Kenworth T55


Traction.  It’s all about traction when designing a trial truck.  Because of this, many builders have tried a number of different solutions in designing their own trucks: differentials, gearboxes, various numbers of wheels, various gears ratios, countless suspension designs, and on and on.  So why would it be any different for my trucks?  Every truck I make is a reaction to some set of problems I have encountered with a previous truck.  This is my current solution.

The full gallery is here. Instructions are here.

The Kenworth T55 started as a proof of concept, and turned into a design of a fictional truck.  I wanted to somehow see if there was a way to use differentials in a successful trial truck.  For this to work, two things had to be accomplished.  First, there had to be a way to keep the tires from spinning uncontrollably when they lost contact with the ground.  And second, the torque going through the differential had to be low enough that it would not shred the gears inside the differential when the truck encountered an obstacle.   Could I make axles that had a limited slip differential while having all of the gear reduction at the hub?

Enter turntables.  I have seen some ideas before, most sigificantly from Borec, including this truck, so I went to work.  I designed a mount for the wheels and tires, and placed the universal joint as close the wheels as I could.  Then I used a limited slip differentail design, and sent the driveshaft back to the body.  A simple steering design was used, and the two fuctions passed through the common design of using a differental body through a turntable.

The chassis was designed to keep the weight low, both is mass and location.  As is common, I used the Power Functions XL motor for drive, and I decided to use the 8878 Battery box as it was significally lighter than the other design.  In addition, I would have a drive shaft and a steering shaft running the length of the truck, so the motor and power pack needed to set on both sided of the truck.  Also, because I used the turntables, the drive shaft was very high.  This gave great ground clearance, but I needed to keep the heavy components low.  By having both large components on the side, I was able to keep a short wheelbase of 30 studs, and keep the mass centered and low.  The driveline was complete.

Ever since my GMC 2500, I have held to the belief that a linked suspension is the most efective setup for four wheel trial trucks.  The design keeps all four wheels firmly planned, and does not have the wobbling feel of many pendular suspension designs.  But as is often the case, I ran out of room to place a link rod between the two wheels so I opted with useing the rubber connectors to keep the axles level.    I added a body, and a steering motor, wired everything together, and I was done.

So how did it work.  As you can see in the video, the suspension was effected by the steering and drive shafts.  This further confirms my thoughts on the linked suspension.  Second, the turntables did not really add much.  They added a lot of friction to the driveline, and though they kept the differentials safe because of the tall final gear they did not really isolate the forces on the differential as much as I would have liked.  Third, the limited slip differentials worked well, but still allowed for too much wheel slip.  I ended up replacing the rear with a locked axle, which seemed to work well.  So, for the next truck, bring back the linked suspension, and find a better differential solution.  Maybe then I can find some more traction.

LMTV Mini Truck Trial


As I start this new blog, I thought it may be helpful to post some of my older models to show what I have done, and give a little history to my designs.  In the midst of getting into Trial Trucks, I decided it was time to make a new smaller truck.  I designed a mini truck, to test my abilities with smaller functions based on the Oshkosh LMTV military truck, and to take my mind off the design that was taking most of my time.

I chose the six wheel truck as it would make the suspension of the truck a little more simple than other designs.  The rear axles would be tied together on each side, with a pivot point in between each wheel; one for each side.  A single axle would connect both sides through the dual pivot points, and would be powered by a worm gear directly from the drive motor.  This set-up did not require any additional suspension components, and this would allow the front axle to use an unsuspended pendular set up.  The PF M drive motor was placed above the rear wheels and drove an axle that would go to both the front and rear axles.

Like the rear, the front axle would use a worm gear directly from the motor to drive the wheels.  From the worm gear, a 8z gear was used to drive two 12z double bevel gears for each wheel.  Both 12z gears would drive two more 12z double bevel gears to which allowed for drive through the steering axis.  A simple link connected the two steering wheels, and used a rack and pinion setup to transmit the steering function.  It worked well, and allowed the steering motor to slip at steering lock.  A PF M steering motor was placed above the front axle in the cabin.

I created a small bed, and cabin for the truck and decided to build the truck in blue.  As I was browsing through my Brickshelf folder, I noticed I had too many red vehicles, so something blue would be good.  I placed the 8878 battery box, and PF receiver between the cab and the bed.  This allowed for proper center of mass, and gave me a fully functioning bed.

The model worked well.  For such a small vehicle, it took a lot to stop the truck, in part because of the worm gears.  The steering worked well, but the turning radius was limited, due to the poor steering lock.  I was a fun model, and it is still one of my more popular projects on Flickr.  See the full brickshelf gallery for a more complete view.

GMC 2500


This trial truck vehicle was my first modern truck.  It was the first time I used a number of verious developments in the new truck, including studless frame and body, functional portal axles, power functions, and the new 94.8 tires.  I built this truck with function in mind first.  Because of this the truck ended up looking like a GMC HD, rather than being designed after it.

I started the build with the axles, and then filled in the driveline.  I place the battery box behind the rear axle to keep the majority of the weight low and in the rear.  I linked the front and rear axles so the suspension would work together to add to the stability of the truck.  This also keeps the truck from tipping as an independent setup would have.  Gearing was 5:1, with 3:1 portal axles.  The truck was powerful, light, stable, and quick enough.

With so much focus on the stability, the truck could have used a little more traction.

All pictures can be found here.