Self-propelled funnycar dragster

[nerdsforprez]

Here is my rendition of a drag racer “funny car.” Not built for looks, but for acceleration. This was achieved by both including raw power but also limiting the weight of the model (989 grams), and also reducing friction as much as possible (thus the odd front wheels). This build was somewhat inspired by Lego 42050. Contrary to many, I actually really like Technic set 42050. Not for looks to be sure, but at least the looks were consistent with the name usually given to these cars (i.e. “funny car”; TLG mercifully titled the 42050 “drag racer” but I think “funny car drag racer” fits better).

The obvious lacking in the set was any speed. How can you have a drag racer and no speed? I did not want to depend on PF components, nor RC motors. IMO, they lack the authenticity of an engine that at least sounds like a real engine. So I turned to my pneumatic engine I purchased from Alex Zorko at LPEpower.com a while ago.

I love this little engine. Tons of power but also gives the sounds of a real engine. In fact, in the video below, when played in slow motion you can hear the pistons (pneumatic cylinders) firing independently. I worked with two gear ratios…. 2.8:1 (here in the next few pictures you see the first set of gears in the drivetrain, these are the only ones that were adjusted, the other set remained consistent with a 12t gear as the driver and a 20t as the follower): and 1.6:1:

Surprisingly, it was the latter that worked the best. I say surprised because although I knew the vehicle was light, and the torque of my engine high, I thought I would still need more of it to really get this car accelerating fast. But I did not. Also, I initially thought I would need to double-up the gears to limit slip, but I did not. By only needing one set of gears in the 2L size I had set for myself I also reduced friction (with both sets of gears in an 2L space you can see there would be a lot of friction). As can be seen in the video, I used two PSI figures. At 50 PSI the car started fast, with no tire slippage. At 60 PSI, gearing down to 2.8:1, and getting the tires wet I could actually lay down some rubber. Some Lego rubber!

The drivetrain was incredibly simple but effective. Neutral… and one forward drive gear. This is the gear set-up for most drag racers. This can get complex though, with many sophisticated models allowing a type of clutch that limits slip and only allows a certain amount of torque to be applied at different RPM levels. But for all intents and purposes, for this model, all I wanted was neutral and drive. This certainly tested Lego gears, changeover catch, and drive rings. But incredibly, there was no skipping. This was quite amazing given the amount of torque that was applied to the drive train. This is similar to real drag racers. I remember a documentary I watched a while ago stating that the gears in a real drag racer only last 2-3 races before needing replacement because of the tremendous torque, heat, and stress applied.

The goal of the drive train was to have a neutral gear so that the engine could first rev up. Although the engine is quite powerful in and of itself, the fact that it was already running at approximately 1500 RPMs before the gears engaged really resulted in the car’s speed. The whole set-up looked like what is below. To explain the picture you have the arrows pointing in opposite directions representing the tendency of the drive ring to want to pop out of place because it was trying to engage a set of gears where moving at different speeds (well, one was moving and the other was stationary) and the rubber bands acting against it. Initially, when the engine was started I had an axle placed in the hole where the left arrow is to hold it in place. Placing the axle here would stretch the bands back and disengage the drive. So I would first start the engine, let it rev to approximately 1500 RPMs, put the axle, the bands would pull the catch, engage the drive ring into the receiving gear and….. we were off!

More details, pics, and videos @   http://mocpages.com/moc.php/428357