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Old 12-11-2015, 09:55 AM   #1
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Driveway vs Coach Geometry Calculations

Givens:

20 ft wheelbase of diesel pusher with 10 ft rear overhang
Bottom of receiver hitch is 16 inches above street grade at ride height

Driveway rises 16 inches in first 20 feet from street.

Assumption:

When the front wheels are 20 feet into the drive (16 inches above street grade) the rear wheels are at the street grade.

The bottom of receiver hitch is now 8 inches above street grade as the rear wheels start up the drive. Should make the transition without dragging.

Are these assumptions correct?

Ask a motorhome salesman this question and their eyes glaze over and they change the subject.

Thanks for advice and comments.
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Old 12-11-2015, 10:35 AM   #2
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Assuming the driveway levels out and dose not rise anymore, then yes you should clear the road.
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Old 12-11-2015, 10:36 AM   #3
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Our sons driveway has a sudden rise from the street that puts my hitch about an inch from the crown in the street. I have never dragged coming in or out but always scares my wife when backing out into the street, we use our cell phones to communicate while backing out into traffic and she always stops me thinking it's going to drag, I creep it back slowly while she watches so far no problems.

I carry several short pieces of two by eight lumber for leveling and would use them under the tires if it was too close.


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Old 12-11-2015, 10:53 AM   #4
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Just try it
Backing in may help as you can 'twist' the rear a lot faster backing in at an angle than pulling in forward...

or a small ramp at the lowest point,
or start up at an angle so the inside wheel is lifting the overhang...

or park in the road/at the rv park and drive the toad !
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Old 12-11-2015, 11:02 AM   #5
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The math says your OK assuming the street is not crowned, or you don't have a dip in the gutter.

Raising the front wheels 16" creates a 3.8 deg angle for the coach chassis.

Now when you drop your overhang 3.8 deg it causes the rear bumper to drop 8".

The quick way to think about this is if you had a 40ft board sitting on a saw horse with the sawhorse in the center.Raising one end make the other end drop a equal amount. Since your overhang is 1/2 of your wheel base, the drop is 1/2 of what the nose rises.

Good thin you did not ask about turning radius and tail swing...

Regards

Dan
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Old 12-11-2015, 11:27 AM   #6
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Quote:
Originally Posted by gmcgene View Post
Givens:

20 ft wheelbase of diesel pusher with 10 ft rear overhang
Bottom of receiver hitch is 16 inches above street grade at ride height

Driveway rises 16 inches in first 20 feet from street.

Assumption:

When the front wheels are 20 feet into the drive (16 inches above street grade) the rear wheels are at the street grade.

The bottom of receiver hitch is now 8 inches above street grade as the rear wheels start up the drive. Should make the transition without dragging.

Are these assumptions correct?

Ask a motorhome salesman this question and their eyes glaze over and they change the subject.

Thanks for advice and comments.
That will work unless the gutter at the edge of the street grade is lower than the grade 10 ft into the street where the receiver is located. With road crowns and gutters, I'm not sure it would work.

If it were me, I'd get a 30' string and make a few measurements. Mark the string at 20'. If the bottom of the front tires and the receiver are touching the string, the rear tires should be 2/3 of 16 or 10 2/3" below the string.
Take the string out to the street and put the rear wheel mark at the low point with the ends of the string 20' into the driveway and 10' into the street. At the rear wheel mark, the string should be less than 10 2/3" above the ground. If it's higher than that, the receiver will drag.
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Old 12-11-2015, 11:39 AM   #7
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Tell your saleman that the test drive will consist of a pass/fail atempt to get this monster on the rv pad. He may reply...never mind, no one of even room temperature intelligence gives a crap what a salesman has to say.
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Old 12-11-2015, 12:11 PM   #8
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If you don't have the towbar in place, I would not worry about dragging the bottom of the receiver. After a few years there will be lots of instances where you will drag. An accessory roller device can be added if you don't like the sound of the dragging. A bigger concern is the mud flap.

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Old 12-11-2015, 10:20 PM   #9
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You didn't say what kind of DP you have - is it possible, if you have to, to raise the rear air bags some?
I also think the string technique as in Post #6 would an easy and practical way to evaluate the situation.
All the best!
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Old 12-12-2015, 08:43 PM   #10
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Quote:
Originally Posted by gmcgene View Post


Ask a motorhome salesman this question and their eyes glaze over and they change the subject.
Why do you think a salesman should even know this answer?
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Old 12-14-2015, 09:08 AM   #11
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Geometry?? Holy moly!!

I had to measure my 40' MH 3 times before I figured out it was 3' too long for my 37' driveway................and I still pulled it in to verify my math
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Old 12-14-2015, 09:32 AM   #12
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Quote:
Originally Posted by TonyMac View Post
Tell your saleman that the test drive will consist of a pass/fail atempt to get this monster on the rv pad. He may reply...never mind, no one of even room temperature intelligence gives a crap what a salesman has to say.

Test drive / fitment try would answer the question. Agree.
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