ARB Minutia

[johnc]

From Mark Ortiz's latest newsletter:

 

FOLLOWUP QUESTION TO FEBRUARY ISSUE ON A/R BARS

 

I have a followup question to your comments on sway bars in your February newsletter.

 

It seems to me that at full load, where all the car's weight has transferred to the outside wheel, springs and bars would have the same effect. But while in transition, while the outside is compressing, the way the bar resists that compression is by exerting upward force on the inside wheel. Would that not decrease grip on the inside tire, as long as it would otherwise still have corner weight (downward force) pressing the tire to the surface?

 

Here's a scenario:

- 1700 lbs of weight on the front end, 100 lbs/corner of unsprung weight

- the springs are 500 lbs/in, and there is 1200 lbs of swaybar resistance (for every inch of chassis roll, or 2" of total swaybar flex)

- the car should roll 1" when all 1700 lbs have transferred

 

 

- at 1/2" of roll, approx 425 lbs will still be on the inside corner, and 1275 lbs on the outside

- inside corner should be exerting 425 lbs of force on the inside tire against the ground, giving it

grip

 

- the spring is being compressed by 325 lbs of corner weight, and so it will be compressed .65" by that. The swaybar will be exerting 600 lbs of force in the other direction, and so will compress the spring another .83"

 

Is the amount of force exerted on the tire reduced by the upward pressure of the swaybar, therefore reducing the traction on that inside tire? Is it correspondingly placing load on the outside tire then?

 

The questioner is confused on a number of points. I will try to sort things out.

 

First of all, springs, sway bars, and all other interconnective springing devices are purely displacement-sensitive. None of them have any different effect due to the suspension having a roll velocity. They are only sensitive to roll displacement. Dampers create forces that affect load transfer when the suspension has roll velocity, but sway bars do not, nor do springs.

 

The scenario posited has a number of problems. Conventionally, the 100 lb/wheel of unsprung weight is not treated as transferring through the suspension, but as discussed in previous newsletters, if there is zero camber recovery in roll, the unsprung masses do create a roll moment that the suspension must resist. So for simplicity, let's suppose that the effective mass acting on the suspension in this half-car model really is 1700 lb.

 

If that's so, the weight or load transferred due to cornering is not 1700 lb. It is, at most, the load on the inside wheel. If the half-car is assumed to be symmetrical, that's half of 1700 lb, or 850 lb transferred. At that point, the inside tire is at the point of impending lift.

 

If the wheel rate in roll is 1700 lb/in, 500 from the spring and 1200 from the bar, the half-car has a displacement of only half an inch per wheel at 100% load transfer. Any further roll moment will lift the inside wheel.

 

At half of that load transfer, 425 lb, roll displacement is ¼" per wheel.

 

In either case, it would not matter if there were no bar and the spring rate at the wheel was 1700 lb/in instead. The half-car would act exactly the same.

 

The tires do not know where the roll resistance comes from. They only respond to how much of it there is in total, at a particular instant. The roll resistance may be elastic (from bars and springs), frictional (from intentional and unintentional damping), or geometric (from linkage-induced support forces). But wherever it comes from, it can only hold the car upright by exerting force on the

 

ground, through the tires. There is a simple, inexorable relationship between roll resisting moment, load transfer, and track width:

• Load transfer through the suspension (i.e. less unsprung component) times track width equals roll resisting moment for the wheel pair.

• Roll resisting moment for the wheel pair, divided by track, equals load transfer through the suspension.

This is true regardless of what part or characteristic of the suspension generates what portion of the resistance.

 

The total roll resisting moment from the front and rear wheel pairs together always equals the roll moment created by sprung mass inertia in response to acceleration (and gravity, which nowadays is sometimes considered an acceleration). The relative roll resistance of the front and rear suspensions controls the front/rear apportionment of the total, but not the overall magnitude of the total. Adding roll resistance only at the front increases front load transfer but not the total load transfer for the vehicle. It follows that rear load transfer must be less.

 

So yes, the bar does unload the inside front wheel, and it does reduce front grip, and correspondingly increases rear grip, compared to the same setup without the bar. However, it does not do this any more or less than any other method of obtaining the same front roll resistance.