What is coasting and why is it wrong?

Discussion on Advanced and Defensive Driving. IAM, RoSPA/RoADA, High Performance Course. All associated training. Car training.

Postby Martin A » Thu Sep 18, 2008 2:34 pm


Just trying to find out people's opinions on this matter.
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Postby ROG » Thu Sep 18, 2008 2:52 pm


coasting means that there is no engine braking so therefore the vehicle is 'not under full control' in most circumstances.

The acceptable 'coasting' is when, say, in 5th at 70 on a dual carriageway and approaching a red light.

The light stays on red and you have to stop. still in 5th.

At some point during the braking the clutch has to be progressively dipped enough to prevent stalling.

During the latter part of this, the clutch will be fully depressed and you will be coasting - this is acceptable coasting as the only alternative is to go down through the gears which either means taking one hand off the wheel whilst braking - not a safe idea - or come off the brake, change gear, brake, off brake, change gear etc etc ............. a bit daft I think you will agree :)

Does this help :?:
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Postby Martin A » Thu Sep 18, 2008 3:02 pm


If I am going downhill at the steady speed I want to travel at and can steer the car accurately how am I not in full control? I don't want the engine braking effect otherwise I will slow down and have to waste fuel. Surely in these days of high fuel prices and eco driving we should make better use of the free energy supplied by gravity.
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Postby Mr Cholmondeley-Warner » Thu Sep 18, 2008 3:21 pm


(I'll be the first one to say it ;))

Modern fuel injection systems actually shut off the fuel supply completely on the overrun - i.e. with the wheels driving the engine. If the engine is still being driven (even at idling speed), fuel is being used. If your car has an instant consumption meter, try comparing the two modes. When coasting, you will see a consumption figure - admittedly a high one - maybe approaching 200 mpg. With the car in gear but the throttle closed (gravity pulling the car down the hill), you will see a false reading - either zero or just a line of dashes, normally.
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Postby Jasp » Thu Sep 18, 2008 4:56 pm


Martin A wrote:If I am going downhill at the steady speed


At what speeds and gradients does this happen?

I would have thought the times when gravitational forces and air + rolling resistance are perfectly balanced are few and far between.
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Postby michael769 » Thu Sep 18, 2008 5:55 pm


Coasting is any time the vehicle is moving when in neutral or with the clutch depressed.

Coasting is often unavoidable (eg when changing gear or just before stopping). Unavoidable coasting is OK, avoidable coasting is not.

Why?

Coasting in neutral is, of course very bad. If you found the need to accelerate or for engine braking (say if the brakes failed), you need to waste time engaging a gear, which in some older boxes can be surprisingly difficult. Coasting with the clutch down is less than a problem as you have the option to bring the clutch back in. Problems can arise with this if you happen to be in the wrong gear.

IMHO the main reason we are discouraged from avoidable coasting is that the most common reason for doing it is because the wrong gear is selected. Obviously this argument does not apply to the downhill argument but...

As Mr CW points out coasting downhill is false economy. When coasting you will use 1 miles worth of fuel every 5 minutes as opposed to no fuel at all when on overrun (vehicle moving but accelerator released). This is another good argument for looking well ahead and releasing the accelerator early when you see you will need to slow down or stop as you can gain the same benefits here too.

Even in older carburetor equipped cars, the amount of fuel used is closely linked to the throttle position. When the throttle is released the fuel consumption is reduced to near that that the engine would use when idling.
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Postby Gareth » Thu Sep 18, 2008 7:12 pm


Although people often bring up coming to a stop while in a high gear, I've never been particularly concerned by this providing the clutch is depressed at about the point where the engine begins to labour. After all, the driver is still braking at that point thus retaining full control.

Worrying cases of coasting might include where someone has fluffed a gear change (or just left it too late) and ends up going around a corner or tight bend while not in gear and/or with the clutch pedal depressed, and while not braking.

All responsibility for keeping the car on the road is left to tyre friction, and it's not a classy position to be in when you want to accelerate away.
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Postby crr003 » Fri Sep 19, 2008 10:03 am


Gareth wrote:After all, the driver is still braking at that point thus retaining full control.

I thought this was the definition of good "coasting"?
If you're on the brake, you're not coasting - to the DSA definition.
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Postby ROG » Fri Sep 19, 2008 10:24 am


crr003 wrote:
Gareth wrote:After all, the driver is still braking at that point thus retaining full control.

I thought this was the definition of good "coasting"?
If you're on the brake, you're not coasting - to the DSA definition.


but you COULD be coasting without needing to whilst on the brake - usually done by those that fully dip the clutch at the same time they apply the brake :!:
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Postby zadocbrown » Fri Sep 19, 2008 12:54 pm


Try this analysis of various coasting or non-coastion options.

1. Normal driving In gear, clutch up, right foot on gas or brake. We have instant control of either acceleration or braking, and almost instant control of the reverse effect by just moving our foot accross. This is good.

2. In gear, clutch down, foot on brake Instant control of brake, but to get any power down we have to move both feet and get the clutch engaged, which takes time, and can destabilise the car if done roughly. Also, without the engine engaged, it is harder to know which gear you should be in, or what revs you need to match in. Also possibly reduced sense of speed of the vehicle. This is less good than 1.

3. In gear, clutch down, foot not on brake All the disadvantages of
2, plus you don't have any immediate control of your speed, in either direction. You are at the mercy of gravity etc. This is generally bad.

4. No gear engaged As for 3 but now you have even more work to do before you can get moving. Bad.

Whether any of this becomes an actual problem is down to circumstances. The rules do start to break down at low speeds. I will, sometimes, coast in neutral down a hill in crawling traffic if my speed is not getting above 10 mph, as this avoids a lot of unnecessary pumping of the clutch. The time to really avoid coasting is going into hazards, as this is where you especially need full control, and coasting here tends to result in getting your feet tied in knots when you decide to 'go'!
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Postby Martin A » Fri Sep 19, 2008 2:24 pm


I am aware of the fuel cut off. A competent driver will be planning ahead and will be able to anticipate when drive or braking is necessary so assuming instant go or slow is not needed how is one not in full control.?
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Postby dth » Fri Sep 19, 2008 9:07 pm


Martin A wrote:I am aware of the fuel cut off. A competent driver will be planning ahead and will be able to anticipate when drive or braking is necessary so assuming instant go or slow is not needed how is one not in full control.?


Whatever the level of planning and anticipation going on, any driver can be taken by surprise by an unexpected and unplanned eventuality. In such a case, coasting will not facilitate immediate application of the drive which provides maximum control.

The expert (not simply competent) driver, will always do whatever is necessary to maintain full control at all times and shouldn't make assumptions about safety.

In life, people will do things or not do them and there may never be any consequence. That doesn't mean that to do something or not to do something is right, it just means they haven't been caught out - yet!
Life is not black and white - neither is driving.
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Postby Martin A » Fri Sep 19, 2008 10:37 pm


Just playing devil's advocate here, but surely one could also be caught out when block changing down. So by the same token we should not block change but go down through the gears sequentially to always be in full control. i.e. always be in the gear that can provide acceleration, which for instance fifth won't do at 20 mph say if we are slowing to negotiate a 10 mph turn from 70 mph.

As I said in my original post I was looking for opinions. So far there don't seem to be any reasons that prevent it from being another 'Tool in the box'.

The instant control myth has just been shown to be a red herring or if it isn't then block changing is wrong and the fuel cut off will only be on the over-run, which slows the car down, which I don't want if I want it to maintain it's speed.

Are there any other reasons that the car cannot be 'under full control' in the instant while coasting nd not at some future point when the circumstances change.

Please don't get angry with me if I challenge accepted wisdoms. I've returned to this forum for some intelligent debate. I have my own carefully thought out opinions that are the result of experiment and open minded reasoning because like many who visit this forum I am not satisfied that just because IAM or whoever say something is so that it is.
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Postby Mr Cholmondeley-Warner » Fri Sep 19, 2008 11:04 pm


OK Martin, suppose we turn this on its head.

Why don't you tell us what the benefits of coasting are to you?

(see, we can play Devil's Advocate, too :D)
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Postby dth » Fri Sep 19, 2008 11:10 pm


Martin A wrote:...... So by the same token we should not block change but go down through the gears sequentially to always be in full control.


Shall be interested in your response to Mr CW!

In the meantime, the greatest benefits of block changing are:

less wear and tear

less driver effort

more concentration on planning/dealing with the hazard rather than purely control skills

increased use of brake lights for safety

increased incidence of two-handed steering
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