The Four Most Common Types of Differentials and What You Need to Know About Them
By Ross Bentley
October 20, 2014
Many fans and enthusiasts talk about or hear pro drivers and their engineers discuss the "diff" or "differential." It's a part of the car that is NOT easily changed and adjusted by the average high performance track day driver, but knowing how the different types of diffs behave can help you understand the handling of your car and what you can do to improve it. Then you can get a smart, experienced driver coach to help you drive around the limitations of the diff that's in your car, or hire a smart race engineer to change or adjust it for you. So what is a diff and what does it do?
When a car goes around a corner, the outside rear tire has to travel further than the inside one, because it's on a larger arc. If the rear tires were connected by a solid axle (think of a go kart), then one tire or both would have to slide against the ground because they have to turn the same RPM. This is bad because you are forcing the rear tires to slide and that reduces the traction. We never want our tires to slide against the ground.
A long time ago, engineers came up with a gear arrangement that allowed the tires to turn at different speeds (depending on the difference in torque placed on the 2 axles because of the different RPM demand). It was simple and is still used today in many road cars and race cars.
This "open diff" worked great on the low-powered cars of the day, but when Mr. Porsche designed the Auto Union F1 car with tremendous power, he found that the open diff would spin the lightly-unloaded inside tire, and stop transmitting all his engine power to the outside tire. The car had reduced drive off the corner. It's just how a open diff works. You likely have experienced this in a street car when one rear tire spins on ice or mud, and as you add more throttle, that spinning tire just spins more and you go no place.
We needed a diff that would limit the slip allowed between the axles, but still allow a speed differential, so we could go around corners without sliding the tires. Nowadays, most race cars use some form of a limited-slip differential (LSD). There are many types and variations on the function and design, from mechanical, to hydraulic, to electronic.
How important is getting the diff set up right? I have a personal story about that. Carroll Smith once came back to the pits from watching his car on track at Watkins Glen and pulled me aside. I was racing against Carroll, engineer versus engineer. "Jeff, your diffs all wrong," he said. What ensued was a 3- or 4-year-long conversation between us about diffs. I am sure he forgot more than I ever knew or will know about diffs, but it had to be the main topic between us for all that time. It never ended. I would see him two weeks later and we never said hello; we just continued our diff discussion. He could watch a car and see what was wrong: how the diff unlocked, how it was in the coast phase, then how it locked up and what the driver was doing in the car to work with or fight the action of the diff. He never told me what to do to fix my diff, he made me figure it out. Getting beat by Carroll all the time also helped me figure it out! THE highlight of my early engineering career was years later at Mid Ohio, when we were both running Indy Lights cars. Carroll came to me and said "your car is great center off, what's your diff set up?" You'd better believe I told him...Carroll Smith wanted to know my diff set up...are you kidding me....how cool was that?
I will give you some links below to excellent articles that describe the detailed operation of many types of diffs, so you engineering geeks can see them and the math that goes along with each. If you really NEED to know how they work, please go read the articles and watch the videos. But for now, let's look at a few you may come across in your high performance driving, and discuss what they make a car do and possibly how you can drive your car differently to work around their limitations.
This is the non-limited slip diff. If you have this in your car, let's hope it's low-powered or has massive downforce. If you feel the inside- driven tire slipping as you apply power out of a corner, and feel like you are going no place until the inside tire gains weight, then forward traction, you have this diff. Short of getting a limited-slip diff, what can you do? You need to reduce the roll of the car and weight transfer to prevent the inside tire from unloading. Stiffer springs and smaller anti-roll bars can help. Drive smoothly and try to transfer the weight slowly and over a long time...keep that inside tire loaded so you can get to power and go forward.
So let's just lock the tires together like a kart. That's called a spool and unless you have a vintage Porsche 962/956 Le Mans car or a Indy car set up for ovals, you likely will not run across this, These things push (understeer) like crazy. When you get off the throttle into a corner and turn the wheel, the rear axle wants to go straight; same on the throttle - there is NO differentiating going on. On ovals, we use a bigger outside tire to make it travel further for each revolution (stagger). To illustrate, take a Styrofoam cup, lay it on its side and push it. It acts like the rear axle with a bigger outside tire and turns to the inside of the corner, that's how stagger works. For an oval, that's great, as we only turn one direction. For road racing, a spool is bad and makes it hard to get the car to turn. You have to really "free" the car up with stiff rear springs, lots of rake, and every trick you have to make the car loose. Driving a spool car is not easy - you have to jump on and off the throttle to break traction on the rear and slide the rear around if you have the power. The reason the Porsche 962 used them at Le Mans is because if a open or LSD diff is in the car when a axle breaks, you are not getting back to the pits. All the power is transmitted to the broken axle and it just spins.The spool drives the other side and you can get back to the pits. Nowadays, axle technology is much better and the performance loss of a spool at Le Mans is too great to accept.
TORQUE-SENSITIVE LSD: SALISBURY RAMP DIFF
This diff is used in most of the current racecars running today. It uses a clutch pack connected to each axle. A ramp compresses the clutch pack and causes the axles to lock, depending on the amount of ramp angle, clutch plate set up, and preload used on the clutch packs. See the video link below for a good explanation of this type of diff. It can be tricky to set up, but get it right and it can transform a racecar. You can control the lock of the axles on the deceleration phase as you turn into a corner, you have control over the coast phase, and the power-on phase. Some very basics to look at are:
- A higher ramp angle produces less locking and a more open diff
- More preload on the clutch plates tend to keep the axles locked more on entry on deceleration
- More clutch plates or friction surfaces make the diff lock harder and sooner on power
As for driving a ramp-style diff, it's all about the throttle. Off throttle and you're on the deceleration ramp angle and using the preload. On power, you're on the power ramp and the clutch friction surfaces. You can trick the diff if you can't change what's wrong, by thinking about what your car is doing and what the diff is doing when it does the bad thing. As an example, if the car pushes the second you touch the throttle, then you have too many friction surfaces in the clutch pack or the power diff ramp angle is to low, causing the diff to lock the axles early and hard. You need them to be free and turning at different speeds to let the car keep rotating from the center of the corner to exit. As a driver, you could try to get your turning done early in the corner so you're straighter, sooner. That way, when you go hard to power and the diff locks, you're already almost straight and will not be asking the car to turn when it does not want to, due to a locked rear axle. Better yet, change the diff ramps.
There are many of these type of diffs being used now. Many incorporate stability control and do all the work for you. Taking car speed, steering angle, throttle position, three-axis accelerations, yaw rate and brake pressure into account, the diff ECU map calculates the amount of axle lock needed 1,000 times a second and optimizes it perfectly. The problem is most of these systems, like the ones found on a Ferrari Challenge car, are mapped to keep the driver safe first, and fast second. So they tend to make the car very stable on entry and push on power. The factories hate it when an owner backs his new expensive super car into a fence on some track day. They would rather he see the fence he is about to hit first! To drive around these diffs or tune the car set up around these is difficult. I have been working on that for a full season this year with a Ferrari Challenge team, and have had limited success. Man, I wish I could call Carroll about this. I know he would be able to help. He always did.
And we just scratched the surface....
Check out these links:
- Jeff Braun