Wing mirrors on cars

[boltar]

I own one.

Nonsense. They change UP as the load on the engine is less, not down.

[NY]

I would have thought that it would be easy enough with a computer-controlled auto box to sense "overrun" when the wheels are trying to drive the engine and to change down a couple of gears to achieve some engine braking.

Failing that, you could always use the "L" setting (equivalent to changing down in a manual) which since the early days of auto boxes has forced the gearbox to change down - probably so it uses one or two of the lower gears according to engine and road speed.

The difficulty with older autos was moving the selector from D to L without the car lurching as it changes down and incurs extra engine braking rather suddenly; in a manual you'd be able to increase the throttle to match the lower gear and you'd be able to let the clutch in gradually to "smooth over" any remaining mismatch. I imagine that is less of a problem with a modern auto.

[Dave Plowman (News)]

As I said, you've not driven a decent modern auto. Both my last car - built in 1997 - and my current one change down automatically if your speed increases going down hill on a closed throttle, and you touch the brakes.

[boltar]

My current auto car built in 2008.

I guess you just drive a posh car then. Tho why you would want it to kick down when on overrun is anyones guess. It might be useful on a steep hill once in a blue moon but it certainly won't be going down a motorway incline. If you want to change down , use the manual selector.

Also brake pads are a damn site cheaper than a new gearbox!

[boltar]

How does the computer know if you want to brake? You might have just come off the throttle going down a fast incline. Autoboxes should just be dumb changers. If the driver wants to force it to another gear then use the manual selector as most gearboxes have had for a decade if not more.

[Dave Plowman (News)]

You can do much the same by changing into low gear on an auto while using some throttle. Just enough to prevent a lurch.

[The Natural Philosopher]

Because that puts it in the right gear to accelerate away afterwards

[NY]

You'd want to only change down if there was significant braking as well, to prevent the car changing down *whenever* you come off the throttle - 90% of the time you simply want very slight slowing eg when adjusting to the speed of a slightly slower car in front, or when going down a gentle hill where normal engine braking in top is enough to maintain constant speed.

My driving style in a manual is to remain in top while braking (on the level) and then to change to the appropriate speed as I'm ready to accelerate out of the hazard, and I'd want an auto to do the same - or else to do nothing and let me trigger the change-down manually.

In a manual, you wouldn't change down every time you lifted off the throttle, because you wouldn't want significant engine braking or significant acceleration afterwards.

If the gearbox *did* change down, you'd want it to stay in the appropriate low gear even if you happend to lift off the brake because the engine braking happened to be sufficient to maintain the required speed going downhill.

Having manual control of selecting low gear, which autos have always had, is probably more certain and predictable (no unexpected gearchanges up or down at the wrong moment) than letting a computer work it out.

[James Wilkinson Sword]

I've used engine braking ONCE in my entire life, brakes are cheap and more controllable, so I always use them apart from this one time when I smelt something funny. Descending in the French Alps with my Golf auto. I just knocked it down two gears with the lever and left it there for a mile. Nothing overheating or got broken. That's why the low gear selection is there.

[The Natural Philosopher]

Useless on a race track or rally stage then.

[NY]

Agreed, but people drive on normal roads, not under race conditions, so they don't hare up to a corner as fast as possible (to shave a few seconds off their time) and then brake (using brakes and engine braking) as hard as possible to a safe speed for negotiating and accelerating out of the bend.

Rally/race driving is one extreme, "granny-driving" (painfully gradual acceleration, max speed 40 mph anywhere) is the opposite. Normal, smooth, making-good-progress driving is in-between.

[The Natural Philosopher]

But you still want to be in the gear that allows you the most options - i.e. acceleration or braking, whichever.

I guess you simply aren't te sort pf person who spends time on te motorway in nose to tail traffic looking for the softesr places to crash when the idiot behind you overtakes, cuts in, causing the guy in front to slam his brakes on just as the guy behind starts to really tailgate you.

[Dave Plowman (News)]

Doesn't mean it has a decent auto.

Kick down? That refers to a large throttle opening. Not the over run.

Depends on how steep the motor way incline. But it only happens when you have told the car you don't want it to increase speed on a downhill by touching the brakes. If you are happy for it to speed up going down hill, it won't change down.

Not much point in an auto if you often have to select the gears yourself.

And auto changing down on the over run puts little load on the clutches.

[Dave Plowman (News)]

That's what it does. The car is speeding up downhill on a closed throttle. You dab the brakes and it will change down. To lessen the load on the brakes - if you still need some braking to keep the speed down to what you want. Just the same as a manual. A lower gear may or may not keep the speed constant depending on the hill.

[TMS320]

I have not told you what I think. I have been asking you about a special force you say is imposed on the front wheels.

Hmmm. So you think I need a crash course in Newtonian mechanics? Try the first law - "Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed."

So this torque you mention... what are the back wheels doing?

[TMS320]

Indeed. I have driven one that did. The algorithm behaved as though it was looking for non-decreasing revs when the accelerator was released. But not to the brake. Very irritating it was too, because the car had to be driven down gentle slopes where it would be normal to glide down.

[Dave Plowman (News)]

I've not come across one which changes down going downhill unless you apply the brakes. Why would it?

[TMS320]

I have no idea. Perhaps the person producing the software requirements wanted a downhill feature but forgot to mention the brake. But the rest of the shift program was a dog's breakfast, shifting too early or too late as though they had lifted the rev/accelerator tables from a totally different model.

[NY]

Timing of gear changes is the thing that has let down the (few) automatics that I have driven. In a manual I know when I am going to change gear and I time my gearchanges to happen *between* periods of acceleration rather than

*during* them so I'm changing at time when the engine is not driving the car hard. Automatics tend to change during acceleration so you tend to get acceleration that varies according to what gear you are in: hard acceleration in a lower gear, then less as it changes up, so you press the accelerator a bit harder to achieve the same acceleration as before which triggers it to change back down again - and so on.

[TMS320]

The energy dissipated going downhill must equal the energy it took to go up. The engine is sole producer of power on the way up but can only share the absorption of power with the brakes on the way down. Result - the gearbox cannot get hotter going down than it did on the way up.