Piping up: How nuclear subs gave us titanium exhausts

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As bikes get heavier manufacturers search for ways of making them lighter. Just look at the latest superbikes, such as the Ducati Superleggera or Honda Fireblade, with their titanium pipes.

But what is it that makes titanium so good? Ian Moody, a metallurgist at NeoNickel in Blackburn, gives us the answers.

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“Titanium was first discovered in 1791 but it took almost 200 years for it to find common usage. In fact, it wasn’t until 1969, when the Soviet Union built a nuclear sub with a titanium hull that a flurry of research began, which led to the growth of the titanium industry as we know it today.

“Given the issues that titanium brings – relatively high cost, weldability problems and difficult machinability – the question has to be asked ‘why do we bother’? The simple answer is titanium parts are light.

“Pure titanium has a yield strength approximately equal to structural steel but with half the density. Alloyed titanium grades achieve even higher strengths, which means that structural parts made of titanium can be made thinner and lighter than their equivalents in steel and this weight saving means improvements in performance.

“This all brings us to motorcycle parts. A typical exhaust made from titanium rather than stainless steel will be 40% lighter. Titanium has the added advantage of improved corrosion resistance, increasing the component life. It also has 30% higher thermal conductivity, adding that to the thinner section possible using titanium adds up to pipes that cool a lot faster.”

Colours of the rainbow

The titanium exhaust system on a Honda MotoGP bike

“One additional benefit of titanium is its corrosion resistance. Titanium is so reactive with the oxygen in the air that it forms a microscopic oxide layer on its surface which protects it from further attack.

“Left to its own devices, titanium will passivate with a thin oxide layer and sit unchanging for decades. But apply heat and the protective oxide layer will grow. And as the layer slowly grows, like spreading oil on water, it begins to refract light into all the colours of the rainbow.

“First a light straw yellow, then gold before darkening. The exhaust heat of a bike often equates to the temperature required to eventually create a blue surface layer. Bluing of the pipes isn’t indicative of any defect – this is just a natural response of titanium to its environment as it builds up its oxide layer to protect itself.

“Once pipes have blued, the only way to reverse the process is to mechanically polish the oxide layer off – but be aware, you are thinning the pipe (so reducing its life) and doing nothing to prevent it from starting all over again.

“Titanium has evolved over the years from a niche material into a realistic engineering option. When looking to upgrade your bike, always remember that the cheapest material isn’t always the best choice for lifetime cost.”

Titanium explored:

  • Strength in numbers Adding other metals changes the microstructure. Ti6Al4V is nearly three times as strong as mild steel
  • Big mix up The most common grade of titanium is Ti6Al4V, which is an alloy with small quantities of aluminium and vanadium
  • Super light Titanium has a density of only 4.5g/cm3 as opposed to 7.85 for carbon steel and 8 for stainless steel
  • Clever stuff Of all the pure metals on the periodic table, titanium has the highest strength to density ratio
  • Lots around Titanium is actually the ninth most abundant element on Earth – working with it is the expensive part