It’s raining lies: part 1

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…

4 thoughts on “It’s raining lies: part 1

  1. I think the statement is correct if you add my parentheses. “In the rain, soften the suspension to decrease (the speed of) weight transfer.” Softer suspension spreads the weight transfer over a longer period of time, which gives you more time to react to sudden grip/slip changes.

    As a comparison, I’m thinking of a mercury recoil reducer in a rifle stock. The weight of mercury helps reduce recoil on its own, but the sloshing of it from one end of the tube to the other spreads a portion of the recoil out over a longer period of time. The net effect is less perceived recoil, and an easier to manage muzzle. (The net recoil is the same, nothing is truly reduced, just delayed.)

    To get back to cars in the rain, let me add another variable. I would imagine that hard suspension increases grip (even in the rain), and so when you lose grip, you lose more of it. So to put some numbers on it, maybe the hard car goes from .83g to .70g in a grip/slip situation, while the soft car goes from .81g to .70g. I’d imagine it’s easier to catch a slide with a smaller difference between grip and slip. Maybe this is a minor point, but you can add this to the existing problem of hard suspension that reacts immediately to weight transfer.

    So this softening of suspension does two things: gives you more time to react, and gives you an easier grip/slip threshold to manage. “In the rain, soften the suspension to decrease the speed of weight transfer and minimize differences between grip and slip.” Maybe?


      1. I think an even more accurate statement would be: “In the rain, soften the suspension BECAUSE there is less weight transfer” . Think of one of the ways that anti-roll bars work. Simplistically, they help compensate for unwanted affects on the suspension and tire contact patches that occur due to changes in lateral loads during cornering. So for example, this is why we are taught that stiffening the rear anti-rollbar can correct understeer: You are increasing the magnitude of lateral weight transfer on the rear tires, which in turn decreases the total grip available for the rear tires, which in turn gives relatively more grip to the front and reduces understeer. Ross Bentley gives a nice simple description of this in his books (maybe this will get him back on speaking terms with us! ;) ) At neutral lateral balance he gives the rear left and right tires each an arbitrary traction unit number of 10 for a total of 20 units traction on the rear . When vertical load is transferred to one side under cornering, the outer tire will now have 15 traction units, but the inner tire will have 3 units for a total of only 18 units- so you have less total grip for this set of tires (assuming you had less lateral load transfer at the front). The stiffer the anti-roll bar is, the more total grip you will lose under lateral loading. As you have correctly stated, in the rain there is less weight transfer, so to get the same effects as you would in the dry, you would have to soften the suspension system (including anti-roll bars) to increase the weight transfer. This follows the classic teaching to soften the anti-roll bars in the rain, but as you correctly stated, we are NOT doing it to decrease weight transfer, we are doing it BECAUSE there is less weight transfer in the rain.

        Big fan of your work, I am eagerly awaiting the exciting conclusion to the continuing story of raining lies.


      2. You can also get less weight transfer with a harder tire. But I think if you were on dry pavement with hard tires, you would still prefer a stiffer suspension. Water makes a qualitative change in grip that isn’t simply a lower CoF.


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