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| TVR FAQ - Tyres
The question of tyres frequently comes up and this FAQ provides the answers. Each of the bibles covers this topic in more detail. What does the tyre size actually mean?
Deciphering a tyre size When you go and buy a new tyre, you have to state the size, such as 205/60ZR15, but what do all these numbers mean? 205 The width of the tyre in millimetres. 60 The aspect ratio of the tyre, which allows the height to be calculated. By taking this number as a percentage and multiplying it by the tyre width, the height of the tyre can be determined. With the 205/60/ZR15 tyre, this means that the tyre height is 123 mm (60% x 205). This measurement technique means that it is easy to fit a lower profile tyre with the same circumference — providing the tyre width is increased. TVR have occasionally used this technique on the Griffith and Chimaera, e.g. when the switch to the SO-2 tyre took place. Z The speed rating that defines the maximum speed recommended by the tyre manufacturer. There are several designations for this:
When choosing a tyre speed rating, it is common practice for cars with very good acceleration to choose a higher rated tyre than one simply defined by the car’s maximum speed. (This is why the V6 powered S series cars use a V rated tyre, despite their maximum speed being below this value.) The Griffith and Chimaera all use Z rated tyres and should not be fitted with anything less. R This stands for radial. 15 The wheel diameter in inches. When should I replace the tyre? While tyre performance is important, it is all too easy to forget that bad tyre maintenance or simply wear and tear can quickly degrade adhesion, often with catastrophic results. The most important tyre maintenance tool is your pair of eyes! Too often tyres are left alone and not inspected until there is a sudden experience of a lack of grip or the sight of a bald patch on the tyre. The most important thing to check is the tread depth. This should be at least 2 mm although the legal minimum is now 1.6 mm. Many company fleet car managers insists on tyre changes at 2.5 or 3 mm because of the rapid deterioration in braking distances and grip that low tread gives. Waiting until the tyre is down to the minimum legal depth can be a false economy. The adhesion is greatly reduced and therefore there is an increased chance of losing grip and bumping into something. The problem is that it can be difficult to gauge how much tread is present. Bridgestones, along with many other makes have little bars in the tread that indicate when the tyre is on this minimum depth. While these are fine, there is no indication of the actual depth they represent and how accurate it is. The best thing is to invest in a tread gauge. Mine lives in the glove compartment, ready for use. With low mileage cars, there is also the danger of the tyre wall degrading before the tread wears away. The first signs are usually small thin cracks in the tyre wall. If these are present, the tyre should be replaced irrespective the tread depth. What sort of wear should I expect? It all depends... Tyre wear is dependent on driving style, the power of the car and the type of road surface. With large V8 powered TVRs, rear tyres can be worn down in as little as 4-5000 miles road use or they can last for 10,000 miles or more! However, if they are taken on track days... Without doubt, track days are probably the most fun you can legally have with your TVR except for maybe sprinting or racing... They do have an interesting effect on tyres however. The first thing is that it is recommended to increase the tyre pressures by about 10 to 15% before going out on the track. Remember to lower them afterwards. This should be done when the tyres are cold. This helps keep the tyre beads in place and helps reduce tyre wear. The next problem is that expect to loose some tread especially with the V8 engine cars and especially with the 5 litre Griffs and Chimaeras. It is also highly dependent of driving style as well. With the rear tyres, 1 to 2 mm is quite common although the front nearside (left) tyre will often suffer similar or greater wear as it takes the brunt of cornering. The tread will often be sloughed off the tyre and create a bead that goes around the circumference of the tyre. The tyres can also get so hot that they pick up the rubber debris that covers the track -- the dreaded marbles -- and this can weld blobs of rubber onto the tyre and upset the balance. These need to be removed by carefully cutting away.
Inside edge tyre wear due to spritited driving on a track day! One final warning about track day tyre wear. If you car has 3 mm of tread at the start, and looses 2 mm during the day, the tyres will be illegal for the journey back home. Secondly, the ever reducing tread will degrade the adhesion as you are getting more confident and faster in driving the car. There is therefore an increased risk that the car will loose grip and spin so take care. If the car is driven hard, the tyre will overheat and the tyre will "go off" as they say during motor racing commentaries. The end result is a tyre with less grip and the car will feel wrong as it slides. It will often understeer and can cause an interesting moment or two! The moral is: if the tread is getting low, don't push it. Do I have to use Bridgestone? This gets complicated... The Griffiths and Chimaeras were all fitted with Bridgestone tyres and many users continue to fit them when the originals wear out. However, it is tempting to fit other tyres especially when lower prices are involved. Several of the TVRs that I have owned have been fitted with different makes and I have found myself coming to the conclusion that Bridgestones are probably the most suitable tyres for most uses, especially if you drive one of the modern cars like a Griffith or Chimaera. To complicate matters, all current TVRs (Cerbera, Tuscan, Tamora) are fitted with Toyos and it is tempting to use these. Owners should be careful in that Toyo do not have a ZR rated 15 inch tyre and they have a softer sidewall compared to the Bridestone S02 or S03 tyres. This is one of the reasons that the S03 has gained an unjustafiably bad reputation. It has a stiffer sidewall which when fitted to larger wheels that need even lower profiles results in very hard handling. I have heard many comments from dealers and owners alike that Bridgestones are the best choice for Griffiths and Chimaeras. I have treated this with a little scepticism. However, having owned and driven a Griff 500 hard on track days and expecting a car that was impossible to catch when the back stepped out, I have been more than impressed with Bridgestone tyres, in the way that they progressively slide. As a result, I've found myself also saying when asked, that they are really the only choice for these cars. For other cars, Yokohama are a good alternative. I know of several Wedge owners who prefer them to Bridgestones and claim that the wear better. I had them on my S2 and found them excellent tyres although the tyre dealer reckoned that they were so soft that I would only get 4000 miles from them! Yes they were soft, but boy did they grip! With hindsight, the only real issue I had with them was that when they did break away it was very sudden! For me personally and I stress that this is my own opinion, I think Yokohama are probably the only alternative to Bridgestone I would consider for these cars. However, this does not mean that other makes are not good tyres and should not be considered. I know some owners are happy with Toytos and others that have had bad experiences because of their softer sidewall. In the end, the choice of tyre is dependent on several factors and not just price. Driving style and technique can dramatically affect tyre wear and performance. Track days coupled with aggressive driving - power slides out of corners instead of using the steering wheel are extremely fun but boy, do they wear out the tyres - can reduce a tyre's life by 75%. Here, tyre performance is probably the most important and the tyre wear and higher cost is just part of the price to pay. For a car that is used to cruise the open countryside can expect a far longer tyre life and the tyre performance is not so important. Here cost or tyre longevity are probably the main concerns. A good tyre dealer will often ask what car the tyres are for and what type of driving do you do. Ask fellow owners for their experiences, before making a decision. What is a flat spot? If the brakes are locked or the car is involved in a bad skid, then it is possible to flat spot the tyres where the tread is worn in one place only and thus creates an area of the tyre that is flattened instead of round. In bad cases, this can make the wheels feel like three penny bits (older owners) or 50p coins (newer owners). The symptoms are excessive vibration: with bad cases this can occur at 30 to 40 mph, with mild flattening at 80 mph and so on. The only cure is to get the wheels balanced &emdash; don’t forget to make sure that there are no marbles stuck to the tread. This can often cure or greatly reduce the problem. In very bad cases, replacement is the only real way of removing the vibration. How important are tyre pressures? Very! The tyre pressures should be checked regularly -- the owner's handbook will have the correct values. They should be the same on both sides. Pressure differences of only a pound can make the steering pull to one side in some cases. The camber of the road can also do this by the way! If your car spends much of its time cocooned for the winter or simply in storage, it is worthwhile putting the car on stands to get the wheels off the ground so that they do not deform and thus create flat spots. Alternatively, inflating them to about 35 lbs pressure can also help prevent this problem. Finally, simply rolling the car a few inches on a regular basis will also help. Can I fit different sizes? In most cases, the original tyre sizes are still available but in some, either TVR have changed the size or a manufacturer no longer supplies that size. As a result, there may be need to fit an alternative tyre. The question what will fit and will the speedometer need re-calibrating? As a rule of thumb, reducing the aspect ratio by 5% and increasing the tyre width by 20 mm will give you the same size circumference tyre to within a few mm. The percentage difference is small and is typically around 0.5 percent. However, before changing the size there are several things to bear in mind: The first is that this could be construed as modifying the car and this must be cleared with the insurance company first. The second item is to ensure that the wider tyre will fit and not foul anything, such as the bodywork, brake pipes or suspension. There is a spread sheet and a set of tables that can be downloaded - click here to access. Front tyres need to be checked at the limits of the steering as well. Fitting wider tyres can change the handling due to the reduced lateral stability -- the tyre wall can flex and give more so that on cornering there is more sideways movement. On the other hand, straight line acceleration should be better because of the increased grip but the rolling resistance will be higher and so fuel economy may be decreased. Fitting wider tyres to the front can put greater strain on the steering components and cause premature wearing and a heavier feel. Again, if you can find someone who has already done it, find out the implications before proceeding. And chcek and double chcek that you have the clearance that you need.
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