The Mark Ortiz Automotive

CHASSIS NEWSLETTER

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to the Motorsports Community

March 2011

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WELCOME

 

Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions.  Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by    e-mail to: markortizauto@windstream.net.  Readers are invited to subscribe to this newsletter by e-mail.  Just e-mail me and request to be added to the list.

 

 

FOLLOWUP INFORMATION TO JANUARY ISSUE ON STOCK CAR A/R BAR PRELOAD AND RELATED ASYMMETRIES

 

In the January newsletter, a questioner mentioned Ron Hornaday's truck being higher on the left after breaking a sway bar arm, and wondered if that would imply that the bar was preloaded.  I said it seemed to me that it would.

 

Actually, that would be so if the truck was higher on the left when sitting still, and if no pit stops including chassis adjustments occurred.  If it was only higher on the left on-track, at speed, there is also another possibility.

 

I owe a tip of the hat here to Mike Keena-Levin of Morse Measurements in Salisbury, NC on this.  Morse Measurements does kinematics and compliance (K&C) testing.  They do a lot more work with top-division NASCAR teams than I do (mostly Cup and Nationwide; not so many trucks, so they tell me).  What they divulged to me about current bar setups is evidently so commonplace nowadays that they consider it no breach of confidentiality to share it with me, and for me to share it with my readers.

 

It is illegal in the uppermost divisions to preload the sway bar at static, so nobody does that.  However, they do use highly unequal sway bar motion ratios on the right and left.  This produces some interesting effects, especially when the bar is very stiff and the springs are very soft.

 

The unequal motion ratios are achieved by using an arm on the left end of the bar that applies its force to the lower control arm at a point further inboard than the right one does.  These cars have what is known as a soft link on the left front.  Instead of a conventional drop link connecting the bar arm and the control arm, there is a pad that allows the bar arm to lift the control arm, but not draw it down.  One reason for this is to keep the bar from de-wedging the car so much if the driver puts a wheel on the apron.  Another reason is to facilitate enforcement of the prohibition against bar preloading.

 

 

With a bar arm/control arm connection like this, it is easy to have a selection of bar arms with different bends, creating different top-view offsets, that produce a variety of contact points and bar-end-to-tire-contact-patch motion ratios.

 

If the motion ratio is dramatically less on the left than on the right, some remarkable things happen when the vehicle is on the K&C rig and the front end is displaced in pure ride (equal displacements right and left; car held level in roll but moved up and down).  The wheel rate in ride for the left front wheel can be negative: the load at the contact patch can decrease as the front end compresses.  In extreme cases, the left front tire will actually lift into the air, even as the fender above it is coming down and the overall front tire pair loading is increasing.

 

When a setup like this is running on the track, and subjected to aero or banking loading, with only the relatively meager rear elastic roll resistance to work against, the car leans to the left under such loading, and gains wedge, due to the action of the bar.  If something in the bar system breaks, and the bar becomes ineffective, the car will run higher on the left as it goes around the track.  However, it will not sit higher on the left when stationary, if in fact there was no bar preload in that condition.

 

Another thing that could make the truck sit higher on the left in such a situation, even when stationary, is the adjustment to the rear jacking screws that the crew would very likely make to compensate for the broken sway bar arm.  The crew would need to put more wedge or diagonal percentage into the vehicle, and they would in most cases use the rear jacking screws to do this, since the front ones are less accessible.  They would need to raise the left rear corner and perhaps also lower the right rear, and this would tilt the vehicle to the right.  This would have to happen on a pit stop.  If the arm broke late in the race, and the truck did not pit after that, and it was higher on the left when sitting still, that would suggest a preloaded bar.

 

 

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 " 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.