Downshifting

Cecil's Columns/Staff Reports
1

I have an argument for downshifting, which is completely irrelevant as far as everyday driving goes, but important when you’re doing that mountain road fun driving: if you’re going fast around a corner, it’s safer to control your speed by varying just the accelerator than it is to go back and forth from accelerator to brake. It allows you to quickly go from deceleration (just before entering a turn) to acceleration (while going through a turn) smoothly. If you use the brake to slow down before a turn, you have to* quickly get back on the accelerator, which is difficult to do smoothly (especially in a steenking automatic, which let the engine RPMs drop and went into a different gear while you were braking).

Anyway, lots of people reject this reasoning as well, saying “well you don’t have to drive as fast as possible”, but the argument isn’t about that. It’s about whether downshifting has a purpose, which it does. Besides, I like to drive fast. :slight_smile:

Ok, you don’t have to get back on the accelerator for the turn, unless you want to take the turn as fast as possible*, but that’s often the point of fun mountain road driving, isn’t it?

**Any nimrods wishing to challenge this assertion better start by drawing a free-body diagram of the forces on the point of tire/road contact.

2

I have heard people talk about how they do this (people on a sports car/rec racing list), and it is called the ‘toe heel’ or ‘heel toe’ method. Basically you break with your toe, while using the heel to match rpms to downshift.

Also, using the clutch is very handy on icy roads. You can manage it just using the brakes, but it can be easy to lose traction. Using the clutch and downshifting allows you to slow gently without necessarily getting out of control.

Of course, this doesn’t always work. Driving on some black ice in my RWD car, I downshifted because I was going up a hill (at 30mph no less), and it was so icy (and I had such poor tires) that I actually lost traction. That was a long and stressful trip.

3

While downshifting does has it’s advantages on driving, it has it’s disadvantages as well…(though the reply from galt, ascertaining too having to draw a free-body diagram of tire/road connection and point contacts is ignorant at best), it does give you more control on a tighter turn at higher rates speed, it also destroys your transmission!! unless you are using a triple plate clutch system, (read that as PROFFESIONAL RACE CAR DRIVERS). Most common folk don’t bother to turn their car into a tuned monster on the road for “fun mountain driving”. So unless you ahve the money to drop on a whim to replace your transmission, when it refuses to work for you because you downshift on mountain roads all the time, I would suggest you simply drive it with slight break pressure (you down have to use the clutch to do this), and simply go back to the accelerator when you have come out of the turn.
And any NIMROD who wants to debate this, better start drawing to scale a detail clutch system and BHP on your axle/drive shaft at the rate of 3000-5000rpm, and how it affects the ability to keep traction on the road. Of course take into the fact of tires, I.E., how wide, deep, offset/pitch, material made of, etc… :slight_smile:

4

That is funny Cecil… my 1989 Toyota Corolla has 265,000 miles AND the original clutch.

And I downshift.

I think all this talk of wear and tear is over-rated nonsense. Your car should always be in the appropriate gear for your speed, or else you are not in control of your automobile. Period. This does not lead to excessive shifting; by definition, it is what you should be doing and not excessive. I have never heard of anyone having a clutch replaced (or engine bearing or anything else for that matter) because of downshifting. The most common reason clutches are replaced is simple: the idiot drivers left foot is on the clutch too long.

5

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To slow a stick-shift car, should you brake or downshift? (27-Feb-1998)

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6

quote “I think all this talk of wear and tear is over-rated nonsense.” unquote

You obviously never even took “auto-shop” in school? Downshifting does produce metal fatigue on your clutch and transmission. Excessive downshifting at high rates of speed, will damage your clutch, I didn’t say normal downshifting will damage it fast, but time will. Though Toyotas built from 82-89, use a double plate clutch, most cars have a single plate clutch.

quote “The most common reason clutches are replaced is simple: the idiot drivers left foot is on the clutch too long.” unquote

Very true, and I will not argue this fact, but if you stop downshifting, or even shifting from one gear to the other unless it’s absolutely neccesary…I.E., starting from 0mph/kph, you will alleviate this problem as well. Too many people downshift when coming to a red light when it is not neccesary, use your break While your car is still in gear 3rd or 4th, until you reach 15mph/36khp. Then remove clutch from drive, continue to stop put it in 1st and wait.

There is no reason to shift from 4th gear to 3rd to 2cnd etc… to come to a stop. It’s not required, nor is it smart, and yes excessive downshifting destroys clutch’s.
Ask any mechanic.

7

Okay, so I downshift, but leave the clutch in, and just cycle through the gears. My feeling is that this leaves me ready to go should I need to drop the hammer and go again, but I’m not engaging the engine thoughout.

Does this harm the clutch at all, or is it comparativly normal use?

8

I’m with the school that says “proper shifting (up or down) does not affect your clutch.”

Up or down, you should not let out the clutch until the rpms match - that means both plates are spinning the same and there is no wear on them.

I remember when I was younger enjoying my manual shift car more by not using the clutch at all: up and down. Took a few “grinds” to learn to listen for the matched rpms, but once learned, never forgotten! But, not recommended for emergencies.

Using the engine to slow down does use more gas, but it pobably won’t break your wallet and it’s more fun.

Oh, and I would not rely totally on what they teach in school. They also say that to implement the 2-second follow rule, you need to wait 2 seconds after the car ahead starts before beginning to move from a stop. It’s one of the reasons so few cars get thru left turn signals in FLA.

9

I miss driving a stick shift after driving an automatic for the last 15 years.

I recall that just taking your foot off the gas and letting the engine drop to the bottom of the RPM range for that gear started the car’s slowdown. Then, I would downshift in situations where I didn’t expect to come to a complete stop, so that there was little, if any, extra clutch use.

I did have to replace a clutch once. A friend had borrowed my car the day before. (Otherwise, no-one ever drove my car but me.) I found out that although the car had 4 gears, the highest gear he put it in was 3rd. That in itself wouldn’t damage the clutch (or would it?), but I’ve always wondered whether it’s possible to ruin a clutch in one day.

10

Do not ever, ever downshift to brake on an icy road! Your car has brakes for a reason. Firstly, downshifting only brakes the drive wheels (rear wheels on most sports cars, front wheels on most others), while the brakes work on all four (generally with about 2/3 of the braking power on the front wheels and the rest on the back wheels). Secondly, while brakes are carefully balanced (as required by law) to apply equal braking power on both sides of the car, downshifting distributes the engine’s braking power unevenly because of the differential transmission. The difference isn’t normally noticeable on dry blacktop, but on a slippery surface it can be lethal.

If you have trouble maintaining traction while braking on ice, you can a) apply lighter pressure (not always an option), b) install ABS brakes (or buy a car with ABS preinstalled), or c) simulate ABS by “pumping” the brake (takes some practice).

If you choose to go for ABS brakes, practice using them - the noise and vibration the brakes make when the ABS kicks in is very surprising if you’ve never experienced it before, and you don’t need that surprise to distract you in an emergency situation.

You should also be aware that ABS brakes take longer to stop the car than full braking power without ABS - the upside is that you maintain control of the car and can steer while braking (which you can’t do with locked wheels).

(Ah, the advantages of living in a country where practice in driving and braking on ice is a mandatory part of driver education…)

11

Hey 0-0, this is the scenario I’m talking about:

I’m driving 50mph in 4th (about 3500rpms, say), and I want to slow down a bit for a turn. I either:

A) Blip the accelerator to match the RPMs, pop it in 3rd and let out the clutch with very little slippage, then let off the accelerator to slow myself. To come back up to speed, I merely accelerate – no more clutching.

-or-

B) Hit the brake, slow down to 3rd gear range, push the clutch in, shift into 3rd with as little slippage as possible, and accelerate back to my desired speed.

You’ll notice that both of these options involve exactly one use of the clutch, and they’re both situations where the RPM’s are matched as well as the driver can manage. I’ll grant you that the one that occurs under higher RPM’s will likely result in greater wear, but not by much.

You are correct, downshifting to a stop (or many many other times) is pointless and causes tons of undue wear and tear. And clutches see huge amounts of wear and tear from people who insist on downshifting, but don’t know about RPM matching. But if you can downshift from 3rd to 4th without the passenger even feeling the slightest jerk and then decelerate, you know damn well there is not excess wear on the clutch.

And you’re right, it’s unnecessary and can cause more wear. But driving should be enjoyable, and when done correctly, the small amount of extra wear on my clutch is perfectly acceptable.

If you want to point out how I’m wrong about the forces involved in going around a corner under slight acceleration now, I’m all ears. But please leave the clutch out of it. Nobody is talking about shifting right in the middle of a hard corner.

You should drop all the crap about the tires/BHP/rpms, too. While we’re all very impressed that you know lots of factors in the equation, it’s quite possible to have an intelligent discussion without pinning down every variable.

12

Hey gearheads, all this talk about matching RPMs, and not one of you has bothered to explain how to do it. Maybe you can get off you high horsepowers long enough to do so. There might be someone here that could learn from it.

Regarding acceleration and turns, I definitely feel more stability and control accelerating through turns. Plus it’s more fun.

Yuck, I knew a guy in college who worked as an ambulance driver. He told me that the ambulances were manual transmissions, but nobody used the clutches. They just knew what RPMs to shift at. ACK! So then he bought a manual pickup and didn’t know how to drive it.

galt, I want to know how you downshift from 3rd to 4th. :wink:

13

Great, now I’m bothered.

Details: Been driving a clutch for 8 years. I don’t downshift to control my speed (unless I’m in the mountains) or to try to gain traction on ice. However, I always engage the clutch every time I brake. Don’t shift, just push the clutch in (unless I need to stop; obviously then I shift). Every time, even when I just need to slow for corners or such. Have for years. I’m not as knowledgeable as all you seem to be, and know little about clutches (except that I should replace my fluid ~80,000 miles or so), so my question, friends, is this: Am I damaging my car? I’m a smooth shifter and try to avoid jerks, so I’m not damaging the clutch that way, but are my practices damaging? If so, I’ll hafta change. But it’ll be hard after all these years!

Sidenote: had a '90 Honda Prelude that I treated this way–went to 200,000 miles with that baby. No clutch replacements, ever.

Thanks!
Snicks

14

Hmmm,
Ok galt, Your correct, I need not pin donw all the factors, it was excessive of me, I apologize.

He we go, and I’ll try to include everyone who asked a question or commented, not in any particular order.
Honda Prelude, 200,000 miles and no replacement?? Consider your self very lucky, but I have to say, I used to own one, and I was impressed with it, but I drove the hell out of it!! SO I had to replace the clutch at 140,000!! :slight_smile:
But don’t engage the clutch unless you have to, you apply slight pressure to your breaks.(say for 3-4 sec) with out even having to depress it.

Braking on ice, NEVER, EVER, brake on ice, (two points for DES), this is the worst thing you can do, you are better off merely trying slightly brake while keeping control, also downshifting on ice is just about the same thing, when you downshift (especially if the RPMs don’t match), you will speed up the wheel spin, this creates that “hydroplane” effect you may heard about in Driving school when traveling fast while it’s raining…mind you this is back before they had ZR rated tires, and before Posi-traction became standard on most cars.

Irishman, I would never shift the car without using the clutch…(what your friends who drove those ambulances were talking about), was something a lot of “hot-rodders” used to do, once you got real familiar with your car, you tend to know when the plates are matched…I.E. on most cars in 2nd going to 3rd between 2.3k and 3.2k RPMs, they would simply pull the shifter out of gear and put in the next, but you had to be quick and smmoth, otherwise you would girnd the hell out of the clutch. What glat and I were discussing, was that if you want to down shift, you need to be able to know at what RPM is the correct shifting time to go into a lower gear…(example:, If in 4th, if you let you foot off the accelerator pedal and let the tac drop to say, 1100rpm, then shift into 3rd <-mind you this varies on each type of car and ho tuned it is-> you would be “matching” the plate spin, and thus create no “smaking” of the links when you engae the lesser gear, coming from the higher one.

Mind you all, even in Automatics, there is a CLUTCH. you just don’t manually shift it. the computer in the car is preset to determine what rpm to shift it at with an actuator rod/pin.

knappy- you are correct in the way you drive, but your friend DEFINITELY damaged was tearing up you transmission, and subsequently your Clutch if he never went out of 3rd gear at high speeds.which would only further me to believe when he did downshift, the car was really getting “jerked” around. I wouldn’t let him drive it anymore, unless you don’t mind replacing your clutch again, and sooner than you realize, your transmissions. Your clutch will go first 90% of the time, but when that transmission goes, ouch! Break out the piggy bank!
thinksnow- well yes and no, as long as the gear you’re cycling into when you decide to drop the hammer, is the proper one then you’re fine, but if you are in 4th, say at 60 mph, and you see that the light up ahead it red, but you know it’s going to change etc., etc., and you depress the clutch, and start cycling, if you are now at 40 mph and you’ve cyled 2 gears, and you drop the “hammer”, you will be beating the hell out of your clutch, and tranny. it goes back to know not only what the Tac says but what speed each gear is best at (speed range that is). it takes a while to get used to knowing this. I remember that on 1990 Honda CRX’s had alittle light on the console that would tell you when to shift to the next gear, this was very usefull, in the fact you couold see what speed range each gear was supposed to be controlling. You learned to not even look at it after awhile, just at your speed and tacometer, and then finally, just the “sound” the engine made for shifting, this is what I call “learning the noise” factor.
Hmm, weight ratio, tires size, wheel pitch/displacement.
Add in curb weight (read as your cars shift weight on taking corners at certain speeds), and that is a new topic thats for sure. Oh yes, I almost forgot…
Accelerating on corners…not suggested, you increase your curb weight (think centrifical force)…or better yet…remember the water in the bucket?? and how you spin in circles and the water never seems to goe anywhere, well on a corner you don’t have that bucket to hold your butt in… and thus increase the chance of your car “slipping” out of the turn, just take your foot off the gas, and enjoy the turn until about 3/4 of the way through, then accelerate, and enjoy the feeling of the straight away!

15

I’m going to give you the benefit of the doubt and assume these are all typos:

It’s downshifting that you’re not supposed to do on ice. You have to brake sometimes (if you’re not allowed to use the motor or the brakes to slow you down, the only thing left is a snowdrift). Read DES’s post again.

If you shift without the clutch, you don’t grind the clutch, you grind the gears.

To go from 4th to 3rd, you don’t drop the RPM’s, you raise them. Put the foot on the pedal, don’t take it off.

If you think if it in these simple terms, it’s easy to see why you don’t understand the physics involved. If you’re on the edge of breaking free while cornering, hitting the brakes is one way to break free for sure. Basically, there is a point at which you’re simply going too fast to make the turn, and you have to stay below that line. If you brake during the turn, you have to stay much farther below the line, because braking is imparting a force in the opposite direction from where you want to go, which will lead to breaking free easier. If the force is imparted in the almost the same direction you’re traveling, you can go faster without breaking free.

Here’s a (hopefully) illustrative example: you’re running across a frozen lake, and you want to change the direction you’re going. You’re not going to try to turn and slow down at the same time. If you keep running forward at such a speed that each footstep is planted and doesn’t slide, then you can also impart a tiny bit of sideways force with each one, and gradually change your direction. You could do the same thing while slowing down, but A) you’d have to impart less sideways force, because the sideways force and the slowing down force will combine and overcome the static coefficient of friction sooner, and B) you’re, by definition, making it around the corner slower.

In addition, in a car, braking causes a lot of weight transfer to occur, which make keeping the tires stuck to the ground more difficult.

I highly recommend Physics of racing. There’s even a chapter called “there is no such thing as centrifugal force”. :slight_smile: Yes, it’s about racing. No, that doesn’t mean it’s not applicable for those of us who haven’t “turned their car into a tuned monster”.

16

Here’s a very simple way to think of this. Firstly, what keeps your car on the road is, well, roadgrip. There’s only so much roadgrip to go around - the exact amount depends on the size and quality of your contact patches and the state of the road surface. Secondly, everything you do that changes the way your car moves (braking, accelerating, turning) consumes an amount of roadgrip proportional to the amount of change (how hard you brake or accelerate, how hard you turn the wheel) and the speed of your car.

The immediate consequences of this are: accelerate too hard, and your wheels start spinning loose; brake too hard, and your wheels lock and you start planing; turn too hard, and the car continues in the direction it was going when it lost its roadgrip.

A further consequence is that if you do two of these at once (brake or accelerate while turning), you consume even more roadgrip and the risk of running out becomes much higher. In addition, braking power and acceleration apply unevenly to front and back wheels, so the wheels that are braking or accelerating harder lose traction first (especially if they’re also the ones doing most of the turning, as is the case on front-wheel-drive cars), and you start spinning like a top.

17

I have been reading the post’s on this with much interest. I live in Alaska and have spent the better part of 15 years driving both professionaly and personally on some of the worst roads and road conditions in the whole country. I just finished spending 4 years in the lower 48 putting down as many as 65,000 miles per year professionally. I thought I might be able to add a few things.

Braking on ice. If you have a manual transmission and IF it is a rear wheel drive down shifting on ice is an excellent way of decellerating. When you break (assuming you do not have abs) the brake will try to stop the wheel where as the transmission will continue to provide some power to it during a down shift. This lets you slow down with out ever touching the breaks. It will keep the wheels turning lowering you chances of breaking traction. Last but not least since it’s a rear wheel drive if you should lose traction and start into a slide all that need be done is to engage the clutch and kill all power to the rear wheels. This does not work as well in front wheel drive cars as you lose all steering control if your front tires break lose. This also gives you the option to use your breaks to help with the load if the distance is short or it is safe to do so since most of your breaking power is on the front wheels.

Just a thought, I’m sure I’ll get lambasted on this one but it is the method that the Alaska State Troopers advise.

18

One more clutch question, since I’ve become determined to break myself of my bad clutch habits (using it to balance on inclines in heavy beltway rush hour, clutch braking).

When I come to a light that I know will change in a few seconds, I tend to leave my foot on the clutch and shift the car into first. Should I always shift to neutral and take off the clutch, or is the stress from keeping the clutch down in this case nonexistent or negligible?

19

Argh!

Keeping the clutch down does no harm at all. The wear comes when you transition from down (unengaged, “out”) to up (engaged, “in”). (And a tiny bit when you transition the other way.)

20

That’s what I thought, but not knowing much about the actual mechanics (I consider changing my oil/corroded distributor cap/radiator hose to be an accomplishment) I was taking people at their word when they spoke of stupid people keeping their foot on the clutch as causing wear.

Also, since I was told that keeping the clutch half-way down while in first to make beltway start-stops easier and allow more fine grained control was damaging.