Primate wrote:JTMess wrote:Our (unnamed) vehicle weighed in at just under 2 kg for regionals. We're just starting modifications for states and we plan on using an adjustable weight with more weight for longer distance. We found that this works well because the car will lose less speed at longer distances due to conservation of momentum.
On another note, we calibrated our car on a flat school hallway. The flooring at regionals was unfortunately covered in dust and caused our vehicle to skid for 25 cm more than it was during testing. Is anyone else having this problem? And if not, what wheels are you using to avoid this? We were using CD's coated in electrical tape for traction.
If you're using the threaded-rod breaking system, add some rubber washers to your axle. They'll brake pretty consistently and almost completely eliminate skidding, so friction will impact only the run time, not the distance.
Adjusting weight is a good/workable approach. Technically, its not
conservation of momentum that gets you less speed loss at some (longer) d, its that you have a certain, let's call it momentum appetite (rolling friction-i.e., bearings rolling resistance-"tire" deformation, braking system drag/friction, aerodynamic drag - let's call it total friction loss rate) that's "eating into" the momentum. Plotted against time, this "appetite" will be a curve- greater appetite at higher v; decreasing as v decreases. Momentum is m x v; m is constant, so as momentum is "eaten up", v decreases. That appetite curve is your instantaneous velocity vs time curve. If you have more mass (i.e., more momentum, your instantaneous v at any distance will be greater. The only conditions under which this would not be true is if your overall design means significantly higher total total friction loss rate at higher m. Decent design/engineering will avoid that.
Hope (assume) you mean 25
mm (2.5cm) of additional skidding?.
Also, I see nothing in the rules that would prevent wiping the dust off the floor. Either requesting the Event Supervisors to do so, or requesting permission to do so - just do it at check-in, so its same conditions for all (tather than after anyone's run).
As to wheels, electrical tape is certainly an improvement over the bare plastic of a CD. Its all about coefficient of friction. I suspect the old ring of rubber from a balloon would work a little better, and a rubber band is another, possibly better option. We're using aluminum wheels from an RC race car; about 2" diameter, with neoprene O-rings.
lucwilder42 wrote:So you mean put rubber washers between the wingnut and the stop? Won't that prevent the locking?
No, it will lock up at some point, depending on how thick, how hard, how many. Instead of locking up virtually instantaneously, braking force will be applied (i.e., distributed) over some increment of time/distance. The instantaneous force increases as compression of the washer(s) increases, but right up to near the end of compression, it is significantly less than of a "hard" lockup. If the instantaneous force is reduced sufficiently, no wheel skid. The amount of washer compression you'll get before lockup will vary with velocity (i.e., vary with run distance - which means you'll need to calibrate for that variable at various distances).
