Titanium rods

[rgoodrich]

I have an 81 SC. I'm planning a build to 102mm P&C's. I am considering Titanium rods to lower reciprocating mass to rev quicker. Other than cost, is there any down side to using Titanium?

[Henry Schmidt]

Originally Posted by rgoodrich

I have an 81 SC. I'm planning a build to 102mm P&C's. I am considering Titanium rods to lower reciprocating mass to rev quicker. Other than cost, is there any down side to using Titanium?

Nope

[Jonny042]

And what if cost IS an issue? Just, you know, theoretically.....

[Henry Schmidt]

Buy a set of R&R rods from LN Engineering.

[Tippy]

Aren't they life limited?

[tctnd]

Everything is "life limited". Because of the cost most titanium rods have been used in race cars where they certainly have a limited life span, as do steel rods. In "normal" use they will last just as long as steel rods. I was immortal until I was about 35, then it began to wear off. Now at 68 I am definitely "life limited".
regards,
Phil

[BoxxerSix]

Originally Posted by tctnd

Everything is "life limited". Because of the cost most titanium rods have been used in race cars where they certainly have a limited life span, as do steel rods. In "normal" use they will last just as long as steel rods. I was immortal until I was about 35, then it began to wear off. Now at 68 I am definitely "life limited".
regards,
Phil

Awesome

[Flieger]

Well, ferrous alloys have an ability where if they are cyclically stressed to only about half of their yield stress they will live forever. (Well, not forever but they have been doing tests for a long time and have certain samples that never failed after like a billion cycles).

Titanium and Aluminum alloys do not have this trait.

However, designing something in steel to be stressed so little will mean it will be quite heavy. This is not acceptable for connecting rods and would both limit revs and cause more wear to bearings and such.

Therefore, while steel could have an infinite life, high performance rods most likely do not.

Titanium would not last forever no matter how lightly stressed it was. It is only a matter of cycles.

Steel rods will last longer than Titanium.

[Weissach911]

It is quite difficult to generalize about Fatigue Limits without detailed knowledge of local stress concentrations, surface finish and a number of detailed metallurgical considerations but there are some simple estimates that can be used.

Some materials are said to exhibit a Fatigue Endurance Limit(S’e), which is a stress at below which they never fail and this is well accepted but the data is typically based in frequenices of less than 100Hz around 10^7 cycles and various statistical techniques have been used to determine this limit. In recent years piezoelectric machines have been used to look at frequencies of up to 20kHz and Giga Cycles of load reversals and there is some debate about the validity of the Endurance Limit in the very long term.

Titanium Alloys are generally said to have a Fatigue Endurance Limit although pure Titanium is considered poor in this respect.

There is a reasonably well accepted metallurgical model to explain this behaviour

The problem with Ti alloys is that the endurance limit can be very variable depending on process history and in the early days of using this type of material a very cautious approach was taken and some Ti rods were given around a 40 hour life in Race Engines.

Modern process control has generally led to an accpetance that Ti does have a Fatigue Endurance.

There is also a design concept of 'Fatigue Life' (S'n) used for a range of alloys which can also be helpful and related to the Ultimate Tensile Strength of a material (Su). It is fair to say that S'n is a pseudo limit and these material will eventually fail if loading is continued beyond the lives indicated.


Ferritic Steels S’e = 0.5 Su where Su < 1400mpa

Ferritic Steels S’e = 690MPa where Su > 1400MPa

Titanium Alloys S'e = 0.5 Su

Cast Steel/Iron S'e = 0.4 Su


Magnesium Alloys (10^6 cycle life) S'n = 0.38 Su

Nickel alloys (based on 10^8 cycle life) S'n = 0.35 Su ->0.5 Su

Copper based alloys (based on 10^8 cycle life) S'n = 0.25 Su ->0.5 Su

Wrought aluminium alloys up to 280 MPa (based on 5 x 10 8 cycle life) S'n = 0.38 Su

Cast aluminium alloys up to 350 MPa (based on 5 x 10 8 cycle life) S'n = 0.16 Su

This data was established from simple Whoeler Rotating Bend tests (fully reversed loading) where typically 6 specimens would be used to examine each condition and this data assumes a survival of 50% of the test pieces.

The type of loading also has an influence on fatigue life and is basic engineering steels are considered:

Approximations for endurance limits for three types of loading for steel are as follows

Bending S'e is about 0,5 Su
Axial S'e is about 0,45 Su
Torsion S'e is about 0,29 Su

Any environmental influences may reduce these values significantly and would need to be considered.

I think that in general terms a 4340 material would have a tensile strength of around 1000MPa (145ksi)

A 6AL4V Ti would be similar in strength at around 1000MPa.

I think that this means that the stresses produced for a given cross section would be lower in the Ti rod than the steel rod so for a given fatigue condition Ti may be superior ?

The stresses shown on the FEA plot on the R&R website seem to be a maximum of 55 000ksi whic is well within the operating range for either material and give a reasonable degree of confidence. I would be more concerned about bolt or lubrication failure.

Steel Connecting Rods.

Certainly the Honda NSX has used Ti rods in production for some years and I believe without problems.

[Flieger]

I was too lazy to write all that.

[rgoodrich]

Wow. That's more data than I expected! Ok, they're strong enough. Do they really rev quicker, compared to a steel allow like R&R?

[ONQRACIng]

We run R+R on our GT3 cranks, very nice rods... A+ for the $$

[Weissach911]

Originally Posted by rgoodrich

Do they really rev quicker, compared to a steel allow like R&R?

A typical Titanium Rod is normally about 30-35% lighter than an equivalent steel rod and this must reduce the inertia of the rotating assembly.

For a given torque the angular acceleration produced will be greater for a lower inertia so the engine should increase its revs more quickly with a lighter rod.

It is also fair to say that it will also slow more quickly on a trailing throttle.

To work out the amount of change would need some calculations to be made but as this type of rod is becoming more common in high end race engines there must be some benefit.