mark kibort

I thought this would be a good discussion topic, as most of its concepts are understood by most here, at a high level. It was a topic that started to hijack another thread, so I started its own. I havent done much testing on the subject, other than superficial things I have tried at the track. Its not like the HP-Torque subject, where it is mostly cut and dry, this topic depends on an number of factors for which I thought we could talk about.

What is the concensus here? How much rear braking do you want before it becomes a hinderance for lap time and possibly control. There is a lot of talk about dive, squat in regards to rear brake effectiveness, but from Carrol 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 rears are being, in essence, lifted up and have no where near the capacity, even with the same size tires as the fronts.

I guess what this all says, is that you want the weight transfer to match the grip capability of the front tires. Load transfer=acceleration (g) x weight-lbs x cg "/wheelbase"

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

Just to put it in perspective and plug some common values into the equation, 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 individual tire loading for a 1.2 g braking force. Now I know why my front brakes wear almost 10 times faster than my rears!

Once we understand the physical capabilities, its then up to the driver to use it best. Probably an entirely new discussion regarding the use of driving techniques to best utilize the braking capablities of the vehicle.


Thoughts?

Mk

Larry Herman

I think that there are 2 main variables as to how much rear brake bias to run, and they are pretty obvious. The first is the F/R weight bias of the car; the more rear weight bias the more rear brakes you can run. The second is how you like to drive the car with-in the parameters of how IT wants to be driven. If you tend more towards trailbraking, you are going to want less rear bias. If you are more of a straight-line stopper and only like to rotate the car on the brakes, you can get away with a little more rear bias.

There are all kinds of other minor items that will affect bias, such as tire size, spring rates and shock settings. These will all have some affect on the amount of rear grip available as the car transfers weight to the front, and therefore will impact the amount of rear bias you can run.

IcemanG17

MK
Which bias bar are you running...the stock or 33bar? I have the 33 bar in the widow & I can FEEL it....not really in straight line braking, but in slight trail brake situations the rear end comes around MUCH faster than my stock 928, or the M3....and thats with the monster rear tires..... I do think it contributes to overall better braking too...but only by a little bit...

mark kibort

You know how many race laps i have had with this chassis. get rid of the 33 bar and put in the 11 for awhile. (I have the low one. I think its 11bar if 33 is the higher one). Its not going to be much, if the fronts are about 5-10times more the stopping power anyway. I wonder what the G loading is for our cars on a reasonably hot lap. Im betting near 2 Gs braking. Ill have to review Rylans telemetry output to see what ball park we are in. If its near 2g's, there is not much braking done by the rears. maybe only a little to help stablize the car and add a little "Squat" as Seth has pointed out.
mk

Seth Thomas

I am getting ready for Sebring and will give you guys my quick little snippet on this topic.

On a car with a 50/50 weight distribution I will adjust the rear bias until I get to the point the rear brakes tires lock up. I will then give my car 2 clicks more bias to the front and leave it. This is near perfect. In this same car as the fuel burns off during the race I do have to add a click or two more to the front to keep the rears from locking as the weight disappears. As Larry stated all this depends on the shock settings, the track, springs, and the overall setup of the car. My reasoning behind using more rear brake is two fold. One to use all the brakes that I can to stop the car. Using just the fronts won't stop the car in the same distance as using the fronts and rears. I try to maximize this braking effort all across the car. Two I like to add the bias to the rear so it will help to level the car off more during the brake zone. This helps to keep the car flat and less compression on the front tires. I know that when you brake the front tires get pushed into the ground. This is something you cannot keep from happening. But you can help to limit it as much as you can and one way to do this in a production based car is to add more rear bias.

These are my quick thoughts and I hope to be able to check into the thread later in the week if time and internet connection permits. Have fun with it.

mark kibort

These are the trade offs Ive been dealing with for years now. I have very little rear bias, but when i had just a little more engine braking. (fuel cut off engaged vs not engaged) I noticed more rear wheel lockup on tight trail braking turns.

you cant really get the front to dive any less, right? Its a fixed weight transfer for a given weight, distribution, length and g's of decelleration. by increasing the rear brake bias, you will actually add to the weight transfer, due to it stopping the car faster, right? so, all you can really hope to do, would be to have the rear squat, to lower the cg, but since the force available is such a fraction of the front brakes, it would be a tough argument to soften rear springs or lower shock compression to get some movement. Now, there could be some interesting points made in the area of stablizing the decel with the use of rear brakes. I have to admit, i use almost no rear brakes, but wonder how things could improve if I increased rear brake bias. as I mentioned, I believe I have it right due to my little crude test of the fuel cut off, reducing engine compression braking, and some ebrake checks during some threshold braking runs. I dont have a brake bias selector, but that would be very cool to have to optimize the rear brakes, though it is about 20% factor of the overall braking equation. a lot more when you start looking at 2g stopping deceleration. add some trail braking and i dont know how much is best back there.



mk

mark kibort

a quick calculation, without any rear downforce, the total weight on the rear tires of a 50/50 BMW, would be 60lbs (30lbs per tire) with a deceleration of 2g. So, your car is almost standing on its front tires! I dont know about you, but if the fronts have 2940lbs on them and the rears have 60lbs, you can see that any more than a little rear bias, could cause some serious lockup. on more realistic 1.5g decel, the rears would be more like 420lbs. ( 210lbs per rear tire, like a motorcycle).

I can see why ive seen so many rear tire lock up caused spins and slides in cars in cars in front of me

By the way, what is the greatest rate of constant decel anyone's electronic instruments have measured on their road race car?

Mk

Bryan Watts

Way too high.

See above. 2 is WAY too high unless we start talking about open wheel or sports racers with aero. 1.2 is more in line with what you would see on your Porsche.

Bryan Watts

Easy answer to the thread: Run as much rear brake bias as you can stand.

mark kibort

I agree, and knew it was greater than 1, but less than 2. (so I kind of changed my original post when i started looking at telemetry of the guys I run with).

So, even at 1.5 gs, there is a substantial weight transfer, with cg being around 24" up. even that might be too high. On a formula car its more like 13", so i figure it might be around there. in thinking about that, i bet 15" might be more like it. (like fender hight, or top of engine hight)

Point is, there is some substantial weight transfer under braking in our cars running DOTs, let alone those running slicks.

mk

Bryan Watts

Hooser R-comps aren't all that different from slicks with regards to peak grip...we've had both on our car.

jgrant

100% rear. It really helps with rotating the car, I find. It also really saves on the front pads/rotors.

mark kibort

That would be another interesting data point from some telemetry output

By the way, as far as G loading during acceleration or deceleration. I remeber reading about a 0-60 acceleration number that was 1g. Probably average or peak, but the time was something like 3.6 seconds. Ill have to go back through the numbers and get a value. Obviosly, most any race car can decelerate faster than this rate. I would bet 1.2, of my original number is pretty close to reality for the sake of argument.

mk

jgrant

For what it's worth, here's some data from the 24hrs of Daytona that we just ran:

car: 2007 Cup Car, with slicks
driver: Ross Bentley
Best Lap Time: 1:52.178

Long G's: .61 max (acceleration), -1.33 min (deceleration)
Lat G's: 1.68 max (lefty) 1.55 (righty) (probably due to the banking)


That should give you a rough idea of the types of G forces in play for a 997 Cup.

mark kibort

Well, there we have it.

mk