Slot.it Spur Gear Placement

I had the three Slot.it cars flipped on their backs and noticed something: one of the Fords had the spur gear set up with the teeth on the inside near the chassis, while one [GT40 #11] was set up with the teeth near the wheel.

I noticed that the spur gear was grinding against the chassis on the yellow Ford and the Alfa. It was suggested to put a space in between and leave the spur gear in that position. As a test I switched the two cars that were set up this way.  [Note: see the two light gray scratch marks on the underside of the chassis directly in front of the wheels? That's paint from my track from when the cars go over the edge in a turn, then correct themselves. Pretty cool. They haven't scraped any of the paint off the corner edges yet.

I don't have spacers yet, but I have a list of spare parts I'm collecting. Once I get the spacers I'll flip the spur gears back to teeth-inside and see if that changes anything. I've asked around and haven't got definitive word yet which way is optimal.

I have run the two cars, and they are indeed quieter now that they aren't rubbing against the chassis. Here's what one looks like now:

Here's the Alfa after switching the spur gear so the teeth are near the wheel. It's pretty much right on the edge of the pinion either way it is set up.

I'm going to leave them this way until I get the spacers. I'd rather they not rub and maybe not be ideally setup until I can get the parts to do it.