Ask Kevin: What Do You Think of Aluminum Axles?

1953 Triumph 500cc Rickman Metisse
1953 Triumph 500ccCraig Boone via

Question: I have just assembled a "new" Rickman Metisse utilizing a 1953 Triumph 500cc twin mated to a BSA Goldstar gearbox. The thing starts and runs beautifully. I even managed to get a license plate here in California by adding lights and a horn. In building the rear axle for the bike, I did some research on McMaster Carr's website to discover that 7068 aluminum is "stronger" than 4140 chrome-moly steel, so I used the aluminum to save weight. The axle is 11/16-inch diameter. Did I make a mistake in doing so? I asked Gilles Vaillancourt at Works Performance about it, and he said he had tried to utilize aluminum axles many years ago, but his dirt track racers wouldn't let him. Did they know something we don't know? What do you guys think?

Answer: Congratulations on a beautiful classic machine. The more I look at modern Triumph-inspired engines, the more I value the unmatched look of the real thing.

As to the aluminum axle, I and others went through something similar. The point to be made here is that strength and stiffness are separate qualities. Aluminum is 1/3 the stiffness of steel, no matter what alloy or heat-treat it may be. Imagine we make two one-inch holes in the face of a vertical concrete wall, each six inches deep. Into one we put a two-foot steel bar, one inch in diameter. Into the other, a similar one-inch bar of aluminum, heat-treated to the same 60,000-psi strength level as ordinary, nothing-special steel. We start to hang equal weights on the free ends of these bars, and the bars deflect elastically under the load. We notice, as we add load equally, that the aluminum bar is deflected three times as much as the steel. Finally, at some load, both bars yield. The initial elastic deflection is a measure of stiffness. The eventual yield is the measure of strength.

Another anecdote. When Yamaha TZ750 rider Nick Richichi first had his bike, a machinist buddy made him a lovely front axle out of the usual high-strength titanium alloy 6Al4V. Nick put in the axle for a practice at the old Loudon track. When he came in and took off his helmet, his first words were, "Take it out." It had made the steering feel sluggish. Its low stiffness allowed more fork twist in the quick Turn 2/Turn 3 roll-over, where a rider uses all his strength so he doesn't run out of road in the process.

You may find nothing wrong with the aluminum axle, however. Do bear in mind that steel has what is termed a "fatigue limit," which means that when loaded below that limit, it lasts essentially forever. Most aluminum alloys lack such a fatigue limit, so they accumulate fatigue damage steadily under stress cycling (this is why airframes have a "lifetime"). Notice that the landing gear of commercial aircraft is made of forged steel. I don't know, but am told that in recent years F1 engines have been switched from titanium con-rods to much stiffer yet equally strong rods made of maraging steel.

As a result of the above, non-steel axles give me the willies.

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