Piston Weights

Randy Grein randygrein at comcast.net
Thu Mar 13 18:24:58 PDT 2008

There are a few others, while not builders or engineers have  
significant experience/knowledge regarding racing. Ernie generally  
knows what he's talking about. Z and KC are major builders and racers  
(former); you guys have what - about 40 years experience between you?  
I (usually) know what I'm talking about although there are days...  
(grin). I'm pretty good with the physics of racing and condensing  
explanations into layman terms but leave the actual engine building to  
guys with the specialized knowledge to do so. The racers here  
generally have years of experience and do (most) of their own work.

The knowledge/BS ratio is pretty high here, esp. compared to some  
other lists I've read. A pure BS answer lasts about 30 seconds and I  
can't recall an actual fight about anything. Feel free to chime in - a  
lot of tech discussions are like this one, very collaborative.

Randy Grein, WMRRA #41
Micapeak SV list admin

On Mar 13, 2008, at 3:54 PM, Ernest Montague wrote:

> You are getting decades of experience from a couple of the top  
> engine builders around, neither of whom has a degree in engineering.  
> However their bikes win races. Take that for what it is worth. I  
> know there are a couple engineers on the list. My GF is an  
> engineer.  (PE) Her job is to run a company that monitors the growth  
> of invasive species in the SF bay.
> On Mar 13, 2008, at 3:31 PM, Jake Meyer wrote:
>> 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.
>> Jake
>> 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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