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Twins Technical Talk Technical Talk for Hinckley Triumph Twins: Bonneville, T100, Speedmaster, America, Thruxton, and Scrambler.

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Old 08-06-2011, 06:32 AM   #1 (permalink)
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Lightening the crank ...or the flywheel?

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?
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Old 08-06-2011, 07:39 AM   #2 (permalink)
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Quote:
Originally Posted by Aussiebikerdave View Post
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?
Some have done it.... http://www.classicbike-raisch.de/htm..._vergaser.html

check the bottom of this webpage...
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Old 08-06-2011, 01:15 PM   #3 (permalink)
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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.
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Old 08-06-2011, 01:37 PM   #4 (permalink)
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A scholarly article on flywheel lightening. Incredibly effective mod, specially for acceleration in the low gears:

http://www.pumaracing.co.uk/FLYWHEEL.htm
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Old 08-06-2011, 02:30 PM   #5 (permalink)
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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.
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Old 08-06-2011, 06:24 PM   #6 (permalink)
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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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Old 08-06-2011, 09:03 PM   #7 (permalink)
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Originally Posted by Aussiebikerdave View Post
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?

Bob Farnham racing in the UK lightens the alternator rotor and people say its a good mod. I think robnobrakes has had it done. But you already have more power than he can produce.

http://bobfarnhamtuning.com/bonneville.asp
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Old 08-07-2011, 07:11 PM   #8 (permalink)
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Some have done it.... http://www.classicbike-raisch.de/htm..._vergaser.html

check the bottom of this webpage...
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.
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Old 08-08-2011, 02:59 AM   #9 (permalink)
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Quote:
Originally Posted by Aussiebikerdave View Post
Would be most grateful if one of our in-house multi-linguists would translate this.
Using Google translator and further edited by me to make some sense:

Flywheel weight optimization. We shave off your flywheel (alternator rotor) until its weight is 2.8 KG. The OEM flywheel is 4 KG. This is a weight saving of 42% . This results in a much lighter engine, you can really feel immediately as you wind on the throttle. Completely new engine dynamics! The theoretical disadvantage of poorer idle and slow-running is negligible and you can't really tell. The flywheel is supplied perfectly balanced and ready to install includes a new gasket.
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Old 08-08-2011, 03:20 AM   #10 (permalink)
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Using Google translator and further edited by me to make some sense:

Flywheel weight optimization. We shave off your flywheel (alternator rotor) until its weight is 2.8 KG. The OEM flywheel is 4 KG. This is a weight saving of 42% . This results in a much lighter engine, you can really feel immediately as you wind on the throttle. Completely new engine dynamics! The theoretical disadvantage of poorer idle and slow-running is negligible and you can't really tell. The flywheel is supplied perfectly balanced and ready to install includes a new gasket.
Thanks Ernesto, why didn't I think of Google? ...it sounds like this is the definative answer so I can see this mod being added to the endlessly growing list of power mods. I dropped the bike around to the dyno centre this afternoon to get get the a/f ratios tuned after the fitment of the 994 barrel and Pieman's bored out TB's. Will post the results when they come to hand.
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