You are comparing a car with lawn mower? Yes if you are not belted in, you can go though the windscreen at higher speeds that a stock lawnmower can go. Lets say you are going as fast as you can go on a lawn mower, you may hit a whole 14 mph on flat ground, I'll give you 16 mph going down hill, those are not road speeds. I think the term you were looking for is called INERTIA, maybe not. As far as bolting weight to the frame, you are putting more stress on the axle then if the weight is in the tires. When the weight is in the tires it is unsprung weight, if the weight is on the frame the axles now have more weight on them that is undo stress on bearings, bushings and tires just to name a few parts. As far as rust on the inside of the rims, your rims are painted, if you don't have any chips or scratches on the inside of your rim you should be OK. Now if you put calcium chloride in your tires, that stuff will eat the paint off the rims and you will get rust. But if you put tubes in your tires then put in calcium chloride in the tube you should be OK. You can put 50/50 water/anti-freeze in your tires and have no trouble with rust, anti-freeze has an anti-rust additive to stop rust just like it works in water cooled engines. The term WATER is used to described the fluid in general instead of saying anti-freeze and water 50/50 mix. When a engine manufacturer gives an in general description of the cooling of an engine, they use the word WATER to describe how the engine is cooled, like (air cooled)(water cooled) engines. Putting weight in tires is good for traction and ballast when you are on hills. Putting weight in your tires is the best way to add ballast and you can't get weight any closer to the ground then in your tires which are on the ground.....