New To Trucking How does a driver predetermine CONTROL SPEED for an upcoming downgrade?

[Sam McCloud]

What I've learned so far about gear selection for a downgrade is that it should be the gear that puts the truck's engine just under governed speed at control speed. I suspect governed speed information as well as gear ratio information should be in the truck's operator guide.

Generally, a truck will be in the same gear going down the hill as it climbed up it if the truck is old or a gear or two lower than the climbing gear if the truck is modern.

However, there will times that road goes from a stretch of flat and level (no slope) to a downgrade and there is no uphill climb to determine your gear selection for going over the summit.

Let's say you are in a tractor-trailer with one trailer on a level highway at 80,000 pounds and this highway is a speed zone of 55 mph and you are cruising at this speed. Your transmission is at its normal gear range for this driving condition. You see a sign indicating there is a 5% downgrade ahead, road conditions are dry, it's broad daylight and weather is clear.

What should you now do? How will you calculate your correct CONTROL SPEED for this upcoming downhill run? Which gear will you select?


Lets' change 5% to 10% downgrade in the scenario above, what is your correct control speed now?

Keeping that 10% downgrade, let's change road conditions from dry to wet, what is your control speed now?

Now let's change road conditions from wet to packed snow, what is your control speed now?

Let's now change your fully-loaded truck to an empty truck, what is your control speed now?

Now let's even change your truck from a single to a double, now what should your control speed be?

What is the mathematical formula in mountain driving for determining control speed based upon:

1. load
2. transmission type and governed engine speed
3. vehicle age and design
4. number of trailers/vehicle composition
5. visibility
6. posted speeds
7. grade
8. road conditions
9. traffic
10. weather

There must be rules established for predetermining control speeds for many driving variables to be safe.

A driver will use the the snubbing 5-3 braking method on top of engine braking to maintain control speed downhill.

Engine braking will never be used under slippery road conditions to avoid skids.

 

[r3gulator3]

I had to get off on the goat trails and bypass some wreck around there once.

Just take that fan switch and set it to "on" instead of "auto". It'll save some wear on the clutch.

It's just constant up/down/up/down at low speeds.

I don’t have a fan switch or a Johnny bar in this new truck. I put it in L (which uses the lowest available range without over revving the engine and when jake-ing it locks the fan on for maximum brake horsepower)

 

[Hillbilly Canuck]

I don’t have a fan switch or a Johnny bar in this new truck.

There's something basically wrong with that.

 

[tommyh]

I always run my fan when I hit a big hill

 

[Duck]

I don’t have a fan switch or a Johnny bar in this new truck. I put it in L (which uses the lowest available range without over revving the engine and when jake-ing it locks the fan on for maximum brake horsepower)

How the hell are you supposed to do donuts in a snowy parking lot without a Johnny bar? You'd have to drop the trailer and do them bobtail and that's no fun.

 

[Nomad1]

How the hell are you supposed to do donuts in a snowy parking lot without a Johnny bar? You'd have to drop the trailer and do them bobtail and that's no fun.

I have to go to the tube U and look for some videos now.

 

[Ontario Outlaw]

I always run my fan when I hit a big hill

How come?

 

[r3gulator3]

There's something basically wrong with that.

I agree I was more pissed about those two items than I was about being forced into an automatic.

 

[mndriver]

I got the OEM harness and switch and wired a fan switch into my Detroit.

Most fleet rides are tuned for protection of the motors. Not for a capable driver.