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Lightening the crank ...or the flywheel?

11K views 9 replies 7 participants last post by  Aussiebikerdave 
#1 ·
How many have gone to the trouble of lightening the crank or flywheel for quicker engine pick-up? The question is, what is best, lighten and rebalance the crankshaft when engine is apart or just lighten the flywheel inside the RH engine cover? Any comments from those that have gone this way including how much weight was removed?
 
#8 ·
Thanks Agro. I found the following on your link but my German is a bit rusty, infact it's limited to about three words achtung halt and danke! Would be most grateful if one of our in-house multi-linguists would translate this.

Polrad Gewichtsoptimierung. Wir drehen Ihr angeliefertes Polrad ab und erleichten es auf nur noch 2,8 KG. Das Urspungsgewicht beträgt gute 4 KG. Das sind satte 42% mehr. Dadurch ergibt sich ein wesentlich leichteres hochdrehen des Motors, was man wirklich sofort am Gasgriff spürt. Eine völlig neue Motordynamik ! Die theoretischen Einbußen eines schlechteren Rundlaufs im unteren Bereich sind echt zu vernachlässigen, da man es es faktisch nicht spürt. Das Polrad wird einbaufertig und perfekt gewuchtet incl. neuer Dichtung geliefert.
 
#3 ·
Dave, this can be a good mod and up to 1.5kg can be removed without any mechanical problems. I wouldn't remove more than 1kg for a daily ride as 1.5kg can make bimmbling around in traffic a real pain.

You notice quicker acceleration and especially noticable is the reduction of engine braking when going down through the gears, it makes it harder to lock the rear wheel through engine braking.
 
#5 ·
Good article Forchetto, but it neglects to mention the impact of flywheel weight on tortional vibration in the crankshaft. Many engines mount dampers just to cope with these stresses. Lightening a flywheel excessively can result in increased vibration and ultimately crankshaft failure. Without complex FEA simulations, practical experience is probably the best guide to how much lightening makes sense.
 
#6 ·
NEVER attempt to "lighten/balance" a crank IN THE ENGINE! That is absolutely asking for a blown engine one way or another. It simply cannot be done reliably while in place.

Another disadvantage of lighter rotating mass is unstable low speed tickover. Often, you have to increase the idle or the engine can stumble and die at low revs. Also, engine braking effect is reduced.

The trade-offs are not typically advantageous for street use.
 
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