Monday, July 23, 2012

4 Wheel Drive Truck Design Considerations and Two Face Tires

I bought eight Two Face tires to try out on my own four wheel drive off road truck. Here are some pictures of the truck before and after. You also might notice the shiny Technic style frame that you don't recognize. Those are 37 unit long 303 stainless steel beams from Becreo. They are extremely strong and made building the model really easy. There is no frame flex which helps the suspension actuate properly which is awesome to watch. You also might notice a PSP-Nx-v4 from Mindsensors. This model uses an NXT to power the Power Functions motors with a series of adapters like these and those. The NXT receives information from the PSP-x-v4 in a sensor port to control the model. For the programming part, I pretty much mapped one of the Playstation 2 controller thumb-sticks to speed and the other to steering with some scaling values. It was that simple.

An Overall View

Front Module
The front and rear module are primarily constructed of Technic frames. This makes the construction of the module itself extremely strong. The strong modules in combination with the Becreo frame make the structure effectively invincible. with the Above and below you can see the differential which is powered by two Power Function XL motors. This gives the truck plenty of torque. However, the XL motors snap the differential gears like they are made butter. I have had to replace the differential gears at least 10 times in the four weeks of teaching at InZone because they are so easy to break with that much power going into them. Cursed ABS plastic! I might have to make some metal ones....The other weak point are the Lego suspension parts used to get 4 wheel independent suspension. The suspension parts rely on very small nubs to transfer torque at varying angles. After seeing the differential gears break, I am pretty much waiting to break the half shafts going into the wheels if I keep this model on XL motors. Here is a motor of the truck working and breaking a gear. It's really hard to drive the truck and hold the camera. Sorry for a bit of a dark picture.

The simple way to solve this issue is to put less power into the differential. I originally had Power Functions Medium motors powering the modules, but I put XL motors in the model to increase its off road performance. I had fewer issues breaking parts with the medium motors; I might go back to that configuration to solve that problem. One way to solve the differential issue would be to significantly gear up the motors before they arrive at the differential. This would allow the differential to run at high speed but with low torque. The problem would be that the half shafts coming out of the differential would need to be geared down again...significantly. Having all of those gears would be become extremely inefficient due to friction which would rob all of the advantages of putting more power into the system in the first place. Therefore, putting some Medium motors instead of the XL motors would be simpler. The bottom line is that most of these parts were never meant to handle that much abuse. If you look at Lego's current Technic models, most of them are motorized by one or maybe two motors at most to provide demonstrations multiple functions; they are meant to show off clever mechanics instead of some silly 23 year old trying to do silly things. I broke this when I took the video. You might notice that this is much less of gear.

The next, and more involved idea, would be to change the differential to a spool which is a straight axle connecting the two sides. While this would fix the issue with the differential gears cracking apart, there would be a lot of stress on the half shaft couplings since the wheels cannot turn at the proper speeds. How do you fix that? You ditch the independent 4 wheel suspension in favor of a live axle setup. You might notice that in real life, pretty much all of the serious off road vehicles have dependent live axle setups. This would be similar to something like a Jeep Wrangler suspension. They have that for a reason: durability and simplicity. In my next post, I will talk about building a proper crawler!

Rear Module

Rear Suspension Geometry
You can see some of the Technic Frames here in the construction. It is actually pretty simple. You can also see the arms that stabilize the double wishbone suspension.
Getting some new tires!

New and Old tires
The new tires are pretty cool. They add a lot of ride height to the model. They also make the model go pretty fast since their radii are significantly greater!
Outfitted with New Tires

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