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  • FIRST POST
    • 50Twuncle
    • By 50Twuncle 30th Sep 17, 4:50 PM
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    50Twuncle
    MOT exempt
    • #1
    • 30th Sep 17, 4:50 PM
    MOT exempt 30th Sep 17 at 4:50 PM
    Why on earth has the government decided to exempt all 40+ year old cars, from MOT tests
    Surely - it is these old bangers that are likely to have dodgy brakes and are going to cause accidents
    And what about pollution ?
    These old carburettored cars - are going to emit heaps of CO and other nasties (particularly pre 78 diesels) - we have all seen old ex-taxi mercs emitting smoke
    What a stupid idea
Page 3
    • jack_pott
    • By jack_pott 9th Oct 17, 12:27 AM
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    jack_pott
    Funny thing, that, back-of-an-envelope figures suggest that the pressure on the road surface from a typical bike tyre is actually a little higher than the pressure from a typical car tyre.

    Say 90kg bike + rider, equally split between two tyres with a contact area of 28 square cm each gives 1.6kg per sq cm.

    1500kg car + driver split between 4 tyres with a contact area of 310 sq cm each gives 1.2 kg per sq cm.

    So, yes, per contact area, the bike will be producing higher wear
    Originally posted by Joe Horner
    Erm, the damage to the road is determined by the fourth power of the weight, not the pressure.

    http://pedalfortcollins.com/greatest-demand-on-tax-dollars/
    Last edited by jack_pott; 09-10-2017 at 11:35 AM.
    I'm not upset that you lied to me, I'm upset that from now on I can't believe you.
    Friedrich Nietzsche
    • Joe Horner
    • By Joe Horner 9th Oct 17, 12:44 AM
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    Joe Horner
    Erm, the damage to the road is determined by the fourth power of the weight, not the pressure.

    Originally posted by jack_pott
    Erm, no. It's the weight per unit area (otherwise known as the pressure). Big wright spread over big area doesn't matter.

    Remember that the weight of air is about 1kg on every square cm of ground. A Yaris sized patch of road, 3.95 x 1.7 metres, has an area of 67150 square cm, and is supporting 67 tonnes of air. Big weight but no damage!
    • jack_pott
    • By jack_pott 9th Oct 17, 1:31 AM
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    jack_pott
    Erm, no. It's the weight per unit area (otherwise known as the pressure). Big wright spread over big area doesn't matter.

    Remember that the weight of air is about 1kg on every square cm of ground. A Yaris sized patch of road, 3.95 x 1.7 metres, has an area of 67150 square cm, and is supporting 67 tonnes of air. Big weight but no damage!
    Originally posted by Joe Horner
    From the American Association of State Highway and Transportation Officials:

    the damage caused by a particular load is roughly related to the load by a power of four (see table 1)

    Road damage is calculated in ESALs (equivalent single axle loads), not from the tyre pressure.

    Atmospheric pressure is irrelevant. It's static, and doesn't cause the road to flex as passing vehicles do.
    I'm not upset that you lied to me, I'm upset that from now on I can't believe you.
    Friedrich Nietzsche
    • Joe Horner
    • By Joe Horner 9th Oct 17, 7:11 AM
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    Joe Horner
    All good, except if you notice I wasn't talking about tyre pressure, I was talking about the pressure exerted by the tyre against the road. That is the same as the load per unit area. Static or dynamic, that's what causes damage - which is why big baloon tyres do less damge off road than normal onez. They spread the load over a larger area.

    if you also notice my reply to Car54 (before you "corrected" me) I was also being slightly tongue in cheek!
    • Gloomendoom
    • By Gloomendoom 9th Oct 17, 9:01 AM
    • 13,264 Posts
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    Gloomendoom
    Erm, the damage to the road is determined by the fourth power of the weight, not the pressure.

    Originally posted by jack_pott
    Just a hunch, but I think you need to update your Photobucket account to enable 3rd party hosting.
    Advice; it rhymes with mice. Advise; it rhymes with wise.
    • Herzlos
    • By Herzlos 9th Oct 17, 9:55 AM
    • 6,092 Posts
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    Herzlos
    Most won't, but I can personally think of 3 mid-seventies vehicles (Capri, Cortina, VW camper) which have been parked up for years since failing their tests due to corrosion. All of them start and drive and I would be amazed if all 3 weren't back on the road as soon as they are MOT exempt with nothing more than a fresh tank of petrol, a wash and a jump start to "recommission them".
    Originally posted by ilikewatch
    It'll still be illegal to drive a car that isn't roadworthy, though. There just won't be an annual MOT for them.

    There aren't that many pre-77 cars on the road currently, and those that do are unlikely to cover anything more than negligible mileage.
    • Herzlos
    • By Herzlos 9th Oct 17, 9:56 AM
    • 6,092 Posts
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    Herzlos
    why should an old banger that puts out unlimited amounts of CO and pollutants, get a waiver from tax -
    Originally posted by 50Twuncle
    Because (a) most of the emissions are produced on production and (b) emissions from driving are negligible when you only do 100 miles a year. It's completely dwarfed by even the most efficient modern car doing 10,000 miles a year.
    • jack_pott
    • By jack_pott 9th Oct 17, 11:10 AM
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    jack_pott
    Just a hunch, but I think you need to update your Photobucket account to enable 3rd party hosting.
    Originally posted by Gloomendoom
    I've no idea what that means, when I followed the link it looked as if I was expected to open a subscription account. (The link was working fine for several hours after it was posted.)
    Last edited by jack_pott; 09-10-2017 at 11:36 AM.
    I'm not upset that you lied to me, I'm upset that from now on I can't believe you.
    Friedrich Nietzsche
    • jack_pott
    • By jack_pott 9th Oct 17, 11:34 AM
    • 4,295 Posts
    • 5,518 Thanks
    jack_pott
    All good, except if you notice I wasn't talking about tyre pressure, I was talking about the pressure exerted by the tyre against the road. That is the same as the load per unit area. Static or dynamic, that's what causes damage - which is why big baloon tyres do less damge off road than normal onez. They spread the load over a larger area.

    if you also notice my reply to Car54 (before you "corrected" me) I was also being slightly tongue in cheek!
    Originally posted by Joe Horner
    You have a misconception of how vehicles damage roads. It's not like a stiletto heel on a soft wooden floor, it's by fatigue fractures caused by the repeated flexure of the road. A stiletto damages a floor because the compression strength of the heel is greater than the shear strength of the wood, so the wood shears at the periphery of the heel and you're left with a depression.

    On a road, the compression strength of a tyre is much lower than the shear strength of the road, so that shear force distributes the pressure from the wheel over a much wider area than just the area of contact.

    Think about a diver standing on the end of a springboard, the board doesn't flex any further if the diver increases the pressure by standing on tiptoe, but it will flex further if a heavier diver uses it. The board spreads the load sideways via the shear strength of the steel.

    Balloon tyres off road are another example like the stilettos, as the mud and snow are soft surfaces, unlike tarmac.

    The pressure exerted by the tyre on the road is the same as the pressure in the tyre, it can't be anything other. Old Ike, he say: "Every action has an equal and opposite reaction", so if there's 28psi of air pushing the rubber onto the road, then there has to be 28 psi from the road pushing the rubber against the air.
    I'm not upset that you lied to me, I'm upset that from now on I can't believe you.
    Friedrich Nietzsche
    • bigadaj
    • By bigadaj 9th Oct 17, 12:40 PM
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    bigadaj
    You have a misconception of how vehicles damage roads. It's not like a stiletto heel on a soft wooden floor, it's by fatigue fractures caused by the repeated flexure of the road. A stiletto damages a floor because the compression strength of the heel is greater than the shear strength of the wood, so the wood shears at the periphery of the heel and you're left with a depression.

    On a road, the compression strength of a tyre is much lower than the shear strength of the road, so that shear force distributes the pressure from the wheel over a much wider area than just the area of contact.

    Think about a diver standing on the end of a springboard, the board doesn't flex any further if the diver increases the pressure by standing on tiptoe, but it will flex further if a heavier diver uses it. The board spreads the load sideways via the shear strength of the steel.

    Balloon tyres off road are another example like the stilettos, as the mud and snow are soft surfaces, unlike tarmac.

    The pressure exerted by the tyre on the road is the same as the pressure in the tyre, it can't be anything other. Old Ike, he say: "Every action has an equal and opposite reaction", so if there's 28psi of air pushing the rubber onto the road, then there has to be 28 psi from the road pushing the rubber against the air.
    Originally posted by jack_pott
    Yes but the damage to the road often isn't to the tarmac, or more correctly asphalt nowadays, Tarmac is not only a trademark but most material produced as such is now classed as a contaminated material.

    It has some relationship to the base course but in many instances is a function of the sub base and formation materials on which the wearing course is laid.

    Any design will derive from full or equivalent cbr testing of the in situ materials and or the road building materials as they are emplaced, together with a best guess of the equivalent axle loads that will be incumbent on that surface for its design life. There is very little useful theory behind the road design process, if you look at the equivalent axle load derivation in the highways brown book for example it's a rough approximation of the impact of different lorry weights with different axles and load characteristics, for design purposes car loading let alone cyclists are largely irrelevant.

    Also teh concept if shear strength isn't really apt in this instance, we don't have a sudden failure in mist cases but suffer from serviceability limit issues. These are derived from slow rutting of materials, Tarmac or asphalt is an odd material in that it acts like a slightly fluid solid and so may be subject to creep over time. Continued loading of a specific area means this creep is exacerbated and concentrated. So rather than teh shear strength of the material were more interested in the relevant modulus, potentially shear or secant, as well as the poissons ratio to a lesser extent.
    • Manxman in exile
    • By Manxman in exile 9th Oct 17, 2:33 PM
    • 1,048 Posts
    • 728 Thanks
    Manxman in exile
    Erm, the damage to the road is determined by the fourth power of the weight, not the pressure.

    http://pedalfortcollins.com/greatest-demand-on-tax-dollars/
    Originally posted by jack_pott

    I'm familiar with second power equating to area (eg pressure) and third power equating to volume (eg weight, mass x g) but what is the fourth power? Are we talking time?


    I have looked at your link but it's not clear to me(?) where the fourth power comes from.
    • bigadaj
    • By bigadaj 9th Oct 17, 2:47 PM
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    bigadaj
    I'm familiar with second power equating to area (eg pressure) and third power equating to volume (eg weight, mass x g) but what is the fourth power? Are we talking time?


    I have looked at your link but it's not clear to me(?) where the fourth power comes from.
    Originally posted by Manxman in exile
    It's effectively time, and actually cyclical loading, so a non constant which is attempted to be replicated using the California bearing ratio test, with a result in percentage of a calibrated base load, in practice a single sized gravel sample loaded and subjected to the same number of cycles.
    • Richard53
    • By Richard53 9th Oct 17, 8:10 PM
    • 2,576 Posts
    • 2,192 Thanks
    Richard53
    I'm familiar with second power equating to area (eg pressure) and third power equating to volume (eg weight, mass x g) but what is the fourth power? Are we talking time?


    I have looked at your link but it's not clear to me(?) where the fourth power comes from.
    Originally posted by Manxman in exile
    You're confusing powers with dimensions. Yes, the fourth dimension is time, but that isn't what is being discussed.


    The power of 2 (squared) is A x A
    The power of 3 (cubed) is A x A x A
    The power of four is A x A x A x A
    An hour alone spells freedom to the slave.
    • Robisere
    • By Robisere 9th Oct 17, 10:16 PM
    • 1,931 Posts
    • 2,685 Thanks
    Robisere
    Don't know what most of the preceding has to do with the original question, but talking about 40 yo cars reminds me of the many cars I have owned. In the 70's and 80's I owned Capris, Cortinas and 2 Granadas. Each one was an excellent drive, with the virtue of being fairly simple for the ordinary motorist to service and maintain. That was because there was a limited number of parts and components which the ordinary motorist could not maintain and service, given a Haynes or workshop manual and enough of the relevant tools. Anything more complicated or heavy-duty, was brought to garages like the one in which I was workshop foreman.

    Many of these 40 yo - and older - cars are now rebuilt, renewed and kept running by enthusiasts, and they require specialist insurance, which usually insures their OTR miles per year. Most Classic owners only use them in the warmer months anyway, storing them in the dry during winters. They are treated as valued possessions, washed, waxed heavily and continually checked for signs of wear and corrosion. There is a corresponding decrease in the annual amount of emissions, with of course an associated decrease in pollution.

    These cars are totally divorced from the current wave of "Euro Boxes" which are stuffed with electronic gizmos and therefore exotic materials, which require dedicated Reclamation facilities to deal with them. Sadly, I drive such a vehicle, but really wish I had the time and money to buy and afford to run, a Classic.
    There may be more than one way to skin a cat.
    But the result is always inedible.

    • iltisman
    • By iltisman 9th Oct 17, 11:20 PM
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    iltisman
    I sold my classic Mini to a lady in her 20s I had to explain what the choke did and when to use it. I doubt there is going to be a rush to buy old vehicles with their strange controls and requirement for constant maintenance .
    It's only my cheerfulness that keeps me going
    • oldagetraveller
    • By oldagetraveller 10th Oct 17, 11:37 AM
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    oldagetraveller
    I sold my classic Mini to a lady in her 20s I had to explain what the choke did and when to use it. I doubt there is going to be a rush to buy old vehicles with their strange controls and requirement for constant maintenance .
    Originally posted by iltisman
    "Constant maintenance" no, just routine servicing at the specified time/mileage.
    Granted that is less than a more modern car but at least it is possible to maintain/service properly on much older cars. e.g. an abundance of grease nipples to prolong the life of moving parts. There are even oil lubrication points on my classic's dynamo bearing and an oiling nipple onthe steering rack. Therefore less need of throwing components in the bin when they wear out (sealed for life).
    A lot of components are also repairable or may be swapped out on a reconditioned exchange basis.
    I would suggest that my 55 year old classic is in a far better mechanical and roadworthy condition than a lot of cars on the road.
    Last edited by oldagetraveller; 10-10-2017 at 11:39 AM.
    Did you hear about the politician who had a backside transplant? It rejected him.
    • Shoshannah
    • By Shoshannah 17th Oct 17, 7:43 AM
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    Shoshannah
    http://www.bbc.co.uk/news/resources/idt-sh/how_toxic_is_your_car_exhaust
    • glentoran99
    • By glentoran99 17th Oct 17, 7:46 AM
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    glentoran99
    This also means that road tax will be free as well !!
    What about I buy an old Jaguar - fit a new engine and gearbox and then slowly replace all of the body panels - suspension components, brakes, exhaust etc etc - when does the pre-78 car become new - \and subject to MOT and Road tax ?
    Originally posted by 50Twuncle


    https://www.youtube.com/watch?v=BUl6PooveJE
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