Mystery of Broken Crankshaft Resolved

As you should know we rebuilt, modify and tune motorcycles. Recently, and for no apparent reason, we had a spate of cracked crankshafts on one of our motorcycles. After years of running standard cranks, we’ve never had one break before – and we’re producing big horsepower numbers continually.

It took us a long time and cost us a lot of money to find the cause. In search for lighter weight and marginal gains, little did we know that we sacrificed reliability. Once we looked at what was going on in greater detail, we built an engine with a billet crank just to make sure we weren’t doing something wrong, but we still broke a billet crank, which is virtually unheard of. I went back to the manufacturer of the crankshaft who is well-known worldwide, and they’ve never had one break before, too.

We looked into why this was happening because we weren’t cracking crankcases. We had some titanium bolts made to hold the engine in the frame, fitting them and torquing them up as per the specified figures. The bolts were fine, there were no manufacturing issues.

We used some blue (a dye that engineers use as a trouble shooter) and measured the bolts afterwards, and realized the bolts were over-stretching. So when the engine got hot, everything moved, and something had to give somewhere. The crankshaft is the free-moving component that soaked up the forces and eventually gave way. If you imagine and engine revving at 10,000rpm, racing on the roads is nothing like short circuits where you’re driving through third and fourth gears. We’re flat out for prolonged periods of time – plus the engine is being twisted and contorted through the sheer forces being put through the chassis.

It’s incredible to think that something so cheap and seemingly so innocent could cause such catastrophic failures. What gave us the idea of checking the bolts? We’d checked everything else! Titanium is a useful tool in the correct environment, but its properties don’t always lend themselves to the stresses and strains of maximum performance – and weight saving to the last gram isn’t always the answer….

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