Slick tire (Slick tyre)
Slick is the name given to racing tires that have completely smooth tread. The smooth tread allows for a larger contact area between the tire and the road, thus giving the tire more grip.
The first mass-produced slick tires were introduced in the early 1950s. M&H Tires began producing them for motorsport, specifically dragster racing. Today, they are a common part of virtually all motor racing.
The characteristic feature of slick tires is their completely smooth tread. A slick tire has no tread pattern, so it is completely smooth on the surface (slick). The lack of tread maximises the contact area between the tire and the road, thus ensuring maximum traction for the vehicle. At the same time, these tires are usually made of a special softer compound that further increases the tire’s grip on the road.
Slick tires must not be used in normal traffic! This is due to the risk of aquaplaning when driving in wet conditions. The tread of a slick tire does not have any grooves to channel water away from the track underneath the tire, thus helping to prevent aquaplaning. Another problem with wet track is the rapid drop in temperature below the optimum usable range of the tire, see slick tire temperature.
You will therefore only encounter slick tires in motor racing or karting. A special case is the semi-slick tire, which can also be used in normal traffic.
- slick tires have a larger contact area between the road and the tire, therefore achieving greater grip (adhesion)
- the contact patch is continuous, so there is no bending of the individual tread blocks, resulting in improved tire performance
- have a stiffer sidewall construction for better transmission of lateral forces when cornering
- the tire performs under high stress even at high temperatures
- low noise levels due to the absence of tread
- not possible in wet and rainy conditions
- shorter life due to the desire to maximise traction and the way they are used, mileage of slick tires can be less than 200 kilometres
- to properly exploit the potential of slick tires it is necessary to reach the operating temperature of the tires, below this temperature the tires are virtually non-functional
- the lack of tread makes them sensitive to uneven surfaces
Slick tire construction:
The main design difference between slick tires and conventional tires is the absence of tread. Thanks to the completely smooth tread without cuts and sipes, the tire lies on the road with the maximum possible surface area. Since the tread of a slick is not made up of individual blocks of rubber, but is actually one large continuous block, the tire is better able to transmit large lateral and longitudinal forces. There is no bending of the tread blocks. All of this results in improved handling and traction.
Another design improvement that provides increased lateral stiffness to the slick is the reinforced sidewall. The stiffer tire deforms less under lateral forces. This ensures greater stability, better cornering behaviour and faster steering response.
A very important aspect for the correct use of slicks is the choice of the correct compound hardness, which is directly influenced by temperature. For example, tire manufacturer Michelin offers three hardnesses, graded according to road temperature. Dry slick tires are offered in three different compounds: S7 – Soft, S8 – Medium, S9 – Hard. The different rubber compounds guarantee a wide range of applications from sprinting to endurance racing. The wet compound P2 is designed for the wet track.
Slick tire temperature:
From the above compound table it is clear that the performance of slick tires is highly dependent on the temperature of the tire itself. A warm slick tire has a temperature of around 80 – 100 °C. Conventional tires do not even reach such temperatures and would lose their properties at these temperatures. Slick tires need to be kept within their temperature window during the race, below the optimum temperature range (so called “cold”) they have very little grip. An unheated slick tire has significantly less grip even compared to a normal sports tire.
Keeping the tires in the optimum temperature range so that they are sufficiently warm can only really be done in hard track racing. In normal traffic, the optimum temperature window is virtually impossible to achieve. That’s why slick tires don’t make sense as an alternative on the circuits even in dry weather, not to mention the fact that they are banned for normal road use.
In order to allow the racer to make the most of the tires immediately after the start, so-called tire heating blanket are used. These tire warmers warm up the tire until the last moment before the start. Ideally, the temperature should be the same across the entire width of the tire.
Tire slick cycling/lifetime:
Tire temperature is also related to tire cycling. This is the process by which a tire is heated to its optimum temperature window to achieve maximum grip. The tire then cools down. During this cycling, chemical processes occur and actually harden the compound gradually. How many cycles a tire can last before it gets too hard depends on a number of factors such as: length of warm-up, vehicle weight, driving style, etc.
Pressure in slick tires:
Even contact between the tread and the road is important for proper functioning and tire life. It is important to remember that the size and shape of the tire footprint is primarily influenced by the correct tire pressure. Slick tires have a higher operating temperature than conventional tires, for this reason slicks are inflated to a lower initial pressure (i.e. cold). As the tire warms up during the race, the tire pressure rises to the ideal final value. Correct and stable pressure value = ideal tire shape in the cut.
The idea of filling tires with compressed nitrogen instead of the usual air also comes from the race tracks. Nitrogen, which is a significant component of normal air (78%), is more stable under pressure than air alone and has less thermal expansion. The so-called dry nitrogen in the tire mitigates pressure fluctuations due to thermal expansion of the gases. This is why nitrogen-filled tires are used in racing, where even small pressure changes can affect the handling of high-performance race cars moving at the limit of drivability. The advisability of using nitrogen in conventional or tuning cars is questionable to say the least.
Slick tire wear indicator:
Since slicks have no tread, the depth of the main tread grooves cannot be measured. Yet, logically, tire wear does occur. Circular holes located on the tread surface are used to indicate the tread thickness.
The limits of grip of slick tires:
Slick tires can push the driving limits of a vehicle noticeably further, there can be no argument about that. However, one has to take into account that vehicle breakaway occurs at higher speeds and suddenly. Maximum slick traction is achieved with tire slip of around seven to ten per cent. If the driver does not relax, the slip exceeds the threshold beyond which the adhesion drops sharply. This characteristic of slick tires requires considerable skill and experience on the part of the driver.
Are slick tires really faster?
Of course they are. To give you a better idea of where slick tires can take you, German magazine Sport Auto has published a test. At the Hockenheimring, a sports car with slick tires completed a lap in 1:14 minutes. The same car with stock tires was approximately +4 sec (1:18) and with semi-slick tires approximately +2 sec (1:16).