Downshifting and downgrades

Having just returned from a 1000-mile trip throughout the California mountains, I now have a much better understanding of using lower gears for engine braking on downgrades.

Keep in mine, these lessons are derived from a 2005 Scion xB, and YMMV.

(and only somewhat tongue-in-cheek)

If the sign warning of an impending down grade reads:

5% or less - leave it in 5th

6% - use 4th gear

7% - use 3rd gear

8% - option (a) use 2nd gear and bounce the rev limiter

8% - option (b) use 3rd gear and install new front pads at the bottom of the hill :)

Brake in bursts, and dont buy cheap pads.

Kenny_McCormic wrote: Brake in bursts, and dont buy cheap pads.

^This. Also, I've found that turning the AC compressor on and rolling down the windows also makes an appreciable difference (at speed). How did you bounce off the rev limiter engine braking? Doesn't the rev limiter only cut the fuel supply?

Put the jake on full, drop a few gears and let it roar!

BRAAAAAAAAAP

yes, the rev limiter won't do a thing when the engine is being driven mechanically.

Mmadness wrote: How did you bounce off the rev limiter engine braking? Doesn't the rev limiter only cut the fuel supply?

I didn't. It was strictly theoretical. On the 8% grade in 3rd, the tachometer was about 1000rpm shy of redline, and I was honestly frightened to find out (the hard way) what would have happened with a downshift to 2nd.

I'm not a huge fan of redlining engines in DD, and I think that's irrelevant of whether it occurs under load or under compression braking.

And yes, I know how to brake in burst, and release, in order to give everything at least a moment to cool down.

And no, I really don't need new pads, that was an exaggeration for literary effect.

What I do know is that there was, on long 8% grades, a not unreasonable amount of "pucker effect" in trying to maintain a balance between reasonable speeds and not hitting the brakes only to find out they aren't there due to boiled fluid or any other possibility.

Next time I write something that's a bit tongue-in-cheek, I'll make sure to label it as such - oh wait, I did.

IIRC the main entrance to Laguna Seca is a 16% grade. Going down that hill with the trailer is... interesting.

That's actually a nice thing about the automatic in my wife's TSX: if the accelerometer notices the car is tilted downward and you are trail braking, it will drop down from 5th to 4th until you get back on the gas. It's subtle but effective.

My 2011 fiesta with the PS transmission has some interesting shifts when in the "Hill Assist" mode. Button on the shift lever. It will attempt to maintain the speed you are driving by down shifting and often will down shift when you apply the brakes. works great when hooning on back roads. Down shifts when braking , stays in that gear when coming out. I am curious as to what will happen if the revs exceed the red line on coast. I have gotten close.

Braking down hill: There are a lot of theories on which is better. Pulse braking or light dragging.

When I was driving school bus we were told light dragging was better.

Kenny_McCormic wrote: Brake in bursts, don't take a scion through the mountains, and dont buy cheap pads.

FTFY

It has to be hard pulses. Light dragging heats up the brakes nicely without slowing down the vehicle much. Hard pulses slow down the vehicle a lot - all the heat that's generated doing so was put to good use.

iceracer wrote: Braking down hill: There are a lot of theories on which is better. Pulse braking or light dragging. When I was driving school bus we were told light dragging was better.

Light dragging doesn't make sense to me. If my understanding is correct, brakes convert kinetic energy into heat energy.

So theoretically, slowing a vehicle from 50 to 35 is going to produce the same amount of heat whether you threshold brake or lightly drag them. But by lightly dragging the brakes, you are never giving the pad/disc/fluid a chance to cool from the heat put into the system.

Feel free to criticize if I'm wrong, but this makes sense to me.

The_Jed wrote: Put the jake on full, drop a few gears and let it roar! BRAAAAAAAAAP

Now would be a great time for someone to tell me how to equip my car with a jake brake.

Kenny_McCormic wrote: Brake in bursts, and dont buy cheap pads.

This is pretty much it for a normal car.

Towing stuff would be a different story though.

z31maniac wrote:

iceracer wrote: Braking down hill: There are a lot of theories on which is better. Pulse braking or light dragging. When I was driving school bus we were told light dragging was better.

Light dragging doesn't make sense to me. If my understanding is correct, brakes convert kinetic energy into heat energy. So theoretically, slowing a vehicle from 50 to 35 is going to produce the same amount of heat whether you threshold brake or lightly drag them. But by lightly dragging the brakes, you are never giving the pad/disc/fluid a chance to cool from the heat put into the system. Feel free to criticize if I'm wrong, but this makes sense to me.

The brakes are more efficient as you approach lockup, at which point the tires are doing all the work?

In reply to Kenny_McCormic:

the tires are always doing all the work.

Just to make it clear, I didn't agree with the dragging . I still don't.

codrus wrote: IIRC the main entrance to Laguna Seca is a 16% grade. Going down that hill with the trailer is... interesting.

I can't believe they had you go down that with a trailer, actually. They always send me up the side way and that seems like a much better drive with a trailer. I think the only time I've been through the main entrance is when I did Skip Barber and a few times leaving events at the end of the day in a normal car.

Max_Archer wrote:

codrus wrote: IIRC the main entrance to Laguna Seca is a 16% grade. Going down that hill with the trailer is... interesting.

I can't believe they had you go down that with a trailer, actually. They always send me up the side way and that seems like a much better drive with a trailer. I think the only time I've been through the main entrance is when I did Skip Barber and a few times leaving events at the end of the day in a normal car.

There's a side way? I've been to Laguna probably 20 times, and I've always gone in the main entrance (the one off 68) and up the steep hill. Actually, that's not true, I went in a different way once when I went to Monterey Historics, but that involved 10 minutes of winding around tiny little roads and ended up in a parking lot, not the paddock. (I think it was off Reservation Road?)

The most recent time I went down that grade was coming home from the Miatas @ MRLS event, and I was behind an 18-wheeler that was hauling Mazda's show cars back to Irvine. As interesting as it was in my Silverado with open trailer, I hate to think about trying to take the big rig down there.

when I got my CDL, they taught pulse braking.

codrus wrote:

Max_Archer wrote:

codrus wrote: IIRC the main entrance to Laguna Seca is a 16% grade. Going down that hill with the trailer is... interesting.

I can't believe they had you go down that with a trailer, actually. They always send me up the side way and that seems like a much better drive with a trailer. I think the only time I've been through the main entrance is when I did Skip Barber and a few times leaving events at the end of the day in a normal car.

There's a side way? I've been to Laguna probably 20 times, and I've always gone in the main entrance (the one off 68) and up the steep hill. Actually, that's not true, I went in a different way once when I went to Monterey Historics, but that involved 10 minutes of winding around tiny little roads and ended up in a parking lot, not the paddock. (I think it was off Reservation Road?) The most recent time I went down that grade was coming home from the Miatas @ MRLS event, and I was behind an 18-wheeler that was hauling Mazda's show cars back to Irvine. As interesting as it was in my Silverado with open trailer, I hate to think about trying to take the big rig down there.

Reservation road is the side way I meant. I'm usually on credentials when I go these days and the Reservation Road entrance is often the media/crew only entrance, although sometimes it's public like for the Historics. That way does connect to the same road the main entrance lets you in on, but thinking about it there's a big hill between it and where the main entrance meets the paddock, so it's not much better. (Although at least it's straight and doesn't have a cliff next to it.)

z31maniac wrote:

iceracer wrote: Braking down hill: There are a lot of theories on which is better. Pulse braking or light dragging. When I was driving school bus we were told light dragging was better.

Light dragging doesn't make sense to me. If my understanding is correct, brakes convert kinetic energy into heat energy. So theoretically, slowing a vehicle from 50 to 35 is going to produce the same amount of heat whether you threshold brake or lightly drag them. But by lightly dragging the brakes, you are never giving the pad/disc/fluid a chance to cool from the heat put into the system. Feel free to criticize if I'm wrong, but this makes sense to me.

You're pretty much spot on. In the bicycle world, the people who descend on their brakes instead of coasting/braking firmly/coasting are the ones who blow tires off the rims.

There was an interesting thread regarding truck brakes on an engineering forum that I audit every now and then, too.

mad_machine wrote: when I got my CDL, they taught pulse braking.

^This. The reasoning behind pulse braking in trucks is that the air brakes don't apply as evenly many times due to lash adjusters not being all exactly the same and just the overall design of air brakes. Used to teach that you should descend a hill in the same gear you are able to climb it. Now it is a gear lower due to more power, less overall drag to help slow you down.

Stab braking is pointless in a modern car. Haul and tow all the time, used to teach people to drive truck, drove a 40' semi for quite a few years. Jcamper

Gotta jump in on this one.... Braking (as z31 mentioned) is about turning kinetic energy into heat. Brake dragging will convert kinetic energy into heat the same way that pulse braking does, but the difference is the temperature of the pads/shoes and discs/drums.

If you assume that ALL of the heat is absorbed by the brakes, then both scenarios would be the same. The truth is that a significant amount of heat is absorbed by the wheels, the air, the bearings, the calipers, etc.

Let's say you have two equal rigs going down a long hill in neutral. Tell both drivers they must get to the bottom at the same time, but one needs to do it by maintaining the exact same speed and the other one needs to do it by braking on and off. I promise you the one that maintains its speed will have a much higher brake temperature than the one who pulses.

The reason is simple physics. First of all you have to understand that heat does not equal temperature. A substance can absorb or expel a large amount of heat without changing temperature. The brake friction is creating heat. That heat goes somewhere. Wherever it goes, it increases the temperature of that recipient. At high braking pressures, that heat mostly gets transferred to the metal of the braking system. At low braking pressures that heat shifts more of its demand to the air. Since air is thousands of times less able to absorb heat than the braking system's iron, the temperature increases faster than if you had done pulse braking.

Something that's been oddly missing from this thread is using one set of brakes instead of the other. Doesn't work with tractor trailers, but it works nicely with cars and pickups and motorcycles. Use the rear brake as a drag on an as needed basis (as well downshifting), leaving the front brakes cool and very available for hard braking when needed.