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Why an Open Differential Doesn't Work on Track

C7 Corvette Grand Sport... doing a burnout with a proper diff - Graham MacNeil ©

I wrote a tech post comparing various open and limited slip diffs for a comparison and I found that a lot of people were asking questions. To simplify and make it easier to read, I decide to break them up for a future tech article about handling. Make sure to stay tuned for two posts on limited slip diffs this week!.

WHY DO YOU EVEN NEED A DIFFERENTIAL?

A differential's job is to allow two wheels on the same axle (or two axles in a 4WD drivetrain) to spin at different speeds so a car could smoothly go around a corner. Why do the wheels need to spin at different speeds? Each wheel on a car has to travel a different path to reach the end of the turn. You can see that for yourself every time you drive on snow covered roads or muddy trails.

Four different tracks by four wheels/tires on one car going around a turn - Rams Eye The Track Guy ©

If you take a turn, you'll see that each wheel/tire leaves a separate track (arc) on the road, which means they all have to travel different distances. And since they also have to travel different distances in the same period of time - the time it takes the car to go through the corner - each wheel has to travel at a different speed throughout the turn.

VIDEO OR IT DIDN'T HAPPEN

The video below has been used countless times to demonstrate how a diff works and, although made by General Motors over 80 years ago, it is still one of the best videos I've found that explains very visually how a differential works. It's a long video; fast forward to about 2:00 in (from 2:00 to 2:22 is the problem.differentials solve and explained above and 3:40-6:35 is the working principle/mechanism of a diff).


The diff was a fantastic innovation and solved the problem of two powered wheels traveling at different speeds on the same axle. But it's not without its faults, especially when it comes to demanding conditions such as driving on snow/ice or high performance driving (and ultimately racing).

ONE WHEEL DRIVE

The main limitation with open diffs is that they allow one wheel to spin endlessly, even if the other is completely stationary. The demonstration at 5:30 into the video shows that. If you're on the road with one wheel on tarmac/asphalt and another on ice, one wheel and tire "grips" the pavement (pun intended) due to higher grip level on tarmac compared to ice for the other tire. In other words, if one wheel has higher grip or traction, it's harder for the diff to turn, much like being held still in the video relative to the other one, and instead, the diff will turn the other wheel since it is easier to turn.

That problem isn't just limited to one wheel spinning endlessly while the other is stationary, though. An open diff transfers virtually equal amounts of torque to both wheels. That means that the wheel with little traction will dictate how much torque the wheel with a lot of traction gets. If you give more power (and therefore torque) than the low traction wheel can hold, it will start spinning, because it is easier to spin. This is why if you have a FWD or RWD car with an open diff on snow or ice, some people call it one wheel drive because it only takes one wheel without traction to render the whole car stuck.

SLOWING DOWN...

Once a wheel is spinning (excessively), it can't do much useful work, meaning it can't transfer much power to get you moving or accelerating. This is evident if you watch someone do a burnout, where you can see that the tires are spinning endlessly but the car is barely moving. It will take a very little amount of power to keep an already spinning wheel spinning. Now think about that: that little amount of power that it will take to keep a wheel spinning while the car is barely moving is the maximum you can transfer to the other wheel as well because the diff transfers equal amounts of torque to both wheels.

North American 8th gen Honda Civic Si burn-out (demonstrating how little power it takes to spin a tire...) - Graham MacNeil ©

THE PROBLEM

If you have uneven available grip between two wheels on the same axle, you have one wheel with higher torque carrying/transfer capacity than the other. When that happens in an open diff with no way to unevenly distribute torque, you can more easily overpower the low traction wheel. If you overpower it, it will start spinning. If it starts spinning, it is contributing very little to your longitudinal grip (forward/backward) or lateral grip (sideways). Moreover, the wheel with a lot of traction and, therefore, good torque carrying/transfer capacity is underutilized, because it won't get any more torque than that which is transferred to the low grip wheel (not much).

Nissan 280Z at Atlantic Motorsport Park locking front outside tire - Graham MacNeil ©

The result is limiting how much power you can use to move (or accelerate) as well as making it easier to reduce your available grip by spinning the low-grip wheel, which still contributes to the car's overall lateral and longitudinal grip available. Once it starts spinning, it can't do much. If that happens at the rear axle (RWD), that spinning low-traction wheel means less grip at the rear end and more likely to oversteer. On the front axle, it's understeer.

NOT SUITABLE FOR TRACK DAYS

How does this play out on a track? When you're going around a corner, the inside wheel is unloaded because weight is transferred to the outside wheel. That means the inside wheel has less grip, which means it can transfer less torque than the outside. If you exceed the maximum it can transfer, it will spin. If it starts to spin (excessively), it will have even less grip, so you'll have to use even less power and your corner speed has to come down since one of the tires now has less grip due to excessive spin. Moreover, the outside wheel - which is loaded due to weight transfer and can transfer a lot more torque - is underutilized.

In short, an open diff under-utilizes available traction because it can't unevenly distribute torque. In other words, you can't use as much power as the total that the two tires could transfer because the grip at one wheel/tire is underutilized. It also makes it easier to spin a wheel that has relatively less grip than the other it shares an axle with, making it more likely than you're run out of grip and understeer or oversteer.

Fortunately, you aren't out of luck since there are many solutions for this problem. Stay tuned for a follow up to this!

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