OK I'll bite as well.
The textbooks all say light is right, light should always be faster. But then again, all the textbooks are about on-road, rubber slick tyre-kind of friction, which tapers off as load increases. On asphalt, weight transfer is a bad thing. Weight is bad, period.
At the other end of the spectrum, there are Baja trucks, CORR trucks, sand rails,... Those folks all seem to concur that bigger and heavier is faster. If they were let loose, they'd all be building 5 tonne monsters with 2m wheels. The rules specifically constrain wheel size, and that keeps all the rest (hp, vehicle mass and dimensions) in check. note that these folks have 2 things in common with us: 1) dirt, and 2) they face rough terrain and obstacles which are bigger than the vehicle.
R/C-wise, we've all been busy testing with weights ever since LiPos arrived. The first thing I noticed was that a sheet of lead down low is much better than using heavy NiMH cells which put the weight high up. That's a no-brainer.
Then there's the light car. Around 1500g, close to the weight limit. Personally I've never been comfortable nor (consistently) fast with it. Motoring down and softening up the suspension a little does make it more driveable, but still,.... it always seems fragile and skatey to me. The tyres have a hard time keeping in touch with the ground. And having tyres on the ground is an advantage when racing. It's like what Doughty and Flipside have already posted: wheels have mass as well, and it seems you need a high enough sprung/unsprung mass ratio. Regardless of wheel color.
As a side note: over a year ago, there was a lot of talk about Team AE's "B4 light set-up". It seems to have dies a silent death....
A heavy car ( >1700g) does work. Off-road traction is something undocumented and unpredictable, but pushing spikes or pins into the ground is completely different from rubber adhesion. Off-road, more wieght = more grip. A heavy car is easy to drive, and easy to drive HARD. It's as if more mass is like a bigger stick you can attack the out-of-proportion obstables we call jumps with. The downside is that you need a lot of motor. Things heat up more, components wear out sooner.
The stopwatch tells me that the optimum lies somewhere around 1600g. It depends on the track a bit, on a difficult astro track with lots of bumps and jumps, I usually run around 1650g. Same for low grip dirt. I only run light (around 1550) indoors and on billiard-smooth astro. The thing to keep in mind with those figures is they're for a car that's not ridiculously over-powered. The more power you intend to lay down, the more weight you need to bring. And Tony can help you with that.
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