DES’s post reminded me of something I was going to mention in my last post. When we talk about accelerating through a curve, we’re not talking about romping on it, because that will likely cause you to break free just like braking would. What we’re talking about is keeping your foot on the gas, so you’re accelerating a little. It’s a balance thing.
When you’re closer to exiting the turn…that’s when you romp on it. 
My problem… I drive a 5 speed Saturn. I sometimes don’t hit the gear fully, and get a little grindage. I think this is mostly rushing the clutch release - getting too cocky on shifting and not making sure the shifter is fully in position, plus my shifter has had some problems and sometimes doesn’t want to go. I had it checked out and it was problems in the shifter, not the tranny, and I had part of the cables replaced, but I think some others are a little loose or stretched or whatever.
I learned on a ‘57 Chevy pickup truck - 3 speed column shift, old beater. This thing had power steering - arm power steering. Just crank away and you eventually start turning. This one in particular had a accident that busted one of the control rods in the column and it was never repaired right - dad did it himself. It had about 70 deg of free play in the steering wheel. No foolin’.
Anyway, when learning to drive, I was never taught anything about looking at rpm’s. I was taught to drive by listening to the motor, and then shifting when it “sounds right”. No real explanation for when it sounds right, just experience driving and coaching. Drove that baby for a few years.
After that I drove other people’s cars a few times. The hard part was getting comfortable with how their car behaved, when to shift, etc. Plus after a couple years driving only automatics, sometimes I’d forget to shift, which sucks when you’re braking and the car starts jerking and dies.
When I got my Saturn 6 yrs ago, again it was not a matter of knowing anything about when to shift. It was trial and error, when it seemed to sound right. I am fairly comfortable now with figuring out what gear I should be in based on feel and sound, but occasionally go one too high or one too low when shifting speeds in city driving - slow down, speed up, slow down, speed up, stop.
Anyway, I’m just curious if there’s a better way to learn than “that pitch sounds high enough, I’ll try a shift now”. Thus my question about matching rpms. I have occasionally found shifting out of 5 I hit the right rpm it comes out to neutral without the clutch, but I don’t try forcing 4th, I always use the clutch to engage. And I don’t really drive it that way, coming out of 5th without the clutch.
Oh, my car has one of those shift lights, but I have always ignored it. I was told those are basically idiot lights - if you haven’t shifted now, you really need to. I either beat the light on shifting, or else am transitioning to the lower gear for more power for things like passing and merging.
irishman says:
The explanation of this that was posted was backwards, so it might have confused you more. Here’s my attempt at it.
Say you’re driving along in 4th at a constant 3000rpm, and let’s pretend that’s 50mph. There is a corresponding engine rpm for 3rd gear which will also make you go 50mph. Let’s pretend it’s 4517rpm. So imagine that you know this, and you push the clutch in, move the shifter into 3rd, push on the gas until the tach gets exactly up to 4517rpm, and let the clutch out. You have switched gears with zero wear on your clutch. Once the clutch is fully engaged, let the gas off and you’ll slow down pretty quickly. Not as quickly as if you hadn’t matched the RPM’s, but that would be the Wrong Way to do it. A clutch plate is not a brake pad, folks.
Of course, in the real world, you don’t know what rpm you want all that precisely, and you want to do this quickly, so it becomes a feel sort of thing. You tend to downshift at about the same speeds most of the time, so you get a feel for how hard you have to tap the gas when going from 4th to 3rd, etc. You notice certain patterns. I notice that on my car in 4th gear, the scale of the tach and the scale of the speedometer are such that the needles are always parallel, so it’s really easy to match the RPM when going into 4th.
Incidentally, where did I say “downshift from 3rd to 4th”? I think you misread.
I almost had to stop myself from choking,
But I see someone beat me too it, NEVER, EVER break on ICE.
EVER!, Here, I’ll say it again, NEVER break on ice, and the frozen lake theory is simply ridiculous, think of dowshifting vs breaking like this on Ice. Breaking, Long strides with your inertia still moving you in one direction, which means your long strides, however slow they may be, will take you longer to over come your inertia in order for you to redirect yourself. Downshifting, small steps at a faster pace, I.E. less time for you to redirect yourself do to the greater movement now going in the other direction/ or another direction to over come your initial vector/inertia. Ths is Physic, the basics.
You’re are correct galt, I miss explained myself, The tac should not drop the speedometer should, two points for you.
Breaking vs accelerating on a curve/corner.
Accelerating will make you break out of a turn much faster then breaking, and yes there is something called centrifugal force, think of it like this…
Suppose a person sits on a bus, moving in a straight line with constant speed v. V=velocity the forces involved are the passenger’s weight F1 and the reaction force of the seat F2, and in the absence of acceleration, the two cancel:
F1+ F2 = 0
Suddenly the bus makes a sharp turn, following part of a circle of radius r. If the passenger’s body (read this as your car on the road) is to stay in the seat as before, an extra force must be added, to keep it from continuing in a straight line (its natural tendency)<-- Newtons forst Law-- If ru is a unit vector, directed outwards from the center of rotation, along the radius r, the force F2 exerted by the seat must increase (read road grip) to provide the centripetal force -(mv2/r)ru that makes the passenger follow the motion of the bus:
F1+ F2 = - (mv2/r) ru
The bus and the seat now constrain the passenger’s body to follow part of a circle, and to do this must pull it towards the center. To stay in place (that is, balance the above equation), F2 on it must be increased by an additional force in the direction of -ru, towards the center of the curve.
How does this same event look in the frame of the bus? Adding (mv2/r)ru to both sides of the equation gives, in a similar way to what was done before
F1+ F2 + (mv2/r) ru = 0
Equilibrium is attained and the passenger stays in place, if the equation above is satisfied. This can be viewed as the equilibrium between 3 forces-- F1, F2 and the centrifugal force (mv2/r)ru directed along ru, radially outwards.
It does exist, and if it is not balanced, I.E. not breaking hard, nor even accelerating the slightest, on a corner, you will find out otherwise. Again this is not taking into effect Tire size, wheel base width, Pitch of the tires, or a number of other things, but if it was all basics, you’d be a very unhappy (and possibly dead) driver.
case closed.
It’s amazing how passionate people get over this issue.
My qualifications/biases:
14 years as a semi-professional mechanic, 10 years of serious hot-rodding before that.
nitpicks:
Kyber and JW Kennedy, leaving the clutch pedal depressed puts the full pressure of the clutch spring(s) on the throwout bearing, fork, and crankshaft thrust bearing. Granted, this is not so much a concern with the newer diaphragm clutches as it was with the old B&B units, but still increases wear on these parts. Keep the pedal down the absolute minimum ammount of time needed to safely execute the gear change.
1a)Using the clutch to hold the car on an incline is a VeryBadThing.
Engine braking does not increase fuel usage. High manifold vacuum (higher RPMs, closed throttle plate(s)) uses the least ammount of fuel possible.
A properly executed downshift causes less wear on the clutch than an upshift, and no extra wear on the engine (aside from the thrust bearing already mentioned). The negligible wear on the transmission appears on parts that were designed and included in the gearbox specifically to allow downshifting.
4)0-0, you are so wrong on so many issues I am not even going to address it.
My $0.02:
What DES said.
What Hilander10 said.
I do not recommend downshifting in a front wheel drive vehicle.
Flat-shifting (no clutch):
Don’t do it unless you are prepared to replace a lot of drivetrain components. You only have to screw it up once to destroy some expensive, very hard to find parts.
Shift lights (idiot lights) are usually timed to maximize fuel economy. People don’t like to drive that way. People really hate to be behind someone that drives that way.
IMHO, downshifting provides better control and is safer than upshifting only or driving an auto trans, but requires a commitment to driving. (No cell phones. Oh great, I just opened myself up to another jihad)
I always double-cluch downshift. I live in the city, average 7 shifts per mile, (4 up, 3 down. I’ve been keeping track since I saw this thread) and in the last 11 years, have replaced 1 clutch ($200), 2 throwout bearings ($45), 1 U-joint ($20), and only 3 sets of brake shoes (2 front, 1 rear, $65)
(labor not included, obviously)
7a)I get really rotten gas milage.
It doesn’t really matter. People are going to drive however they want to.
Sorry this post is so long.
All splendid responses, but we have moved the point a little from the original column. The question asked was about whether one should change gear or use the brakes (not breaks as a couple of people have said) to slow down.
The column did not properly address the question, this thread has not properly addressed the question.
Most posters seem to agree that brakes are the thing to use if you want to slow down.
Most posters seem to agree that being in the right gear to accelerate again later is a good idea. True, but both obvious and nothing to do with how one should slow down. Use your brakes to slow down AND then use your gears properly.
Russell
Actually I allways gear down first. I spend most of my time driving on ice where brakes are a big no no… As to which is better that depends alot on how you drive and what the road conditions are.
RusselM: I think this is still pretty on-topic. The point of my original post was “yeah, I agree that for just plain slowing down to a stop, brakes are the right thing to use, but there is an appropriate situation for slowing down by downshifting, and that is when going into a fast turn that you want to accelerate through smoothly.”
pmh: a question on nitpick number 2: which uses less fuel: closed throttle plate at high RPMs, or closed throttle plate at low RPMs? I would think the former, but I admit I’m no expert.
0-0 Knowledge: I’m going to come right out and say it. You don’t know what you’re talking about. All your elementary physics, while perfectly correct, doesn’t support your babbling in any way. Besides, the fact that you can’t make a coherent sentence on your own, but can come up with a pretty well-stated (yet still incomplete) discussion of the forces a passenger feels while riding in a bus makes me think you just lifted that from somewhere else. Adding “case closed” at the end does not make you any more correct.
galt, thanks for the description.
And no, I did not misread. You mistyped.
Your post 2-20-2001 at 9:42 A.M.:
[bolding changed to emphasize quote]
This is the post right before my post that mentions it.
A) Well, yes, keeping the clutch depressed causes another kind of wear, but not what we’re talking about.
B) Yeah, that kinda goes through my head every time the light is red on our local “Snake Hill” (about a 50 degree slope).
probably isn’t worth that much, since I am not a mechanic, or a physicist. Merely someone who drives.
That said: I used to do rallying in New England (the Pirelli series was always fun!) in front wheel drive vehicles, usually small foreign cheap ones. (Rallies aren’t as much fun in sophisticated machinery.) (Think Subaru - cheap subarus were the ideal rally car for the amateur enthusiast.) Anyway, I did a lot of downshifting during rallies. I never had to replace a clutch, and that was putting rougher mileage on a car than most people. Granted I never drove one to 200,000 - any car I owned rusted out before then - but 140,000 miles of downshifting as a regular practice didn’t seem to hurt anything. I did ding and lose an awful lot of mufflers and exhaust pipes; I could be seen in the evenings on my Boston side-street under the car attaching more hangers for the exhaust pipe.
And from a different perspective, motorcycles: again my experience is as a rider, not a mechanic, but in 30 years of riding a bike, I’ve downshifted on every bike I’ve owned (pretty much all small Japanese bikes) and the people I hang out with do the same. Maybe the mechanics of the thing are different for bikes? I don’t know. But motorcyclists downshift frequently. (I don’t know about bikers, ain’t never been one a’ them bikers.)
So - does anyone who knows a lot more about the physics and mechanics of it want to explain to me why it would and should be different on bikes than on cars? Thanks!! (I promise to attempt to remember everything I learned in freshman physics before I switched majors to accounting.)
galt, at higher rpms, the engine can use a fuel-air ratio actually lower than that needed to idle. But it also uses more air. The two modes (decelerating and idling) will use the same ammount of fuel at a certain decelerating rpm (which is different for each motor). Perhaps I should have said “lowest f/a ratio” rather than “least ammount of fuel”.
That said, the difference either way is likely to be negligible.
pmh is right on that point ,but i should also point out that most modern electronically fuel injected engines will actually cut the fuel supply comepletely (or very significantly),under negative torque conditions for fuel economy, and especially emissions reasons, making downshifting use little or no fuel at any rpm, all other issues aside.
Irishman: mea culpa. I’m a bonehead.
pmh: Thanks for that clarification. It’s one of those points that always made me wonder.
BunRab: my only guess about bikers downshifting is that bikers have more of a vested interest in keeping their tires planted while screaming around corners (and proper throttle modulation is a good way to do that). But then, I ain’t a biker, so my guess probably ain’t worth squat.
If you aren't trying to accelerate hard, the shift points are 10, 20, 30, 45. I don't consider the light in the Saturn to be an idiot light--in general, it comes on about the time I would shift anyway, it occasionally comes on when shifting is a bad idea even. (Maintaining constant velocity just below the shift point. It can come on yet I know the engine will barely be able to handle the next gear. While technically you could shift, doing so would leave you with basically no ability to accelerate if needed.)
Every manual Saturn I've ever seen had a tach needle, also. You can see how the engine reacts to a shift.“brakes are for slowing, gears are for going” is one phrase my driving instructor used a lot.
One thing I haven’t seen mentioned is that brakelights signal to other road users that you are slowing down, rather than leaving them to find out the hard way.