are braking while turning—and you will lose control. That is not as much of an issue with exceeding available traction in the front because that simply results in understeer, which is not a big deal—relatively speaking.

So, in a world where there is ample traction, a higher rear bias should be more stable, which is what your, my, and the pro Z4M driver’s experiences illustrate. Since there is limited traction in the real-world, rear bias (and rear braking force) needs to be optimized such that rear grip is never exceeded under heavy braking so that oversteer is avoided, and excessive front braking force does not make the car unstable.

Car manufacturers leave some rear traction on the table and bias the brake system more forward than necessary because they don’t want law suits in case someone loses it under braking and oversteers. That so-called “safety margin” is how BBKs can decrease stopping distances in emergency stopping tests (not because they are “big” but more because they have more rear bias); they eat into it.

I agree with the rest of what you said in your recent post.

[Z///Ms]

Originally Posted by Shipkiller

It was a backhalf cage. The seats are the reason that you could not make this a daily driver. The seat tops were very aggressive. The seats were not Sparco's or Simpson. They were a different brand and I can't remember the name. He said that they were a much better fit for a MC than the others.

He also related that you really need to use the VAC motorsports mounts. It just simplifies everything.

The seats in my car are Cobra Evolutions. They fit with no mods or cutting, but really recommend the VAC seat mounting kit. The 4 pt cage by McMahan Autosport, Akron OH (rollcageguy.com) is a work of art. Full weld in. Cost about $900.

I also experience a rear end wiggle under hard braking. It happens with the initial hard application of brakes and it stops with a slight release of brake pressure. I'm running stock brake calipers and rotors with PF 01 compound pads front and rear. The suspension is set up for the track (Bilstein PSS10 coil-over, Ground Control sway bars front and rear, Turner camber plates) with the car corner balanced and suspension adjusted at the track by a shop that supports a number of club racers. After talking with Shipkiller at VIR I’m thinking of trying PF 06 compound pads on the front with 01 on the rear. The 06 compound is suppose to have slightly less initial bite. I ran my 2002 Z3 M Coupe on the track for a season with different compound track pads front and rear as recommended by HMS Motorsport and it work well. My next event will be Summit Point in April. I’ll let you know how it goes.

[lucid]

There could be many other reasons for the wiggle. Dynamic toe changes under compression (and static suspension alignment specs), worn out or loose bushings, tires, etc. It can be a pain to figure out this kind of stuff. Let us know what the pad changes yield.

[Z///Ms]

The different brake componds (PF06 front, PF01 rear) did not change the wiggle under hard braking at Summit Point. I then had new delrin rear bushing installed and the wiggle was still there this past weekend when hard braking at Mosport. So I'm still looking for the cause/solution.