More fun calculations

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Ace VenTRa
Posts: 86
Joined: Sun Feb 23, 2020 8:32 pm

More fun calculations

Post by Ace VenTRa »

Having a little time on my hands I decided to reconsider whether or not I should bother having a set of rods made for the V. Conveniently there are calculations out there to help determine if the changes made to your motor will effect it positively, such as lighter piston, rod, etc. In short, the issue with reciprocating weight is what they refer to as inertia and it considers what happens at TDC and BDC when the piston stops and starts rapidly https://www.racepagesdigital.com/piston ... ur-engine/. The calculation to find primary inertia is:

.0000142 x Piston Weight (lb) x RPM2 x Stroke (in) = Inertia Force

If you trust my calculator and scale the inertial force of each piston assembly in an oem VTR is 4552 lbs at 10k rpm.

If you then figure the correction factor using this formula: Crank Radius÷Rod Length = .2598

“Because of the effect of the connecting rod, the force required to stop and restart the piston is at maximum at TDC,” says Fussner. “The effect of the connecting rod is to increase the primary force at TDC and decrease the primary force at BDC by this R/L factor.” This factor is added to the original inertia force for the upward stroke and subtracted on the downward movement."

So...after all that what we find is that at TDC the forces are 5735 lbs and at BDC they are 3274 lbs. Why does anyone care? Because with a new, lighter forged piston assembly from JE (which is lighter) the forces at TDC are 163 lbs lighter and at BDC 95 lbs lighter. That is the result of an overall decrease in weight of 15.8 grams per piston assembly. Now how does that help influence my decision?

Well, the rod is exposed to either tension or compression depending on which way it's traveling. The sudden transition at TDC causes tensile loads and is extremely hard on not only rods but rod bolts. So, if I visit ARP and getter stronger bolts that do not fatigue as easily as oem bolts and I have my rods cryogenically treated like everything else, I have not only reduced the tendency of the rods and bolts to fatigue, but I have reduced the inertial forces acting on them both. In other words; I made my stock rods stronger and reduced the amount of work they have to do and in consideration of that, I think I should keep my stock rods and save $500 bucks that I can spend elsewhere.

Note* This works only if you maintain the same redline. Raising redline raises inertia exponentially.

AV
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Mav617
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Re: More fun calculations

Post by Mav617 »

The maths that matter to me are saving 500 bucks.....
I'm so unlucky that if there is such a thing as re-incarnation I'll come back as me!
tony.mon
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Location: Norf Kent

Re: More fun calculations

Post by tony.mon »

Peening the rods used to be popular, to reduce microcracks.
Is that worth consideration while you're there?
It's not falling off, it's an upgrade opportunity.
Ace VenTRa
Posts: 86
Joined: Sun Feb 23, 2020 8:32 pm

Re: More fun calculations

Post by Ace VenTRa »

Peeing works but it's really a surface treatment, the cryo treatment penetrates completely and has the same effect. Peeing does make them look pretty though.

AV
Ace VenTRa
Posts: 86
Joined: Sun Feb 23, 2020 8:32 pm

Re: More fun calculations

Post by Ace VenTRa »

FYI, If anyone is rebuilding a motor any time soon...ARP does not specifically list a rod bolt for the V but as luck would have it, the V shares a rod bolt with the Hayabusa. It's an M9x.75 bolt and requires 45 lb/ft, you'll would likely need to check the big end dimensions and or have them resized accordingly. The kit for the Busa is # 271-6301 and provides 4 bolts.

AV
tony.mon
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Joined: Wed Jul 25, 2007 10:46 pm
Location: Norf Kent

Re: More fun calculations

Post by tony.mon »

What does the cost work out per bolt that way? And are they a stretch bolt?
Honda ones are reasonable, when I was building storm engines regularly I kept a stock of them, there's probably some left in the garage somewhere.
It's not falling off, it's an upgrade opportunity.
Ace VenTRa
Posts: 86
Joined: Sun Feb 23, 2020 8:32 pm

Re: More fun calculations

Post by Ace VenTRa »

With shipping it works out to about 10$ a bolt. If I remember correctly that's about 2$ more than the Honda bolts but that's not really the point. I think the OEM bolts are likely fine in most cases. I went with ARP because they are very high quality, affordable, and will ensure as much as possible that the big end does not distort under heavy extended loads and cause havoc. They are not a stretch bolt and are torqued to 45ft/Lbs as per ARP if using ARP thread lube.

AV
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