RV.Net Open Roads Forum: Tow Vehicles: Any engineers in the house

RV Blog

  |  

RV Sales

  |  

Campgrounds

  |  

RV Parks

  |  

RV Club

  |  

RV Buyers Guide

  |  

Roadside Assistance

  |  

Extended Service Plan

  |  

RV Travel Assistance

  |  

RV Credit Card

  |  

RV Loans

Open Roads Forum Already a member? Login here.   If not, Register Today!  |  Help

Newest  |  Active  |  Popular  |  RVing FAQ Forum Rules  |  Forum Help and Support  |  Contact

Search:   Advanced Search

Search only in Tow Vehicles

Open Roads Forum  >  Tow Vehicles

 > Any engineers in the house

This Topic Is Closed  |  Print Topic  |  Post New Topic  | 
Page of 5  
Next
Sponsored By:
4x4ord

Canada

Senior Member

Joined: 12/23/2010

View Profile


Offline
Posted: 04/09/12 08:50pm Link  |  Print  |  Notify Moderator

I was playing with the calculator and came up with some figures that seem to make some sense. Based on my experience with the truck I think the numbers are likely right but if there are some numbers guys out there that want to kill a little time I don't mind being corrected.

Based on:
11 mpg towing a 15,000 lb RV at 60mph on a level road with no wind;
and 18 HP hrs/gal (fairly optimistic efficiency for a diesel engine)
6.7 Powerstroke with 3.55 rear axle and 20" wheels
1.52:1 third gear and 2.32:1 2nd gear


These are the numbers I calculated:

HP required to overcome wind drag and rolling resistance and driveline loss is 98 HP on a level road with no wind.

Torque on the crankshaft at 60 mph is 365 ft lbs in 6th gear at 1411 rpm assuming level ground and no wind

lbs of pull between the drive tires and road surface is 490 lbs. (this assumes 20% power loss through the driveline).

Now start climbing a hill. A 7% grade and a 24,000 lb truck and trailer would put an additional 1676 lbs of pull on the surface of the drive tires.
This would equate to 335 hp at the crank (assuming 20% driveline loss)

So the total engine HP required to pull a 15000 lb trailer up a 7% grade at 60 mph would be very close to 433 HP.

To go down at 60 mph there would be the 1186 lbs of pull on the surface of the tires to overcome. This would create 249 ft lbs of torque on the crank assuming 3rd gear and 20% power absorbed into the driveline. To create that torque the exhaust brake would be required to hold 46 psi back pressure at 2920 rpm. The total power required to hold the unit back would be 139 HP from the exhaust brake plus 98 HP absorbed by the driveline, wind etc.

Now if you assume you go down at only 45 mph. The wind resistance would drop drastically so that the power holding the vehicle back due to wind, driveline and rolling resistance might be only 50 hp instead of 98. At 45 mph this equates to 417 lbs of pull on the outside surface of the rear wheels.

The hill and weight of the truck and trailer would still put 1676 lbs of pull at the surface of the pavement and rear wheels. So if you subtract the 417 that the wind and rolling resistance account for, and then account for the power absorbed by the driveline there would need to be 173 ft lbs of torque on the crank in order to hold the unit back from accelerating down the hill. (assuming the truck is shifted down into 2nd gear - 2.32 :1)

173 ft lbs of torque on the crank would require 32 psi back pressure on the exhaust brake. The engine would be turning at 3655 rpm and be creating 121 negative HP. The total negative HP required to hold the unit back is the 121 from the exhaust brake plus 50 HP that the wind and drag are responsible for plus the 30 HP absorbed by the driveline.

My truck will maintain 60 mph up a 6% grade. It will not hold back on a 7% downhill grade in 3rd but holds more than enough on a 7% grade in 2nd while pulling a 15,000 lb rv.


2011 F350 SRW short box 4x4 CC 6.7 PS King Ranch
B&W TurnoverBall and Companion
2003 Citation Supreme 34 RLTS


PapPappy

Wilmington, NC

Senior Member

Joined: 12/23/2007

View Profile



Good Sam RV Club Member

Offline
Posted: 04/09/12 09:16pm Link  |  Print  |  Notify Moderator

OUCH!!! My brain just cramped

What was your question


Bill & Claudia / DD Jenn / DS Chris / GS MJ
Dogs: Sophie, Abby, Brandy, Kahlie, Annie, Maggie, Tugger & Beau
RIP: Cookie, Foxy & Gidget @ Rainbow Bridge.
2000 Winnebago "Minnie" 31C, Ford V-10
Purchased April 2008 FMCA# F407293
The Pets


Turtle n Peeps

California

Senior Member

Joined: 06/23/2008

View Profile


Online
Posted: 04/09/12 09:30pm Link  |  Print  |  Notify Moderator

PapPappy wrote:

OUCH!!! My brain just cramped

What was your question

There was not a question mark in it.


~ Too many freaks & not enough circuses ~


"Life is not tried ~ it is merely survived ~ if you're standing
outside the fire"


RedRocket204

CO Front Range Mnts - Got MTB trail?

Senior Member

Joined: 05/22/2010

View Profile



Posted: 04/09/12 09:36pm Link  |  Print  |  Notify Moderator

Is that an African or European TT?


I love me some land yachting

4x4ord

Canada

Senior Member

Joined: 12/23/2010

View Profile


Offline
Posted: 04/09/12 09:45pm Link  |  Print  |  Notify Moderator

Woops, the questuon is are my assumptions reasonable and are my calculations right?

#40Fan

Colorado

Senior Member

Joined: 02/13/2011

View Profile


Offline
Posted: 04/09/12 09:55pm Link  |  Print  |  Notify Moderator

Ford EB's aren't very good is the answer to the equation.


2013 Arctic Fox 22GQ
2011 Ram 2500 CC LB CTD G56 3.42 Mineral Gray

4x4ord

Canada

Senior Member

Joined: 12/23/2010

View Profile


Offline
Posted: 04/09/12 11:11pm Link  |  Print  |  Notify Moderator

#40Fan wrote:

Ford EB's aren't very good is the answer to the equation.


Actually it seems pretty good but it would be nice if it would work like the Banks speed brake where you can set your speed and the EB would open or close to keep it constant. Even if you could manually adjust the level of braking would be nice. I wonder how much back pressure can be produced before the valves start to float? It wouldn't surprise me if all three of the manufacturers are using similar back pressure settings.

The Mad Norsky

Yankton, South Dakota

Senior Member

Joined: 04/14/2003

View Profile



Posted: 04/10/12 01:35am Link  |  Print  |  Notify Moderator

#40Fan wrote:

Ford EB's aren't very good is the answer to the equation.


I was under that impression also, based almost entirely on that "truck tow out" test where they used I-70 in Colorado and tested Dodge, Ford and Chevy going downhill from the Eisenhower Tunnel.

However, further reading and reviews seem to indicate it is just as good as the other two, albeit with the limiting factor that the starting decent speed on the Ford must be no more than 50 MPH. In the "truck tow test" I believe they started their decent at a speed higher than 50 MPH with all vehicles.

Now back to our regular program. I certainly do not have the know how to contribute any more to this.


The Mad Norsky, Doll, Logan and Rocky
2014 Ram 3500 w/ Cummins/Aisin
2003 Hitchhiker Discover America Quad Slide
2008 SnowRiver 10-8 D/S Torklift, FastGuns, StableLoads
RV'ing since 1991

I took the road less traveled .....Now I'm Lost!

Old-Biscuit

Verde Valley

Senior Member

Joined: 06/20/2009

View Profile



Posted: 04/09/12 09:56pm Link  |  Print  |  Notify Moderator

Regardless of all the calculations & assumptions........isn't this what really matters?

"My truck will maintain 60 mph up a 6% grade. It will not hold back on a 7% downhill grade in 3rd but holds more than enough on a 7% grade in 2nd while pulling a 15,000 lb rv."


That's what counts in my world......what it does!

MitchF150

Puyallup, WA

Senior Member

Joined: 07/13/2002

View Profile


Offline
Posted: 04/09/12 10:18pm Link  |  Print  |  Notify Moderator

I press the gas pedal and it goes... It'll downshift into 2nd and yes, the RPMs are north of 3000, but it's pulling good and I'm passing the semis and making good time..

That's all I know!

Mitch


2013 F150 XLT 4x4 SuperCab Max Tow Egoboost 3.73 gears #7700 GVWR #1920 payload. 2003 Prowler Lynx 722F #5000 GVWR and weighs every bit of it! Happy Camping!


This Topic Is Closed  |  Print Topic  |  Post New Topic  | 
Page of 5  
Next

Open Roads Forum  >  Tow Vehicles

 > Any engineers in the house
Search:   Advanced Search

Search only in Tow Vehicles


New posts No new posts
Closed, new posts Closed, no new posts
Moved, new posts Moved, no new posts

Adjust text size:

© 2014 RV.Net | Terms & Conditions | PRIVACY POLICY | YOUR PRIVACY RIGHTS