CorvetteForum - Chevrolet Corvette Forum Discussion - [ANSWERED] How to Decipher eLSD Display

The original question is here.

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jcthorne asked:
Can you tell us what the information in the eLSD display screen means and how one might make use of it or understand what its trying to tell us.

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Tadge answered:
Good question, jcthorne. Many people have asked about that display. Electronic limited slip differentials (eLSD) have only been introduced relatively recently, so many are unfamiliar with the details of their operation. The Corvette is a very sophisticated machine and we try to provide instrumentation that is accurate, readable and informative about the hardware. When we first decided to add eLSD to the 7th generation Corvette, we started thinking about how we would inform drivers about its operation and decided an added display might be interesting to some customers.

Generally speaking, we try to calibrate our chassis controls so that they are virtually invisible to the driver. The idea is to integrate our electronic and mechanical systems to the point where the Corvette just feels like a naturally great handling car - intuitive and benign without unwanted interventions that take the fun out of driving.

Normally I write answers on the forum myself and check in with appropriate experts for additions or suggestions. I can't take any credit this time. Our eLSD integration engineer, Jason Kolk, provided a detailed answer below. He is one of several chassis controls engineers who calibrate the chassis controls to great effect on the Corvette.

Also, I would be remiss if I didn't remind everyone that the eLSD does some of its most important work in some pretty aggressive maneuvers. That is not the time to be looking at the display.... First priority is to keep eyes on the road!

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Jason Kolk answered:
eLSD is standard on all Stingray Z51, GrandSport, and Z06 models. The eLSD display (the bottom page in the Performance display group) was added in the 2015 model year to Corvettes equipped with eLSD. There are two pieces of information on the display, eLSD clutch coupling percentage value on top in yellow, and the slip percentage of the rear wheels on the bottom in white shown in the bar graph.

eLSD Percentage in the top/middle:
In the center there is an image of the car, two wheels and the differential. The differential lights up as the eLSD coupling increases. The value shown is a percentage of the full locking coupling capability, and it is the actual value reported by the actuator. When we calibrate eLSD, we work in units of torque. 100% corresponds to 2000 Newton-meters (1475 ft-lbs) of break-away torque (every 1% is 20 Nm (14.75 ft-lbs)). Said another way, while holding one wheel stationary it would take 2000 Nm of torque on the other wheel to make the clutch between the two wheels slip if the display read 100%. For reference a C6 mechanical differential clutch pack was roughly 120 Nm (88 ft-lbs). The actuator is very quick to respond and is able to change from open to locked (0 to 100%) 150 ms (.15 sec) in order to respond to any dynamic situation.

There is a lot going on behind the scenes in the software to come up with the eLSD coupling that you're seeing here. There are a number of algorithms that are running at the same time to collectively decide how much coupling is needed for the different vehicle dynamics situations that they each monitor and control. We have some logic to decide which one of them wins out or which ones add together to deliver the final command that you see on the display and feel in the car.

At the most basic level, the eLSD can have a subtle but profound effect on the handling of the car. We really consider it to be a 'base chassis' component. It's something that plays a big part in setting up the character of the car.

Off-throttle, more eLSD coupling adds stability, but too much can be a bad thing. The eLSD is connecting the two wheels so in a turn it's trying to slow down the outside wheel and speed up the inside wheel. In other words the eLSD clutch coupling is trying to oppose the direction that the car is turning, so setting this off-throttle level is pretty important to keeping the car feeling agile. In a steady turn this can help tune the amount of understeer the car has. In highly dynamic maneuvers, this results in something that we call yaw damping where it will reduce the rotation rate of the car.
When the driver is on-throttle, the eLSD clutch can shift torque from the inside wheel to the outside wheel. This has the combined effect of minimizing or eliminating inside wheel spin, but it also controls how much it feels like the car turns with the throttle. More torque on the outside and less on the inside will help the car turn - to a point, but that's the balance we're constantly searching for while we tune the software.

Each package is tuned individually so a Z06 won't have the same values as a Stingray Z51, for example. Automatic and Manual transmissions have different calibrations, and even suspension and tire packages like FE3 and FE4, FE6 and FE7 do differ from each other.

eLSD is fully integrated with the stability control and Performance Traction Management (PTM) systems.

Note that changing from Tour to Sport to Track has no effect on eLSD mode. eLSD mode does change automatically when the Traction Control button is pressed. No unique input from the driver is required.

eLSD Mode 1 is the standard mode when the vehicle is started. It is optimized for how torque is delivered with Traction Control active and off-power there is an emphasis on vehicle stability. Mode 1 is also used in Performance Traction Management Wet mode.
eLSD Mode 2 is engaged when both Traction Control and Electronic Stability Control are turned off. This calibration provides more nimble corner turn-in, and is optimized for traction out of corners.
eLSD Mode 3 is engaged when Performance Traction Management is in Dry, Sport 1 & 2, and Race modes. Off power this is a nimble calibration with similar functionality as eLSD Mode 2, however, it is integrated to work with Performance Traction Management when the driver is on power.
eLSD Mode 4 is engaged when Traction Control is selected off, but stability control remains on. Vehicle stability is still the priority, while allowing for optimized traction out of corners.

Here are some examples of what you may see if you watch this screen. I'll talk generally, so the numbers may not match exactly what you see, but the trends should be there:

Bleed events. The actuator needs to bleed small amounts air out of the hydraulics every few keys cycles to keep things operating consistently. As a driver you may see a couple of spikes to 100% at very low speeds while going straight. This is totally normal and can only happen in a relatively small range of steering on-center so you won't feel it in tight parking lot maneuvers.
Driving straight down the road, we have some speed based preload to add stability and on-center feel. This is going to be relatively small and you will only see a small amount around 10-15% at highway speeds. You'll notice that when you do steering inputs and simple lane changes that it drops down slightly and then pops back up when you are going straight again. This strategy is to improve steering feel and agility. We can be more open at low speeds than previous fixed clutch packs (C6 was fixed at 120 Nm (88 ft-lbs)), and then add more at very high speeds to add stability.
On larger throttle applies you will see eLSD clutch torque grow, and these could be the largest amounts of eLSD coupling that you'll see under normal circumstances. On track this could go as high as 40-50%. The goal of this algorithm is to maximize rear traction while cornering and tune the feel of how much the car is turning while you're on power.
The largest eLSD coupling will happen under very extreme lane changes and slaloms where we can nearly lock the eLSD clutch to add stability at just the right moments, but open back up to allow the car to steer through double lane changes at just the right times.
If you were to drive your car in the winter and start with one wheel on ice and the other on bare pavement, you may see clutch torque build in response the wheel on ice slipping to keep it under control and to maintain smooth acceleration.

Wheel Slip percentage on the bottom of the display:
The important thing to know here is that this slip display is not directly connected to the eLSD software. This wheel slip display has its own calculation and it's showing the average rear wheel slip compared to the average front wheel slip. It's not showing how much slip is occurring across the eLSD clutch (between the two rear wheels), even though I can see how a driver could make that connection. ` Think about this as showing how much rear wheel spin you're getting at the drag strip in a burn-out box, for example.

There may be rare instances where you see this display flicker when you first start to move or when you come to a stop. Don't be alarmed by that, it's function of the math behind the display when the numbers get really small.