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calculating bob weight

General engine tech -- Drag Racing to Circle Track

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Darin Morgan
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Post by Darin Morgan » Sat Dec 03, 2005 7:04 pm

There is a trend to under balance these days. In pro Stock its common place but I don't know about other classes or forms of Motor Sports. By considerably cutting down the diameter of the CW and inserting heavy metal in order achieve a 45% balance (5% under balance) you lower the moment of inertia, decrease windage and increase acceleration. Yes, it does show up on the dyno and the track. I use this method for any engine operating at high engine speeds ( 5000-6000fpm). On a pro stock engine the balance seems to phase in about 7000rpm and is very smooth all the way up to 10000. Below 7000rpm you can definitely feel the engine vibrating. We have tried 44% and that just vibrates all the way up through he RPM range.
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Post by SchmidtMotorWorks » Sun Dec 04, 2005 1:07 am

A given Recip. weight gets heavier (inertia weight) as the RPM increases.
And so does the counterweight, an object or mechanism can be balanced without rotating it, this is done in CAD all the time. If it is perfectly balanced at any speed, it is perfectly balanced at all speeds.

SInce balance doen't change with speed, the "gun leading" analogy doesn't fit the dynamic balance concept at all. Dynamic balance isn't a moving target, it is a reduction of imperfection target.

As for overbalance reducing bearing loads, no way, it can only be a coincidence. Here's why.

Let's say you balance a crank perfectly with no overbalance, then add weight to the bobweights for overbalance. Now you spin upthe crank and it will show the CWs to be light at about 155 degrees opposite each of the end rod pins.

So how will you correct this? You could do it by adding a slug to any of the counterweights that have material at the 155 position.

Kepp in mind the end counterweights on most V8s are about 30% heavier than they would be if they were in a 90 V-twin with the same parts. The other counterweights are about 50% too light. There just isn't enough room oin the block to make them heavy enough.

If you look at your crank from the end view (down the snout) you will notice that the end counterweights are biased to one side relative to the rod pin. Notice, they are biased in the direction of where material is needed on the other counterweights. This arrangment makes the crank lighter because the CW weight is much more effective at resolving dynamic unbalance at the ends of the crank than it is at the middle.

So from this you can know that all the CWs in a cranks are at least 100 grams away from what they would be to make a perfectly internally balanced crank. (By internally, I don't mean without external flywheel and balancer weights, I mean main journal to main journal balancing).
If you look at where V8 cranks burn first on the 2nd and 4th mains, you will notice it is opposite the split between the 2nd &3rd CWs. This is because they are never big enough. Balancing a crank with overbalance and adding weight to the end CW only makes it worse.

Next time you see one of those cranks with big center counter weights that are indexed in alignment with the end counterweights rather than opposite the center rod pins, you know that it is carrying a lot of extra weight that hurts bearing loads. The only benfit is it is easy to balance in the balancing machine (except for lifting it).

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Post by machine shop tom » Wed Dec 07, 2005 3:28 pm

Todd H wrote:While we are at it then lets bring V6's in to discussion, Some of them IIRC are balanced at 50% and some of the old Busch stuff balanced at 36.6%. I never could get the logic of why the difference? Rpm's perhaps, Even vs. odd fire? Todd H
The percentage used for V-6 engines depends on a few factors. 60 degree, 90 degreee, split-pin, balance shaft or not, all are factors to be considered.

The Chevy/GM 4.3, for instance, is listed as 40%, 50% being used for those equipped with a balance shaft. However, we have found by testing using OEM components and weights, we have found that the non-balance shaft engines are factory balanced at 36%. The 200 and 229 V-6 Chevys are listed at 46%. Most other V-6s are listed at 50%.

Mecury balances the 2.5L outboard V-6 at 30%, with 50% being used for racing.

Some of this information is from the Hines Balancing Guide.

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Balance weight

Post by Adger Smith » Wed Dec 07, 2005 4:44 pm

Tom,
Yep thats what we have been using to. I use 50 1/2% on my v-6 engines that I race. (odd fire) It appears to work O.K. It just shakes a little at idle & low rpm. It seems to smoothout when in the operating range,7,400 - 9,600.
Thanks Darin & Schmidt. It is refreshing to be able to see a different approach. Hummm, counterweight inertia is more a factor than reciprocating. thanks, now I see th light.
Darin, any formulas to get a guy started in figuring where to start with diameter of the counterweight?
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Re: calculating bob weight

Post by Caprimaniac » Wed Mar 25, 2020 4:29 am

talon wrote:
Tue Nov 29, 2005 8:14 am
Good Morning Gents

when calculating bob weights for a 90* V8 - is the formula

100% of rotating weight or mass(big rod ends and rod bearings)+ 50% of recipricating weight or mass(pistons,pins,locks and rod small end)

have any of you ever had to balance the CHP SBF 347 cast or forged crank - I heard they were a bit of a challenge.

How difficult is is to make one of these externally balanced cranks a internally balanced unit?

Thanks
Jim

Yess, yess- old thread. But right on the spot in what I just needed.

Data input.
Piston: 452g

Pin: 117g

Rings and Locks: 41g

Rod small end: 232g

Rod big end: 368g

38 + 368 +(232+41+117+452)/2

I end up With 827g bobweight using formula above...? Cannot be right...

Online website, "Performance engine Tech" http://www.performanceenginetech.com/ I get bobweight 1.658g which seem more correct. BTW: Anoynoe on here running that site? No name or any details in "about me" or "Contact me".....
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Re: calculating bob weight

Post by PackardV8 » Wed Mar 25, 2020 11:16 am

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Re: calculating bob weight

Post by SchmidtMotorWorks » Wed Mar 25, 2020 12:24 pm

Caprimaniac wrote:
Wed Mar 25, 2020 4:29 am
talon wrote:
Tue Nov 29, 2005 8:14 am
Good Morning Gents

when calculating bob weights for a 90* V8 - is the formula

100% of rotating weight or mass(big rod ends and rod bearings)+ 50% of recipricating weight or mass(pistons,pins,locks and rod small end)

have any of you ever had to balance the CHP SBF 347 cast or forged crank - I heard they were a bit of a challenge.

How difficult is is to make one of these externally balanced cranks a internally balanced unit?

Thanks
Jim

Yess, yess- old thread. But right on the spot in what I just needed.

Data input.
Piston: 452g

Pin: 117g

Rings and Locks: 41g

Rod small end: 232g

Rod big end: 368g

38 + 368 +(232+41+117+452)/2

I end up With 827g bobweight using formula above...? Cannot be right...

Online website, "Performance engine Tech" http://www.performanceenginetech.com/ I get bobweight 1.658g which seem more correct. BTW: Anoynoe on here running that site? No name or any details in "about me" or "Contact me".....
38 looks like a typo unless it is the bearing

don't divide anything.
Include the big end of two rods and bearings
Include only one set of the reciprocating parts
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Re: calculating bob weight

Post by Dan Timberlake » Wed Mar 25, 2020 12:38 pm

Caprimaniac wrote:
Wed Mar 25, 2020 4:29 am

Yess, yess- old thread. But right on the spot in what I just needed.

Data input.
Piston: 452g
Pin: 117g
Rings and Locks: 41g
Rod small end: 232g
Rod big end: 368g
38 + 368 +(232+41+117+452)/2

I end up With using formula above...? Cannot be right...

Online website, "Performance engine Tech" http://www.performanceenginetech.com/ I get bobweight 1.658g which seem more correct. BTW: Anoynoe on here running that site? No name or any details in "about me" or "Contact me".....
==============.

827g bobweight would be right for a single Rod and Piston assembly.
For V8s with common rod journals fitted with a single bobweight 2 X 827 = 1654.
Add a little oil and vinegar to taste, and there you go.

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Re: calculating bob weight

Post by BillK » Wed Mar 25, 2020 1:00 pm

Capri,
Maybe this will help. Here is a bobweight card filled out for a Chevy engine but you will see the basic idea.
You do not have the required permissions to view the files attached to this post.
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Re:

Post by Truckedup » Wed Mar 25, 2020 2:03 pm

SchmidtMotorWorks wrote:
Tue Nov 29, 2005 2:09 pm


Regarding Harley balancing, when some Harley engines seem to run smooth at certain speeds it is the result of the vibration frequency syncing-up with the resonance frequency of the entire bikeand engine mount. If you mount the engine on a very rigid dyno, you will find there is no smooth out speeds becuase the resonant vibration frequency is higher than the engine can rev.

With rigid mount engine street bikes the balance factor is adjusted for rider comfort. Frame design plays a role here with some being better at resolving vibration. Yes, always a compromise.
Motorcycle land speed racing... wearing animal hides and clinging to vibrating oily machines propelled by fire

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Re: calculating bob weight

Post by Caprimaniac » Wed Mar 25, 2020 3:02 pm

Dan Timberlake wrote:
Wed Mar 25, 2020 12:38 pm
Caprimaniac wrote:
Wed Mar 25, 2020 4:29 am

Yess, yess- old thread. But right on the spot in what I just needed.

Data input.
Piston: 452g
Pin: 117g
Rings and Locks: 41g
Rod small end: 232g
Rod big end: 368g
38 + 368 +(232+41+117+452)/2

I end up With using formula above...? Cannot be right...

Online website, "Performance engine Tech" http://www.performanceenginetech.com/ I get bobweight 1.658g which seem more correct. BTW: Anoynoe on here running that site? No name or any details in "about me" or "Contact me".....
==============.

827g bobweight would be right for a single Rod and Piston assembly.
For V8s with common rod journals fitted with a single bobweight 2 X 827 = 1654.
Add a little oil and vinegar to taste, and there you go.
Yesss, Dan. I didn’t think it through. X2, ofcourse.
Tha online calc used 4g of oil, 0g vinagar.... So, it all ads up.
Only thing left to do, decide if crank can stay 5% overbalanced or not. Bobeeight (as balanced from «factory») is 1750g. (Sbf stroker crank).
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Re: calculating bob weight

Post by Caprimaniac » Wed Mar 25, 2020 4:00 pm

Yes, Bill- 2x ...

38g bearing, correct.
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Re: calculating bob weight

Post by My427stang » Sun Apr 05, 2020 10:25 am

The rod big end looked a little light (but I am an FE Ford guy), and I didn't see bearing or oil values, however, high 1600s is where I ended up too if your numbers are correct
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