MR + 4WD + Lightweight = Understeer

[kickin_on]

MR + 4WD + Lightweight = Understeer

This all relates to weight, the percentage of the total weight the engine makes up, and the position of that weight in relation to the type of drivetrain layout.

So why the understeer?

Because of the MR layout, there is hardly any weight over the front wheels. This is a problem, as being a 4WD, the front wheels are also being used to drive the car (not to mention steer!), and therefore it is extremely important to have weight over them, so they can have enough grip to actually do their job (i.e. make the car handle well).

Acceleration further intensifies this problem too, as even more weight is taken off the front wheels as it gets transferred to the rear.

Now the important part - Because the car is extremely light, the weight of the engine itself makes up a huge percentage of the total weight. No matter what car, or how much it weights in total, if the engine itself makes up a large percentage of that total weight, the handling of the car is going to be greatly affected, differing in type of effect depending on the position of the engine.
In this case, its near the rear, greatly disadvantaging 2 of the wheels that are used the drive the car and maintain desirable handling, which diddnt have much weight over them in the first place due the the car's overall low weight. This is why the percentage of the total weight that the engine makes up is vitally important, not only in this configuration, but in all drivetrain/engine layouts.

This shows how the fairly rare configuration of MR / 4WD / Lightweight is very tricky to engineer, tune, and drive. As frustrating as it is though, when you get it right, the results are pretty amazing.

An example is the TommyKaira ZZII, which I posted about in another forum, though I feel this derseves its own thread in the forum where more of the engineering tech-heads hang out lol (not that im one of them or anything lol)
Anyone know of any other examples? Ford RS2000? Lamborghini?

Thanks for reading, I appreciate any comments/discussion 👍

 

[uMadson?]

Interesting discovery. I found this in the Lambos; I dunno if 1.5 tons is light enough, but hell the Murcielagos are such whales.

 

[Rotary Junkie]

Personally I think making a lightweight MAWD, regardless of how it drives, is fairly amazing in terms of engineering.

And incredibly stupid. The less weight you have, the less power you need, the less traction you need. Every MAWD "supercar" I've met would be at minimum better to drive (if not faster around a track) if it didn't have the extra weight of the AWD system and power going to the tires that only really need to steer the car.

I'd kill for a RWD Gallardo, even though mine is bloody hilarious to drive specifically due to 700+hp and the AWD setup and relative lack of grip. 90 degree powerslides are fun and I can keep away from them if I want to... It just takes willpower I lack.

 

[XDesperado67]

^^what he said to a certain extent^^

On a racetrack AWD is of limited and questionable value. On a rally car or drifter they make more sense.
Real world driving over roads that can have a variety of problems not typically found on a racetrack they can be an aid to the driver if set up right and driven properly.

 

[F1 VET]

Try setting the center diff to 40% front and 60% rear. Understeer is when the front tires have no grip but the back ones do. Problem solved my friend!

 

[Schadenfreude13]

Personally I think making a lightweight MAWD, regardless of how it drives, is fairly amazing in terms of engineering.

And incredibly stupid. The less weight you have, the less power you need, the less traction you need. Every MAWD "supercar" I've met would be at minimum better to drive (if not faster around a track) if it didn't have the extra weight of the AWD system and power going to the tires that only really need to steer the car.

I'd kill for a RWD Gallardo, even though mine is bloody hilarious to drive specifically due to 700+hp and the AWD setup and relative lack of grip. 90 degree powerslides are fun and I can keep away from them if I want to... It just takes willpower I lack.

The understeer of the Gallardo is something that bugged me almost to the point of obsession. It took me awhile to finally giving up on trying to dial out that understeer. Now i just use the RKM tune and throw it into corners like a hoon without any worries of spinning out

 

[kickin_on]

Try setting the center diff to 40% front and 60% rear. Understeer is when the front tires have no grip but the back ones do. Problem solved my friend!

Im not sure if this will exactly be "problem solved" as the car is still going to have a huge engine weight percentage at the rear.

This concept would also apply even if the car was purely MR. Any lack of weight or pressure on the front wheels during acceleration or turning is going to result in very poor handling.
4WD just means that the front tires have one more thing they are supposed to do, yet will severely struggle with.

Kinda like driving a car with a wheelie tune to describe it really basically.

Any other input, wise GTP members?

 

[V4.SKUNK]

Increase rear and lower front damping to help shift weight of car to front when accelerating hard.

 

[oppositelock]

Because of the MR layout, there is hardly any weight over the front wheels. This is a problem, as being a 4WD, the front wheels are also being used to drive the car, and therefore it is extremely important to have weight over them, so they can have enough grip to actually do their job (i.e. make the car handle well).

Any lack of weight or pressure on the front wheels during acceleration or turning is going to result in very poor handling.

Weight does not increase lateral grip. If it did, people would race dump trucks filled with gravel. (Ok, that was sarcasm, but you get the point) This myth needs to die an ignoble death. The fact that the Yellowbird has the inverse problem should be proof of this. If a mid-engine AWD understeers it's because of the power to the front wheels (see: friction circle) and the front LSD which resists the front wheels moving at different speeds. Personally, my Lamborghinis feel loose as as a goose.

 

[kickin_on]

Thanks for the input oppsitelock, this is an issue thats been milling around in my head for a while so its good to get some discussion on it (even if we disagree, which I do lol)

I say that weight *does* increase increase lateral grip, that is, if we arent dealing with situations involving excessive inertia like drifting and/or bad driving. And is especially important on the wheels responsible for turning.

To your dump truck analogy - I think people WOULD race dump trucks filled with gravel, if that particular amount of weight provided optimal handling for that trucks power, drivetrain, engine position etc etc etc.
The key here is "optimum". Too heavy and its, well, too heavy (you can figure out the implications). Too light and we have the situation outlined in the original post (and quoted in the post above).

Extreme scenarios will always result in poor performance, no matter what end of the scale. My opinion is that in the TommyKaira ZZII, and potentially other Light/MR/AWD cars, the position of the engine and the percentage of the total weight it makes up creates a scenario heading toward a fairly extreme end of the scale, meaning that an increase in weight over the front wheels would, in fact, improve handling.

I reckon V4.SKUNK is thinking well with his suggestion of damper settings to reduce the shift in weight off the front when accelerating.

Lets keep this discussion going guys, I reckon this is an issue which could use a bit more to-and-fro in this forum

 

[XDesperado67]

Just my two cents for what its worth...

It's not really a question of weight its a question of maximizing the cars ability to make optimum use of a tires grip. The perceived benefit of weight added to one end of a car is felt because the car is keeping more of the tires contact patch in contact with the road resulting in more grip. Adding weight however is not the optimal solution to this issue but should more often be used as a last resort if other steps can't gain maximum grip from a given tire type.

On the subject of Dampers being used to control weight shift I think your off. Adjusting the spring rate will act to help control weight shift and also insure that the tires stay firmly planted. The Dampers are for controlling the amount of travel in the springs so you get neither a bottoming out of the springs from over compression nor wheel hop/bounce from the springs continuously hitting maximum extension.
An adjustment in ride height can also affect the vehicles weight shift.
Finally having properly balanced Camber and anti-rollbar settings along with your spring rates will help to maintain the tires ability to place as large of contact patch on the road and keep it there when cornering.

 

[kickin_on]

I would say that it really is a question of Weight. Amount/location/transfer may not be the only *answer* to this problem, but it sure is a major *cause*. Increased weight over the front can be used in conjunction with other tuning practices to provide better handling. Whatever weight that is, it is the optimum weight for that application. Not the stock weight (and its distribution/transfer) as the stock setup is what results in the poor handling issues we're talking about.

Good input on the subject of the transfer of weight. Thats a fundamental factor in this issue, so I'd imagine suspension tuning plays a big role in fixing the inherent problems, so thanks for some great insight into that

I havent got all the solutions, but I can certainly can see the inherent weight problems these types of cars have which greatly impact their handling.

 

[oppositelock]

I say that weight *does* increase increase lateral grip, that is, if we arent dealing with situations involving excessive inertia like drifting and/or bad driving. And is especially important on the wheels responsible for turning.

I suppose you could make a case that in the wonky world of GT physics this might be true, but that's a stretch. In the real world, it's not an opinion, it's a scientific fact. The centrifugal force pushing a kilogram of automobile to the outside of the corner is always greater than the increased friction that that kilo creates at the contact patch. Until someone proves conclusively that GT works otherwise, I'll stick with that.

To your dump truck analogy - I think people WOULD race dump trucks filled with gravel, if that particular amount of weight provided optimal handling for that trucks power, drivetrain, engine position etc etc etc.
The key here is "optimum". Too heavy and its, well, too heavy (you can figure out the implications). Too light and we have the situation outlined in the original post (and quoted in the post above).

The optimum weight for any vehicle is as close to zero as possible. Think of a remote control car which weighs around a kilogram. This RC car will change direction faster and corner harder than any real race car on the planet. If you were to scale up the performance to a full sized car it would probably injure or even kill it's occupants. It doesn't need thousands of kilos pushing down on the tires to make it turn, and neither does a real car. It's not a parabolic relationship between grip and weight, it's more or less linear.

Extreme scenarios will always result in poor performance, no matter what end of the scale. My opinion is that in the TommyKaira ZZII, and potentially other Light/MR/AWD cars, the position of the engine and the percentage of the total weight it makes up creates a scenario heading toward a fairly extreme end of the scale, meaning that an increase in weight over the front wheels would, in fact, improve handling.

It's funny that you use the ZZII as an example, because this particular car has perfect 50/50 weight distribution. Which, oddly enough, likely increases understeer. The Lamborghinis, with their heavy rear weight bias, are far more neutral imo.

 

[brave bear]

For all of my AWD vehicles i drive with my torque diff at F30 R70. That is my "default" setting. Handling the weight transfer for me usually means firming the suspension up and lowering the car. Depending on the car and how it handles or i wish it to handle the front or rear is raised a minimum of 5mm. The front is raised in order to induce oversteer and vice versa. My swaybars are also tightened and that also depends on the car. My gallardo which is the only lambo i have tuned as yet is quite a showboat and loves to get loose in the corners. she generally runs a stiff-ish suspension and a wing to keep her tail quiet. For my lsd settings i lowered the values in the front.

 

[P Irons]

Try the Ballast

 

[shmogt]

Go max out a Lamborghini Murcielago LP 640 2009 and drive it around and see how insane it is just like your saying. Than apply this https://www.gtplanet.net/forum/showthread.php?p=5535816#post5535816 and see how the majority of the problems go away. I will say all MR car and M4WD are the hardest to tune. They always have either a over steer or under steer and sometimes both. I don't think it matters about weight too much tho because the lambo is insane and weights so much while the Evora is just as crazy and weights nothing.

 

[kickin_on]

the optimum weight for any vehicle is as close to zero as possible. Think of a remote control car which weighs around a kilogram. This rc car will change direction faster and corner harder than any real race car on the planet. If you were to scale up the performance to a full sized car it would probably injure or even kill it's occupants. It doesn't need thousands of kilos pushing down on the tires to make it turn, and neither does a real car. It's not a parabolic relationship between grip and weight, it's more or less linear.

Woah hold on a sec man, you make good points in some areas but this part just isnt true im sorry.

When turning, front tires require a certain amount of vertical force on them in order to counteract the lateral forces which want to make the car continue in a straight line (The forces on the tires is lateral as they are turned a different direction to where the car is pointing).

If there is not enough vertical force (weight) on the front tires, the inertia generated from the car as a whole will get to a point at which it overcomes the vertical force and the tires begins to lose grip, and guess what? It understeers!

If this weight was zero, it would require almost zero force to make the tires lose grip. Proved by your point below:

The centrifugal force pushing a kilogram of automobile to the outside of the corner is always greater than the increased friction that that kilo creates at the contact patch. Until someone proves conclusively that gt works otherwise, i'll stick with that..

Now, neither the weight on the font tires, nor the lateral force generated is static. Both change during driving:
- Vertical weight shifted forward and back during accel and braking
- Lateral force generated from cornering

However, the lateral forces will always vary by a much larger margin that the forward/back forces, due to having much more ability to do so through *inertia* (think scandinavian flick, J-turns, drifting, and plain ol' under/oversteer - as opposed to suspension travel).

Inertia is a powerful thing. Because the inertia generated varies depending on the way the car is driven, enough weight needs to be over the front tires to match or counter the varying lateral forces. This is why bad drivers who do not drive smooth and do not follow racing lines are always sliding all over the place.

MR cars struggle with this because of their weight distribution, and adding 4WD into the mix complicates matters even further as 2 of the wheels that are being relied upon to get power down, are already struggling just with getting the car to point in the same direction as they are!

Wow. Epic post lol

 

[XDesperado67]

When turning, front tires require a certain amount of vertical force on them in order to counteract the lateral forces which want to make the car continue in a straight line (The forces on the tires is lateral as they are turned a different direction to where the car is pointing).

If there is not enough vertical force (weight)

Okay here is your problem your confusing vertical force with weight. While weight can be used to apply vertical force it is definitely not the same thing. Just as weight and mass are not the same things but are related.

The vast majority of vehicles both in game and in real life have more than enough weight (term used loosely as mass is probably the more correct term but is measured in pounds or kilograms) over any given wheel to keep the tire and its contact spot firmly planted provided the suspension is properly tuned. That said when the mass of the vehicle creates lateral g forces that exceed the tires ability to generate grip that tire will lose traction. Wider stickier tires have greater grip so they can resist higher lateral forces. When you add weight to a vehicle you are increasing the amount of lateral force that will be applied to the tires which only have a finite amount of grip they can provide.
So your solution should be to tune the suspension properly to fully utilize the tires available traction/grip.

The Porsche 911 is probably one of the best examples of a car that had a very lightly loaded front end. Yet despite this the enginees at Porsche were able to produce a car whose ability to corner was among the best in the world.

 

[oppositelock]

If this weight was zero, it would require almost zero force to make the tires lose grip.

If the vehicle weight is zero, then there's no vehicle inertia, hence no lateral force to cause the tire lose grip (assuming it stays in contact with the road). We can talk about theoretical nonsense scenarios all day long, it doesn't change the fact that every race car designer on the planet leans towards my point of view. I can see you're trying to understand though, and that's good. Some of this stuff is counterintuitive, it helps to forget what you think you know and browse a book or website on vehicle dynamics or race engineering even if you don't understand all of it.

 

[kickin_on]

My brain hurts lol.

Great input Desperado, example of the Porsche is a good one.

I stand by my theory of these weight problems being inherent in MR and 4WDMR cars (and RR ), I suppoes just a case what you can do with the tuning to counteract these characteristics (counteract / manage / work to your advantage)

Adding and/or shifting the distribution of weight will still help this cause, but I can see how other methods may be more effective, making changes to weight distribution more of a supplementary technique used in conjunction with more advanced suspension tuning techniques.

Tricky drivetrain/engine layout to tune eh?

Thanks to opposite lock too - we may disagree on a couple of points but that still just helps everyone (including us!) understand this stuff