More on electric cars.

[John Williamson]

Look closely at the part of a tyre which is in contact with the ground, and you will see that due to the way the rolling radius varies along the flat part, there must always be a small amount of slippage between the tyre and the ground which may or may not be absorbed by the elasticity of the tyre material. This applies to all wheel/ ground interactions where the wheel and ground are not perfectly rigid.

Incidentally, in the 19th Century, experiments were done which gave the hauling capacity of a horse at walking speed on a level surface as (roughly) 4 tons on a metal tyred, thin wheeled cart on a good hard road surface, 7 tons on a railway waggon and 10 tons on a narrowboat. The road figures may have changed slightly when the pneumatic tyre was invented and came into use.

[The Natural Philosopher]

seems a lot. I built a model plane that would move 24" span at 70mp on only 100W...

Mind you, that was edge on..

[The Natural Philosopher]

No, it doesn't mean that at all.

which may or may not be absorbed by the elasticity

No, it doesnt.

What counts with tyres is deformation, not slip

a lot. about 10 times as much rolling resistance for a tyre on concaret

[<me9]

Eh? about 20%. Coal and nuclear are usually a higher percentage.

See which has been discussed here before.

[The Natural Philosopher]

Gas used to...but gas got expensive

coal is running far more of the baseload, and RWEs converted-to-imported-biofuel coal plants (see 'other' )

Gas is now only being sold into peak demand periods, or as hot standby for when the wind drops again..and again..and again...

If you look at the time graphs gas has lost out to coal over the last year.

Nuclear is actually increasing slightly - EDF have spent money doing preventative maintenance on the old AGRs and they are running at better capacity factor now.

Almost makes up fort the last but one Magnox that's closed..

[Grimly Curmudgeon]

They are. Rubber squirming under load put on and taken off and even in the contact patch there is movement.

[Grimly Curmudgeon]

TNP has a fizziks degree; he knows better than everyone who doesn't.

[The Natural Philosopher]

No I have an engineering degree, which means I talk about whats important, not what might theoretically be one ten thousandth of the observed and meausured result.

The dominant rolling resistance of a tyred car is down to deformation and hysteresis. That is not normally called 'friction'. Nor is what 'friction' the tyre generates due to pico slippage in any way more than such an infinitesimal fraction that it makes any sense to talk about it unless of course you are in danger of looking a prat and losing an argument.

End of.

[Dave Plowman (News)]

And deformation of the tyre generates friction...

[The Natural Philosopher]

No it doesn't. Not in the accepted sense of that word, because deformation of a perfectly elastic medium is possible without energy loss at all.

E,.g a coil spring does not get very hot ....

[Dave Plowman (News)]

Which tyres ain't.

[Andy Champ]

Nope. Getting planing is all about the wave systems. Once you are planing things change; 10 tonnes being pushed off a jetty isn't planing. Once planing the hull shape is also pretty important - try tying a bucket to an outboard leg, and see how well a motor boat planes. And think how small a proportion of the wetted area that is.

OK, so just how much does the air compress around a car at 70MPH?

The compressed air in the tyres isn't relevant to the air drag.

Andy

[Bill]

Well yes, the Omegas and Carltons were all petrol 2.2 or 2.0 litre. The current thing is a 1.9tdi Skoda, which is still fairly long, so I suppose petrol may be better at higher speeds. Be interesting to compare notes with a petrol Octavia.

The crucial thing for me was that a change in lifestyle brought the need to do innumerable short trips with a lot of waiting time. The Omega on almost permanent full choke did between 6 to 9 mpg, compared with its

40mpg at 70 on the motorway. The Octavia on a good day with all the lights in its favour has done 70mpg across Wales and back at a moderate speed and does about 40mpg on the stop wait start cycle.

I take issue with the motoring writers who constantly say that you have to do a huge mileage for a diesel to be sensible. Of course if the turbo falls to bits into the engine, I might change my mind.

[The Natural Philosopher]

depends. Maybe a few PSI.

Ask McLaren.

Or an aircraft designer. Enough to create a pressure difference big enough to keep a plane up.

[Dave Plowman (News)]

Sounds like poor design. My 3.5 Rover auto - '85 - does 15 mpg on short trips, like down to the shops in winter. Even an old 4.2 Jag I had on carburettors with an auto choke did better than your Omega.

[Dave Plowman (News)]

Planes have wings specifically designed to produce lift. Only the most stupid card designer would use the same effect.

[Grimly Curmudgeon]

Unless he was an ace at it.

[geoff]

In message , "Dave Plowman (News)" writes

Well, aksh

(turn thenm upside down and ...)

[The Natural Philosopher]

I am beginning to wonder about you Dave.

You aren't Drivel under a new name are you?

All of formula 1 is about aerodynamics - using air pressure to produce lift downwards..

Air pressure changes are inevitable in any body moving through air. When these result in turbulence and vortices these become where the energy goes that causes the actual profile drag.

Skin friction is also an affect, but at car sizes and speeds is not a huge component.

What dominates low speed drag is a more or less constant rolling resistance due to tyre deformation: What dominates higher speeds is air resistance associated with pressure changes and vortex formation round the bodywork

[Dave Plowman (News)]

;-)