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”.

4 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.

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  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.

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  3. Scrolling through your content and I ran into this post. Ross Bentley featured a guest on his podcast named Randy Beikmann. He wrote a book called Physics For Gearheads. I can’t recommend it enough. Note that it’s deceptively dense. Sounds like it’s up your alley.

    I can’t comment with pictures but there is a chapter in regards to how vertical loads impact traction. Comes with a graph that is plotted out. The example used was understanding how where the weight is placed will determine traction levels AND load transfer. Both traction extremes were used.

    A Porsche 911 and a modern Honda Civic have similar weight distributions on each axle just on opposite ends obviously. The Porsche will have about 65% of its total weight over the rear axle and the Civic will have about 62% of its total weight over the front axle. This also happens to be over both of these vehicles’ drive wheels.

    In the graph it showed the load transfer (or lack of) when in the snow. So around .22 Gs of acceleration. The Civic and Porsche had relatively similar acceleration potential because of where the vertical load on the drive wheels is. .22 Gs is so low that significant load transfer isn’t happening to the civic to remove its traction advantage on the drive wheels (front) vs say something like a Miata which has 50:50 weight distribution. The Miata won’t have as much grip potential under low grip circumstances as say the Civic because it can’t take advantage of load transfer over the rear wheels. (Load transfer being influenced by wheel base length, COG height, and total mass) The Porsche has its engine over the rear axle which are the drive wheels. Ever notice that rear engined Porsches always have impressive hookup and low 0-60 times? Or how the C8 Corvette hooks and goes vs the front engined versions?

    The reason your Yaris does better in the wet is for all the reasons you mentioned about it being easier to drive, but also because under low traction situations, vertical load where traction is needed matters most.

    Why are 2wd rally cars often front engined? Or even better, why does an open diff 2wd truck suck so much in the rain? And how do we get them to hook up? Throw weight in the bed.

    As a motorcycle racer, when I’m racing in the dirt in really loose conditions, why does placement of my body to weight the end that needs grip matter so much? Go ride a dirt bike in the mud and sit on the rear fender and notice how it doesn’t turn and plows.

    People make fun of Audi and Volkswagen for their front biased all wheel drive systems. And some might mock Mazda and the Speed6 AWD system only providing 30% of power to the rear wheels once the front wheels start losing traction. But based off of understanding load transfer and distribution, would a car making less than 1G of grip need all that power moving back? Hence why they are engineers…

    From what I’ve noticed in assetto corsa, they get this load transfer and weight distribution relationship pretty bang on. Take a Porsche 911 GT3 street car and notice how it wants to get tail happy when really hard on the brakes. Drop the rear down and that makes trail braking much more effective. And in the front wheel drive Audi TT car, brake balance is so biased towards the front because hard on the brakes easily locks the rear tires on corner entry. There is minimal vertical load. And the locking is worse depending on grip too in the game. Pretty impressive.

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    1. Thanks for all the thoughtful thoughts. Wasn’t the RR Alpine a great rally car? I’m not sure I follow your reasoning, but it sounds like a Yaris and a 911 would be equally good in the wet as they both put load on their drive axles. To the extreme, on slippery surfaces, it’s better having 100% of the weight on the drive wheels and none on the others? While that may be true for acceleration, I don’t think it works for braking or cornering. For braking, you want all of the tires to have as equal weight as possible since they are all doing work. RR is better than FF in this regard because when the weight goes forward, you get close to 50/50. This would be true in wet or dry conditions although you get less weight transfer in the wet. For cornering, I think you want to be close to 50/50 so you don’t experience extreme oversteer or understeer.

      If you watch the video of me racing in the rain, you’ll see that two of the fastest cars out there were the Yaris and the Supra I was following in the first part of the video. We passed a huge number of FF and FR cars (and some AWD as well). The Yaris wasn’t faster because of the FF layout anymore than the Supra was faster because of the FR layout. The main reason is that most people can’t drive in the rain.

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