portal axles | Thirdwigg.com

Zil 132

May 4, 2012 4 Comments

A couple of months ago I was struck by a new design by Waler. It was refreshing to see a well made Trial Truck based on something a little different. I wanted to make a model of my own. Thanks to him for the inspiration, and for the great ideas on the cab and the fenders.

From the beginning I knew this truck was not going to be a serious off road contender, but I wanted to redesign the whole drivetrain. I decided to go with a pendular suspension for the first and second axle and a trailing live axle for the third axle. All three axles would have a differential and a a set of portal axles. The first and the third axle would also have steering linked together. As is often the case with my trucks, I had the pendular axles held by a turntable with the steering function passed through the turntable by use of a differential. The second axle was held by a turntable in the front, and the steering differential passed through to provide steering to the final axle. The drive function powered all three axles and would connect to the transmission and motor in between the first and second axle.

The third axle was a suspended live axle that had a trailing setup created with the new 8110 pieces. This would allow for rotational and vertical articulation while connecting the drive shaft and giving space to the steering function above. The steering shaft would allow for movement via a CV joint. The Power Functions M steering motor was placed in the rear, and used a simple 1:9 reduction.

A Power Functions XL was used for the drive funtion and was placed between the front two seats. The motor was mounted on a sliding assembly for the gearbox function, much like the design pioneered by ATRX. I used my three speed changeover design to move the motor through three gears, for a final ration of 1:7.5, 1:4.7, and 1:3. The gearshift worked perfectly. While the drivetrain was a little complicated, the gearing was rather simple. The battery boxes were place above the second axle side by side. This kept the weight centered, and as low as I could get it.

Finally I added a cabin and a cargo area. The cabin was straight from Waler’s design, as was much of the fender area. I used technic panels to create the cargo area, which also gave me a space to place the two IR receivers. This also hid the two battery boxes, and the wiring, and generally cleaned up the truck. I created two small doors in the top to assist with picking up the ZIL. I was done.

Over mostly level ground the ZIL was one of my better designs. The differentials and steering worked flawlessly to make the ZIL drive easily. The gearbox worked well and eased the drivetrain over slight irregularities. But once the pavement turned to dirt the ZIL struggled a little more. It was not designed to have too much suspension travel, and this showed. It struggled on some of the bigger bumps, as the tires would scrape the wheel-wells. Overall, I was pleased with the design, and was happy with the way it turned out. It looked great, it was fun to build, and it was a blast to drive.

The full gallery can be viewed here. Also, a big thank you to The Lego Car Blog for posting this model on their blog.

Filed under Trial Truck Tagged with Differential, Driveline, Gearbox, live axle, MOC, PF M, PF XL, portal axles, power functions, remote control, steering, suspension, technic, trial truck, visorak

Kenworth T55

April 28, 2012 8 Comments

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.

Filed under Trial Truck Tagged with 8878, Differential, Driveline, Instructions, Limited Slip, PF M, PF XL, portal axles, power functions, remote control, steering, suspension, trial truck, Turntable, visorak

GMC 2500

April 20, 2012 1 Comment

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.