[Fot] FW: Flywheel wt & crank harmonics -TR3/4

[Peter Vucinic]

Hi Tim,

Good explanation. And 100% right on the effects of the flywheel weight. Not to discount the pulley at the front of the crank and its effect also.

 

What you are feeling at around the 5200 mark is the 2nd harmonic of the crankshafts natural frequency. The 2nd harmonic is usually the most destructive harmonic and this is the crankshaft bending the most during rotation. Usually, but not always around the mid distance crank. But can be around the 2nd crank journal in the 4 Cyl engine. You will see that a lot of people will fit billet, machined, large main bearing cap around mid-crank length as they have had issues with either bearing ‘wipe’ of broken cap bolts. They are trying to counteract the 2nd harmonic by making the bearing journal stiffer. 

 

You don’t need to complete a complex calculation to understand what RPM’s you should minimise holding your duration at. The crank is suspended and ‘struck’ and the natural frequency that the crank vibrates/resonates at will give you it’s harmonic characteristics. Just like a tuning fork. This is the basis for setting your RPM’s and multiples of that RPM to stay away from. OR you need to reduce the effect at those RPM’s. Say, using a harmonic balancer design to reduce the harmonic effect at certain RPM’s.

 

Just as Joe Alexander describes, the first harmonic is in the 2800 RPM range, but it is the least destructive so you almost miss feeling it.

 

Hope that helps?

Have a great day

 

 

"You can Make a Small Fortune From Motor Racing. As long as You Started With A Lage Fortune!"

Kind Regards

Peter Vucinic

TR4 - TR7 V8 - Spitfire MkII

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From: Fot [mailto:fot-bounces at autox.team.net] On Behalf Of timmmurphh--- via Fot
Sent: Friday, 22 May 2020 9:08 AM
To: fot at autox.team.net
Subject: [Fot] FW: Flywheel wt & crank harmonics -TR3/4

 

 

 

From: timmmurphh at gmail.com <timmmurphh at gmail.com> 
Sent: Thursday, May 21, 2020 6:06 PM
To: 'TeriAnn J. Wakeman' <tjwakeman at gmail.com>
Subject: RE: [Fot] Flywheel wt & crank harmonics -TR3/4

 

The weight of the flywheel will definitely affect the harmonic frequency.  The heavier the flywheel, the lower the harmonic frequency will be.  The flywheel contributes a significant amount to the total rotating inertia due to its weight and that the weight is at greater radius than the other rotating masses.  Inertia increases as the square of the radius.  It is not a linear function of the radius.

 

An RPM of 5200 was mentioned as the harmonic resonance.  I had always heard that it was more like 6300 RPM.  I have never seen a calculation for it.  It would be complex.  Note that lighter pistons and connecting rods would tend to lower the harmonic frequency as they would reduce the inertia in the engine.

 

Tim Murphy

 

From: Fot <fot-bounces at autox.team.net> On Behalf Of TeriAnn J. Wakeman via Fot
Sent: Thursday, May 21, 2020 2:44 PM
To: FOT <fot at autox.team.net>
Subject: [Fot] Flywheel wt & crank harmonics -TR3/4

 

Something I have wondered about for years.

Does the weight of the flywheel affect the RPM and intensity of harmonics inherent in a TR3/4 stock crank? I've never found an answer to that question.

TeriAnn

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