The physics 'text' book sounds rather woolly & imprecise from OPs comments - get a better text book!
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>>A physics text book that I have been shown
Would you be kind enough to post a reference for the book? I'll see if I can get my hands on a copy to read the article in context.
From the description given, it's not clear exactly what point the book is trying to make.
As an example, from the data available at the launch of the Calibra (I know!, I know! ancient history now), the 16v version had a higher Cd_A figure because it was fitted with wider wheels.
Number_Cruncher
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There is also the concept of rolling resistance, this is not drag due to air flow but the effort required to constantly flex the walls of the tyres as the wheel rolls along the road. Back in the 80s and 90s various cars promoted low rolling resistance tyres (usually higher profile) as a means of improving fuel efficiency.
StarGazer
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Low profile tyres often use larger wheels within the same outer tyre circumference as the standard ones. So they are unlikely to make any difference to air resistance. Anyway, in normal road use, any reduction, even if measurable, would have a negligible effect, even at 70 mph.
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Number_Cruncher
The book is Intermediate 1 Physics by Arthur Baillie and Andrew McCormick, published by Hodder Gibson.
(This book would be aimed at children of about 14.)
On page 117, in the section on 'Forces' there is a page on 'Streamlining'
It says the following:
In racing cars air friction can be reduced in a number of ways:
- a wing at the back to help produce a downward force
- thinner tyres, called low profile tyes, also help
Edited by tyro on 12/02/2008 at 18:50
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"thinner tyres, called low profile tyes, also help"
Aha, the devil is in the detail? Narrow tyres would not be conventionally known as low profile tyres!
Narrow tyres would decrease air resistance but by negligble amounts and at the expense of grip, cornering ability etc so would therefore complicate the example of streamlining.
I will leave the numbers and calculations to others but as with everything there is a compromise that altering 1 aspect of a vehicle will have another effect elsewhere on the same vehicle. The very stuff that F1 engineers (and others) spend hours trying to "cure"!
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It says the following: In racing cars air friction can be reduced in a number of ways: - a wing at the back to help produce a downward force - thinner tyres called low profile tyes also help
Quite frankly, if that's the best they can do then I really hope that this isn't a reflection on the general standard of text book used to educate our youth.
A wing at the back of the car will increase downforce, assuming its mounted the right way round, but it won't reduce air resistance.
Thinner tyres are thinner tyres, lower profile tyres are lower profile tyres, unless the small quote from teh textbook has been taken much out of context then O do hope that no taxpayers money has been spent buying such textbooks that are stating facts that just aren't fact.
Next, we'll be educating people that speed kills and the ill educated masses will believe it.
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On page 117, in the section on 'Forces' there is a page on 'Streamlining'
It says the following:
In racing cars air friction can be reduced in a number of ways:
- a wing at the back to help produce a downward force
- thinner tyres, called low profile tyes, also help
Oh dear!
I appreciate that you need to dumb things down for 14 year old (a bit like you have to dumb things down for physics undergraduates these days!) but, you shouldn't do this at the expense of the truth!
The problem with things like this in books is that most people tend to believe the thing they were told first, so this type of mis-information just leads to problems down the line. Many magazines and quasi-technical books about cars are, sadly, filled with similar myths and nonsense.
Making wheels thinner does help reduce drag - by at least two effects, so it's a stronger effect than you might think - firstly it entrains less air, but it also reduces the frontal area of the vehicle.
IIRC, the difference in tyre size between the 8v and 16v Calibra made a difference of about 0.02 to a drag coefficient which was about 0.28 or so (forgive my poor memory, it has been some time!)
Number_Cruncher
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So let's just make sure I have got this right.
What you are saying is that "Thinner tyres do reduce air friction - but thinner tyres are not the same as low profile tyres"? Or, to put it another way, the physics is correct, but the definition of 'low profile tyres' is not (as yorkiebar & spikeyhead say)
(The illustration in the book definitely shows low profile tyres, by the way.)
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What a waste of time. Please quote correctly so that we can dismiss passages from inaccurate & misleading sources - the book obviously cofused thinner ( i.e. narrower)
tyres with low profile tyres. Tch.
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Thanks all for your answers.
You may be interested to know that the physics teacher tells me that this is not the first time this text book has been wrong.
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>>not the first time this text book has been wrong.
The teacher can use this to teach the kids an invaluable lesson - sometimes, the text books are wrong!, and you always check another way.
I've found errors in the sorts of text books which many engineers consider to be beyond reproach*, and use formulae from without any further checking as a matter of routine - I've had to return safety critical calcs which I have been tasked with checking and approving because the engineer has used an incorrect formula from such a source without checking it.
Number_Cruncher
* Roark's "Formulae for stress and strain" (I think the newer editions correct many of the errors, but it's always best to check)
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On the Mondeo 225/45 tyres increase the CO2 grm/km from 154 to (IIRC) 159 and hence (if applicable) into a higher BiK bracket.
Though that is not because they are lower profile, it is mainly the increased rolling resistance caused by a large contact patch and. in a small part. increased drag caused by the the tyre being wider.
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>>increased rolling resistance caused by a large contact patch
How does this work?
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How does this work?
Its not straight forward NC because larger contact patch and same weight means less load for given area etc.
Sounds like your domain NC, go on NC you tell us?
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Well, the main thing is that bigger tyres doesn't equal more contact area - the contact area is given by the vertical force (weight) divided by the tyre pressure. So, if the tyre pressure remains the same, so does the contact area.
The contact area changes *shape* when you fit a wider tyre, but doesn't necessarily change its size.
Edited by Number_Cruncher on 13/02/2008 at 15:58
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Well the main thing is that bigger tyres doesn't equal more contact area - the contact area is given by the vertical force (weight) divided by the tyre pressure. So if the tyre pressure remains the same so does the contact area. The contact area changes *shape* when you fit a wider tyre but doesn't necessarily change its size.
So, if weight and tyre pressure are constant, then with a wider tyre, the contact area is the same so must be wider and also shorter front-to-rear. The weight is transferred to the contact area of the tyre by the sidewall. With a shorter front-to-rear contact area, the section of sidewall transmitting that weight is smaller.
Nothing controversial so far I think. Time to start speculating... Same weight transmitted through a smaller section of sidewall means greater stresses in the sidewall. Greater stress implies more energy lost hence higher rolling resistance?
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The weight is transferred to the contact area of the tyre by the sidewall.
To give you a clue as to why you're wrong, take the air out of the tyre and see how well the sidewall supports the car. After that, the rest of your argument loses its potency.
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The contact area changes *shape* when you fit a wider tyre but doesn't necessarily change its size.
Depends on the relative flexibility and elasticity of the two tyres, the thickness of the tyre wall and tread are, the tread pattern, too many variables, though everything being equal I guess you are right.
So a wider contact patch of the same area provides a greater rolling resistance? Why else would two identical cars but for the tyre profile and width have different CO2 grm/km figures? Cos it is surely not only down to the increased frontal area from an aero drag perspective.
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Presumably a wider tyre requires a wider wheel which is heavier than a narrower wheel. Also, decreased tyre profile implies a larger diameter wheel as well, so more weight. This would increase the fuel consumption and therefore the emissions.
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>>too many variables
If you now claim there are too many variables, your original point can no longer stand either - which way do you want it?
Here we are again, a point of dubious technical merit is raised - and the person who raises it does not explain either the source of the information or the technical logic behind it.
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Reminds me of a telephone exchange that was going to be extended by adding another floor on top! Fortunately someone spotted the error caused by one of the 'weaker sex' getting a decimal point two spaces out.
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Low profile tyres are usually on the wide side. Wider wheels have more drag than narrower ones. Sometimes the standard fuel consumption figures of different trim levels of the same car differ slightly for that reason alone. QED.
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Increase your alloy wheel circumference by 1" * and lose 1 MPG.
According to What Car.
*whilst keeping the wheel+tyre circumference the same.
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Low profile tyres are usually on the wide side. Wider wheels have more drag than narrower ones. Sometimes the standard fuel consumption figures of different trim levels of the same car differ slightly for that reason alone. QED.
Clearly the width has an effect though I would not have thought that 10mm on the tyre width would have a marked effect from aero alone.
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I would not have thought that 10mm onthe tyre width would have a marked effect from aero alone.
Perhaps there's a difference in rolling resistance too. I don't really know. You can make a measurable difference to fuel consumption on the road by keeping the tyres at their high-speed, full load pressures rather than the town comfort single-passenger squashy pressures.
Of course it makes the tyres wear better and makes the car handle better too, at the cost of more banging and rattling over the billiard-table surfaces of this wonderful town...
Edited by Lud on 13/02/2008 at 20:32
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