Chris Wilson Posted May 22, 2015 Share Posted May 22, 2015 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 ~@ all about the reasoning, so I decided to try and gather something more substantive together. The original link: http://www.autoguide.com/auto-news/2015/05/nissan-gt-r-lm-nismo-specs-detailed.html Something 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 tyre load at a greater percentile rate than they add front tyre 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. Maybe. 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 tyres 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 tyres 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 tyre put power down. So Mr. Bowlby has gotten the tyre 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 tyres tend to deliver less rearward acceleration when they are worked less equally. However, when the front tyres are bigger than the rears, the situation is not so bad. The tyres 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 outbrake the rear drive alternative? Answer: yes, at least in the upper speed ranges – provided the front brakes hold out. Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted May 22, 2015 Author Share Posted May 22, 2015 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. Case #1 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. Case #2 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 Case #3 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. Case #4 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 lockup. As in #3, car is decelerating at 3.75g. Case #5 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 tyre 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 tyres 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 tyre 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. So now you know, if you haven't nodded off Quote Link to comment Share on other sites More sharing options...
grahamc Posted May 22, 2015 Share Posted May 22, 2015 Interesting read and I liked the info on the front wheel drive choice, so very interesting stuff in there. FWD yes, but thats not the whole truth.... The engine will drive the front wheels - 550hp and there is an electric motor(s) (which harvest from the front KERS) to drive the rear wheels - 700hp, which all adds up to a tidy 1250hp. http://www.topgear.com/uk/car-news/nissan-gt-r-le-mans-car-2015-02-02 Considering that the GTRs are RWD with an active 4WD (a good system, but keep in mind that there are 2 drive shafts, 1 to the back and 1 to transfer power to the front), the LMP1 system fits in perfectly with the "guessing" for the R36 GTR, which is over 700hp hybrid (engine driving one set of wheels and electric motors driving the other set) - this would then save a LOT of weight on the GTR (albeit they would add it all back with the electric motors and batteries). Also in the above article:- We have used the fact that there is no engine driving the rear wheels to allow us to have a through duct aerodynamic solution," explains Bowlby. "We duct the air that comes from underneath the front splitter - underneath the nose of the car - all the way through to come out above the diffuser at the back of the car. EDIT - Forgot to mention that I saw this car, it was "unveiled" at the Nurburgring 24h Race to guests of Nissan, got to have a good close-up look around Quote Link to comment Share on other sites More sharing options...
grahamc Posted May 22, 2015 Share Posted May 22, 2015 Pic of the front wheels:- Illustration of the front and rear wheels:- The rear diffuser:- Quote Link to comment Share on other sites More sharing options...
Wez Posted May 22, 2015 Share Posted May 22, 2015 Mental machine and nice read, thanks Chris Quote Link to comment Share on other sites More sharing options...
T2 MSW Posted May 22, 2015 Share Posted May 22, 2015 Interesting read. Im looking forward to seeing how it performs as there are a few slow corners in the Le Mans setup now and be interesting to see how the front brakes hold up. I also like the way thats been designed. The turbo setup is ace with the cold air feed behind the cooling for the intercoolers and they are ideally placed to be easy changed should something go wrong. Quote Link to comment Share on other sites More sharing options...
pedrosixfour Posted May 22, 2015 Share Posted May 22, 2015 What if it rains??? Quote Link to comment Share on other sites More sharing options...
BoGlE Posted May 22, 2015 Share Posted May 22, 2015 Great explanation. I love how Nissan/Nismo are always trying something from outside the box! I remember being at le man afew years ago when they ran the 3 wheel thing.(cant remember it's name) It amazed me how fast it was seeing it go round with the lmp1 cars. This will be interesting to watch over the races and I hope it does well. It would really mix things up abit if it does. Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted May 23, 2015 Author Share Posted May 23, 2015 Great explanation. I love how Nissan/Nismo are always trying something from outside the box! I remember being at le man afew years ago when they ran the 3 wheel thing.(cant remember it's name) It amazed me how fast it was seeing it go round with the lmp1 cars. This will be interesting to watch over the races and I hope it does well. It would really mix things up abit if it does. Ben Bowlby, the designer of this car, also designed the Le Mans Delta car (the "3 wheel thing"). When I was more active racing in Special Saloons this young guy tipped up with a Kawasaki powered tiny little Special Saloon based on a small Suzuki car. It looked 90% diffuser, and the rest F3 and a huge effort in weight reduction. Most laughed as it was wheeled through scrutineering. 2 hours later they were no longer laughing. Fast as hell, didn't break, and it greatly upset the old guard. I remember a friend of mine and myself talking in the bar later and wondering who had designed it. Bowlby overheard and said something like "It's hardly "designed", more a fag packet effort for my degree, I have some much better ideas in the pipeline...". The lad was dead right, he had! The Suzuki is shown part way down the page here: http://www.motorsport-fanatic.co.uk/combe-specialGT-assorted2.html Quote Link to comment Share on other sites More sharing options...
lewysdanny Posted May 23, 2015 Share Posted May 23, 2015 Hey mate , great article ( even if most of the facts and figs were over my head ) . Me , Dr Jekyll , and a couple more Welsh cars are off to Le Mans for this years race and will report back on this Nissan and others, if we are capable ! Quote Link to comment Share on other sites More sharing options...
grahamc Posted May 23, 2015 Share Posted May 23, 2015 DeltaWing is not really 3 wheeled http://www.smcars.net/attachments/plans-jpg.140921/ Quote Link to comment Share on other sites More sharing options...
Wez Posted May 23, 2015 Share Posted May 23, 2015 The DeltaWing is quite special, I have seen it a couple of times, some very clever out the box engineering Quote Link to comment Share on other sites More sharing options...
grahamc Posted May 23, 2015 Share Posted May 23, 2015 The DeltaWing is quite special, I have seen it a couple of times, some very clever out the box engineering Nissan seem to be quite good at that aspect.... Quote Link to comment Share on other sites More sharing options...
Kendo11 Posted May 23, 2015 Share Posted May 23, 2015 Although Nissan copied Panoz for the deltawing design The GT-R was discussed a while back at length on jalopnik, it's for some serious potential. Quote Link to comment Share on other sites More sharing options...
Hammer Posted May 24, 2015 Share Posted May 24, 2015 Interesting read. Thanks for the effort put in Chris. Quote Link to comment Share on other sites More sharing options...
Ian C Posted May 24, 2015 Share Posted May 24, 2015 Jolly good writeup Top Gear magazine covered this and actually did a good job of the technicalities of it. They didn't QUITE put in as many case examples though Loving the potential 5G stopping power, doubt even an astronaut could withstand that over a 24hr period Quote Link to comment Share on other sites More sharing options...
Gaz6002 Posted May 28, 2015 Share Posted May 28, 2015 This car is a headline-writer's dream, but the reality is it's not really FWD at all. Clever aero though. Hope they manage to start/finish the race! Quote Link to comment Share on other sites More sharing options...
grahamc Posted May 28, 2015 Share Posted May 28, 2015 This car is a headline-writer's dream, but the reality is it's not really FWD at all. Clever aero though. Hope they manage to start/finish the race! Definitely Quote Link to comment Share on other sites More sharing options...
Ian C Posted May 29, 2015 Share Posted May 29, 2015 This car is a headline-writer's dream, but the reality is it's not really FWD at all. Clever aero though. Hope they manage to start/finish the race! If the front wheels are the driven wheels then I don't understand your assurances there? So it's RWD but just going in reverse? Quote Link to comment Share on other sites More sharing options...
Wez Posted May 29, 2015 Share Posted May 29, 2015 I think its supposed to be 4WD with a hybrid system running the rears, but since that is not ready / working and only the front are being driving then I say its FWD Quote Link to comment Share on other sites More sharing options...
Gaz6002 Posted May 29, 2015 Share Posted May 29, 2015 I think its supposed to be 4WD with a hybrid system running the rears, but since that is not ready / working and only the front are being driving then I say its FWD Yep. All wheels are driven. In theory Quote Link to comment Share on other sites More sharing options...
nolizma Posted June 1, 2015 Share Posted June 1, 2015 Great read! Thanks for sharing! I'll also leave this here: Quote Link to comment Share on other sites More sharing options...
Thorin Posted June 3, 2015 Share Posted June 3, 2015 If the front wheels are the driven wheels then I don't understand your assurances there? So it's RWD but just going in reverse? Exactly that. http://sniffpetrol.com/2015/06/03/nissan-in-radical-le-mans-reverse-plan/#.VW7O1XZVhBc Quote Link to comment Share on other sites More sharing options...
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