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There are six types of differentials being used in racing; the open dill, the cam and pawl or ZF type, the clutch locker, the Detroit Locker, the Weismann and the spool. The only reason to run an open diff is if the regulations require one. Locking the open diff is very simple--you weld the spyders solid. You will get caught, eventually. I have not personally run an open diff since the days when I didn't know that there was anything else and I do not expect to ever run one again. Therefore, I know nothing about the techniques used to trick them so that they will partially lock. I do know, however, that this has been done in SCCA production racing. I neither know nor care how it is done. The reader who is interested should be able to find out without too much trouble.

So much for the open differential.Street cars need differentials between the driven wheels because the outside wheel in any cornering situation must travel on an arc of greater radius than the inside wheel, and so will have to revolve more times in negotiating any given corner. If the two driving wheels are locked together, the unladen inside wheel will be forced to rotate at the same speed as the inside one and will therefore hop along like a rabbit. This makes a funny squeaking noise and upsets the handling of the vehicle. Street cars typically operate at low force levels so the open differential does not normally present a problem--and it is maintenance free. However, there are times--like trying to get up a steep hill in winter when one wheel happens to be on glare ice--when the limitations of the open diff become very apparent. With the open diff, the torque from the engine takes the easy way out and if, for whatever reason, one of the driving tires has exceeded its thrust capacity, all of the torque will be delivered to that wheel and it will spin--while the other tire does nothing and the vehicle goes nowhere. This condition occurs on the race track all of the time-- while we seldom end up with one tire on a good surface and the other on a slippery one, lateral load transfer accomplishes the same end by unloading the inside tire. Since no one tells the open differential about this state of affairs, as we try to accelerate out of a corner, the diff keeps transmitting drive torque to the unloaded tire until the torque becomes more than the tire can bear and it starts to spin. About then the diff sends all of the torque to the spinning tire and none to the laden tire and we go nowhere. The problem becomes more acute as the power to weight ratio raises, but even Formula Fords get inside wheelspin out of slow corner. Wings, by keeping the unladen tire partially loaded with aerodynamic downforce, make the problem less acute.

However, all racing cars, in order to realize their potential, require some sort of limited slip or locked differential--and always have. What we need here is a differential that will be open--or will differentiate--on a trailing throttle, so that the rear wheels can rotate at the required radius speed during corner entry, but will start to lock as the driver comes back on the power to stabilize the car, thus providing a degree of built-in understeer by driving the inside rear wheel, and which will gradually lock all of the way as the power is increased so that there will be no inside wheelspin. At the time of writing, no one has quite achieved this goal.There are five types of differentials in use in racing cars today--the locked dill, the cam and pawl or ZF type, the clutch pack or Salisbury type, the Weismann locker and the Detroit Locker. With one exception, each has advantages and disadvantages. We'll start with the exception.

To my knowledge, the Detroit Locker has no advantage over any other type of differential except the open diff. It is an abortion. Its functioning can be compared to that of the ratchet on a chain fall. As load is transferred it is forever locking and unlocking, causing great lurches and changes from understeer to oversteer. The best thing to do with a Detroit Locker is to remove the center cam and run it locked. Period. End of discussion. We all know that the Indy cars, the dirt cars, Nascar and Porsche use a totally locked diff and they go like stink.Most of the IMSA type large sedans also use it. Why then, is the spool not used in sophisticated road racing machinery except by Porsche? Not because people haven't tried it! The problem has to do with corner radii, weight distribution and how much we are willing to sacrifice. The high banks and the two-and-one-half-mile ovals tolerate the spool because at the corner radii we are talking about there is virtually no difference in rear wheel rpm--and the tire stagger makes up for most of that--when we only need to worry about one corner radius, we can, by making the outside rear tire larger in diameter than the inside (stagger), arrive at an equal tire rpm situation and therefore neutralize the drag moments about the center of gravity on a trailing throttle corner entry situation. So long as the driver picks up the throttle smoothly and progressively, we can then tune out the full throttle understeer caused by the drive on the inside rear wheel. It is also very important that the driver not apply sudden power during a time when he has understeer lock (toward the corner center) on the front wheels or he will understeer immediately into the wall--thump. When, as in road racing, the radii of the various corners vary considerably and the amount of the braking and turning combination taking place also varies with the nature of the corneL it is no longer possible to achieve equal rear wheel rpm in most of the corners

extract from "tune to win" Printed in about 1978?
written by
Carrol smith

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