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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: email@example.com. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.
MORE THOUGHTS ON ZERO-DROOP SETUPS, AND RELATED OVAL TRACK SPRING SPLIT ISSUES
In the February 2007 newsletter, I responded to a question regarding zero-droop setups. These are setups where the suspension is not allowed to have any droop travel from static position. I said that these were bad for everything except controlling ground clearance in order to have consistent aerodynamic ground effects. As a result of further reflection, and conversation with a number of people, I have concluded that there is another benefit.
That benefit is that when the car is in a rolled condition, the inside wheel pair has greater pitch resistance than the outside wheel pair. This has effects similar to left-stiff spring splits on an oval-track car. The car de-wedges when accelerating rearward while also accelerating laterally, and it gains wedge when accelerating forward while also accelerating laterally. That helps the car get itself rotating in yaw on entry, provided the driver is decelerating or braking while entering, and helps it put power down on exit.
This would apply to a car with zero-droop setup at both front and rear. If only the front is zero-droop, effects would be confined to corner exit.
This benefit does come at a price in the wheels' ability to follow bumps, but if the surface is smooth enough, it may be worth it.
A similar effect, but more subdued, can be had by making the wheel rate increase in droop, either using rocker geometry or using snubbers. Note that this does not mean that the spring load or force increases in droop; it means that the force decreases at a greater rate with respect to droop displacement, as droop displacement increases.
A third way of creating a rising rate in droop is used on dirt Late Models, and could also be applied to road racing cars, although I have yet to hear of it being tried for road racing. This third way is to have two springs stacked on top of each other on a coilover, separated by a slider. The slider is
arranged to top out against an adjustable collar on the coilover threads, at some point as the suspension extends. This adjustable collar has a smooth sleeve above the shoulder that the slider seats against. This provides a smooth surface for the slider to ride on, and protects the threads. These devices are usually used on the left front in oval racing.
The rate increase occurs in a single step. It would be possible to create two smaller steps by using two sliders and three springs, but I have never seen this.
Elsewhere in oval racing, one still often encounters the erroneous belief that the way to tighten the car (add understeer/reduce oversteer) on exit is to use a right-stiff front spring split. This belief stems from misunderstanding of the relationship between spring rate and spring force, in a situation that causes an extension displacement of the spring.
To understand this sometimes confusing concept, imagine a car on the drag strip, with the suspension on the left front wheel locked solid (spring rate effectively nearly infinitely large). When the car launches, the locked suspension will be unable to extend, and it will be quite easy to lift that wheel off the ground, while the right wheel, whose suspension can extend, will stay on the ground. All load remaining on the front wheels will be on the right front, and the car will have more than 50% of its weight on the right front and left rear.
The more we increase the spring rate, the more closely the suspension approximates a locked condition. The greater the spring rate, the more the load increases with compression, and the more the load decreases with extension. Consequently, a stiffer inside front spring increases load on the outside front and inside rear tires, and tightens the car (adds understeer) on corner exit.
HIGH OR LOW PANHARD BAR?
Can you discuss the advantages and disadvantages for various heights of Panhard bar? For example, on my stock car, I could run the Panhard bar as high as 12½", or I could run it as low as 10¾". In either case, I could get the car balanced by changing the combinations of springs, crossweight, etc.
In many cases, there is an additional variable: rear anti-roll bar stiffness. In many stock car classes, rear anti-roll bars are now prohibited, but they are still used in the top NASCAR divisions, and in many stock-car-related road racing classes. So we have at least four variables that play off against each other: Panhard bar height, springs, diagonal percentage, and anti-roll bar if allowed.
To start off, let's take a simple case: we are considering whether to use a high Panhard bar on an oval, or stiff rear springs. The high Panhard bar with softer springs will ride two-wheel bumps better. It may or may not ride one-wheel bumps better. That depends on how flexible the tire sidewalls are. The higher Panhard bar creates more lateral tire scrub on one-wheel bumps. If the tires have a lot of lateral flexibility, (e.g. dirt tires with tall sidewalls, running low pressures) that
may not matter a great deal. If the tires are stiff laterally (e.g. low-profile road racing slicks, as on a Trans Am car), it matters more.
On the other hand, if we are seeking rear downforce, we may opt for stiffer springs, and accordingly a lower Panhard bar. The stiffer springs will keep the rear end of the car higher, resisting the influences of banked turns and aerodynamic downforce. The higher dynamic rear ride height will let air out from under the car better, and get the spoiler and rear deck up into the airstream more.
The higher rear ride height will also add some drag. If we're running Daytona or Talladega, that may matter more than downforce, and we may want the rear to squash down as much as possible. In that case, we want softer springs, and a concomitantly higher Panhard bar.
Another difference relates to torque wedge: the tendency of driveshaft torque to load the right front and left rear tires under power. A higher Panhard bar, with softer rear springs, gives us more torque wedge. A lower Panhard bar, with stiffer springs and/or anti-roll bar, reduces the effect.
In general, torque wedge is our friend in a left turn, and hurts us in a right turn. Therefore, for road course work, we want to minimize the effect. Especially with low-profile tires that have stiff sidewalls, we will want a really low rear roll center, and plenty of rear anti-roll bar.
As for diagonal percentage (right or outside front plus left or inside rear tire load, as a percentage of the four-wheel total), for oval track applications this plays off against both geometric roll resistance (Panhard bar height) and elastic roll resistance (springs and anti-roll bars). We can run more diagonal percentage, and balance this with increased rear roll resistance (or decreased front roll resistance) from geometric or elastic sources, or we may run less diagonal percentage, and correspondingly less rear roll resistance relative to front roll resistance.
The key to finding a good combination here is getting the car consistent as track conditions vary. A setup with relatively little diagonal percentage tends to go loose on slick surfaces and get tighter as grip improves. This is the most common pattern. A car with high diagonal percentage may go tight on slick instead. In my experience, it takes considerably more than 50% diagonal to get a stock car to do this. Somewhere between these extremes is a combination that changes its handling balance relatively little as grip levels vary.
Additionally, a car with ample static diagonal percentage and relatively great rear roll resistance tends to be tighter on both entry and exit, and freer in the middle of the turn, than one with a combination using less static diagonal.
The reason for both of these effects is that static diagonal is a starting point for load transfer and does not change with lateral acceleration, while roll resistance balance affects the way dynamic diagonal percentage varies with lateral acceleration. A setup with ample static diagonal and ample rear roll resistance has relatively great dynamic diagonal when cornering force is moderate (low grip; entry and exit) and relatively little dynamic diagonal when cornering force is great (high grip;
mid-turn), compared to a setup with less static diagonal and a less rear-stiff roll resistance distribution.
It will be apparent that this is a complex game, and there is no one-size-fits-all solution, especially with the wide variations in driver preference. But when we understand how the variables interact, we improve our ability to tailor the setup to the driver and the track, and to keep the car more consistent as conditions change.