aluminum rods vs. titanium rods: fatigue life

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iScamp

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I did do a search on this subject. Got some information but, it's scattered among build threads and takes a while to dig through. So, if you don't mind I'd like to bring up the subject again.
This is not for an automotive application.

Question: All things being equal with the engine (rpm, horsepower, etc. ) which will last longer?. Aluminum connecting rods or titanium

Question: What has been some real world experience with either kind of rod?
such as:
blew up second pass on the 1/4,
been running circle track all season, no problems.
you'll be playing with the grandkids before a set of xx rods will fail.
 
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MoparLeo said:
If not for automotive, what for ? All things being equal...
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2 cylinder air cooled engine for a small amateur built aircraft.
37 h.p. , 3200 rpm redline.
Reducing total aircraft weight even by a pound is a big deal in this application.
These engines are currently built with forged steel rods.
 
iScamp said:
2 cylinder air cooled engine for a small amateur built aircraft.
37 h.p. , 3200 rpm redline.
Reducing total aircraft weight even by a pound is a big deal in this application.
These engines are currently built with forged steel rods.
Click to expand...
I would be talking directly to a HP rod mfr. Al-U-minimum has superior lubricity against a steel/iron crank, much more forgiving in any lubrication issue that could arise, but fatgue life is iffy. There are new alloys all the time, and there are Members here that have stated they run them on the street. If chosen I'd discuss a replacement interval as routine maintanance with the mfr.
Titanium is not as light, but is far superior in fatigue strength, however to quote David Vizard..."w/o proper care & lubrication it will weld itself to anything it rubs against, if dry it will stop just short of welding itself to wet ice". Again, there are always alloy developments going on.
Talk to the experts..
 
Good advice. Certainly will gather much more information before embarking on this.
I know titanium has some odd properties, didn't know anything such as what Vizard said.
I do wonder why no one has done lightweight rods before.
I knew I could get some information from the folks on FABO, even for a esoteric subject as this.
 
Make sure it'll glide real good.
 
iScamp said:
2 cylinder air cooled engine for a small amateur built aircraft.
37 h.p. , 3200 rpm redline.
Reducing total aircraft weight even by a pound is a big deal in this application.
These engines are currently built with forged steel rods.
Click to expand...
Someone has been flying around Hickory, NC area in a small plane maybe like what you're describing. When I first heard it, I thought airplane but then I thought it didn't sound exactly right. I walk outside to see what it was and it was flying really slow maybe a little faster than an ultralight.
Post some pictures.
 
Find other places to save weight. Put a tried and true forged steel rod in it and never worry about it again.
 
tinman2 said:
Someone has been flying around Hickory, NC area in a small plane maybe like what you're describing. When I first heard it, I thought airplane but then I thought it didn't sound exactly right. I walk outside to see what it was and it was flying really slow maybe a little faster than an ultralight.
Post some pictures.
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Hummel Ultracruiser

ultracruiser.jpg
 
If you want to start down the rabbit hole, search 'SN curves' and do some reading. You'll be lucky to find good graphs of certain materials and it will become obvious pretty quickly why most rods aren't aluminum.
Forged steel is tough to beat. I'm sure there could be some tool steels or high-strength steels which could be used (many high-strength steels lack fatigue life - it's a trade off for this super high strengths, but not always).
If you can increase the strength, obviously you can reduce the mass which can further reduce the load which reduces the mass etc. This level of optimization can be quite an engineering challenge, and inputs to calculate an acceptable performance window are not always readily available. Lots of minor stuff also becomes a concern - stress risers, imperfections, finish methods etc.
Quite a few high output OEM engines are using powdered metal rods these days, but tooling and testing can eat up $millions in development. Still, papers on those things might help size some of the loading to do some initial investigation.
 
a friend of mine made a set of titanium rods for his funny car (note it wear out tooling left and right ) , but titanium doesn't grow with heat as much as the crank did . it lock up the crank on the burnout to heat the tires , so i would say cool if you go that extra mile with a titanium crank as well . what kind of use are you needing the material for rods ?
 
Have you priced titanium lately?
Alot of good machine shops, will machine the parts, just for the shavings off the part.
 
The first rule of thumb: Ti is far more ridged. Thus excellent strength to weight ratio. Your bearings aren't going to like them. But in a flat out race engine? Bearings are replaced frequently.

I run BME 7.6" Hemi rod in my high compression 451. These rods are designed for Top Alcohol. So it's an over kill to a N/A engine build. I got them for $500 on BME over stock listings. Thus I believe no brainer compared to $600+ Eagle steel rods. (All prices 5+ years ago. Im sure both all are x2 today.) The other advantage is the race Aluminum rods are very close to the same weight as stock RB 7.6". Thus I made no counter weight change to crank. The Eagles were much heavier.

So in closing? Aluminum are easier on your crank bearings than Ti. MUCH less expensive. And an over kill is available to rest any fatigue concerns. Hell, BME makes Aluminum rods for fuel cars.

Just my limited experience.
 
Titanium, like many other elements, has other elements alloyed with it in varying percentages to give it the desired 'Titanium' characteristics reqd. Strength, wear, heat effect some/all of the above
It is a waste of time talking about Ti strength & other features without knowing the rest of the mix.
 
RustyRatRod said:
Oh my goodness! That's cool! Looks like a Volkswagen engine!
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Yes, most are powered by a 1/2 vw engine. They take a Type II flat four and cut off the back two cylinders.
There are some purpose built 2 cyl. engines, Mosler and Global, (out of production but, still available here and there) that are of the same configuration.
The Global uses a forged SCAT crank, forged rods. I think a Chevy 350 con. rod will work, also. VW titanium and aluminum rods are available off the shelf, last I checked. Which was several years ago.
 
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tinman2 said:
So... is it you I'm seeing?

I was walking my little boy Shih Tzu about 9:55pm and heard and saw lights of a small plane. It was fairly quite and didn't appear to be moving very fast. But, I couldn't tell what it looked like.
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No, not me. Maybe I'll get there, one day.
 
pomonamissel said:
a friend of mine made a set of titanium rods for his funny car (note it wear out tooling left and right ) , but titanium doesn't grow with heat as much as the crank did . it lock up the crank on the burnout to heat the tires , so i would say cool if you go that extra mile with a titanium crank as well . what kind of use are you needing the material for rods ?
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Off the shelf are or were available. Just planning on purchasing those. Don't really want to do too much R&D on this.
 
cojohnso said:
The first rule of thumb: Ti is far more ridged. Thus excellent strength to weight ratio. Your bearings aren't going to like them. But in a flat out race engine? Bearings are replaced frequently.

I run BME 7.6" Hemi rod in my high compression 451. These rods are designed for Top Alcohol. So it's an over kill to a N/A engine build. I got them for $500 on BME over stock listings. Thus I believe no brainer compared to $600+ Eagle steel rods. (All prices 5+ years ago. Im sure both all are x2 today.) The other advantage is the race Aluminum rods are very close to the same weight as stock RB 7.6". Thus I made no counter weight change to crank. The Eagles were much heavier.

So in closing? Aluminum are easier on your crank bearings than Ti. MUCH less expensive. And an over kill is available to rest any fatigue concerns. Hell, BME makes Aluminum rods for fuel cars.

Just my limited experience.
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Thanks. Just the kind info I was looking for. Lots of great replies, here.
 
Just because a top fuel or NASCAR team uses something, doesn't mean it is the best for all applications. Those engines get torn down and rebuilt after every run or every race. The only durability they really care about is getting through the current event. I think using some of these exotic materials in an airplane engine your life depends on is a fool's errand unless you are intimately familiar with the material properties. I don't think asking for opinions on a forum dedicated to 50 year old cars is the way to gain that knowledge. Current engines are using forged steel for a reason. You want to save a pound or two off the airplane, eat less and walk a couple miles each morning.
 
gzig5 said:
Just because a top fuel or NASCAR team uses something, doesn't mean it is the best for all applications. Those engines get torn down and rebuilt after every run or every race. The only durability they really care about is getting through the current event. I think using some of these exotic materials in an airplane engine your life depends on is a fool's errand unless you are intimately familiar with the material properties. I don't think asking for opinions on a forum dedicated to 50 year old cars is the way to gain that knowledge. Current engines are using forged steel for a reason. You want to save a pound or two off the airplane, eat less and walk a couple miles each morning.
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I understand about the race engines torn down often and blow up, occassionally. My reason for asking here is there are some guys that are building/running modern equipment. And there is a knowledge base. I'm on a forum dedicated to the aircraft/engine I posted earlier. However, those guys/gals are pilots and builders. They don't have any previous background to speak of.
I'm just getting some basic information here. There are 2 individuals, I know of that have extensive, decades long knowledge with those 2 cyls engine. One has been racing air cooled VW's for decades. The other guy builds specifically for that aircraft application. Before I pester them, I'd like some real world background info.
FABO guy/gals do have knowledge with modern race and street applications even if it's in an old Mopar.

I hear ya' about the weight. Sure could use more exercise, no doubt. However, part of the weight requirement is per FAA regulations. To meet that requirement, builders will delete the plexigas canopy,brakes, set cushion, etc.. (brakes are not needed on a grass runway). That being said, it's still a weight reduction in addition to what I can do personally. 2 pounds is actually slightly noticeable performance difference in that aircraft.
 
iscamp just make sure the crankshaft is of a similar material , so they grow at the same rate .
 
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