Planetary Tech

There are a number of planetaries which have been re-tasked for use in the monster truck industry. Most of these are decades old designs scavenged from obsolete construction equipment. However this may eventually change. Contemporary planetary axle designs offer superior engineering and materials as well as greater gear reduction.

To my knowledge the first planetary to find it’s way onto a monster truck was the Rockwell PS250. These were first mated to 5 Ton Rockwell military axles in 1984 on a little known monster truck named the ‘Texas Armadillo’.

In this early example the PS250 hubs were actually mated to the 5 ton spindles as opposed to the subsequent use of flanges to mate the planetary ends to the desired axle housings.

Monster truck owners soon realized the performance advantage when planetaries found their way onto high profile trucks such as Bearfoot and Bigfoot. PS250s are found with both one piece and two piece knuckles, some on flanged champagne cups and others on fixed housings.

As planetaries became a standard component of a monster truck cost and availability became a major concern. At that time the most readily available planetary was found to be the Clark 20 Ton. As a result nearly 20 years later this has become the industry standard. In addition to being plentiful these were also long considered to be bombproof. That is until recent years. Today these have become scares and weaknesses have surfaced. Axle shafts have been a weak link when used in combination with Rockwell F106s. This is due in part to the Clark’s roughly 3.5:1 reduction. In addition new weaknesses have been found such as spindles and even drive flanges!

Another planetary which found it’s way onto monster trucks very early in the game was the Rockwell PS115. These first appeared on Fred Shafer’s Lil’ Bearfoot in 1984. I personally favored the PS115 due to its greater 4.3:1 reduction and relative light weight (as compared to the PS250 and Clark 20 ton).

Though these have high strength champaign cups the weak point is the knuckles themselves. We found a way to sufficiently strengthen these but today David Smith of King Krunch has developed billet steel knuckles which have eliminated this weakness. It is also my understanding that he has found a 5:1 gear set which is a direct replacement. Unfortunately the PS115 too has become hard to locate.

Now the lesser known planetaries.....Mixed in with Rockwell and Clark planetaries was the sporadic use of the Pettibone planetary already discussed by Scott Bryant. (See earlier post)

Arguably the largest planetaries to ever find their way onto a monster truck were incorporated by Jeff Dane on his Awesome Kong II. These massive planetaries he claims were salvaged from a 170 ton Navy beach recovery vehicle. These were so large in fact that they just fit within the 32" diameter wheels of his Goodyear 73" tires.

A little noticed planetary in use today is the Gallion. These compact planetary ends have been quietly used for years (and very successfully I might add) by Dennis Anderson on his Grave Digger trucks. One advantage of this planetary is that it can be adapted to a standard Clark 20 ton knuckle. These are also lighter than the standard Clark 20 ton hub. However once again the draw back is their availability.

Here is another variation of the Clark which was brought to my attention by Paul Winkleman (Evolution & American Thunder monster trucks). These are actually an aluminum version of the Clark 20 ton planetary. It is my understanding that these hubs will also mate directly to standard Clark 20 ton spindles and knuckles. However I am not aware of anyone who has tried to run these on a monster truck as of yet.

Now the mighty minis....

Few monster truck components have been so coveted as the ZF APL365. Though there were a few issues which had to be worked though, who could not be impressed by their spectacular performance. These small planetaries were first implemented (pun intended) by Jack Willman back in 1989 on his Taurus Racer due to their light weight. There were however several other benefits. First and foremost these feature a 6:1 gear reduction. Secondly they are of a very compact design which reduces leverage on the spindles, knuckles and king pins. As a result these have proven to be extraordinarily strong. The weakest point of these was their spindle nut which was prone to stripping. Tom Meets at the wheel of Paul Shafer’s Monster Patrol and later his own trucks (Maximum Destruction, Hot Wheels, etc.) has beaten these to a pulp and appears to have addressed every potential weakness. If these planetaries were actually readily available they would surely be the standard.

Other similar ZF axles have been and are used by a few teams. Fred Shafer (Bearfoot) received a sponsorship from ZF back in 1991 which allowed him to obtain very rare APL 735s. Later Bob Chandler secured the ZF sponsorship for his Bigfoot team but chose the larger APL 755 with integral wet brakes. Predictably these are also very scarce.

The latest mini to glean interest is the Clark/Hurth. As I mentioned earlier these were first utilized by Alan Pezo on his Predator race truck back circa 1994. However he experienced reliability issues with the axle housings. In recent years Jimmy Creten experimented with the Clark/Hurth on his wife’s Scarlet Bandit truck. Kirk Dabney is also running complete Clark/Hurth axles under his latest Overkill truck.

Mac Plecker has also been successfully running the Clark/Hurth on sheet metal axle housings with custom built knuckles and Ford 9" centers for over a year now. These axles were built by Matt Heady as prototypes for his incredibly light Big Gun monster truck. His truck features chromoly axle housings with billet aluminum knuckles and billet aluminum hubs with Clark/Hurth spindles and planetary gears. I believe this is the most extreme example of planetary axles to date. Though Clark/Hurth planetary axles are uncommon it is my understanding that replacement parts can still be ordered over the counter.

There is a common thread which runs though this entire post. It is the lack of availability of these re-tasked tractor parts. As the supply is expended builders will need to resort to other means to keep the industry going. Custom built sheet metal housings such as those offered by Dan Patrick are now replacing the Rockwell F106 housings used for over the last decade.

Pablo Huffaker (Grave Digger, Blacksmith) now offers custom billet spindles, David Smith offers custom billet knuckles, SCS offer custom inner and outer axle shafts, and Matt Heady single handedly fabricated s billet aluminum version of a production planetary. Therefor, how far off could we really be from purpose built monster truck planetary axles? I believe the industry has been inadvertently inching towards this for some time. If a concerted (combined) effort was placed into the vision of a standardized planetary axle (though the biggest hurdle will be agreement on the specifications) and I am confident that this could (and will) be accomplished in the foreseeable future.

Marty G.
Overkill Racing