Crank balancing

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Harvey
Posts: 107
Joined: Thu Dec 29, 2011 1:05 am
Location: Coffs Harbour. Australia.

Re: Crank balancing

Postby Harvey » Mon Jul 14, 2014 3:38 am

I agree with Graeme and Nigel, that there is no fixed factor, it really depends on the frame and the way the engine is to be used.
To get a handle understanding why we balance you have to go back to basic functions. The piston force going up and down can be balanced completely. The bob weight that we add to the flywheels moves opposite to the force of the reciprocating piston to balance it in the vertical plane, but in doing so we produce another problem.
When this bob weight rotates to the front of the engine there is no reciprocating weight opposite to balance it, same happens when the bob weight is rotated to the rear of the engine, so we develop an out of balance in the horizontal plane.

There has to be a compromise made, if we don’t completely balance the reciprocating weight the horizontal force won’t be as bad, we could say 50% to go both ways, but as there is a difference in the effects of the two different forces we can favour the one that counts. The horizontal forces are felt greatest at low rpms and reducing as the rpm increases. The vertical forces increase as the rpm increases. So this is where we make the compromise to suit the frame and use.

The frame can dampen or amplify an out of balance; it may reduce the effects of the horizontal force, so more of the vertical force can be balanced. Or if the engine is to be used at lower rpms, less of the vertical force is balanced to keep the engine smooth at lower speeds. High speed engines a 70+% balance factor is used as they are running in the higher rpm, and don’t spend much time at low rpms.

The main point in doing a balance is to reduce the reciprocating mass as much as you can, to reduce the amount that you are going to balance the first place, as this reduces ALL the forces. The 88mm Wisco piston that I used was reduced to 470gms, a 70% factor made it very good at speed, 1000km rides were a possibility, but stopped at traffic lights the front brake had to be held on to prevent the front wheel from bouncing around from the horizontal balance.
Harvey.

machten
Posts: 507
Joined: Sat Jan 01, 2011 12:57 pm

Re: Crank balancing

Postby machten » Tue Jul 15, 2014 3:02 pm

Got the crank out tonight. It sure is shiny. Someone had way too much time on their hands! :)

Image

Balancing has been done before...

Image

All looks good inside the engine. The big end feels perfect (Nice one, Nigel!) - a few chips off 2nd gear main shaft in the usual places (not drive faces) that I'll just tidy up and the main bevel shaft shimming was a bit loose for my taste and the 6203 bearings I suspect are not C3 rated and show signs of suffering, but otherwise, looking good.

Anyone recognise these marks stamped on the case surface?

Image

Just an aside...the Electrex generator...might run a spotlight on it...

Image

Kev

joe46ho
Posts: 247
Joined: Mon Jun 20, 2011 10:45 pm
Location: Erlanger, Ky
Contact:

Re: Crank balancing

Postby joe46ho » Sun Dec 07, 2014 2:35 am

Has anyone ever measured the weight on a stock borgo 86mm piston (with rings/pin/clips) ?
This was brought up earlier in this thread but no one responded...
A Borgo oem replacement oversize piston should be ok too... Just curious as to what these cranks where balanced with at the factory, and an OEM 450 piston is one of the things i dont have at all...

Joe
Too many projects to list...
12 Ducati singles currently

machten
Posts: 507
Joined: Sat Jan 01, 2011 12:57 pm

Re: Crank balancing

Postby machten » Sun Dec 07, 2014 12:55 pm

Is it the same Borgo piston was used in the 900 twins? (in which case I think I can measure one). Having run now with a 70% balance for a while, I have to say for a street 450 springer, that's worked out quite nicely.

Kev

Eldert
Posts: 771
Joined: Sun Nov 01, 2009 3:23 pm
Location: Hazerswoude Rijndijk Netherlands

Re: Crank balancing

Postby Eldert » Sun Dec 07, 2014 3:46 pm

Hi Joe

i just put a 450 Borgo piston with a 22 mm wristpin on the scales and it was 545 gram .

Kev only the very late 450's with the B type conrod used the same piston as the beveltwins . wristpin was 20 mm

Eldert

machten
Posts: 507
Joined: Sat Jan 01, 2011 12:57 pm

Re: Crank balancing

Postby machten » Sun Dec 07, 2014 5:08 pm

Thanks for clarifying Eldert.

Kev

machten
Posts: 507
Joined: Sat Jan 01, 2011 12:57 pm

Re: Crank balancing

Postby machten » Sun Dec 07, 2014 5:18 pm

Following Eldert's info, my 450 is in the last produced - it does have the "B" rod and a 20mm wrist pin. After a bottom end failure, the rod was re-sleeved by Nigel (who I'm sure will tell you that's his approach of last resort!), Happily, that bottom end remains perfect, (onya, Nigel!),

In summary however, any measurements I could provide are probably irrelevant,

Kev

Eldert
Posts: 771
Joined: Sun Nov 01, 2009 3:23 pm
Location: Hazerswoude Rijndijk Netherlands

Re: Crank balancing

Postby Eldert » Sun Dec 07, 2014 5:44 pm

Hi Kev

your engine is a very late type . the case bolts in front and at the back of the barrel are spaced further apart as earlyer cases . engine number should be 46xxxx

Eldert

joe46ho
Posts: 247
Joined: Mon Jun 20, 2011 10:45 pm
Location: Erlanger, Ky
Contact:

Re: Crank balancing

Postby joe46ho » Mon Dec 08, 2014 4:35 am

Eldert,
Thanks for the info, does that weight also include the piston rings and wristpin circlips ?
The stock pistons seem to be pretty heavy. This makes me still think about using one of those (heavy) Wiseco 900ss pistons, I still have not bought one, but the 450 im building will be for street use only, and will be ridden very little. Im more than comfortable with removing 30 grams from it if needed (the wiseco) but if it was over 100 grams heavier than i might re-think using one... The only reason to use one instead of buying an omega or Arias would be cost, plus i have used wiseco pistons several times in dirtbike engine rebuilds and they seem to be of very good quality especially for the price... Any thoughts on this ? I know a lot of guys on here lean toward parts that are suited for racing but there are a lot of 450's out there that never see the track (and my 450 MK3 will be one of them)
Joe
Too many projects to list...
12 Ducati singles currently

machten
Posts: 507
Joined: Sat Jan 01, 2011 12:57 pm

Re: Crank balancing

Postby machten » Mon Dec 08, 2014 2:53 pm

You're on to it Eldert.

462965

Kev


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