Z00

I've just been reading posts and I don't know what the difference is. Anyone care to enlighten me? It seems the spools is not good for driving on the street but I don't understand why.

Also, when would you go for a spool over a posi?

<small>[ April 10, 2002, 09:58 AM: Message edited by: Z00 ]</small>

Fireball

iTrader: (21)

spools are a machined piece that does not allow relative rotation between the back 2 wheels. if the right wheel turns at say 5 rpm, the left wheel will ALWAYS turn at 5rpm. This makes it "unstreetable" since when the car is turning around a corner, the outside wheel is travelling a larger turning circle than the inside wheel. Thus the outside wheel needs to turn faster. But the spool won't allow this so the tire will "chirp" as it skips so it can turn at the same speed. For drag racing, this is the best setup...full traction to both tires with no complicated mechanisms to break.

A posi is a mechanism that allows the wheels to turn independently of each other when there is enough torque bias. Instead of the tire "skipping" when it becomes overloaded, it will disengage the clutch in the posi and will turn faster than the inside wheel. Once equal speeds are reached it will "lock" again so both drive wheels turn at the same speed. A big problem with this is that all the engine power will be transmitted through little spider gears that control relative rotation of the tires. If you break these spider gears...you are done...call a tow truck <img border="0" title="" alt="[Roll Eyes]" src="images/icons/rolleyes.gif" />

A third option is a detroit locker. Though I'm not 100% familiar with how it works, its essentially the same idea as a posi. But there are no spidergears or clutches, it is a ratcheting mechansim that physically locks the rear tires together and can "unlock" when necessary. Unit is VERY strong but its noisy and jerky during operation (I personally run a locker in my 9")

<small>[ April 10, 2002, 10:12 AM: Message edited by: Fireball ]</small>

Chris Spiess

Good explanation, Fireball. I'd like to expound on the 'posi' definition though.
'Posi' traction was a term coined by Eaton back in the '60s and brought into common use by GM who used Eaton's 'Posi' differentials in it's 60-70s muscle cars and other vehicles. It has become a common term to describe all type of limited-slip differentials, which operate basically like Fireball describes above. The differential works to engage both drive wheels while they are turning at a similar RPM, but still allow differentiation in axle speed for turning and the like. There are a vast number of designs and styles available from different manufacturers.
The most common units are pre-loaded clutch based designs (Eaton Posi, Auburn) where a pre-load mechanism (usually a set of springs) forces a set of clutches together to lock the axles together. The clutches then slip against each other when the axle speeds differ enough to overcome the fictional coefficient of the clutch packs. The amount of pre-load affects the performance of this design by applying more or less pre-load to the clutch packs. More pre-load or higher friction clutches require more difference in axle speed to allow differentiation. Pre-loaded clutch type differential never really disengage, they simply allow the clutches to slip against each other. Obviously the clutches ultimately will wear out and the differential must be rebuilt or replaced. Typically they require less and less difference in axle speeds to allow differentiation until eventually only one wheel is actually driven.
There also clutch/viscous type designs. These are normally in a disengaged state. These designs also rely on a clutch pack to provide engagement, but the clutches are actually disengaged and the differential drive only one axle at a time like an open differential. These units are based on fluid viscosity and temperature. The fluid in the differential is designs so that the wheel speed difference between the non-driven wheel and the driven wheel causes the fluid viscosity to change and gradually engage the clutch pack to drive both wheels. The unit will also wear their clutches out eventually but since they are not forced to slip under normal driving conditions they typically have a much longer life span than pre-load designs, but also offer less effectiveness in performance applications. A common application for this type of unit is in non-electronic all-wheel drive systems.
Another type of limited-slip differential are gear driven units (Detroit TruTrac, Torsen). These units drive both axles under normal conditions and allow differentiation based on frictional forces between a mesh of internal gears. The bias ratio of gear based limited slips are varied mainly be the design of the gears and their relationship inside the differential case. However, altering the fluid viscosity can affect the available bias ratios as well - i.e. higher viscosity promotes higher torque bias requirements between the axles, lower viscosity requires less torque bias to permit differentiation. Some of these units are based primarily on spider gears that resemble a typical open differential (i.e. Detroit TruTrac) while others have much more complex gear arrangements and interactions (i.e. Torsen). These units obviously do not have clutches to wear out and as such have a virtually unlimited life span short of actually breaking a gear in the unit. Another advantage of these units is that they can be designed to allow partial disengagement, that is they can continue to deliver torque to each axle while still allowing differing wheel speeds. Clutch type units can do this as well, however are much less effective.

<small>[ April 10, 2002, 04:08 PM: Message edited by: Chris Spiess ]</small>

NHRAFORMULA00

iTrader: (2)

In simple terms, Posi for the STREET, SPOOL is for all out drag cars, you wont like it for the street!