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I'm taking the driving license and my instructor told me that I have to downshift to slow down the car, without using brakes. He said that I have to use brakes only when needed (when I need to stop or in case of emergency).

Is it correct? Could downshifting without braking damage the clutch?

PS: I'm also doubtful about this because when you downshift without hitting the brake, the red lights on the back of the vehicle do not turn on, so people behind you cannot understand that you're slowing down.

SyncroIT
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  • Take a look at https://mechanics.stackexchange.com/questions/1210/does-downshifting-engine-braking-cause-extra-wear-and-tear – Yannick Huber Jul 17 '18 at 13:57
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    I suppose his intention was to tell you about engine breaking. That is really important for prolonged descents as overheating brakes tend to "fade", reducing braking effectiveness up to the point where you could loose control of your car. – Martin Jul 17 '18 at 14:28
  • I just asked if this is correct or not, I already did read the other question but it's more technical and I cannot fully understand what the people says in the answers because I'm not a mechanic engineer. Sorry. – SyncroIT Jul 17 '18 at 14:31
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    Your clutch is expensive and hard to replace. Your brakes are cheaper and easy to replace, what with being at the wheels instead of between the engine and transmission. Which one would you rather wear out first? (Every time you shift, some wear on the clutch disc occurs.) – 3Dave Jul 17 '18 at 15:31
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    @3Dave: A set of rotors and pads for my front brakes costs me 250€ - A new clutch 50€... Also, with engine braking my clutch held for 100.000 km and counting, whereas I had to change the brake at ~80.000 ... – Daniel Jul 17 '18 at 15:33
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    @Daniel And how much does it cost do have the clutch installed? On one of my cars, it's close to 1000 USD. Brake install cost is 100USD around here. (I do this stuff myself, but not everyone does.) – 3Dave Jul 17 '18 at 15:35
  • @3Dave: That´s the point, it very much depends on the car (and how skilled your mechanic is). For me it costs nothing - a friend has a shop where we´ll do this after-hours for some beer. What kind of car do you have, if I may ask - 1000 USD that´s 1-2 days work? Sounds exessive! – Daniel Jul 17 '18 at 15:37
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    @Daniel The $1000 - actually, about $950 - was on a C5 Corvette, about 15 years ago. But, given that car's unconventional drive train (front-engine, rear transmission), it's a little harder to do. I imagine it would be much less on the Solstice, but even the clutch on that car cost me about $350. (But it's an upgraded clutch, so... not apples-to-apples.) – 3Dave Jul 17 '18 at 15:52
  • My dad had a habit of downshifting in his 82 VW Scirocco, it had original brakes at 140K, but 2nd and 3rd gear syncros were worn slap out. – Moab Jul 17 '18 at 16:34
  • Some cars have the ECU's setup so that there is almost no engine braking, effectively the computer applies some throttle to prevent or greatly reduce engine braking. It seems to be more prevalent with automatic transmissions that can be manually shifted. – rcgldr Jul 17 '18 at 19:22
  • @Daniel - I'm curious what car is that for? My experience matches the conventional wisdom cited by 3Dave. And FWIW if I'm doing the work myself, I'd much rather be replacing the brakes than a clutch. – Nat Bowman Jul 17 '18 at 20:53
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    @Nat Bowman: For my Toyota Aygo 1 liter a clutch kit is starting at ~50€. Audi A4b5 about 70 € for example. Seems I was wrong with the Brakes though - they are much cheaper than stated above. Maybe I was remembering price installed. – Daniel Jul 17 '18 at 21:38

4 Answers4

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So there are several aspects to keep in mind if you use engine braking.

  1. No brake lights! Don´t do this when somebody sits at your back!
  2. Don´t over-rev. Only downshift to an appropriate gear. Never use force to engage the gear.
  3. Downshifting and releasing the clutch when cornering hard can lead to unexpected behavior. You are braking only one axle, you´ll get oversteer/understeer depending on the kind of drive-system. No Anti-lock!
  4. You´ll trade clutch-lifetime against brake-lifetime. You can reduce wear on the clutch if you give the gas a little pinch before re-engaging the clutch. The clutch wear only happens during the short slipping moment upon engaging - so this is a good trade off when you are going downhill for example.
  5. You use absolutely no fuel, when engine-braking as opposed to the motor idling while disc-braking.

This leads to using the engine brake only in certain situations. When you are cruising and see a red light or speed limit up ahead to gently slow down. Or when you go downhill, to save the brakes from overheating.

It´s not a good alternative in any situation where you need a swift or finely controlled slowdown!

Mark Stewart
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Daniel
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  • So given the number of tailgaters then you won’t be using engine braking.... – Solar Mike Jul 17 '18 at 15:05
  • @Solar Mike: not much during rush hour at least :) – Daniel Jul 17 '18 at 15:34
  • I have heard that it is good to use in Icy Conditions because it doesn't risk the lock up like brakes traditionally would. Is that true? – Nick Jul 17 '18 at 16:32
  • @SolarMike I drive an EV with regenerative braking so when I let off the "gas" it slows down more than downshifting did when I drove a manual transmission. Anyway I find people tailgate me less now; presumably because they think my brake lights don't work. It's great. I highly recommend. – Dean MacGregor Jul 17 '18 at 16:45
  • @DeanMacGregor maximising regen involves looking ahead - I had one ... – Solar Mike Jul 17 '18 at 16:48
  • @SolarMike I'm not talking about maximizing regen. I'm just talking about the incidence of tailgating when people behind you don't see brake lights. I suppose you can't avoid brakes to the same extent with engine braking as you can with regen braking so people will invariably see your brake lights. – Dean MacGregor Jul 17 '18 at 17:06
  • @DeanMacGregor the « I had one » was the relevant part... – Solar Mike Jul 17 '18 at 17:14
  • @Daniel - Could you highlight your conclusion, where you actually answer the question? I would write, "No, in general, use braking in preference to engine". Which is what I was taught in 1990 by my instructor, who said engine braking was 1960's drum-brake mentality. – Nigel Touch Jul 17 '18 at 18:06
  • "You use absolutely no fuel, when engine-braking" How so? – Acccumulation Jul 17 '18 at 19:30
  • Yeah hybrid car braking is great, as you let up on the gas it switches the electric motors polarity and you get magnetic resistance as it becomes a generator-- all on top of coordinating with the cars clutch and downshifting as usual. – Scott Kramer Jul 17 '18 at 20:12
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    @Acccumulation: The energy for turning the engine is comeing from the momentum of your car, so you do´t need fuel to keep it running - that´s actually the foce that you feel as engine braking - the force your motor normally needs to burn fuel for when ideling. – Daniel Jul 17 '18 at 21:43
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    @Nigel Touch: Maybe you should read the whole post again. Engine braking is still essential when going downhill. You should understand the whole mechanics behind it - there is no simple No or Yes. If it´s too complicated, one should refrain from operating a car. Moving two tons of metal in public at deadly speeds should be accompanied by some basic understanding of its modes of operation. – Daniel Jul 17 '18 at 22:12
  • @Daniel First of all, you're confusing energy and momentum. Without friction, no energy is needed to keep the engine turning. And the frictional slowing of the engine is the same for both cases. Engine braking doesn't come from turning the kinetic energy of the vehicle into rotational energy of the engine; if it did, the engine would quickly reach thousands of RPMs. Engine braking comes from forcing the engine to expend more energy pumping gases from the intake manifold into the exhaust manifold. – Acccumulation Jul 18 '18 at 15:08
  • @Acccumulation: Turning the engine over consumes energy. Just try it yourself by hand, it´s surprisingly hard work even at 1 rpm. When idling normally that energy comes from fuel. When engine-braking no fuel is injected. Your engine is linked in its rotations to the wheels through the driveshaft. The momentum of the car pushes the wheels forward, the resistance of the engine works against that, that is what you feel as braking force (and that´s why the braking effect is higher at high rpm). An yes, part of that resistance comes from pumping the air trough the engine, so you got that right! – Daniel Jul 18 '18 at 15:24
  • @Daniel "Turning the engine over consumes energy. " The engine turning, in itself, doesn't consume energy. Firtional forces created when it turns consumes energy. "Just try it yourself by hand, it´s surprisingly hard work even at 1 rpm." It takes energy to go from not rotating to rotating, but that's different from it taking energy to keep it rotating. "An yes, part of that resistance comes from pumping the air trough the engine" Pretty much all of the resistance comes from pushing gases through the engine. "no fuel is injected" You have done little to reduce my skepticism of that claim. – Acccumulation Jul 18 '18 at 15:43
  • @Acccumulation: You should read up on the internal combustion engine. Compression is one of the four strokes in four-stroke-engine. You can´t turn the engine without that - it´s not designed to skip strokes. As for your skepticism, that´s really not my problem! – Daniel Jul 18 '18 at 15:46
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Personally, I use engine braking in my current gear to slow the car down, essentially coasting, then use disc brakes (assuming I can plan ahead a bit). This reduces brake wear without increasing wear on the clutch.

I would not engine brake through all of the gears as this will significantly increase clutch wear. Clutch jobs are not fun, nor are they cheap.

masospaghetti
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    You can shift down, match engine speed with road & gearbox speed without having to slip the clutch at all. And, if you are any good you can change up or down through the ‘box without even using the clutch... – Solar Mike Jul 17 '18 at 15:09
  • Clutch jobs are not fun, nor are they cheap. Depends on the model. Older Opels/Vauxhalls used to have a clutch that you could slide in with little effort and was dirt cheap... – Daniel Jul 17 '18 at 15:11
  • @Solar Mike: I've only done it on my motorcycle for convenience\speed, but doesn't clutchless shifting put a ton of extra wear on the synchronizers, even if it feels smooth? – SyntheticAbyss Jul 17 '18 at 15:54
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    Done it on a gearbox that did not have synchros to get worn - if it ain’t right the teeth tell you.... – Solar Mike Jul 17 '18 at 16:11
  • @Solar Mike: That's certainly true. It takes more effort, but your point is 100% valid. – masospaghetti Jul 18 '18 at 15:45
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It's strange that neither of the answers mentioned transmission synchronizer wear.

If you don't double-declutch (i.e. press the clutch, change to neutral, release clutch, blip the accelerator, press the clutch, switch to lower gear, release the clutch while rev-matching), you cause synchronizer wear to the transmission.

I would be more worried about synchronizer wear rather than clutch wear, as most manual transmission drivers know how to rev-match.

Worn synchronizers mean that gear-changing requires more force than it used to require, and in extreme cases, you can hear grinding sounds when changing gears.

Clutch replacement is cheap in comparison to fixing worn synchronizers.

The summary is: don't downshift at every intersection. Just let the RPMs fall below whatever RPM the injection starts at, then press the clutch.

juhist
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  • One other aspect to consider, the cost of replacing a worn clutch is substantially greater than the cost to replace worn brakes! – fred_dot_u Jul 17 '18 at 14:30
  • Can you find any source that concludes that downshifting actually wears synchronizers? From my understanding, correct shifting will not wear a synchro ring at all, since the synchronizers don't actually touch (the oil has enough hydrostatic pressure to match the speeds as the synchronizers get closer). Assuming the car is properly driven and maintained, i.e. proper rev-matched shifts and gear-oil changed as specified, the synchronizers shouldn't wear. – Shamtam Jul 17 '18 at 18:11
  • It's from my experience of driving a 1989 Opel Vectra, with transmission oil never changed. The wear was very noticeable and prompted me to change my driving habits. Perhaps it was due to the old transmission oil? – juhist Jul 17 '18 at 18:13
  • I've not encountered a need to double clutch in any vehicle with syncros. (Read: anything that is likely to still be on the road that's worth less than $1M and isn't pulling a 40-ton trailer.) In 25 years of driving manuals, I have yet to have a transmission blow due to this. – 3Dave Jul 17 '18 at 19:16
  • @juhist Yes, very likely. Just because the transmission isn't shifting as smoothly doesn't mean the synchronizers have worn. I (and many others) have great luck with smoother shifting after changing the gearbox oil. Oil breaks down and can potentially lose viscosity over time, which will show the symptoms you describe in your answer. – Shamtam Jul 17 '18 at 20:29
  • As 3Dave says, any modern vehicle with synchros does not need a double-clutch to properly downshift. In fact, unless the driver is well-trained, double-clutching could actually cause more harm than good. I agree with your overall summary, though: coast until you're near idle, then shift to neutral. – Shamtam Jul 17 '18 at 20:31
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People have not mentioned that you almost certainly have to downshift instead of braking when coming down a mountain pass say from 11,158 feet (3,401 m) to 5280 feet (1,609 m).

Mark Stewart
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Sam
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  • Almost? According to dictionary, that means very close to downshift, but not quite downshift. I certainly wouldn't almost downshift; I would downshift. – juhist Jul 17 '18 at 18:04
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    @juhist instead of being snarky you could simply use the edit button to fix it. – Sam Jul 17 '18 at 18:40
  • Sam, I almost never use the edit button to modify content, just spelling and such. If you agree with @juhist, edit your answer. Otherwise... – 3Dave Jul 17 '18 at 19:18
  • @3Dave, I have edited it, and it was a missed word, well within the uses of the edit button. If juhist had not wanted to use the "edit" button he could have, like most civil people simply said "your grammar makes no sense, I think you missed a word" – Sam Jul 17 '18 at 19:21
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    Well... we try (here) not to put words in other's mouths. I would not presume to speak for you or to alter your speech (posts) as that feels like over-stepping. Something that looks like a mistake to me may not, in fact, be a mistake. – 3Dave Jul 17 '18 at 19:23
  • lol, I'm done interacting with you, any reasonable person would see I made a mistake and would see that juhist is being snarky instead of helpful. – Sam Jul 17 '18 at 19:25
  • @Sam: I would classify your scenario as going downhill so yes, I kind of did mention it in my answer. But good that you mention it expicitly! – Daniel Jul 18 '18 at 15:30