The other day I was pulling out of my work parking lot and there was sand on the road from some construction thats going on and I felt my Model S do the right thing with the tires — one spun with no grip on the sand and the other pushed me through the patch and it was all automatic and trouble free. That got me thinking. How does Model S traction control work?

Before I get into this, a caveat: I’m an electrical/software engineer and not mechanical. I can change a tire, I used to change my own oil many years ago but otherwise I let the shop (usually a dealer) do all the work and I find one I can trust. The information below is from my own research as I wondered how it all worked.

Open Differential

I had a very basic concept of how the AC induction motor drove the wheels on my Model S. I thought it was just a motor you applied positive and negative current to and that drove the shaft one way or the other (forward and reverse) which then drove the wheels. This kind of thinking comes from the hobby kits you can get at Radio Shack and other places. I hadn’t thought about how that may translate to driving two wheels, nor how it could drive them at different speeds or how any of this could handle tricky situations like sand and ice.

The answer to that is the drive shaft that comes out of the Model S motor goes into an open differential gear box that then drives the two wheels. This gearing allows the outside drive wheel to rotate faster than the inner drive wheel during turns. Without this gearing either the inside wheel will rotate too fast or the outside wheel will drag and this leads to tricky handling, damage to your tires and a lot of stress on the drivetrain. HowStuffWorks has great animated images showing how this stuff works in action.

Tesla describes the gearing well (albeit in the more-detailed Roadster area of their site):

The motor is directly coupled to a single speed gearbox, above the rear axle. The simplicity of a single gear ratio reduces weight and eliminates the need for complicated shifting and clutch work. The elegant motor does not need a complicated reverse gear – the motor simply spins in the opposite direction.

Limited Slip

The downside of the open differential approach is that torque (force if you will) is evenly applied to both wheels. The traction control system limits the maximum amount of torque to the largest amount that will not make the wheels slip. It doesn’t take much torque to make a tire slip on sand, ice etc.

When the wheel with good traction is only getting the very small amount of torque that can be applied to the wheel with less traction, your Model S isn’t going to go far.

While you can turn traction control off, essentially telling the motor not to back off when one of the tires is spinning, this can often result in a lot of tire spinning and will not necessarily get you unstuck. It could also get you dug in deep. On the other hand it can be fun if you want to burn off that last bit of rubber before getting some new tires 🙂

Usually this problem is solved with a much more complicated system called a Limited Slip Differential which replaces the open differential and allows for different amounts of torque to be applied to the different wheels. That approach involves much more complicated gearing which involves more parts and weight and may not be as reliable, especially with the high torque the Model S motor produces. Tesla chose to go with a simpler and sturdier open differential approach. But they needed to solve the slip problem.

Tesla solves this problem by selectively applying the rear brakes to transfer torque to the the non-slipping wheel. This is tied into the electronic stability control system which also needs to brake the rear wheels independently to improve the handling of the car under various conditions.

When a wheel starts slipping, Tesla applies the brake to the slipping wheel giving the “impression” that the tire has traction and allowing torque to be applied to the other wheel without spinning the slipping wheel. This is all done super fast with computers watching how things are going and backing off on torque, applying brakes where needed, etc.

Tesla describes both traction and stability control this way:

Model S Traction Control is designed to ensure maximum contact between the road and the tires. Whether you are accelerating off the line, zooming along the winding roads of the Rockies or find yourself in a Gulf Coast rainstorm, Traction Control prevents loss of traction and maintains control. Stability Control reacts in moments of under-steer or over-steer by reducing torque and applying the brakes to individual wheels for enhanced control when cornering.

Summary

People think mostly about the Model S as a premium electric vehicle and sometimes miss the rest. Tesla is about re-thinking the entire car. If you start from scratch and question every design decision, every part, every ounce of weight, every approach and use some of the best minds on the planet what kind of car will you end up with? The Model S is Tesla’s current answer — and not their last. The 17″ display is one example of re-thinking what has been tried and re-tried many times. The unique approach to battery placement is another and there are many more.

Tesla’s traction control and emulated limited slip functionality is another case of Tesla replacing complex static hardware with lighter and simpler hardware while replacing the functionality advanced software algorithms that can take advantage of today’s computing power.

Next time you slam that “go” pedal and take off like a space ship, think about all the technology thats helping you launch into the future.