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Front crank pulley and fan extension


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Hi one and all 

I know this has been covered many many times and I have been reading all the appropriate posts and supplied information from way back concerning the front pulley and fan extension acting as a crank damper. However the old grey matter is finding it hard to understand how the Triumph set up works. All the crank dampers Ive come across over the years are constructed of crank boss / rubber bonding / v pulley  and one of the service points we carried out was check rubber bonding on crank pulley every 6000miles. So question is where is the rubber bonding on the 4a pulley assemble? surely not the 8 rubber fan mounts?   The reason I ask is I have a non damped narrow belt conversion with the existing fan assembly . If my understanding is that the extension is just to carry a fan closer to the radiator I will remove it and keep the existing undamped pulley (electric fan is already fitted) unless anyone can enlighten me further

Chris

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Hi Chris,

The 4 pot engine (TR2-4A) does not damp the crank using rubber to absorb vibration from wherever.

It is suggested that it does it by increasing the mass. This supposedly modifies vibration and its harmonics to make them 'harmless'

Some owners also suggest that if you remove the extension that the crank will break due to torsional loads.

The crank does and will break given time but torsional loads are not the cause and the removal of the extension didn't kill it.

My engine has done  +300,000 miles 

the first 89,000 miles with the extension.

Then 170,000+ miles without the extension

And now 50,000+ miles on a new prototype crank without the extension

No cracks in either crank.

You do not need a damper and the extension can be removed for normal driving  in my view.

 

Roger

 

Edited by RogerH
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The extension is just meant to bring the fan closer to the rad, there's no other explanation to be found. I strongly suspect that this is why it's called "fan extension".

Edited by Geko
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Hi Roger and Stef

Thanks for the replies. These were my thoughts as well unless the damped pulley has been specifically designed for the appropriate crank its fitted to its almost as good as not and Im sure the ones sold in the kits haven't been thoroughly tested and designed for the cranks they are suppose to fit so in my opinion and only my opinion maybe do not give much benefit.

Chris

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You may recall my article in TRaction (Issue 320, March/April 2020, "Crankshaft Torsional Vibration").   To read it, see https://www.tr-register.co.uk/tr-action-index and enter "Torsional Vibration".

I studied the crank dampers on Triumph six cylinder engines, but in doing my research and demonstrating how I can measure this vibration and the effect of damping, I learnt of the controversy about earlier T4s and their fans.    That discussion seemed to me to be mostly opinion,  to lack actual data and to be sorely in need of some measurement!    I made contact with a TR4 owner who allowed me to take the dimensions of this fan and extension, to design bracketry that will temporarily attatch my sensors to such an engine.     Sad to say, my contact has ceased, I know not why, and I need to find another TR3/4 owner who will let me measure the vibration on his engine's crank.

I live in Lancaster.    I will be most grateful if anyone who would be willing to join this project, and who lives within a  'reasonable' distance, would please PM me?   I will be glad to explain and discuss how, without any permanent changes to the subject engine, I hope to provide objective data on this subject.

JOhn

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Hi John

unfortunately your too far from me but that would be an interesting experiment to conduct on a tr4 however as I understand the 2s and 3s have a shorter fan extension so surely if the extension and fan form part of a crank balancer/ damper shouldn't they all be uniform as they fit to the same engine all be it in different models? 

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Thnak you, trchris!   I wasn't aware of this difference, but it would be good to know how even one model behaves.

I have great respect for a Neil Revington opinion, but his is based on experience, not evidence!

John

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If  the whole engine, flywheel and clutch assy has been balanced then type of pulley fitted without fan extension should make no difference. However with an engine of unknown balance the result must be luck of the draw which could be why some cranks break and some don't. Surly to go to the expense of rebuilding an engine and not getting it balanced is false economy, they certainly run much smoother when they are. 

Chris

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3 hours ago, john.r.davies said:

Thnak you, trchris!   I wasn't aware of this difference, but it would be good to know how even one model behaves.

I have great respect for a Neil Revington opinion, but his is based on experience, not evidence!

John

Is it relevant that the TR2-3-3A (upto about 1959) flywheel with the shrink on ring gear is lighter by about 20% than that used on the later TR3A-4A which had the bolt on ring gear.?

Old sidescreen 'wisdom' was that there is a 'crank snapping RPM' around 4200 - Which if the engine were held at for sustain periods would cause failure - As 4200 rpm is over 100 mph in o/d top I never managed to sustain it.

I removed the mechanical fan from my TR in 1975 and have not refitted it.

Peter W

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There was also a theory at one time that crank breakages were caused by poor use of overdrive i.e. dropping it out on a trailing throttle.

Stuart.

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14 minutes ago, stuart said:

There was also a theory at one time that crank breakages were caused by poor use of overdrive i.e. dropping it out on a trailing throttle.

Stuart.

That could certainly cause a bang and jarring on the all the parts in the drive train from front to back.

 

Roger

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Thanks all for the input and comments I agree with the consensus that it makes no difference if the extension and fan are there or not and theres no need for a damped pulley unless pacifically made for the TR2/3/4 crank  the benefit is minimal if any  

Chris

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Just now, Malbaby said:

IMHO.....fitting a harmonic balancer is good insurance. I have a non TR one fitted to my engine.

Hi Mal,

where is the logic in that.  What frequencies are you trying to damp.

Just more things to go wrong.  The rubber in the TR6 dampers goes soft and the timing marks move.

 

Roger

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13 hours ago, Malbaby said:

IMHO.....fitting a harmonic balancer is good insurance. I have a non TR one fitted to my engine.

There is only any point in fitting a harmonic one if you had it balanced along with all the rest of the rotating components.

Stuart.

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And if the balancer is tuned to the frequency(s) you wish to dampen.  Will a harmonic balancer from an MG do the job on a TR?

I have no mechanical fan and no harmonic balancer, nor does Neil Revington on his 4-pot TRs.

Ian Cornish

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Raising this controversy again has shown that, despite the erudition of our learned and knowlegeable co-members, they rely on their expert opinions!  No data!

I've had one volunteer(thank you trchris!) but we live far apart.   Is there no one in the North West of England or Southern Scotland who will help me measure torisonal vibration in these engines?

John

Edited by john.r.davies
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For what it's worth I've also ran my TR4 without the mechanical fan and extension for over 10 years, with no problems.

The only difference of note is that the engine runs a darned sight smoother.

Cheers,

JeffR

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