Piston Weights

Jake Meyer meyerjme at gmail.com
Thu Mar 13 15:31:22 PDT 2008

I know I'm a n00b here, but I have to ask - how many people on this e-mail
list are engineers?  mechanics?  I'm just wondering how many years of
design/build/repair/modify experience is sitting in my inbox on any given
day...  I'm a mechanical engineer with only about 7 years experience working
on cars.  Zero experience on bikes :) - so thanks to everyone for sharing
your knowledge and experience.


On 3/13/08, KC Gager <kc at brgracing.com> wrote:
> Spot on Randy
> The problem with piston weight is the surface area that the rod has on the
> crank. It is design for X weight and the oil film will hold X weight. If
> you
> make x heavy old film goes away and boom.
> KC Gager
> BRG Racing Products
> "We Have Sickness For Quickness"
> KC at brgracing.com
> http://www.brgracing.com
> (925)672-5789
> -----Original Message-----
> From: Randy Grein [mailto:randygrein at comcast.net]
> Sent: Thursday, March 13, 2008 6:19 AM
> To: Suzuki SV650 Mailing List
> Subject: Re: Piston Weights
> No. Neither reciprocating nor rotating mass affect torque. An increase
> in rotating mass evens out power output making it easier to feather
> the throttle (like spinning coming out of  a corner), and does provide
> a slight increase in acceleration (measured torque and power on a
> Dynojet dyno), but the few grams we're talking about here between
> pistons isn't enough to notice. Piston weight does affect the optimal
> crank balance; go light or heavy enough and you'd increase vibration)
> but the primary impact is on crank/rod stress. Big heavy pistons break
> cranks at high rpms.
> Your drive line deformation argument is interesting, but except for
> the clutch basket springs and sprocket cushions there isn't any to
> speak of at this level. Parts that deformed enough to materially
> impact power delivery, even instantaneous power delivery would be
> deforming significantly and break in short order. Yes, there is some
> deformation, which is why we don't make cranks out of aluminum, but
> for the effect you're thinking of to be noticeable deformation would
> have to be plastic and on the order of 10-20 degrees of crank rotation
> every half  engine rotation.
> Randy Grein, WMRRA #41
> On Mar 13, 2008, at 5:35 AM, Ilya A. Kriveshko wrote:
> > Steve Robertson wrote:
> >> So do those boat anchor Wiseco pistons make good torque :-)
> >
> > I understand it's a joke. But pretending for a second you were
> > serious,
> > let me ask the gear heads on the list: would a heavier *reciprocating*
> > mass affect torque figures? And how?
> >
> > My own gut feel is that unlike heavier rotating mass, heavier
> > reciprocating mass would actually reduce output torque.
> >
> > Heavier *rotating* mass would increase the mean torque over the cycle,
> > by borrowing from the sharp and short power peak and spreading it
> > around
> > the ~3x wider valley. Normally, the sharper the power peak (i.e. the
> > quicker it changes), the more of its energy gets dissipated via
> > material
> > deformation in the drive line. Lowering and widening the peak allows
> > the
> > various stresses to rise and fall at a slower rate, thereby wasting
> > less
> > energy on component deformation, and delivering more of it to the
> > output
> > shaft.
> >
> > However, it seems that heavier *reciprocating* mass would not have the
> > same full-cycle peak spreading effect. It would rather amplify the
> > harmonic of positive and negative inertial torque effects that is
> > overlaying the combustion torque curve. If anything, that would
> > produce
> > more sharp combined torque rises and falls, creating peaks and
> > valleys,
> > producing more stress, sapping energy and wearing components. The
> > torque
> > measured on the output shaft would probably fall.
> >
> > Anyhow, that's how I explain it to myself. Barring confusing
> > terminology
> > and lacking clarity, do I have the right picture in mind?
> > --
> > Ilya
> >

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