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I know there are some articles on the internet regarding whether a billet or forged crankshaft is better for a racing application, but was wondering what people's opinions and experiences were here? I'm either looking at either a forged crank from Farndon Engineering or a billet crank from Arrow Precision. The crank is 1275 standard stroke. I know the Arrow crank has lightening holes drilled in the rod ends to help counter balance. They are both about the same price. Thanks

Mini Spares say Forged is better?

http://www.minispares.com/product/Class ... .aspx|Back to shop

A quality Billet item with no stress raisers from poor design / machining would I am sure be very similar....

I'm no expert, but from my understanding of metallurgy:

Both use the principles of deformation-hardening, where the cast or raw material is highly deformed , causing "locks" in the crystalline structure of the metal. Alloying also makes locks by inserting different sized or shaped molecules within the crystalline structure.

Billets are a casting of a large chunk of metal which is usually rolled into the final "billet" shape. From there it is machined to specifications. The rolling process is done at a temperature lower than recrystallization temperature, ensuring the structure locking. If it is rolled in two or three different directions, the material will be very "locked", very hard and very tough.

Forged cranks are highly deformed during the actual shaping process where a piece of previously rolled (usually in two directions) metal is cold shaped (less than recrystallization temperature) by bending, twisting, squashing.

Either way, the final product is nearly equally deformed and equally hard and tough. The difference would then be in potential large flaws or crack starting points. These will more likely show up on the forged crank simply because the surface is mostly "as forged" rather than machined. But a good forging will not have any surface flaws, at which time it gains the advantage of the surface not cutting across the granular structure of the metal, where intra-granular-voids are often found. These would be found nearly everywhere in machined billet parts, but the voids are small compared to a forging flaw.

So I guess, the forging would be better, but check for flaws (cracks) first before accepting the part.

If anyone would like to add / correct any of this, feel free.

There shouldn't be any debate, a forged component will ALWAYS be stronger & more resilient that one machined from a billet.

That is not to say that billet components can't be well up to the job, they certainly can be.

M

My understanding is that the molecular structure is altered by rolling/whacking.

First of all, you have to compare like with like. So, if you have a billet which was treated to X by a physical process, you have to treat the same crank with the same pressure in order to alter it's molecular characteristics to a similar degree.

By using physical processes, you can alter the crystal structure of a steel in a way which makes it very strong. So, a super billet will be able to be machined into a very strong shaft. If you take a bit of steel and "forge it", you are essentially applying a similar physical process to it which in the case of forging creates a special sort of grain which enhances the tensile strength. This isn't so much of a recrystallisation process, but a sort of "compacting process". The thing is moderately plastic and is hammered into shape which gives you a superior tensile strength compared to an equal unidirectional billet.

You can get very high grade billets which are superior materials for other purposes, but forgings appear to be better in practice for what they are. I suppose the only way to provide a definitive answer would be to destructively test many of them.

Personally, my preconception and partially informed opinion would lead me towards the farndon.

Failing that, you could see where titanium bronze could take you. (I read about it in a very arcane engineering book)

i cannot really comment on what a farndon cranks costs / looks like nowadays, but the cranks that arrows do for minispares are a lovely thing

Are you sure the Farndon crank is an EN 40B forging ?

Thing that does need to be considered here, is the 3 bearing crankshaft........... A billet will allow a very small amount of flex, but a forging won't.

251 ENG wrote:Are you sure the Farndon crank is an EN 40B forging ?

Sorry, you are correct, after speaking with Farndon, the crank they have is billet crank as well.. for some reason I always assumed that the Farndon cranks were one in the same with the "Paul Ivey" forged crankshafts. Are Farndon and Paul Ivey associated? I guess I'm confused. Who are the current manufactures of forged cranks?

thanks for the info

Think it's more to do with the process, which iirc isn't allowed now thanks the tree huggers..........

I thought the nasties were in the whatever it is surface hardening. Probably nitriding,

Smiffy wrote:Thing that does need to be considered here, is the 3 bearing crankshaft........... A billet will allow a very small amount of flex, but a forging won't.

One must remember that ALL things flex. Which btw is usually better than breaking. And personally, of course without actually testing, I would think that with essentially similar materials, flexing should be similar.

edit: I have a strong suspicion that the materials, though similar, are in no way like. Alloying, degree of deformation, consistency of deformation, linearity of deformation, critical point dimensioning etc all play their part. Billet is linearly deformed (maybe to a couple hundred percent*) and then machined, where forged is probably deformed maybe 50% while rolling to a cylinder (rod), again while bending and twisting to make a rough crankshaft shape maybe 50-80%, then again when squashing to make a final shape, maybe 30%
*Rolled in one direction, then in another direction, then lengthwise.

Though dimensioning (how thick are the webs) has a large effect. But along with thicker webs comes more rotating inertia and "slower" acceleration. (minutely)

In the shed wrote:I thought the nasties were in the whatever it is surface hardening. Probably nitriding,

EN40B is considered a nitriding steel, and they may well use that process, but it limits the usability of the crankshaft. Surface hardening by inductive heat-hardening is better with a depth of around 2mm. Nitriding is cheaper and harder but only goes to about 0.4mm making a regrind no longer an option.
Some other auto manufacturers have done this. The result is older motors are no longer economical to repair for a high quantity basis.

"Nasties" however are unlikely to be found there, but in transition areas (sharp bends). Or in the case of Billet, internal flaws can be exposed by the machining process at any place on the crankshaft.

guru_1071 wrote:i cannot really comment on what a farndon cranks costs / looks like nowadays, but the cranks that arrows do for minispares are a lovely thing

Are you saying Arrow are the ones forging currect Minispares cranks?

What about Paul Ivey - is he still doing cranks?

sandman wrote:
Are you saying Arrow are the ones forging currect Minispares cranks?

we dont get 'arrow' printed on them for a laugh......

they are doing the work on the forgings we supply them

Frogeye61 wrote:

Smiffy wrote:Thing that does need to be considered here, is the 3 bearing crankshaft........... A billet will allow a very small amount of flex, but a forging won't.

One must remember that ALL things flex. Which btw is usually better than breaking. And personally, of course without actually testing, I would think that with essentially similar materials, flexing should be similar.

A good point. A crankshaft does flex and twist in normal operation. The crank should be designed such to withstand these as a matter of normal operation. To build a crank that doesn't flex or twist will fail in a very short space of time.

LOL who's seen a billet snap ? I've seen at least 3 snapped en40b cranks.........

guru_1071 wrote:i cannot really comment on what a farndon cranks costs / looks like nowadays, but the cranks that arrows do for minispares are a lovely thing

Like this?

Gorgeous!

Vegard wrote:

guru_1071 wrote:i cannot really comment on what a farndon cranks costs / looks like nowadays, but the cranks that arrows do for minispares are a lovely thing

Like this?

Gorgeous!

the current ones dont have the larger counterweights as they are made using the 'blank' std forgings which wont accomodate them, they are still back drilled and done to a very very high standard though

Smiffy wrote:LOL who's seen a billet snap ? I've seen at least 3 snapped en40b cranks.........

Yeah, I've seen quite a few, some because of harmonic damper problems.

Spider wrote:Yeah, I've seen quite a few, some because of harmonic damper problems.

Anything can go wrong if other factors aren't right.........