For those who are parking lot racers, or who are just concerned with spurs and pinions being chewed up by rocks and such, i found a very simply method which saved many spurs gears from death
I've done this for more than 2 years now.
Just using scrap lexan from your body and mounting it on the screw next to the spur gear. I too race in parking lots and found that this saved my spur gear dramatically, probably reducing 98% of all rocks entering the danger zone
Here is my example on my evo4MS: