Forum Discussion

4x4ord's avatar
4x4ord
Explorer III
Oct 08, 2018

HP required to pull hills

Here is a formula that will give a rough idea of the flywheel HP required to tow a fifth wheel up a grade at speed.

HP= s(53000+gw)/31800

s = speed (mph)
g= % grade
w= weight of truck and trailer

It should give a fairly accurate number for most 5vers traveling 50 - 65 mph.
Tow Vehicles
  • Cummins12V98's avatar
    Cummins12V98
    Explorer III
    Oct 20, 2018
    ShinerBock wrote:
    Cummins12V98 wrote:
    ShinerBock wrote:
    Plugged the formula into Excel. This is what I come up with for 65 mph.



    Looks like mine is off the charts!


    I made that chart for my conditions, and I do not tow more than 22k GCW. I don't like DRW trucks so anything above 22k GCWR does not concern me.


    I "FEEL" discriminated against. :B
  • ShinerBock's avatar
    ShinerBock
    Explorer
    Oct 20, 2018
    Turtle n Peeps wrote:
    Thank you OP for the formula and thank you Shiner for the chart.


    No problem. If you want the Excel spreadsheet then PM me your email and I will send it to you when I get back to work on Monday. The chart I posted changes the HP needed by inputting the speed, but I also made one to chart speed at different grades/weights by inputting HP.
  • Turtle_n_Peeps's avatar
    Turtle_n_Peeps
    Explorer
    Oct 20, 2018
    So I finally had time to run the numbers on one of my trucks.

    I ran the formula backwards to see where it came out.

    I didn't have solid numbers but I had a pretty good WAG where they were. I knew pretty close what the grade was, what the total weight was and what the HP was. I knew exactly that I was going 28 MPH up this mountain.

    So I punched in all the numbers and hit go and...……………….28.7 MPH!!! WOW!!! Now that was cool!

    I was very impressed!

    Thank you OP for the formula and thank you Shiner for the chart.
  • ShinerBock's avatar
    ShinerBock
    Explorer
    Oct 20, 2018
    Cummins12V98 wrote:
    ShinerBock wrote:
    Plugged the formula into Excel. This is what I come up with for 65 mph.



    Looks like mine is off the charts!


    I made that chart for my conditions, and I do not tow more than 22k GCW. I don't like DRW trucks so anything above 22k GCWR does not concern me.
  • Cummins12V98's avatar
    Cummins12V98
    Explorer III
    Oct 20, 2018
    ShinerBock wrote:
    Plugged the formula into Excel. This is what I come up with for 65 mph.



    Looks like mine is off the charts!
  • rjstractor's avatar
    rjstractor
    Nomad
    Oct 20, 2018
    Even at the lower weights I'd say it's pretty accurate. I found a drag/horsepower calculator online, and at 65 mph it takes about 90 hp to move a high profile vehicle (80 square feet frontal area, 0.6 drag coefficient) down the road, without adding anything for weight or rolling resistance. On the same site there is also a rolling resistance calculator. For an 8000 lb vehicle add in another 20 hp for a total of 110 just to maintain 65 mph. This explains why smaller vehicles with engines that need to rev way up to get significant horsepower struggle so much with high profile trailers.
  • ShinerBock's avatar
    ShinerBock
    Explorer
    Oct 19, 2018
    Plugged the formula into Excel. This is what I come up with for 65 mph.

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