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The Delta Wing - NOT Rear-Steering

Article 5 of 7 by Scott Morris
Tuesday, March 09, 2010

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Delta Wing Differential
Before delving into this installment, we do want to say that we are/not formally endorsing the concept at this time, but do feel that the design has been misconstrued by many. We will address a number of these issues in our summary article, but this installment does deal with one misconception: the car is rear-steer.

This was fairly easy to mistake, as it has been mentioned several times that the design of this car incorporates what is known as an "active" differential (rear end)

This is a design that allows the torque to be directed to the wheel of choice, by setup, active computer control, or even control directly by the driver.

Why would this be necessary, or even an advantage? Well, there are a few reasons, but one of them is not traction control.

One of the biggest challenges in a racing car, especially as it applies to oval racing, is that the outside tires on an oval setup will be as much as 1/2 inch larger diameter (taller) as those mounted on the inside wheels relative to the track. This is known as "stagger."

If you ever placed a cone cup on a table and tried to roll it, of course it would not roll straight. It turns toward the pointy side. With the smaller tires on the inside, the car turns more naturally to that direction, which here in the USA would of course be to the left.

The geometric condition is that the outside wheel is travelling a further distance than the inside wheel, so it must be sped up just a bit to keep the rear stable and consistent in handling. If any of you have ever raced karts, you know that you have to drive the kart hard enough to get that inside rear wheel off the ground just a little bit to get the kart to handle well. That is not really a good solution in an IndyCar however.

Input torque is applied to the ring gear (blue), which turns the entire carrier (blue), providing torque to both side gears (red and yellow), which in turn may drive the left and right wheels. If the resistance at both wheels is equal, the planet gear (green) does not rotate, and both wheels turn at the same rate.
Having a tire on one side that is a little larger than the other sounds fairly simple, but is not quite so. Tires are pliable deformable objects that are very susceptible to many external factors such as humidity, temperature and even light that can slightly change the dimensions of the tire. We have all heard of a team at one point or another talk about getting a bad set of tires.

The manufacturers and teams do their best to find tires that match and keep them together as set. This can be a very tricky and time consuming ordeal, and introduces a higher cost and element of possible error.

Normally, IndyCars have, for all practical purposes a limited slip differential. They use this for the road courses, and then add something called a "spool" for the ovals, that essentially causes the two half shafts to turn at the same speed.

All of these factors create an additional cost. Also, there really is very little relevance to road-going cars, giving manufacturers one less reason to be in the sport. The sport needs to be relevant to what they produce and sell. After all, that is where the sport came from.

All road cars by 2012 are required to have an active differential that provides stability control for the driver. Currently, most cars use a system that applies the brakes to one wheel or the other, instead of incorporating an active differential.

Incorporating such a system in a racing car just incorporates one more element that keeps manufacturers interested in racing as a development platform.

If the left side gear (red) encounters resistance, the planet gear (green) rotates about the left side gear, in turn applying extra rotation to the right side gear (yellow).
An active differential can be adjusted such that the outside wheel rotates at a different rate, without the need for fitting specially sized tires. It can be adjusted on-track as well, so the driver can adjust "stagger" through a given run.

We are told that when Andretti heard about this feature, he said "That's a big deal...that is huge."

The other interesting aspect of this is effectively the same effect, but a slightly different application. Since you can adjust the torque from one wheel to the other, you could effectively steer the car much in the same way a boat captain steers a boat with the throttles, or a airliner can be steered with just the engines, by varying power from one to the other.

Imagine how this could be applied by the driver on any track to get the car to turn better. This would even work on a road course.

Applying this measure to tune the car is also going to be much easier on the tires as well. Making various adjustments to camber, toe and caster can have adverse effects on tire wear. Sometimes, you just accept this as a cost of achieving a desirable setup.

With this system, the driver could actually tune the setup of the car on-track. It is a case where the technology doesn’t take anything away from the driving task, and in fact adds a new facet to the required skill set.

The active differential is just another way the Delta Wing represents a revolutionary change in racing technology. The ironic thing is, for a car that is so revolutionary and seemingly radical, it has much more design relevance to a road car than any other purpose built racing car.

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