This week I attended a Ross Bentley webinar titled “The Art and Science of Racing in the Rain”. He runs webinars several times per year with a cost somewhere in the $50-100 range. Is it worth it? Yes, I think it is. If you’re serious about racing and improving your lap times, $59 is one of the cheaper expenditures you’ll have. Looking back, I’ve attended a bunch of his webinars: Speed Secrets, Tires, Drive Faster, Reading Your Car, Chalk Talk, and now Rain. That may be all of them. I’ll be the first to admit it, I’m a huge Ross Bentley admirer. But I’m also here to tell you he lies. OK, so that’s maybe too strong a statement. It would be more accurate to say his theory is sometimes incorrect. But this is YSAR and we write provocative shit here, so yeah, Ross Bentley is a goddamn liar.
Before some other Ross Bentley fanboi punches me in the face, let me explain (yes, I said other and I’m a little worried about hitting myself in the face as I write this). I don’t dispute Ross’ advice on driving in the rain. I’m going to do exactly what he says. What exactly did he say? Well, you have attend the webinar for that. I’m not about to pirate his content. But I will reference the parts that need critique.
In the rain, soften the suspension to decrease weight transfer.
— Ross Bentley
Softening the suspension does not decrease weight transfer. The more the vehicle pitches to the side, the more weight is transferred because the center of gravity moves more. One of the attendees wrote the equation for that in the chat box and it stopped all chat for a while. Nobody wrote “Ross, you’re full of shit” because we all respect Ross too much. But let’s be clear, softer means more weight transfer, not less. It is true that in the rain there is less weight transfer compared to dry, but that’s because the corner speeds are lower, not because the suspension is softer. So why, I ask you, should one ever soften the suspension? You’ll have to wait for that answer…
In the rain, lateral grip is affected more than longitudinal grip.
— Ross Bentley
This is not my experience. I find that braking works nearly as well in the wet as the dry. I use pretty much the same braking markers. Now it’s true that my straight speed is slightly lower in the rain, and pick up a later apex, but the grip is still darn good. Don’t take my word for it, or anyone’s word for it. Look at the data. In the image below, the blue line is dry and the black line is wet. The downward slope of the lines in the braking zones are nearly identical. The longitudinal G-forces in the 2nd panel show that peak Gs are similar, as you would expect.
Have you ever stepped on the throttle a little too eagerly in the rain? The car spins around without giving any warning. The grip under braking and accelerating are totally different in the rain. Note that this is from my experience driving high performance street tires not F1 racing tires. Since I’ll bet that you’re racing on tires sort of like mine, I think the difference between braking and accelerating grip is a very important distinction. My experience with corner grip is that it’s not as bad as you might think. The graph above backs that up. If I was going to put some subjective numbers on comparative grip levels in wet vs dry, I’d say braking has 9/10 grip, cornering is 3/4, and accelerating is 1/4. Although Ross didn’t put such numbers on these, he ranks them as braking > accelerating > cornering. So who is right? Turns out we’re both wrong.
Back in 2012, Car and Driver did a really nice comparison of 9 performance tires. For example, on the Bridgestone tire, the skid pad grip was 0.89G in the dry and 0.83G in the wet. That doesn’t sound like a very large change in grip level. They also reported 50-0 mph braking distance as 80 feet dry and 101 feet wet. Putting those distances in terms of Gs, that’s 1.04G dry and 0.83 wet. There are actually two lies we need to debunk here. The first one is that cornering grip is more adversely affected than braking. It isn’t. In terms of Gs, 0.06 is smaller than 0.21 by a metric shitload. To put this in terms that you might appreciate more, you can go 75.7 mph around a 200 ft radius circle at 0.89G. At 0.95G (plus 0.06) you get 78.2 mph. Racers would throw loved ones under a us for a 2.5 mph corner speed advantage. At 1.2G (plus 0.21) speed is 87.9 mph. I don’t think I have the macabre imagination required to describe what a racer would do to get a 12 mph advantage.
WHAT THE FUCK IS GOING ON HERE? This doesn’t mesh at all with my driving experience, the data above, or Ross’ instruction. Corner grip is less affected than braking? It’s true. It’s right there in the numbers. So why do we feel like it is less? And why do the telemetry traces tell a different story? Sorry, but you’ll have to check back next week for those answers.
What’s the other lie? It concerns the friction circle. The way the friction circle is explained, your tires have a certain amount of grip and you can divvy that up between lateral and longitudinal axes. So you could go 50/50 or 90/10 or 100/0. But it’s not symmetric, and therefore not a circle. Tires actually have more grip under braking than cornering. In the example above, 1.04G braking and 0.89 cornering. Circle shmircle. What’s one more drop in a bucket of lies?
Check back if you want to see how this mystery resolves…