Monster Truck Axles

In this months installment of Monster Tech I will cover one of the single most critical components of the modern day Monster Truck.  That being its Axles.

The average family pick-up truck utilizes 1/2 ton axles to which a 29" to 31" tall tire on a 7" wide wheel is affixed.  These axles were designed to withstand the typical loads placed on them by consumers.  But, as off-road enthusiasts began mounting larger tires and wider wheels on these trucks the additional leverage often resulted in axle failure.  This led to the utilization of stronger 3/4 & 1 Ton axles.

As we crossed the threshold into true Monster status, trucks typically began sporting 48" tall tires with a 25" tread width mounted on 20" wide wheels.   This created its own new set of problems.  The leverage factor simply proved too great for even the strongest 1 Ton axles.  The spindles, steering knuckles and king pins were never designed for such loads.  Furthermore, in order for the fenders to clear the larger tires, trucks were lifted to the point where driveline angles became quite severe.  These problems prompted a search for a suitable replacement which could address these issues.  The solution came in the form of big olive drab 'Rockwell 2 1/2 ton' military troop transport axles.  In addition to having stronger knuckles and king pins, these axles had a third member which was inserted from the top side of the axle housing ("Top Loader") which incorporated a 90 degree pinion thus reducing driveline angles.  These axles also were equipped with a 6.72:1 gear ratio which worked very well with the 48" tire combo.


 2 ton
Rockwell 2 1/5 Ton "Top Loaders" feature a 90 degree pinion.


The Rockwell 2 1/2 ton axles did an adequate job but as more horsepower found its way under the hoods of these Monsters, weaknesses were soon found. Of primary concern was the fragility of the axle shafts. And, as drivers became more aggressive many experienced spindle, knuckle and king pin failure once again.  Some found a solution in an older military truck axle, the 'GMC 2 1/2 ton'.  These proved to be a little stronger than the more common Rockwells, however they had a conventional pinion arrangement making them less than ideal in most cases.

Most builders however opted to stick with trusted Rockwell technology this time in the form of '5 Ton' military axles.  These substantially larger axles were also "top loaders" and were available in 6.40:1, 8.30:1, and 10.26:1 gear ratios.  These axles proved to be bomb proof when combined with the 48" tire and allowed for the progression to car crushing with the larger 66" tall tires we are so accustomed to seeing on today's Monster Trucks.


5 ton
Although similar in design, Rockwell 5 ton "Top Loaders" are substantially larger and heavier than their 2 1/2 ton siblings.


But, just as the jump to 48" tires created problems, so did the 66" especially due to their tremendous width.  As Monster Trucks began "Wheelstanding" the same weaknesses found the 2 1/2 Tons once again appeared in the 5 Tons.  Weak knuckles required additional gusseting and greater negative offset wheels were often utilized to reduce leverage on the fragile king pins and spindles.  The 43" wide tread on the tires also provided a great deal of traction which when combined with high horsepower was often simply too much for the stock axle shafts to bare, therefor custom oversize hardened axle shafts and locking differentials ("Lockers") were installed.  But as Monster Trucks got bigger and heavier, and drivers became even more aggressive, a new solution was needed which addressed all of these weaknesses. This solution revolutionized Monster Trucks as we know them and greatly increased their performance capabilities.  The solution was 'Planetary Ends'.


Planetary hubs are easily spotted protruding through the centers of Monster wheels.


Some heavy equipment such as Off-Road Loaders utilize axles with a Planetary gear reduction hub in which the axle shafts are connected to a sun gear which turns bull gears within a ring gear affixed to the hub.  This arrangement typically provides approximately a 4:1 gear reduction, greatly reducing the load placed on the axle shafts.   Furthermore, the design parameters for these axles mandate extremely strong spindles, knuckles, and tie rods.  Seemingly the only down side to the axles was their conventional pinion arrangement, however in many cases the ends were flanged to the housings.  This allowed enterprising Monster Truck builders to fabricate flanges which were then welded to the Rockwell 5 Ton axle housings to which these Planetaries were then mated utilizing custom axle shafts.  Planetaries such as the 'Rockwell PS250' and its lighter lower ratio brethren the 'PS115' became commonplace.  However, by far the most popular model to this day is the 'Clark 20 Ton'.  This innovation provided the best of both worlds but it came at a price....weight.


5 plan
Rockwell PS250 Planetary Ends are flanged to 5 ton Rockwell axles just outside of the U-bolts.


As with most new solutions come new problems.  Although the axle shaft, spindle, and knuckle problems were solved, they now had in addition to the 1000 lbs. of wheel and tire, an additional several hundred pounds of planetary out on each end of the axle housings.   This additional weight in combination with the inadequate suspension dampening of the day often resulted in bent axle housings.  To combat this, builders welded internal as well as external gussets pushing the weight of these already heavy axles to over 2500 lbs. each! 




Caption: Here we see the result of a heavy foot and inadequate gusseting of the flanges coupling Rockwell 5 ton axles to Clark 20 ton Planetary Ends.

Some builders believed that the real problem was the low quality casting material utilized for the military axle housing so they developed a 'Right Angle Drive' system which allowed for the use of the entire planetary axle assembly. There were a variety of such methods employed ranging from mounting Corvette rearends yoke to yoke above the housings, to custom built SCS 'Drop Boxes' attached to the pinions.  These arrangements proved to be very strong but were still quite heavy.


Behind the billet aluminum bow tie differential cover sits a Corvette rearend mounted on its side linked yoke to yoke to a Rockwell PS115 Planetary axle assembly.


Note the drive shaft entering the Corvette rearend far above axle centerline illustrating an advantage provided by a 'Right Angle Drive' system.


As true racing Monster trucks were developed builders searched for a lighter alternative and the solution was found in a most unlikely place.  Underneath big yellow school buses!

Most common school buses are equipped with 'Rockwell F106' rearends which are substantially lighter than 5 tons or Planetary axle assemblies and are available with a wide variety of gear ratios.  Although they incorporate a conventional pinion arrangement the third member is plenty strong enough to handle high horsepower when combined with suitable planetary hubs and custom hardened axle shafts.  And by this time racing Monster Trucks utilized lower engine placement and drop box transfer cases which reduced driveline angles anyhow. 


Here we see the majority of the components (spindles, knuckles, hubs, end plates, etc.) which make up Planetary Ends. 


This Rockwell F106 axle housing has been fitted with flanges in order to attach Planetary Ends.



The F106/PS115 combo weighs as little as 1,250 lbs.  complete with steering cylinders, Tie rods, and brakes.


The F106/Planetary End combination requires some gusseting of the axle housings but is still sufficiently light weight enough to become the standard for today's Monster Trucks.  


This modern day racing Monster Truck utilizes Rockwell F106 axles with Rockwell PS115 Planetary Ends.


The latest step towards lighter and stronger axles became possible as the result of the increased popularity of 4 wheel drive farm tractors.  Manufactured by companies such as 'ZF' in Germany these newer axle designs incorporate a very small planetary hub typically with a 6:1 reduction ratio combined with a high tech axle housing design cast from superior materials.  Many of these housing include an integral steering cylinder/tie rod assembly, and some utilize 'wet brakes' located within the planetary hubs themselves.  There are a wide variety of such axles available and lighter versions weigh in around 900 lbs..


Light and strong by design, these ZF farm tractor axles feature an integral steering cylinder/tie rod assembly.


On the horizon with foreseeable improvements in suspension dampening I believe racing Monster Trucks will soon be utilizing 'Sheet Metal Axle Housings' mated to ZF type planetaries, with light weight third members (possibly aluminum) transferring power through custom hardened axle shafts. These Sheet Metal Housings will be custom fabricated out of chromoly plating and should prove to be the ultimate in light weight.  It is likely that under a light weight truck with proper suspension dampening, these axles should weigh in at less than 700 lbs., 25% that of the axles utilized a decade earlier.


Sheet Metal Axle Housings may be fabricated with superior strength and lighter weight if properly design.


When it comes to axles it is clear that form follows function.  Where original designs relied on size, contemporary designs rely on engineering.  And as such, today's racing Monster trucks bare little mechanical resemblance to their car crushing forefathers.