Tyres. All you need to know
Tyres are one of the most important parts of the suspension on your Alfa Romeo; they are the only part of your car in contact with the road and as such the amount of grip you have is entirely dependent on them. The tyre is also a major factor in how comfortable your car is and how well it handles.
All car tyres are a compromise of a number of factors for example as the sidewall of a tire decreases in size, it becomes a lower profile and you lose the flex of the tyre wall and so the car will turn in better but the ride will be firmer. While if you use a softer compound it will grip better but will wear out quicker. All of these things are taken into account by the manufacturer and your choice of tyre is also dependent on what you require your car to do.
There is also a lot of information on the sidewall of the tyre the most important of which is the size and specification markings, as an example 205/50 ZR 16 87:
- 205 is the tyre width in mm,
- 50 is the aspect ratio, in this case, the height of the sidewall is 50% of the width of the tyre ie 102.5mm,
- Z is the speed rating of 150 mph,
- R means it is a radial tyre,
- 16 rim diameter in inches,
- 87 load capacity rating the maximum load permissible load at maximum pressure which is 545 Kg on this tyre.
Further, around the tyre, there is the Department of Transport mark, DOT FU TA H76x3501, the last 4 digits of this give you the date of manufacture of the tyre which is the 35th week of 2001.
On most tyres, there will also be tread wear indicator marks around the outer edge of the tyre at intervals near the tread and if you look at the tread in these positions you will see small raised bars in the base of the tread. When these are level with the surface of the tread you have reached the wear limit for that tyre.
The Law on tyre wear and damage is:
- You must have 1.6mm of tread over 75% of the tyre's width in one continuous band and the cords in the tyre must not be exposed
- You must not have any cuts in the tyre which go to the carcass or cords
- You must not have any bulges in the tyre.
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Over the last five years or so you may have noticed that some manufacturers are claiming that their tyres can save you fuel by lowering the rolling resistance of the tyre, this is done by adding silica. You may think that adding something to a tyre to reduce its rolling resistance would also reduce the amount of grip it had and this used to be the case with adding more carbon black but by adding silica this can be avoided.
The grip is affected by the degree to which a tyre is distorted at high frequencies - in other words, the degree to which it hits small stones and unevenness in the road surface and can adapt to them. Grip is best served by rubber compounds that absorb high levels of energy and can adapt to these small imperfections in the road. Rolling resistance, on the other hand, is affected by low-frequency distortions - the deflection of the tyre as the area in contact with the ground changes as the wheel rotates. This requires compounds that absorb low quantities of energy. This contrast is why it has been impossible in the past to provide tyres that both reduce rolling resistance and increase wet grip.
With the addition of silica, however, tyre engineers have been able to produce compounds that are high hysteresis at high frequencies but low hysteresis at low frequencies. The use of silica can result in a reduction in rolling resistance of 20% and more. Assuming correct tyre pressures are maintained and making allowances for varying speeds and different driving characteristics, a 20% reduction in rolling resistance equates to a 4% fuel saving, which, according to Michelin in their promotion of the Michelin Energy, can save the average motorist £65.00 a year. The use of silica can also improve wet skid performance. By incorporating silica in their winter tyre range, Vredestein claims to have improved wet skid performance by as much as 15%, substantially improving braking distances at the same time. Silica also provides substantial benefits in winter tyres and all-season tyres. Compounds using silica are more elastic and flexible at lower temperatures allowing better grip and braking during wintry weather. In the UK we generally only use summer tyres and there is no legal requirement to change your tyres in winter however in much of Europe it is necessary to use a winter tyre, however, as a visitor to any country in the EU you don't by law have to change your tyres to use them over there as long as you can manage to make satisfactory progress which effectively means that you have a set of snow chains handy to get yourself out of trouble. Snow tyres tend to contain more natural rubber which does not tend to freeze at extremely low temperatures and the tread blocks have lots of tiny grooves in them which help to grip the snow.
On newer Alfa Romeo's (post-2000) there is another issue with tyres which has come to our attention recently which is difficult for people to get their heads around. This is when they fit one or two new tyres the ABS (anti-lock brake) or VDC (vehicle dynamic control) warning light comes on. This is caused by the new tyre having a different diameter to the old one and while it rarely comes on with a tyre change it will defiantly come on if you change the aspect ratio of one tyre. The ABS and VDC systems work by counting the number of signals that they receive in a certain period of time and if one is going faster or slower than the others by more than a prescribed amount then is signals that up as a failure of the system and turns the system off fitting the wrong size tyre causes the wheel to go round faster if it is smaller and slower if it is bigger and so triggers the fault light on the system.