winders

Yes, but the dynamic "wedge" is different than static "wedge". It is equal in left and right hand turns. Without that difference, you would not be altering the handling dynamics of the car properly for left and right hand turns. You can't equate this to changing the static cross weights or at least I don't see the correlation.

So I guess I don't understand the point SundayDriver is trying to make. I want to....I just don't see it.

Scott

winders

I hang my head in shame. It's been a few years since studied this stuff and I realized that I have been misusing the term "roll couple".

Roll couple: A force equal to the lateral force times the distance between the center of gravity and the roll axis exerted on the sprung mass.

I should have used "roll couple distribution" or "roll couple percentage" instead of "roll couple". Changing roll stiffness on one end of a car does not change the roll couple, but it does change the roll couple distribution.

Scott

Larry Herman

You know, I was just about to nail you on that one.

SundayDriver

I was a bit too quick to move away from the aero example. I is valid and here is why.

Car A - 2000 lbs cornering at 1G. 2000 lbs (force) downards and 2000 lbs (force) lateral.

Car B - 1000 lb aero car cornering at 2 Gs. Assume linear tire grip curve so the car is making 1000 lb of downforce. 2000 lbs (force) downards and 2000 lbs (force) lateral.

The tire sees exactly the same loads in this case.

J richard

Car B only has a 1000lb horizontal component tho...

SundayDriver

The point is that it does not. It is a 1000 lb car being 'pushed' at 2 g's. The lateral (horizontal) force is equal to the mass times the g load - it is 2,000 labs because of the higher g and corresponding higher speed.

Similarly, if you take the 2,000 lb car and corner at 0.5 G's, it does not have a 2,000 lateral load, it has 1,000 lbs. Take it all the way to the extreme, of 0G (straight line) and the lateral load is 0.

J richard

The g force is only additive to the vertical component if you are on a banked curve for 2g component it would need to be a 30deg banking, without the banking the horizontal component is equal to the mass x centrifugal force which the aero downforce doesn't enter into, it adds force but not mass.

SundayDriver

Let me try again. The additional downforce allows the car to go faster around the corner -we are USING the downforce to corner faster. The centrifugal force is, therefore, higher as is he lateral load. Car A takes the corner at 100mph but car B takes the corner at 141.4 mph. That is the difference between 1 and 2 g's.

sig_a

---------------
Here is a simple description of what's happening in body roll.

F. Puhn --"The total roll couple (f+r) acts to rotate the sprung weight around the roll axis, and this motion is resisted by the roll stiffness of the suspension. Notice that the total roll couple can be resisted by ether the front suspension, the rear suspension or a combination of both."

Larry Herman

Ok, here is the article - edit away!
---------------------------------------------

Tire Grip

One of the biggest difficulties in understanding how to improve or adjust the handling of one's car is the comprehension of grip: what it is, how it works and how to work with it. Grip is derived by placing weight (or load) on the tire in a vertical direction, which will produce cornering ability in the horizontal direction. When I talk about balancing the car to an advanced group of drivers, I like to present the basic premise, devoid of aerodynamics or other influences, that a car has the most grip when it is sitting in the paddock! Pretty profound huh? Each axle has fairly even weight on both sides; therefore the tires have the same pressure against the tarmac, so they are both contributing equally, providing maximum grip. Why is this so? Because of the fact that as you increase the weight (downward loading) on a tire, its grip (ability to resist sideways loading) will increase as well but not quite as much in relation to the load. Here is a graph to illustrate what I am talking about. The numbers are purely for demonstrative purposes.

Larry Herman

This phenomenon of diminishing returns is why a car when cornering hard, transferring much of its weight onto the outside tires, actually has less overall grip. It is also the biggest reason that light cars generally handle better than heavy cars.

This single most fact is what balance is all about, and why a change in roll rate, either accomplished through springs, sway bars or even through shock dampening can make a profound difference in the understeer/oversteer characteristics of a car. As the car accelerates, brakes and corners, it is constantly transferring weight from back to front, and from side to side. This changes the respective balance of grip from front to rear, and creates the tendency for over or understeer. And depending on the car’s front and rear roll rates, and the amount of weight being transferred, the change in grip can be pretty dramatic.

Let’s take a simplistic example to illustrate what I am trying to explain. A Boxster or Cayman is a fairly neutral car with about a 50/50 weight distribution and similar sized front & rear tires. If we were to drive this car around a skid pad, as our speed is slowly increased, more weight will be transferred from each inside tire, to each outside tire until they get to maximum grip. If the we continue to feed in a little more speed, the slight shift of weight on the back tires should provide enough extra grip so that the front will start to slide (understeer). If we lift off even infinitesimally, the weight will shift back towards the front and the rear will begin to slide (oversteer). Most drivers have a pretty good grasp of this. But now how do we tune this so that we can still have a little oversteer as we are adding throttle? By adding roll stiffness to the rear, or reducing it at the front. I am a big believer of adjusting the end of the car that I want to control, so if I want to add oversteer while the car is accelerating, I will stiffen the back end.

Why does increasing the rear roll rate do this? In my classrooms many hands get raised; “because it transfers more weight to the outside tires” is the typical answer. And that would be incorrect. Remember that there are only 3 things that affect the total amount of weight transfer in a car: #1 vehicle weight, #2 center of gravity and #3 track width. So what is going on here?

Let's take our Cayman example whose corner weights are:
800 800
800 800

Imagine that we are in a right hand corner that transfers 1000 lbs total, with equally stiff front and rear springs, the corner weights would be:
1300 300
1300 300

Now if we change the rear roll stiffness either through sway-bar adjustment or spring rate such that 70% of the total weight transfer (or 700 lbs) happens at the rear, the corner weights would be:
1100 500
1500 100


See how the balance has changed from front to rear, and the weight differential across the front is much more even than across the back? It is pretty clear that the front now has a lot more grip and the rear a lot less. This car is going to oversteer bigtime!

How does this rear to front weight shift occur? Increasing the roll stiffness at the back of the car will help keep the overall platform more level, rather than having it “sit down” on the outside rear tire. This keeps more weight on the inside front tire, thereby increasing the grip at the front of the car. You can try it yourself with a simple test. Take your iPhone (or something similar) and pretend it is the car. With the phone long ways, hold each end with your thumb and forefinger on each corner. Lift up your thumbs, tilting it up on one side slightly to simulate a car leaning. Now gently lift the fore-finger that you imagine is outside rear corner. See how it immediately pressures the inside front, across the diagonal? This “leveling of the car” is what transfers load from the inside rear to the inside front, reducing grip in the back and increasing grip at the front. It is important to remember that we have not changed the total amount of weight transferred, we have just shifted it forwards.

Keep in mind that what I am explaining here is not the definitive word on all of the forces in play, but a primer to help with getting a grasp of the basics. I hope that this provides a little clearer idea of what is happening with grip and balance.

claykos

But tires are not linear. In real life, the 1000 lb car with 1000 lbs aero (2000 lbs vertical load on the tire), can probably only corner at 1.8 or 1.9 G if 1G was maximum cornering with 1000 lbs vertical load. If you are making your weight in downforce it doesn't mean you can corner twice as fast, it means you can a little less than twice as fast. Which is a whole lot better than if we just added weight to the car!

Your logic states that if a 1000 lb can corner at 1G max, then a 2000 lb car can also corner at 1G max on the same tires.

Larry: The effects you write about are generally correct, but I don't really like the way you explain what is going on as there really is not any diagonal weight transfer from outside rear to inside front (I mean look how much more weight is on the outside rear in the second case...). The iphone, for me is not a very good example because it is a totally different phenomena. The reason you feel the weight shift with the iphone is just because of gravity as you are changing the angle, which is not the effect with changing spring rates.

However, it does get the main ideas across, as in stiffening the rear reduces grip at the rear AND increases grip at the front...

Maybe I am just too much of an engineer so I am bothered by things that aren't 100% correct!

SundayDriver

If you reread what I said, I was making a point that weight does not equal lateral load and specifically said this was a tire with a linear load curve. Yes, it is imaginary and read tires don't act that way. But sometimes we have to hold certain things constant to make a point or try to explain something.

This whole part of the thread started with someone said (I think) that the fall off in grip is drastic and I disagreed. We are dealing with words that are not precise, but my interpretation of 'drastic' is on the order of 1000 lb capacity dropping to 200 or 500 pretty suddenly, as opposed to dropping to 800 in a more gradual manner. I have data, from an aero car, that shows the later to be the case and various folks have said it is not valid because the lateral load is always equal to the car weight, which is simply not the case as the aero car goes faster and makes more lateral load from the increased speed.

The other thing being missed (Larry you state this in the article) is that many seem to assume that max grip is at the very lightest load. Tires are engineered and sized to product max grip at something greater than the static load at a corner of the car. If we do it right, the car with no weight transfer does NOT have the max G capacity.

Larry Herman

I agree that the iPhone example is not accurate, because you actually are transferring weight across the diagonal there, but it does serve a conceptual purpose. In my explanation though, I do not put forth diagonal weight transfer, and even used your example (albeit a tweak) to show how the outside weight transfers rearwards, and the inside weight moves forwards. Sometimes to be 100% accurate will totally obscure the basic premise for the masses.

Thanks for the input. Anything else that maybe I should address?

SundayDriver

Here is my tire explanation...

Race tires (and all tires to some extent) have three factors that affect grip (ability to take lateral load). I am going to use the example of someone standing on a high bank of a race track. Stand there with new leather soled shoes and you slip and tumble down the track. There is not enough COF to stand there.

Now change to rubber soled shoes, but it is hard rubber. You still can't stand there but you have more grip. That is traditional COF.

Now let's switch to a soft rubber. Not only have we gotten and increase in traditional COF, the soft rubber starts to mold itself into the pattern of the tarmac. We have added a velcro like effect and have even more grip.

A person that weighs 150 lbs will not push the rubber into the pattern of the tarmac as much as the person who weighs 250 lbs. The heavier person has more grip. So this leads to rubber (tire) choice. Let's now give the smaller person smaller feet and/or softer rubber on the shoe and they will push the rubber into the tarmac pattern and get as much grip as the heavier person.

But there are diminishing returns in the real world because the rubber is slipping. So if we add load, at some point the grip starts to fall off.

So here is a question - are you going to choose a tire that gives max grip at the static load or one that maxes out at a higher load? If you want to go faster, you choose a tire with higher max load point. SO as the car rolls (or other load transfer) the grip actually increases on the outside wheel, to a point. At the same time it is dropping on the inside tire, but since it has much lower normal force, the overall impact is less.

In Larry's article, if we have a tire that max's at 1300 lbs, no matter how much the 300 lb tire loses, we will still get max cornering force. Again, real world - there is far more going on so we have other trade offs, but it shows the idea.