TYRE TECHNOLOGY Q & A
Wed, 01/20/2010 - 03:22 — admin
TYRE TECHNOLOGY Q & A
We have said it many times before and it still bears repeating that tyres are indeed the single most cost-effective way to change the entire character of one’s car. We take an in depth look at how and why it works.
1.What are tyres made of?
Contrary to popular belief that tyres are merely injection moulded items like a car’s plastic parts, they are a complex composite of rubber, fabric, nylon, steel, oil, carbon and silica. There are several types of rubber compounds used in any one tyre and though their exact composition is secret, they can be classified into three main types. Styrene-Butadiene Rubber (SBR), Butyl Rubber (BR) and Natural Rubber (NR). SBR and BR together with another synthetic rubber, Cis-Polybutadiene are used exclusively in the tread compound. Natural rubber is commonly used in the carcass to keep costs down as synthetic rubber is more expensive.
Apart from rubber, steel in the form of steel braids is usually used in the tread bed to provide a stable foot print for the contact patch. Use too much and the tyre’s weight and costs go up, use too little and the footprint deforms when stressed. Lately manufacturers have been resorting to high tensile steel with a smaller diameter. While costs go up the tyre’s footprint can be made more stable but yet lighter and more flexible. Some tyres have had the steel belts substituted by Kevlar for even lower weight and higher strength but this exotic material is expensive.
Nylon is wrapped over the steel belt to maintain the overall diameter during high speeds because nylon has the curious but useful ability to shrink or contract with heat. In the sidewall we find a composite of rubber impregnated fabric like rayon. The rayon strips are arranged in a radial fashion hence the term radial ply. They stretch over the tyre bead which has a circumfrential steel cord that holds it down to the bead seat. From the bead seat designers can stiffen the sidewall by introducing a stiff composite wedge called the filler bead.
How do I read what is on the sidewall?
1.215/45 R 17 87 W
205= Cross sectional width in mm
2.How does a tyre develop grip?
Tyres develop grip at two levels, the macroscopic level and the microscopic level. When a tyre compound is soft enough to “key” into the surface asperities of the road, the grip that is developed is termed macroscopic. The softer the compound the easier it keys into the road roughness the more grip it can provide. However the more it is able to do this, the more wear it experiences. It also increases rolling resistance.
Microscopic grip is the stickiness of the compound not unlike sticky tape occurring at atomic level. The high grip compounds tend to use this mechanism to aid the mechanical grip of keying into the road surface. Often after a drive, these high performance tyres pick up road gravel on the flat surfaces of its tread.
The compound’s tear resistance must also be higher than the forces developed by the grip mechanism otherwise the tyre leaves a lot of rubber behind and limits the grip available. Heat is the greatest culprit here where the surface of the rubber literally melts off, leaving streaks of melted rubber behind especially obvious when one locks up the wheels during braking.
Q= 160 km/h
*These are from the previous speed rating system. A VR tyre only needed to stay intact for a short period just over 210 km/h. A ZR tyre only needed to stay intact just over 240 km/h. Many tyre manufacturers exploit the confusion of the rating system and pass off their tyres as ZR, which under the new system only rates as W. The choice of tyre manufacturers to start their rating system near the end of the alphabet has caused problems because car manufacturers have now exceeded the 300 km/h mark.
It would seem that in Singapore with a speed limit of 90 km/h would only need tyres rated at Q. However the tyres with W, Y ratings come with significant technical enhancements that can be useful at speeds far below 300 km/h. The construction is usually state of the art and the quality control is second to none as these tyres have to keep stable, balances and intact at very high speeds. Their ability to withstand heat and produce less heat is also of a high order. Since these tyres will most likely to be fitted to high performance machines, the tread compound will be the high grip variety.
Dry grip depends heavily on the tread compound and its contact area. The ultimate dry tyre is the slick, full tread contact with a broad and stable footprint. As described before, the softer and the stickier the compound the higher the grip. However to put that to good use, the contact patch must remain stable and not buckle under the heavy loads like braking and cornering. A stiff contact patch is preferable but it takes away comfort so a compromise must be made. To soften the rubber compound, special oils act as plasticizers when mixed with the rubber. Please do not attempt this with engine oil as it damages the rubber.
Carbon Black has been used far a long time but recently the Silica based compound has taken prominence. Silica like Carbon Black is used as a filler. This filler effects the degree of plasticity and durability over a wide load and temperature range. It seems that with the Silica based compounds a good compromise between dry and wet handling with a long tyre life is attainable.
Carcass construction is very important as the design also must be able to distribute the force over the contact patch evenly, with the help of proper inflation pressure the tread compound can be pressed evenly onto the road surface. The latest carcass designs increase contact area as you brake hard and reduce it on the move to reduce rolling resistance.
Water has a deleterious effect on the tyre’s grip mechanism. It cuts the stickiness or microscopic grip mechanism to near zero levels. We are then only left with the macroscopic mechanism of the mechanical keying of the soft compound into the road surface asperity. Hard compounds, those usually chosen for long tyre life cannot key very well into surface asperity well and rides over the tops of the road asperity and have poor wet grip. Soft compounds provide far better wet grip but do have higher wear rates.
Computer aided tread pattern design helps with high-speed water drainage and this extends the useful speeds that a tyre may be used in wet conditions. Aquaplaning, a topic covered later, severely limits he abilities of any tyre at higher speeds regardless of how good the tread compound is.
Many use the terms interchangeably, which is wrong. Something with high grip can handle poorly and something with low grip can handle well. These two aspects seem almost diametrically opposed. Handling is not just due to the tyre whereas grip is very much dependent on the tyre. Handling is the balance developed between the interaction of the tyre with the car’s chassis. Good handling means the tyre behaves in a progressive fashion directly proportional to the driver’s inputs together with a high degree of predictability. Poor handling is the opposite with resultant responses from driver inputs being not what you want. For example you steer and the response lags, you steer more and the tyre slides out, you brake lightly and it responds well but you brake hard and it locks up, this is poor handling at its worst. However there are tyres with low grip but they respond proportionally and predictably to driver’s inputs and are deemed to have good handling.
Grip is the adhesion force measured at the very limit of the tyre's ability to keep the car on a chosen corner or ability to allow rapid retardation of the car when one applies the brakes. It does not define handling in any way. Handling is all about how it gets to the limits and back. How smooth, how progressively, how predictably and how informative determines handling.
The trick is to obtain high grip and retain good handling. This needs state of the art designs and compounds. Slapping on race-compounds onto a $50 tyre is not going to do. To get a good handling tyre, assuming one’s car has equally good handling characteristics boils down to the carcass design. There are a lot of trade secrets in the seemingly innocuous sidewall. We have now moved away from sheer rigidity in the sidewall, that’s old hat. The trick now is to build in flexibility but increase stability. This not only improves ride comfort it handles better as well. Manufacturers apply little tricks to improve turn-in response by allowing small deflections in the rear tyres to aid the front tyres. These nuances are a mark of a state of the art design and are kept secret for obvious reasons.
Only a handful of tyres can generate high grip and have excellent handling. With the grippier compounds come higher stress levels on the carcass. It must be stiff enough not to buckle under severe load but yet it must provide comfort and be able to roll over road irregularities to maintain constant contact. Make the carcass and tread too stiff and a mere pebble can lift the tyre from full contact. In addition the higher grip loads up the car’s suspension which then may compress further, roll more or flex more. All these aspects must be countered by stiffer springing and stiffer bushings. As you can see it is not a simple matter.
It was not too long ago that 26 psi was a normal inflation pressure to use. Today it is moving toward 30 and 32 psi. Inflation has a direct bearing on the tyres ability to carry a load. No pressure and you crush the tyre and destroy it. The sidewall is not usually strong enough to hold up the car’s weight. Only today’s run-flat tyres have such stiff sidewalls to the extent its doesn’t look or feel that the car has a flat tyre which creates its own set of problems.
Low inflation pressures(22-26 psi) increase comfort significantly, increase rolling resistance, increase heat build-up, reduce load carrying ability, reduce cornering stability and makes the steering heavy. Strangely low pressures unless too low do not effect braking too much and may produce slightly better wet grip and handling.
High inflation pressures(32 to 36 psi) decrease comfort considerably, reduces rolling resistance, lowers heat build up, increases load carrying capacity and lightens steering load. It also increases cornering stability and results in better handling as it compensates for poor sidewall design to some extent. However its ability to produce sharp responses in braking and cornering in the wet upsets many tyres. It is always better to be smooth in wet conditions to extract the maximum performance from tyres.
Inflation pressure changes with higher temperatures. The tyre pressure can increase several psi when it has been driven over a distance. Smaller tyres with a smaller internal volume have greater rises in inflation pressure for the same rise in temperature. Larger tyres have a lower rise, its simple physics in an enclosed volume.
Not necessarily. Bigger isn’t always better. Within limits a wider tyre will provide more grip but this is not analogous to improving handling. It is highly advisable to keep close to the original tyre height or rolling circumference while changing up in wheel and tyre size.
Wider tyres can improve braking performance significantly but depending on the car’s suspension, it may not improve cornering significantly and may even reduce handling. The wider the tyre the more critical is the suspension’s ability to keep geometry changes to a minimum. However the greater grip results in more forces on the suspension and this results in bigger geometry changes and counteracts any improvements in grip. Keeping changes reasonable is one way around this problem or a suspension upgrade becomes necessary. A plus one upgrade has little consequence but with a plus two upgrade the suspension must be looked into for suitability and for a plus three upgrade is inadvisable without a proper suspension upgrade.
By merely lowering a tyres profile does not necessarily improve performance. Everything being equal, it does not improve braking, may improve cornering, sharpens steering, becomes harder riding, and lowers load-carrying ability.
However with low profile tyres manufacturers take the opportunity to use better construction, higher grip compounds and ask for higher prices. So even within a single model that appears as 60, 55 and 50 series, construction and compounds used are going to be different. Typically the lower profile tyres have improved performance characteristics at a higher price.
At high speed wet conditions bring about another problem; aquaplaning, the phenomenon where the wedge of water in front of the rolling tyre cannot disperse sufficiently at a high speed. Water actually manages to lift the tyre away from the road surface. The ability of a tyre to resist this depends on its tread pattern design(and its width). For a long time the V-groove pattern represented excellent water dispersal ability but recently the state of the art tyres use deep circumfrential grooves. While having deep, wide and numerous grooves improve aquaplaning resistance, it cuts down contact area and creates narrower, less stable tread blocks. A compromise must be made and this problem gets worse with a worn tyre. Even before reaching the 3mm depth indicators, wet weather performance would have dropped off significantly as the grooves become shallower and more importantly the tread compound hardens with age.
Aquaplaning is a very dangerous problem as it occurs at high speed and since the tyres do not contact the road, all the car’s clever electronics are useless. You cannot do anything to save the situation except pray that you regain traction before you spin off the road.
13. Is long tyre life a good thing?
Beware the guy who boasts he has been using his tyres for the last 3 years and thinks he needs to see the carcass before he gets new ones. Rubber compounds have a use-by date, they harden with age thanks to the effects of leaching, heat, acid attack and ozone attack. How can one trust a tyre that has been exposed to the filth on our roads for over 3 years? Typically tyre performance especially wet performance falls off after 10,000 km. Dry performance fortunately is relatively stable over the life of a tyre. Tyre life is a good thing when they mean it lasts a longer distance but they do not mean that it should be used for a longer period of time. Unfortunately here we take it to mean more years where in Europe they take it to mean more miles. To wear down a tyre in a short period means you have a constantly fresh tyre surface throughout. To still have half the tread left after 3 years means the tread rubber has become so hard and shiny it has next to no wet grip. You won’t want to be in front of Mr. Loong Tread Life when it rains.
14. Does low noise mean low performance?
For a long while it did because manufacturers put aggressive(noisy) tread patterns on their performance tyres because they believed that helped to sell their tyres. Today they have moved away from aggressive designs and sales have not dropped. So while they look demure, they have low noise characteristics but possess high levels of performance.
Again today’s high performance tyres can be comfortable but this was certainly not the case with tyres of a few years vintage. Carcass and belt construction is the key as is low weight. The best tyres today do not offer race compound like grip but they offer a considerable jump in comfort to offset the mild increase in performance. As a bonus, handling has improved as a consequence of the better carcass designs.
Choosing bigger wheels and tyres is more a function of looks rather than sheer performance. One can get near-race tyre-like performance in a 14 inch size that would whip today’s 18 inch tyres. What concerns most upgraders is how wide and how low can they afford to upgrade to. Admittedly it is not cheap to go plus one or two sizes up.
First of all, the choice of an upgrade is usually driven by the attraction of smart looking rims, not good looking tyres. Today 17 inch is a common size and 18 inch getting increasingly popular. However there are limits as 18 inch wheels are not usually fitted to the typical Japanese car because the original tyre size is only slightly taller than the naked 18-inch rim. Mounting 35 or 40 series tyres is very difficult as there is no sidewall flex to help. Also with just a thin sliver of tyre to cushion road shocks is like going back to solid rubber tyres.
Follow the Plus One or Plus Two principle where the overall height or rolling circumference of the tyre does not change along with a change of rim size. Most tyre fitting outfits have an equivalency chart, which they can consult and advise you on your choice.
If you can afford it please pick a reputable brand and consult Torque’s tyre round up for the best tyre reference in town.
60 Series: Comfortable, usually OE tyres, performance compromised for good ride comfort.
Upgrading is certainly an enticing prospect but it is prudent to obey the laws of the land and more importantly the laws of Physics. Although you may be able to evade the long arm of the law but you may have to face the rather harsher punishment should you attempt to defy gravity. Performance tyres should be seen as adding a bigger margin of safety rather than to utilise all that extra performance. Remember increased grip does not translate into better handling. With more grip comes more speeds and bear that in mind because if anything untoward should happen the car will be skidding off at a far higher speed and with rather dire consequences. This is because the kinetic energy contained in the skidding car rises in proportion with the square of its velocity.
Oftentimes the levels of grip that can be provided with high performance tyres is beyond the original design limits of the car. Slapping on a set of 17 inch wheels with 45 series tyres does not make an Impreza an STi. The STi’s suspension is a carefully considered system with much finer tolerances and fewer geometry compromises since comfort is hardly in the equation unlike the stock Impreza’s. Ditto to all the Civics trying to be Type Rs. A proper suspension upgrade is advised if you want to use profiles lower than 50 series.
Also the mere fact you can fit a set of cool looking wheels and tyres does not make you a race driver. It will definitely stand you in good stead if you attend some of the advanced driving courses being conducted. It will teach you besides the requisite skills, some dedication and discipline. Do not tempt fate by racing on our streets, the life you save may be yours.