Building a speed bot, advice?

Hello, I am competing in a classroom competition on who can build the fastest bot.
I have some questions…
Should I use tank tread or wheels for the drive? Which works better?
Should I use chains and sprockets for gearing or normal gears for a really fast drive train?
Any other advice?

Our current setup is a four motor drive train with a 1:5 gear ratio using high strength chain and sprockets, with 4 of the standard 4" wheels.
We are considering switching out the two front wheels with omnis.

Also, we are allowed a maximum of 10 motors.
Thanks in advance.

What surface is the race going to be on?

Try to make the robot as light as possible

If you do not need to turn us turbo internals and gear it up faster externally

If you do need to turn us speed internals and gear it up externally

Thanks for the response, the surface will be polished concrete (the hallway) and the race is a linear track, so no turning is needed.

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Use wheels. Treads would have more friction and are slower. Using sprockets vs chain does not really matter. Is the bot only a chassis? If so you could probably run 10 HighSpeed motors geared 3:1(480 RPM) on 4 inch wheels. If that stalls use 3.25 inch wheels. Keep it really light. If it is extremely light you could probably gear it higher.

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Use the smooth green 4in wheels. Go for turbos with a really fast external gear ratio, and have it be really light.

A 10 motor chassis can probably handle a 5:1 if not more on 4" wheels. Depending on the length of the race you could go for an even faster gear ratio (will burn out after a bit of driving).

Tank tread is really slow. Wheels all the way (smooth green 4" wheels if possible, omnis otherwise). As for chain vs gears, I’ve always used chain (allows for more ease in motor placement) but there’s no real difference, as shown with high speed flywheels last year.

Use clutches. that’s the sound that the robot makes, but that’s what’s saving the system.

The trick here is that the lightest robot with the most power will win.

So if I were you, I would use a 3:1 external gearing with turbo gear insides with all 10 motors. Use clutches to save the motors. Make the drivetrain as thin as possible since you dont need to turn. Use the smallest wheels. Link the axles of the wheels to drive straight

Is there any chance of you hitting anything during the driving?
Honestly 5:1 with torque motors is the best way to go with 4in high traction wheels.
If you go any higher you will have to get creative with the ratio which would mean complicated gear setups and slop.

Be very very careful when you start from the stopped position though and ease your way into full speed instead of flooring it as flooring it may make you go slower in the long run

Non-Vex motors but still

For this competition I would suggest connecting an axle all the way across the drive so that you can’t turn. Since you don’t need to, this will keep it from arcing while driving. I might go for a 10 motor 1:7 or something like that. If you don’t want to use a huge gearbox, you can use fewer motors on a 1:5 but use as many as possible. Make sure you test whatever you make before the competition to make sure it actually works.

I would say use the standard 4 inch wheels as a standard if you are racing on marble floor. However, if you have some time before the competition, I would measure the weight of each of the wheels that you have available and then use the wheels that are the lightest and have the greatest diameter. I would help you do this but I don’t have access to any wheels right now since my school’s season ended for the year.

Regarding the gearing, I think you would be able to go as high as turbo or high speed motors with 1:5 or 1:4 without stalling if you use all 10 motors. Just for your reference: high speed motors are 1.6 times faster than standard high torque motors and turbo motors are 2.4 times faster. Ask me if you have any questions. Good luck!

If not full Vex rules why stop at 10 motors? Y off to a few more.

Other non-legal Vex methods of performance enhancement.

  1. Bridge the PTC so it is shorted (twist it or blob of solder, jut remember to undo this before your team is DQ’d in competition)
  2. Waaay overcharge the battery. Not sure the upper limit. Search for why Vex battery chargers are the only thing and see what others used to get their batteries up to without frying the Cortex.
  3. Use something to get a nice grip on the wheels to avoid slipping/spinning. Dirt will not be good on the concrete floor. Clean them before every run. This one is Vex legal as long as it is not chemically altering the wheel.

The wheels need to be free spinning. So make sure they can freely spin to not shank you left or right.

Do you have to use motors? You can use a rubber band or tubing wind-up system. Or just launch the thing with a slingshot.
Oh, and tune the acceleration by playing around. Program with timers and use a camera and full batteries. I would also look for the best batteries. Borrow or “borrow” :slight_smile: batteries from others and test.

Or you can make a wall extend and block the other robots…

You might also want to create a ratchet system so that when you turn the motors off, you don’t destroy the motors from the momentum. The speed of the bot and its mass will keep it moving and more than likely chip gears in the motor. I am saying this because this happened to us on our flywheel before i added the ratchet, so it would be a great thing to add.

I ran 1:5 with 4" wheels and 4 motors (just a chassis). No doubt with 10 motors you could run 1:7 or higher with 4" wheels.

Ratchets would add weight and as long as they are careful you don’t need them

Accelerating will be the slowest part, and probably what sets apart the winner in a race like that. You could make a ratcheting rubberband drive so that it only helps with the take off, but does not slow it down after that. Then use a ridiculously high ratio to stay ahead.

I would use 2.75" wheels. Smaller diameter wheels have better acceleration. While using smaller wheels means you need a larger gear ratio to have the same top speed as larger wheels, your acceleration will be better.

Making sure friction everywhere in the system is reduced as much as possible is critical. This cannot be stressed enough.

Stick to one battery on this. If its somewhat of a drag race setup (which it sounds like) the benefit of having a second battery to pull from won’t make up for the weight that battery will add.

Honestly you could just drive the back wheels and have some small idlers on the front. Look at drag racing cars and such. This would also help reduce weight which will help you go faster.

If you wanted to be really efficient, if you can get the overall wheel base size pretty small, you could drive both sides together so it wouldn’t even be able to turn. Any kind of veering will slow you down so if both sides spin together that would help.

One thing I would worry about is throwing 10 motors on one shaft with both wheels on it. I would definitely use a high strength shaft in that situation or you could twist the shaft.

So, small wheels have better acceleration for the same reason that large wheels have better top speed. Wheels and axles are basically levers; the longer the lever, the lower the mechanical advantage (faster but less torque).

This isn’t completely accurate though as it isn’t an exact relationship like your statement makes it sound like. In general with drives of the size of VEX (and even FRC) maximizing acceleration tends to be more important than other aspects.