Speed-Talk is running on www.Speed-Talk.com

IMPORTANT: Update your bookmarks to https://www.speed-talk.com/forum/
(Right-click the URL and select "Bookmark this link")

piston weight

General engine tech -- Drag Racing to Circle Track

Moderator: Team

David Redszus
Guru
Guru
Posts: 8459
Joined: Tue Nov 27, 2007 9:27 am
Location: Chicago
Contact:

Re: piston weight

Post by David Redszus »

like everything its best to use only what is needed as far as wrist pin go. People try to push the limits and get burnt as there is no simple formula to tell you. Personally i think the reciprocating weight is way overrated in the majority of applications as far as importance, more potential pain than gain by making things light.

There are several simple formulas. And then there are accurate, complex formulas. Your choice.

The blind pursuit of lightness brings component failure along with it. A smart designer will always provide
a margin of safety to allow for measurement and calculation errors.
digger
HotPass
HotPass
Posts: 2223
Joined: Sun Jan 27, 2008 6:39 am
Location:

Re: piston weight

Post by digger »

David Redszus wrote: Mon May 03, 2021 6:39 pm
like everything its best to use only what is needed as far as wrist pin go. People try to push the limits and get burnt as there is no simple formula to tell you. Personally i think the reciprocating weight is way overrated in the majority of applications as far as importance, more potential pain than gain by making things light.

There are several simple formulas. And then there are accurate, complex formulas. Your choice.

The blind pursuit of lightness brings component failure along with it. A smart designer will always provide
a margin of safety to allow for measurement and calculation errors.
there are no accurate simple formula. there are no complex formulae that you can plug simple numbers into and get an accurate answer.

There are design processes that involve use a physics and mechanics of solids approaches that can be used to design a wrist pin. The complexity of the process depends on how optimised one wishes to be.
David Redszus
Guru
Guru
Posts: 8459
Joined: Tue Nov 27, 2007 9:27 am
Location: Chicago
Contact:

Re: piston weight

Post by David Redszus »

The physical properties of wrist pins has been well known for over 75 years. They can be modeled
very accurately. The bending and distortion effects can be calculated on your desktop. That's the easy part.

The more difficult part, is to know what forces will affect the wrist pin, what the distortions will be,
and what effect it will have on the piston.

Twenty five years ago, Mahle showed that a lightweight, flexible wrist pin will, under load, cause
a expansion and contraction of the piston skirt, in both the thrust and non-thrust axis.

Simple beam theory indicates that a short, fat, wrist pin will deflect much less than a long, smaller
diameter pin. Pin ID being fairly insignificant compared to OD and length.
piston guy
Expert
Expert
Posts: 989
Joined: Mon Sep 10, 2012 1:55 pm
Location: Anaheim, CA

Re: piston weight

Post by piston guy »

LSP wrote: Mon May 03, 2021 7:48 am
piston guy wrote: Tue Apr 27, 2021 1:00 pm
LSP wrote: Tue Apr 27, 2021 12:33 pm

No, cracking was due to them not designing the piston properly to handle the rpm in the first place

The white paper was toilet paper, used to clean up the mess on the ground after another engineering disaster..
Usually due to inadequate wall thickness on the wrist pins.
Being the simplist part to design in the piston, pin, ring package system, I would hope that the perceived superior intelligence group would at least get that correct.
About 17 or eighteen years ago , the "piston industry" was experiencing "deck" and pin tower cracking . Knowing cracks in aluminum are caused by flex, the "book smart" engineers "added mass to save their ass" but the parts kept breaking. "My" opinion was the "load wear" in the pin bosses showed pin flex so I suggested to a 410 sprint car engine builder a switch to a .180 wall wrist pin and the problem went away over night. We then went back and undid all of the "fixes" we first did. The parts still didn't crack, telling me the pin was the issue.
digger
HotPass
HotPass
Posts: 2223
Joined: Sun Jan 27, 2008 6:39 am
Location:

Re: piston weight

Post by digger »

David Redszus wrote: Tue May 04, 2021 11:42 am The physical properties of wrist pins has been well known for over 75 years. They can be modeled
very accurately. The bending and distortion effects can be calculated on your desktop. That's the easy part.

The more difficult part, is to know what forces will affect the wrist pin, what the distortions will be,
and what effect it will have on the piston.

Twenty five years ago, Mahle showed that a lightweight, flexible wrist pin will, under load, cause
a expansion and contraction of the piston skirt, in both the thrust and non-thrust axis.

Simple beam theory indicates that a short, fat, wrist pin will deflect much less than a long, smaller
diameter pin. Pin ID being fairly insignificant compared to OD and length.
they can be modelled accurately but it is not the same as using simple bending, shear and deflection formulas based on beam theory. The dimensions of the wrist pin and the type of support do not lend to that method being useful or accurate for more than a school assignment.

Case in point you state "Simple beam theory indicates that a short, fat, wrist pin will deflect much less than a long, smaller diameter pin. Pin ID being fairly insignificant compared to OD and length." that is not very useful for optimisation of a design. Like all real world things being stronger, stiffer, more durable is a relative thing but means nothing without absolutes as to whether it is actually strong enough, stiff enough or durable enough which requires specific criteria. Without accurate methods you can not optimise with confidence. the classical methods end up being far too conservative

the forces are straight forward to calculate with a 1D simulation, the load history is more time consuming to categorise, but proper analysis involves relatively complicated FEA to model the system rod, pin and piston assembly to capture the compliance of each part and local load transfer between each part.
David Redszus
Guru
Guru
Posts: 8459
Joined: Tue Nov 27, 2007 9:27 am
Location: Chicago
Contact:

Re: piston weight

Post by David Redszus »

digger wrote: Tue May 04, 2021 5:54 pm
David Redszus wrote: Tue May 04, 2021 11:42 am The physical properties of wrist pins has been well known for over 75 years. They can be modeled
very accurately. The bending and distortion effects can be calculated on your desktop. That's the easy part.

The more difficult part, is to know what forces will affect the wrist pin, what the distortions will be,
and what effect it will have on the piston.

Twenty five years ago, Mahle showed that a lightweight, flexible wrist pin will, under load, cause
a expansion and contraction of the piston skirt, in both the thrust and non-thrust axis.

Simple beam theory indicates that a short, fat, wrist pin will deflect much less than a long, smaller
diameter pin. Pin ID being fairly insignificant compared to OD and length.
they can be modelled accurately but it is not the same as using simple bending, shear and deflection formulas based on beam theory. The dimensions of the wrist pin and the type of support do not lend to that method being useful or accurate for more than a school assignment.

Case in point you state "Simple beam theory indicates that a short, fat, wrist pin will deflect much less than a long, smaller diameter pin. Pin ID being fairly insignificant compared to OD and length." that is not very useful for optimisation of a design. Like all real world things being stronger, stiffer, more durable is a relative thing but means nothing without absolutes as to whether it is actually strong enough, stiff enough or durable enough which requires specific criteria. Without accurate methods you can not optimise with confidence. the classical methods end up being far too conservative

the forces are straight forward to calculate with a 1D simulation, the load history is more time consuming to categorise, but proper analysis involves relatively complicated FEA to model the system rod, pin and piston assembly to capture the compliance of each part and local load transfer between each part.
Yes, of course you are right. My simple historical overview does not permit the accurate optimized design of a wrist pin
for a specific application. That is particularly true when designing an articulated or coke bottle shaped pin.

However, even simple beam theory can provide much more insight than racing folklore or marketing bafflegap.

And even FEA has now become a desktop computer computation.
agertz1
Member
Member
Posts: 70
Joined: Wed Aug 14, 2019 7:43 pm
Location:

Re: piston weight

Post by agertz1 »

Doesn't a lighter recip. mass grab rpm faster ? Doesn't a crankshaft have a certain amount of "flywheel effect " ? Aren't they solely propelled by high pressure gas. ?
Post Reply