I was wondering if I could get some help designing a drive train for gateway:). I normally find this fairly easy however I had a design for a 6 motor drive but I needed at least 2 more motors for my intake. Oh ya, here is the biggest problem. It needs to be powered by 269’s or 3 wire motors :o .

So you are saying that you need a drivetrain that would be powered by four 269/2-wire motors? If you already designed a 6-motor drive, I would think a 4-motor drive would be just as if not easier to design, no?

Very good point. I suppose I should have also mentioned that the main difficulty i’m having is what gear ratio to use. The robot is probably going to weigh a good 8-10 pounds.

8-10 lbs is pretty light for VEX robot, really. If you’re wondering about gear ratios, the thread murdomeek posted has quite a bit of discussion on it. (https://vexforum.com/t/gateway-drive/19230/1)

Thanks so much legomindstorm:) That helped out a lot but one other thing. Couldn’t you just find out the friction by hooking up a wheel to a piece of metal and put an axle lock on it? Just wondering seeing as you don’t think that the weight affects your results.

Yes, and I don’t believe weight should come into play there. We recently tried out this experiment at a meeting but didn’t really have the ideal tools for the job, so I won’t be releasing the numbers. (I don’t think they’re nearly accurate enough.) Hopefully I’ll be able to try it out again soon, and will release my numbers for the static friction of the different wheels.

Sorry one last thing. My team has gone on summer break and I am really bad with gear ratio’s and well anything related to gears. If I got 13 out of your equation would that be a 1:13 gear ratio? And if so what would those be in actual gear sizes? Also if that was wrong could you tell me wear I went wrong. I first got a random tangent(45 degrees = 1) and then used the 4inch wheels with a radius of 2. 1 x 2=2. Now I chose to use 4 3wire motors which= 26 and divided it by 2 which = 13.

If you got a 13 that would be gearing your robot up 1:13, which would be crazy.

Okay, so you need to take weight into consideration. You got 1 as your coefficient of friction, so multiply that by the weight, which you said would be around 8-10 lbs, so let’s say 10. 1 x 10=10, so now you can take 10 and multiply it by 2, to get 20. Now remember that 26 is your stall torque of all your motors together, so you want to multiply that by around 50%-60% (let’s say 60%) to get 15.6. Now you can finally take 15.6 and divide it by 20, to get 0.78.

So all in all, assuming that your coefficient of friction between your wheels and the foam tiles is 1 (pretty unlikely), you would have to gear down your drive motors 1:0.78.

No problem! And yes, I know you were using 45° as an example, and if you do happen to do this test please post your results, if you would. They could help me in finding the average coefficient of frictions for the wheels, and then in turn help the entire VEX Forum community, and I’d be very grateful if you did.

Again you are very welcome, and I’m glad that I could help you and your team out with this.

I think your final division is backwards, but I’ll have to rewrite it with units.
Max static Traction is Normal force (typically weight for a flat field) * Cof:
Cof = 1 (or ~.8 as mentioned elsewhere by Jordan) (COF is a unitless ratio)
and weight = 10 lbs: 110lbs = Max static traction = 10lbs
Multiply by 2 (2 inch radius of a 4" wheel) to get 20in-lbs of torque max that can be applied from wheels to motor, or vice-versa, without slipping the wheels.
I assume that 6.5in-lbs of stall torque on each of the 4 motors you are using is what provides the 46.5 = 26 in-lbs of motor-shaft-torque.
Since 26 in-lbs torque applied > 20 in-lbs max traction, it is likely that the wheels will slip momentarily when starting up at max-torque.
Assuming a lower Cof of 0.8, and this “problem” gets worse.
Using a speedier gear ratio to lower the torque from the driving shaft would reduce this “problem”.
Note that startup-wheel-slip is only one of a host of issues to consider, and could be countered by ramping up the power more gradually.
And this simplified model assumes one 4x motor driving one wheel with all the weight on it: independently driven wheels and weight shift between wheels during robot acceleration will also affect a more detailed calculation.