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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.
YAW MOMENTS FROM DIFFS WITH FRONT-WHEEL DRIVE
Do your comments on clutch type vs geared LSD's [in the October 2008 newsletter] apply to front wheel drive cars, especially in regard to solo2 or autocross conditions? I have used clutch types in FWD D Mod cars that are very front biased in regard to weight. They do demonstrate the darting problem from uneven grip with high preload. I have heard that the geared types do not provide enough total grip for autocross with very light and powerful modified cars, though I do not have experience with them. I run lots of preload.
Yes, the same principles apply with front-wheel drive, and with all-wheel drive. Differing thrusts on the right and left sides of the car create a net yaw moment, whether the thrust comes from the front wheels, the rear wheels, or all four.
Additionally, when the front wheels have differing thrusts, this creates forces in the steering. Not only does the driver have to apply corrective steering inputs to keep the car headed in the proper direction, he/she also has to wrestle with erratic feedback torques in the steering to keep the front wheels from steering themselves in an uncontrolled manner.
With front-wheel drive, we always have the problem that load transfers off of the driving wheels under power. If we are cornering at the same time, the lateral and longitudinal load transfers combine to unload the inside front wheel. We combat this by having lots of static front weight, and lots of rear roll stiffness, but when the accelerations are large and the engine is strong, the inside front wheel still tends to spin. Without preload on the clutch pack, we may face the problem of not getting enough torque to adequately trigger the diff's torque-sensitive lockup.
The questioner is taking the right approach: use lots of preload, and just live with the twitchiness and steering fight. It gets old in a street car unless you value performance very highly, but for racing it's the way to go fast.
TURN-IN ON SLIPPERY SURFACES
What is the foot dance routine to bring a car into a corner sliding sideways like in rallye driving? I've read that you should be on the gas and brakes at the same time and then release the gas first. I've tried this but can't get it to work. Maybe on rallye cars the have a very large rear brake bias.
I can do it on snow and dirt to some extent but not on pavement to the extent that rallye drivers do it. When I do it on snow or dirt I do not need to be on the brake and gas at the same time and then come off of one. I can do a 360 deg on snow but the last 90 deg is hard.
Car is a '93 Mustang.
Iím not a rally driver, but I can tell you for sure that it differs greatly depending on whether the car has rear-wheel drive, front-wheel drive, or all-wheel drive.
I can see why applying throttle and brake together doesnít help your car turn, or help it slow either. I think of left-foot braking with simultaneous power application as being mainly a front-wheel-drive technique. With rear drive it generally adds understeer. Oval racers sometimes use it, but thatís to keep the car tight entering large-radius turns on the brakes at over 100mph. Right-foot brakers use less rear brake percentage instead. The advantage of using left-foot braking and adding some throttle is that if youíre skillful you can adjust the effective brake bias with your footwork. The disadvantage is that you use up more fuel and rear brake pad.
It might be that with rear drive and a lot of rear brake, a driver might want to do as you describe: gas and brake together for straight-line braking approaching the turn, then off the gas to get full rear brake for turn-in, then off the brake and on the gas for a controlled powerslide. This would only work well with lots of rear brake.
With front drive, the idea is to use engine power to fight the front brakes and make the car act like it has a lot of rear brake. You turn in with the front wheels either driving gently or at least not braking strongly, and the rear wheels either locked or using a lot of their traction envelope for retardation so
theyíll slide easily laterally. Once the car is rotating, you get off the brakes so it wonít completely spin, and you now have some yaw velocity established and are cornering power-on.
With rear drive and good brake bias for straight-line stopping, usual technique is to get your braking and downshifting done in a straight line (slippery surfaces will develop your ability to get smooth heel-and-toe downshifts), and turn in off the brakes while trailing the throttle to get engine braking. Manual transmissions work much better for this than automatics. If you are serious about driving in the slippery stuff, you really donít want an automatic. (You really donít want a nose-heavy car like a Fox-body Mustang either, but with a manual you can at least get it to turn by trailing the throttle.) Once the car is rotating, you balance it and control its trajectory with steering and throttle.
To loop the car using the brakes, you may need to use the handbrake. Depending on brake system properties and road surface, you may be able to do it with the foot brake as well, or just use engine braking and crank the wheel, perhaps applying enough throttle to spin the wheels once the car is rotating. Thatís the usual way of doing donuts on a snow-covered parking lot. With a bit of practice, most people can get two or more loops that way.
Tires make a big difference. You definitely want snow or gravel tires on all four wheels. Where legal, on snow there is no substitute for studded snows all around, tail-heavy weight distribution, and rear-wheel drive. All-wheel drive is potentially even better, if itís designed to let you throttle-steer like you can with rear drive.
On pavement, it is possible to do donuts by braking and applying lots of power in first gear, at nearly zero speed. In this case, the engine power isnít just fighting the rear brakes, itís overwhelming them to the point of spinning the wheels. Actually, once the wheels are spinning, you donít really need the brakes on to get the car to spin, but you do need them to keep the rear wheels spinning while keeping the speed to a crawl. But at speed, and short of the point of wheelspin, adding power while braking gives you the equivalent of less rear brake, and sticks the rear rather than sliding it. At speed, spinning the wheels is generally all you need to throttle-steer the car.