Rod half life shorter by rpm or torque (2012)

Status
Not open for further replies.
The evolution of motorcycle frames from steel to aluminum showed the advantages/disadvantages of aluminum.
Although it is 1/3 the weight of steel it is 1/3 the strength. The advantage was design flexibility - the ability to add mass to benefit the intended use without a weight penalty. The lesson learned was to build a structure from aluminum just to lower its weight didn't always work out.
 
mdt-son said:
Use of Hiduminium RR 56 was pioneered by Edward Turner for his Speed Twin project during the years 1936-38. There were several novelties in his application, especially the application to a machine turning twice the speed of an aircraft engine. The RR designation means the alloy was forged to a Rolls Royce specification.

"Ultimately, fatigue was not an issue in Triumph's choice of forged aluminum rods. Virtually every British parallel twin that followed the Speed Twin also featured them, and most held up superbly. Many of those engines have survived a half-century of use and abuse, including racing, and are still going strong." (Linday Brooke: Triumph Motorcycles - A century of passion and power)

I am not necessarily supporting that statement .... but to my knowledge, most rod breakages are due to loss of lubrication, not fatigue. The AMC twin rod which I know the best fails due to elongation (plasticity) of the aluminum cap which kills off the lube to the big ends. Obviously the engineers forgot to check the stress level under heat .... Turner avoided this fault by specifying steel caps, and Norton followed suit in due course.

According to Wikipedia, the composition is as follows:
Aluminium 93.7%
Copper 2.0%
Iron 1.4%
Nickel 1.3%
Magnesium 0.8%

I have two land speed racer Triumphs..None of the current top running Triumphs I know of use stock Triumph rods...
Silicon 0.7%
Titanium 0.1%

Nearest modern equivalent I have found is 2618-T61.

-Knut
Click to expand...


I have two land speed racer Triumphs..None of the current top running Triumphs I know of use stock Triumph rods...
 
Stock Alum rods will get you by. But the Norton needs a longer rod to reduce the jerk/stress (rod length to stroke ratio). And a light small end as well. The problem is that you can't get aftermarket forged aluminum rods for a Norton - they are all machined billet (not as strong as forged).
 
Dutch said:
The evolution of motorcycle frames from steel to aluminum showed the advantages/disadvantages of aluminum.
Although it is 1/3 the weight of steel it is 1/3 the strength.
Click to expand...

Strength is actually less than 1/3rd. Welded aluminium rarely exceeds 125 MPa unless the entire frame is post-weld heat treated. Today's top notch maraging steels (e.g., Reynolds 953) surpasses a yield strength (Rp) of 1000 MPa, that's 8 times higher than aluminum. Even if PWHT is applied, the strength relationship is about 5:1.
Even more important is the stiffness ratio. Aluminum has only 1/3d the stiffness of steel, and to compensate this disadvantage, section sizes have to increase considerably, which has a weight and space penalty. Especially space is limited on a motorbike, and I've often asked myself, what is the logic of using aluminum? I think it has a lot to do with fashion.

-Knut
 
mdt-son said:
I am not necessarily supporting that statement .... but to my knowledge, most rod breakages are due to loss of lubrication, not fatigue.

-Knut
Click to expand...

That may be, Knut, but in my (limited) personal experience with stock Commando aluminum rods, the only failures I have experienced (two cases) were at the small end, and not lube related, but due to fatigue. Both were discovered during dye penetrant crack testing, which showed a network of cracks in the small end just below the pin bore. There were no signs of a lubrication failure on the pin or pin bore, so I assume the cracks were due to fatigue. Both were from race engines. I have seen catastrophic failures with aluminum rods in other folks Commandos and Triumphs, where the failure was at the big end, both stock rods and MAP rods, which certainly look like they could be the result of lubrication failure and seizure. Big end bolt failure might also be a cause. There were some pretty questionable aftermarket Commando rod bolts on the market at one time. And I did have one stock Commando rod bolt show a crack during Magnaflux testing, back in the '70s.

I've run a lot of stock Commando rods in race bikes over the years, with no disasters, but retired the last one of them a couple of years ago. I'm sticking with steel rods on current and future engines. I do have stock aluminum rods in one of the street bikes, but will probably go with steel for any future street builds. It just seems like good insurance to me. I'd still be comfortable with stock rods in a mild street bike engine, ridden sensibly, but that's not the kind of bike I seem to end up with:rolleyes:

Ken
 
The trouble with this stuff is that stronger rods only build in reliability, they don't make the motor more powerful. So if it has never exploded why bother with something more expensive ? If I was racing frequently, I would definitely consider finding titanium conrods for my Commando engine. The reason my Seeley Commando 850 sat unraced for 20 years was I could never believe that such shit could stay together. I am very careful not to over-rev it. I think a lot of things are stressed to the limit in a Commando motor.
 
Last edited:
Status
Not open for further replies.
Back
Top