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Downshifting to slow down
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Posted by: Slow Nova

Correct me if I'm wrong. I'm no physics expert, but it is my understanding that friction is the only thing that can slow an object in motion. On another forum, there's a guy on there trying to explain to me that downshifting while rolling down a hill doesn't put any additional stress on the drivetrain, because vehicles have always been designed to do this, and that all you're doing is "saving your brakes".

My thinking is completely opposite - The brakes were specifically designed to slow/stop a moving vehicle. Anything else used to slow the vehicle, is taking on additional stress/friction. Brakes are cheaper and easier to replace than engine/transmission/differential parts. So why not use them for their intended purpose?

Quote:

Originally Posted by His Words


	downshifting doesnt hurt the drivetrain at all,vehicle drivetrains have been designed to handle this for as long as cars and trucks have been made. as long as you are not slamming the shifter into a lower gear at too high an RPM or towing beyond the rating of the truck it will do no harm and is recommended in the manual as well

Posted by: No Rice Allowed

Quote:

Originally Posted by Slow Nova


	Correct me if I'm wrong. I'm no physics expert, but it is my understanding that friction is the only thing that can slow an object in motion. On another forum, there's a guy on there trying to explain to me that downshifting while rolling down a hill doesn't put any additional stress on the drivetrain, because vehicles have always been designed to do this, and that all you're doing is "saving your brakes". My thinking is completely opposite - The brakes were specifically designed to slow/stop a moving vehicle. Anything else used to slow the vehicle, is taking on additional stress/friction. Brakes are cheaper and easier to replace than engine/transmission/differential parts. So why not use them for their intended purpose?

I would think Downshifting would put alot of stress on the engine and the tranny..Engine would rev higher when going into a lower gear to quickly, causing wear and tear on both the engine and tranny...Now if your racing, downshifting would be the way to go in some situations, less braking to keep the car from spinning out. Downshifting in the rain same principle. This would save from wrecking your car, if speed was an issue.

Posted by: Slow Nova

His reasoning is that the friction is spread over a bunch of parts, many of which are lubricated, so the individual stress at each point is negligible, for instance, it's spread over everything between the piston and the wheel. And that when it comes to braking, the entire force is contained within the tire, wheel, lugs, rotor, pads, & caliper.

Of course there's going to be significantly more wear on the brakes - they are designed to wear.

Call me crazy, but I'm just more into preserving the expensive, hard-to-get-to parts, and replacing the cheap, disposable, readily-available, easy-to-replace parts more often.

Posted by: Slow Nova

Now the guy's changing his tone, and saying that he's doing it to reduce the effects of brake-fade, which is a valid reason, especially if hauling a load.

Posted by: DeckSetter

My take:

I don't think just spinning the engine without using power is going to hurt it much. Everything stays lubricated because the oil pump is still pumping as long as the engine is turning over. Since the vehicle is slowing down because of the engine, the combustion taking place in the chamber isn't causing as much downward force on the piston, so your bearings/piston pin/whatever shouldn't be wearing as much. You're still getting the same compression (that's what's really slowing you down), but the compression's not near as strong a force as the combustion would be, so I don't think it would wear it as much.

Think about all those little 5 speed manual toyotas and hondas that are well over 20 years old with 300,000 miles and still running good. Not only do a LOT of people downshift to slow down (I know I sure did with mine...) but they run higher rpms at the same vehicle speed in comparison to most american cars (especially v6 or v8). Example: My Dakota runs 2000rpm at 70mph in top gear. My old '86 Celica ran 2000rpm at 45mph in top gear. At 175k miles, I still ran that little Toyota up to 6000+ rpms regularly and it felt tight. If run in top gear only for the full duration of those 175k miles, it would take the Dakota 272k miles to turn the engine over the same number of times.

Also most newer vehicles (and a lot of older ones) are overhead cam, so there's not as much resistance in the valvetrain (less moving parts). Less resistance = less wear.

Add this to the fact that you don't do it very often. Say you're driving in the city, you may spend 30 seconds out of every 5 minutes stopping. If you're downshifting, 1/10 of your engine revolutions are used slowing the car down. On percentage, that's the same as the difference between a 45k mile or 50k mile car, a 90k mile or 100k mile car, or a 180k mile car and a 200k mile car. By the point there's a very big mileage difference, you are normally more concerned about how the car was maintained if you're going to purchase it anyway. I'd rather have my own car at 200k miles and know how it's maintained than get one with 180k miles just because I thought it would run longer.

As for the force acting on everything between the tire and the engine, yes it will, but it's probably not going to be any more force than acceleration. It's just going the opposite direction, but that's just like putting the car in reverse.

THAT SAID, whenever I gear my car / minivan down, I don't put it in a really low gear. I don't rev it up high, mainly because I don't like the extra noise and it can get a little jerky. I also try to match my rpms so I don't wear the clutch out too quick (it's just like doing a reverse burnout, right?)

Also I wouldn't do this in an automatic car because automatic transmissions are wayyy too finicky and if you overheat the fluid you can kiss your trans goodbye.

Posted by: 65fairlane

I have heard it stated like this:
Braking puts wear on your brakes. Downshifting puts wear on your clutch. Wear out the one you are most willing to change first.

PS I downshift to slow down anyway. It just feels like the thing to do when driving.

Posted by: 1LowCav

You are merely using the engine's compression as a suppliment to the brakes. It's talked about in all the books I've been reading in my CDL education. And I've always downshifted anyway. If done properly (the next lowest gear when the engine is near idle) it will not hurt the engine or drivetrain. Large trucks are very prone to brake fade on grades, so it is proper procedure.

Posted by: Grape Ape

Downshifting to slow the vehicle is part of driving a big truck. If you don't downshift, you'll be killing breaks. Particularly when under heavy loads, downshifting is a good thing. For a big truck, there is a technique to use.
1) Allow the engine to slow till about 1,200rpm.
2) Depress the clutch and shift into neutral.
3) Rev the engine to about 1,600rpm(generally only takes one motion of pushing the throttle pedal to the floor and letting off again).
4) Depress clutch and shift into next lower gear.

Takes some time to get the hang of it and it can be done without clutch pedal too. The same thing can be done with a standard trans small auto. You'd use higher rpm of course. Also, the light vehicle has more synchronizers in the trans and it's easier to shift than a truck. Revving isn't nearly as needed because of that issue.

All said, it shouldn't be any problem to use the engine as a means of braking the vehicle. It is the same as accelerating in many respects, but it's less stressful in most cases. That just means it shouldn't cause harm. If you get into the wrong gear... That can be big problem. Done correctly though, no biggie for most vehicles. The best part... When done right, it definitely works!

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