V-Twin Flywheels 1

[swall]

On V-twins with built up cranks, like Harley, the flywheels, crankpin and output shafts are assembled.Then the assembly is put into a fixture to check for flywheel run out (side to side). If runout is not in spec, you take a lead hammer and whack the flywheels until it is.This has always bothered me.It seems that if you can make the assembly run true with a few whacks of a lead hammer, then it could be easily un-done by the normal operational forces of the engine. What do you guys think?

 

[etatv]

This is why the guys that race them will weld the crank pins to the crank(fly)wheels after they have been trued (with the lead hammer). Pretty common practice in the motorcycle world.

 

[Metalguy]

Somehow the thought of taking a lead hammer to a "certain V twin engines" brings a big smile to my face! <g>

Crank welding had best be done by experienced people who are well aware of the problems that welding can create with typical crank steels.

 

[Tmoose]

I think the forces resulting from combustion operating thru the crankpin would have a hard time shifting the flywheels.

Stock Pre evolution HD flywheels were cast iron with mainshafts and crankpin retained with nuts pulling the shafts' deep into fitted tapered joints. Early PH (power side) mainshafts had a drive key too, but that is no defense agaist a loose creeping taper. In the 80s they went to much more secure BIG taper, BIG nut torque, and no key. The crankpin has a small key, but on the RH side to align an oiling hole, not to share some kind of load. At some point The racig XRs had integral mainshafts and flywheels.

Some Japanese multis use crank "flywheels" forged of steel with integral mainshafts.

Drag racers, and VERY abusive street riders could tweak/shock the multi cranks out-of-true with aggressive starts with sticky tires, unless the pressfitted joints are welded at the ends. HD big twin drag racers sometimes brazed the left hand output shaft to the forged steel S&S flywheels.

 

[swall]

Interesting comments.I get the idea that most of these remedial measures are designed to keep the crank assembly from coming apart, rather than go out of true.

 

[GregLocock]

What forces will knock it out of true? (I know the answer, just wondered if you do)

Cheers

Greg Locock

 

[patprimmer]

I would suggest that if it were to go out of true, the resultant effect on balance would quickly tear it apart.

Regards
pat pprimmer@acay.com.au
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[GregLocock]

I don't think a tilted, but centred, disc has any particular forces on it that are harmful... do you? I can see that the non-existent centrifugal force will tend to straighten it back up, but that's about it.



Cheers

Greg Locock

 

[patprimmer]

I thought the indexing of the crank arms on the journals could be twisted out of phase by the torsional forces when the clutch is suddenly dropped with some revs up, if the clutch bites and the tyres stick. It used to be a problem when drag racing air cooled VW Beetles with pressed together roller cranks.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[swall]

On operational forces, I was mainly thinking vibration. If you true the crank with a few bangs with a lead hammer, I was wondering if vibration might undo the trueness. But some of these comments about welding the crank pins have got me thnking about weld shrinkage pulling the crankshaft pins out of square. Also, what about the effects of driving a timing gear onto the crankshaft pin?

 

[GregLocock]

You are right Pat, there will be some bending stress in the crank (and hence main bearings), until the disk straightens up.

But I still don't see many significant forces trying to knock the thing out of true in the first place.

The only one I can think of is gyroscopic precession due to cornering.

Cheers

Greg Locock

 

[notnats]

Greg,
Those bending forces in the crank are the reason why a shaft which is statically balanced may not be dynamically balanced. The bending forces rotate with the crank.

Jeff