We have one of these about every year so I will be the guilty one and start this year's thread.
Our Cub Scout Pack's derby is next Saturday. I have run it the last couple of years and this year instituted a Dads/Siblings class. Same rules as the rest of the Cubs this year but I may go 'unlimited' next year for that race. Differentiating makes check in a little challenging as you have to keep the different cars apart.
Our rules are pretty basic. no more than 5.0 oz, use what is in the kit. Meet the dimension requirements. 4 flat on the track, profile of the wheels must not be modified but light polishing is allowed. If you want to go to the district race you have to use groves for the axles provided in the block. If you do not it is okay to drill new axle holes. In the event that you win and you have drilled axles guides then the 2nd place car goes to the district (assuming it is legal).
That drilled axle thing personally I think is a bit of 'luck' because our fastest car last year was standard. The closest competitor was drilled. Then next was standard. We only have a few folks who choose to do that. I did not do it this year to my car. We built two for my son which we are finishing up tonight and tomorrow I hope. I built one for myself and one for my 4 year old to race with.
We use tungsten weights toward the back end of the car but trying to balance the car just forward of the rear wheel. I do polish the wheels with graphite and debur and polish the axles. Also if they need straightening I do that. Get it to 5.1oz and then back it off a bit...
I let the boy do as much of the work as possible. He uses my scroll saw pretty good - next year he'll be able to do most of the work himself I think. He's approaching the ability to use the dremel tool too but I'm a little wary of that one (all supervised of course). He's not too good at the finishing part. Again, I direct and help there but that tends to cause a little frustration. So I usually back off a bit. Pinewood gets the worst of me sometimes...
Here is this year's entry for me. Pictures of his are coming too as we finish them up.
My wheels will go on there tonight.