Axial-based Solid Axle Monster Truck Project

By AR on Oct 17, 2015

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Solid axle monster trucks patterned after real monster trucks have a long history in R/C. And while these big rigs have been around a long time, it has always been the case that you need to make it yourself to get what you really want. Creative modelers have fashioned a plethora of scale versions of the legendary Bigfoot and Grave Digger trucks and numerous other popular trucks. At the heart of this segment is a creativity that we can all appreciate. Lately, builders have turned to Axial to build high performance competition worthy monster trucks. The most common part they’re reaching for are a set of Axial AR60 axles. One of the early movers in the Axial-based monster truck movement was RC Truck Stop. Their build used a variety of Axial parts. In fact, the drivetrain is just about all Axial. Check out the article and photos below:

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Could you imagine showing up to the track with a 25-year-old RC car? Me either. Like all other club racers, I’m a sucker for having the latest and greatest, and when I wanted to build a new monster truck for side-by-side racing, I couldn’t bear to start a new build with outdated equipment. But, that’s exactly what the vast majority of solid-axle monster truck racers do when they lineup for side-by-side action. I am, of course, making reference to the Tamiya Clod Buster and its legions of loyal owners. I am a huge fan of the Clod, and you probably are too. Tamiya’s classic car crusher is truly iconic and one of the most significant releases in RC history, but while we celebrate its 25th year, it seems odd to race it in anything other than the vintage class. I knew with some tinkering and some creativity, I could build a modern monster for racing.

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CHANGE OF CHASSIS
In my opinion, side-by-side monster truck racing is a lot like rock crawling. Huh?! Lower that eyebrow and let me explain. I see the appeal of monster truck racing being in both the performance and scale appearance of the trucks. As such, many of the race chassis just don’t do it for me because while they perform well, they don’t look anything like a real monster truck’s tube frame. Going the other way, while not all are super heavy, steel or even brass tube frames are generally not very light or built with a low center of gravity. They look great, but aren’t the first pick for racing for a reason. I wanted the lightness of fiberglass, graphite or plastic and the realism of a tube chassis. As such, I went with HPI Wheely King plastic side plates. They are feather weights and look like the real deal. All I used from the stock Wheely King are the side plates. To get a more custom look and inspired by the way the full-size Grave Digger’s chassis is painted green, I painted the frame rails with a plastic-specific spray paint. The cross members I made are solid Delrin rod that I cut to length and drilled for 3mm hardware.

The only possible downside to the Wheely King plates is that they allow a noticeable amount of flex. Even with five Delrin rod cross members spanning the chassis, the finished frame can be twisted by hand. It’s unlikely this will negatively impact handling, but it’s worth noting.

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SKID PLATE
While the frame rails are what most people initially notice, the real heart of the truck is the use of Axial Wraith drivetrain components (axles and transmission). I’ll outline the benefits of the Axial components a little later, but the transmission required a custom skid plate to be fabricated. While I would eventually like to get a higher quality skid plate professionally machined, I was able to fabricate my own out of plastic. I started with some blank plastic stock that I received from my friend and fellow RC’er Tom St-Onge and from Pin Shop Hobbies in Oakville, CT.

When designing the new skid plate, I used my preferred method–CAD or Cardboard Aided Design. In addition aiding in marking the transmission mounting holes, I used the stock Wraith skid plate as a guide for how to position the lower link mounts. Axial’s engineers know heaps more about suspension geometry than I do, so I saw no need to mess with their design. One I had all the measurements I needed, I made cardboard template. And, after transferring the template to the plastic, I cut the template (hacked at times, to be honest) with my bandsaw and Dremel rotary tool fitted with a reenforced cut-off wheel(See safety note below).

WHY THIS WIDE?
The determining factor in how wide I built the skid plate was actually the axles. I wanted to use the AR60’s stock shock mounting positions to maintain as much of Axial’s geometry as possible. So, I built the skid plate to position the side plates about 4″ apart. The link geometry of this truck is the same Wraith with the exception of link length and the upper inner link mounts.

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SUSPENSION
As I have said, I took as much as possible from the suspension design of the Axial Wraith. I did vary from the formula when building the link length. I made the links longer to achieve the 13-inch wheelbase I desired. I also made the rear links slightly longer than the front to increase front of center weight bias.

The shocks I used are HPI Wheely King aluminum upgrade pieces. I removed the purple anodizing to make them better fit the look of the rest of truck and filled the shocks with 30-weight shock fluid after rebuilding them using Team Associated Green Slime on the O-rings. I also installed limiters under the pistons to decrease the extended length of the shocks and lower the truck a fairly low race-ready stance. I used Axial shock bushings for the top mounts and trimmed (narrowed) the shocks lower plastic balls to allow them to fit in the AR60 lower shock and link mounts. I used Axial’s aluminum lower link and shock mounts because they offer more adjustability with three holes for the shocks and three holes for the links.

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Most monster trucks benefit from sway bars, but they are absolutely essential on a shaft driven monster truck. I bent and cut music wire in a variety of thicknesses. The music wire is attached to the lower links with cable ties. This is how many people add sway bars to trucks like the Wheely King and is similar to the factory sway bars on Tamiya’s TXT-1.

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DRIVETRAIN
The only things Clod Buster about most racing Clods are the axles. While the motor-on-axle (MOA) design of the Clod Buster axles have the huge advantage of no torque twist, the axles were never intended for competitive use. The Axial AR60 axles have many of their own advantages. There is no denying that shaft driven trucks are plagued by torque twist, but it can be tamed and in the case of the AR60 axles, I strongly believe the pros outweigh the solitary con.

AR60 Advantages:
> Streamlined design
> Option of sealed, tunable differentials
> Replacement parts readily available
> Significantly durable
> Wider than most other modern axles
> Easily adjustable caster
> Numerous hop-up parts available
> Easy to service
> Excellent steering geometry
> Easily made straight or steerable
> Gearing options

I installed Axial EXO differential parts (Thank you, Axial) to replace the stock locked differentials. I did this because I preferred actual differentials for this application, but also because No Limit RC–one of the sanctioning bodies I will be racing in–requires front and rear differentials. I have started with simple black grease in the diffs, but thicker grease or silicone fluid is a possibility.

I installed Axial’s heavy duty bevel gear sets, front and rear, and I took advantage of the fact that I could run different gear ratios in each axle. Using an under drive (43T/13T) gear set in the rear axle and the stock ratio (38 to 13) is a common trick used in rock crawling to control torque twist.

While the AR60 axles are wider than most 1/10-scale axles currently being made, I needed a wider stance for racing. I used RC4WD’s 12mm to 12mm aluminum wideners (Z-S0449). These wideners increase the width on each side 1.25-inches for a total increase of 2.5-inches.

AXIAL PARTS LIST*

  • AR60 OCP Front Axle Set >> AX30831 >> (2X)
  • AR60 OCP Machined Link Mount >> AX30830 
  • Aluminum Servo Horn 24T (Hard Anodized) >> AX30835 
  • Heavy Duty Bevel Gear Set – 43T/13T >> AX30402 
  • Heavy Duty Bevel Gear Set – 38T/13T >> AX30395
  • WB8 Driveshaft Set (2pcs) >> AX30794 
  • Differential Bevel Gears >> AX30390 >> (2X)
  • Differential Shaft >> AX30170 
  • Differential O-rings >> AXA1162 
  • AX10 Locked Transmission Set >> AX30487 
  • Steel Outdrive Shaft Set >> AX30435 
  • AX10 Slipper Clutch Set >> AX30414 

 *List does not include all possible parts and hardware needed as builds can vary

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STRC PARTS PACKAGE
There are a number of manufacturers that offer aftermarket parts for the Axial AR60 axles (Axial included). One of my favorites is STRC. The reason I like STRC is they make a wide variety of direct fit aluminum parts with a quality finish, and best of all, the parts are durable. Since the axles take a real beating on a truck like this and because I wanted rear steer, I upgraded the axle C’s and knuckles with STRC’s aluminum pieces. I also replaced the plastic diff cup retainers and diff covers. My upgrades didn’t end there. I added STRC’s upper link mounts and aluminum steering links.

STRC PARTS LIST

  • CNC Machined Precision Aluminum Steering Knuckles for Axial Wraith (1 pair) Black >> STA80061BK >> $31.99 (2X)
  • CNC Machined Precision Alum. C-Hubs for Axial Wraith (1 pair) Black >> STA80062BK >> $30.99 (2X)
  • CNC Machined Alum. HD Diff Cover for Axial Wraith (Silver) >> STA80070DS >> $15.99 (2X)
  • CNC Machined Alum. Internal Diff Holders (1 pair) for Axial Wraith Silver >> STA80070S >> $$15.99 (2X)
  • CNC Machined HD Alum. Front Servo Mount Block/Upper link mount, Axial Wraith (Black) >> STA80072FBK >> $23.99
  • CNC Machined HD Alum. Rear Upper Link Mount, Axial Wraith (Black) >> STA80072RBK >> $23.99
  • CNC Machined Aluminum Servo Mounts (1 pair) for Axial Wraith (Silver) >> STA80072SS >> $14.99 (2X)
  • STRC Heavy Duty Aluminum Steering linkage set for Axial Wraith (Silver) >> STA80073S >> $12.99 (2X)

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GETTING GRIP
In RC racing, I often preach that tires are the single most important part of setup, so why would a monster truck built for racing be any different? To get improved traction over the stock rubber, I selected RC4WD Rumble tires (Z-T0015). These are direct a fit on the odd ball sized Clod Buster rims. The Rumble tires look like shaved full-size monster truck tires in that the backside of each lug have been removed. The compound is significantly softer than stock, and the tires include foam inserts. Even though the Clod rims have vent holes, I added three small additional holes to the tires. The goal is to allow the tires to compress and distort when landing or cornering without causing bounce or not returning to their proper shape immediately.

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BODY
Traditional thought is that looks don’t matter in racing, but you better believe appearances matter in monster trucks. Looks may never win a race, but the body on a monster truck is its personality–the basis of its persona. I envisioned a truck that looked bad to the bone, but wasn’t a dime a dozen like RC Grave Diggers. And, I wanted to avoid a typical pick-up truck body, so I selected the ’55 Bomber body (182) from McAllister Racing. This gives my truck the styling of the popular Advance Auto Parts Monster Jam Avenger truck, but the simple black paint job sets it apart. The McAllister ’55 Bomber body is a little wide, but is otherwise perfect for this application.

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MORE MOTOR
As much as I personally love solid axle monster trucks, the reality is that this segment isn’t short course racing. What I mean is there isn’t a race class at every hobby shop. So, I have to go where the racing is and play by their rules. And, that’s where it gets complicated. To maximize my potential racing opportunities, I designed this truck to be able to compete in No Limit RC competition and at RC Monster Truck Race Series (RCMTRS) races. Both series require 2.6-inch Clod rims, but the similarities between the rules pretty much end there. Ironically befitting the name, No Limit RC has no wheelbase or track width limitations. In contrast, RCMTRS limits wheelbase to 14″. For now, I’m sticking with a 13″ wheelbase but may change to 14″. More on that later.

The biggest dilemma I faced was that No Limit RC requires dual motors and RCMTRS does not. I’m not a big fan of running two motors when one can get the job done, but I needed to be able to comply when needed. I installed Level3-RC dual motor mount for the Axial transmission. Level3-RC makes two versions, and I selected the one that positions the motors one on top of the other because it makes for the most compact arrangement.

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ELECTRONICS
After an initial round of testing, it was clear I needed to go with a sensored brushless system for my single motor setup. The local RCMTRS affiliated club, RC Monster Truck Challenge of NY, limits motors to 5700Kv and limits the battery to 2S as opposed to no motor limit and a 3S limit outlined by RCMTRS. At first I tested with a sensorless 5700Kv 4-pole motor just to push the suspension and general setup. The low speed hesitation (often referred to as cogging) was unbearable, so I installed a Duratrax DE10 speed control (DTXM1300) and a sensored Duratrax 8.5-turn motor (DTXC3425) which is rated for 4450Kv. To tap into the full potential of this speed control, I also used Duratrax’s handheld digital programmer (DTXM13500).

With 4WS, I needed exceptionally strong servos, so I bolted in Hitec HS-7950TH servos (37950S). These servos feature coreless motors, titanium gears and deliver 403 oz./in. of torque at 6V and can put out 486 oz./in. of torque on 7.4V.

To control the truck, I went with a Spektrum DX3R Pro. This 3-channel radio is easy to program and allowed me to quickly setup my on-demand rear steer. I programmed Switch B which is normally used for steering trim (moved to switch C) to control my rear steer. Switch B is easy to reach while driving and I have to set up in such a manner that when turning right I flip it to the right for more steering and vice versa for turning to the left.

Because the battery sits high, I am using a MaxAmps.com 3250mAh 2-cell (part no. varies). This pack is incredibly thin and lightweight, but will easily keep up with the power needs of the truck and offer plenty of runtime–more than enough considering I’ll be doing side-by-side runs.

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PERFORMANCE
Since this project is more of a custom creation than, say, a modification of an existing platform, I was exceptionally eager to test it. While the concept seemed sound in my head, there were no guarantees that it would even work. My first tests quickly proved that my fear of torque twist was very real. Adding sway bars, as planned, immediately helped, but were not enough. I even doubled up the rear sway bars at one point. I also experimented with extremely stiff spring setup on the right rear and left front shocks. While stiff springs can be used to tame the torque twist, jump handling is diminished too much for this to be a viable solution. Ultimately, I installed the under drive gears in the rear axle and switching to the milder Kv motor. With the rear axle geared lower than the front and the slightly lower Kv motor (also less torquey 2-pole), the truck was pretty close to dialed. Thicker sway bars are needed and in the works. I simply underestimated how thick the wire needs to be to control torque twist at launch.

Further testing included a short run on carpet at Pin Shop Hobbies in Oakville, CT. My impression of the truck on carpet is that it handled extremely well, but covers a lot of ground quickly. Quickly enough to make me nervous. I didn’t hit anything too hard, but I now know what the full-size drivers feel like when racing in the small arenas. While not every carpet track is the same, I was surprised how well the truck rotated on the fuzzy stuff.

My first real test that featured some competition took place at a Touch-A-Truck event that the Eastcoast RC Trucks club was participating in. In addition to demonstrating the truck, I raced it on the club’s timed course. Even while running at a fairly conservative pace, I was happy with how competitively the truck performed. My times were among the fastest recorded and I wasn’t running all out. After the timed runs (I ran two official runs to settle a tie-breaker), I started to get braver with my freestyle antics. The truck jumps great and responds extremely well to throttle inputs. It also recovers well from less than perfect landings. If you’ve ever driven or seen a truck like this run, you know how much fun watching one handle like the real trucks.

Only once during my day of time trials and freestyle, did I encounter any sort of breakdown. A particularly nasty end-over-end landing popped the front steering link off. I snapped the rod end back in place, made a mental note to add a washer as a retainer and was back on my way. But, speaking of breaking stuff, previous to the Touch-A-Truck, I did have some minor parts carnage. The early victims were the body mounts. Even though the body mounts are fairly short, the thinner 1/10-scale touring car style ones I first tried pretty much instantly broke. So far, the slightly beefier replacements have held up, but I’ll be watching them and thinking of an even stronger solution. I also broke the original Clod Buster rims–all four. I have no idea how old these rims were, but they didn’t last too long. Pin Shop Hobbies came to the rescue with a set of new Clod Buster rims from Tamiya. The “un-aged” ABS have worked out great and withstood all sorts of abuse.

The best way to describe the overall experience of driving this truck is that it’s fun. I know that’s vague, so to be more specific, it drives, reacts and generally behaves like a real monster truck. All of that makes it–a lot of fun. It’s a handful off the line when too aggressive with the throttle, but I’m working on that. As described above, jumping is a blast. Hit a jump square and it flies perfect, is easy to control and lands on all four without even the slightest bounce. Fly through the air crooked and it’s even more fun as you land one wheel first, bounce the front up and power out like the real deal.

Unlike most race Clods, this truck has rear steer. The rear steer adds to the fun, but I have to admit I largely ignore it even though I have the rear steer throw turned down to less 25% of the front steering. I’d have to get tons of wheel time and develop some real synergy with the truck to use it in racing, but it rocks in freestyle type use.

While the testing will continue and I will certainly learn even more when I race it side by side with other trucks, I can say I am highly satisfied with the truck thus far. At this early stage, it isn’t perfect, but this truck clearly has potential to be a legit racer.

FUTURE PLANS
While I’m sure there will be changes that I can’t predict as getting more real racing experience with the truck will show what needs to be done–or not done. I do hope to have a skid plate professionally machined. I would also love to have new side plates cut out of G10 fiberglass. My design for the fiberglass side plates would mimic a tube frame and the look of Wheely King plates, but will have more tuning options for the upper shock mounts and upper link mounts. Even better, it will be more durable.

I also plan to cut down the lugs of the RC4WD tires and balance the tire and rim setup. While these tires are proven to be exceptional thus far, I would like to also try Imex’s Baja tires for the Clod Buster rims.

The one item I am on the fence on is the rear axle and whether to keep the rear steer or go with a straight axle. On testing and experimentation will reveal which is best. I have lightheartedly dubbed this project the Clod Killer or CK-1 as it’s the first version. Maybe CK-2 will be based on the Axial XR10 if I can talk someone into making a differential for the axle.


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