FWD vs. RWD rain: part 2 (thanks Paul)

I have to thank YSAR reader Paul for sending me down this path, because it’s been really fun. I truly appreciate feedback that makes me look critically at a problem. In this part 2, I do some testing in Assetto Corsa, and come away with some surprise.

Testing scenario

To do the fwd vs. rwd and dry vs. wet experiments, I had to choose a track, two cars, and two grip levels. I like to use Brands Hatch Indy and the NA Miata as a baseline. Sometimes I use the Street 90s tire and sometimes the Street tire. The Street 90s are a couple seconds slower. When you have the AI drive the car, both tires have the same lap times. I think it uses the default (Street 90s) tire. So that’s what I did too.

For the FWD car, I chose the Chevy Monza Classic 500EF. This model is a free download. One reason I chose it is because the dry lap times are very similar to the NA Miata when both cars are on their default tires.

For the wet grip, I reduced traction from the default 0.98 to 0.75. That figure is a little bit arbitrary, but I’ve seen various tables that show a reduction of about that much.

  • Track: Brands Hatch Indy
  • RWD: NA Miata
  • FWD: Chevy Monza
  • Dry – 0.98 grip
  • Wet – 0.75 grip

How to modify Assetto Corsa grip

There are three ways to modify the grip of cars in AC that I know of: run a server, change tires, change track surface. The easiest is the last, but for completeness, I’ll describe the other two first.

If you set up your own server, you can set the grip level of the track. This requires a separate program running as the server. That’s why I’m not recommending it. But on the plus side, it’s just one line of one file.

If the cars are developed in the legacy way, they have editable text files for individual components like tyres (yes, that’s spelled with a ‘y’ because AC uses the British English spelling rather than American English). Most cars these days have binary files that aren’t easily edited. Both the Miata and Monza use binary files. This is why I’m not recommending this way.

If you look in a track folder, you will find a surfaces.ini text file that you can edit. A track may have several surfaces. For example the Brands Hatch Indy file has 11 surfaces. Before you go editing this file, first make a backup copy so that you can restore it to its original configuration later. The grip levels of the various parts of the track range from 0.98 on asphalt to 0.6 for grass. To simulate rain, I set everything to 0.75 because I was lazy and didn’t want to multiply everything by 0.75. But that would be a better way I suppose. However, I planned on driving on the track, not grass or curbs.

AI driver

The first thing I wanted to test was how much the AI driver was affected by reduced traction. Here are the values.

  • RWD -7.31% loss
  • FWD -6.95% loss

There is more loss in RWD than FWD. To put it into the perspective of a typical lap, if your dry time is 2:00 minutes, your RWD wet time will be 2:08.78 and your FWD wet time will be 2:08.34. 0.43 seconds is pretty significant in a sprint race, but we’re not talking about 10 seconds here. It’s just a little time. However, this is the AI driving. What about a human?

Human driver

Move over AI, it’s time for Ian to step into the car.

  • RWD -9.06% loss
  • FWD -6.92% loss

That looks a bit more significant. Let’s put this into perspective of my Toyota Yaris at Thunderhill last May. My fast dry time was 3:43. If we multiply these 223 seconds by 1.0906 and 1.0692 we find that the difference between RWD and FWD is nearly 5 seconds. That’s pretty significant! Given that my Yaris is heavier, higher, and less powerful, than a Miata, the Miata has all the advantages on a dry day, but given some rain, the advantage just might tip in my direction.

Here are the graphs for the simulation experiments.

However, this is a human driving a simulator, what about in real life?

More data diving

Let’s look at the actual laps from the race. On a dry track, I was averaging about 3:50 in traffic. Bring on the rain and that drops to 4:20. So about 30 seconds. I had to make a lot of passes, and when I had a clean lap, I got down to 4:03, which is a loss of just 9%. Driving around slow cars in the rain really kills your lap time.

Some of the fast RWD cars I passed included the Miata of Eyesore and the Celica of Uncle Joe’s. Eyesore’s fast lap was 3:29 but in traffic it was typically 3:35-3:40. They dropped to 4:35-4:40 in the rain, a loss of 60 seconds. Uncle Joe’s fast lap was a 3:34 and it’s traffic laps were in the 3:40-3:45 range. In the wet, they dropped to 4:25-4:30, or about 45 seconds.

Two of the fast FWD cars I passed were the Integra of Big Test Icicles and the Neon of Neon Pope. The Integra went from 3:50 dry to 4:25 wet. The Neon was 3:45 and 4:30.

The race winners, Shake and Break (E30), were typically lapping at the same speed as Eyesore in the dry (3:35) but much faster in the wet (4:10).

Let’s take a look at the relative losses of these cars.

  • Yaris -13%
  • Celica -20%
  • Miata -28%
  • Integra -15%
  • Neon -20%
  • E30 -16%

Summary

Given equal lap times on a dry track, a FWD car definitely has an advantage over a RWD on a wet track. How much? I think it depends a lot on the skill of the drivers. At the high end, maybe 0.5 sec per lap. At my level, a couple seconds. At the “you can’t drive for shit in the rain” level, I think it’s less about which wheels are connected to the engine and more about the driver lacking the skill and confidence to maximize traction in the rain. Pedal mashers who over-brake and then hammer the throttle are the ones most severely affected. A Miata doesn’t normally spin when you stomp on the throttle. But it does in the rain, and if one’s driving style isn’t very nuanced, rain will be very unkind to your lap times. However, in a FWD car, stomping on the throttle may cause a bit of understeer, which is easily mitigated by lifting. FWD cars are more noob friendly. I’m not a noob, so I don’t see that FWD and RWD are that much different. But to someone not used to sliding their car around, RWD could be a major disadvantage.

I just watched the “you suck at racing in the rain” video again asking myself “where does the Yaris have an advantage?” The expectation is under acceleration. But that’s not where I’m catching people. It’s under braking. There is no FWD braking advantage. If you’re thinking it’s because my car is newer than the others and has ABS, that’s a good idea. However, you can hear the tires sliding in some corners when they lock up because my ABS has been broken for a while.

So to sum it all up, the reason for Yaris Rain Domination (YRD) is a little bit of FWD advantage and a shit-load of “most people suck at racing in the rain”.

2 thoughts on “FWD vs. RWD rain: part 2 (thanks Paul)

  1. I agree with your conclusions. You’ve made the point several times that compared to most sports, most drivers actually have very little practice time (IOW a driver with 10 years of experience has driven far less than a tennis player with 10 years of experience has played). Is it possible that most humans just aren’t that experienced in RWD rain driving? Even people who do sim practice most likely practice 99% in the dry.

    Reminds me of Ross Bentley’s video ( https://www.youtube.com/watch?v=3t4l_4z99sw ) where he just walks his way up through the field on a wet day at COTA.

    Like

  2. Yeah, nobody gets much rain practice. The problem with sim racing on a slick track is that it’s more predictable than a real track. Real racing in the rain is even more difficult.

    Like

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