RWD vs. FWD: Part 5 – Snow-ish

If you believe common wisdom, RWD is better than FWD with one exception: snow. We know that’s not quite true from some recent experiments here on YSAR, at least in a simulation environment, because FWD can actually be faster than RWD on asphalt. Today I’m going to show some experiments on how overall grip affects the performance of RWD and FWD vehicles. In our test environment (Assetto Corsa, NA Miata, Brands Hatch Indy) we found that a FWD “Miata” with a 60/40 static weight distribution has nearly identical performance to the actual Miata. Let’s find out what happens when we start reducing grip from asphalt to snow (tire grip values reduced to 50%).

Impressions

Driving a RWD vehicle with 50% grip is quite challenging. It took a lot of concentration, which turned some of the fun into frustration. It is a great training exercise for oversteer recovery though. I spun a couple times and nearly spun about a hundred times.

The FWD vehicle with 50% grip was also challenging, but in a totally different way. It felt like I had to pay a lot of attention to optimizing entry speed and position. There was no way to work my way through a corner. Either I started the corner well and ended it well, or I started it poorly and ended it poorly. I ran wide at Paddock twice trying to get through there as fast as possible.

Lap Times

As before, I ran 20 laps in a row and selected the best 10 for analysis.

  • RWD fastest 1:21.069, average 1:21.531 (0.378)
  • FWD fastest 1:19.790, average 1:20.353 (0.255)

The FWD vehicle was about 1.2 seconds faster in both fastest and average measures.

Squiggles

In the graph below, red is RWD and blue is FWD. The 3 fastest laps of each are shown.

FWD spends more time on the brake pedal and also more time on the throttle. RWD spends a lot of time managing the steering wheel and throttle to prevent spinning. FWD also has to manage the throttle, but just to optimize traction.

Conclusions and Thoughts

People sometimes bash FWD vehicles by saying phrases such as “wrong wheel drive” and “the rear wheels just prevent the ass from dragging on the ground”. But the experiments I’ve done here suggest FWD is as fast on a grippy surface and markedly faster when things get slippery. Let’s bring out the caveats.

  • This is a simulation
  • The vehicle is light and low powered
  • The tires aren’t very grippy
  • There is no aero package

It may be that under high power and/or high grip, the FWD advantage goes away. One reason to suspect that is because FWD is more limited in how much power it can put to the ground: weight shifts to the rear under acceleration.

Looking at the data, I’m left with a new perspective on what makes FWD work. And it’s actually the reason most people give for why FWD doesn’t work. It is said that FWD asks the front wheels to do too much: brake, corner, accelerate. That’s true, but it leaves the rear wheels doing just one thing very well: lateral grip. FWD vehicles don’t have to share cornering and acceleration on the way out of a corner the way RWD vehicles do. That’s why the traces above show FWD beating RWD on corner exits. A driver in a RWD vehicle is balancing throttle and steering and hoping he won’t spin (or maybe he’s enjoying the drift). A driver in a FWD vehicle is just managing throttle to maximize traction and has no thoughts of disaster (or fun).

That’s all for now, but what’s next? Power, grip, or aero? Hmm…

4 thoughts on “RWD vs. FWD: Part 5 – Snow-ish

  1. This makes rational sense. Corner exit you can just mash it and there’s essentially no no consequences. I always felt FWD inherently safer and therefore felt more brave. This is likely a good test for your representative skill, wonder if it changes as you start to get to pro level capability.

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    1. It would be interesting to do these tests with drivers of differing experience. I’m pretty sure FWD would be better in the hands of a novice, but the pro thing is a big question.

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  2. Good stuff. Would be interesting to see the same comparison with a RWD at 40/60 weight distribution like a mid engine/rear engine car. I’d think that would be the best combo for low friction surfaces. The one downside is understeer if you can’t get the weight transferred forward (high speed corners or increasing radius corners or very low friction conditions).

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