How can cars with All-Wheel Drive go in the ditch?

It’s that time of year again. In the Northeast, we are getting snow (finally). Not that I’m a huge fan of the stuff, but growing up in WNY, I have learned to cope. It seems like we have the same discussion at the beginning of every winter. “How many people will we see drive off the road today?” It is staggering some days. On one commute last year, I counted 14. FOURTEEN, with many of them being SUVs.

I always thought SUVs, with their mass and All-Wheel Drive (AWD) capabilities, were supposed to be better in slippery weather until I started thinking about it. All-wheel drive gives power to the wheels, so if I need to stop quickly, all the power in the world isn’t going to help. In fact, with that much more mass and inertia, it may even be more difficult to stop.

What does getting stuck in a ditch have to do with SOLIDWORKS? Well, using SOLIDWORKS Simulation, I can prove that AWD does not always help if you need to stop quickly.

I created a VERY simplified model of a car. A Subaru, as a homage to Derek. I Google image searched a 2015 Impreza and traced the sketch picture in SOLIDWORKS using style splines, lines and arcs. I wasn’t concerned with exact geometry; I just wanted the side view to look like a car.

Here is the block after a few other features. I obviously paid very close attention to the drive train and suspension… (sarcasm)

I spent just about the same amount of effort on the wheels.

I did spend my time making sure the vehicle dimensions and mass were close.

Why do I care about mass and dimensions? Because SOLIDWORKS Motion, that’s why! I want SOLIDWORKS Motion Simulation to prove to me that AWD may help you accelerate but do nothing for stopping distance.

So, I created a “road” component and assembled my car on there. Using the SOLIDWORKS Motion add-in allowed me to run a kinematic study using friction, gravity, and motors.

I set up contact sets between the ground and the tires and between the wheels and the body. In the contact definition, Motion allowed me to specify kinetic and static friction coefficients. Since I wanted my study to start with the wheels at full velocity, I could essentially ignore static friction. The friction of cold rubber on ice varies quite a bit, so I just chose a value and ensured it was consistent across both studies (AWD and FWD).

I added motors to all four wheels and gave them a rotational velocity of 200 RPM.

Given:

• 28" tire diameter
• circumference = d * pi
• 5280 feet in a mile
• 60 minutes in an hour.

My simulated car should top out at about 16.7 mph. It doesn’t really matter how fast we go as long as I run the simulation long enough to gain traction, roll at constant velocity, slam on the brakes (no ABS), and slide to a full stop.

For the FWD study, I suppressed the back two motors, allowing them to freewheel. At 17 seconds, all four wheels “lock up.” You cannot have a motor at zero RPM, and suppressing the motor would allow the wheels to spin freely. To adjust for this, I changed the motor speed to 0.001 RPM.

For the AWD study, all four wheels start at 200 RPM and “lock up” at 17 seconds.

I calculated both studies and plotted Velocity in the X-direction. For clarity, I copied the data to Excel and overlaid the plots.

Exactly what I expected! Stopping time was completely unaffected! The AWD car was able to get up to speed faster, but with no ABS and the same mass, both slammed into the car in front of them. :)

Obviously, I made many assumptions here, and anyone living in the Snow Belt knows that there is more to snow driving than slamming on the brakes. The point of this study was to use Motion to prove that AWD may not always be a safer bet in a low-friction environment. I got to show you some cool tools in SOLIDWORKS and create some neat plots.

Thanks for reading, and as always, Happy Simulating!