Q: This weekend I did my first event in a Formula Continental. This is a step up from the Formula Ford I drove last year. The FC cars are faster, corner with more lateral acceleration, and stop better. In the afternoon race on Saturday at Thunderhill, I had two snap spins going into T10. The first time it happened, I was trying to go a little deeper before braking, and I did mash the brake pedal pretty hard. The second time, I went maybe a little deeper, and got on the brakes what I thought was with moderate pressure.
I’ve always associated such snap spins under braking with rear lock-up. On Friday, during a few test sessions, I had some wiggling under braking at T14, so after the two spins on Saturday, I wanted to check the bias pressures. We have an Aim data system on the car, with pressure transducers on the brakes, so we adjusted the rear down about 50 psi on Sunday morning. I didn’t have any problems on Sunday, but I didn’t push it either. I’m really not confident the fronts will lock first. I realize brake bias is a moving target, but do you have any guidelines for how to set it and how to monitor it? Would anything else cause these snap spins under braking?
A: It sure sounds like the brake bias was too far to the rear. That would explain the “wiggling” on Friday, and the spins.
With the data, find the longest, hardest brake zone on the track, and overlay the front brake pressure over the rear pressure. Put the cursor just after maximum braking and read the front and rear pressures. Turn that into a percentage ratio (i.e., front is 55% and rear is 45%). Every car is different, so I’d start by comparing Friday’s bias to Sunday. My guess with that car is that it should be somewhere around 54-55% front, 44-45% rear. But it could be as much as 53-47% to 56-44%. Ideally you would go on track with older tires (ones you can afford to lock up), and purposely lock up the brakes and make note of which lock up first, dialing it in to the point where the fronts lock up just a fraction of a second before the rears (that’s more stable and easier to correct for). Then you would take the bias reading from the data and know that’s your starting point.
Different conditions will mean different biases, too. With less track grip (rain, especially), you want more rear bias (maybe 1% more to the rear). Why? Because you can’t transfer as much weight to the front tires when you have less grip, so the rears will do more work. With more track grip, you might be able to dial more bias to the front. As tires wear, sometimes the fronts will go off before the rears (or vice versa), so in that case you’d adjust the bias to the rear slightly and gradually as the tires wear.
Another method you can use is to put the car up on jack stands and manually check the bias. Have someone sit in the car to use the brakes (or reach and pull the brake pedal by hand). Have them push down on the brake pedal gradually and slowly as you turn/rotate the front tire. As soon as you can’t turn the wheel any more, have the driver hold that pressure on the pedal. Go to the rear and turn the wheel – you should be able to just barely turn it. This will mean the fronts have a little more bias than the rears. This technique might take a few times to get it right, and it’s something that you get a feel for, but it’s surprisingly effective. Again, then turn on the data and take reading right then as to what the numbers are and compare that to past readings. Over time you’ll learn what you and your car likes best.
Brake bias can make a big difference!
Fantastic, thanks Ross! My next car has a brake bias control and all I have found so far is “don’t touch it until you understand what it does”.
If you have in-car bias adjustment, you might want to start a session with more bias to the front, and then adjust backwards. Front lockup, while not good for the tires, will tend to be safer and easier to control (you’ll feel the steering go light, and the car will want to go straight until you release the brake).
You can then walk the bias backwards in small increments until the braking is balanced (and if you feel that wiggling again, you’ve gone a bit too far).
Don’t forget that bias might change again when new pads or rotors are installed.
So, would you say, “Wiggling” under heavy brake pressure or “Hunting” can be a symptom of too much rear brake bias that could be a snap spin warning ? I had a snap spin at COTA going into turn 11 after we bled more air out of the rear brakes last year and found too much rear bias when we checked it out as you have described.
Yes, that’s a good clue. I can’t say it’s 100% for sure, because it depends on where you feel the “wiggling” or “hunting.” If you feel it in the rear, then yes, it could very well be too much rear bias. Of course, it could be other things, too (i.e., bump steer in rear suspension). But I’d start by looking at brake bias.
You put car on the jack stands to manually check the bias, but the load distribution, especially dynamic load distribution is not necessarily even between front and back, and some cars run on staggered tires, so no matter which situation, the grip limits are different, I don’t understand why it’s a practical approach to test the bias, since it’s only a test of how the pads grab the rotor, not about which end will lock up first
The test and check on jack stands will “rough” it in to the right bias, but with experience can be surprisingly accurate. As I wrote in my answer, you want the fronts to lock with a certain pedal pressure, and at that same pressure have the rears turn just slightly. That means there is slightly more bias to the fronts. And that bias to the front is what addresses the load transfer. If there was no load transfer, you’d want the bias to be equal, 50-50, front to rear. But because the fronts do more work when braking, due to the load transfer to the front, they need more “bite” from the brakes. If you follow the procedure in my answer, you’ll find the ideal for the specific car, in those specific conditions. But it will change, as I mentioned in regard to rain. And every car is different. Some could be very close to 50-50 (a rear engine Porsche, because the rears are doing more work due to all the weight at the rear of the car), some closer to 60-40. If you find what works on the track, then test/check it on jack stands by hand and make note of the way it feels, you can tear the entire brake system apart, put it back together, and then set it on jack stands and know you’ll be very close to the ideal bias – to start. And be prepared to adjust as you drive to suit the conditions, and fine-tune it.