pushrod strength test - alum vs steel

I recall seeing a video showing a carbon fibre wheel on a race bike turn into powder as the rider was leaned into a corner at very high speed.
Carbon fibre has a real cache with the Sport bike crowd but I remain unconvinced that it is a full on replacement for metals.

Maybe we should instead think of it as a high strength, lightweight plastic, which is essentially what it is.

Glen

worntorn said:

I recall seeing a video showing a carbon fibre wheel on a race bike turn into powder as the rider was leaned into a corner at very high speed.
Carbon fibre has a real cache with the Sport bike crowd but I remain unconvinced that it is a full on replacement for metals.

Maybe we should instead think of it as a high strength, lightweight plastic, which is essentially what it is.

Glen

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I think the B2 bomber, which is carbon composite, is pretty convincing evidence that carbon composite is in fact a full on replacement for metal - not all metal, not all applications, but some metals in some applications. There is obviously a huge difference between someone slopping together some carbon fiber and room temperature cure epoxy in their garage vs the aerospace pre-preg layups made with specialty resins, cured under pressure and at very high temperature.

mdt-son said:

Jim,

Sorry to question your experiment, but I truly believe your finding is questionable. Being a specialist in structural mechanics and stability of structures in particular, I can tell you this:

* Forces in a set-up like yours are distributed according to effective stiffness E*I . Determination of E*I (Young's modulus times area moment of inertia) doesn't require an experment. Thus, I think one rod was subjected to a higher load than the other.
* Straight line loading is paramount. I am not convinced your setup ensures completely axial loading due to the eccentric placement of specimens and the apparatus itself. Testing one by one in a professional testing machine would give more confidence in the result.
* Boundary conditions play a significant role in achieved stability load. In this case, deformation of the aluminum ends and shear load due to friction may have increased the collapse load of one vs. the other. You really need to mimic the simply supported end conditions of the genuine pushrods by providing a ball and cup type of condition at both ends, thus eliminating adhesion, end deformation and misalignment.

I would be most interested in your findings, but I also think simple calculations will provide a reliable answer. I may provide a calculation shortly.

Kind regards,
Knut Sonsteby

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Yes I was thinking the same.
two dial gauges over each pushrod and recording the deflection. Or a pressure gauge used to show the fail rate @ PSI of each type, during an individual test. Food for thought. :idea:
Cheers,
Thomas
CNN

The test in the video uses alum with 3Xs the thickness of the steel. Its the wall thickness that gives the alum better bending/buckling resistance to steel.

Cut a section of each tube (steel and alum) in 1/2 the long way. Cut them in 1/2 again the long way (quartered). Do it again so you have an 1/8th segment etc. The alum piece with the thicker cross section will be stiffer when bent in the direction of the cross section (because its 3xs the thickness/dimension).

I used steel tube pushrods in my 500cc Triumph engine for many years. It's stroke was 63mm and it used to rev to 10,500 RPM regularly. However the lift rate on the cams was very slow and the total lift was not very high. Titanium his it's problems - it loses dimension if it is impacted repeatedly. In a Commando engine, revs will usually kill the motor in other places long before the valve train gives a problem. In any case, I don't believe a 'full race' cam is ever used which would run at the revs which would require the valve train to be lightened.

Sorry but that's not really a true test, any slight variation in angle , pressure applied, distance from the jack, squareness of the press will exert more pressure on to one rod

Size for size, there is a very considerable difference between a steel pushrod, and one of aluminium with conventional steel ends.
Apart from the weight difference, which leaves the all steel item the heaviest by some 288% , the steel pushrod has a load carrying capacity before buckling of 29% better than the aluminium item, and in both axial and lateral stiffness the steel pushrod is superior by 290%.

The really interesting comparison is between the axial and lateral vibration natural frequencies, where the steel pushrod is again superior, but by the exceedingly small margins of 0.2% axial, and 0.6% lateral. It is these latter two frequencies which should not be permitted to coincide with the valve spring natural frequency anywhere within the working range of engine speeds.

The above calculations were made for no particular pushrods, details were selected at random for the excercise, the above variations could differ were other dimensions selected such as longer steel ends and shorter tubes

Snotzo said:

Size for size, there is a very considerable difference between a steel pushrod, and one of aluminium with conventional steel ends.
Apart from the weight difference, which leaves the all steel item the heaviest by some 288% , the steel pushrod has a load carrying capacity before buckling of 29% better than the aluminium item, and in both axial and lateral stiffness the steel pushrod is superior by 290%...

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I agree that size for size steel is superior - there is no question, but what I'm trying to get at is the strength per weight. If size per size gives steel a 29% advantage but alum is 2-1/2xs lighter than steel (size for size) then it seems like alum is stronger weight for weight.

If steel has 290% more lateral stiffness but it is heavier than alum by 288% - those figures show alum and steel to be nearly matched.

And what about wall thickness? A 1" wide flat strip of alum that is 2-1/2 times as thick as a 1" wide strip of steel will resist bending much more than the thinner steel. An equal weight of alum to steel pushrod tubing (with the same OD) give aluminum 3xs the wall thickness.

How do you figure wall thickness stiffness into the calculations?

I would like to see a real life test that is more refined and scientific than my quick and dirty example.

JS
using a flat strip for comparative purposes in this instance is completely irrevelant.

Since you are interested in strength based on equal weights, I have run another calculation, which again shows the steel pushrod to be superior in all aspects under consideration.
For my calculations I had to reduce the wall thickness of the aluminium tube by some 32% to end up with an equivalent overall weight of the steel item.

Not the massive difference as indicated in the size for size calculations, but a useful improvement everywhere, and particularly in the axial and lateral vibration natural frequencies of 1.0 and 1.1% respectively.

Is 4130 steel tube suitable for pushrods available in wall thickness of 0.022 thou?

Snotzo said:

JS
using a flat strip for comparative purposes in this instance is completely irrevelant.

Since you are interested in strength based on equal weights, I have run another calculation, which again shows the steel pushrod to be superior in all aspects under consideration.
For my calculations I had to reduce the wall thickness of the aluminium tube by some 32% to end up with an equivalent overall weight of the steel item.

Not the massive difference as indicated in the size for size calculations, but a useful improvement everywhere, and particularly in the axial and lateral vibration natural frequencies of 1.0 and 1.1% respectively.

Is 4130 steel tube suitable for pushrods available in wall thickness of 0.022 thou?

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The flat strip was just one example and I used it because its easy to visualize and test. Go back to my earlier post which reads:

"Cut a section of each tube (steel and alum) in 1/2 the long way. Cut them in 1/2 again the long way (quartered). Do it again so you have an 1/8th segment etc. The alum piece with the thicker cross section will be stiffer when bent in the direction of the cross section (because its 3xs the thickness/dimension)."

Now we are talking about shape as well as cross section. When you cut the tubing lengthwise you have an arc. An arc is relevant to a piece of tubing - and a small segment of alum arc with a thicker wall cross section has more bend resisting strength than a small segment of steel arc with a thinner wall cross section.

The tests I made with the press were in favor of the steel tubing because the steel was about 2% heavier than the alum. with 3/8" OD pushrod tubing the closest you can get is .095" wall alum and .028" wall steel (.022" wall steel is not available -you would have to ream it to size). When testing, I was careful to align & square & parallel everything carefully and placed the jack point as close to center as I could measure. I repeated the test by offsetting the jack slightly toward the alum so it would see more load and sure enough - the aluminum failed.

I could refine the real world test with ball bearings on all the pressure points and spend a lot of time making everything exact. Whether I do that or not - so far the strength per weight comparison between alum and steel (of equal weight) seems to be closer than expected. So close that I wouldn't bet money on it either way. This is a work in progress. Whatever the calculations may show - they still need to be confirmed with an accurate real world test.

Snotzo said:

Is 4130 steel tube suitable for pushrods available in wall thickness of 0.022 thou?

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When I was more actively looking at making steel pushrods as well as aluminum ones, I concluded that I would need to use .025" tube to get the same performance as the stock Commando aluminum rods. That was from the basic Euler equation comparison. At that time, the closest I could find was .035". The biggest problem I had was not so much the tube size, but finding pushrod ends to fit the thinner wall tube. At Jim Comstock's suggestion, I tried making my own from aluminum bronze. That works, but is way too time consuming for anything more than one-offs. I got some quotes from a local CNC shop and a heat treat shop for making a batch of suitable steel ends, and it was way too expensive for the quantities I needed.

In the end, I let the project go idle. Steel rods do look better in theory. But in real life, I've never had a problem with aluminum push rods in any of my race engines, and it doesn't look like there's any need for anything else in a street Commando. The only place I think it might make a difference is in some of the much higher revving super short stroke builds, like the ones documented here in the forum, as well as some in process now. Particularly with the really aggressive cams (think N480). And it might not even be an issue there.

Ken

I suggest that building a high-revving short-stroke Commando engine is a pretty delusional exercise. It is easy to build something which would scare the crap out of you. Taming it is another story. I did the exercise with the 63mm stroked 650 Triumph engine. It turned me into an instant dud. I love my methanol-fuelled 850 Commando engine - with the close ratio box, it is excellent - fast enough to win and easy to ride. That short stroke Triumph was really bad news, it crashed me all over the place and I am probably not a bad rider.
The theory behind the short stroke Commando is great, the reality is probably different.

Alan, not so with the short stroke, the most successful Norton in Australasia has a short stroke engine, its currently on its way to the UK/Europe for a campaign over there . I doubt they would have done as much winning with it and sending it overseas if it was dangerous.

Scroll down the .pdf file and look for Shorai Report from Flick, young guy James Flitcroft (ex Moto3 rookie)who will ride it overseas.

http://www.hamiltonmcc.org.nz/anytime/p ... edp314.pdf
Regards Mike

JS
if you are interested in meaningful tests, you need to avail yourself of a Lloyds tensile tester, or equivalent. Such equipment was at the disposal of Prof. Gordon Blair when he was working through the development of his 4StHead software, and is still in use at The Queens University, in Belfast.
You need something a little more sophisticated than a hydraulic jack and a toggle bar!

Regarding the usefulness of such tests, the results, while coming out in favor of steel pushrods overall, is certainly not the be all and end all of the question. Most would go for steel pushrods where a cast iron cylinder is used, and alternatively, aluminium pushrods would normally be used in conjunction with aluminium cylinders.

Usually, but not allways!

Brooking 850 said:

Alan, not so with the short stroke, the most successful Norton in Australasia has a short stroke engine, its currently on its way to the UK/Europe for a campaign over there . I doubt they would have done as much winning with it and sending it overseas if it was dangerous.

Scroll down the .pdf file and look for Shorai Report from Flick, young guy James Flitcroft (ex Moto3 rookie)who will ride it overseas.

http://www.hamiltonmcc.org.nz/anytime/p ... edp314.pdf
Regards Mike

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Great quote from the rider:
"The feeling of riding the painstakingly prepared SBR Norton 750 classic bike is indescribably unique, having the feeling and handling of a GP 125 modern, yet the speed of a GP 250 and torque of a modern 600" !

Snotzo said:

JS...
you need to avail yourself of a Lloyds tensile tester....

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I am interested in buckling strength under compression - not tensile strength. Tensile strength measurements are made with a solid rod that is stretched until it breaks (without bending loads). But when you make compression tests - the results will change when you change the shape and wall thickness of the material - a small diameter rod will buckle/bend sooner under compression than a larger diameter tube of the same material and weight. In this case tensile strength ratings are misleading. I'm only interested in comparing the compression strength of 2024 alum to 4130 steel tubing of the same outside diameter and weight - without spending a fortune. For that I need an accurate pressure sensor (expensive) or a balanced application of pressure between the two different types of tubing (cheap toggle bar - with the addition of ball bearings at the pressure points).

JS

my reference to a 'Lloyds tensile tester' was an unfortunate error on my part, as it infers that the machine is a tensile tester only. I should have used it's correect name, which is a LLoyd's material tester.
The machine will conduct tests in both tension and compression, varying capacity load cells being available to cover a large range of testing requirements.

Snotzo said:

JS

my reference to a 'Lloyds tensile tester' was an unfortunate error on my part, as it infers that the machine is a tensile tester only. I should have used it's correect name, which is a LLoyd's material tester.
The machine will conduct tests in both tension and compression, varying capacity load cells being available to cover a large range of testing requirements.

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Sounds like the best way to get accurate measurements. Of course I'd love to have one - or make use of one - or have someone make the comparison test for me. Any Norton owners out there have access to one?

Brooking 850 said:

Alan, not so with the short stroke, the most successful Norton in Australasia has a short stroke engine, its currently on its way to the UK/Europe for a campaign over there . I doubt they would have done as much winning with it and sending it overseas if it was dangerous.

Scroll down the .pdf file and look for Shorai Report from Flick, young guy James Flitcroft (ex Moto3 rookie)who will ride it overseas.

http://www.hamiltonmcc.org.nz/anytime/p ... edp314.pdf
Regards Mike

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When you say 'short stroke' are you talking about 75mm or less ? I can see advantage in the 82mm (?) stroke factory engine, however when the stroke gets down far enough, the motor is usually nasty and intractable. There is much more to winning races than the blast down the straights. There is nothing worse than getting mid corner with a nasty short-stroke motor and have it fall out of the power band. Often when you bring it back on, you are likely to get launched. There are theoretical advantages in short stroke motors - they can be revved much higher to give more power and are often more reliable, however riding them well can be a problem. I did that stuff with my short stroke Triton and I regret it. These days I cope well with any difficulty when racing, so something good probably did come out of the exercise. But the time when I did it was when I was young and enthusiastic - that is when you need the GOOD bike - Not now when I am 75.

Sounds like the best way to get accurate measurements. Of course I'd love to have one - or make use of one - or have someone make the comparison test for me. Any Norton owners out there have access to one?[/quote]

Jim,
Is there an engineering school in the area? When my son was in college he had access to all sorts of equipment. That machine you guys are referencing sounds like something that might be in a Strength and Materials lab. You might be able to get access to it if you find the right person.
Pete