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Old 2014-08-21, 04:00   #34
Xyzzy
 
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Quote:
Applying pressure to the accelerator pedal to reduce speed.
If you are coasting down a hill in a vehicle (moped?) that has a centrifugal clutch, and the engine is at idle thus causing the clutch to disengage from the engine output, the vehicle will freewheel faster and faster. If you "burp" the throttle enough to overcome the spring tension holding back the clutch shoes then they will engage the engine which results in compression braking.

Quote:
Applying pressure to the brake pedal to increase speed.
If you have an open differential and you are stuck in sand, mud or whatever, and only one wheel is spinning, and you are not going anywhere, gently applying the brakes will redirect the power from the driven wheel that has no traction to the undriven wheel that (hopefully) has traction.
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Old 2014-08-21, 04:10   #35
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I suppose it's all a matter of the amount of power being applied....and the size of the gravel. I remember seeing, on the downgrade side of mountain highways, runaway truck traps. My memory is that they had gravel in them, but it could have been sand. They were spurs off the shoulder of the highway, ramps which provided increasingly deep "resistive material".

I also remember a time, in an 70s Oldsmobile, in icy conditions, in Springfield, IL. I was being cautious with the power, starting up an overpass, but even so it started wagging its tail, as it were. (This is rear wheel drive, of course.) I just coaxed it over the peak and coasted down the ramp, and right into a very fortuitously placed motel parking lot.

After my traveling companion and I checked in, we were directed to drive around to one side of the rather large building so as to park close to our room. Unfortunately, the building was on the peak of a hill, and our path had us going directly across a fairly abrupt down slope. As soon as we were fully on the slope, the car commenced sliding straight sideways on the icy asphalt, which it continued to do until it fetched up against the grass at the edge of the parking lot where it stopped fairly gracefully.

(My part of Chicago to Springfield is about 208 miles. Google says 2 hours, 25 minutes. It had taken us 6 or 8 hours.)

We pulled our bags out, locked it up, and very cautiously crept up the pebble-like surface of sleet embedded in freezing rain. I had been driving since Chicago in snowfall, which turned to wintry mix just north of Springfield, so I was absolutely exhausted. I'm sure I knocked back a few of more than one substance.

In the morning, the sun was shining and a guy from the motel came out with a bag of salt and threw it under our tires and on the slope above.

Quote:
If you have an open differential and you are stuck in sand, mud or whatever, and only one wheel is spinning, and you are not going anywhere, gently applying the brakes will redirect the power from the driven wheel that has no traction to the undriven wheel that (hopefully) has traction.
Way Cool! I think part of my problem in the Olds was that it had Positraction, which made it fishtail on the ice.

Last fiddled with by kladner on 2014-08-21 at 04:21 Reason: details, details
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Old 2014-08-21, 08:36   #36
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Quote:
Originally Posted by retina View Post
1) Applying pressure to the accelerator pedal to reduce speed.
Perhaps you're thinking of changing down from a higher gear to a lower one, which might be part of reducing speed.

When still living in the UK in the 1980s Nick and I owned a couple of Morris Minor 1000 Travellers from 1971 and 1970 respectively. They had started to become classic cars even back then, and 1971 was in fact the year the Traveller version stopped being produced.

The cars had no synchromesh on first gear. Normally this was no problem because you would engage first gear only when stationary to start off, and you would avoid changing down from second to first when the car was moving. The exception to the latter rule would be on a very steep hill, either upwards or downwards, when first gear was indeed needed at a time when the car was still moving. The technique required was called double declutching, and it was commonplace a few decades before that when cars had no synchromesh on any of their gears so that you had to do it all the time when changing to a lower gear (that was before my time). It involved putting the clutch out to disengage the previous higher gear, then giving a quick blip on the accelerator pedal in neutral to give the engine sufficient revs, before putting the clutch back out and engaging the lower gear.

I did it twice in the 1971 Morris on a journey Westbound on the A39 in Somerset and Devon. Climbing Porlock Hill, which has a couple of hairpin bends with 25% gradient, second gear sufficed for the stretch between the bends but first gear was needed for the top hairpin with as little loss of momentum as possible. The double declutching went fine, we rounded the bend in style, and I was quite gratified. Fifteen minutes further on it was time to come back off the mountains via Countisbury Hill, a long straight descent of 12-20% gradient with a 25% final sting in the tail. After a lengthy period with engine braking in second gear we reached the warning signs about the hill steepening to 25% and I confidently did my double declutch routine for a second time. SNAP! The clutch pedal fell lifelessly to the floor. I had broken the clutch rod. We coasted down the 25% remainder of the hill on the Minor's dodgy drum brakes, managed to freewheel to the side of the road, and called the garage.
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Old 2014-08-21, 12:48   #37
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Quote:
Originally Posted by Xyzzy View Post
If you are coasting down a hill in a vehicle (moped?) that has a centrifugal clutch, and the engine is at idle thus causing the clutch to disengage from the engine output, the vehicle will freewheel faster and faster. If you "burp" the throttle enough to overcome the spring tension holding back the clutch shoes then they will engage the engine which results in compression braking.
I hadn't thought of that one specifically but it sounds plausible to me. But how about for an ordinary family car?
Quote:
Originally Posted by Xyzzy View Post
If you have an open differential and you are stuck in sand, mud or whatever, and only one wheel is spinning, and you are not going anywhere, gently applying the brakes will redirect the power from the driven wheel that has no traction to the undriven wheel that (hopefully) has traction.
This is precisely what I was thinking of. You get extra credit for that.
Quote:
Originally Posted by TheMawn View Post
1) If you're in exceptionally gooey terrain mashing the accelerator could cause your wheels to lose their grip. Then, instead of helping you to propel forward, the wheels are useless and slow you down as they plow through the mud or sand.
Somehow I don't think it will quite happen like you suggest.
Quote:
Originally Posted by TheMawn View Post
2) Downhill on an icy slope, the friction between your wheels and the ice might control your speed whereas if they were suddenly locked, the lack of friction allows you to accelerate faster under the force of gravity.
I guess this is correct but you would lose all control of the vehicle so on that basis I would say not really a good answer.
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Old 2014-08-21, 12:51   #38
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Quote:
Originally Posted by Brian-E View Post
Perhaps you're thinking of changing down from a higher gear to a lower one, which might be part of reducing speed.
I wasn't thinking of that. The double clutch thing doesn't count because as you apply the accelerator the effect on speed is nothing. I'm looking for a situation where applying the accelerator is the impetus that slows you down. In the double clutch scenario it is the gear selection and engine braking that slows you down.
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Old 2014-08-21, 19:18   #39
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Quote:
Originally Posted by retina View Post
I wasn't thinking of that. The double clutch thing doesn't count because as you apply the accelerator the effect on speed is nothing. I'm looking for a situation where applying the accelerator is the impetus that slows you down. In the double clutch scenario it is the gear selection and engine braking that slows you down.
Methinks those moving goalposts need some impetus to slow them down.

Alright then, I was wrong again.
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Old 2014-08-21, 19:32   #40
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Quote:
Originally Posted by retina View Post
There are some other driving "tricks" also.

Extra credit to those that can spot the following situations:

1) Applying pressure to the accelerator pedal to reduce speed.
Throttle induced oversteer (e.g., drifting)? Skilled drivers maintain control of the car at all times during this maneuver (to often amazing precision) and mashing the gas (while carefully steering) definitely slows them down (at some cost to the tires).
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Old 2014-08-21, 20:06   #41
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I hadn't thought of that one specifically but it sounds plausible to me. But how about for an ordinary family car?
We would posit that there are many thousands of mopeds around the world that serve as family cars.

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Old 2014-08-21, 22:46   #42
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Quote:
Originally Posted by bsquared View Post
Throttle induced oversteer (e.g., drifting)? Skilled drivers maintain control of the car at all times during this maneuver (to often amazing precision) and mashing the gas (while carefully steering) definitely slows them down (at some cost to the tires).
I'll have to take your word for this as I have no experience here. But I will fallback on my previous comment about the solution being practical. The above is clearly for the petrol heads and boy racers and thus entirely impractical. There is a far more common situation. I'm not talking about esoteric specialised scenarios that only hyper-skilled people could undertake.
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Old 2014-08-21, 22:49   #43
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Quote:
Originally Posted by Xyzzy View Post
We would posit that there are many thousands of mopeds around the world that serve as family cars.
Hehe, indeed. I've seen it firsthand also. Kind of stretches the meaning of car though. I'll give half credit for the humour content.
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Old 2014-08-23, 22:01   #44
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Originally Posted by only_human View Post
Ok, here is more information:
http://en.wikipedia.org/wiki/Risk_compensationmy bold emphasis
Typical academic economist - taking credit for a concept that is in fact much older.

(The wikipage focuses on the financial aspects of MH, but the generalization to other areas of game-theoretic risk/reward is self-evident).

Last fiddled with by ewmayer on 2014-08-23 at 22:25
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