randys blog
Limited Slips: Gov-Lock Vs. Yukon Dura Grip
In the early 1970s, real muscle cars were fading away, and average Americans were becoming more concerned with a plush ride than with high performance. In the 1973 model year, General Motors decided to start using the Gov-Lock in trucks, in an effort to eliminate chatter associated with conventional spring preload clutch type positractions limited slip differentials. This was the one of the few solutions available using 1970’s technology. Thus, for GM trucks produced after 1972, the only traction-enhancing option was the Gov-Lock (produced by Eaton). These governor actuated units provide a locking differential without chatter. Great for the average truck owner, but not for those who really use their trucks off-road. Often referred to as a “time bomb” or “Timex Posi”. These locking differentials work fairly well for occasional use when traction is of small concern, but not when power transfer and traction are the main goal. The Gov-Lock can fail in any number of ways, and often does. Usually, the case will break in half, but sometimes small internal parts will break. I have no intention of defending them except to say that the Gov-Locks used in 3/4 & 1 ton trucks hold up fairly well due to their sheer size, and especially well when compared to the smaller designs. For readers looking for something stronger, Yukon makes redesigned units that are worth showing off. The Yukon Dura Grip is not actually a “new” design so much as a vast improvement on the old design. The basis for these new units is the heavy-duty positraction that was originally produced for big block GM muscle cars in the 60s and early 70s. Dura Grips now come with composite clutches that are more durable than their steel counterparts. New units also use stronger spider gears produced with Net Form Forging technology. These stronger spider gears look very different compared to standard cut gears. They are made from 8620 steel to increase strength and durability. Net Form Forged gears perform at least 20% better than standard cut gears in both impact testing, and testing for fatigue under heavy loading. In addition to the other improvements, Yukon has improved the case designs. The original 12-bolt truck positraction was very similar to the passenger car design but was not nearly as strong as it’s passenger car counterpart. The new 12-bolt truck cases are about three times stronger than the original ones used in the early 1970s, and all Yukon Dura Grip cases are now made of nodular cast iron with increased material thickness in critical areas. The new 10-bolt cases now have larger bearing journals that increase the case strength and use bearings with more rollers than stock. Overall, the new Dura Grips are superior to the admired and sought after original OEM GM positraction. By using 90’s technology, Yukon has built a stronger, harder working positraction that does not chatter. Yukon has developed Dura Grip limited slip differentials for: Chrysler 9.25″ Dana 44 Dana 50 Ford 7.5″ Ford 8.8″ Ford 9.75″ Ford 10.25/10.5″ GM 7.5″ GM 7.625″ GM 8.2″ GM 8.5/8.6″ GM 9.5″ GM 11.5″ GM 12 Bolt Passenger Car GM 12 Bolt Truck GM 10.5″ 14 Bolt Truck Nissan Titan CI Corvette 17 Spline Popular Resources: Ford 9 Inch Differential Information Four Critical Ring & Pinion Settings
Positraction Chatter
When looking for great off-road traction an aggressive positraction is one of the most popular ways to go. But, when driving on the street every day that great off-road posi may be annoying. I have found that it is very difficult to make a positraction work really well off-road and still not chatter during every day driving. Many say that they can do it but my question is can they do it consistently without any customer complaints? Positraction function has been described well by many before me so I will skip that for now. I use the words “positraction” or “posi” as generic terms for all limited slip differentials (not including lockers). Posi chatter happens when there is a lot of friction in the clutches. The friction causes the clutches to bind during a turn and twist the axles. Then the clutches let go and the energy stored in the twisted axles is violently released shaking the whole vehicle. It is kind of like a bomb with energy stored in the twisted axle instead of being store in an explosive. Posi slip happens when there is not enough friction in the clutches and they do not hold when they are needed. Sometimes this can be helped by applying the brake or parking brake lightly in order to add resistance load to the carrier assembly which increases the clutch loading. Even with aid from driving technique a slipping posi is disappointing when off-road and really disappointing when stuck with one wheel still having good traction. The ultimate positraction would slip during every day driving and then sense that one wheel is slipping a lot relative to the vehicle speed and then apply more friction. Many manufactures have attempted to achieve this through many different types of positractions. Every time I see someone promote the final solution they find a way to upgrade it a year or two later and promote the replacement as the final solution. For those of you who are really serious about off-road I suggest getting some sort of locker but that can be left for another time. Popular Resources: New Gear Break-In Properly Setting Preload with Side Adjusters Size Matters
Worms Can Help Off-Road Traction
Even though the Gleason Torsen limited slip differential has been out of production for about thirteen years now, I still get quite a few questions about it. The Gleason Torsen was a gear driven, torque-sensing limited slip differential that was very smooth and reliable, as long as it was not used with tall tires. Although Gleason decided it was too difficult to make a strong yet economical unit, Tractech (maker of the Detroit Locker and EZ Locker) has been able to develop their own line of torque-sensing limited slips they call the Detroit TrueTrac. Detroit TrueTracs are similar in design to the Gleason Torsen , only the TrueTracs hold up well with reasonably tall tires. They use worm wheel gears that work on the same principle as a worm gear. When the unit is loaded, the worm wheels are forced away from each other and against the case. They develop locking torque by the light friction between the worm wheels and the case. This light friction is multiplied by the ratio of the worm wheel, which depends on the size of the worm wheel relative to the size of the side gear. The amount of torque biasing or lockup can be changed by the manufacturer by simply changing the diameter of the gears, or the pitch and spiral of the teeth. A larger difference in the size for the worm wheels and side gears (smaller worm wheels and larger side gears) will increase the lockup force. Increasing the pitch and spiral of the teeth will also increase the lockup forces. Like everything, there is a limit as to how much lockup force is reasonable. Making the unit too “aggressive” will cause tire scrub during everyday driving and will cause the unit to wear out too quickly. All of the feedback that I have received from customers indicates that the unit works very well for mud and snow, while going unnoticed during every day driving. When used in situations like rocky trails where one wheel gets off the ground, the unit will not lock up 100%. Light application of the brakes will help the differential engage more transferring power to the tire that is still on the ground. For extreme situations where the vehicle will have one wheel in the air often a locking differential provides better power transfer to the wheel on the ground. Detroit TrueTracs are designed for medium duty 2WD and 4WD applications. They work well in the front and rear, and are so smooth that there is no hint of resistance in the steering wheel when used in front drive steering axles. Like the Gleason Torsen , the Detroit TrueTrack does not bang and clunk, has no clutch chatter, does not wear out like clutch type positractions, does not need limited slip additive, and needs no preload or clearance adjustments. Sounds too good to be true? The only drawbacks to the TrueTracs are limited applications and their inability to lockup 100% like a locking differential can. Although Gleason no longer makes the Torsen differential, the Detroit TrueTrac is a proven design that is available for many popular applications.