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Longevity of Aluminum rods

Posted By: ewgoforth

Longevity of Aluminum rods - 05/21/10 2:53 am

Hello,

I know Jim Schmidt sells what seem to be really nice Carillo rods and you can also buy conventional Carillo rods from other folks, and a lot of racers use them.

I've had material sciences classes and from them I know that Aluminum doesn't have an endurance limit, like steel does. That is, if you plot the level of applied stress on the vertical axis and the number of times to failure on the horizontal axis. You'll see that the graph starts out near vertical and becomes more and more horizontal as you reduce the stress.

For steel, the line eventually does become horizontal, which means that if you keep the level of stress below a certain limit, it will last forever. For aluminum, the graph never becomes horizontal. It flattens out quite a bit, but it keeps going down, which means that eventually an aluminum part will fail.

What I'm wondering is though, how common is it for high mileage aluminum rods to fail in street use, without a big-end lubrication failure? Our British vertical twins came with aluminum rods from the factory, but outside of this application, they're generally considered super trick, short lived parts. I think that the only other folks who use aluminum rods are some drag racers, since they're the lightest thing out there.

I know there are folks who've run Commandos over 100,000 miles with stock rods, but I was just thinking about this as my bike is getting close to 100,000 miles on it's stock rods.

-Eric
Posted By: Tiger

Re: Longevity of Aluminum rods - 05/21/10 10:38 am

Change them ?
Posted By: phantom309

Re: Longevity of Aluminum rods - 05/21/10 4:44 pm

I would say very rare to break one on the street, over Heating I would say is main cause for aluminum to fail.Steel is for tractors :-)
Posted By: Peter R

Re: Longevity of Aluminum rods - 05/21/10 5:03 pm

Concerning aluminium rods in the Commando, I remember a discussion some time ago of breakage problems due to an inferior manufacturing process.
These rods were used in the 850 Commandos from 1974, and have an encircled "D" cast in the side of the rods.
This has nothing to do with the fact that these rods were made of aluminium, just the manufacturing process was to blame here.
Posted By: HawaiianTiger

Re: Longevity of Aluminum rods - 05/21/10 7:02 pm

Right, the drag racer that used to balance my cranks swore that the only good rod was a well used aluminum rod, to hell with the steel ones. His idea was that they were far more forgiving.
A different take on the oriental wisdom "In high winds you must bend like a willow rather than break like an oak tree, grasshopper."
The only aluminum rod I have seen that broke was due to a spun bearing although I know it does happen.
Bill
Posted By: gtx

Re: Longevity of Aluminum rods - 05/23/10 3:07 am

You can burn nitro w/ stock rods,World champs have.
Posted By: johnm

Re: Longevity of Aluminum rods - 05/23/10 8:54 pm

Hi ewgoforth,

I cannot give you any hard data but I do think you have a good point.

As you say aluminium does have a finite cycle life no matter how low the stress - unlike steel. See the red line in the first graph in the attached.

http://en.wikipedia.org/wiki/Fatigue_limit

The D rod issue is discussed on Les Emery's site under Technical D rods be gone! As Peter points out this was a different (but related) problem and produced the same result!!

http://www.norvilmotorcycle.co.uk/

I use after market aluminium rods in my Dominator 500 firstly Nourish rods and now Thunder Enginereering from the UK. Nourish is not making his rods any more. I replace them about every four or five years of racing. Never had a problem but doen't want one either.

If I were you and either had a engine which fell within the age range of D rods defined by Emery or had a 100,000 mile Commando I would very seriously consider new rods. Not sure if I would pull the motor down just to do it but would defintely replace them if I was in there anyway. More so if the bike has ever been raced or pushed very hard.

I bet the world champs burning nitro had a preventance maintance programme. That how you get to be world champion. And while I agree with many excellent things Hawaiian Tiger has said over the years on this one I have to disagree. I dont think his friend is right about "well used aluminium rods". The metallurgy is against you. New aluminium rods - yes. Well used aluminium - no.

Best regards

John
Posted By: phantom309

Re: Longevity of Aluminum rods - 05/24/10 2:11 am

I bet the world champs burning nitro had a preventance maintance programme. That how you get to be world champion. And while I agree with many excellent things Hawaiian Tiger has said over the years on this one I have to disagree. I dont think his friend is right about "well used aluminium rods". The metallurgy is against you. New aluminium rods - yes. Well used aluminium - no.


Plus one john :-)Maybe we could get high dollar for our old rods john:-)
Posted By: johnm

Re: Longevity of Aluminum rods - 05/24/10 5:52 am

Hi Tim,

They make good trophies !

For me it is an insurance policy. A broken rod can break almost everything. Including the rider. Im conservative because I was a rider and I know how much broken bones can hurt!!

Cheers

John
Posted By: Tiger

Re: Longevity of Aluminum rods - 05/24/10 11:16 am

I broke an alloy rod on a road bike once at 4200 RPM, stripped the engine and found that some arsehole had fitted narrow shank pre '68 rods.

Think into this a bit and wonder where the wide shank rods went and why the bastard fitted castoff parts to my engine ?

Most likely from an old pre-unit that had been neglected and seized and otherwise abused ?

Pretty much blew the engine to bits and I was lucky to be upright at the time, in a corner it could have been fatal.

If good replacement rods are available and you are unsure of history spring the bucks for new ones, cheap compared to new cylinders and cams and crankcases and such.
Posted By: oilyrag

Re: Longevity of Aluminum rods - 05/28/10 9:47 pm

Hi all,
Yes they go, and this is how it looks:-
[Linked Image]

[Linked Image]

[Linked Image]

This engine was doing approx 3800rpm when this failure occurred. The history of the rods was unknown but they had been X-rayed 2000 miles before and looked ok. As Tiger says:-
Quote
If good replacement rods are available and you are unsure of history spring the bucks for new ones, cheap compared to new cylinders and cams and crankcases and such.


Wise words.
Posted By: ewgoforth

Re: Longevity of Aluminum rods - 05/29/10 1:46 am

Hey oily rag,

Was there any evidence of of the little end seizing up? It looks like it snapped in tension or from bending. With perfect bearings on both end, the rod should be subject to pure compression and tension. the compression forces should be much greater than the tension forces. If it failed in compression and you'd have the break at 45 angle to the long direction of the rod.

Do you see evidence of fatigue? If you look at the fatigue fracture (possibly with a magnifying glass) you can typically see little lines where the tip of the crack kept inching forward, like the rings in a tree. The final brittle failure will look kind of frosted.

-Eric

Originally Posted by oilyrag
Hi all,
Yes they go, and this is how it looks:-
[Linked Image]

[Linked Image]

[Linked Image]

This engine was doing approx 3800rpm when this failure occurred. The history of the rods was unknown but they had been X-rayed 2000 miles before and looked ok. As Tiger says:-
Quote
If good replacement rods are available and you are unsure of history spring the bucks for new ones, cheap compared to new cylinders and cams and crankcases and such.


Wise words.
Posted By: oilyrag

Re: Longevity of Aluminum rods - 05/29/10 12:02 pm

Hi again,
Ewegoforth asked:-
Quote
Hey oily rag,

Was there any evidence of of the little end seizing up? It looks like it snapped in tension or from bending. With perfect bearings on both end, the rod should be subject to pure compression and tension. the compression forces should be much greater than the tension forces. If it failed in compression and you'd have the break at 45 angle to the long direction of the rod.

Do you see evidence of fatigue? If you look at the fatigue fracture (possibly with a magnifying glass) you can typically see little lines where the tip of the crack kept inching forward, like the rings in a tree. The final brittle failure will look kind of frosted.


It was a straightforward fatigue crack as you describe, small end was perfect as was the gudgeon (wrist?) pin, the bend is post-failure as the rod smashed its way through the crankcase, it hit the camshaft hard enough to bend that as well, never found the piece of case with the breather valve in it, its on the M56 near Helsby if anyone wants to have a look.

BenG said:-

Quote
That was an old, old rod. It is the kind that Norton used on the early 650cc and very early Atlases, I don't think they used them after 1963, so it may have had a lot of time on it.


Even older than that, actually, the ally bearing cap and pressed-in small end bush should give it away to all as an 88/99 rod, I had actually tried Carillo rods but the vibes were terrible so I went back to what looked like the best set of original rods I had. I run a hybrid 650/99 motor in that bike now with new Commando rods.
Posted By: gtx

Re: Longevity of Aluminum rods - 05/29/10 12:32 pm

Rod failure was never a issue in "k" town bikes.We did however,replace the rod bolts and nuts.
Posted By: jim schmidt

Re: Longevity of Aluminum rods - 05/29/10 4:51 pm

The proprietary Carrillo rods I sell have lighter small ends than the reqular Norton Carrillo rods. This is because the bronze bush is eliminated in conjuntion with a DLC coated pin that needs no bush at all - saves 30 grams on the small end. Feels good to be able to reduce the vibes and still have super reliable steel rods.

[Linked Image]

http://users.gotsky.com/jimschmidt/nortonrods.html

Jim
Posted By: jaycee

Re: Longevity of Aluminum rods - 07/02/10 3:09 am

you know jim those things are so beautiful its almost obscene
Posted By: jaycee

Re: Longevity of Aluminum rods - 07/02/10 3:37 am

i feel my wallet twitching
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