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PostPosted: Wed Dec 12, 2012 3:21 am 
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Something i did for the gt4oc so thought i would share it with you guys as well :)

I’ve been saying this for countless years to my customers not to fit the alloy crank pulleys in replacement of the standard Damper Pulley. Toyota did this for a reason and I’ve seen countless failures of the crank and quite a few with my own 2 eyes but up till now I have not recorded a failure, just did the repair as instructed. Well I had a customer come to me with a knocking coming from the bottom end of his GT4 ST205, the knocking sounded very much like a bigend rattle so I advised that if this was caught early enough we could drop the sump and as long as the crank was in perfect condition and the journal measurements came within specs and a plasti-guage final check we could get away with re-shelling the conrods, which basically means fitting new bearings with the crank in-situ and just bolt it all back up. If done correctly this is a perfectly adequate repair and will give many years of trouble free motoring. Conrod bearings will wear because of a few reasons, Age and power is the main one which is the conditions that give the most successful repair of this nature. The other is foreign material or metal entering the bearing or lack of oil, these will typically damage the crank which requires a bottom end partial rebuild.

Upon inspection of this car I noticed it had an alloy crank pulley fitted (pictured below)

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The sump was dropped and the bearings were inspected, #1,2,4 were all in perfect condition. #3 was in a terrible state, extensive wear and a deep grove down to the base metal of the bearing. This was the cause of the knocking. The crank was inspected and seemed in good condition but found a small lump on the top of the journal which you could just feel with your finger tip, at the time it was facing the top of the engine so I was feeling blind. Believing at first to be a part of the bearing stuck to the surface I rotated the crank to give a better view but found this damage instead. There was a fine Hairline crack which ran almost 3/4 the way around the bearing journal of the crank, and in one part the surface had pushed upwards creating that small lump felt earlier, and the cause of the deep grove created in the bearing. Damage pictured below

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The job was completed with a new set of bearing throughout, a replacement crank that was rebalanced, journals polished for clearance and rebuilt. The alloy pulley was also replaced with a factory item and the customer collected his car, Which had an initial fire-up and running-in by my to set the bearings and 300miles of normal driving before replacing the oil and going back to normal full boost operations.

The car this happened to only modification was an exhaust system; boost was even standard so power wouldn’t have exceeded 260bhp at the most. Yet the crank had failed. To put this into perspective we’ve done drag cars running over 900bhp with standard cranks without a single failure or even bend. Some models for testing were also extremely machined to remove any metal we could for revablity, they were machined like pieces of paper and at still exceeding 900bhp still not a failure. All these were fitted with the standard damper pulley. Yet this example failed at 260bhp with a pulley that could control the harmonics which had lead to its premature failure.

The reason behind the failure was Harmonic Resonance Failure. Without getting to in-depth I’ll keep the explanation simple but there are a few good examples for the technical minds amongst us on wikipedia.
http://en.wikipedia.org/wiki/Resonance
http://en.wikipedia.org/wiki/Harmonic_oscillator

Basically everything in the universe vibrates, whether it is the human body, sheet of metal, or in this case a crank. This is called its structural resonance frequency. Now this frequency is best expressed as a wave, with its peaks and troughs. And if you can generate an additional wave with the matching frequency (so the peaks and troughs match) the amplitude of that wave will grow, or the item will vibrate with more force, it’ll keeps growing until it structurally fails on a molecular level. To put this into everyday context you need to picture a swing with a fat man on it and your trying to push the swing with your little finger, Normally this isn’t going to happen but if you can get the swing started adding the small amount of force at the correct period, just at the peak of the swing you can over time make the swing oscillate higher and higher. This swing is the oscillation frequency and at the top of each swing is the peaks of the wave form and the base of the swing would be expressed as its trough. Now if you continued to keep adding this small force at the top of each swing the arc would eventually get higher and higher until either the swing broke or the fat man fell off, this would be the expression of the structural resonance failure. To stop this you need a simple damper that will effect outside frequencies (in our case the engine) or crank rotation from matching the frequency of the crank resonance. Going back to our fat man on the swing the damper would be effectively stopping the outside force, (you with your little finger) pushing on the swing at the correct time, for example before the swing meets its peak point, This would do nothing to aid the swing and certainly wouldn’t add force the its momentum making it arc higher and higher.

This is what the Toyota standard Harmonic Damper Pulley does, it removes harmonic resonance from the crank and additional frequencies causing the failures were seeing above.

For curiosity purposes I went all over the crank looking for more hairline cracks and found another around #2 journal leading into the counterweight but it was only very small, around 1cm in length but if left unattended would have grown like #3. So to see how far the resonance damage had gone I caused a controlled failure of the crank by putting in it a press until it snapped on the cracked areas. The pictures of this can be seen below. If you analyze the pictures you will see there are 2 different types of metal texture. The shinier, almost polished appearance is the damaged caused by the resonance; this is a failure on the molecular level and as you can see it quite extensive in #3 and starting to spread inwards on #2 just being visible on the outside edge. The other type is a more harsh texture, this was good metal until the press finished it off, this is typical of a pressure break where the cast material of the crank has basically shattered and all this was caused by me so ignore that. But it does show the 2 different types of damage, and how close this celiac came to complete destruction, where instead of a hairline crack and a rattley bearing the crank would have snapped pushing the rods out the side of the block casing and the entire engine unit would have been a total write-off.

Before people jump the gun and say “well I’ve got one and running 400bhp for years” this isn’t a situation where it’s a loaded gun at your head, where you fire you will die. It’s a situation where failures can/normally happen eventually, ranging from a matter of weeks to years, and from standard power up to 1000bhp. It’s a device which doesn’t address the base foundation of structural physics where the original Toyota unit does.

I hope this has enlightened people to the dangers of fitting an item like this, yes you will save around half the weight of the stock pulley but this really doesn’t add up to any noticeable gains in performance or revability and all its doing it radically effecting the longevity of the engine.

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Tim :)

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PostPosted: Wed Dec 12, 2012 9:41 am 
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A very interesting read tim. I can just imagine anyone with a alloy pulley running to their car and hastily tearing it off! Lol!

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PostPosted: Wed Dec 12, 2012 3:17 pm 
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alot of people on the OC are doing that actually.

its something i have warned against for a long time, but this time i had a car come in where i could document it properly. 260bhp st205 and this happened. Sure everyone would agree its not meant to do that lol.

Tim
TB Developments

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PostPosted: Wed Dec 12, 2012 6:19 pm 
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I would hazzard a guess that resonance is at an RPM that he used a fair bit, e.g. his normal speed on a motorway.

How much extra force did it take to get the cracks to break all the way through ? I'm guessing he's very lucky to still have a block and pistons.

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PostPosted: Thu Dec 13, 2012 1:55 am 
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as you can see it broke in 2 places. the first one i just dropped it onto the concrete from about 4ft high, just under gravity didn't really slam it down and that snapped. The other i used the press but didn't take huge amounts. Mine doesn't have a pressure gauge on the press but going from the amount of feedback coming through the pump handle it was less than is needed to push wheel bearings out the hub.

Yes very lucky, i've seen them before snap in situ and totally destory the engine, I just dropped the engine did a very simple bottom end build, new polished crank and factory pulley and put it all back together. Did all this for £550.00 so think he had a bit of a deal, despite the stupid deadline i had, basically came in thursday to reshell the bigend because of a slight rattle, found this and it drove away monday afternoon :)

Tim
TB Developments

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PostPosted: Thu Dec 13, 2012 1:32 pm 
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If a lightweight pulley can cause this, then how big is the chance this will happen with a lightweight flywheel?

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PostPosted: Thu Dec 13, 2012 3:14 pm 
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the front pulley is a damper and its the resonance that causes this. The flywheel standard or lightweight isn't designed to stop anything of this nature so your going to fine fitting those.

Tim

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PostPosted: Fri Dec 14, 2012 11:31 am 
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Great write up Tim :D

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PostPosted: Fri Dec 14, 2012 1:38 pm 
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Hi,

what Kris said 8)

greetz

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PostPosted: Fri Dec 14, 2012 3:10 pm 
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:)

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PostPosted: Tue Jan 08, 2013 11:41 pm 
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Celicagt2, its a torsional damper - the problem is not so much the reduction in weight as the loss of the rubber-suspended ring on the outside of the pulley. the crank oscillates in twisting, the mass on the outer of the pulley resists this but without the rubber connection the energy would simply build up in resonance. with the rubber the energy is lost in shearing the rubber (the same way a pencil eraser heats up if you bend it too and fro lot). if you lighten your flywheel you may, however, affect the resonant frequency of the crank assembly which may push it outside the frequencies attenuated by the pulley, so there is a risk, but qualifying that would be pretty difficult.


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PostPosted: Wed Jan 09, 2013 12:08 am 
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It's always good practice to zero balance the flywheel assembly and crank assembly anyway so the harmonic damper pulley should still be effective

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PostPosted: Wed Jan 09, 2013 12:37 am 
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The main requirement for resonance is when a wave motion travelling along the object is reflected back at each end, and the reflection is in phase with (adding to) the original excitaion pulses. Effectively, the energy is reflected back and adds to new energy being introduced. In the case of a crankshaft, that energy is pulses from each firing stroke.
I've been looking for a video of the classic skipping rope demonstration from my old o levekl physics and so far drawn a blank. Basically, tie a rope to a fence, then the other end flick right and left. You will see the pulse travel down the rope, then reflect back. If you tie the other end with another,thinner rope you can get to a point where the energy is absorbed and it doesn't reflect back. That is the equivalent of a damper pulley.
In electronics, we have the 'transmission line', e.g. networking cables, aerial cables etc. and you will hear the term 'characteristic impedance'. - if you put a resistor of that value at the end you will get no reflection. Open circuit or short circuit and you will get a reflection.
Excuse the most boring speaking voice in the world, but this is about the best I've found as an example of a wave travelling along:

http://www.youtube.com/watch?v=c9bhjlC_51g

And showing a 'damping' termination

http://www.youtube.com/watch?v=eLDIJvTEBhg

When the frequency of the excitation is exactly the same (or a multiple) of the time taken for the wave to travel down and back, then the size of waves can grow to be much larger than the original excitation.

e.g.

http://www.youtube.com/watch?v=NpEevfOU4Z8
http://www.youtube.com/watch?v=2DVdxvk3xs8

A very expensive lesson in resonance.

http://www.youtube.com/watch?v=j-zczJXSxnw



A characteristic of resonance is that it takes time to build up and decay. The bigger the resonance 'gain' (and therefore more destructive), the longer it takes to build up and the more precise the exciting frequency needs to be.

In the case of a crank, in 'race' conditions the engine is rarely at a constant RPM for very long so the chances of a destructive resonance building up is small. On a road engine, if the RPM for resonance happens to be on a common cruising speed then fatigue will happen fairly quickly. I did hear an example that it's known that a certain engine used in a one make series self destructs if held at a certain RPM for too long (5900 from memory)

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PostPosted: Wed Jan 09, 2013 9:37 am 
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Could this happen with a rotary or am I just being dumb?

Just notice that alot of rx7 modifications include lightened pulleys etc...

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1992 celica gt4 st185- td04 340hp/330lbs
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1998 pug 1.9 td- mile muncher!
1993 rx7 542hp savage monster!!
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PostPosted: Wed Jan 09, 2013 3:13 pm 
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I know the rx7 is even more funny with lightened parts as they have to have balancer shafts added to there eccentric shaft (crank) as standard to balance the rotors, so i know lightweight flywheel selection is selective because of that also.

Tim
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