Calculating TLLTD for non rocket scientists?

[adambrouillard]

I've been trying to find out how to calculate TLLTD from various sources and I can't figure out the correct method.

Some methods seem to only use the difference between the CG and roll axis at the CG and others seem to take into consideration the CG at each end of the car. Millikens book is probably the best method, but I can't figure out the equations.

I know my wheel rates, roll centers, and overall CG.

Could someone explain in simple terms how to calculate the TLLTD from this?

[Goran Malmberg]

There are only one Centre of gravity, so in any case tis is the starting point.
The rest is depending on what you want to know. You may be looking for the deflection, the weight transfer or the jacking effect. It is a rather complicated situation and this is probabley the reason for you not to see any straight formula. It is a dynamic situation where Ic is constanley changing together with the loading of the wheels. So even finding the numbers for input is a story for itself.
A good program is a great help, but it has to be a good program then.

Regards
Goran.

[adambrouillard]

Mainly what I am interested in is finding the out all the effects that would change the balance of a car and how much of an effect they have. My main question deals with ride height at each end of the car.

Popular opinion is that a higher ride height at one end of the car will transfer more weight and thus have less grip at that end.

The roll center would raise at that end and the weight on that end would raise as well. The roll moment should basically stay the same so the only effect would be the weight raising. This would raise the CG of the car a little, but if the only change was the CG of the car, then this would not effect the balance of the car only the overall grip level.

This would mean that effectively changing rake on the car should not effect the balance assuming your roll centers basically moved with the cg, is this true?

[Goran Malmberg]

Originally Posted by adambrouillard

1
Mainly what I am interested in is finding the out all the effects that would change the balance of a car and how much of an effect they have. My main question deals with ride height at each end of the car.
2
Popular opinion is that a higher ride height at one end of the car will transfer more weight and thus have less grip at that end.
3
The roll center would raise at that end and the weight on that end would raise as well.
4
The roll moment should basically stay the same so the only effect would be the weight raising.
This would raise the CG of the car a little, but if the only change was the CG of the car, then this would not effect the balance of the car only the overall grip level.
This would mean that effectively changing rake on the car should not effect the balance assuming your roll centers basically moved with the cg, is this true?

1
I have heard of ride height balans adjustments but is no friend of the idea.
However, ia a racing class for cars that ar pretty much allike, special way of adjusting may devellope and turn out to work.
If the car has groundforce changing the rake would affect the aero balance maybe to a dramatic extent.
2
If the car use doublle A-arms the Rc will alter and more weight will be tranfered over the geometry while the sprung weight transfer will stay more of the same. This is the same phenomenon as I discussed in an earlier tread about the roll axis beeing higher at the rear. So, in place of rising the ride height we could as well alter the A-arm attacching point to the chassis.
3
Check what you see when cornerweighting the car....
4
As allready said, the geometric weight transfer will alter.
5
There are a number of different A-arm layouts, some with heavy camber compensation built in, some cars are lowered resulting in less good geometry
etc. How great the balance effect will become is hard to tell then. Myself I use all a-arm angle layouts with great care in order to lessen all of those balance effects from ride heights, and adjust blance using sway bars springs and shocks (And aero). Maybe some slight higher rear Rc for turn in steering respons.
Regards
Goran Malmberg

[adambrouillard]

Ok, I suppose what I don't understand then is how changing the roll center at one of the car effects balance. I thought you measure the difference between the roll center and CG at that end of the car to get the roll moment. But if only the overall cg matters, what do you measure the roll center to?

[adambrouillard]

Ok I have been reading other threads and it seems that changing the roll center at one end would effect jacking, camber gain, and the rate at which the tire loaded because it is undamped ( ie transients). But it would not change the overall balance of the car. Is this correct?


Only changing the springs, bars, effects the mechanical balance of the car, correct?

As an example. Let's say the front roll center is at ground level, the rear roll center is at 10". The car has equal tires all around, equal weight distribution, and equal springs and motion ratios. Then this car would be totally neutral and moving roll centers around would not change that, correct?

[adambrouillard]

2. Moving the Roll Center Positions

A. Lowering the front roll center or raising the rear will make the rear take more roll
couple and the rear will saturate sooner. Balancing the brakes to give more on the
rear may have the Same effect.

E. Dropped throttle tuck-in will be decreased if the rear roll resistance is reduced or
the front increased. This can be done by lowering the rear roll center or raising the
front roll center. This will make the car more understeer; more ofthe total roll
moment will be resisted on the front.


I got this directly from Milliken's book. Is he wrong?

[Goran Malmberg]

Originally Posted by adambrouillard

1
Ok I have been reading other threads and it seems that changing the roll center at one end would effect jacking, camber gain, and the rate at which the tire loaded because it is undamped ( ie transients). But it would not change the overall balance of the car. Is this correct?

Only changing the springs, bars, effects the mechanical balance of the car, correct?

As an example. Let's say the front roll center is at ground level, the rear roll center is at 10". The car has equal tires all around, equal weight distribution, and equal springs and motion ratios. Then this car would be totally neutral and moving roll centers around would not change that, correct?

Well, it do influence even the balance since both axles is affecting each other. If we raise Rc at one end by an inch, the roll axle will be at 1/2" over the ground right under the Cg. This means that the roll lever arm is shortened by 1/2" thereby putting less stress on ALL four springs. So, the sprung balance remains the same, but not the quatities. What happen is that the axle with the higher Rc is geometrically transfering all the load that is NOT going over the spring system.
This Geometric weight transfer is very harsh-direct since it is hitting the ground like a steel bar running from the Cg down to the contact patch.
But even so, it is still transfering its given portion of load during the whole cornering period, while at the same time creating a lifting effect on the chassis at the axle in question.

Well, I better stop here to hear what you think. This is not very easy to explain in short termes without creating missunderstandings.
Goran

[adambrouillard]

I believe I understand what you're saying. I have just been told for so long that changing changing the roll center at one end of the car alters the steady state balance, including many published books. It's amazing so many people got it wrong.

Just to make sure I understand correctly, it does not change the steady state balance, but changing roll centers heights can effect transient behavior because it is acting directly throught the suspension members and is undamped.

[Goran Malmberg]

Originally Posted by adambrouillard

1
I believe I understand what you're saying. I have just been told for so long that changing changing the roll center at one end of the car alters the steady state balance, including many published books. It's amazing so many people got it wrong.
2
Just to make sure I understand correctly, it does not change the steady state balance, but changing roll centers heights can effect transient behavior because it is acting directly throught the suspension members and is undamped.

1
Well, what I am saying is that the balance actually IS changing, but from and in the form of geometric weight transfer. The axle that is getting a higher Rc will geometrically have to take care of the weight transfer alone that would otherwice have been distributed over the spring system to both axles.
2
So, raising the Rc changes the steady state balance, but not over the sprung system, it happen over the UNSPRUNG way of weight distribution.

Lets say that we raise the Rc for both axles to the height of CGH. Then there will be no roll at all present. But we will still have the same amount of weight transfer during cornering. Weight transfer is only dependent on (CGH*G*W)/Tw. And the transfered weight could be transfered over both the sprung and geometric systems to different proportions.

To answer your statement "It's mazing so many people got it wrong".
The Rc and its effect has been reviced over the latest years. Many book writers is looking at other writers and its no wonder that so many people show the same statements. I know about this "front and rear cgh", but dont want to make any comments here. I am not saying that I am the one that is right. I have been working with and studying real physical models, and know pretty well what happen, BUT, my conclusion of what I seen may be more or less correct. Lets say I know the result of an action pretty well, and is having a theory of why this is so.
Regards
Goran Malmberg

[adambrouillard]

Ok, well then how do we measure the geometric weight transfer then? We measure between the roll center of each end and what? The main CG of the car?

[Goran Malmberg]

Lets say that the CGH is 400mm. The Tw is 1600mm. G is 1. W is 1000 kg. Roll axle height at CGH is 100 mm. (200mm for the axle)
The weight transfer will the become 400*1*1000/1600=250kg.
No matter what we do to the Rc, at 1G the Wt will be 250kg
from the inside to the outside wheels.
The roll moment will be ((400-100)*1*1000)/1600=187,5kg.
The geometric load transfer is 250-187,5=62,5kg.
In short, from start with ground Rc the weight transfer is an equal 125kg each axle. With the raised Rc the axle of the raised Rc will transfer (187,5/2)+62,5kg=156,25kg. This means 156 + 94 for the respective axle = 250kg. This is quite a difference but 200mm raised Rc is a big number.
Goran.

[adambrouillard]

I'm sorry I think we must be having a language barrier problem.

Let me ask the question in another way dealing with the tire loads.

Let's say we have a neutral car with 100 kg on each tire.

In steady state cornering we transfer 100 kg total.

Now the inside tires are at 75 kg and the outside tires are at 125 kg.

If we raise just the rear axle roll center but leave the front the same. Would the tire loads change at all?

[phoenix]

Originally Posted by adambrouillard

Now the inside tires are at 75 kg and the outside tires are at 125 kg.

If we raise just the rear axle roll center but leave the front the same. Would the tire loads change at all?

Rasing the rear roll centre will result in:

1) the total weight transferred to the outside tyres will remain the same

2) more of the total weight transfer will be geometric: through the rear suspension links, directly to the rear tyres via the hub.

3) front geometric weight tranfer will be the same as before raising the rear ride height

4) total weight transfer through springs/bars will be reduced by an equal amount to the increase in geometric transfer seen in 2) above

5) weight transfer through the springs/bars will be in the same proportion front/rear as before raising the rear ride height.

6) there will be an overall increase in anti-roll stiffness, so there will be less roll.

7) overall, less weight will be transferred to the outside front tyre and more weight will be transferred to the outside rear tyre.

So the answer to your question is given in 7) - the tyre loads will change.

BUT

Vertical loads will be less

Side load (at the rear) will be greater

[adambrouillard]

Ok, well then going back to my original question, how to we calculate the change? What do we measure the roll centers against? I've have been trying to figure out the equation in Milliken's book, but it is not the most easy to follow book in the world.

[adambrouillard]

Originally Posted by adambrouillard

Ok, well then going back to my original question, how to we calculate the change? What do we measure the roll centers against? I've have been trying to figure out the equation in Milliken's book, but it is not the most easy to follow book in the world.

Ok I was looking through another book and it seems you measure it to the ground, then multiply by the g-load and divide by the track. Is that right? What happens if your roll center is below ground? You would have a negative number and this would subtract roll resistance then?

[adambrouillard]

I had a Eureka moment, and now I have some calculating to do. "how to make your car handle" explains it pretty simply.

[phoenix]

Originally Posted by adambrouillard

I had a Eureka moment, and now I have some calculating to do. "how to make your car handle" explains it pretty simply.

Just a reminder to calculate the unsprung weight transfer at the front and the rear separately from the sprung weight transfer and remember that the unsprung weights front and rear can have different c of g heights from each other and from the sprung mass of the car - in fact none of these heights are likely to be the same.

[Goran Malmberg]

I see that phoenix allready has come up with a good answer.
Point 7.
For an average racecar raising the Rc an inch for one axle could make some 20p more Wt for that axle. According to my own simplified equation just ot get a finger & thumb figure.
It would be nice with a simple calculation formula, but it isnt that simple. Or, to put it another way, it depends of how exact one want the outcome to be.
The geometric Wt is dependent on the tan of the forcelines, and the left and right Fl is going to alter in different directions during roll. The outside Fl will give + and inside - jacking effect, (moost of the times). Futher more, it is load (tire grip) dependent. While the roll is a direct function of Wt, like taking 100kg of one side of the car and putting it on the other side. The roll pattern will look like a sume of the two.
Goran

[Goran Malmberg]

I might try to find out some sort of formula for a breif Wt calculation. In fact I have been thinking of this for a while as it would be nice to have in my book.
One problem is where to start the formula. Should for example the A-arm geometry with all their mounting points be used for inputs? Or should we start directly using the angle of the forcelines? Then we have the wheelrate including sway bars calculations which also could be concidered solved or to be calculated.
However, the main problem is the CGH, TW, geometric and sprung weight transfer equation. And probabley at a static loaded situation where the Wt side load% has to be estimated for starter.
What do you members of the forum say here?
Goran