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Flywheels - cast iron v steel

Flywheels - Cast Iron v Steel

Before you start reading this article please bear in mind the fact that the owner/builder of a Tractor Puller is liable for any injuries or damages caused in the event of an accident!

Italian flywheel/clutch failure 130908

Click on the image to view the video. Click here to see the second video. There is a video taken from an alternative angle on the same web page.

At the time of writing (September 2013) there have been two catastrophic flywheel failures in Italy so this article is not scare mongering, just giving you the cold hard facts.

Oh, and another one for you to think about. Nearly 1,000 bhp being pushed through a 'grey' (cast) flywheel.

John Deere Flywheel Fail in Canada

2017 - John Deere in Laurierville, Quebec, Canada

A broken cast flywheel

Don't bother to try a jigsaw on this one. The rest is somewhere out on the track...

Why do cast flywheels fail? Three very simple reasons: -

I am using the Ford 380 as an example, not because there is anything inherently wrong with them but firstly because numerically they are the most common and secondly because I have all the facts and figures to hand. The first thing to bear in mind is the age of the casting. ‘D’ series 380’s were manufactured from December 1969 to March 1981 and the Cargo from March 1981 to January 1993 therefore the oldest 380 would now be 44 years old and the youngest 20.

If you know the Model Series number of your Ford lump, have a look at this table to see when it might have been made. Dependent on whether it was normally aspirated or turbocharged the red line would have been somewhere around 3,000 rpm so if you were to run one at 4,000 you are now working it 33.3% beyond its design limits, however just for your information, stress rises as a square of the rotational speed, which means this figure is actually 77.6889%.

Just for the record, at 4,000 rpm the centrifugal force being exerted on the circumference is 2.54922 Newton’s (Kg-metre/second²) or 5.73087 lbs-ft/second² For a 20-year old casting that’s bad enough but if the diesel is over 40-years old then catastrophic failure is inevitable. Click here for the calculation tool.

A Ford 380 sitting in a truck is only subjected to normal loads while accelerating up through the gears however when you stick it in a Tractor Puller the loads become extreme and totally abnormal. For example, trying to pull in too high a gear or all the heat generated when the clutch is slipping with the engine running flat out! Two prime reasons the cast flywheel will cry enough and disintegrate.

Now let’s have a look at the part metal fatigue plays in cast flywheel failures…

What is metal fatigue? Metal Fatigue occurs when the metal is subjected to repeated or alternating stresses below the material’s static yield strength. Fatigue failure occurs BELOW the material's ultimate tensile strength. Parts that are exposed to alternating stress cycles, such as engine crankshafts, may break even though they were never stressed near their ultimate strength. How does one know if a part is close to fatigue failure?

Fatigue life is determined by the number and magnitude of the stress cycles. Fatigue cannot be inspected - unless you know the past history of a part, there is no method of determining how many stress cycles and therefore how close to fatigue failure the part is at. (source ‘Metal Fatigue, Cracks, and Turbo Mallards’,

Unless you’ve owned the truck the engine came out of from new, then you have no way of knowing how well or how badly it was treated and just how much abuse has been thrown at the flywheel. The one thing you can be sure of is that the casting has been repeatedly subjected to a tensile, cyclic load and vibration in the normal course of event with acceleration and deceleration and the effects of combustion and all this time the time bomb is merrily ticking away. Now if you add the extreme loads put on it by Tractor Pulling you can understand why they fail.

Cast iron (4.5% Carbon) has a yield strength of 130 MPa (megapascals) and an ultimate strength of 200 MPa. Mild steel has a yield strength of 248 MPa and ultimate strength of 841 MPa. (source

S355 (EN50B) steel plate is a high-strength low-alloy European standard structural steel used to produce steel flywheels which has a yield strength of 355 MPa and an ultimate strength of 630 MPa, or 173.2% stronger than the original. (source

The ETPC Rule book states that a 13” steel flywheel can be run at up to 9,000 rpm and I’ve yet to see a Ford 380 spin up quite that fast.

A competition steel flywheel

If you look at the Specialised Links page of this web site you can find a number of people who can supply steel flywheels however if you decide to make one yourself or get someone you know to do it make sure you have the all the paperwork for the steel and don’t forget to check your flywheel and clutch yearly. You are the person who is liable if something goes wrong.

If you're still stupid enough decide to stick with your cast flywheel, for God's sake do not exceed 2,700 rpm. You think I'm being harsh calling you stupid? Please wake up an smell the coffee! If you decide to (probably) amputate you toes, that's entirely your choice BUT if the debris from the exploding flywheel hits either members of the track staff or the spectators, two things will happen: -

Further reading

'Design of Flywheel' (Reprinted from Design Data, PSG Tech, 1995) -

'High speed flywheel design' - Tobias Kamf, Department of Engineering Sciences, Uppsala University.


Phil Cavanna and Steve Cox, SWTPA

♣ Note: The reason we have been so fortunate in this country is because if the majority of Independent Pullers suffer an engine failure with a Ford 360, 365 or 380 rather than rebuilding it they simply replace the whole lump with another one gleaned from a commercial Recycling source.

Related Links Clutch Shatter Blankets (UKTP)
Useful source (converts RPM for a given diameter into Linear Velocity).


I am indebted to Onky (Garry Ankers) for these three photos he posted on Facebook on the 23rd February 2014. He said: -

"Here's a very good reason why you shouldn't run a cast flywheel. The crack has gone right through.

That was out a road going truck that revs no more than 2000 rpm imagine that it a tractor reving at 4000 rpm the result could be fatal !!!!!"

I rest my case m'lud...