Power Functions 4×4 8081
May 18, 2012 9 Comments
For most LEGO enthusists, when they purchased the set 8081, they quickly modified the set with a Power Functions drivetrain. It makes sense. LEGO models are a little more exciting when they are motorized. But I guess I went a little backwards. I wanted to do the fun stuff first, and make the most complicated and compact drivetrain I could make. I posted the instructions here, and they can also be viewed on Rebrickable.com.
But the comments kept coming from people who wanted to see my model motorized. So I thought it might be a fun addition. I added a two PF M motors, a 8878 Battary Box, and an IR receiver. I tried to keep the modifications simple, so I could easily add the motors to the MOD, and take the system out if I wanted to. The drive motor was placed on a simple mount that connected to the frame. The power was fed thought a 8z gear to a 24z gear which then connected directly to the V8 driveshaft. The driveline was unchanged from the V8 down. The steering motor was mounted laterally in front of the rear seats. A 20z double bevel gear drove a 16z gear, then a worm gear moved the final 8z gear which was mounted on the existing HOG steering axle. I removed the passanger seat which is where I placed the battary box, and created a simple mount for the IR receiver. The added weight required a new shock absorber, so I added that as well.
The model worked alright. The drivetrain did well to handle the new power, and I could easily control the Crusier. The steering motor was a little too powerful for the upside down facing steering rack. It skipped a little under load, which was a problem over rougher terrain. The drive motor was a little taxed, so a PF XL would have done a little better. I guess I could add that, but I am ready to move on to my next model. Stay tuned.
The full gallery may be found here.
Thirdwigg.com 
The mods look great, though I agree, an XL motor would have worked better. I like that, unlike many people, you did not take out the engine to motorize it. I also love the new suspension and 4WD.
Agreed. I have found the XL should be used in most drive applications. Thanks for your praise on the 4WD. Feel free to build a copy of your own. Thanks for following.
Hi, I liked your mod. but I can’t understand the mechanism of steering, could you make a photo from a different view of steering mechanism with M motor. I rebild my 8081 by your instruction and now want motoriz it, I ad XL motor, the power is great and now is thinking about steering, but can’ understand your idea. I will be very grateful
Sorry Braburs, the model has been disassembled. You may want to check out this picture and this gallery. In the photo, the motor steering unit is placed directly on the “Hand of God” steering shaft that comes out of the top. The motor has a simple 20z to 16z reduction that powers the worm gear. This worm gear drives a 8z gear mounted on the “Hand of God” shaft that is in the model. I hope this helps.
It’s very very nice, but why didn’t you try using 8110/9398 ball joints? I say this only ’cause I’ve broken many cardans on drivetrains like this one… It’s a very sad thing! They’re quite expensive.. And didn’t front diff. transmission have much backlash? the front axle doesn’t have a very robust gear case -.-” However you’ve done a really pretty model. N.B.:nice the anticonformist idea to use m motor for drive, seen that the moc doesn’t seem very heavy
Thanks for your comment Gabry. I did not use the ball joints because of space for the front. The rear would have worked, but would have required too many changes to the frame. I have had no problems with the U-joints or with the front gear case. It all works well. Yeah, the M Motor is a little unorthodox, but functions quite well. And again, it saves a little space as well.
Thanks!
Hey I was wanting to know if the instructions for the 4 wheel drive version are still available somewhere. When I click the link it’s dead. Big fan of your work by the way! It amazes me what you guys can do
Hi Josh, Brickshelf is currently down, and should be up soon. When it is back up you will find the instructions still posted there. Happy building.