Ask the Geek: Aluminum Alloy and Tire-Slippage

The word billet refers to the raw metal and indicates it is in bar or plate form - a solid block as opposed to something cast. The Aluminum Association allocates and registers aluminum alloys following an ANSI (American National Standards Institute) standard, which defines the numerical designation based on the chemical composition and properties of the finished alloy. The first digit represents the main elements added to the aluminum. For example, the first 6 in 6061 means it contains magnesium and silicon. 7 indicates zinc, and 2 indicates copper. The second digit represents any modifications from the standard alloy, while the last two identify a specific alloy in the series. Many aluminum alloys gain strength through heat treating, aging or cold working; the T6 designation means the material was thermally treated and artificially aged, and T3 indicates solution heat treated, cold worked and naturally aged material. In summary, 6061-T6 is widely used as it is strong, machines well and welds easily. 7075-T6 is stronger but is more expensive and not as easily machined. 2024-T3 is not as strong as either, cannot be welded and has poor corrosion resistance, but is more malleable than either leading to better fatigue resistance. Of these, you would want 7075-T6 for a rear sprocket for maximum strength to reduce wear. Aftermarket engine covers are usually fabricated from 6061-T6, while Barnett machines its clutch baskets from 2024-T3.

Titanium alloys have very roughly the strength of steel but half the density (aluminum has approximately one-third the density of steel). This material is very costly and difficult to machine or weld, but is used to make some products where light weight and good corrosion and temperature resistance are priorities - exhaust pipes, for example. Sidewinder offers sprockets made from titanium; as you'd expect, they are more expensive and slightly heavier than aluminum parts, but strong enough that the company can offer a lifetime guarantee. Inconel 625 is a brand name of a nickel-chromium alloy made by Special Metals Corporation. According to the company, it "is used for its high strength, excellent fabricability (including joining) and outstanding corrosion resistance. Service temperatures range from cryogenic to 1800 degrees F." Very good thermal-fatigue strength makes it a good choice for exhaust systems. For some parts, there are diminishing returns with stronger material. Inconel exhaust tubing can be thinner than titanium and potentially be lighter even though the material itself is denser. But you can't make a sprocket thinner (and thus lighter) to take full advantage of a particular material's strength as thickness is determined by chain size.

On a race-prepped Suzuki GSX-R1000 with Pirelli Supercorsa DOT race tires, our Racepak data acquisition shows tire slippage of about 5 percent at 150 mph on Willow Springs' front straight, for a rear wheel speed of 157.5 mph. We calculate slip by comparing GPS speed with wheel speed, although to be accurate the bike must be upright (because of the changing tire circumference at lean angles) and tire deformation can be a factor. Here are some additional numbers for thought: Heading up the hill toward turn 6 at Willow, tire slip with the bike vertical is greater than 10 percent at 85 mph. In some turns, taking into account wheelspin as well as the smaller tire circumference, slip can be as much as 40 percent. Under braking, we typically see negative slip between 15 and 20 percent.

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