Smallest bit ive ever seen! i have got to make one of these!

Sporttracguy · Post by **Sporttracguy** » Thu Jun 09, 2005 6:55 pm

well the problem i see with mounting the tire directly to the motor is the space issue. The idea is to make this thing as small as it can freaking be and in order to mount that wheel on the motor you would have to have the motor mounted in front of the pcb and that would make the wheel base longer although im sure it would be much more efficient.. The other hard part would be mounting the opposite wheel and tire to match up correctly..certainly an interesting idea though, it would be alot faster for sure

Damn, now you got me thinking,, what if i were to mount both the motors in the back and mount a wheel to each one,, could have a bit of a mini dragster. each wheel would have its own power so more power to one would steer it,, man thats sneaky. I think i need more of these tanks to try this idea out.. Damn it Finks lol why did you have to go and get me thinking!?

Something like this

i dont think it would steer nearly as well but it looks damn cool

codesuidae · Post by **codesuidae** » Thu Jun 09, 2005 7:06 pm

Can't mount the wheel directly on the motor because you'll get squat for torque. It has to be geared down.

If the motor has a shaft of 0.75mm and the wheel has an outer diameter of 8mm, then you get a drive ratio of 10:1. If the motor spins at 10kRPM you get 1kRPM on the wheel (with 10x the torque, not taking into account the uber-crappy power transfer you'll get running it like this). Rim speed of the wheel is like 25M/min

If the wheel is directly mounted on the shaft the drive ratio is 1:1. If the motor runs at 10kRPM you get 10kRPM at the wheel, and a rim speed of about 250M/min. But the delivered torque will be bupkis and the thing won't move.

It might be worth splining or knurling the motor output shaft for more grip on the tire, elsewise you'll have to put a pretty high lateral load on the shaft to keep it from slipping too much.

codesuidae · Post by **codesuidae** » Thu Jun 09, 2005 7:20 pm

Sporttracguy wrote: I doubt a normal bit pcb would work as well if at all. You need to be able to control two motors (front and back) and our bits only do one. The steering wires would be pretty much useless unless you could figure out how to re wire them.

In theory you could swap the existing steering transistors for n-channel FETs, then wire in two p-channel FETs in place of the existing coils and then wire a motor across the middle and you'd have an second full h-bridge controlled by the steering outputs.

The hard part is finding a place for the new FETs. The most compact arrangement would probably be to super-glue them upside down on top of the two steering FETs, then air-wire to the leads and secure the wires with hot glue.

SuperFly · Post by **SuperFly** » Thu Jun 09, 2005 7:21 pm

codesuidae wrote:Can't mount the wheel directly on the motor because you'll get squat for torque. It has to be geared down.

If the motor has a shaft of 0.75mm and the wheel has an outer diameter of 8mm, then you get a drive ratio of 10:1. If the motor spins at 10kRPM you get 1kRPM on the wheel (with 10x the torque, not taking into account the uber-crappy power transfer you'll get running it like this). Rim speed of the wheel is like 25M/min

If the wheel is directly mounted on the shaft the drive ratio is 1:1. If the motor runs at 10kRPM you get 10kRPM at the wheel, and a rim speed of about 250M/min. But the delivered torque will be bupkis and the thing won't move.

It might be worth splining or knurling the motor output shaft for more grip on the tire, elsewise you'll have to put a pretty high lateral load on the shaft to keep it from slipping too much.

I don't know. when you take away the weight of the chassis and everyting else, and you've got a whole motor going to each wheel, there might be enough torque to drive without gearing down. Kit racer gears get you down into the 3:1 range, and those will move (slowly and torqueless, granted) on a stock chassis. Mosfets make them great for large tracks. Perhaps mosfets would be enough to do the trick. Will you need two batteries or just run one parallel?

codesuidae · Post by **codesuidae** » Thu Jun 09, 2005 7:42 pm

It might work, but keep in mind that if you run them with their rated voltage but don't let them get up to speed they'll draw way more current and overheat faster.

The faster the motor turns, the more back-EMF it generates, which opposes the applied voltage. The point where the generated back-EMF is equal to the applied voltage is top speed. Ideally the motor draws no current at this speed (in reality friction losses keep it below that ideal peak RPM, so it continues to draw enough current to maintan speed against the friction losses). If the motor is designed to operate mostly in the top, say, 85% of its RPM band, then it will be designed to dissapate only the amount of heat generated by the current drawn at that speed. If you overload it by under-gearing it, it will still run, but it'll draw way more current than it is designed for, possibly resulting in thermal destruction of the moto. On the other hand, it may just run hot and eat a lot of power.

At a couple bucks for the motor, who cares if it burns up anyway?

DarkTari · Post by **DarkTari** » Thu Jun 09, 2005 8:39 pm

I remember that from way back from the same site Namiie is talkin about.

Post by **CaboWabo** » Thu Jun 09, 2005 8:55 pm

Sporttracguy wrote:yeah, i had one of those pen RC cars and it was cool for a few minutes but then i got bored with it. The problem was that it would go straight forward and to turn you hit reverse and the front axle would pivot to turn. I liked this because it can steer right or left in forward and reverse

lol I know dude. I was suggesting that the pen r/c might be a source for bodies, that's all.
Happy hunting.

stagg · Post by **stagg** » Thu Jun 09, 2005 9:32 pm

Yea i picked up a Pen RC thing for like 5 bucks awhile ago, lasted 5 mintes.

crazydave · Post by **crazydave** » Thu Jun 09, 2005 9:46 pm

codesuidae wrote:
Sporttracguy wrote: I doubt a normal bit pcb would work as well if at all. You need to be able to control two motors (front and back) and our bits only do one. The steering wires would be pretty much useless unless you could figure out how to re wire them.
In theory you could swap the existing steering transistors for n-channel FETs, then wire in two p-channel FETs in place of the existing coils and then wire a motor across the middle and you'd have an second full h-bridge controlled by the steering outputs.

The hard part is finding a place for the new FETs. The most compact arrangement would probably be to super-glue them upside down on top of the two steering FETs, then air-wire to the leads and secure the wires with hot glue.

You could probably make that work, but I've been trying and I can't figure out what I'm doing wrong.

My Big Lots still has a ton of goofy stunt RCs left from Christmas. $7 and you get the tank style controller, PCB with the dual h-bridges, stand alone charger with mini jacks, and a dual cell pack. I grabbed a few for parts. I think that's some good parts right there.

Another alternative to the pen RC for the body, is a wrist watch RC I saw at Walgreens.

Sporttracguy · Post by **Sporttracguy** » Mon Jun 13, 2005 12:36 am

Holy Damn!

Its alive! Im still testing it but i have the rear motor mounted. I was a little worried if this would work at all with the friction style "gearing" but it really does work although im sure it doesnt have the low end torque. I dont have a body yet that i like so i cant mount the front axle but tomorrow ill go and find one for sure,, just thought i would give you guys and update.

STG

Sporttracguy · Post by **Sporttracguy** » Mon Jun 13, 2005 2:54 am

ya know, as i was building this i had an idea. Could style of steering be used to make a 4 wheel drive bit? Think about it, right now im using one motor on the front left wheel and one on the right rear,, equil power to both makes me got forward or backward and a little less to one or the other lets me turn.

Whats to say i dont splice into each motor and mount two more? i would splice a motor into the right rear and place it on the front right side and a motor spliced into the front left and place that in the rear left. So now i would have power to all 4 wheels and this would give me more of a tank style driving. Although the tank controller i have has a forward, backward and a left and right like a normal bit.. you push forward and right and it will just give less power to one side of the car making it turn... Just an idea i had,, not sure how well it would work though

Sporttracguy

codesuidae · Post by **codesuidae** » Mon Jun 13, 2005 9:43 pm

Should work, but you'll be doubling both your power consumption and the board space required for your motors. Might not be a problem depending on the body you intend to use, but if the focus is on small size you might be barking up the wrong tree.

Sporttracguy · Post by **Sporttracguy** » Mon Jun 13, 2005 9:51 pm

Yeah, if i did 4 wheel drive id up the size to that of a zipzap. I got this little bugger running and it works,, steers right and left, forward and reverse (acceleration is awsome but there is a problem. Because of how the drive system works with the shaft resting on the tire i dont get any coasting.. So in other words, i let go of forward and it instanly stops,, takes a little getting used to,, i still dont have a body as of yet but im working on it,, ill get pictures soon

STG

codesuidae · Post by **codesuidae** » Tue Jun 14, 2005 12:03 am

Be careful about extending the wheelbase, the longer it is the more you have to skid the tires to turn, and the more skidding requires more power from the motors.