Updated 20th
August 1998 - Tyre size and circumference
tables addedThe question of tyres frequently appears on the email list. This FAQ addresses the majority of questions from new and prospective owners.
What does the tyre size actually mean?
When should I replace the tyre?
What sort of wear should I expect?
Where can I get the best value?
Do I have to use Bridgestone?
Why can't I use RE71?
Why can't I use SO-1 Expedia?
What is a flat spot?
How important are tyre pressures?
Can I fit different
sizes?
Where can I get a good deal?
Legal bits
Updated 20th
August 1998 - Tyre size and circumference
tables added
Updated 23rd March 1998 - Tyre dealer details added.
Almost without exception, all modern cars are today fitted with radial tread tyres and not the older cross ply tyres. This change over started in the mid 1960s and it is reasonable to say that TVRs prior to that date were fitted with cross ply tyres, and afterwards radials became the standard. For example Granturas were fitted with cross ply but the later 1800S and Vixens were fitted with radials. During this period there was a lot of interchange and therefore, as with most things TVR, it is possible to see both types fitted.
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?
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205 |
This is the width of the tyre in millimetres. |
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60 |
This is the aspect ratio of the tyre and 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. This measurement technique does mean that it is easy to fit a lower profile tyre with the same circumference providing the tyre width is increased. This is a technique that TVR have occasionally used.
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Z
|
This is a speed rating that defines the maximum speed that the tyre manufacture recommends. There are several designations for this:
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Please note that when choosing a tyre speed rating that it is common practice for cars with very good acceleration to choose a higher rated tyre than one chosen simply by the car's maximum speed. This is why the V6 powered S series cars use a V rated tyre despite the fact that their maximum speed is below this value.
|
R |
This stands for radial. |
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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.
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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.
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.
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Mail order can be a good way. The tyres are sent by courier and then they can be fitted locally.
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All current TVRs are 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.
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 AVS 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.
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.
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The reason is simple: they are no longer made!
There should be an E mark on the tyre as well to comply with a new European directive that comes into effect in June 1997. This requires that all tyres sold in the UK must have the passed the European tests and therefore carry this symbol. This is one of the reasons why the Bridgestone RE71 tyre has been replaced by the SO-2: the RE71 was not tested to the European standards although it passed the US and Japanese ones, which are just as stringent. As it was going to be replaced by the new SO-2 tyres, Bridgestone decided that it was not worth testing to the European standards.
Due to pressure from the manufacturers, the European directive has again been delayed and does not come into force until the end of 1997. As a result, it is still legal to sell RE-71 tyres and therefore if you look hard enough, you may still find some!
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You can! Please note that SO -1 tyres may also follow the same fate -- they are getting harder to find and may also be replaced by the SO -2 in due course.
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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.
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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.
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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.
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.
To help make a decision about replacement tyre sizes, the two tables contain the circumference for most tyre sizes used by TVRs. The Excel spreadheet that was used to calculate this can also be downloaded and allows a tyre circumference to be matched.
Tyre circumferences for 15 inch rims
Tyre circumferences for 16 inch rims
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These tyre dealers have repeatedly provided TVR owners with good
prices for tyres and can usually deliver mail order. If you are
looking for tyres, a good place to start would be with this list.
Tyre prices can change and vary so it is important to ring round!
Slough Tyres 01753 525488
Micheldelver Tyres 01962 774437
Feltham Tyres 0181 890 7138
Sheen Tyre Services 0181 876 8688
Stapletons 01296 415651
Elite (Reading) 0118 9504100
Charletts (Oxford) 01865 842727
Wedge Automotive 0114 281 7507
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