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Crankshaft Lightening and Balancing

Lightened and Balanced FZR400RRSp Crankshaft

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Lightened and Balanced FZR400RRSp Crankshaft

Above is a lightened and dynamically balanced FZR400RRSp Crankshaft. It has also been polished and the webs have been knife edged.The rods have been lightened, balanced end to end and total mass, polished and then shotpeened.

This crankshaft has been lightened by 1.6Kg, and hence its inertia has been reduced significantly because most of the mass has been taken from the perimeter of the flywheel webs. A lot of people will tell you that this upsets the balance of the crankshaft with respect to the pistons being balanced by the crank. Taking cylinders 1 & 2, when 1 is at tdc (trying to lift the crank) , 2 is at bdc (pushing the crank down). This would be fine if both the pistons were in the same cylinder but they are seperated by about 100mm. Hence, this acts like a see-saw and tries to rotate the crank about the mid point between the conrods (in this case clockwise). The opposite happens when 2 is at tdc and 1 is at bdc, and the rotation is then anti clockwise. This is called a 'couple' and applies likewise to cylinders 3 &4. The bob weights (or webs) are on the crank to reduce this couple. This is why the outer webs are bigger than the inner ones, they are further from the center of the couple and have more effect. If these Webs were to counterbalance the pistons, the inner and outer webs would be the same and could then be made smaller and then have less inertia. As can be seen the crank acts like a pair of twin cylinders and only has to be balanced to itsself.

The size of the Webs is another matter.
The cost of doing this to a good crankshaft and rods is 460.00+vat

To sub arc weld regrind and balance + supply/size and fit all big end shells 240.00+vat (includes resizing the rod where possible)

To repair (weld/grind fit shells), and lighten/balance/polish/shot peen etc 600.00+vat