It’s raining lies: part 2

Where Were We Anyway?

If you recall, three weeks ago I did the following:

• Called Ross Bentley a liar
• Committed career suicide (see above)
• Claimed that wet tires have 9/10 braking grip, 3/4 cornering grip, and 1/4 accelerating grip
• Showed telemetry traces that support said claims
• Calculated the G-forces in a Car and Driver tire test and found that braking loses much more grip than cornering
• Lied that I would resolve the mystery the following week

In my defense, the series is called “it’s raining lies”. So let’s get back to our watery tale and see how this story resolves.

The Braking Mystery

Why do I feel like the car brakes equally well in the wet and dry when the data shows dry grip is so much better? I believe this is pretty simple. When the track is dry, we aren’t braking as hard as we could. Thinking back a couple weeks, let’s be Paul Gerrard and see if we can get to the root cause. It’s certainly not physics holding us back. Do we fear excessive Gs? Not exactly, what we fear is flat-spotting a tire. Our team races on a small budget and tires are the largest expense. Flat-spotting a tire is a huge no-no. Everyone on the team is acutely aware of that. Because we are afraid of destroying tires, we don’t brake as hard as we could.

If the car had ABS, we would probably brake harder in general because ABS prevents flat-spotting. Braking is so much easier with ABS: just mash the pedal and let the computer take over. Surely the Car and Driver tests were done with an ABS-equipped car. It makes the testing procedure much more repeatable if you minimize the human element. And why not use ABS? ABS systems probably brake better than you do. Nannies in cars are getting better and better. For most drivers, having various nannies on is faster and safer than driving fully analog.

Have you ever noticed that WWII fighter planes have wings that slope up and modern fighter jets have wings that are straight out or even slope down?

A dihedral wing, one where the wing tips are higher than roots, is inherently stable. But an anhedral wing is not. Build a paper airplane with an anhedral wing and it will flip over and fly as a dihedral. It’s very difficult to fly a plane with an anhedral wing angle. Human pilots can’t do it. There isn’t enough compliance. They need a fly by wire system that makes hundreds of tiny adjustments per second to keep the plane flying level. So why have anhedral wings? Because the inherent instability makes the plane want to turn, making it more agile. Could cars be tuned the same way, so twitchy that no human could drive one without nannies? Surely. The evolution of performance driving will someday see computers outperforming humans at every level of the sport. When that happens we’ll become even better drivers as we learn from computers whose AI can explore the parameter space more deeply than we can.

Let’s return from my crystal ball and recap: we under-brake in the dry because we are afraid of flat-spotting our tires. I’m happy to make this compromise for 3 reasons.

1. A flat-spotted tire is a waste of money
2. A tire that fails on track could cause a crash
3. Time spent fixing things in the pits is time not lapping

Brake Bias

When tuning a car for driving in the rain, one parameter that is often changed is the brake bias. Since there is less overall weight transfer on a wet track, there’s less weight on the front wheels. With more weight on the rears, more braking is possible out back. In my old E30, I installed a manually operated prop valve. To adjust the bias, you lift the hood and twist a dial. Real race cars put the bias adjuster in the cockpit so the driver can make changes mid-race. Until you’ve tried an adjustable prop valve, you probably haven’t experienced how much it changes the handling of your car on corner entries. If you don’t trail-brake, you won’t notice much at all, but if you do, it’s basically an oversteer tuning dial. Want more oversteer? Add more rear brake. It’s really that simple. If you don’t have a prop valve, you can still tune your brake bias with different pad compounds, but the resolution is much lower.

The main problem with adjustable bias is forgetting to dial it back when the track dries. This can lead to disaster. The rears will lock up first, causing the back of the car to wander when braking in a straight line. This can even happen on a wet track if the brakes are horribly out of proportion. Early ABS systems were kind of crappy and just kept the rears from locking up. If you’ve got such a system and the ABS computer is defeated or the fuse is blown, the bias is dangerously out of whack. Watch below as the fast POV is destroyed by a slow BMW that loses control while braking in a straight line.

How did the BMW team not realize their brakes were so horrible? Probably because they usually brake very gently. The rain moved the lock-up G-force threshold lower and the driver found himself in unfamiliar territory. How do you mitigate this? That’s a very good question. On the one hand, you can tune the brakes appropriately with a prop valve, pad compounds, or ABS. That fixes the problem with the car. But there’s another problem, which is how to fix the driver. Should the car be good enough that the driver doesn’t matter? Or is it the responsibility of a driver to work around problems with the vehicle. Probably a little of both don’t you think?

So how do you get practice driving a car with horrible brake bias? How do you get experience with locked up rear tires? If you want to train yourself for disasters, you have to put yourself in disastrous situations… without wrecking other peoples’ property, your car, or your body. The answer, which you can guess if follow this blog, is simulation. Not every car has adjustable bias even in a sim. Find one that does and then experiment with brake bias. Once you experience how useful and fun it is to tune your corner entry oversteer, you may want to install a prop valve in your race car. It takes all of 30 minutes and costs less than $100.

There’s still more to come in the “it’s raining lies” series. We still haven’t discussed why you soften the suspension in the rain. Check back next week for the resolution (or possibly more lies).