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Alloy crankshaft pulley

Alloy crankshaft pulley

Alloy crankshaft pulley

Hi guys,
I don't know if i'm writing in the correct topic if not sorry...
I would like to ask if someone can resolve my question wich is
The crankshaft pulley has a specific material, weight and diameter given by the manufacturer...when we change it for a light weight ones and smaller diameter don't we put stress to the crankshaft and piston rod's bearings?
For an instance, when an engine run at 7000rpm the sympathetic vibration of the crankshaft must be absorbed from the flywheel and...the pulley.
If not why the manufacturers created the double mass pulley (like the flywheel)???
p.s. sorry if my english are not so good...

RE: Alloy crankshaft pulley

That's right. The inertia of the pulley is part of the crankshaft, changing it could cause a bending or torsional failure.

That is an acceptable risk for a racing engine, possibly.


Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Alloy crankshaft pulley

I'm pretty sure that engines are designed to be balanced well enough that a harmonic balancer should absorb/balance out a lot of the vibration.  Reducing the weight of the pulleys actually reduces the inertia the engine has to overcome to accelerate.  Reducing the engine pulley diameter slows down the rotating speed of the accessories, further reducing the load on the engine.  Not quite sure what you mean by a double mass pulley, but I think you might be talking about the harmonic balancer. (Round disk usually sandwiched between the pulley and the front of the engine?)  From my experience with engines, the more mass you can remove while preserving part strength, the better off you'll be.

RE: Alloy crankshaft pulley

Did the engine have a damper type pulley when it was stock?
If it did, I'd expect a modified engine will need one too.

Generally speaking, some inline 4s get away without torsional dampers, but inline and V sixes and 8s do not.

Drag racers can get away running damperless because each race is so short, and time spent at any of the dangerous rpm (or any particular rpm) is brief, but I think other forms of racing require much greater duty cycles  

RE: Alloy crankshaft pulley

Acceptable risk in a racing engine? Of course, depending on the engine.  I certainly would not use a lightweight pulley on my Mini...That would guarantee a catastrophic ending!  On the other hand, many Ford based engines such as the Lotus twincam, Cosworth BDA type do away with the crank pulley and replace it in most cases with a ~2" dia Gilmer belt type drive.  These engines often see the North side of 10,000 in road races.  Most F-1 engines use alloy or do without and they look very often at 20,000.

All this information is on line or in "How To" books. Unless you are building a racing anomaly like a M20 Pobeda...(I'll bet no one even knows what that is...Hint---GAZ.)winky smile


RE: Alloy crankshaft pulley

Bear in mind that lightening the flywheel will also affect the crankshaft dynamics, to some extent. In fact if you look at main bearing forces there isn't much you can do to an engine that doesn't change them, and usually change means worse if the system was properly designed in the first place.

Many engines will have a satisfactory durability AT THEIR ORIGINAL RED LINES if they don't have a TV damper. But of course for a race engine you don't care about the original red line. if you spend most of your time at an RPM that doesn't line up with a resonance then the harmonic damper won't be helping much anyway. (There is a small reduction in TV away from the resonance, but the levels seen are usually OK).

Also bear in mind that crank bending, as opposed to torsionals, is a perfectly good way of destroying engines, for instance the LT5 would eat itself without a crank bending damper.


Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Alloy crankshaft pulley

Thank you all for your answers and your time!
M20 Pobeda, good one haha!(i can't get a smilie...)
The reason i asked is that a guy i know told me that racing engine's (let's say for a hillclimb) don't need any damper and with the alloy pulley they are fine, with no wear in the cranks bearings.
Something that in my opinion can't happen...And that's why racing engines must be open after one or two race's for examination.
Torsional vibration rise as the crankshaft size rise right?

RE: Alloy crankshaft pulley

"racing engines (let's say for a hillclimb) don't need any damper and with the alloy pulley they are fine, with no wear in the cranks bearings."

I am not sure all is optimum or even well Just because the main bearings bearings look OK after the first race.
I suspect the crank bearings may be among the last components to show the effects of prolonged operating at a torsional critical speed.

I guess Modern engines sometimes have dampers (or is it sometimes the flywheel?) with features that make them effective at taming lateral or bending crank (or whole engine?) vibration.

I believe Historically dampers were usually torsional dampers.  I base that on SAE papers that introduced engines of the 50s and 60s (Chevy, AMC, and Pontiac V8, Corvair) including charts showing dampered and damperless crankshaft angular deflections at certain crank rpm, but with frequency  many times operating rpm.  Typically like this -

If a damperless engine is run at full power at an rpm where a torsional resonant frequency exists, the amplified angular twist/untwist/reverse twist motion will subject the crank to a million fatigue inducing overstress cycles in a few hours, and that leads to cracks and eventually breakage.  That's bad enough.
How about the possibility of more insidious effects on valve train, especially overstressed valve springs, and ignition timing?

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