Get out of the flat country and hit the hills, sooner or later you’ll find a sign warning of a “steep grade,” often associated with a percentage number. Road grades seem mysterious at first, but really are simple.
Simply put, road grade is the amount of rise or drop over a given distance. A 5% grade means over 100 feet, the road will rise or fall 5 feet. In real-life terms, a sign reading, “5% downgrade next 4 miles” indicates that you’ll lose 1,056 feet in altitude over the 4 miles of run. Here’s the math: 5,280 feet (per mile) X 4 miles = 21,120 feet X .05 (5% grade) = 1,056.
Should you be concerned about steep grades? For RVers, road grades are extremely important. Going up a long, steep grade can lead to overheating your engine and transmission. Heading down a long, steep grade requires preparation: An RV, heavier than most automobiles and trucks, must be kept in control. “Brake fade,” resulting from overuse of brakes can lead to an out-of-control situation. Being aware of your rig’s handling on a grade is an important part of safe RVing.
So what’s a steep grade? Grades are typically marked when they reach 5% or more. On the US Interstate Highway system grades are not allowed to be over 6%; on other roads and highways there is no limit. RVers generally agree that the longer the grade the greater the concern. We’ve been over short-length double-digit grades that gave us no trouble, but even a five-percent grade can be worrisome if it goes on for miles and you or your vehicle are not prepared for it.
How do get ready for a steep grade? Going uphill keep an eye on your engine comfort. If you’re dealing with a long grade you may need to switch off your air conditioner to keep your engine cool. Watch your temperature gauge and–if you have one–your transmission temperature gauge. If things start heating up back off the throttle and down shift. The same is true if your engine begins to lug, drop down a gear.
Going down a steep grade means keeping your rig under control. The old trucker’s adage, “You can come down the hill too slow many times, but you can come down the hill too fast only once,” applies well to RVing. It’s much easier to start out at the top of the grade slower than you “think” you should–once you build up downhill momentum things can get out of hand very fast. The rule of thumb says whatever gear you required to come up the pass is the one (or one gear lower) you’ll need to head back down. Beware, diesel engines don’t have nearly the compression braking of a gas engine.
Ideally the gear you choose for the downhill run should “hold” your rig at a comfortable speed, not allowing it to gallop away. Some truck drivers advise the use of aggressive braking: Keep the vehicle under control with the proper gear and figure a “safe” speed. When the rig hits the safe speed, bear down hard on the brake pedal and reduce speed by five miles per hour. Get off the brakes and hit them again when the safe speed is reached. NEVER ride your brakes–it’s a sure way to overheat them and lose braking power.
photo:Â jodastephan on flickr.com
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Unless you’re on I 70 in CO where the grade is 7% to the Eisenhower tunnel.
If you have a Ford F-53 motorhome (and probably other vehicles with Tow/Haul mode) you may find this helpful:
We always use Tow/Haul mode while driving in D and never manually shift gears. Tow/Haul is turned on and off by a pushbutton on the end of the gear shift lever. When Tow/Haul is on it lights a yellow light on the dash. When that yellow light is off then Tow/Haul is also off.
Tow/Haul mode delays the shifting pattern so the vehicle remains in lower gears when accelerating to build speed faster. We use Tow/Haul even when not towing as do many other people. There is no real reason to not use Tow/Haul 100% of the time.
When going down a hill in Tow/Haul mode, each time you tap the brakes the transmission will downshift one gear, even into the gears you cannot manually select on the gearshift with the V10. Tow/Haul is the poor gasser owner’s version of an engine brake.
If your speed is building up going down a hill, stab the brakes hard-ish to quickly get back to about 5 MPH or so BELOW your DESIRED speed and then come off the brakes. You do not want to be on the brakes for more than a few seconds if possible to avoid overheating.
That “stabbing of the brakes” when in Tow/Haul mode will cause the engine to rev higher as the transmission downshifts one more gear but you cannot over-rev the engine. The computer handles it all. That’s why there is no “redline” on the V10 tach.
If you think it downshifted too much, just turn Tow/Haul mode off and several seconds later the transmission will resume normal shifting and then turn Tow/Haul back on OR step on the accelerator and the transmission will upshift after several seconds. Yes, stomping on the gas while the engine is screaming seems counter-intuitive but it does work. We sometimes do that at the bottom of a hill although it’s usually not needed.
Tow/Haul mode must be turned back on each time you start the engine.
Until you get used to it, you’ll pull out of a rest area or a gas station and not remember that you need to turn Tow/Haul back on. We are in the habit of reminding each other.
We NEVER manually shift gears on ours. There’s just no need.
Great APP called Mountain Directory which not only shows grades but describes in detail the descent with what to expect.
The “%” given on the road signs should not be confused with the mathematical (trigonometry) angle in degrees. A 5% road sign is a mathematical 0.5 degree angle.
A 100% slope is 45 degrees. “Rise over run” is the way it was taught to civil engineers back in the olden days.
We’ve got a 17% short grade in our area that we went up and down today. I’d swear that thing is close to 45 degrees. Our kids, when little and sitting in car seats in the back seat, would scream in terror as we crested the hill to go down the steep part. I finally sat back with them, put my head at their eye level and I saw the problem. It looked like we were driving off a cliff from their height.
A 5% grade is actually 2.86 degrees above or below horizontal. You can check that here: https://www.omnicalculator.com/math/right-triangle-side-angle
The exhaust brake on our Ram diesel has an automatic setting. It has worked extremely well on the long 10% downgrade on the west side of the Big Horns in Wyoming (US 14A), on the 6%, 9-mile long Eisenhower Tunnel westbound downgrade, and on the sharp curves of the 2-lane eastbound downgrade on Wolf Creek Pass. With that TV, I have never had any problem holding my speed or overheating brakes when towing our fifth wheel. If an exhaust brake works that well here in Colorado, it will work anywhere!
I’m intrigued by the statement about compression braking. First: any discussion of engine braking should note that diesels have either an exhaust brake (somewhat effective) or an engine (Jake) brake (extremely effective), neither of which is available on gassers. But re: compression braking: your engine “brakes” at zero fuel input because it’s being driven by the drive-train, i.e. “compressing” air in the cylinders to absorb energy. Gas engines typically compress air in a ratio of around 9 to 1. Diesels compress the air to at least 12 to 1, which heats the air hot enough to result in fuel ignition when (if) fuel is injected. So theoretically, a diesel should have more compression braking than an equivalent size gas engine.
For those who believe gassers have more compression braking, I’d love to hear how this can be explained. This Engineer thinks that may be a fairy tale…
Agree completely! Arguably the best thing about towing with a diesel is its’ ability going downhill not up. The engine brake on my 6.7 lets me go down the steepest the Rockies have without needed to use the brakes and where I live that means every trip out of the driveway.
Not all diesels have this, like Sprinter based RV’s. I thought the same though about compression until I researched it some. Here’s one explanation: https://www.quora.com/Why-do-diesel-engines-have-engine-braking-when-they-dont-have-an-ability-to-restrict-air-flow
Truth as I see it, gassers WITHOUT a turbo brake well. Diesels and turbocharged gas engines do not not brake well when compared to naturally aspirated engines of EQUAL horsepower. My 3.5L turbocharged engine does not brake nearly as well as the 6.0L naturally aspirated engine with comparable output. Just my take on the subject.
Very simple. The engine brake on a gas engine is the throttle plate. When the throttle is closed, it restricts air flow to the engine. A high vacuum is created via the open intake valve, and the piston speed during the intake stroke. The higher the piston speed the greater the vacuum created. This is why gas RVs downshift going down a grade.
With the throttle plate closed on gas engines you are primarily operating in a partial vacuum, just a small amount of air gets past the throttle plate as when the engine is idling.
I can’t count how many RV’ers I have come across that have “under trucked” their tow vehicle. I “over trucked” my 35′ Montana with a Chevy 3500 dually diesel. Going down a steep grade is a simple as setting the cruise control to a slow manageable speed (maybe 30-50 mph, depending on grade) and letting the engine brake do 100% of the work. I seldom use the truck or trailer brakes in these situations. Just so much safer.
Agreed, my better half’s F350 is easy to run in the mountains as the truck is oversized for the trailer (the trailer is a ‘half-ton towable’ model, he laughs at that as it’s at the top end of his old F150, partly why he has the 350 now). He sets cruise to a comfortable speed for the hill, pops the exhaust brake on at the top of the hill (he legally has to stop at brake checks, something about how his truck is licensed for work), and rides down without touching the brake. Been over the rockies that way and never had issues in the 7+ years we’ve been hauling a trailer
Towing is a big issue too. We run a gas class A towing an F150. At first we didn’t realize our braking system for the road is designed to not brake downhill unless you brake hard. I have experimented manually braking the toad for five seconds periodically on a long downhill. We checked temperature at the bottom to make sure we weren’t overusing the toad brakes. Alternatively, disconnect and drive the hill, up or down separate.
Your brake controller can be adjusted to apply more or less trailer brakes with pressure on your truck brake. If it’s not braking the trailer unless you “brake hard”, then it’s probably not adjusted properly, which may be dangerous. I’d be checking that…
The old saying about same gear going down as up is not always right. Some times the terrain on one side of a pass is different than the other. Do your research.
Better too slow than the alternative!
Also fine tune your brake control settings for your trailer, so the trailer brakes slow the trailer and the tow vehicle brakes slow the tower. They should work together, not one or the other doing all the work.
Great discussion Russ, but I have some questions:
1. “highway systems grades not allowed over 6%” ? I’ve been on plenty of highways that are way over that number. Utah state 12 comes to mind at 14%. Which highways are you talking about, only Interstates?
2. Many of the grade-ratings I’ve seen are extremely over-stated. Do you think that the number on the signage is perhaps the maximum grade within the distance? For example, “6% grade next 8 miles” would result in a half-mile drop in elevation (2600 ft), which rarely if ever actually occurs.
Good point, Greg. Yes, that restriction is for the interstate highway system. As to overstated grades, my guess is these posts may refer to the maximum percentage overall. Been across a couple in the last month that weren’t posted that felt a bit more than six!
The authors were extremely clear about the 6% max grade – “On the US Interstate Highway system grades are not allowed to be over 6%”.
Do not know how they could have stated that more accurately, and they certainly did not write, nor even imply, that this max applied to any state, county, or local road.
Most of our travels are in the western states. I’ve come to rely on the altimeter function of my Garmin to indicate the start of a grade. While climbing a grade I monitor the engine temp indicator very closely. With my 300hp engine I find 2250 rpm in 3rd gear usually works well on a 5% grade.
Exhaust braking with the variable vane turbo is a wheel brake saver. The best approach to handling a grade is slow down.
I think you miss-spoke. Gas engines have less engine compression braking than diesel engines. Other than that… a fine piece of writing.
Sorry Ken, the author was right. Gas engines have more compression braking than diesels, UNLESS the diesel has has a Jake or some type of compression brake !
Yes Ken it was a war on words. But acting like the gas engines compression braking is anything compared to the diesels exhaust braking is just silly. Article was very misleading on that line. Diesel engines are so much more capable of holding the load steady on steep grades it’s not even a discussion. It’s two totally different systems.