For years I have practiced freewheeling with any manual car I've owned. I always completely disengage the gear (i.e. not just hold in the clutch).
In times past, with carburettors, I can understand how this would keep consumption down, but on newer vehicles, with engine management, what exactly happens when
(a) you freewheel with engine idling
or
(b) you simply go onto trailing throttle. Does the management system / fuel injection actually shut down completely?
I say this because in a friend's car the computer showed zero consumption when he was on trailing throttle.
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The answer to b) is 'yes'
If you freewheel with the engine running the engine is fuelled just as it would be if you were stationary.
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Been there, done that.
www.honestjohn.co.uk/forum/post/index.htm?t=62298&...f
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Also :-
www.honestjohn.co.uk/forum/post/index.htm?v=e&t=64...6
Should keep you occupied for a few hours.
If you can slow down on a trailing throttle you will use no fuel until the engine gets to just over tickover speed & will give the best MPG.
If you want to travel the furthest on the least fuel then rolling in neutral with the engine ticking over will give the highest MPG.
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From:
www.iam.org.uk/pressroom/drivingtips/drivingtip18....m
"But never coast to save fuel; vehicle control must not be compromised."
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Passat diesel shows zero fuel consumption while coasting, pressing the clutch in makes it change to show a figure (usually somewhere between 50 - 200 mpg depending on speed).
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If you can slow down on a trailing throttle you will use no fuel until the engine gets to just over tickover speed ...........
That doesn't sound quite right. You're implying that for a time the engine isn't firing. If the engine is firing it must be using fuel.
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>> If you can slow down on a trailing throttle you will use no fuel
That doesn't sound quite right. You're implying that for a time the engine isn't firing. If the engine is firing it must be using fuel.
I think he might be refering to a diesel, L'Es, that will be the case... it is discussed further down the thread....
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The snag for oldies like me, who have freewheeled for years - I once managed it on a long hill in Utah for 11 miles - is that we choose gentle inclines where a car will happily settle at (say) 50mph in neutral. On a trailing throttle, especially with a diesel, the car will gradually slow towards engine-idle speed, which is not what is required. We must adapt to steeper slopes, I suppose.
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More to the point I can't think of any 11 mile hills locally - I don't find the slight slowing down any problems on the roads I drive normally, the hills are too short! - Even my cruise control does it (on trailing throttle that is, not coasting!).
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Back in the 60s, Rover produced many cars (90, 95, 110 series) with a free-wheel facility. Saab also did this with early 1970s 99s and 93s. I had the Saab version and could easily get 50mpg from a 1700cc engine (which was basically a Triumph Dolomite engine).
One advantage was when the clutch cylinder packed in - the free-wheel allowed you to change gear by simply backing off the accelerator. You had to start the engine in gear, but as soon as the road speed exceeded the revs, the freewheel cut in.
Unfortnuately Saab dropped the idea in 1972 when they released the rubber-bumper model with the 1800cc engine. (The Dolomite one was rubbish anyway!)
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>>Unfortnuately Saab dropped the idea in 1972 when they released the rubber-bumper model with the 1800cc engine. (The Dolomite one was rubbish anyway!)
Our 1977 Saab 96 with the V4 Transit engine had a freewheel, and rubber bumpers. And Kermit Green paint.
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Heh heh, my wife's cousin had a lime-green one like that.
That Ford V4 must have been the worst big-volume engine ever made. Bottom end lasted about ten minutes.
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The other colour choices weren't that great - purple, beige and orange are what I recall. Not that we had a choice, ours was a second hand bargain. Never let us down though, and the four-on-the-tree gearchange was very ergonomic. Brilliant in the snow.
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Pre war Rovers had freewheells as did my 1946 Rover 16, 1954 Rover 75 BUT my 1959 Rover 110 had overdrive and no freewheel.
I lived in Scotland near teh Highlands and anyone who used the freewheel down some of the hills with drum brakes (the 16 and 75) discovered what few modern motorists ever have experienced: brake fade and the smell and sight of brake linings cooking and red hot drums..
Never again!
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Ditto. I thought the freewheel was a brilliant idea until I encountered a 1 in 12 downhill. The sensation was not unlike the 'broken lift cables' ride at Universal Studios. The little t-handle was pulled out never to be pushed in again.
Edited by Manatee on 31/10/2008 at 20:15
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Tee hee, fading drum brakes could be quite amusing, yes...
But six-cylinder Rovers had Rolls-Royce style overhead inlet, side exhaust valve gear, which was smooth but not especially frugal. Intelligent use of the freewheel - note the word intelligent - could make the whole exercise a bit more economical and even smoother. WB of Motor Sport liked the feature. A lot of fifties Rover drivers used it. But the roads were emptier then. The mimser is the curse of fluid economical rapid driving.
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Drum brakes could be regarded as an early form of ABS. If you overcooked the brakes on a long downhill at least there was little chance of a lock up understeer moment. I reckon they taught you just how fast cars will actually go round corners if required......
Edited by Humph Backbridge on 31/10/2008 at 20:34
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Or how what looked like someone's drive exiting on a sharp bend could suddenly become an escape road...
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>Or how what looked like someone's drive exiting on a sharp bend could suddenly become
>an escape road...
I seem to remember that some A-roads with long steep declines used to have runoff areas ending in sandtraps to stop runaway vehicles.
Do they still exist?
Kevin...
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>>I seem to remember that some A-roads with long steep declines used to have runoff areas ending in sandtraps to stop runaway vehicles
Now dignified with the name 'arrester bed' - there's one on the link from the A41 to Hemel Hempstead, amongst others.
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There are quite a few in the Peak District, especially on the A6 between Taddington and Ashford in the Water.
My brakes failed on the Winnats Pass which seems to be about 1 in 1 when you drive down it. I was in a lwb Nissan Patrol and the gearbox didn't seem to help.
I was panicking and pumping at the same time. Fortunalely I got some bite and pulled off to investigate. It had completely thrown a front pad and I stopped with piston against disc. I had some used pads in the toolbox, put one in and continued my journey.
Quite a fright tho' !
Ted
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There's one on the steep descent into Dover from the A2, isn't there?
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There's one on the steep descent into Dover from the A2 isn't there?
Yes and on the Driffield to Scarborough road where it drops an escarpment to join the A64.
Long descents on French motorways seem to have a "Voie de detresse" at regular intervals. They also have signs in four languages advocating the use of engine braking. In German simply "Motorbremsen" while the Italian version needs a sign of it's own!!!
Edited by Bromptonaut on 01/11/2008 at 14:10
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hi - on the A69 approaching Carlisle there's an emergency run-off in case you've lost your brakes and i'm sure i've seen one or two others in t' north. The run-off is basically a sand pit which should slow you down effectively, although the've cleverly left a very substantial tree in place at the end of the run-off. So, if the sand doesn't do the job, the tree certainly will.
Following from coasting etc, on modern engines the fuel is cut off when the accelerator is released (hence 999.9 mpg on my corsa diesel), as per other contributor fuel is required to keep engine turning at tickover (ie when coasting) so coasting has no value at all and can comprimise safety anyway - note contributor from iam.org. Cheers
Snipquote.
Edited by Dynamic Dave on 14/04/2009 at 13:38
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Following from coasting etc on modern engines the fuel is cut off when the accelerator is released (hence 999.9 mpg on my corsa diesel) as per other contributor fuel is required to keep engine turning at tickover (ie when coasting) so coasting has no value
Dont forget that mpg is "miles per gallon". You make the assumption that the same distance will be travelled when coasting as you would in gear with the engine on tickover. You will travel a greater distance with the car in neutral.
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Yes. We have one on the A259 going into Eastbourne coming down from Beachy Head.
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And there's a run-off on the outskirts of Penryn on the A39 into Falmouth.
Edited by Optimist on 14/04/2009 at 16:16
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Of course drum brakes were contemporary with crossply tyres. Crossplys had less ultimate grip but it broke away much more progressively and controllably. A scenario in which overheated brakes caused a higher than desired entry speed into a corner at the bottom of a hill could often be relatively easily controlled albeit with some fairly lurid lateral sliding and sawing at the steering wheel. My Wolseley Hornet was equipped with both features and on an odd occasion........
The Keystone cops come to mind if a visual reference is useful.....
;-)
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There are some nice gravel traps on the Buxton to Sheffield road.
None as long or as needed as the downhill from Pietermaritzburg to Durban road in SA: a 1,000 metre descent in about 5 miles...iiirc.
P4 Rovers rolled a lot sorry a LOT... so cornering was safe but uncomfortable with leather bench seats and no seat belts.
Mind you those wide front seats and column mounted gearchanges on the early P4s made car passion much easier and the front seat slid back a long way. Trouble is , leather could get cold quite quickly...but then I carried a tartan car rug for such emergencies and to avoid ruining the leather..
Oh happy days: -)
Yes P4s were not very economical but then I can recall as a student my shock when pertrol rose from 2/3 to 2/6 per gallon... (or about 12p) . Oh even happier days...:-)
The Golden Age of Motoring is dead.
Edited by madf on 01/11/2008 at 07:41
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I recall many year a go a driver being prosecuted for free wheeling down Bird Lip Hill he was followed by a police and they notices the slightly smokey chuffing exhaust ( tick over Austin Maxi) and excessive use of brakes. This resulted in a accident when he lost control of the car left the road and hit a wall, from memory a convent ( 1969/70) I believe is it illegal to freewheel unless the option is specific to a cars spec, like the old Sarb 93/6. Thing about it if you freewheel and use your brakes the servo runs out of vacuum and at tick over the vacuum will not fully replenish. This is a dangeroues practise. Regards Peter
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>>the servo runs out of vacuum and at tick over the vacuum will not fully replenish.
Sorry Peter, there's plenty of vacuum at tick over.
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Trailing throttle??? Que?
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In gear, clutch engaged, foot off accelerator.
When rolling downhill you use no fuel (in a diesel, not sure re petrol)), everything that comes off the engine works (servos, power steering, etc) and by placing your foot back on the accelerator you have instant response....
Edited by b308 on 02/11/2008 at 09:40
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In gear clutch engaged foot off accelerator. When rolling downhill you use no fuel (in a diesel not sure re petrol)) everything that comes off the engine works (servos power steering etc) and by placing your foot back on the accelerator you have instant response....
Thanks for clearing that up!
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>There's plenty of vacuum at tick over.<
That was the problem with the (very) old vacuum-operated wipers, which were fitted to American cars at least until the 60s - they just didn't work when accelerating, as there was no vacuum. A serious problem when overtaking in heavy rain ...
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Not just to American cars, unfortunately. Ask any former owner of a Ford Pop/Prefect/Anglia about the comedy arrangements.
I had a two-stroke Wartburg with a lockable freewheel but I rarely locked it because it was easy to get used to. And, of course, there was no need to use the clutch once you were moving. Mind you the Hillman Imp ordinary manual gearbox was like that - shame manufacturers seem to have lost the knack...
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When I was driving through Norway in 2001 in my Bora 2.0 petrol, I freewheeled more or less all the way down a (gently sloping) mountain road and saw my average consumption go from about 32 to nearly 60. Very very satisfying......
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