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Calculating Required Torq an HP

  1. Sep 10, 2010 #1
    I want to build a buddies son a Rock Crawler out of his Power Wheels Jeep. Mainly as we just finished the 74 CJ 5, and his son is trying to treat his power wheel the same way his father (my friend) treats his jeep. So we figure its time for a real metal frame and tires.

    Heres my question.
    I have several issuse that determine the design of this, including weather to use a central power plant with front an rear drive lines, or 1 motor per axle. We could also consider 1 motor per wheel. Also weather to go small gas or electric. Can a 1/2 hp wead wacker motor be enough to push this thing after being geared and maintain requested speed.

    To start this off, the first issue I see is how to properly calculate the required HP and Torq to push this thing.

    What we do know is the following
    tires are about 10inch tall and weigh about 2 or 3 pounds each.
    If Electric, 2 Optima Batteries about 30 pounds and 12v each (Deep Cycle). Should we go 12v in ser or 24v in par?
    Do we need 2 bat's?
    Are target weight is no more than 100 pound including batteries.
    MAX speed is 5 miles per hour (Thats how fast we go thru the trails)
    Child current weight is 40 pound. BTW: He is 4yrs old

  2. jcsd
  3. Sep 10, 2010 #2
    Ok, so I just had a thought that I need to learn how to figure out to assist in the begining design. How to determin RPMs needed at the axle to meet the desired speed. The speed may not be abtainable via smaller tires due to input RPMs needed. May need to reconsider tire size.

  4. Sep 10, 2010 #3
    If I have done my math correctly
    There is 63 360 inches in a mile.

    10 in tire has 31.4 in Cir and would require 2018 rotations per mile (33.6 rpm's)
    15 in tire has 47.1 in Cir and would require 1345 rotations per mile (22.4 rpm's)
    (assuming 1mph on both above numbers)

    So I need a max rpm of 336 to get 10mph with 10in tires and 224 rpms for 15 inch tires.

    Wow, this is seeming feasable. I was thinking far higher.

  5. Sep 11, 2010 #4

    Ranger Mike

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    Gold Member

    there are a lot of new tech regarding batteries..you can get 16 volt batteries,,drag racing..you can get nice gel cell low weight batteries...check out SBS ...22 AH our battery at 20 pounds..optima is bullet proof but HEAVY
  6. Sep 11, 2010 #5
    I dont know if you have ever been Wheeling, but we feel we need Bullet Proof due to terain. We want this to follow us in the trail, and needs to be able to take the abuse that Wa trails can give it. Further more, I have seen 1 optmia blue top run a winch on a jeep with a dead engine up a 35 degree incline for almost 50 yrds and still have power to start. Were hoping that 1 will provide enough capacity that this can be ran all weekend and not need a recharge.

    Its all in the air. In my following post, will talk about my questions of gears which may change my options.

    Thanks for your comments, I will look into it, but still feel a blue top has proven its self.

  7. Sep 11, 2010 #6
    Ok, so in looking into this, I have come to some decisions I need to make.

    Considering the following..
    At max final output RPM (wheel), I need 336 rpms

    Can someone explain why I wouldn't want a 1:1 ratio at the axle so that drive line speed matches axle speed? In cars such as my jeep, the drive line rotates 3.73 times faster than the axle due to gears in the axles.

    I am finding high torq electric motors that rotate 200 rpms. What would be the best way to accomplish my goal. Is there a reason to not run 1:5 in a tcase, spin the drive lines at 1000 rpms, then use 3:1 worm gears in the axle to obtain my max rpm of 333 at the axle?

    Can any one provide me benefits and draw backs? How much inertia force in the drive shaft helps? I understand with every change if momentum, I loose force due to efficiency isnt 100%.

  8. Sep 17, 2010 #7
    Any other insight for me too follow?
  9. Sep 17, 2010 #8
    Is there something I am missing?
  10. Sep 24, 2010 #9
    Good torque at such a low rpm draws lots of current. You are not going to achieve good battery life without a significant gear reduction. Pick a different motor.
  11. Sep 27, 2010 #10
    Yes. Every time you step up or down you lose efficiency and add weight.
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