Rear End Technical Information

Pro Gears and Street Gears

Pro gears are made from 9310 and then heat treated. Itis a softer alloy than the 8620 Street gears. The softer 9310 alloy allows the gear to absorb higher impact loads that are generated in drag racing without developing cracks. A harder 8620 street gear could shatter under the same loads. As a side effect the "Pro" gears are not the best choice for street use as they will wear faster. Also available in the "Pro" gears is a large pinion with a 35 spline shaft for high powered applications. This requires a bearing change in the pinion support as well as a 35 spline pinion yoke. Gears termed 9-1/2" are also available for the 9" thirdmember. They offer a slight strength advantage over a standard 9" gear.

About Different Axle Designs

Flange Axle Design - The most common flange axle design has a single piece axle, flange and shaft. The axle bearing has a press fit on to the axle shaft and a bearing lock ring is then pressed on behind the bearing. Housing ends are welded to the housing tube and the bearing slips into the housing end. The bearing is then retained by the caliper mounting bracket. Wheel studs are installed in the flange of the axle where the wheel bolts directly to the outside of the axle flange. With this design the axle and axle bearings must support not only the vehicle’s weight and torsional loads, but also the bending loads from acceleration (rear wheels try to “toe-in” during acceleration).

Safety - In the event of an axle failure the axle along with the wheel and tire can separate from the vehicle. In the case of a break outboard of the bearing, the wheel will definitely come off. A break inboard of the bearing and the wheel will stay on only until the caliper mount breaks and the bearing is free to come out of the housing end. This will also take out the rear brakes and the caliper and brake lines will be torn off.


Full Floater Design -The full floater design incorporates a hub with a bearing installed in each end. Wheel studs are installed into this hub and the assembled hub is then slipped on to the floater spindle. The floater spindle is then welded to the housing tube. The floater spindles are tubular and are large enough for the axle shaft to pass through. The drive plate is splined in the center and slides over the wheel studs and seats against the hub. The axle shaft is splined on both ends, one end engages the drive plate and the other end engages spool in the thirdmember. With a full floater the axle shaft only sees torsional loads since the wheel and tire are bolted to the floater hubs, which are independent of the axle shaft. All other loads are taken in the hubs and floater spindles

Safety - In the event of an axle failure the wheel and tire will remain with the vehicle (as stated earlier the axle is independent of the flange and hub which are mounted on the floater spindle). The rear brakes would not be affected by axle breakage.


Live Axle Design - With this design the axle shaft is a large diameter, thin wall tube and becomes more of a structural member. The shaft is splined in the center to engage the spool and splined on both ends to accept a splined flange on each end. Both ends have a threaded area for the flange retaining nut. Most live axles use a single tapered roller bearing on each end, which must be preloaded with the flange retaining nuts during assembly. This design is most commonly found in Top Fuel car applications. With this design the axle shaft must carry all the load.

Safety - As is the case with the flange axle, in the event of an axle failure (unlikely due to the size of the axle) the wheel and tire can separate from the vehicle. This would also take out the rear brakes and the caliper and brake lines would be torn off.

Which Type of Rear End?

Ford 9"- The Ford 9" is definitely the most popular of the rear ends in drag racing. It offers good strength characteristics and the largest variety of gear choices. The thirdmember type center section allows a racer to have multiple gear ratios available and is much easier to change than a rear loading type rear end. Work can be done on the rear while it is out of the car without major disassembly. With a full-floater style rear the brakes do not even have to be removed to remove the thirdmember. The downside to the 9" rear is it is approaching its strength limits for pro-mod and alcohol applications.

Mark Williams 11"- The Mark williams 11" is by far the strongest of the three types. The 11" system can run virtually maintenance free through entire race seasons. With dimensions and offsets very similar to the 9" Ford, very little modifications are necessary to change over to the 11". The downside to the 11" is that it weighs about 35-40 Lbs. more than a comparable 9" Ford, depending on ratios and components. Gear changes can be rather difficult also, so we reccomend a spare center section with alternate ratios. It is recommended that the 11" be run in situations where gear changes are not frequent. The cost of replacing worn gears and the dependability of the 11" make up for its downsides.

GM 12 Bolt - The lightest of the three, the 12 bolt offers a large selection of gear ratios (although not as large as the 9") and good dependability. 12 bolt modular housings are also narrower than a 9" or 11" at 14" wide instead of 16" wide. The 12 bolt is mainly run in situations where weight and efficiency are major factors, such as Comp and Comp Eliminator. The disadvantage to the 12 bolt is that gear changes can be difficult.

Spool, Locker and Posi-Traction

Spools - 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 - 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 - The 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 - 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.

Open Carrier - 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.


© 2018, Mark Williams Enterprises Inc.

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