The Nine-inch Carrier (Differential)
When it comes to the carrier for a 9-inch Ford, there are three basic options available: Spool, Posi-Traction and Locker, and of course a series of variants in each (we’ll get to that in a bit). You can also get an open carrier, but honestly, this isn’t really a performance choice, at least for anyone contemplating a drag race situation. Mark Williams notes: “An open carrier uses a set of gears to allow slip. The thing to remember with an open carrier is that torque is always equal between both wheels. This means that if one wheel is slipping, the other will only be able to apply as much torque as the slipping wheel.”
Williams goes on to state: “A spool is single piece carrier and does not offer any compensation for different rotating speeds in the wheels. Because of additional stresses created by a spool, it is not a good idea to run stock-spline axles with a spool. Spools should be run in race only type applications and are not intended for use on the street.
“Lockers use a ratcheting technique in combination with a cam to ensure that both wheels are locked together. The locker will not allow the wheels to spin at different speeds as long as there is forward torque on both wheels. The unit allows the outer tire to ratchet while turning a large radius such as cornering.
“Torque Sensing differentials use mechanical means to control slip. They are rated with a bias, such as 5:1, that rates the amount of torque the unit is capable of applying to the non-slipping wheel. For instance if you are spinning with 20 ft/lbs of torque, the non-spinning wheel will be able to receive 100 ft/lbs in a 5:1 ratio. In a case where there is no torque on the loose wheel, the differential will not apply torque (this is why they recommend off-roaders apply the brakes when they slip). With an adjustable bias you can tune the differential to your needs.
“Posi-Traction is similar to an open carrier and uses a set of clutches to apply torque to both axles. The clutches are pre-loaded by springs and the separating action of the spider gears increase the pressure on the clutch discs. Different clutch materials can be used as well as different static pre-loads to change the amount of torque needed to make the wheels slip.”
For a strip only combination, then your best bet is a spool. Williams manufactures both steel and aluminum spools. On the steel spool side, the 4140 steel forgings are CNC machined, and heat-treated using the same austemper process as axles (we discussed it in more detail several issues ago). During the machining process, the ring gear register and the bearing diameters are precision ground to ensure zero run out on the ring gear mount surface. The cross-section beneath the ring gear register is increased in order to prevent ring gear deflection. Lightweight steel spools incorporate lightening holes drilled through the hub of the spool as well as a profile milled ring gear flange. This reduces the weight by as much as 25% over the standard steel spool, and it doesn’t sacrifice strength.
Standard steel spools for 9-inch Fords are available in 28, 31, 35 or 40-spline axle configurations. Lightweight steel spools are only available in 35 or 40-spline configurations. 35-spline spools require a stock 2.893-inch case or a 3.062-inch case. 40-spline spools require a 3.250-inch bore case, 45-mm wheel bearings and matching housing ends.
Aluminum spools are manufactured from 7075-T6 alloy forgings (they’re gold coated following machining). An aluminum spool will weigh approximately ½ the weight of a profile milled steel spool. They’re available for 28, 31, 35 and 40-spline axle combinations (keep in mind the guidelines for case and wheel bearing sizes mentioned above still apply). How strong are aluminum spools? They’re regularly used in Pro Stock, Competition Eliminator, Super Stock, Stock and Super Gas eliminators.
In the next issue, we’ll take a closer look at pinion supports and ring and pinion gears.