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CORRESPONDENCE REGARDING CORNER EXIT UNDERSTEER
I run a Porsche 996 in PCAís GTB1 class, a hotbed of activity these days. The car runs at 2650 pounds dry, 1300 pound front springs and 1150 pounds rear, with Penske double adjustable shocks. I run the rear bar on full soft, and the front on one-off full stiff, with 9Ē front rims shod with 250/650-18 Yokohama slicks, and rear 11Ēs with 300/650 Yok slicks. All in all, a really well balanced package after years of trial and error.
My question concerns corner exit understeer. Per Carrol Smith, Iíve always tried to follow the logic of ďfix the end that misbehavingĒ, as opposes to destroying the grip at the other end to bring the car into balance, but whomever I ask for advice on how to solve corner exit understeer, everyone wants me to fiddle with the back. I run fairly stiff bump and fairly soft rebound up front, and the car has great corner entry point-in, without upsetting the back, but once power goes down, the front just ďfloatsĒ. Not badly, but just a little too much. Iím already on -20 out of 30 settings on the Penskeís for rebound, so there room to go, but I donít want to have an experiment going really wrong and damage something or have an event. So, my inclination is to soften the rebound, to allow the tire to remain more in contact with the pavement, as opposed to it being ďlifted upĒ by the surging front end of the chassis as the weight transfers to the rear. Again, itís a light float that still allows for accurate placement at track-out, but if the front could hook up just a little better, I could go to power-down that much sooner.
I agree with not balancing the car by throwing grip away, but that doesnít necessarily mean you donít touch the end that sticks better. Each end affects the other end.
Does the car pick up the inside front wheel under power? Most 911/996 style cars do, correct?
What do you run for rear toe? How about the diff? I expect itís a clutch pack limited-slip, correct? Are you able to play with ramp angles or preload?
Are you happy with the steady-state balance?
Yes, the inside wheel does pick up. Barely in most cases, so Iíd describe it as skimming the surface with the ARB set on one off full stiff. To your points, I learned along the way that front roll stiffness is a requirement for the back of a rear engine car to behave. If I go to full stiff, the effect is more pronounced, but the entry and exit understeer becomes more pronounced.
The toe is 1/8 inch each side, or a ľ inch total.
It is a Guard clutch pack diff described at 50/80, so I donít know whether thatís ramp angle or lockup percentage, but the diff works great on corner entry; you can pound the car down into an apex with the brakes on hard and the back end just sits down and takes it, key to running hard and long without something reaching up to bite me. And on exit, the back moves just enough but not enough to begin to step out. But no, playing with ramp angles is beyond the scope of me and my amateur capacity.
As to the steady state, the balance is darn near perfect for fast sweepers; little blips front and rear that can be adjusted with throttle.
Hope that helps.
Have you tried less rear toe-in? I would expect that to add some oversteer in all conditions, but most under power.
No, but I will. With the fairly stiff springs, I donít get much toe out under power, so limiting the toe might help.
Itís funny. I remember standing in the hairpin at Sebring and watching the RSRís launch in first gear. It seemed like the front end would raise up and the wheels would stay on the ground, almost as if there was three inches of droop. I was assuming that was very little rebound control. Is that a bad assumption?
I would be surprised if you get toe-out under power. Generally, designers avoid having either bump steer or compliance steer in that direction. Itís pretty difficult to get compliance toe-out under power even if you want to. Ordinarily that happens in decel or braking.
I actually have bump steer readings from a K&C rig kinematic test of a 996. This is a stock street-driven cabriolet, so itís a much softer setup than yours and probably about an inch higher ride height as well. The rear wheels do toe in in compression and out in extension. Over the first inch of
compression, each rear wheel toes in about 0.15 degree. Over the second inch of compression, which would probably be roughly the first inch on your car, each wheel toes in about an additional 0.30 degree; the rate of change increases. The wheels toe out in extension, also at an increasing rate. The bump steer curve is S-shaped. On the commonly used 28Ē toe plates, half an inch is about a degree. An eighth of an inch per wheel is then about a quarter of a degree. Measuring at the wheel rims, an eighth of an inch is more like half a degree. The 996 in the K&C test had an average of about a quarter of a degree per wheel at static, which I would imagine would be a factory recommended setting.
Regarding front end rise under power, assuming that the driver can stay on the power long enough so the car is able to reach dynamic equilibrium, the front spring rate, or more precisely the front wheel rate in ride, is what determines how far the front end rises. The shocks determine how fast it rises. Softer springs make it rise further. Softer low-speed rebound damping makes it rise faster. Hold-up shocks with soft rebound and stiff compression help keep the front end from coming down as much during shifts.
When weíre going straight, there is a moderate advantage in having the nose lift, at least in the lower speed ranges. The c.g. is a little higher so we get a bit more rearward load transfer. At higher speeds, we may lose more aerodynamically than we gain through rearward load transfer. However, it takes a powerful car to produce a lot of front end rise at high speeds.
To combat a power push, we probably want the nose to stay down. For one thing, that will help outside front wheel camber a little. The camber goes toward positive a bit when the nose lifts. This isnít a big effect on a 996. The camber only changes something like a quarter of a degree per inch in the range the suspension would be in during exit on the outside front wheel.
And the springs youíve got are pretty stiff already. The street 996 has a wheel rate of about 178 lb/in at the front. The spring rate would be higher than that, but probably no more than 250 lb/in. At 1300 lb/in, your racing version is about five times that. The wheel rate must be upwards of 1000 lb/in. I donít know what the vertical spring rate of your racing tires is, but the tires on the street version were around 1670 lb/in, so youíve stiffened the springing to the point where the tires are about a third of the suspension in ride, and more than that in roll. Therefore, I donít think you can reduce nose lift or exit understeer much by going even stiffer on the springs.
To some extent, a power push is just a characteristic of a car with a lot of rear percentage. Front engined cars tend to be easy to point with the throttle. They develop power oversteer gently but at modest power application. My mid-engined Corvair, which has about 60% rear, stays at light understeer on exit through a wide range of power application values. If you feed in more power, within reasonable limits you hardly have to do anything with the steering; the car just eats up more road laterally if you get on the power harder. A car that has 63% rear or more, like a 996, tends to develop understeer under power. All rear drive cars will transition to wheelspin oversteer at some point. The more tail-heavy the car is, the more power can be applied before the car reaches that point, but also when the power breakaway comes, the more violent the transition is.
I would discourage playing with extreme damping strategies in a road racing car. Letís see what happens when you take a little rear toe-in out of the car. That may very well give you just the modest change youíre seeking.