Slowing by Downshifting

In My Humble Opinion
21

Replacing brakes is cheaper than replacing your clutch. That’s pretty much the end of the story.

[QUOTE=Bearflag70]
My brother claims to have seen someone driving an automatic trying to stop in a hurry by slamming on the brakes, locking them up, then quickly throwing it in reverse, then flooring the accelerator. He said it make a lot of smoke, but it did the trick.
[/QUOTE]

Doubtful. Anti-lock braking, for example, works by actually turning off the brakes momentarily because tires give you the most traction–and thus braking power–when they’re near the speed of the ground beneath. When they lock up or even go backwards, you’re essentially hydroplaning on rubber.

I mean, I suppose that if your brakes have locked, making them go backwards might help marginally, but I doubt it. More likely you’re not going to have any measurable difference, but have ruined tires and run the risk of suddenly backpeddling into what’s behind you.

22

[QUOTE=Bearflag70]
Here’s an aside:

My brother claims to have seen someone driving an automatic trying to stop in a hurry by slamming on the brakes, locking them up, then quickly throwing it in reverse, then flooring the accelerator. He said it make a lot of smoke, but it did the trick.

I think he was pulling my leg.
[/QUOTE]

Considering that the coefficient of static friction is always higher than the coefficient of kinetic friction, one will always stop faster when the bottom of the tire is stationary with respect to the road.

Besides breaks being cheaper to replace than transmissions (they’re designed to wear out, for one), there’s another advantage to using your brakes instead of slowing down solely with your transmission: people behind you know you’re slowing down!

23

Apropos of nothing, has anyone else ever seen those signs that say “No Engine Braking”? I’m guessing it’s an anti-noise thing aimed at truckers, but don’t really know.

24

[QUOTE=BellRungBookShut-CandleSnuffed]
I do it to slow down on ice, but wasn’t exactly taught it, so the concept I have of it is that you should have your clutched (ie neutral) RPMs match where they will be when you release the clutch (similar to upshifting, actually).

[/QUOTE]

Downshifting on ice is not the best idea. If your rear wheels lock, your car can swap ends in a big hurry. I speak from experience.

25

[QUOTE=Sal Ammoniac]
Apropos of nothing, has anyone else ever seen those signs that say “No Engine Braking”? I’m guessing it’s an anti-noise thing aimed at truckers, but don’t really know.
[/QUOTE]

That’s pretty much the gist of it. The difference is that gasoline engines can utilize engine braking by simply letting off the throttle, but diesel engines can’t, unless the flow of gas through the valves is modified. More info here: Compression release engine brake - Wikipedia

(Jake Brake is a brand of engine brakes.)

26

[QUOTE=Santo Rugger]
Considering that the coefficient of static friction is always higher than the coefficient of kinetic friction, one will always stop faster when the bottom of the tire is stationary with respect to the road.

[/QUOTE]

If that we true, then we wouldn’t observe that a car stops faster at the threshold of lockup than in a full-on skid. It’s the first exercise they have you do at track school. With your wheels locked up, you slide waaay past your threshold braking distance marker.

27

[QUOTE=Pork Rind]
If that we true, then we wouldn’t observe that a car stops faster at the threshold of lockup than in a full-on skid. It’s the first exercise they have you do at track school. With your wheels locked up, you slide waaay past your threshold braking distance marker.
[/QUOTE]

That’s what I just said. When the bottom of the tire is stationary with respect to the road, the tire is still moving. When it’s stationary with respect to the car, but the car is still moving, the wheels are locked up, and the bottom of the tire is moving with respect to the road.

28

So what’s the consensus?

Enging braking places extra wear on your clutch, and brake jobs are cheaper than clutch jobs, but modern cars are pretty robust mechanically, and you probably won’t have to replace your clutch for 3456879765 years anyway?

I’m starting to wonder - brake jobs are cheaper, but they are still a dent in the beer budget. Maybe it makes more economic sense to just use the clutch and be able to postpone the brake jobs?

Eh, anyway I’m currently driving an automatic, so it’s moot for me.

29

Clutch jobs are a huge dent in the beer budget. I’m not sure what the average life of a clutch is, probably somewhere over 100,000 miles.

IF you synchronize the engine and transmission RPMs properly the clutch can last indefinitely, but that’s a skill that not everyone would have.

30

[QUOTE=control-z]
Clutch jobs are a huge dent in the beer budget. I’m not sure what the average life of a clutch is, probably somewhere over 100,000 miles.
[/QUOTE]

This is what makes me wonder. I don’t drive so much these days, so I’d probably have a different vehicle before 100 000 miles. On the other hand, I’d expect to be having the brakes done occasionally. I’m reasonably gentle on clutches. I don’t do maniacal starts, I know my revs, and I know how to blip the throttle for a smooth downshift. I’m starting to think it does make more economic sense to hand over a large percentage of the brakes’ work to the clutch and drivetrain. Or slow using a combination of both (which is how I tend to drive) and spread the load over the various components.

31

If it’s economic thing then why not just never use the right turn signal?

I mean it really doesn’t do anything that useful and you could save a couple of bucks over the life of the car.

32

[QUOTE=Santo Rugger]
That’s what I just said. When the bottom of the tire is stationary with respect to the road, the tire is still moving. When it’s stationary with respect to the car, but the car is still moving, the wheels are locked up, and the bottom of the tire is moving with respect to the road.
[/QUOTE]

I got you now. I misread the word ‘stationary’ as meaning locked up. I should stop drinking so heavily while still at work. :smack:

33

It’s not such a big deal for automobiles and light trucks, particularly now that most have disc brakes. However, proper use of engine braking is essential to preclude brake fade in larger vehicles. It was part of the test I took for a CDL eons ago, and was part of my on-road evaluation when qualifying to drive 10 wheelers and triaxles on a part time basis two years ago. Whether the vehicle is a stick or has an Allison, I will always downshift to decelerate, using a Jake (unless prohibited) alone or in combination with an output retarder on the automatics.

34

Warning: Anecdote ahead.

[QUOTE=Pork Rind]
I got you now. I misread the word ‘stationary’ as meaning locked up. I should stop drinking so heavily while still at work. :smack:
[/QUOTE]

Drinking at work? Must be nice! No worries, though, it is a bit of a mental leap to switch between WRT (with respect to) the car and WRT the car. I knew you knew what I was talking about from the way you worded your reply, but was trying to make myself more clear for those who didn’t.

I’ve got a story about coming down a mountain in a “cherry picker” (bucket truck, the type the phone company uses to get to the top of a pole). It had air brakes, and the travel was adjusted in such a way that too much air had to be used to engage the brakes. I was coming down the mountain, pulsing on the brakes to use as little air as possible. Unfortunately, even though I was limiting the amount of time the brakes were engaged, the truck started to get away from me. I tried to downshift, but the wheels were moving way faster than the engine was, so I got stuck in neutral. I tried to rev the engine up to get it into gear, but was only able to get it back into the higher gear. I knew the sharpest (and steepest overshoot!) turn was coming up, so I took a deep breath, and slammed the cherry picker into the side of the mountain! When I stopped bouncing around, I opened the hood, and caught a whiff of sulfuric acid that almost knocked me out!

I sent a “911” page to the foreman, and he didn’t even answer it, he just came up to where he figured I had wrecked. After a stern talking to about how to conserve brakes, calling the mechanics up the mountain to get the truck started again, and waiting for the compressor to charge, he had me drive his truck down the mountain while he drove the cherry picker.

When we got to the bottom of the hill, he slowly rolled to a stop, so I pulled up next to him to see why he stopped. Turns out, the same thing happened to him, and he barely got it down the mountain! He apologized to me, and the truck ended up in the mechanics shop for the next week! The moral of the story is, given brakes, transmission, or the side of a mountain, I much prefer either the brakes or tranny!

35

[QUOTE=danceswithcats]
<snip> in combination with an output retarder on the automatics.
[/QUOTE]

What’s an output retarder?

36

[QUOTE=Chefguy]
Downshifting on ice is not the best idea. If your rear wheels lock, your car can swap ends in a big hurry. I speak from experience.
[/QUOTE]

Eh, I dunno. I’ve downshifted for situations where at the same speeds my brakes have caused me to skid or fishtail. ABS only takes you so far, and now that I have a vehicle that doesn’t (I don’t think) have ABS, it’s all the more important. Why would your rear wheels lock when downshifting?

37

[QUOTE=TheLoadedDog]
So what’s the consensus?

Enging braking places extra wear on your clutch…
[/QUOTE]

I don’t think that’s part of the consensus. Properly rev-matched downshifts put virtually zero wear on the clutch. The wear isn’t exactly zero, but it’s so low that proper downshifting technique will allow the clutch to outlast almost every other wear item on a car, no matter how many shifts are performed.

The trick is actually doing the rev-matching…very few drivers do it, or do it right. That means heel-and-toe’ing before turns, and blipping the throttle for straight-line deceleration. I’m not claiming perfection myself, which is what a lot of cocky drivers with superiority complexes will try to do in threads like this. I make an effort to match every shift, but I get sloppy and lazy and miss the mark often enough.

Despite my not-quite-flawless technique, my attempt at doing it right has paid off. I still have only had to replace one clutch in the last 15 years, and that was on the car I learned how to drive on. I started on that clutch when it had 140,000 miles of unknown treatment, then used it to learn. No wonder it was shot by 180,000

And the brakes? I’ve had one set of front brake pads replaced in the same time period…that’s over three cars and about 250,000 miles. And I don’t drive like a grampa, just ask my passengers.

38

One set of brake pads in 250,000 miles. :eek:

You must live in the middle of nowhere. :smiley:

Brake pads usually last about 30,000 kilometers on my cars on average, brake disks about 80,000. And those are ABS vehicles.

And yeah, I do downshift, but not all the time. If I’m just cruising along towards a red light, I’ll let it coast in whatever gear it is in until I’m almost at a stop, then engage the clutch. If I’m having a bit of a fun ride, I’ll downshift and engine brake as well, but for the most part I do that to have the car in the right gear for engaging a corner (nothing messes up your grip levels like entering a 40 km/h 90 degree corner in 4th gear). Plus, you’re then in the right gear to power out again. I do try to match the revs as best as I can (riding a two cilinder motorbike is a great training in this regard, you HAVE to match them or you’ll risk locking up the rear), and I’ve got a pretty good feel for it. As far as heel-and-toe goes, no way. I’m far too clumsy for that. :slight_smile:

39

Well, to be fair, only replacing brake pads once doesn’t mean I didn’t wear out more than one set. It just means I had a knack for getting rid of cars right before they were due to be replaced. I know at least one of my cars got sold with very little left on the pads…so for that car, I really went through two sets of pads.

But still, I’m very easy on brake pads, adn I think that’s mostly because I use the drivetrain to bleed off most of my speed most of the time.

40

[QUOTE=Santo Rugger]
What’s an output retarder?
[/QUOTE]

It is a braking means which uses hydraulic pressure and a friction clutch(es) to slow a vehicle. Operation is controlled by a foot button to the left of the brake pedal-retarder is ON when the button is held depressed.