MT Axles: What's Out There and What's Being Used?


Well-Known Member
Okay, here's one for all of you tech guys in here. I've noticed more and more threads about axles lately, mostly about what's for sale or somebody is looking for some. What I want to know is, just exactly how many different axles (modified or custom) are being used by monster truck teams? Now I can safely say that I have my fair share of general monster truck knowledge, but axles are just something that I've usually sort of got lost with. I'm familiar with the Clark, ZF, and Pettibone's, but I'm sure that's just the tip of the iceberg when it comes to axles.

So if anyone here is willing to, would you mind telling me (and possibly a few others) what other kind of equipment is out there, who uses them, what can be the differences or advantages between axles, and maybe even preferences? I'd like to know! Thanks. :)


Well-Known Member
Is there a link for that? My first initial thought was the industry-related sub-forum at the front page, but there aren't even any threads there. :confused:

Christian Riedel

Well-Known Member
A post by Scott Bryant regarding Pettibones:

"Concerning Pettibones, for the price and what the current state
of the Industry is (Show business) I think they are one of the better
buys in terms of components. This "Sport" isn't really a sport....yet.
Like I said earlier, it's show business. You work with promoters to get
bookings and you negotiate you fee with them ahead of time so your
performance at the event isn't directly tied to your pay. You basically
set your fee based on your reputation and popularity with the fans. If
you're known for being a good performer, you and ask for a higher fee,
if your somewhat unknown or new or kind of a dark horse, then promoters
aren't going to be willing to pay as much for you. They basically get
one or two "big stars", maybe a couple of other "popular" trucks within
the industry and the rest of the field is filler. Now none of this has
much to do with what planetary you run other than I mention it to make a
point. When you're starting out in this business, the most important
trait you need to portray is reliability. There's already plenty of
folks that run mega hard and thrash their equipment to be the big star.
So promoters have plenty of those to choose from. What they need is
good reliable operations where the team is professional and the
equipment will last the entire show. Promoters don't care much whether
you win or loose, they basically want you up and running at the end of
the show just as you were at the beginning.

Now that is not to say that you should drive super conservative
and not push the equipment because you do need to put on a good show.
But the focus should be on constructing reliable equipment that can take
the abuse that the Industry currently demands of it so that you can push
it and learn it's limits without having chronic reliability issues. In
my opinion the Pettibone is one of the best components for the price
weight and reliability considering what shows are like today.

Now, more specifically, what makes them different than other
planetaries? First let's look at the outside. Overall, they are about
the same size as most other "big" planetaries, they are basically
somewhere between a PS-115 and Clark 20 ton in size and in weight.
Also, they have 12 wheel studs versus 10 that are found on Clarks so
that's better wheel retention, very important when the wheel and tire
combo is basically the size and weight of a grizzly bear. Also, as you
may have noticed the wheel flange is in the middle of the hub instead of
at the inboard edge. Thus the wheel flange is further outboard allowing
the wheel center to be placed closer to the middle of the wheel, which
in turn, makes the wheel stronger. Also having such a less hub
protruding through the wheel center makes it incredibly easier to mount
and dismount (Especially dismount the wheels in tiring up and down.

Moving internally, the Pettibone is much different than other
planetaries used in Monster Trucks. With your background, I will assume
you are familiar with a planetary gear set so I won't risk insulting
your intelligence with a lengthy explanation on how they work. Rather I
will explain the differences between the Pettibone and others. On every
other planetary hub that I have seen used on an MT, the sun gear is
attached to the end of the axle shaft, with the planet gears revolving
around the sun gear but attached to a drive plate, which in turn, is
attached to the end of the hub. The ring gear is a stationary piece,
inside the hub, that splines onto the spindle. Thus, as the axle shaft
spins, the sun gear also spins. The sun gear propels the planet gears
around the ring gear and as they revolve around, the drive plate that
the planet gears are mounted to, rotate the hub and wheel. The gear
ratio of the planetary is the combined ratio that is the product of the
ratio between the sun and planet gears and the ratio between the planet
gears and the ring gear. Also, since the ring gear is stationary, the
planet gears revolve the same direction as the sun gear and axle shaft.
Thus the hub spins the same direction as the axle shaft.

In the Pettibone planetary, the sun gear is attached to the end
of the axle shaft just like others, but from there it's opposite of what
other planetaries are like. The planet gears are attached to a
stationary carrier, inside the hub, that splines onto the spindle. And
the ring gear is actually incorporated into the hub itself. So, the
axle shaft and sun gear spins one direction and that spins the planet
gears the opposite direction. Since the planet gears are held
stationary and the ring rotates with the hub, the hub is spun the
opposite direction of the sun gear and axle shaft. So basically,
Pettibones reverse the rotation of the axle shaft. Like other
planetaries the overall reduction of the hub is the product of the ratio
between the sun and planet gears and the ratio between the planet gears
and the ring gear. Because the Pettibone planetary reverses rotation,
they may make it easier to layout the drivelines in certain chassis

Other internal differences include the Pettibones having four
planet gears instead of just three found in most other types (Some ZF's
have four planet gears too). This spreads the torque over four gears
instead of just three which improves reliability. Also, I have been
told the Pettibones have larger spindles than the Clark 20 ton. Indeed,
I have NEVER seen a Pettibone spindle fail and I've watch Dan Evans
abuse his tremendously.

Additionally, Pettibones are more readily available. It is
getting harder and harder to get Clarks and parts for them as the
Industry eats up the remaining ones left, they are no longer in
production. However the Pettibone, which is found on a large variety of
equipment, is both common in industrial salvage yards and still in
production. This makes it much easier to acquire parts for them and
they are cheaper both in initial cost and in parts than Clarks.

Those are the basic differences between the Pettibones and other
planetaries and essentially my reasons for going with them. They are,
by far, not the lightest and if I were to design a planetary from
scratch for MT's there would be allot of things I would do different.
But for someone starting out on a limited budget, such as myself, and
for what is currently being asked of teams and equipment I feel they are
the best buy going and will go along way towards creating good overall

Marty's post on other planetaries:
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

Paul curtis

Well-Known Member
not to steal the thread or anything, but I saw something very interesting in person the other day, not really a Monster Truck Axle, but I saw a pretty wild looking 80s Chevy truck that had Top Loader axles, but they were smaller than 2.5 tons and the axles were Offset. if I had been thinking I would have taken a picture, but anyway... would this have been a custom set of axles, or is there an actual production axle like that?

Christian Riedel

Well-Known Member
They're generally known as the "Pie Pan" style...they have a part number, but I can't think of it off the top of my head. The planetary cover looks like an upside down pie pan.


Well-Known Member
Thanks for that post, Christian. That helps a bunch. :cool: Just a random thought, didn't Dennis Anderson use a unorthodox style axle for Digger #2? I think I remember him saying on the "Racing To The Finish" video that they originally came off of an aircraft mover or something like that.

Christian Riedel

Well-Known Member
Prior to the "Pie Pan" style pettibones used a rockwell planetary on its larger equipment, PS250 IIRC. These were sometimes badged as Pettibone. As far as the "Pie Pan" style (I could dig up the numbers, but hopefully Chad will chime in here as he knows them off the top of his head) there are some small variations of them. A few things I have noted:

-Some are a "rough" knuckle where there are raised portions where the bolts secure the two pieces together (they use a 2 piece knuckle). Others are smooth meaning they have extra metal between the bolts, making them stronger.

-Inside the planetary you will find that there were two different spindles used. One is a symmetrical bolt pattern, the other has the bottom two bolts have a space between them. I know of no way to tell this other than to take them apart. The symmetrical bolt pattern spindle is reportedly the stronger of the two.

Mine were off an MK-30, and were the smooth knuckle with symmetrical bolt pattern. Unsure of the year of the MK-30 though.

These aren't light, they weigh in around 550 pounds per corner without brakes.

Non smooth style knuckle.

Non symmetrical bolt pattern. Notice the thickness of the spindle in comparison to the dime on top of it. These spindles weigh around 50 pounds by themselves.

I'm not sure if I have any good pics of my planets before I had to sell them, I'll try and dig some out within the next couple days. Hope this helps.