Why GT-R extra weight isn't good for cornering

[cmhs75]

A while back an argument was made that the extra weight of the current Nissan GT-R helps it create more grip for cornering whereas a Formula 1 car, being so light, can rely on downforce for creating grip so it can be as light as they want it to be. (For those interested in the thread: http://www.nsxprime.com/forums/showthread.php?t=128804&highlight=nissan+GTr+interview)

I was recently reviewing some old material and I remembered the GT-R argument and decided to have a go at it, so for those interested as well, here is my analysis:


Looking at the force diagram shown here, there are 3 main forces at play while cornering:

Fcentripetal = Centripetal Force (the force that wants to push your car out of the turn)

Ffriction = Friction Force (the force that stops the centripetal force from pushing your car out of the turn)

Fn = Normal Force (The force that pushes your car downwards)



The frictional force (grip) does truly benefit from a heavy car due to its linear relationship to the mass of the car but at the same time the centripetal force also increases by the increase mass of the car. Moreover the centripetal force is quadratically related to velocity so the faster you go into a corner the worst the situation. So in conclusion to that, more weight will create more grip but this gain will be nullified by higher centripetal forces.

The above case is for any car in general. For the case of cars with high aerodynamic downforce (i.e. F1 cars) the disadvantage of heavy weight is even clearer. F1 cars rely on their weight for grip as well as downforce. So the total grip increases dramatically due to the extra component of downforce increasing the normal force and thereby increasing grip. But while this downforce increases grip, unlike weight, it has no effect on the centripetal force at all. So you successfully increase grip while not increasing centripetal force and that is where the power of aerodynamics shine.

So in conclusion, no you don't want more weight on your car for extra cornering grip. But in defense of the GT-R team maybe they hit an odd balance of compromises at exactly the weight they had. Who knows.


Disclaimer:
I in no way post this as a bash thread against the GT-R or against Mr. Kazutoshi Mizuno, both of which I respect greatly. Also everything here is my take on this so I might be totally wrong.


And please if anyone knows how to post the attached picture blown to size in this thread, please do so, I have no idea how to do that myself.

 

[Moses]

 

[bboxer]

Thanks for bringing this subject back to the front page. While I certainly do not want to question your theory and logic nor question the 2009 remarks of Primers including our resident racer celebrity, I would like to chime in modestly as an GTR owner (3 years now) and an ex FIA/SCCA racer. I don't pretend to fully understand the dynamics of the GTR on a road course. I have nevertheles experienced first hand its phenomenal ability to mix composure, traction with aerodynamics to allow that incredible capacity to lap a given track faster than most.

I see things in a much simpler context: three areas where the GTR excels:

1. Approaching and entering a corner: The GTR is amazingly composed under deceleration and/or braking. Perhaps its weight helps it in this phase. Its quick gear changes, guaranteed RPM blips and excellent braking make for a smooth apex approach. Its composure also helps avoid overbraking and helps it carry good speed thru the apex.

2. Exiting a corner: one word: ATTESA. Traction is unique. A little understeer turns into a neutral trajectory with the application of more power. The car seems to go where you want it to go under increased acceleration.

3. Straight line behavior: Weight/power ratio, traction and aerodynamics (one of the best cd's on the road) all make for great acceleration and low drag at high speeds.

From my simple perspective that does it ! Most people who drive a GTR for the first time in anger will quickly express the view that "it feels so much lighter than it ireally is ". Most are a little uneasy about the car being a little counter-intuitive but that disappears after a few laps and a glance at the stopwatch.

Mizuno has certainly proven his theories. He had even envisioned a great improvement 3 years down the road and he has certainly delivered. I have driven quite a few cars at the Ring and can still not fathom how the 2011 GTR can do a 7:24 on a partially wet track.

 

[dayABR11]

Sorry, but that analysis is flawed. Great try though.

Take this into consideration when not looking at downforce (which doesn't include mass anyway):

You slide when Fmaxfriction < Fcentripetal
when
uFn < mv^2/r
umg < mv^2/r

well the m's cancel out, so you are left with
ug < v^2/r

The mass makes no difference in how hard you can corner. Now, if the mass was squared instead of the velocity, then it would have a huge effect as it would leave a mass variable in the Fcentripetal, but that's not how it works.

What it does make a difference is in the total force and Kinetic energy: The greater those numbers are (increased with increased mass), the greater the wear and tear on parts that act on those forces. Tires wear faster, brakes wear faster, suspension parts are under greater stresses.

The mass also obviously slows acceleration and makes deceleration more difficult as the KE level at a certain velocity is higher.

That's why a Lotus Exige isn't super super awesome compared to the GTR. Now if the Lotus had the same brake components and tires, it would see significantly longer wear on those components.

Also, momentum, which dictates transitions and such would be better in a lighter car. Of course with the correct chasis set up, spring rates, and such, that can be negliable. Think of a scrawny computer nerd and a top football player. The scrawny computer nerd would have much less momentum when transitioning in side to side transitions, but I'll bet the heavy but properly built football player will be much faster.

 

[Midnight_Raven]

Sorry, but that analysis is flawed. Great try though.

Take this into consideration when not looking at downforce (which doesn't include mass anyway):

You slide when Fmaxfriction < Fcentripetal
when
uFn < mv^2/r
umg < mv^2/r

well the m's cancel out, so you are left with
ug < v^2/r

The mass makes no difference in how hard you can corner. Now, if the mass was squared instead of the velocity, then it would have a huge effect as it would leave a mass variable in the Fcentripetal, but that's not how it works.

What it does make a difference is in the total force and Kinetic energy: The greater those numbers are (increased with increased mass), the greater the wear and tear on parts that act on those forces. Tires wear faster, brakes wear faster, suspension parts are under greater stresses.

The mass also obviously slows acceleration and makes deceleration more difficult as the KE level at a certain velocity is higher.

That's why a Lotus Exige isn't super super awesome compared to the GTR. Now if the Lotus had the same brake components and tires, it would see significantly longer wear on those components.

Also, momentum, which dictates transitions and such would be better in a lighter car. Of course with the correct chasis set up, spring rates, and such, that can be negliable. Think of a scrawny computer nerd and a top football player. The scrawny computer nerd would have much less momentum when transitioning in side to side transitions, but I'll bet the heavy but properly built football player will be much faster.

You're almost there but you’re downplaying the affects of weight a bit.This is a dynamic situation we are looking at. In real life the grip is not identical amongst each “individual” tire, and the center of gravity of the car is subjected to moments and that's where the downfall to weight lies.

Your tires are the only thing that determines how much force the car can apply to the ground. Sum up the grip force that each tire is able to provide at that instance and that's what simplified force is counteracting the centripetal force acting on the car. The problem is that when your cornering that centripetal force is also applying a moment about the most outward contact point of the car. This causes the car to lean outward and you are losing grip from your inside tires due to the transfer of weight. Therefore the more weight your car has the less grip you'll have because you are losing the extra grip force you could have had from your inner tires. Your two outer tires grip should increase but at a certain point the force applied to it overcome the two "individual" tire’s grip force and you lose cornering performance. That’s only consider moment about its front to rear axis. Considering that you also have moments about its side to side axis and that shows you that you can severely reduce grip just by having uneven distribution of weight due to the mass of the car. You could tune the suspension and stiffen it up to keep a planted stance but you will still be slower around a corner with a car that has the exact same setup with less weight because of the reason mentioned above. It might not be a significant increase in corner performance but still it will be faster.

I had an illustation but I screwed it up lol. I'm kind of in a hurry to leave so I'll get back it later.

 

[dayABR11]

You're almost there but you’re downplaying the affects of weight a bit.This is a dynamic situation we are looking at. In real life the grip is not identical amongst each “individual” tire, and the center of gravity of the car is subjected to moments and that's where the downfall to weight lies.

Your tires are the only thing that determines how much force the car can apply to the ground. Sum up the grip force that each tire is able to provide at that instance and that's what simplified force is counteracting the centripetal force acting on the car. The problem is that when your cornering that centripetal force is also applying a moment about the most outward contact point of the car. This causes the car to lean outward and you are losing grip from your inside tires due to the transfer of weight. Therefore the more weight your car has the less grip you'll have because you are losing the extra grip force you could have had from your inner tires. Your two outer tires grip should increase but at a certain point the force applied to it overcome the two "individual" tire’s grip force and you lose cornering performance. That’s only consider moment about its front to rear axis. Considering that you also have moments about its side to side axis and that shows you that you can severely reduce grip just by having uneven distribution of weight due to the mass of the car. You could tune the suspension and stiffen it up to keep a planted stance but you will still be slower around a corner with a car that has the exact same setup with less weight because of the reason mentioned above. It might not be a significant increase in corner performance but still it will be faster.

I had an illustation but I screwed it up lol. I'm kind of in a hurry to leave so I'll get back it later.

Right right, true enough. I was illustrating from a simple newtonian physics view point. I mean down to it there are thousands of factors the weight will affect. Tire hysteresis, faster heating of materials, contact patch shape, etc etc. I mean, we're not gonna cover everything in a forum.

 

[bfrank1972]

Right right, true enough. I was illustrating from a simple newtonian physics view point. I mean down to it there are thousands of factors the weight will affect. Tire hysteresis, faster heating of materials, contact patch shape, etc etc. I mean, we're not gonna cover everything in a forum.

Well right - but the way I read your other post is "The mass makes no difference in how hard you can corner". Not true, and Midnight was pointing out why. It's not just increased wear, and it's not just a centrifugal force vector on the plane of the tire contact patch. There's vertical center of gravity, tire flex, chassis flex, suspension flex, loading/unloading of the suspension,etc. All components that can affect how hard you can corner. Nissan went to great lengths to mitigate these factors in an unusually heavy 'sports car', and were very successful. There's more than one way to skin a cat, but in the end, increased weight certainly affects handling in a negative way.

 

[cmhs75]

First of all, Thanks to every single one of you for your replies. They were all fruitful.



But I was also hopping someone would comment about my second point (i.e. the downforce increases the normal force and so increases grip while having no effect on the centripetal force)


Am I right about this point?

 

[bfrank1972]

Well actually I believe it does affect centripetal force, but not the centrifugal force.

Centrifugal force describes the inertial forces applied outward on a mass rotating around a central point. Basically anything that is traveling in a curved path will exhibit inertial resistance to the change in direction as it travels along the curve. This is centrifugal force.

Centripetal force describes the force required to counter the inertial forces to keep that object on the curved path. Centrifugal and centripetal forces are related but not the same.

Basically, in a car, centripetal force is generated by the friction of your tires on the pavement, to counter the centrifugal force generated by the car's path.

So to answer your question:

Downforce CAN affect centripetal force. A car turning but not at the threshold of grip is generating enough centripetal force to counter inertial and continue on the driver's intended path. When the tires begin to lose grip, they can no longer provide sufficient centripetal force to counter the car's inertial - its tendancy to keep going in the direction it is currently traveling (hopefully not toward that tire wall). Add downforce and you increase that threshhold, so at the limit, dowforce can increase the centripetal force, increase capability to overcome greater inertial, which translates into greater cornering capacity.

Downforce does NOT affect centrifugal force - that is related to the mass and velocity of the car alone.

Downforce diverts some of the force from air resistance (caused by forward motion of the vehicle) to press the car to the pavement, like a big invisible hand pushing down. Doesn't affect mass though (other than the added weight of the wing ).

Caveat: Everything above regarding a car is very simplified. Everything above assumes the car is riding on a tiny but very very sticky point contact patch that doesn't change shape. Real cars ride on 4 independant contact patches under constant change. Which is why setup and driving a real race car is a whole science (and art).

 

[heathbar0]

Whoa!




I'm sure this is interesting to some.... but...uh, Wow.

I think I would rather watch 2 guys fish.

 

[bfrank1972]

Whoa!




I'm sure this is interesting to some.... but...uh, Wow.

I think I would rather watch 2 guys fish.

What can I say, I like knowing why my car can turn

 

[tof]

I don't want to Google or trying to recall my high school physics let me just add....

The GTR is heavy because all that technology, luxury, and safety is heavy. I'm pretty sure the GTR is as light as the engineers at Nissan could make it while still achieving their targets for performance, comfort, and safety. I don't think one ounce was added as "ground hugging weight", as the old Buick ads once described it.

The GTR handles like it does not because of its mass but in spite of it.

 

[BD]

Long live the R34!!! Best GTR ever!

 

[bboxer]

Long live the R34!!! Best GTR ever!

Fair enough. Now can you give us a good and rational reason why. Thanks

 

[socalx]

best looking gtr for one !

 

[bboxer]

best looking gtr for one !

That is subjective Rurherford. Some people think Pamela Anderson is a beauty queen. Some dont. On a more serious note i was lucky to test drive a red NNA R34 before it got crushed the following evening. Nothing special IMHO. Certainly outdated and vastly outperformed by the R35. It tickles a lot of poeple' fancy mostly because it is unnatainable.

 

[BD]

Fair enough. Now can you give us a good and rational reason why. Thanks

- R34 is a Nissan, not a Renault. It is a Skyline GTR, not just a GTR - Legacy factor.

- R34 with minor tweaks can easily reach 500 plus HP while maintaining OEM reliability.

- R34 is lighter, roomier, cheaper (when new, about half the price of R35), and with a no restriction ECU. I cannot imagine when you buy a car, Nissan knows exactly how the car was driven simply by plug in the computer. That is a form of intrusion.

- It is a better looking car (for most people).

- R34 is the final evolution of R32 and R33, so it progressively evolved into perfection. R35 is essentially a different car, with a different engine.

Finally, it seems like independent tests all rank that car less than what Nissan has presented and/or want you to believe. When that car goes head to head with ZR1 and 911 Turbo, it has to take a back seat.

Don't get me wrong, when R35 was introduced, it was a hell of a bargain. After the price increase and the discovery of the transmission fragility, at least to me, that car is just another over presented, under engineered car, yes, under engineered. Nissan did not put all the gizmos on that car because it is a technological show case, they did that to off set the basic design flaw - An obesity problem, which is a big no no in sports car design. The foundation, the blueprint, and the approach of the design is all wrong.

Most of the cars will grow on me even with initial negative impression. Somehow the R35 never did anything for me. Same goes with Porsche Boxter/Cayman, Mini Cooper, and Dodge Viper.

It may have the "cool" factor for nerds and geeks, but it is an empty suit.

Unlike the NSX, regardless of the value, real car enthusiasts cannot deny the fact that the NSX is how a sports car should be designed and built. When you look at the R35, you are looking at a big blob of metal with a major identity crisis.

 

[bboxer]

Thank you for your valuable analysis. You have convinced me.

 

[powderballz]

I'm too lazy to read everything above of me but I am almost positive that this point has not been made.... Tires achieve maximum grip when a very specific load is applied, in the case of the F1 car, the mass of the vehicle alone cannot achieve that load on the tires, aka under-loading the tire. For the GTR however, the opposite will be in effect, the mass of the car is too great and over-loads the tire.


THE ONLY WAY the GTR will corner better with more mass, is if the tire is designed for a higher load than a less massive car can produce.

As for mass vs. cornering..... The equation for centripetal acceleration(cornering speed) is Ac= (v^2)/r. Mass is not a part of the equation. Mass does not matter, the only thing mass effects is how much you load the tires.

 

[tof]

I'm too lazy to read everything above of me but I am almost positive that this point has not been made.... Tires achieve maximum grip when a very specific load is applied, in the case of the F1 car, the mass of the vehicle alone cannot achieve that load on the tires, aka under-loading the tire. For the GTR however, the opposite will be in effect, the mass of the car is too great and over-loads the tire.

THE ONLY WAY the GTR will corner better with more mass, is if the tire is designed for a higher load than a less massive car can produce.

As for mass vs. cornering..... The equation for centripetal acceleration(cornering speed) is Ac= (v^2)/r. Mass is not a part of the equation. Mass does not matter, the only thing mass effects is how much you load the tires.

Your formula is for calculating the rate of acceleration. This has nothing to do with the limits of V in an arc or the side vector force imposed on the body moving in that arc. In other words it does not address how fast a car can move through a corner. That variable IS a function of mass, coeficient of friction, curve radius, etc.

As to your other point, what do you mean by "overload the tires?" Is this related to tire deformation vs. grip or something along those lines? To me a tire is overloaded when it ceases to maintain a managable slip angle.