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Old 12-16-2020, 07:15 PM   #1
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Driveway maximum grade calculator?

Greetings,

We are looking at land and one factor will be grading the driveway for a future motorhome. The land is somewhat elevated from the existing road. Was wondering if there was an easy calculator or formula available for figuring what the max angles could be at the top and bottom of the hill?

I assume the variables would be Wheelbase, bumper height, and front and rear overhang lengths? Just not sure how to run the math....
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Old 12-16-2020, 07:28 PM   #2
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1. Somewhat elevated, doesn’t tell anyone what the total height difference is between the road and where you might be parking.
2. Future motorhome doesn’t tell anyone how much clearance or overhang to take into consideration.

I would just propose this.........strictly as a wild suggestion

Do whatever I takes to absolutely minimize the effort it would take for someone in a wheelchair to make the climb.
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Old 12-16-2020, 07:45 PM   #3
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I see what you are saying. Not enough detail for a specific answer. I was hoping for more of a generic formula where I could plug in specific details of the rig to arrive at the max angles of the road.
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Old 12-17-2020, 01:32 AM   #4
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Quote:
Originally Posted by Elessar View Post
I see what you are saying. Not enough detail for a specific answer. I was hoping for more of a generic formula where I could plug in specific details of the rig to arrive at the max angles of the road.

Too many variations in design. Our Georgetown has an overhang that slopes up in the rear to reduce the chance of the rear dragging or getting hung up but others do not. Then you might have to factor in how much the jacks hang down and how much they're offset from the axles. You also would have to consider loading and suspension compression.

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Old 12-17-2020, 07:36 AM   #5
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Originally Posted by NXR View Post
Too many variations in design. Our Georgetown has an overhang that slopes up in the rear to reduce the chance of the rear dragging or getting hung up but others do not. Then you might have to factor in how much the jacks hang down and how much they're offset from the axles. You also would have to consider loading and suspension compression.

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Old 12-17-2020, 08:41 AM   #6
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7% is a magic number.

Roads should not exceed this slope. However some roads do. We found the first one near Branson. Thought the Ram diesel was going to blow up. The Ram turned the ac off and cooling fans for the radiator we did not know about, extremely noisy.

Lawnmowers, tractors, generally use 6%. Or they tip over easily.

The problem on vehicles is approach or departure angle. Causing scraping. Distance from the bumper bottom to the front wheel.

Less is better.
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Old 12-17-2020, 11:15 AM   #7
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My son came up with this simple formula. basically calculating the angle of a triangle. for example: One side (H) is the height of the rear bumper from the road, the other side (RD) is the distance from the center of the wheel to the end of the rear bumper, along the ground.

tan-1 (H / RD) = angle in degrees

It's hard to picture this without a diagram, but if this is correct, then my hypothetical motorhome with a rear overhang of 119 inches and a rear bumper height of 15 inches would clear a road angle of 7.18 degrees, or just under 12.5% grade..... Not that I would ever give the driveway that much...Hoping to stay at 6% max.
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Old 12-17-2020, 01:22 PM   #8
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As a retired general engineering contractor I have designed and built hundreds of roads and driveways. You have received some really off the wall responses so far.
First of all I need to know the elevation of the road, the elevation of the intended parking spot and how far back from the road that spot is going to be. Next is the property wide enough to make the driveway transverse the property in an effort to lengthen the distance to reduce the final slope if needed.
Since this is a driveway you will be using the lowest gear and going slow, given that you could easily climb 25% or more, the key to not dragging is the use of vertical curves on the design of the driveway. Hope this helps.
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Old 12-17-2020, 02:12 PM   #9
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You are on top of approach, departure and breakover angles. I'll skip that part other than to say your math on the departure angle seems suspect.

You mention a motorhome, but I'll respond for both a trailer and tow vehicle (TV) or a motorhome. In a motorhome, power may be the deciding factor. A big class C or A on an F450 chassis may be underpowered for anything too challenging.

Our driveway has about a 10% grade. See photos.

~ When we moved here, the moving van didn't have the power to climb the driveway. They had to unload partway up the hill and hand-truck everything in on foot.

~ My TV, a RAM 1500 with a 5.7 Hemi, struggles to haul my 6000# (wet) rig to the top of the hill. The truck is rated to tow 7700 pounds. I get a running start, at about 20 mph or more (fast) in 1st gear (shifted manually), and I keep my foot in it until the TV crests the hill at the switchback, then ease off a bit so I don't overshoot. I COULD NOT JUST MUSCLE MY WAY UP THIS HILL as I could with my previous 4000# rig. The angle of approach from the county highway makes this technique possible. If my driveway intersected with the road at a 90 degree angle, I would not be able to handle this camper.

~ There's no way whatsoever I could back the rig up the hill, so I must spin it 180 degrees at the top of that steep grade to park it...a feat in itself. I use an overstressed Trailer Valet and the most powerful 1/2" corded drill I could buy to run it...and spin the rig in place. I COULD back a motorhome down the hill, but not a trailer. Vision for backing is thwarted by the breakover at the top of the hill, and my paved driveway is only 10' wide instead of the standard 12', and it has a stone retaining wall on one side and a steep drop-off on the other...not to mention the huge tree.

If you're driveway is too steep, lack of power in your TV or motorhome may prevent you from climbing the hill and maneuvering once up there. Even with barely enough power, torturing the transmission is a recipe for expensive repairs.

So two key factors to understand are:

1. Your trailer tow vehicle or motorhome may not have adequate power to climb a steep grade. It's one thing to hit a 7% grade with some momentum on a highway where you can keep your foot to the floor and make the climb. But on a LONG grade or one where you need to start from a dead stop, the grade may be too steep for the power-train to cope. The last section of highway I hit on my way home is called Crow Hill. It climbs about 1000 feet over about 2.5 miles of 7% grade, and I have to climb it at 30 MPH in 1st gear with the 4-ways on. No amount of running start momentum lasts long enough to even stay in 2nd gear. My truck can safely go 45 mph in first gear...about 500 RPM below redline...but the coolant temp begins to climb, and by the time I'm at the top of Crow Hill, it's approaching the red on the gauge.

2. You'll need GENEROUS space to maneuver at the top of your driveway...parking area...so you can turn around, park sensibly, and still hook on/disconnect (trailer).

When I upgraded from my PUP to my new trailer, it took a lot of careful planning to pick a rig small enough and light enough to fit my parking spot and driveway climb. The new one just barely fits on both fronts. I have already dinged up the spare tire cover on the steep rocky embankment on the uphill side of the parking area, because I have little room for error when I spin it.

Your driveway and parking area need to be well engineered, and they will consume a substantial portion of your lot. They will even impact where you site your house, well and septic. Plan carefully, and you'll be happy. If not, you'll be parking your motorhome at a storage area.

P.S. I must overshoot the paved portion of my driveway by about 10 feet to have enough room to back the trailer up into the space where the pickup is parked in the first photo. This gives me a second bite at the apple to get the trailer positioned on a level-enough spot in the parking area to have the hitch coupler about even with the hose reel in photo 3...level enough to disconnect and not have the trailer roll downhill. That "overshoot" maneuver happens on a dirt extension to the area behind the parked campers, and I often have my left front tire airborne when I stop. I'm in 4WD-Auto so I can back out of there. It's "just barely" all the way around. Make your life easier and give yourself more room.
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Old 12-17-2020, 02:15 PM   #10
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Driveway maximum grade calculator? REPLY

As another retiree from engineering I agree with aircommuter . If your property has that much height between the road and your parking area you definitely NEED to consult a civil engineer and possibly a surveyor to help design your driveway. Many factors come into play including maximum grade (usually 25% for driveways), vertical curves (transitions between different grades), pavement type, retaining walls?, erosion, etc. The cost of proper engineering is small compared to the total investment that you are looking at.
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Old 12-17-2020, 03:24 PM   #11
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Quote:
Originally Posted by jimmoore13 View Post
You are on top of approach, departure and breakover angles. I'll skip that part other than to say your math on the departure angle seems suspect.

You mention a motorhome, but I'll respond for both a trailer and tow vehicle (TV) or a motorhome. In a motorhome, power may be the deciding factor. A big class C or A on an F450 chassis may be underpowered for anything too challenging.

Our driveway has about a 10% grade. See photos.
Ppl
~ When we moved here, the moving van didn't have the power to climb the driveway. They had to unload partway up the hill and hand-truck everything in on foot.

~ My TV, a RAM 1500 with a 5.7 Hemi, struggles to haul my 6000# (wet) rig to the top of the hill. The truck is rated to tow 7700 pounds. I get a running start, at about 20 mph or more (fast) in 1st gear (shifted manually), and I keep my foot in it until the TV crests the hill at the switchback, then ease off a bit so I don't overshoot. I COULD NOT JUST MUSCLE MY WAY UP THIS HILL as I could with my previous 4000# rig. The angle of approach from the county highway makes this technique possible. If my driveway intersected with the road at a 90 degree angle, I would not be able to handle this camper.

~ There's no way whatsoever I could back the rig up the hill, so I must spin it 180 degrees at the top of that steep grade to park it...a feat in itself. I use an overstressed Trailer Valet and the most powerful 1/2" corded drill I could buy to run it...and spin the rig in place. I COULD back a motorhome down the hill, but not a trailer. Vision for backing is thwarted by the breakover at the top of the hill, and my paved driveway is only 10' wide instead of the standard 12', and it has a stone retaining wall on one side and a steep drop-off on the other...not to mention the huge tree.

If you're driveway is too steep, lack of power in your TV or motorhome may prevent you from climbing the hill and maneuvering once up there. Even with barely enough power, torturing the transmission is a recipe for expensive repairs.

So two key factors to understand are:

1. Your trailer tow vehicle or motorhome may not have adequate power to climb a steep grade. It's one thing to hit a 7% grade with some momentum on a highway where you can keep your foot to the floor and make the climb. But on a LONG grade or one where you need to start from a dead stop, the grade may be too steep for the power-train to cope. The last section of highway I hit on my way home is called Crow Hill. It climbs about 1000 feet over about 2.5 miles of 7% grade, and I have to climb it at 30 MPH in 1st gear with the 4-ways on. No amount of running start momentum lasts long enough to even stay in 2nd gear. My truck can safely go 45 mph in first gear...about 500 RPM below redline...but the coolant temp begins to climb, and by the time I'm at the top of Crow Hill, it's approaching the red on the gauge.

2. You'll need GENEROUS space to maneuver at the top of your driveway...parking area...so you can turn around, park sensibly, and still hook on/disconnect (trailer).

When I upgraded from my PUP to my new trailer, it took a lot of careful planning to pick a rig small enough and light enough to fit my parking spot and driveway climb. The new one just barely fits on both fronts. I have already dinged up the spare tire cover on the steep rocky embankment on the uphill side of the parking area, because I have little room for error when I spin it.

Your driveway and parking area need to be well engineered, and they will consume a substantial portion of your lot. They will even impact where you site your house, well and septic. Plan carefully, and you'll be happy. If not, you'll be parking your motorhome at a storage area.

P.S. I must overshoot the paved portion of my driveway by about 10 feet to have enough room to back the trailer up into the space where the pickup is parked in the first photo. This gives me a second bite at the apple to get the trailer positioned on a level-enough spot in the parking area to have the hitch coupler about even with the hose reel in photo 3...level enough to disconnect and not have the trailer roll downhill. That "overshoot" maneuver happens on a dirt extension to the area behind the parked campers, and I often have my left front tire airborne when I stop. I'm in 4WD-Auto so I can back out of there. It's "just barely" all the way around. Make your life easier and give yourself more room.
No offense but I think your slope percentages are inaccurate.
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