Bontrager unveils an all-new downhill tire developed by legendary tire designer Frank Stacy and the athletes of Trek World Racing, dubbed the G5. Proven by Trek World Racing’s Aaron Gwin throughout the 2012 World Cup calendar, the G5 blends Stacy’s knowledge of tread and compound design with the performance demands of the two-time-defending World Cup champion. Built specifically to maintain consistent, predictable traction at top speeds over the gnarliest of conditions, the G5 works best in loose terrain, regardless of whether its bone dry or pouring rain. Riders will instantly notice the new tire’s highly predictable nature when getting sideways through rough corners.
“Bontrager has invested an incredible amount of R and D in tire design in the last year and the G5 is a prime example of the huge payoff we're seeing from it
,” said Frank Stacy. “Collaborating with Trek World Racing this year gave us the feedback of world-class athletes who can constantly push the tires to their limit, and are always searching for that little bit more.
The G5 got its first win at the Fort William World Cup this year, when Gwin needed predictable cornering traction for the course’s notoriously varied terrain. After just one practice run on the new tread, Gwin kept the G5 prototype on his rear wheel for his race run and rode it all the way to the top step of the podium for his first win at the legendary World Cup track. He continued the G5 momentum, riding them on both the front and rear to wins at Mont Saint Anne and Windham.
|It's not often that a company allows you to design a product exactly the way you want it. Bontrager gave me that opportunity with the new G5 tire and it turned out to be the best tire I have ever ridden. - Aaron Gwin|
We'll be reviewing Bontrager's new G5 downhill tire shortly, but in the meantime we sat down with the man behind the G5, tire engineer Frank Stacy, to pick his brain on tire design and rubber compound. What is the idea behind manufacturing the same tire out of different compounds?
Technically, two tires aren’t the “same” if they do not share compound. Other variables include casing material, bead material, as well as the rubber compound. When we develop a tire, we study its intended use and then determine the “spec” based on that intended use. A great example is the Bontrager XR4. The 26 x 2.2 Expert looks a lot like the 26 x 2.2 Team Issue, but they ride like two completely different tires. That’s because they are. We often see tires employ compound designations such as "50A", among others. What do these numbers actually mean, and how much of a difference is there between them?
Using 50A compound as an example: the “50” is the durometer, or rubber hardness. The higher the number, the harder the material. The “A” stands for A-Shore. Shore is a measuring scale developed by Albert F Shore, and is used by almost all tire manufacturers across multiple industries (bicycle, motorcycle, auto, etc.). There are twelve different scales, with A being used for softer rubbers like tires. Contrast that with D, which is used for things like skateboard wheels. The numbers in this system can be deceiving because, due to the nature of all tire manufacturing, a tire labeled a given durometer can vary up to +/- 3 durometer points based on the manufacturer’s formula. Formulating a tire compound involves carefully selecting the right proportions of four main ingredients. These are:
• Polymer. This is the rubber, but there are different types of rubber: natural, SBR, EPDM rubber... or even a mixture or all three.
• Oils. There are several types, but the type and percentage of oil depends on the tire’s intended use. Oil also has an effect on tread life and traction.
• Carbon Black. This is what makes the rubber black but it also adds strength, wear resistance and has an effect on traction. White carbon is used for coloured tires and does not have near the wear resistance of carbon black.
• Fillers. These are used to prevent cracking and increase ease of manufacturing.
How much effect will a tire's compound have on performance relative to tread design, casing, width, etc?
Rubber compound plays a key part in a tire’s overall performance, but it's only part of the recipe. It's a lot like making a cake... if you don’t get the proportions right, it won’t taste very good. That’s why it’s so difficult to judge a tire by one variable (tread design, weight, durometer), and one reason why the expertise embedded in every Bontrager tire results in a consistently incredible experience. How does rubber rebound speed relate to how soft or hard the compound is?
Again, this depends greatly on a tire’s intended use. Typically, DH tires use a very slow rebound formula to reduce deflection on high speed terrain. The downside is that the slow rebound rubber typically means higher rolling resistance and faster wear. Harder compounds typically roll faster and last longer but can have less traction on some terrain. Bontrager has two new rubber compounds that have bridged the gap between slow rebound and fast rolling. These new compounds are found in our Team Issue XR1, 29-1, XR4, and 29-4 tires.
Is this something that the average rider can benefit from, or does one need to ride at a pro level to experience it?www.bontrager.com
While Bontrager tires are developed with professional athletes, we also test extensively at other levels of rider ability. We also test on different types of terrain, in both wet and dry, and everywhere in between. Testing at the professional level is crucial because those riders can push the tires beyond the limits of most people. The average rider can’t tear a tire apart in an afternoon, but Aaron Gwin certainly can. This level of expertise and development certainly benefits the average rider. Although the average rider doesn’t ride even close to that hard, he or she ends up benefitting greatly in areas like rolling resistance, wet traction, and puncture resistance. Does tread design have an effect of a designer's compound choice?
Yes, especially in terms of overall knob dimension (height, width and depth). If you’re intending to use a very soft rubber compound, you'll need to support the knobs by design otherwise they will tend to flex easily. Or if you’re intending to use a harder compound, you can minimize the overall knob dimensions.
What is the purpose of a tire using multiple compounds?
There are several options for multiple compounds. The most common is the dual compound where you have a harder rubber in the center that rolls faster and wears longer, with a softer rubber on the shoulder knob for higher cornering traction. Typically there is a 5 - 10 point difference in rubber durometer, center to shoulder. Bontrager Team Issue XR1, 29-1, XR4, and 29-4 tires use this technology.