mark kibort

signed NDA in your in box!

I am pretty exited to try them out. These are the kinds of things you bolt on that can have dramatic effects. Not as much of a sure thing as my engine mod, but with the larger rotors and the new pads, it should be much better. I think the biggest difference will be the stopping power at the end of the braking zone, where the oranges just gave out. (only with the extra HP ) You could just feel their lack of stopping force when they got smokin hot. Like turn 2 at laguna. It was a joke. a race earlier without the HP, no problem. Now, 5-10mph faster down the main straight and its a big problem. During some bedding routines, you can tell they have very little fade characteristics, and squeal like pigs at the last 1mph before a stop.

mk

mark kibort

Also, the opposite can be true. A lot of trail brakers have big problems with too much rear brake bias or rear brake grip. A lesser pad or less bias in the rear can help cure this problem.

Why would a "heavy" car favor rear brake bias? I would think that a heavy car, with weigiht balance toward the front, will create much more weight transfer to the front tires and require less rear braking forces. At least thats the feeling with my 56/44 weighted, heavy car. I have very little rear brake force, (as seen by the wear ratio and rotor temps, front to rear) and wonder if there is additional gains, before hurting trail braking capabilities, that I could realize if I had the guts to increase my rear brake bias.
At what point do you have too much rear brake bias? stepping out under brake application, rear lock up, rear step out under trail brake?

I think this year im going to swap out my 11bar rear brake bias regualtor for a 33 bar version and see what happens

mk

Veloce Raptor

Mark, reread my post. I said that the E46 M3 was heavy AND has way too much front bias. Adding more bite in the back really helps the car squat under braking, and makes it much more stable, allowing throttle much sooner in the cycle.







Professional Racing and Driving Coach

mark kibort

I read it. But, my question was what the "heaviness" had to do with it. What if it was light and with too much front bias? Same problem?

My question is, is the "squat" better under threshold braking, than getting as much weight transfered to the front tires? As soon as you are off the brake, you are decelerating at a lesser rate, so there is weight transfered from the front to the rear to allow for application of throttle. Does this weight transfer happen faster than you can lift you foot off the brake and put it to the gas. (even with front bump and rear rebound settings that are slow) If you did get more squat, would you have less braking force overall, and would it be a trade off with the ability to possibly add throttle earlier?

Questions Ive always pondered.

mk

Edit

Veloce Raptor

The prodigious weight further exacerbates front dive (and thus weight transfer) under hard braking, even with track suspensions, thus further increasing front bias.

Also, FYI, contrary to your statement above, when you lift off the gas, weight is NOT transferred ot the rear, but rather to the front.







Professional Racing and Driving Coach

Brian P

Is there a site that indicates what size PFC pad to buy for various Porsche models? I.e., if I want to put PFC pads on my Boxster S, which pads should I be buying?

flatsics

Brian,
a couple of links


http://www.hrpworld.com/client_images/files/39_134.pdf


http://ogracing.com/catalog/2-Car/28...BOXSTER-CAYMAN

Seth Thomas

100% correct.

mark kibort

I meant to say, "lift off the brake". (giving a lesser rate of decelleration). (corrected on post). Kind of obvious mistake, as I was just talking about weight transfer to the front with threshold breaking on the previous sentence.

Getting back to the weight. I would guess, all other things being equal, the percentage of dive would be the same for a lighter car. Sure, if you just add weight to a car, a problem of increased dive could be caused by the over driving the springs and shocks.

mk

mark kibort

What if you pull the ebrake only? . So, 99% correct.

Anyway, it was a typo.

mk

Seth Thomas

Cars are not motorcycles. We do not want our front brakes to do all the braking for us. We want the fronts and rears to work together. So if we have them working in unison then they will cause the rear of our productions cars to squat under braking while the front dives. This allows the front tires to not be as loaded as they would be with a car that has a lot of front end dive. This also allows the contact patch of all 4 tires to be similar in shape giving us ultimate grip. This is basically what a F1 car does with downforce. Unfortunately the cars we are dealing with don't have all the cool tricks an F1 car has. We are dealing with production based cars that have a lot of front dive in them for the general public. We are then trying to engineer this rear squat into them under braking. We have done some of this on our World Challenge cars but this called for moving the pickup points around a lot on the car. So our cars under braking feel like they are sinking into the ground instead of throwing us forward like a Showroom Stock car. Big front dive is not optimal for a car. A car diving down on the front tires whether it is a rear engined or front engined car is not good. This overloads the front tires and will make it tougher to turn the car when releasing the brakes. Also more dive, in the case of a 911, makes it easier for the rear to swap ends while under braking. This also can cause the rears to be more likely to lock up. This is where finding the right brake pad for your car is important. Different pads front and rear on a car with lots of dive may be what it takes to keep the rears from locking up or oversteering under braking. I personally try not to put different pads front rear on any car that I run because the different rates where different pads start working usually cause problems. Having the same front and rear you don't get this. The pads will work together as they have the exact same characteristics.

Larry Herman

Another reason for very stiff springs!

mark kibort

But, from Smith, " Not all the kings horses nor all of the anti dive or anti squat geometry in the world will significantly reduce the amount of load transferred under a given linear acceleration unless the vehicles ride hight or cg hight is reduced or its wheel base is lengthened." He goes on to say that when braking, the fronts are doing most of the work as the rear are being, in essence, lifted up and have no where near the capacity, even with the same size tires. The fronts take the brunt of the punishment, which is unfortunate.

I guess what this all says, is that you want the weight transfer to match the grip capability of the front tires. beyond this, you would want to shift the weight of the car to the rear, but that is a weight and balance design issue. Its quite possible the reason that some 911s stop so well is that its a better matched system vs some front engine cars that over use the front tires, or are limited by them, due to weight transfer to the front under decel.

load transfer=acceleration (g) x weight-lbs x cg "/wheelbase"

I think the anti dive and anti squat are more about the rise of cg than it is for its abiltiy to apply rear brake force, but thats just a knee jerk opinion.

You are making me think about what I have going on with my car. certainly, im riding on the front brakes with little rear brake bias. am i at the limit of the fronts and could add more rear braking, or are the fronts so efficient, that any increase in rear braking might just be locking up the tires in inopportune times?
Might have to do some testing to know for sure.

mk

edit: I also disagree with the concept of using the same pad for braking predictability. If you noticed, both front and rear pads are working at grossly different operating tempuratures and bias levels. in no way will any same pad work the same at different caliper force pressures as well as tempuratures. Most of the time, sports cars can show GLOWING front rotors only. If the tempurature curve vs grip curve of the pad was a straight line, it would make sense, but thats not the case. I have gone both ways with more aggressive and less aggressive pads in the rear. They basically did what i wanted them to do. another thing to consider with more aggresive pads, is that they need to get hot to work, and sometimes they can be operating at below optimum temps and have less clamping force than the pads they replaced!
Lots of a factors!

Larry Herman

Just like with handling, total weight transfer under braking is purely a function of wheelbase, CG and how many Gs you create while braking. It is the amount of movement of the body, and the suppleness of the shocks that determine how much F/R grip you maintain while transitioning to maximum braking force.

mark kibort

But, you cant change the weight tranfer with shocks and springs. you can keep the nose off the bump stops, but the weight transfer is fixed to the factors you list.

Just to put it in perspective, a car that weighs 3000lbs , with a cg of 24" with a 55/45 % weight disti, can transfer 950lbs front to rear on a 1.2G stop.

thats taking 950lbs off the rear 1400lbs, leaving only 450lbs, or 225lbs per wheel weight! up front, those 800lb corners how have 1270lbs on them! (or 2535lbs up front!). There is nothing you can do about this, and have to have the bias right to match the wheel loading. 1270 vs 225lbs front to rear for a 1.2 g braking force. Now I know why my front brakes wear 10 times faster than my rears!

Ive actually did a test one time to guess how much more rear bias I can live with. I was able to lockup the rears under a near threashold braking run, by only a little bit of rear e-brake. not scientific, but an indication. I also noticed that when i had my off-idle fuel cut off running, the lack of fuel would lock up my rear inside turn tire. with the fuel cut off not engaged, that hasnt happened since. (with a deep trailbraking turn)

Im still going to step up the rear bias as i certainly am running very little rear brakes , to see what happens. hey, I can always change it back easy enough.

mk