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Thread: why is Nissan racing FWD at Le Mans ?

  1. #21
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    It definitely isn't a publicity stunt or a "just for the sake of being different" type of thing. I think this is the result of some smart guys sitting down with nothing but a 2015 rulebook, a ton of coffee (or maybe cocaine), and no internet connection, and designing the theoretically perfect car for this year's Le Mans.

    The video that jnathan68 posted covers most of it. Things that guy didn't talk much about:

    1. Because FWD prototype cars have basically never existed and because the rules creators probably never thought anyone would make a FWD prototype car, there are some gaps in the rules that heavily favor a FWD layout.

    2. Nissan has said from the beginning that the car is built specifically for Le Mans- a very high speed track with few low-speed technical sections where they'll have to fight mechanical understeer. If FWD can work anywhere, it's at Le Mans. They know they're going to struggle to be competitive at the other WEC circuits, but I don't think they care. They want to win Le Mans.

    3. If you've done much research on how to reduce drag on your own racecar, you know that "putting the air back together" at the rear of the car is the most important part of drag reduction. That's hard to do when you have an engine, transaxle, and super wide tires at the back of the car. Putting the drivetrain in the front of the car and the narrow wheels in the back opens up HUGE potential for drag reduction, which is super important at Le Mans.


    They've been making all of their last minute decisions to favor reliability over speed, so I'll be surprised if at least one of the three cars doesn't finish the race. I'm a huge fan of this car, but I think they have almost zero chance of winning this year. They just haven't had enough time to test and develop the car. If the rules don't get re-written to ban or severely change this car, I think they can win it next year.

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    RaceTape Ninja Force McCocken's Avatar
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    Maybe Renault cribs the data and takes it for their own championship next year?
    Yer pal,
    Force

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    BMW Master bawareca's Avatar
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    Quote Originally Posted by Loose Caboose View Post
    Easy there big fella!
    Not even mentioning this one, one of my all time favorites.

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    Quote Originally Posted by Loose Caboose View Post
    Easy there big fella!

    Attachment 8813
    Your point?
    WWW.949RACING.COM
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    Aside from their cost I never understood why people race them.
    But obviously I just dont get it. -fatbillybob

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    I wanna go fast! thepass's Avatar
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    Tunnels man.....

    Tunnels.
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    what's more shocking is that this thread got to page 2 before William made a FWD joke involving me
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    Copied from another forum.

    Quote Originally Posted by Mark Ortiz
    May 2015
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    LOGIC OF NISSAN NISMO

    It would be interesting to hear your thoughts on the Nissan front wheel drive LMP1 car. Is this mainly for marketing, to take advantage of aero rules, or what?

    Peter Wright has a very good article about the car in the May Racecar Engineering based on discussions with designer Ben Bowlby.

    Evidently, the idea is indeed 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 center of lift/downforce needs to be behind the center of gravity – more so with rear drive than with front drive. If the center of gravity is further forward, the center 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 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 outbrake 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 math. 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 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 outbrake 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 outcorner 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 favor 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.

  8. #28
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    Interesting, logic-based approach. I still wouldn't bet money on that car to finish the race.

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    Quote Originally Posted by emilio700 View Post
    Your point?
    Many Nissans are not hard to look at.

  10. #30
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    I thought the car is a looker from the get go, but with the retro livery...

    le-mans-2015-group-shot-01.jpg

    Nissan goes retro for 2015 Le Mans car livery | Evo

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    I don't think any of the LMP1 cars are particularly attractive.....but the Nissan is the ugliest of the bunch...and the slowest.

    I have no idea what they were thinking.
    Scott Winders
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    It wasnt a FWD car really it was just the RWD is only needed on corner exit over a FWD, once you have aero load a FWD will hook and with the electric motor stuff you can engage the rear wheels on exit (4wd) then let FWD take over from there and keep all the aero benefits of the FWD layout for the aero. Make sense? The only problem was the engine program side totally failboated and they ended up not having the rear wheels driven at all.
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    Le Mans 24 Hours test day: Hartley leads all-Porsche top three - WEC news - AUTOSPORT.com Seems off the pace to me.

    My favorite LMP cars are still the first few editions of the Audi R8. Those things were built like tanks too. Always bumping other cars out of the way without taking any damage.

    Ofcourse the dream series was the FIA GT back in the day with the F40, McLaren F1, Mercdes CLK GTR and the Porsche GT1. We need that back with the new trio of supercars and maybe Pagani, Lambo and some others could join the mix also.

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    1:46.3X @ BRP CW13 - 11/12/15 (400 whp, TD )
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    I am going to be the first to call it:

    Nissan is going to replace the whole front end (engine, suspension, brakes, tires, radiator, etc..) every 3rd stint (front tires will last 3 stints).
    that's like a new car every couple of hours !!! as there is nothing in the back of the car.
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    20 secs behind Porsche currently, 2.5 secs behind P2 leader.
    They sound very optimistic in terms of getting well ahead of P2... if it means anything.
    I'm with Andrew, car had tons of potential playing the rules, but it sounds like engine dept dropped the ball big time.

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    "Fastest is a straight line"

    So were the Minardis back in the day.

    My guess is that they are finding that their downforce generation is very costly, so they've simply trimmed out the car to make the best compromise of what they have.

    Now, if we could just get rid of the chicanes on the Mulsanne Straight...

    Steve

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    Quote Originally Posted by bellwilliam View Post
    I still don't get it ? someone care to explain ? is it because regulations favors FWD. if you have FWD, you are allowed 1,000,000 whp ? . . .
    Their more detailed explanation is primarily based upon aerodynamic advantage.


  20. #40
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    Quote Originally Posted by SteveLevin View Post
    "Fastest is a straight line"

    Now, if we could just get rid of the chicanes on the Mulsanne Straight...

    Steve
    Yes, I would LOVE to see the CFD of the wake behind the Nissan. Significantly less form drag than any of the other cars in its class.

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