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Topic: 4WD vs 6WD when Turning (Read 4704 times) previous topic - next topic

Tamulmol

Hi!
I've a 4WD mobile platform,skid steering,Wild thumper wheels,Hobbyking 285RPM 34:1 Motor.

The motor seems powerful for moving straight but it can hardly turn, it doesn't have problems on smooth surface, but on offroad It needs great speed to be able to turn. 

Would it make a big difference if add two more wheels on center and make it 6WD? I just wanna be sure before I buy.

Eagle3ye

Well both 6wd and 4wd is good on offroad but I think that 6wd can consume more power than 4wd,but if you want 4wd,you can switch to ackerman steering methods of you want it to turn fast but if you need maneuverability and you want it to turn on a dime I guess you can go for caterpillar tracks

TomGeorge

Hi, with skid steering what do you do with the motors, also is it one motor driving both left wheels and one driving both right, or is it one motor on each wheel.
To turn left you would have

  • right side run forward

  • left side either slower than right or if you need tight turn you would stop the left or even reverse left briefly.


To turn right you swap the action to the motors.
Also wide tires will make it harder to skid steer.
Traction is what you are looking for, on smooth surface you have lots of friction to use to turn, with off road you have much less traction and so skid steer with 4WD is more difficult.
Weight will give you traction but it may not be good for the chassis of your 4WD.

Tom.... :)
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Tamulmol


Hi, with skid steering what do you do with the motors, also is it one motor driving both left wheels and one driving both right, or is it one motor on each wheel.
To turn left you would have

  • right side run forward

  • left side either slower than right or if you need tight turn you would stop the left or even reverse left briefly.


To turn right you swap the action to the motors.
Also wide tires will make it harder to skid steer.
Traction is what you are looking for, on smooth surface you have lots of friction to use to turn, with off road you have much less traction and so skid steer with 4WD is more difficult.
Weight will give you traction but it may not be good for the chassis of your 4WD.

Tom.... :)


2 Motors on Left are paralleled to drive left side and 2 on right is also paralleled. I can make it turn without any problems, it looks even very powerful when on smooth surface but on grass it stalls when turning.
I'm just wondering if making it 6WD will really help when turning, not because it'll be more powerful but being the two wheels on middle.

on tight turns why  reverse just briefly, why not reverse all time while turning? thanks for the reply

oric_dan

I assume if you go to 6WD, the wheelbase will be even longer, and then it will be even "more" difficult to get it to turn. With wheels, the longer the wheelbase is compared to the width, the harder it is to turn. Recently, I had a similar problem with a similar 4WD base, not the Thumper. I ended up going to motors with higher torque - unfortunately also lower speed. I've only been using mine inside the house so far. With the 280 RPM motors, it would turn on hardwood floor, but not on carpet. With 76 RPM motors, it turns on carpet now.

I think what you have to do with these sort of platforms is greatly jack up the motor duty cycles when turning - unfortunately. They don't tell you that in the marketing brochures.

Tamulmol

#5
Nov 20, 2013, 03:53 am Last Edit: Nov 20, 2013, 03:55 am by Tamulmol Reason: 1
Thanks for the reply. So the best solution is more powerful motor. I thought it'll be easier to turn when there are wheels on the middle, I thought it's the reason why they make 6WD.  I'm going to move my wheels closer, see if it'll help. Thanks!

zoomkat

What is the rated voltage for the motors? If you are using one of the non MOSFET h-bridges, be aware the ~2v can be lost in the h-bridge itself. If this is the case, you may need to use a high voltage battery.
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Tamulmol


What is the rated voltage for the motors? If you are using one of the non MOSFET h-bridges, be aware the ~2v can be lost in the h-bridge itself. If this is the case, you may need to use a high voltage battery.


I'm using  tip122/127 H-bridges. I'm feeding the motor more than it needs, it's a 6V motor but my Supply is 3S Lipo. I've moved the wheels closer together, just waiting for the glues to dry :)

oric_dan


Thanks for the reply. So the best solution is more powerful motor. I thought it'll be easier to turn when there are wheels on the middle, I thought it's the reason why they make 6WD.  I'm going to move my wheels closer, see if it'll help. Thanks!

Also, you can get 2 different sets of motors with the same gear ratios, but with different torque values - and much different currents. Ah, I see the 285, 34:1 is already the high-current motor,
http://www.pololu.com/category/115/25d-mm-gearmotors

I am a little surprised, as I'd have thought it would turn better out of doors, as the wheels will be able to slip better than on hard floors or carpet, like mine.

The bipolar transistor h-bridges might not be the best. Pololu uses heavy-duty contorllers,
http://www.pololu.com/product/1564

Tamulmol

Thanks all for the help. It's turning easily now, didn't know moving the wheels closer together will make big difference :)

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