4WD, Tank treads and gear ratios.
 

Hey all. I posted these three posts on the VEX forum but since you guys have so much experince building robots (of which I have none), I'm posting them here also.

One:
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I'm familiar with gear theory (torque and speed). But what happens if you connect the wheels directly to the motor? (i.e. the same driveshaft from the motor is the one inserted in the wheel) Does this give good torque? Speed? I don't have any gear ratio involved so I don't know. Is it advisable to always use gears or sprockets, and not to connect the wheels directly to the motors?
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Two:
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What happens when I use four motors to make a "4WD robot". This is suggested in the VEX Inventor's Guide. That is, two motors on each side spinning in the same direction. One motor per wheel. I don't like the idea because I expect that the two motors driving one side of the robot will not be completely syncronized.

Is it necessary for the motors on the same side of the robot to be completly in synch. Or is the factory difference in speeds good enough for this arrangement?

Also what is the benefit of this arrangement, since the speed is "the same" on both motors on each side. I suppose I can expect a torque increase but not a speed increase. Is this correct?

Finally, can I expect greater torque (and traction) with four motors when using the VEX tank treads?
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Three:
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I purchased the VEX tank treads and have so far been dissapointed. Although I have not been able to build a suitable drive train for them yet, I suspect that they are a waste of money. I say this for 2 reasons.

1. They are made of plastic, and therefore they cannot get a good grip on most surfaces.

2. They don't "roll" that easily, so they require considerably more torque than a wheel of the same "diameter".

So my question is: Do any of you have experience with the VEX tank treads? Am I missing something here?
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Thanks in advance for your help.

Re: 4WD, Tank treads and gear ratios.
 

I believe JVN is the guy you need to get in touch with. He is GREAT with that stuff.

Re: 4WD, Tank treads and gear ratios.
 

it only gives good torque if your motor has good torque, and it depends on the wheel size the bigger the faster you go but less torque, the smaller and the slower you go but more torque

you would have to program it really good to have both sides in sync on a 4wheel drive robot, this is because the motors naturally spin faster in one direction because of how they are wound

the benifit of the 4WD arrangment is your traction and your speed/torque

- if you ever have less then all 4 wheels contacting the surface your dfriving on then you have wheels that are driving even if it is one wheel, it will still help alot

-depends on how you gear it you can go the same speed but have 4 times as much torque if you put the wheels right on the drive shaft, but if you gear it down you can go 2 times as fast with the same torque, it is all about gearing

i dont have expirience with VEX tank treads but if they dont roll easily see if something is wrong with them like spray some WD-40 on it see if they move easier or you can exchange them for new ones

the traction you gain with them is debatable because you have more surface aea of contact, but less friction

-it depends on your surface your driving on if you have something like on a rocky surface where a rock or something can get caught between the wheels you should use treads

-if you wanna make it so you can drive over things easily you should have treads

---my 2 cents

Re: 4WD, Tank treads and gear ratios.
 

sparksandtabs is mostly right except for a few things I believe.
You shouldn't have to program anything as far as I know. The default program should be able to handle 4 drive motors.

As for matching motor speeds, as far as I know the Vex motors are neutrally timed meaning they spin the same speed in forward and reverse. Also, for pairinbg motors together, it is done all the time in FRC. With identical motors, you are correct in saying that there will be differences but these differences are so extremely small that it is not of any concern. (maybe if you were building some sort of fine clock it would be). In FRC, we even match differnt motors together quite often with no problems.

For the tank treads, I don't know what to tell you without actually seeing the problem. Maybe you can describe it a little more. I wouldn't go spraying them with any sort of lubricant though. If you are looking for more traction out of them, it is possible you could adhere some sort of rubber to each link of the track.

For mounting the wheels, the benefit of having it driven by gears or sprockets and chain is that it is independent from the motor. The motor is supported by it's own mount and the wheel is supported by it's own mount. If you put a wheel right on the motor, you will be side loading it which is not good. I'm not exactly sure what the vex kit offers in the way of shaft couplings but if you put the wheel on it's own thaft that is inline with the motor shaft, then connect the two with a coupling, that would work just fine.

Re: 4WD, Tank treads and gear ratios.
 

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