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Crank damper survey


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Posted Today, 01:23 PM

Earlier this year, I asked people to contribute to this survey, and I can now publish the results.

Rather than take up everyone's bandwidth, I've done so on Sideways, at: http://sideways-tech...damper-pullies/

Turns out that particular forum has some odd restrictions, so you may not be able to reply there, so I started another thread at ST, for people to do so: http://sideways-tech...damper-pullies/

Or, of course, here!

Looking forward to plenty of criticism/comment!


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So mnay contributed to the survey, and ther was so much interest - has no one any comments?

Here's the whole thing:


A reciprocating engine is subject to different types of vibration.    The crankpin is inherently unbalanced, so web extensions in the opposite direction to the journal can achieve static but not dynamic balance, as the shaft spins and journals in different planes cause a twisting moment about the shaft.

The crank will also be subject to primary and secondary vibration due to piston movement.    The piston accelerates at each end of its stroke, reacting against the crank, and that motion is faster at the top than at the bottom of the stroke, so that secondary vibration occurs twice as fast as primary.   

The straight-six engine is the equivalent of two three-cylinder engines with all those forces opposing each other, so that it is uniquely smooth running, but the longer crankshaft allows torsional vibration to become important.     As each piston descends in the power stroke, against the load or just the inertia of the rest of the drivetrain, it bends the journal on the main bearings of the crankshaft, which rebound and oscillate in the same way as a struck tuning fork, at the ‘natural’ frequency of the part.

A vibrating tuning fork will cause another of the same size to resonate, at the same frequency as the first.    If the input frequency into the journal is the same as its natural frequency, then it will resonate, amplifying the vibration.  Resonance can shatter a wine glass at its resonant frequency, and there may be several “critical speeds” at which a crankshaft will resonate with the frequency of firing strokes.

 Torsional vibration dampers absorb the energy of vibration and dissipate it as heat.  There are many different designs, which depend on:

Viscous friction

Solid Friction

Fluid viscosity

Tuned elastomer mass dampers

The last is most commonly used for production car engines, as it can be made cheaply, is relatively light in weight and requires no maintenance.

Survey of crank damper pullies

A survey was devised and placed on the website Survey Monkey.    It was publicised via websites and message boards in the UK and America, and on Facebook pages, all devoted to the Triumph marque.    Users were invited to visit the survey and complete it.

The purpose of the survey was to find a denominator for the incidence of failed crank damper pulleys in Triumph engines.     To promote compliance, it only contained five questions, four factual and a last question on their opinion if the broken crankshaft was due to the faulty damper.


The Survey ran from the beginning of January to the end of February 2018.   A total of 207 owners responded to the survey.




Q1. Duration of ownership




Q.2. Models owned


Q.3.Faulty crank dampers seen




Q.4. Known fractured crankshafts


Q.5 Opinions on cause of crank failure




Q.6. Free Text

This unstructured text box allowed responders to write what they liked.   109 responders offered opinions, difficult to categorise, but showing several had owned more than one Triumph for many years, rebuilding up to seven engines, all without any sign that the dampers were faulty.     A rebuild involves finding TDC, to time the camshaft and ignition, so a faulty damper would be found if the builder compared that with actual piston position.

Several commented that the outer edge of the rubber in the damper showed signs of deterioration, being swollen and cracked (see title page).    Some had replaced the damper, either by having it rebuilt, with another, better looking damper or with a non-Triumph damper pulley.    One had shaved off the swollen cracked edge, found the rubber beneath in good condition and used it again, with no apparent ill effect.

Three commented that they had seen broken crankshafts, one twice, but they were in racing Triumphs.     Another had seen one while the car was still under the manufacturer’s warranty.     The last had seen one in an earlier Triumph, a TR3A, but that was an earlier, four-cylinder engine with no damper.

Others commented on seeing faulty dampers on other cars, including the Volvo B21 red-block engines, Ford F150 V8 and Triumph Stag.    



The owner who had seen a crank broken under guarantee thought that this might have been due to a faulty damper, but this seems unlikely so early in the car’s life.  It is more likely to have been faulty manufacture of the crank, not deterioration through age.

The survey showed that nearly two thirds of responders had owned one for more than twenty years, so their experience may be expected to be typical.  However, they owned a range of those vehicles that is not in line with the known production figures:


% Produced of Total

% Owned in last 20 years

Saloon 2L/2.5










The attraction of a “British sports car” as a preservation project in the TRs and less so in the GT6 and Vitesse models contrasts with how few of the saloons have survived.      This, however is not relevant, as all these cars had very similar viscous rubber mass dampers, only varying slightly as the 2.5liter engines had wider pulleys

Of those who responded, there were 41 (20%), known crank damper failures, and 32 known crank failures (16%), but only 12% of responders considered the damper failure as the cause of the crank fracture.     They were not asked to amplify what other cause might have led to such a major malfunction, but despite the obvious association there are many other potential causes, and a list would have been only confusing.

The opinion of experts, while not contradicting the experience of owners, contradicts each other and is not helpful in this matter.  The statistics must decide, and that a fifth of the owners of surviving cars had known of a failed damper is a large proportion.   The concern of owners on this problem is justified.

However, the worried owner has no method of resolving his concern.    Inspection of almost any used Triumph damper will show apparent deterioration of the rubber, but four out of five will still function adequately.     A method of testing dampers is needed, to reassure owners, or to demonstrate that the expense of a new damper is essential.   


This project justifies further research to discover such a method.

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