Has anyone any comments on the article in the 2006C Austin Seven Clubs Association Magazine on crankshaft failures? (page2&3)

What are your thoughts on the matter Ken?

Cheers

Ruairidh

Hi Ken,

I have many mixed thoughts about Nick Beck's article on his flexible flywheel.

I have also observed the flywheel reaching a vibration pitch whilst reving up the engine which could indicate that the flywheel is out of balance. I would be interested to know whether this still happens where the flywheel and crank have been dynamically balanced.

In a 'normal' engine ( one with three or more main bearings ) the crank is not allowed to flex as much as in a two bearing engine; the flexing of the crank must alter the axis of revolution of the flywheel as they are physically joined. Therefore, the question is does the flywheel 'vibrate' because it is out of balance or does it appear to do so because the crank is bending? ( or of course BOTH )

I know Nick has been using this modification for many years with success in that he hasn't broken a crank since, even with his high mileage. But there are many others who also do a high mileage and have not broken a crank either.

Having posed more questions than I am able to answer I would make one comment in favour of Nick's flywheel. A lot of noise is generated by the flywheel 'flapping' or vibrating ( no matter what the cause ) therefore Nick's modification will reduce engine noise no matter what else it achieves in crankshaft survival.

Perhaps if I go down the same path as Nick I will be able to have my radio and CD player in my 12 volt car!

I jest ! ( I think ! )

Hi Malcolm and all,

I will attempt answer some of your questions. I haven't read what dad put in the grey mag but here goes.

Any shaft is a spring-mass system and as such has a series of natural frequencies.

The seven crank goes into its first harmonic causing the stroke on 3 and 4 to get longer (pistons hitting the cylinder head on a high comp engine).

The gyroscopic action of the flywheel counteracts this and causes the vibration and bending load on the end of the crankshaft which causes the crank to fatigue.

The stiffer the crank the higher the speed at which the shaft goes into the first harmonic. This is the reason why the 1 1/2" cranks are much smoother running than the thinner cranks in the normal operating range

Lightened flywheels have a reduced gyroscopic action for a given speed so will cause this vibration at a higher speed. At this higher speed the vibration causes higher stresses in the crank is at a higher frequency, this dramatically reduces the fatigue life of the crank.

The floppy flywheel allows the rim of the flywheel to run true and the crank to whirl without the vibration and so removes the fatigue load.

A dynamically balanced shaft is smoother up untill the crank goes into the first harmonic. Balancing will not prevent this.

The floppy flywheel does make the engine dramatically smoother and quieter. The numb hands and bum usually caused by travelling in a seven for more than an hour are also prevented.

I know dad has had quite a few phone calls etc asking for more details on the design of the flywheel so there may be a few more around before long. I will be making one for my own car soon. It will be about MK12 i think, there have been lots of changes from the first one!!

Regards

Rob

Hi rob

Very interesting information thank you.
I have read the article and admit to being keen to find out more.

Lots of modern engines have "harmonic crank balancers" I believe some of them take the form of a large front pulley, some of which are made in two pieces joined with rubber? Mounting the flywheel as described would seem to be a logical use of this idea.
I note that you or your father have made a few before getting it right? I'm quite sure you know how dangerous a flywheel can be! We used to lighten flywheels were I once worked and were always carefull not to weaken the flywheels structure!
Another thought I had concerns the "sprung mounting", which must have its own frequency at which it would vibrate? I guess if the spring steel mounts were the wrong thickness you could arrive at a resonant condition which would make matters much worse!!!

Regards

Steve

Hi Steve,

You are right about the spring connections giving rise to a new natural frequency. But the spring leaves that I use are so light that the natural frequency is below any of the A7 running speeds so the flywheel 'centres' itself and does not flap about.

Nick

Nick,

Thanks for the information re harmonics, I think I can just about get my mind around it. I must admit the first time I saw an article about you dad's flywheel ( can't remember where it was now ) I thought it showed a lot of merit and if my lathe had a bigger throw I would have given it a go but my lathe is just too small.

Does you dad intend putting together some technical notes? I would love to see the detail on the fixings of the springs. Was any thought given at any time to using an A 7 clutch plate as this is made from spring steel?

When you next speak to your dad give him my regards, we had a very pleasant Scotish holiday with him. I hope he didn't get too wet going North from Applecross !

kind regards
Malcolm