Titanium Connecting Rods?

Competition engines and ancillaries - general discussion
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keithwwalker
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Titanium Connecting Rods?

Post by keithwwalker »

Warning, more a philosophical post than a 'wanna build supermotor'

I don't see titanium connecting rods mentioned too much on Fiat DOHC race engines, whereas the older smaller Fiat 600/850/A112 blocks they are available. Is it such that the DOHC rods will safely go up to 9500 rpm and are not a limiting factor rpm?

Keith Walker
Lancia Scorpion
Guy Croft
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Re: Titanium Connecting Rods?

Post by Guy Croft »

Warning to you too, respected member who has agreed to the Protocols!

Links by my direct permission only denied in this case & deleted. Discussion of cost is forbidden.

I have done the fundamental stress calcs on rods of steel race material in 90mm stroke units, they are nowhere near failure even at far higher speeds (11,000rpm+).

What are the weak points? There are only two we should concern ourselves with. At those speeds 3/8" rod bolts are about 75% of the endurance strength of the steel used typically by good mfr. The piston pin boss is close to the limit and actually is most at risk because piston alloy materials fatigue, ie: their life to failure is governed by number of cycles. The stresses naturally increase according to the mass of piston assembly and the upper rod section itself so if you are aiming for extremely high rpm reduction in weight does indeed become very critical.

Pure Ti is only about the strength of alluminium alloy but Titanium alloy rods are near half the weight of steel and some like the complex alloy Beta C (9Ti - 3Al - 8V - 6Cr - 4Mo - 4Zr) can achieve very high tensile strength, say 1300Mpa - about the same as one of the preferred rod steels 4130 chrome moly steel, thus they can deliver a good mass/stress ratio and a reduction in mech losses. But on the other hand, they do suffer fatigue (600 Mpa limit) as many non-ferrous materials do, and the things must be (at very least) religiously taken out of the engine and crack-tested and their bolts too. Moreover - even if run under their life-cycle stress limit, often the weakest point with them is the female thread for the bolts which is difficult enough to form at all in Ti never mind to form to the highest level of perfection that a rod demands. Titanium is beautiful to turn and mill but a real 'fright' to thread (and saw through - it galls and seizes) and if you overheat it during machining you basically weaken it so much you might as well bin it. Threads are subjected to shear stress where the metal strength is usually about half the tensile and analysis must take account of this. An impaired female thread causes local distortion & overloading of the coupled thread pair (male/female) and this can lead to bolt failure per-se, normally between the 2nd and 5th engaged threads. I have seen this happen in high rpm F1 engines (16,000 +) and if you see an F1 blow-up that will invariably be the cause. Assessment of life-to-failure is reasonably straighforward and given the cost of Ti rods you'd be foolish not to do it or at least insist on accurate data from the supplier.

Steel rods of course have their limits but swapping for the very costly Ti option without doing the maths on the engine is a rather cosmetic and pointless exercise, something merely to brag about in the trackside bar in my view.

G

edit note: this is worth reading, I was looking up coatings for Ti to prevent galling (which Ti rod cheeks must have):


http://mechanicsupport.blogspot.com/201 ... usage.html
Biancorally
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Re: Titanium Connecting Rods?

Post by Biancorally »

Hi Guy,

brilliant explanation, as always, very detailed. I say this because almost everybody knows that titanium rods are lighter, but no one knows the whole truth about the Ti rods.

Guy, thank you very much for sharing your experience about titanium rods.

Regards

Daniel
GC_17
keithwwalker
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Re: Titanium Connecting Rods?

Post by keithwwalker »

Great synopsis, Guy.

I looked around the web a bit more - "must not hyperlink" says to himself (while restraining wrist, lol).

One of the more interesting pages was a comparison of different connecting rod weights for Ducati's. I will summarize the values:
Ducati Stock 4V steel rod - 542.6 grams
Carrillo Ducati 4V steel rod - 408 grams (presumably 4340 steel or similar?)
Ducati 4V Pankl F1 H-beam Titanium rod - 379.5 grams

So the titanium rod provides only a 7% weight savings compared to the steel rod - at a much greater cost.
Most cost effective is a lightweight steel flywheel. So if one would examine the weight savings with a steel flywheel and titanium connecting rods, the rods may only contribute ~1% of the rotating mass reduction.

Regarding galling, I thought that manufacturer's would address that with capped bolts, but I think that would create a stress concentration point (stress riser), so even current F1 rods all have female threads.

My conclusion for tonight is that you would only use these on a race engine where the crankshaft is at the limit. That is why titanium rods are available for Fiat 600's, with their 3 main bearing blocks, but don't make sense for a 5 main bearing block like the Lampredi unit, with its good quality crank design.

It's always fun to play armchair engineering while saving up for the real thing!

Keith Walker
Lancia Scorpion
Brit01
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Re: Titanium Connecting Rods?

Post by Brit01 »

Interesting read GC about the Ti properties.
That is why titanium rods are available for Fiat 600's, with their 3 main bearing blocks
I wonder if these rods are available for the Alfa flat four as they have just 3 main bearing blocks also.
Just curiosity only.
pastaroni34
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Re: Titanium Connecting Rods?

Post by pastaroni34 »

I think you'll find that technically titanium rods are available for anything, including Fiats and Alfas. Look closely at the definition of 'available' though. I say this because I highly doubt anyone keeps Ti rods for Fiats in stock, on the shelf. I'm certain that most Ti rods are custom made to your specs. While cool, this is the part that worries me. If I am spending $20k+ on a motor, I want a rod with proven history, not the prototype. If you have silly money to blow, then by all means!

Some companies find listing parts like this a useful marketing technique. The idea being say you have 3 types of part to sell, good ($200), better ($400), and best ($2000). Adding the 'best' option makes the 'better' option suddenly a lot more attractive than if you were only presented the 'good' and 'better' options. By this logic Guy should go ahead and list all form of exotic parts ;) (if only he wouldn't get emailed about them constantly!)
-Jason Miller
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4v6
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Re: Titanium Connecting Rods?

Post by 4v6 »

Thank's for that eye opening analysis on Ti rod's Guy, as ever it seem's you have to read the fine print!
Tony Warren. GC #96.
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