Crank pulley question-- for you engineer types

[ALLAN]

There is a guy on Ebay selling all aluminum crank pulleys, he claims that the all aluminum acts as a harmonic balancer, I have my doubts about this, Any thoughts on this???

 

[derickson]

I seriously doubt it. In general the response of the crankshaft to harmonics (or vibration) is determined by mass, not material, although if the crank harmonics were high enough frequency the material could resonate differently, but I doubt this is the case. It could have some effect, though, since the pulley is really no different than the flywheel (from a moment of inertial perspective), which when aluminum does change the response of the engine. Mostly this effect is to make the engine more responsive (read faster to accelerate), not damp out harmonic effects. I suppose if you have an aluminum flywheel, and really want to go all the way, you could use an aluminum pulley. I can't imagine the weight difference is significant, though.
Technically, a harmonic balancer is different than a crankshaft damper. A harmonic balancer has an energy absorbing component (like rubber) to absorb vibration. Neither the aluminum flywheels or crank pulleys really damp crankshaft harmonics much (in fact they may amplify certain frequencies). Reducing the mass simply makes it easier for the engine to turn over.

 

[trfourtune]

absolutely not!!!
the stock cast extension is a couterbalance for crankshaft bending, the flywheel is the other. aluminum or removal of the stock extension will make crank matters worse. harmonic balancers DO have elastomer in them and work in torsion and up/down, side/side, front/back motion. he can challenge this if he wants to but don't believe him.
rob

 

[ALLAN]

He says that the thin hard rubber on the Triumph balancer doesnt really do much and that the aluminum absorbs vibration, he says there are 50 people running his pulley including his MG and no complaints yet. I probably wont go with it however I dont see how the TR6 balancer is a counterbalance---it is evenly weighted all around?

 

[trfourtune]

its more of a couterweight,ie if you consider the front bearing as a fulcrum and part of the crankshaft is the weight on one end and the "extension" out the front is the other. he probably won't get complaints since sixes are basically inherently balanced. what you get is more wear in your engine than you would with a harmonic balancer. v8's are not inherently balanced nor are inline 4's. they must have them.
rob

 

[derickson]

where's the harmonic balancer on my TR4? I just reassembled the engine, and didn't see anything with a compliant element to damp harmonics.

 

[Dave Russell]

Your TR4 doesn't have a harmonic damper, only a pulley. I believe it is red lined at 5,000 rpm. The crankshaft is purported to have resonate modes at 5,200, 5,800, & 6,200 rpm. If you are planning to run it for any length of time above 5,000 there are a couple of "real" aftermarket harmonic dampers available. One of the sources is British Frame & engine (BFE). I think they are fairly expensive though, which likely explains the omission from the original engines.
D

 

[trfourtune]

you have that steel extension to the fan and the weight of the fan only which is supposed to take care of the motor. that is why they are meant to stay under 5000 rpm because that "couterweight" won't deal with the real harmonics as previously mentioned(5200,5800,6200). so, if you remove the fan and extension with an electric fan you should put on a harmonic balancer. BFE sells them for $250 for engines running up to 6200 rpm, ie stock crank. some say (whoever "they" are) if you remove the fan but keep the steel extension that you are still ok for 5000 rpm.
rob /ubbthreads/images/graemlins/thirsty.gif /ubbthreads/images/graemlins/cheers.gif TGIF