May be of interest to some, Nissan's latest Le Mans contender is a front wheel drive car. I was asked off forum why this might be, and I gave the guy a link to a brief article. In hindsight it was a link that told ***[email protected]
all about the reasoning, so I decided to try and gather something more substantive together.
The original link:
Nissan GT-R LM NISMO Specs Detailed
Something better, I hope, to get your teeth into :
Evidently, the idea is to take advantage of the aero rules. I don’t think Nissan intends to promote front wheel drive, as they are not really more associated with that than any other manufacturer is. The car does stand to have a marketing advantage of sorts, however, simply because its unusual design attracts attention.
Of course, it does no good to attract attention, and then fall on your face. This isn’t a show car. It has to work. Just making it novel doesn’t get the job done.
So what is the functional logic of the design, and does it make sense?
It is about the aero rules. The existing rules very strictly control the design of rear wings and diffusers, and are more lenient on front diffusers. The thinking presumably is that the rear downforce automatically limits the front downforce, because if the designer creates too much front downforce with respect to the rear, the car will be aero-loose: it will have high-speed oversteer unless its suspension is set up for understeer, in which case it will understeer excessively at low speed. Therefore any attempt to increase total downforce by increasing only front downforce will be self-defeating.
To get an acceptable understeer gradient at all speeds, the drag and lift forces on the car must add rear tire load at a greater percentile rate than they add front tire load. With rear wheel drive, we need an extra dose of this effect, because in constant-speed cornering at high speeds the rear wheels are using are using a significant portion of their traction circle or performance envelope for propulsion,
and they consequently have less grip available for lateral acceleration. With front drive, we have a similar effect for the drive wheels, but they’re at the front.
In simple terms, the centre of lift/downforce needs to be behind the centre of gravity – more so with rear drive than with front drive. If the centre of gravity is further forward, the centre of lift/downforce can also be further forward. If the rules limit rear downforce but not front downforce, then a nose-heavy front-drive car can have more total downforce without being aero-loose. Voila! More downforce; more grip; faster corner speeds; car wins races.
The kicker is, this advantage has to be big enough to trump the ever-present disadvantages of front drive for a race car. And those are considerable.
The fundamental problem is that rearward load transfer under power works against us with front wheel drive. The car is therefore at a disadvantage for forward acceleration, up to the speed where it becomes power-limited rather than traction-limited.
To minimize this disadvantage, front-drive cars are always made nose-heavy – typically from 58 to 62 percent front. Typically, they also have equal size tires front and rear. The result is that they invariably understeer, even when set up to corner on three wheels.
I read that the Nismo is even more nose-heavy than that: around 65% front. However, the front tires are much wider than the rears. The car reportedly does still corner on three wheels at times, at least in the lower speed ranges where downforce is moderate. That’s as it should be. That helps the inside front tire put power down. So Mr. Bowlby has gotten the tire sizes and roll resistance distribution right. That will definitely help.
In addition, he has gotten the wheelbase right: he’s made it unusually long. That reduces the rearward dynamic load transfer under forward acceleration. The car therefore has the two main characteristics needed to minimize the disadvantages of front wheel drive.
Despite this, the car will still have less of its weight on the drive wheels dynamically than a rear-engined car when powering out of low-speed turns.
The other big drawback of front wheel drive is that the necessary nose-heaviness is a disadvantage in braking. The front wheels have to do most of the work. Due to load sensitivity of the coefficient of friction, the tires tend to deliver less rearward acceleration when they are worked less equally. However, when the front tires are bigger than the rears, the situation is not so bad.
The tires are only one limiting factor in braking. The other main one is the brakes themselves. It is easier to keep the brakes alive if they share the work fairly equally. If the front brakes have to do most of the work, they have to be awfully good to survive an endurance race.
Now, all of the foregoing assumes that the front drive car has similar aero properties to its rear drive counterpart. But what if the front drive car has a lot more total downforce? Won’t it then out brake the rear drive alternative?
Answer: yes, at least in the upper speed ranges – provided the front brakes hold out.
The design team has run simulations that they say support their decisions. I can’t speak to that, but I can do simple maths. Let’s run some quick numbers. These won’t necessarily exactly represent reality, but they will be close enough to illustrate basic principles and relationships.
Rear-engine car of weight W, at low speed, disregarding any aero effects; 60% rear statically; longitudinal coefficient of friction µx = 1.4; c.g. height 1/6 of wheelbase
Forward load transfer = (1.4/6)W = 23.3%W
Dynamic normal force distribution 63.3/36.7
Front brakes need to do about 65% of the work, since the car should be set up so the fronts always lock before the rears.
Similar to #1, but for front-drive car with long wheelbase; 65% front statically, c.g. height 1/8 of wheelbase
Forward load transfer = (1.4/8)W = 17.5%W
Dynamic normal force distribution 82.5/17.5
Front brakes need to do about 85% of the work
Similar to #1, but at high speed, with serious aero: 1.5W in downforce, distributed 30/70, and .5W drag force acting at c.g. height; µx = 1.3
.5W/6 = 8.3%W rearward load transfer due to drag
1.5(.30)W = .45W added to front
1.5(.70)W = 1.05W added to rear
Rearward force at contact patches = 2.5(1.3)W = 3.25W
Forward load transfer = (3.25/6)W = 54.0%W
Front normal force = .400W – .083W + .450W + .540W = 1.31W
Rear normal force = .600W + .083W + 1.050W – .540W = 1.19W
1.31/2.50 = 52.4% dynamic front
1.19/2.50 = 47.6% dynamic rear
Front brakes need to do only about 55% of the work, but it’s a lot more work. Also, if the car has constant brake bias, this will need to be close to 65/35 to avoid rear lockup in lower speed ranges.
Car is decelerating at 3.75g.
Front-drive car as in #2, but with same downforce and drag as #3, except downforce distributed 60/40
.5W/8 = 6.3%W rearward load transfer due to drag
1.5(.60)W = .90W added to front
1.5(.40)W = .60W added to rear
Rearward force at contact patches = 2.5(1.3)W = 3.25W
Forward load transfer = (3.25/8)W = 40.6%W
Front normal force = .650W – .063W + .900W + .406W = 1.89W
Rear normal force = .350W + .063W + .600W – .406W = 1.19W
1.89/2.50 = 75.6% dynamic front
1.19/2.50 = 47.6% dynamic rear
Front brakes need to do about 77% of the work if the car has active brake bias control. If not, they still need to do about 85% to avoid low-speed rear lock up.
As in #3, car is decelerating at 3.75g.
Front-drive car as in #2 and #4, but now let’s suppose that we have the same rear wing and diffuser as in #3, and we get 60/40 downforce distribution by adding front downforce. Let’s suppose that the added front downforce acts slightly forward of the front axle, so that net rear downforce is slightly diminished, even though the rear wing and diffuser are making the same forces. Let’s also suppose that both configurations have a similar lift/drag ratio. We now have 2.5W downforce total, 1.50W front/1.00W rear, and .8W drag. That’s a lot more tire loading, so let’s suppose that µx = 1.25.
.8W/8 = 10.0%W rearward load transfer due to drag
2.5(.60)W = 1.50W added to front
2.5(.40)W = 1.00W added to rear
Rearward force at contact patches = 3.5(1.25)W = 4.38W
Forward load transfer = (4.38/8)W = 54.7%W
Front normal force = .650W – .100W + 1.50W + .547W = 2.60W
Rear normal force = .350W + .100W + 1.00W – .547W = .90W
2.60/3.50 = 74.3% dynamic front
.90/3.50 = 25.7% dynamic rear
This hypothetical car is decelerating at 5.18g! It will clearly out brake the rear-engined car with the same rear wing and diffuser – provided we can keep brakes and tires under the thing, and provided
the driver’s eyeballs stay in his skull. It will also out corner the rear-engined car, except perhaps at low speeds
Again, these are hypothetical examples, presented to illustrate general principles. But it should be apparent that, at least in theory, the front drive approach does make sense if it buys us a big total downforce increase.
I am reminded of another great exercise in outside-the-box thinking, the Chaparral 2J “sucker car” of 1970. It achieved more downforce than its competitors, by using powered evacuation of the underside of the car. It was wicked fast as a result – but only for a few laps. Then the brakes would quit.
Now we have carbon brakes, which didn’t exist in 1970. Will this technology make it irrelevant whether the rear brakes do a substantial amount of the work? Will it mean that tire grip is now the only thing limiting braking? I guess we’ll find out.
Is the Nismo uniquely suited to Le Mans, and uncompetitive elsewhere? Actually, I would expect that in its current state, the Le Mans circuit is less suited to this car than it would have been years ago. Lots of chicanes and wiggles have been added to keep speeds down. There is now much more low speed braking and forward acceleration in a lap than there used to be. The sort of track that would really favour the Nismo would be one where a large portion of the lap is spent in high-speed cornering, and there is relatively little need for low-speed braking or digging out of slow turns – a track with a lot of sweepers, like Spa in the old days, or like Goodwood. Or maybe, Indianapolis – the "rectoval" part, not the infield part.
One other thing is important to note about the Nismo: it was not originally conceived as a pure front-drive car. The idea is to have a kinetic energy recovery system (KERS) braking and powering the rear wheels. The car will run without that this year because it isn’t ready yet. So the car will be an interesting case study in the possibilities and limitations of pure front wheel drive, but actually that was not the original design intent.
I hope you find this useful, if you're still awake.