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 Post subject: Coasting
PostPosted: Thu Oct 22, 2009 15:24 
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When I was doing my driving lessons (about 8yrs ago) I remember well that my instructor frowned upon coasting (in neutral or just with the clutch down) as he said I didn’t have proper control of the car. More recenlty, whilst taking some motorbike lessons, the same theme came up: coasting means you don’t have proper control of your vehicle.

I’d be interested to explore the pros and cons of coasting, whether it really is dangerous or not, and whether you really don’t have proper control of the car/vehicle or not. Are there situations when it is aceptable to coast, and are there situations where it's dangerous?

I sometimes coast towards lights if I can see they are red. Does coasting stop me form having proper control of the car? I know I’m not going to be speeding up in the immediate future so don’t need the engine power, but I will be slowing down and the car has brakes to do this.

Thoughs and discussions welcome :)


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 Post subject: Re: Coasting
PostPosted: Thu Oct 22, 2009 15:41 
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I coast but only to lighten up a boring drive sometimes. When I'm on my own, I'll be doing 60 and see how long it takes me to stop.

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 Post subject: Re: Coasting
PostPosted: Thu Oct 22, 2009 15:42 
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I always COAST but never coast. :)

The vehicle control point is valid and, as modern cars use zero fuel when engine braking, it's probably more economical than with the engine idling.

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 Post subject: Re: Coasting
PostPosted: Thu Oct 22, 2009 16:02 
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The only thing you can't do when coasting is speed up so it must be safe :D

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 Post subject: Re: Coasting
PostPosted: Thu Oct 22, 2009 16:15 
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Flynn wrote:
The only thing you can't do when coasting is speed up so it must be safe :D

You can if going downhill - and then your brake servo might not work either :twisted:

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 Post subject: Re: Coasting
PostPosted: Thu Oct 22, 2009 16:19 
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Care to elaborate on the control issue? What could the car do that wouldn't be what you want it to do?


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 Post subject: Re: Coasting
PostPosted: Thu Oct 22, 2009 16:26 
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If you were not in gear, then it would take you longer to re-engage the drive train if you needed to accellerate suddenly. You will also have no engine braking available to control the speed of the vehicle.

The real question though is why would you want to coast?

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 Post subject: Re: Coasting
PostPosted: Thu Oct 22, 2009 16:58 
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Brakes are there for stopping the car and are usually what I first resort to if speed needs to be reduced, though there are times when engine braking can be a better method of reducing speed. If some quick acceleration is called for then very often a downshift is also needed. If the car is in neutral then you’ve already done half of the gearshift so is it really going to take longer to re-engage the drivetrain and accelerate? I generally find myself coasting when I am slowing down gently and don’t need engine propulsion (eg, red lights somewhat ahead).

I’m not trying to say that coasting is a good thing to do or even if it is beneficial, more just wanting to explore the ins and outs of it and if it does indeed mean you don’t have proper control as my instructors pointed out.


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 Post subject: Re: Coasting
PostPosted: Thu Oct 22, 2009 18:03 
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malcolmw wrote:
The real question though is why would you want to coast?

When done correctly it can save money by increasing mileage (burn and coast), but if done wrong it can be at the expense of faster wear on the clutch.
Of course this (the fuel saving) is redundant when wanting to slow down; as you rightly say malcolmw, it can be a false economy.



When maintaining a certain speed when going downhill, it is always good to be in gear such that engine braking takes effort away from the brake disks so preventing them from getting so hot; hot disks increase the risk of brake fade when an emergency stop is required.

The following is something I never realised until now.
I believe another problem of coasting is the knock on effect of when an engine dies, specifically with the steering and brake servos.
If in gear, the engine is forced to turn over so always providing power needed for power steering (if applicable) and brake boost, even if the engine fails in some way.
If coasting and the engine dies, power steering will be lost and the brake boost is at risk of being lost (depending on usage and reservoir); you likely won't know about these losses until you need them - which will be too late.

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 Post subject: Re: Coasting
PostPosted: Thu Oct 22, 2009 22:52 
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samcro wrote:
Brakes are there for stopping the car
Well not 'only' of course ! One can brush the brakes to check and steady the car, slow the car, to improve balance, send the car into an intended slide, hold or alter the handling, and control advanced skids inc hand brake turns and J turns - of course. :)
It's all about improving friction & control.
samcro wrote:
and are usually what I first resort to if speed needs to be reduced, though there are times when engine braking can be a better method of reducing speed. If some quick acceleration is called for then very often a downshift is also needed.
When engine braking you are still in gear and so much control is retained and when you need power you have some or lots depending on all the obvious factors. Might often be better to downshift first, (when appropriate), given environment and anticipation of conditions. :)
samcro wrote:
If the car is in neutral then you’ve already done half of the gearshift so is it really going to take longer to re-engage the drivetrain and accelerate?
I am not convinced that this is the best way to look at it albeit in the 'positive'. :)
.. But if you had been in gear already you would have totally negated the need to re-select a gear at all, nor have to take the hand off the wheel, which isn't ideal.
Were you to coast in gear but with clutch engaged that would be marginally improved, perhaps but why 'bother'?
Plus if you fumble a needed gear selection, you might end up in neutral just when you now need to be in gear.
In an emergency and with the correct rev's only, one can 'slam' it into gear, but hopefully this would never (ever) be necessary.
The extra wear and tear, on the gear box and drive train/s might catch you out later on - the cost of coasting & not applying COAST!
And as Steve points out, the potential loss of other controls if the engine dies or indeed, needs to be turned off.
samcro wrote:
I generally find myself coasting when I am slowing down gently and don’t need engine propulsion (eg, red lights somewhat ahead).
Are you saying clutch depressed and braking ? - however this would still leave a gear selected.
samcro wrote:
I’m not trying to say that coasting is a good thing to do or even if it is beneficial, more just wanting to explore the ins and outs of it and if it does indeed mean you don’t have proper control as my instructors pointed out.
No problem, to discuss and explore. :) To disable the ability to 'drive' forwards prevents options if and when necessary.
I'll have to think of advantages but at the moment I can think that is might be more of a psychological advantage, of the seemingly temporary 'feeling of traveling free' and perhaps slightly 'less noisy'. :)

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 Post subject: Re: Coasting
PostPosted: Thu Oct 22, 2009 23:18 
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It is worth examining such rules from time to time, there may be circumstances in which there is at least nothing wrong with coasting and perhaps something useful. I have occasionally kept the clutch depressed for a few seconds when I was about to change down to say third and something in the road picture changes which means I may need second instead but this is still not certain. I will also sometimes dip the clutch for the last few seconds while coming to a stop where I was in fourth or fifth gear as otherwise I would need to select second to prevent the engine stalling or juddering at very low rpm for the last few yards. To me a car always feels wrong if cornering out of gear, this though may be due to it being outside my normal practise.

It may be a little like the do not brake in a corner rule, this is generally good advice, however there are times when braking is the better course.

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 Post subject: Re: Coasting
PostPosted: Fri Oct 23, 2009 00:19 
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malcolmw wrote:
I always COAST but never coast. :)

The vehicle control point is valid and, as modern cars use zero fuel when engine braking, it's probably more economical than with the engine idling.


I've often wondered about this. It's true most modern cars have an over-run fuel cut-off feature. So you use less fuel (no fuel!) on over-run. HOWEVER, you are also engine braking under those conditions. If you dip the clutch and coast, you use more fuel because the engine is idling, but you go further because you're not engine-braking! I suspect there's a complex equation somewhere that will have a fair few variables but would tell you, in any given situation, whether you'll use less fuel by coasting or on over-run!


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 Post subject: Re: Coasting
PostPosted: Fri Oct 23, 2009 09:28 
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I think what Toltec describes is typically all the coasting that I'd do; deliberately slow down changes or pauses in gearwork when it's not immediately obvious which gear will be the best for an approaching part.


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 Post subject: Re: Coasting
PostPosted: Fri Oct 23, 2009 20:17 
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What happens with an electric or hybrid car when you come off the throttle? I would assume there would still be some losses due to bearing friction and air movement within the motor, however unless regenerative braking was engaged the vehicle would be coasting. These vehicles presumably have electric power steering and braking vacuum pumps.

On a similar tack could a car with auto stop/start be set up to stop the engine if the clutch is depressed for more than a few seconds, this would again assume electric PAS, vacuum and water pumps etc.

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 Post subject: Re: Coasting
PostPosted: Fri Oct 23, 2009 21:12 
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toltec wrote:
What happens with an electric or hybrid car when you come off the throttle? I would assume there would still be some losses due to bearing friction and air movement within the motor, however unless regenerative braking was engaged the vehicle would be coasting. These vehicles presumably have electric power steering and braking vacuum pumps.

I don't think there would be any coasting in a leccy car. There's no need to disengage the drive train because there wouldn't be anywhere near as much engine braking; internal combustion engines have a lot of internal friction, much much more than brushless leccy engines.

I would also guess the brake/steering servos (if used in leccy cars) would be driven from their own motors simply because the main drive motor likely won't be spinning when the vehicle is stationary (unlike today's combustion engines) otherwise a clutch and other linkages would be needed.

Some leccy cars have their drive motors integrated within the wheel hubs, so there's no chance of getting any mechanical feed from them

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 Post subject: Re: Coasting
PostPosted: Fri Oct 23, 2009 22:39 
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Steve wrote:
toltec wrote:
What happens with an electric or hybrid car when you come off the throttle? I would assume there would still be some losses due to bearing friction and air movement within the motor, however unless regenerative braking was engaged the vehicle would be coasting. These vehicles presumably have electric power steering and braking vacuum pumps.

I don't think there would be any coasting in a leccy car. There's no need to disengage the drive train because there wouldn't be anywhere near as much engine braking; internal combustion engines have a lot of internal friction, much much more than brushless leccy engines.


It depends on how you define coasting, with an electric motor not supplying much braking torque the the vehicle would feel like it was coasting, then again the motor is still linked to and spinning with the wheels. I was thinking it might make an EV feel rather odd to drive more than anything else. With no clutch your left foot is free to use the brake pedal so it would be easy to apply small amounts of braking for brief intervals without dodging your right foot between pedals.

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 Post subject: Re: Coasting
PostPosted: Sat Oct 24, 2009 00:30 
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The ones I've come across do, indeed, have a separate electric pump for the power steering and and electrically driven vacuum pump for the servo. Pretty much all of them have some degree of regenerative over-run (as opposed to regenerative braking) although one is a subset of the other. They don't slow down much when you lift off the throttle - about the same as lifting off in an ordinary car when you are one gear higher than you ought to be. The ones I've been involved with are pretty darned heavy, so they wouldn't slow down that quick anyway! It's hard to have much regenerative over-run because if you have too much it can become a problem when you lift off the throttle on an icy road! No chance of any left foot skulduggery as the power cuts out automatically as soon as the brake is touched - which is easy to do an a "leccy" car!


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 Post subject: Re: Coasting
PostPosted: Mon Nov 23, 2009 15:38 
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How many accidents are caused by 'coasting'?

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 Post subject: Re: Coasting
PostPosted: Wed Jan 20, 2010 14:19 
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samcro wrote:
When I was doing my driving lessons (about 8yrs ago) I remember well that my instructor frowned upon coasting (in neutral or just with the clutch down) as he said I didn’t have proper control of the car. More recenlty, whilst taking some motorbike lessons, the same theme came up: coasting means you don’t have proper control of your vehicle.


I’d be interested to explore the pros and cons of coasting, whether it really is dangerous or not, and whether you really don’t have proper control of the car/vehicle or not. Are there situations when it is aceptable to coast, and are there situations where it's dangerous?

I sometimes coast towards lights if I can see they are red. Does coasting stop me form having proper control of the car? I know I’m not going to be speeding up in the immediate future so don’t need the engine power, but I will be slowing down and the car has brakes to do this.

Thoughs and discussions welcome :)


Hi everyone,

Being a Driving Instructor this is one of the faults we have to pick up on quickly, the amount of times the learner turns into a side road to the left or right and doesn't bring the clutch pedal up causing the vehicle to go into the junction too fast narrowly missing the bollard or other vehicle coming out of the road is very frequent.

Two reasons for getting the learner to understand this is;

1). It does give more control as said earlier you make use of engine braking which can help your brakes to slow the car down. You would probably agree that when going to a lower gear you will find that the steering will feel more positive as well, which gives more overall control.

2). The second reason the examiner will pick up on this fault on your driving test, you will get driving faults each time you coast possibly leading to a serious fault which would be a fail or a dangerous fault(again fail).

What types of coasting would be acceptable then? Well if you are approaching a set of traffic lights or traffic that is queuing you would put your clutch pedal down when the gear you are in can no longer cope with the speed, but on the other hand if you are about 50 car lengths away this wouldn't be the correct way because then you should be dropping gears so as to gradually reduce your speed, we all coast a little when changing gears, approaching T junctions but that is acceptable.

Hope this might help keep discussion going.

...Roy

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 Post subject: Re: Coasting
PostPosted: Wed Jan 20, 2010 14:30 
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malcolmw wrote:
If you were not in gear, then it would take you longer to re-engage the drive train if you needed to accellerate suddenly. You will also have no engine braking available to control the speed of the vehicle.

The real question though is why would you want to coast?


Think in olden days the answer might come from the nickname -

"Aberdonian's Overdrive ".( Believe this was in early daysdone with engine off ,till introduction of servo brakes/power steering)

What I've done on long empty stretches of road is to display instantaneous MPG on my setup ,and see how coasting vs driving in gear with little throttle ,or as little as needed to keep a constant speed compares -and it's very favourably .Again I've tried this vs driving to the road ahead -obviously there are severe dips on inclines ,but allowing earlyfor the inclines makes a great difference .Perhaps something thecoasters might think about . :D

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