Mavic Engineers Talk: Road Tire Performance
1. The function of a road tire
Although it may look rather unassuming on your bike, a tire has six main functions that it must perform in order to offer a high level of functionality to the rider. It must:
– Roll well (providing an efficient and consistent ride).
– Absorb shocks (the road surface isn’t always good).
– Support the load of the rider and bike.
– Steer and change direction (following the trajectory given by the rider).
– Transfer pedalling and braking power, especially when it is raining.
– Be durable.
During the development phase of a tire it is important to continually check throughout the process that each of these functions are met. They may differ between tires, depending on what function is of most importance to the intended final use of the tire itself. For example a tire designed for time-trialing or triathlon may have a lower rolling resistance (as speed is a high priority) but this will come with an offset in lower adhesion so would not be as versatile as a tire designed to perform well across all conditions.
2. Tire materials
A tire is made up of several different materials, each of which are carefully selected for their specific properties and performance attributes. The aim is to make all of these components work together through the right mixing, manufacturing and assembly process. The different materials that need to be considered are:
– Carbon black
It is very important to carefully develop the optimum mix of materials as each play a fundamental part, not only in how the tire will finally perform on the road but also how it actually fits to the rim as well.
3. Road tire construction
There are four main components that make a road tire, each of which have a specific job whilst at the same time they must complement one another in order to raise the overall level of performance of the tire.
This is the part of the tire that is visible on the sidewalls, and is an assembly of nylon fibre and rubber (to hold it together). Each casing is classified by its TPI (Threads Per Inch) which is in relation to the density of fibre and rubber used. Typically you may see anything from 30TPI to over 200TPI.
Beads are an assembly of Kevlar® fibre with a very high tensile strength. They help the tire hold its form and are the critical link that holds the tire firmly in place on the rim when it is inflated.
Different materials can be used for this layer with the main objective being to improve the puncture resistance of the tire. The material can vary in thickness and density but it’s important to find the right balance of each. Kevlar® is a very good choice to increase puncture protection but it also significantly decreases rolling resistance. For this reason a macromolecule polyamide material may be better suited to maintain a low rolling resistance.
This is the most important parameter and defines the adhesion (grip), rolling resistance and durability of the tire. It is also the part where a lot of development hours are spent since it is important to find the exact right compound that is able to deliver the best possible combination of each of these three elements.
A single tire cannot fulfill every type of riding style. It is for this reason that we develop specific tire constructions with variances on the rubber formula, casing TPI, breaker layer, size and weight depending on the intended application from racing all the way through to every day cycling.
It also goes one step further since a front tire will require a different level of performance (higher grip) compared to a rear tire (lower rolling resistance) so Mavic engineers develop each individually to ensure that the performance when used together is to a higher overall level.
4. What attributes define performance?
Now that we have identified the basic construction of a tire we’re going to look at how certain elements can be tuned to provide specific performance characteristics.
Low rolling resistance:
Increasing the TPI is a good way to reduce rolling resistance since more threads per inch means less casing thickness. Every 1/10mm that can be saved between the road and the tire casing will result in a lower rolling resistance. It’s for this very reason that it is also important to reduce the tread thickness and puncture protection layer. Another way to improve rolling resistance is by developing the tread compound to give more rebound to the rubber. Tread pattern also helps to optimise the contact patch with the ground.
Grip (in the wet) and traction:
In general a more supple casing, that’s one with a higher TPI, will result in a tire with a better ‘feel’ for the rider but it doesn’t significantly improve grip. The rubber compound is the most important factor here but this in itself works directly against rolling resistance since the rebound of the rubber needs to be reduced. It is important to fully assess the elastomeric nature of the rubber, using a combination of carbon black and silica to reinforce it. A higher air volume is also beneficial at improving grip. Increasing the contact patch on the road (for example a 25mm tire compared to a 23mm tire). If designed correctly it is also possible to make the tread pattern more flexible which in turn gives the rider a better feeling at the grip limit.
A higher air volume is always a good way to improve comfort. More rubber on the tread compound helps but only at the detriment of rolling resistance. Softer rubber on the tread can reduce the feeling of vibration, especially when the road surface is of poor quality, but will reduce durability. Finally, lowering air pressure can increase comfort but not so much that the rider may incur pinch-punctures. As you can see it’s about finding a happy medium.
The best way to increase the tire’s life is to add more rubber. It sounds simple, but in doing this it is also important to consider the tread shape which could take on a square-edged profile after some use. When adding more rubber the casing (sidewalls) and beads (securing point on to the rim) must also be reinforced. The result may offer better durability but the tire will feel slower due to a higher weight and rolling resistance.
This is achieved through two different ways:
– The rubber thickness on the tread (providing more material for the object responsible for the puncture to penetrate in order to reach the inner tube).
– The use of strong fabrics in the breaker layer which can be used on the tread or casing sidewall to reduce the chance of cuts.
Whether rubber thickness or fabrics are used, the negative impact will be on the rolling resistance since the overall stiffness of the tire will increase.
5. Laboratory measurements and field testing
The laboratory offers a controlled environment where engineers can specifically analyse the characteristics, properties and performance of each material both on their own and when constructed to form a complete tire.
In this instance Mavic’s rubber laboratory looks at:
– Mechanical properties (including Young’s modulus of elasticity, resistance to stretching and fatigue testing).
– Shore durometer test to analyse hardness.
– Dynamic Mechanical Analysis (DMA) to determine the viscoelastic behavior of the material when a sinusoidal stress is added.
– Rebound rate.
– Durability subject to ultra violet and ozone exposure.
Field testing is focused towards extreme conditions whereby multiple tires can be tested back-to-back to gain direct rider feedback in a real world, yet controlled, environment. For this Mavic engineers have established a protocol to gain more subjective and qualitative data to go alongside the quantitive data from the lab.
In both instances, further details on our laboratory and field testing is explained in our previous article where we talk about assessing the performance of a road tire.
6. What about tubeless?
Theoretically tubeless tires have a lower rolling resistance as there is no lose of power between the tube and the tire. They can also be run at lower pressures for improved comfort and traction without the risk of a pinch-puncture, and no inner tube could also mean lighter. So what’s the catch? In reality the benefits are offset as more rubber is required on the casing in order to keep it airtight. This extra material increases the overall weight (by about the weight of an inner tube) as well as the rolling resistance making a stiffer, less supple, tire. They’re generally regarded as more difficult to set-up and more difficult to repair out on the road should the rider puncture.
It is not possible to have everything in one tire – low rolling resistance with the best levels of adhesion, comfort, durability and puncture protection. The objective is to balance each of these attributes to provide a range of tires that are fit for their intended purpose and that fully satisfy the need of the rider, from road racer to every day enthusiast. We are all unique, riding different disciplines, with varying road surfaces and in different temperature and weather conditions. Identifying your needs as a cyclist is the first step in deciding which type of tire may be best for you.
Jérémy Queffelec (Tire Product manager)