RWD vs. FWD: Part 1 – Preliminaries

There are a lot of reasons why RWD cars should be faster than FWD. When accelerating, dynamic weight shifts to the drive wheels, providing them more grip. When braking, the additional static weight in the rear leads to more equal loads on all tires, and therefore more grip. In a straight line, RWD wins every time. And in corners, RWD gets to share duties (braking, cornering, accelerating) to both axels while FWD has to rely so much on just one. That said, surely you’ve heard that FWD vehicles have an advantage in the rain. Exactly why is that? In this series of posts I’m going to solve the mystery! Well that’s the plan anyway.

The Plan

People have compared RWD and FWD before, but there’s usually so many differences between the vehicles that the drive wheels are only part of the equation. For example, I can recall a comparison of a Toyota 86 and a VW GTI. Well a Toyota 86 isn’t anything like a VW GTI in shape, wheelbase, track width, gear ratios, weight, horsepower, etc. How can we make an informed decision about the relative merits of RWD and FWD when so many of the variables are changing? We can’t. Not unless we pin down a lot of variables.

Let’s imagine the ideal testing scenario. Both the RWD and FWD vehicles need to be nearly identical. However, there are a few typical differences between RWD and FWD that we need to take into account.

• CoG – RWD vehicles have their center of gravity around 0.5 but FWD are about 0.65-0.7
• ARB – The stiffer anti-roll bar is usually on the non-driven axle
• Track Width – RWD vehicles tend to have a wider track width at the rear while FWD are wider at the front
• Brake Bias – RWD vehicles have more rear bias to take advantage of their static weight distribution. For example, a RWD might be set at 0.67 front while FWD might be 0.8 front.
• Spring Rates – FWD vehicles have more front spring compared to RWD because of the added weight in the front
• Weight – FWD vehicles tend to be a little lighter
• Driveline – FWD vehicles tend to have less loss in the driveline

So where do we start? By choosing a platform for doing experiments. Well, Miata Is Always The Answer, right? So we’ll start with a Miata and turn it into a FWD vehicle! In real life? Unfortunately, no. While a FWD Miata would make an awesome Lemons vehicle, we’re going to do this in the virtual world using Assetto Corsa. It’s relatively easy to make these kinds of changes in AC.

In order to make testing as simple as possible I use the NA Miata straight out of the box. That is, I don’t tune it at all. So Street 90s tires, 24L fuel, stock alignment, whatever that is, and base weather. Yes, it’s possible to make the NA Miata faster, but since I’m after relative differences between RWD and FWD it doesn’t matter. It’s more important that I don’t forget to load a setup before testing.

FWD Miata

Here are some parameters of the NA Miata in Assetto Corsa and how I changed them to make a FWD Miata.

• RWD -> FWD
• CoG 0.515 towards the front -> 0.67
• Brake Bias 0.67 towards the front -> 0.80
• ARBs 9502 front, 4259 rear -> swapped 4259, 9502
• Track Width 1.410 front, 1.427 rear -> swapped 1.427, 1.410
• Springs 19300 front, 17900 rear -> no changes made
• Mass 1080 -> 1060 (loss of 20 kg)
• Power 130 bhp -> 136.5 bhp (increase in 5%)

Let’s compare the RWD and FWD vehicles at two of my favorite test tracks: Brands Hatch Indy & Karelia Cross. First up is Brands Indy. Here are the top 3 lap times for each. As you can see, RWD is about half a second faster than FWD.

• RWD: 1:02.49, 1:02.55, 1:02.66
• FWD: 1:02.98, 1:03:05, 1:03.06

In the speed graph below, RWD is red and FWD is blue. It’s pretty clear where FWD loses out: mid-corner speed. In short rotation corners it’s less of a problem, but whenever sustained cornering grip is needed, FWD is left behind. FWD can catch up a little on straights because it is slightly lighter and more powerful, but in the end, the overall grip of RWD wins.

Karelia Cross

Karelia Cross is a fantasy rally circuit with a slippery dirt surface. It’s not too bumpy, which is why it works fine for non-rally cars like a Miata. The top 3 lap times of the vehicles are pretty similar. RWD has the 2 fastest lap times, but also the slowest.

• RWD: 1:03.95, 1:04.01, 1:04.51
• FWD: 1:04.15, 1:04.26, 1:04.34

In the graph below, I have plotted the 3 top laps of each platform. As you can see from the speed (top panel) and time (bottom panel), both RWD and FWD are evenly matched. However, the RWD driver is doing a lot more work. I should know, I was the one sawing the wheel! The middle graph is steering angle. Look at all the steering corrections. Half the time I was steering in the opposite direction. In contrast, the FWD experience was simply managing a bit of understeer.

Preliminary conclusions and future directions

I think the biggest surprise for me was that RWD was faster than FWD on dirt. I wonder if it’s faster in the rain too? Maybe the difference between RWD and FWD isn’t so much the vehicles, but what they demand from the driver.

This post represents the first of several experiments I’ll be doing with the FWD Miata. I want to explore performance as a function of grip. Is there a point where FWD wins, and if so, when? Specifically, at what level of grip does the advantage switch from RWD to FWD? I also want to investigate performance as a function of power. You don’t tend to see many high powered FWD vehicles. I want to explore what happens in that unusual space.