Effective Driving while going down a steep grade


#1

Hello all:

I am interested to get your advice regarding an effective way of driving down a grade of 8.5% (for about 4.5 miles).
I use a combination of driving at a higher gear (4th/3rd) and using my brakes (on and off) to slow down; however, the last time I was driving with a friend, he mentioned that engine braking is probably not a good idea. According to him, using the brakes is more efficient since the brakes can be changed as and when needed, but shifting to a higher gear creates unnecessary load on the engine and especially if the A/C is on. Is he right? Should I use my brakes primarily? Sometimes, my brakes get a little soft (when driving down the grade) and regain their original grip after they have cooled down.
I do not travel down the that road everyday, but do it several times in a month. Any and all responses are appreciated. Thanks and regards.


#2

The rule of them is to go down the hill in the gear you went up it. There’s no extra strain on the AC, it’s spinning either way. You DON’T want your brakes going soft…


#3

On a steep downgrade, the issue that would concern me the most is the possibility of overheating the brakes. Overheated brake rotors can warp, and overheated brake fluid can boil (literally!)–resulting in loss of braking ability–if moisture condensation has diluted the brake fluid.

If your brakes are getting “soft” on downgrades, this indicates that they are heating up too much, and this is potentially very dangerous. If this has happened on your car, and if you have not had the brake fluid changed in the past 3 years, you need to have your mechanic change the brake fluid.

So–if you are driving on a long steep downgrade, such as described, I would suggest shifting to a LOWER gear (your terminology is skewed), and also using the brakes to control the speed of the car. Don’t worry about the engine or the A/C, as they are not affected by downshifting to a lower gear.

Of course, if you decide to downshift to too low a gear, then your engine will be turning at much higher RPMs, so the idea is to downshift one gear at a time in order to find the correct lower gear. If your transmission is a 5-speed, shifting to 4th will not be adequate on a steep downgrade, and a downshift to 3rd is probably necessary. Downshifting to 2nd would be too extreme unless you are driving at…let’s say…less than 30 mph.

For those who believe that using engine braking on a steep downgrade is damaging to a transmission, I can tell you that I have used a combination of downshifting and braking on steep downgrades for the past 46 years on a wide variety of vehicles–both foreign and domestic–and I have never had a transmission problem. Zero, zilch, nada. Of course, I do change the fluid and filter every 3 yrs/30k miles, so that is a part of my flawless record with automatic transmissions.

If this practice was damaging to transmissions, don’t you think that I would have had at least 1 or 2 transmission failures in 46 years?


#4

8.5% grade for 4.5 miles…Is this a paved public road?? Even in Colorado, such a road would be hard to find…But by all means, use engine braking to take the load off the brakes…


#5

Engine braking does NOT put additional strain on the engine, unless you’re allowing the engine to rev too high.

Shift down to 4th or 3rd gear and let the engine slow the vehicle. Use the brakes if necessary for additional slowing, but don’t rely on the brakes only. That’s a good way to overheat the brakes, which could be disastrous.


#6

Your friend is right, BUT, the extra strain is almost immeasurable. But what if it was measurable. Lets say it put as much strain on the engine as going up the grade at full throttle in third gear. If you did that everyday, you might shorten the life of the engine by 1%.

What if using the lower gears going down the grade did shorten the life of the engine by 1%. If the engine would have lasted 200k miles, you would have lost 2000 miles of engine life. You would have only gotten 198k.

Now it is true that you can replace the brakes periodically, but what if you decided to save this overly estimated 1% by using your brakes instead of gearing down and one day the brakes overheated and failed part way down the grade. What could you end up loosing then?


#7

If your brakes are getting soft now, you’re already living on the edge (perhaps literally). Using less engine braking as your friend recommends could be disastrous.


#8

In addition to everyone’s good advice (ignore your friend) I’ll recommend you get your brakes inspected and your brake fluid flushed. Hot brakes going soft are a symptom of water in the brake fluid. When was the last time you had the brake fluid changed?


#9

And when you change it, have it changed to DOT 4 or DOT 5.1 (NOT 5) fluid. That will help your brakes not go soft. You might also think about putting higher performance brake pads on your vehicle. They aren’t that expensive. I like Porterfield R4-S. Very good stopping power without sacrificing low-temperature braking ability.

Tell your friend that it’s hard to change the brakes when they fail as you’re halfway down the hill.


#10

Engine braking will put no extra strain on the engine unless you over-rev it. I’d use a combination of engine braking and “snub braking” technique (braking in a slow pulsing rhythm). Also, keep your speed within reason and don’t go too fast! The brakes work by turning the car’s kinetic energy into heat. If they overheat, they fade, meaning they lose all braking power. If you can feel them getting “soft”, you may be close to the fading point. Be careful, and by all means, ignore you friends advice.

3,000 lbs. rolling downhill must be treated carefully.


#11

The biggest problem is letting speed build up prior to having to use engine braking and foot brakes. I disagree that you should use the same gear coming down as going up. Transmissions are so different it is difficult generalize. Automatics are notorious for providing insufficient braking when needed unless you start slow and use a lower gear than anticipated because of the slippage. Manuals are much easier to match as they are always locked but even then, there is a differense between accelerating uphill or maintaining a constant speed. I would use a gear I would use to accelerate up the hill, for down hill braking, which means traveling at a lower speed then anticipated.

For example, with a car with a 5 speed auto that could climb a hill in 4 th gear, I would use third gear to avoid excessive manual braking. Maybe it’s because I tow and use tractors a lot, but I tend to think of motors as braking adjuncts and would always try to drive with the transmission making the biggest contributor to down hill speed control. I use brakes only to help me stop but use the motor / transmission to contol the traveling speed, up hill and down. Just using a number for a gear shift choice is insufficient compared to feeling the transmission/motor is now in complete control of your cruising speed.


#12

If there is an overdrive switch drop out of over drive first. And I find it less troubling to passengers to brake down to a comfortable speed and then down shift to the gear that will not push the RPMs past 3500 at the desired speed. Doing so nearly totally eliminates braking and there is little drama to disturb those who are already uncomfortable looking at the blind curve a mile down the steep grade. And the mountain passes in Colorado were beautiful in May.


#13

Thank you all so…so much for the valuable comments.
As most of you noted, the optimum mode is to use both foot and engine braking - another question, with my car - automatic transmission, 5 gears, 265 HP and 253 lbs of torque - should I be concerned about the RPM and what it should it optimally be while gong down; Like 3500 as Red Knox noted above?

More importantly, getting my brakes checked tomorrow.


#14

Given your hp/torque numbers, I’m guessing you have a fair sized engine (around 3.5L?). Keeping your RPM around 3,000 should do it. Just remember to take your time.


#15

Thanks doubleclutch!!