Spacers - Polo Gti

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  • Re: Spacers - Polo Gti

    Postby steeldonkey » Fri Jun 18, 2010 10:05 pm

    Just thought that i would post up what my Pirelli pal posted on the forum we moderate.

    Liquid wrote:As you all know, tyres come in many shapes and sizes. Even though each and every tyre has to adhere to strict construction regulations regarding their dimensions and the materials used, there are various takes on how the tyre will be finally constructed by the manufacturers involved which will be suited to what they feel is the best balance of performance over a strict and sizeable selection criteria for the application in question.

    Tyres of yesteryear were high-profile in terms of their sidewalls. It wasn't uncommon to see cars rolling around on 70 or even 80 series tyres. Some modern cars still have sizeable sidewall profiles, particularly those with smaller wheel diameters like the Ford Ka etc.
    Obviously, it's not uncommon for performance cars from about 20 or so years ago to have had tyre sections as low as 40 series which was a big deal back then. These days, this has reached as low as a 15 series tyre which Kumho managed with their KU31's.
    The construction of 'low-profile' tyres (typically 45 series and below) is different to that of higher profile tyres due to the physical area of tyre that has to cope with the weight of the vehicle and the forces that it will generate. Deflection (the movement of the tyre when in use) is far greater in a low profile tyre.

    Imagine bending a straw. If you hold it at both ends and gently bend it, it will bow in the middle as the whole length of the straw absorbs the force. Now, try that but with your fingers about 2" apart. The straw will invariably fold instead of bend. The principle is the same in a tyre.

    This can be called many things, but lets call it a hinge point for clarity. If you were to do the same banding trick with something smaller and tougher; say, a credit card, it takes more force to get it to bend as it's more resilient. However - if it's overflexed - it will create a hing point (as shown by the area turning white when the plastic stretches) which will weaken over time when the same force is applied.
    This is what happens to a tyre to cope with the forces that will act upon them but it has to be strong enough to cope with those forces, yet remain pliable enough to deform over surfaces to give you the ride quality you expect. That's before you get into any other areas of performance.

    As an example, here's two photos of tyre sections that show the difference between higher and lower profile tyres. It's worth remembering how the sidewall profile is calculated. It will be the percentage of the nominal section width which is the distance between the sidewalls - not the tread area. So a 245/40R17 will have a nominal section width of 245mm and the sidewall 'height' will be 40% of this figure. As a result, not all 40 series tyres will have the same measurement.

    This is a higher profile tyre (60 series) followed by a lower profile tyre (40 series).

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    Here's a close up to show you the difference in construction between the two with regards to their sidewalls.

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    The principle difference here is the size of the bead core - the bundle of steel cords that are responsible for holding the tyres on to the rim. In the lower profile tyre, the bead core is larger in order to cope with the increased stresses exerted through the lower sidewall profile.
    In addition, the chafer (the material cords that run around the bead core) has two runs in the lower profile example providing twice the strength in order to help maintain the shape of the sidewall and reduce flexing.

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    The rubber (filler) in this area of the sidewall needs ot be pretty tough for the reasons outlined above. As such, this will limit the tyres flexibility which anyone who has switched from smaller to larger wheels will attest to. As mentioned with the credit card analogy earlier, this area has its limits.

    Ok, so what happens when the tyres are stretched? Here's the 60 series tyre.

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    Because the sidewall is of a different construction, it actually bends quite easily with the minimum of effort. The sidewalls are thinner with regards to the filler involved as well as having a more flexible chafer.

    As a contrast, here's the 40 profile tyre.

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    Because of the stiffer construction, it takes much more effort to force the sidewalls into this unnatural position. In addition to this, look how the tread area distorts when it's flexed this way. The tread becomes concave as the stiffness of the sidewalls helps to pull the material in when they are over-extended.

    The inside of the tyre is lined with a layer of butyl. This is incorporated solely for maintaining the air pressure within the tyre given it's flexible nature and tighter molecular construction. As with any rubber, it will have its limits. When over exerted, it can become porous and even tear if the tyre has a sufficient enough impact through force or an acute enough angle. This will of course lead to air leaking from the main chamber of the tyre into the tyre structure. This will normally manifest itself as a bulge in the sidewall. Something that won't be that apparent if the sidewall is stretched .

    When this happens, the load carrying capacity of the chamber of the tyre will be diminished meaning it won't be able to support the weight of the car effectively - essentially running in a deflated condition. Running tyres in this state will cause them to overheat. As we all know, air expands when it gets hot which results in a build up in pressure. If unchecked, this will get to a point where the pressure will build up and act upon the weakest spot it can find - like the bulge that has formed in the sidewall. It's then a matter of time before you get a blow-out.

    The above is a typical example of what is probably the most common type of tyre damage and is not an designed for scaremongering. You run a much higher risk of this scenario happening with stretched tyres compared to tyres that are correctly sized for your wheels. Naturally, tyre maintenance and driving style will impact massively on the life of the tyres but as mentioned, no two tyres are really the same. There's no certainty in saying the tyres you've stretched onto your rims have the sufficient strength to cope with being stretched - letalone be put into use after. It's easy to say that you don't drive fast or corner hard with these tyres fitted so it won't affect you. You alone know how you will drive with these tyres fitted.

    Some manufacturers (Yokohama springs to mind) are specifically manufacturing tyres to be stretched in order to cater for/cash-in on the stretching craze. If you wish to go down this road, then I would urge you to consider these sorts of tyres to minimise any risk of failure/issue.
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    steeldonkey
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