TR2/3/3A Narrow belt conversion. I can't understand why...

[TFB]

On the issue of dampening and the fan's rubber bushings, any thoughts about changes in dampening if poly bushings are used? I got one of the BPNW narrow belt kits last year and it will go onto my in progress car, along with a Macy fan. - Walter

I installed the Moss thin belt because my car came with no useable generator,radiator ,fan or control box,so after reading all the great info here I went alternator with wizard rad that came with elec.fan.If my car came with a nice original set up I would have tried to just maintain it .
The thin belt crank pulley is listed as a "dampened crank pulley".I don't know how this differs from a harmonic balancer.
As far as the dampening properties of the mechanical fan,is there some technical evidence or documentation of this affect for the TR34?
I think that the typical used fan is more likely to induce vibration than to reduce or cancel it,but this is only based on experience with repairing boat propellers,where slight out of balance,and usually more important out of pitch can cause huge amounts of vibration.
No idea about poly bushings but my guess is they would make it worse.
Tom
Tom

 

[CJD]

Crank dampening involves controlling very high frequency vibrations. I would think the vibrations you feel from an out of balance fan are very low frequency in comparison. Also, an out of balance is more of a radial load (tending to bend the crank), while harmonic vibrations are more of an axial load (tending to twist the crank) Plus, I would think the stresses from an out of balance fan at the front would most load the front bearing...not so much the rear, as shown in Randall's pic, and where it seems most crank failures occur.

There are so many cases of the fan being removed with no ill affects that I have my doubts that the fan really helps much as a harmonic damper...but I also have no real support for that statement. Just rambling thoughts...

 

[Tropical TR]

I seem to recall the fan hub extension being mentioned as key in the dampening effect; in absence of which (i.e. when installing an electrical fan) one must use a pulley with harmonic dampener. I opted for the narrow belt kit from BPNW, replacing all the pulleys. The crank pulley is a one piece alloy NON-harmonic version since it is being used in conjunction with the extension and a mechanical fan. Might have stayed with the wide belt but I needed a replacement for my original bent two-piece pulley. The set was the only way to get an alloy replacement at the time. Believe I recently saw a wide belt alloy crank pulley being marketed somewhere.

 

[TR3driver]

There are so many cases of the fan being removed with no ill affects that I have my doubts that the fan really helps much as a harmonic damper...but I also have no real support for that statement. Just rambling thoughts...

My thought is that it only becomes an issue under fairly specific circumstances, which most people are unlikely to have. It's not that the additional torsional vibration instantly snaps the crank; but rather that the torsional vibration at a specific resonant frequency (rpm), plus full engine output (higher than stock), stresses the crank beyond the fatigue limit. Then that situation has to continue long enough for the fatigue to weaken the steel before it actually breaks; which could be several hours (depending on how far past the limit it goes). Unless the resonant rpm just happens to match top speed (and you run for long periods at top speed), you're not likely to see the problem. But, that doesn't mean it is not real.

I believe it was Ken Gillanders (British Frame and Engine) who wrote long ago that he had an engineering analysis that showed the stock fan with rubber bushings was an effective dampener for the TR2-4 motor. His comment at the time was that the folks (including him) that were eliminating the rubber bushings were not doing themselves any favors. Ken is a long time TR racer (if you have a few hours to spare, ask him about his TR2 drag racing trophies).

OTOH, Kas Kastner doesn't seem to believe any dampening is necessary on the 4 cylinder motor (although he says the stock damper is inadequate on the TR6 motor and should be upgraded).

So, you pays your money and you takes your choice, as the saying goes. Personally, I'll use a damper if/when I ever get a "hot rod" motor built. In the meantime, I've been running for a long time with no fan or fan extension, and no dampener (on fairly tired motors that probably don't make even as much power as original). Only problem I've had is that one of the pulley hubs worked loose on the shaft and tore things up a bit. But if I did snap a crank, that would hopefully only give me an incentive to build a better motor.

 

[CJD]

While I was reading your post, Randall, it occurred to me that the crank made a distinct ringing pitch when it fell out of the truck last week. I started to wonder if there is any correlation between the natural pitch of the ringing and the natural frequency of the crank? If so, you could take a pitch meter...like every musician has these days...and find the frequency of the crank. Then convert the hertz of the crank to RPM (or vise versa), multiply by 2 pistons firing per revolution, and come up with the "critical" RPM for our cranks.

Piston planes frequently have RPM's that they recommend avoiding for cruise, because they match the natural frequency of the crank and can lead to fatigue failure. So, we may be able to come up with an RPM that would expedite crank breakage on our TR's...or prevent it if we avoid that RPM.

Next time I have a free crank I'll measure the ringing and let everyone know if the calculations make sense...or are out of the ballpark!?!

 

[TR3driver]

I think there is a definite relationship there, but it's most likely a harmonic one (hence the "harmonic" dampener). And of course the resonant points will move around somewhat with flywheel weight, crank preparation and so on. IIRC there are several critical rpms, but it seems I failed to save the list. Again IIRC, the one described as being most troublesome is just above the 5000 rpm redline, around 5200-5500 rpm. Obviously not a speed that most people use very much.