What does downshifting to slow down wear out?

[Phax]

I used to pop the car into neutral and coast a stop. Then I got over wearing out the brakes. After that I used to be downshift guy, and then I got over listening to the engine rev. What I do now is just take my foot off of the gas and let the car slow itself down. When it's about to stall, I pop it out of gear and then come to a stop.

[Mameluke]

when you're going at a higher speed say in 4th gear like 90km/h, the engine/transmission will have a lot more intertia cuz of the higher gear therefore it will be able to coast at that speed for a longer time. if the speed is the same and you shift to 3rd you will have less and it will slow down faster, and to 2nd, etc.

[PantherRacer]

I read all of that...actually hoping to learn something....for a second there, I forgot where I was lololol....everybody conflicts with someone else....get it right the 1st time

[SloMoe]

wow. i cant believe i just read this whole thing.

some people on here, im not gonna mention names, should go buy a book, and learn what they are talking about before they give advice.

and besides, everybody knows the only thing downshifting wheres out is the leather on your shift knob.

Ok, so what you're saying is that the synchros wear on upshifts but not downshifts? Please explain...

[youngturk]

^_^

good lord, i started this thread in the first place because I was really confused with all the different theories.....

but when I say downshifiting, I implied WITHOUT rev-matching

will the physicist please stand up. why does downshifting (no rev-match) slow the car down. where is the kinetic energy going? thus which part of the car is taking the beating?

Hey chickentendah,

I've read the whole thread and, quite honestly, not being too knowledgeable about the mechanics of trannys, dog teeth, synchro's, etc., I am left lost and confused by what I have read. However, I think I know enough about cars to answer your questions. The fact that you are thinking about kinetic energy makes me think that your thinking is on the right track and will understand the dynamics of this process very quickly. The following is my opinion only based on experience only, not training.

-Why does downshifting slow the car down? Let me try to answer this question with an example. My ’92 240SX’s engine revs at exactly 3,000 rpm at 72 mph in 5th gear. If I attempted to downshift to 4th gear while still maintaining my 72 mph speed, I would have to rev the engine to…my guess…4,100 rpm and then shift from 5th to 4th. So, in 4th gear at the same 72 mph speed, the engine would need to be revving at 4,100 rpm (due to being a smaller gear). This is Point #1.

I also know that in 4th gear my car would probably be going approx. 55 mph at 3,000 rpm. This is Point #2.

In both of the Points above, the engine is driving the car. For purposes of illustration and dynamics, imagine (conceptually) the engine and the car (by itself without the engine) as two separate entities influencing the dynamics of this process.

Now, here is where the two Points come together:This is what happens when you downshift from 5th to 4th gear at 72 mph WITHOUT increasing the engine’s rpm (i.e. rev-matching with more gas). Lets assume you just shifted from 5th gear into 4th gear while going at 72 mph and engine speed is at 3,000 rpm (the engine speed is probably slightly less due to the small time you spent shifting gears; the engine speed fell slightly as you shifted without stepping on the gas pedal).

As soon as you shift into 4th gear and let go of the clutch, the engine and driveshaft (and wheels) have become engaged. In 4th gear and an engine speed of 3,000 rpm, the car should be going 55 mph. But it is going 72 mph. This is when the tug-of-war between the car and engine begins.

Process A: From the car’s perspetive, since it is going 72 mph but the engine speed is only 3,000 rpm in 4th gear, the car begins to apply reverse force on the engine to raise it’s rpm toward 4,100 rpm (see Point #1). In effect the speed of the car (it’s kinetic energy) gets converted into a reverse force on the engine through the driveshaft and transmission. The car, going 72 mph, is trying to bring the engine’s rpm to 4,100 rpm to maintain it’s speed and not lose its kinetic energy.

Process B: The car is going 72 mph but it should be going only 55 mph at the engine speed of 3,000 rpm in 4th gear (see Point #2). Simultaneously as Process A, the engine – which is being forced to raise it’s rpm by the car – is resisting increasing it’s speed because there is no additional input from the gas pedal to enable the engine to rev up (it is only getting enough gas to rev at 3,000 rpm or less). The engine is trying to slow the car down to 55 mph. The engine’s resistance to rev up produces a reverse force on the driveshaft which is transmitted to the wheels, which causes a decelerating force on the car, slowing it down.

As Process A and B happen, the car slows down a little and the engine rpm comes up a little. The proportion of the car’s kinetic energy vs. the engine’s resistance determines how much the car slows down and how much the engine’s rpm goes up.

When you use the clutch to lengthen the time you take engaging the engine to the driveshaft, you give the engine more time to resist raising its rpm. The engine's resistance to increase its revs slows the car down. You slow the car down the fastest when you don't give the engine any gas, increasing it's resistance to increasing its rpm.

Quick Observation NoteTry resting your foot gently (without any down force) on the brake pedal as you downshift without rev-matching. You will notice that it will actually move downward by itself as you downshift and the engine resists increasing it’s revs. I don’t know for sure why this happens but I think it is because the car itself is applying a reverse force on the engine, thus causing negative pressure (vacuum) in the brake lines. Just an observation.

-Where is the kinetic energy going? The kinetic energy of the car is converted as a reverse force on the engine to increase it’s rpm WITHOUT any increase of gas entering the combustion chambers.

-Thus which part of the car is taking the beating?This is where I know a lot less, but I can guess.

Clutch or related parts: May be worn down more because of the greater forces applied to the individual components during shifting. Normally, when 1- shifting up or 2- shifting down (with rev matching), I would guess that the clutch receives little to no abuse. Downshifting without rev-matching and the resulting tug-of-war of forces between engine and car has got to put more stress on clutch-related components. Whether those greater stresses actually do anything to shorten their life-span relatively I don’t know.

Transmission: Instead of the engine driving the car, the car is driving the engine. These reverse forces have got to cause some additional wear on the transmission compared to not doing downshifting (without rev-matching)

Engine: Same comments as Transmission

Hope this helps.

[navysnail]

i dont know if this has been said or not but, downshifting was used back in the day of rapidly fading, poor stoping drum brakes, modern brakes are more then capable of stoping the car fast enough

down shifting does have its place, but i wouldnt recomend it for daily driving

[chickentendah]

youngturk,

Thanks for your lengthy and physics-oriented post. I was just reading the "How Manual Transmissions Work" article on HowStuffWorks.com, and I think you put things into the right perspective. Conservation of Energy right?

initial translational KE(car) + initial rotational KE(flywheel,tires)= final translational KE + final rotational KE

*translational KE goes DOWN i.e. the entire car slows down*rot KE from tires goes DOWN (tires rotating at a lower rate)*rot KE from flywheel (and its interconnecting parts) must go UP by conservation of energy ===> engine rpms shoot up (which was the obvious fact we all know from experience)

So, IGNORING the fact that in downshifting you have to slip your clutch like in any other shift and also IGNORING that your synchros must work to match up the dogteeth, is downshifting really a bad thing?I mean, isn't an engine designed to go through its stroke-process anyway?

I'm not very knowledgeable on the transmission and engine internals, so I'm pretty sure I'm overlooking something. Anybody want to step up to bat?

[youngturk]

"...is downshifting really a bad thing?I mean, isn't an engine designed to go through its stroke-process anyway?"

I don't think engines are designed to run or withstand extreme forces in reverse mode. Some backpressure and reverse forces from going uphill or towing something are obviously normal and expected. However, if your car is going 72 mph in 5th gear and you downshift into 3rd gear, forcing the rpms from 3,000 towards 5,500 rpms, the reverse forces on the clutch, transmission, and engine components cannot be ignored in my opinion. Repeated - I mean over a 100,00 miles - use of downshifting as a form of braking has got to reduce the ife of some components.

I agree with previous writers: Use your brakes - that's what they were designed to do (and they are cheaper to replace). The engine was not.

Last note: Braking through downshifting is braking through the rear wheels only. If you do it too hard, the rear tires may lock up, lose contact, and slide. In hard braking (deceleration), most of the weight has shifted or is shifting toward the front of the car. Which is why 60-70% of braking force in any car is applied through the front brakes. Which why the bigger braking force is on the front calipers. By braking through downshifting you are missing out on this advantage. I brake via downshifting only in emergency situations. And I do it by stepping on the brakes at the same time, thus taking advantage of maximum braking on both and rear tires.