Fastest Possible Drive

The thread about the NbN drive trains had me thinking: How fast can an actual, VRC legal robot go? Would it be better to use lexan for structure to lose that extra weight, at the expense of strength? Would it be better to not have all 12 motors? Would the extra battery of a power expander be worth its weight? Would we get to the point where air resistance mattered?

Assuming it doesn’t need to do anything else, what is the ideal VRC “racecar”? How fast could it go?

i would say a 12 motor 1:7 drive, with traction wheels…
rip nascar

i can’t edit my post, but i was meant to say 7:1

the issue with using lexan as drives is that cutting it in the required shapes creates thousands of microscopic cracks, ive seen first hand how fragile vex metal shaped lexan is.

yes an extra battery is worth it, by spreading out the load the voltage drop is less and you can go faster, the increase in power is well enough to justify the extra weight

I’d design/build myself, but I have neither the time nor the parts to. So I’ll just leave my wild speculations here. I think that a 12 motor tank drive using lexan with holes drilled in it for structure would be the fastest possible. I’m not sure as to the gear ratio, because a higher gear ratio means a) more weight, and b) more space taken up, requiring more structure, wiring, etc. Turbo gearing would probably ideal.

@PixelToast I can’t quote :frowning:

(Fortunately?) I’ve never had to push lexan to that limit. Would it be feasible if you doubled up on the lexan? Is aluminum heavy enough to warrant using lexan?

At a competition a local team had 4.8:1 drive on 3.25 wheels with 8 motors. It was insane.

@JustinM We thank you.

But that was still only 8 motors. We can almost certainly push this racecar further than that.

If you did precision cuts that didn’t cause many cracks and not in the shape of vex metal (all those squares) it would be enough for a drive depending on how heavy it is

Hehe rip 8 motors that won’t go on a robot again because of my NBN drive…

More motors doesn’t necessarily increase the speed, but it adds more torque into the system, thus allowing higher gear ratios to be used. So 12 motors would be a must.
In vexU, since we are allowed to remove internal gears, we can theoretically achieve faster ratios just by removing internal gears instead of adding more weight with external gears. This will help a lot with maximizing the speed.
Also you probably would want to put it in an x drive configuration to further increase speed and maneuverability.
In addition, larger wheels increase the speed, but require more torque and add more weight.
There is a lot of optimization, calculations, and testing you’d have to do, but in theory you could make an insanely fast drive.

more motors will increase speed but with diminishing returns as you get closer to the maximum speed of the motors

While our bots were being shipped to worlds last season we thought it would be fun to build a sort of rc car with vex parts, rack and pinion steering, rear wheel drive with differential and 11 motors, i could be wrong but i want to say the ratio was 25:1 with all 1:1 internally geared motors. It somewhat worked in that it drove around fairly fast but due to poor build quality it most likely didn’t work to half of its potential.

This thread really makes me want to revisit that project. This time utilizing polycarb or derlin for the main structural composition and using turbo geared motors and a simpler gear ratio possibly just 7:1. I want to see how far VRC components can possibly be pushed in terms of speed. I should probably finish my Starstruck bot first though…

You have too keep in mind its acceleration. If it can’t reach it’s max speed in sqrt2 * 12 feet, then there is no point.

@Vex Vortex That is true in terms of competition viability at least. But personally I just want to see how fast a VRC based robot could possibly go that was still competition legal not competition practical. I think it would just be a fun little project :smiley:

This is kind of a tangent, but I just wanted to add to this point. You should not have cracking or weakening in lexan by bending it. If that is the case, the bend radius you are using is too tight and should be larger if you have the ability to adjust that.

If you are trying to form the exact profile of Vex metal parts, that can be tricky because the sheet metal bend radii can be tighter than what lexan allows. But lexan shouldn’t be weakened by bending it, unless you are doing it incorrectly.

How about to “heat the polycarbonate to aid in bending.” That would be easier :wink:

The microscopic cracks shouldn’t be too much of an issue because with what you’re saying, how would Ri3d build polycarbonate hooks and make it handle enough weight if it has thousands of microscopic cracks? Is there any machine you suggest to limit the amount of polycarbonate microscopic cracks?

There is no limit on speed, because (excluding torque) you can use cogs to increase speed indefinitely.

They want to see how fast they could get vex motors to make the robot go… Which is a limited speed

The free speed of motors is set at a given rpm, increasing the motor count will never change this value.
A 240 rpm vex turbo motor will be the same free speed as twelve 240 rpm motors turning the same shaft.
However when you put motors under a load the actual speed is less then the free speed due to the torque demand of the load. So increasing motor count adds torque, thus decreasing the gap between free speed and actual speed. Thus the observed increase in speed is a byproduct of the additional torque, the additional motors are not directly adding more speed.