B25 Crank shaft balancing

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hammen.2008
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B25 Crank shaft balancing

Post by hammen.2008 » Wed Jul 18, 2012 9:46 pm

I'm building up a B25 race engine and seem to have a problem getting the correct balance.The fly wheels have been machined and I'm using a Carillo conrod and JE piston.These are a fair amount heavier the than standard items and it seems like I actually need to put weight back onto the fly wheels.Looks like I need to start from scratch using stock flywheels.Anybody shed some light on this matter.Any one here in Australia actually built up a B25 race engine? I'd like to hear from you for a bit of advice.

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Re: B25 Crank shaft balancing

Post by minetymenace » Wed Jul 18, 2012 9:58 pm

<135 to the forum hammen.2008

You have come to the right place. Sounds like you need some Mallory metal, but there are pleanty here that will tell you what you have to do and how to do it.

We already have a thriving Oz membership, would you update your profile please and let everyone know roughly where you are?

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Re: B25 Crank shaft balancing

Post by Barry Creary » Wed Jul 18, 2012 10:41 pm

Hi hammen
I'm building a B50 and the crank is getting balanced now to should be done next week , I had a lot of trouble finding some one to do it in Sydney :( , if you are in Sydney PM me and I'll give you his number <033

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Re: B25 Crank shaft balancing

Post by hammen.2008 » Wed Jul 18, 2012 11:53 pm

Barry Creary wrote:Hi hammen
I'm building a B50 and the crank is getting balanced now to should be done next week , I had a lot of trouble finding some one to do it in Sydney :( , if you are in Sydney PM me and I'll give you his number <033
Hi Barry I'm located in central Victoria(Bendigo)I can't find anyone that balances motorcycle crankshafts. I'm aware that I will need to add Mallory metal and that is where I'm puzzled.I followed advice that I was given to lighten the fly wheels and now it seems that I need to put weight back on???.I got in touch with an automotive crank balancer and the price he was asking for Mallory metal is more than a set of standard flywheels.I really can't afford to pay somebody to keep doing the machining just to be told something is wrong with the specs I've been given.I've wasted hundreds of dollars and two sets of flywheels and the guy doing the machining will not do a 3rd set for me as he feels I'm throwing my money away.
cheers Lew.

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Re: B25 Crank shaft balancing

Post by minetymenace » Thu Jul 19, 2012 12:42 am

I have updated your profile, now I'm thinking of moving this topic to the Tech sectio, I'll do this in a few days or once you have posted again...please don't be offended, I'm just housekeeping, so keep posting :grin:

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Re: B25 Crank shaft balancing

Post by roadplough » Wed Jul 25, 2012 1:44 pm

Throw your money AT him.

Not doubting your efforts Lew but I'd be highly surprised if there's no back-yarder in Bendigo with 50 years experience balancing racing flywheels.
I had the problem in this little QLD town so I got in touch with a member of the district vintage motorcycle club who put me in touch with the right man for the job.
Would never have found him otherwise, phone book etc.
But as Barry has said, Ed V could probably advise best.

YAY - housekeeper.. go for it!?
reason: needs better exposure and you need your fix
Last edited by minetymenace on Wed Jul 25, 2012 5:54 pm, edited 1 time in total.
Reason: Mooooved!

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Re: B25 Crank shaft balancing

Post by steve m » Thu Jul 26, 2012 8:50 am

i've got a mate in Kyabram who has been in the industry for a long time. i will see if he can come up with a local for you.
surely it cannot be that critical that you can't leave a bit more weight on the wheels before balancing :???: . just a suggestion (that i'm sure you have thought of).
anyway i'll get back to you.

steve

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Re: B25 Crank shaft balancing

Post by koncretekid » Thu Jul 26, 2012 11:29 am

As you cannot balance a single cylinder motor perfectly, the balance factor is actually only a percentage of the reciprocating mass. If your balance factor was 60%, that means that an amount of weight equal to 60% of the reciprocating mass (piston, pin, and top half of the connecting rod) was added to the opposite side of your crankshaft. You can't use 100% because then the motor would be way out of balance in the fore and aft directions when the piston is at half stroke. So 60% is a compromise. So now if you are using heavier rod and piston, your new balance factor will be a little lower (the balancing weight that was added to the opposite side of your crank will now be less than 60%). But that doesn't mean your bike will vibrate any more, depending on what RPM range you are generally turning. BSA and others tried different balance factors and just came up with the one that produced the least amount of vibration at the most common speed, say 60 mph at 4500 RPM. Now you want to go racing and you'll be spinning that motor at say 6000 RPM at 60 mph, most of the time. My Bonneville prepared bike was supposedly balanced to 65%, but because I'm running it at 8000 plus RPM, I got blisters on my hands after about 8 runs down the salt. Just maybe, your reduced balance factor will be fine. Unless you're entering endurance racing, you're probably going to be too busy to notice the difference in a 30 minute moto. If the new flywheels are lighter, you will get more vibration simply because there's less mass in total to resist the inherent imbalance. If you're going to race, you're going to have to get pretty good at tearing that motor down, so I would just build it and try it.

BTW, dynamic balancing is better because it makes sure both flywheels are balanced the same. But the old guys just supported the crankshaft on bearings, added 50%, 60% or 65% on a "bob" bolted on the crankpin, and then added weight to the other side, probably lead, and welded a cap over the lead. I'm sure they still vibrated!

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Re: B25 Crank shaft balancing

Post by minetymenace » Thu Jul 26, 2012 11:51 am

When the Triumph factory was still going at Meriden, I had a tour. We eventualy got to an old guy balancing the cranks. He was standing next to a fancy dynamic balancer which wasn't in use. He would clip a couple of weights to the big ends and roll the crank on a set of parrallel bars, chalk mark where they stopped, roll it back, mark and once more. He then put the crank under a pillar drill fitted with a large bit and graunched a hole in the flywheel, back on the bars for a quick check and onto the next one. Little did he know that in a short while the dynamic balancing machine wouldn't be the only thing that was redundant.

The guy painting the coach lines on the tanks was a joy to behold. He had a line of tanks (not all Triumph ones, and I'm not sure if the odd Yam/Kwak/etc tank in the queue earned the co-operative or the individual much needed cash), each with a label on with a sketch or note of what needed to be done. He had what looked like gold powder and some sort of solvant or sieze, he dipped his brush in one then the other, read the note, eyed up the tank and in one clean stroke applied a perfect line. A quick wipe with a cloth to tidy up the ends saw a tank coachlined in less than 15 seconds. Probably could't string two words together, but boy could he paint a smooth line!

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Re: B25 Crank shaft balancing

Post by hammen.2008 » Fri Jul 27, 2012 5:53 am

koncretekid wrote:As you cannot balance a single cylinder motor perfectly, the balance factor is actually only a percentage of the reciprocating mass. If your balance factor was 60%, that means that an amount of weight equal to 60% of the reciprocating mass (piston, pin, and top half of the connecting rod) was added to the opposite side of your crankshaft. You can't use 100% because then the motor would be way out of balance in the fore and aft directions when the piston is at half stroke. So 60% is a compromise. So now if you are using heavier rod and piston, your new balance factor will be a little lower (the balancing weight that was added to the opposite side of your crank will now be less than 60%). But that doesn't mean your bike will vibrate any more, depending on what RPM range you are generally turning. BSA and others tried different balance factors and just came up with the one that produced the least amount of vibration at the most common speed, say 60 mph at 4500 RPM. Now you want to go racing and you'll be spinning that motor at say 6000 RPM at 60 mph, most of the time. My Bonneville prepared bike was supposedly balanced to 65%, but because I'm running it at 8000 plus RPM, I got blisters on my hands after about 8 runs down the salt. Just maybe, your reduced balance factor will be fine. Unless you're entering endurance racing, you're probably going to be too busy to notice the difference in a 30 minute moto. If the new flywheels are lighter, you will get more vibration simply because there's less mass in total to resist the inherent imbalance. If you're going to race, you're going to have to get pretty good at tearing that motor down, so I would just build it and try it.

BTW, dynamic balancing is better because it makes sure both flywheels are balanced the same. But the old guys just supported the crankshaft on bearings, added 50%, 60% or 65% on a "bob" bolted on the crankpin, and then added weight to the other side, probably lead, and welded a cap over the lead. I'm sure they still vibrated!
Koncretekid,what are you on?Your reply is of no use whatsoever.I'm needing advice for MY BSA B25 CRANK BALANCE All the stuff you mention I can read in a book.I think the blisters weren't from your handle-bars.

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Re: B25 Crank shaft balancing

Post by roadplough » Fri Jul 27, 2012 6:31 am

Lew, in that case I guess my PM reply was no use to you either ? <201
If you need really specific advise you'll need to submit appropriate data.

Bearing in mind that any advise you get here will be free of charge.

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Re: B25 Crank shaft balancing

Post by steve m » Fri Jul 27, 2012 8:33 am

roadplough wrote:Bearing in mind that any advise you get here will be free of charge.
and posted by people who are doing their best to try and help you, whether you think otherwise or not.

my mate in Ky did know someone but he no longer works in the industry. he gave me a couple of 3'rd party names in melbourne and i could ring them to get the other contact details, but they would be in melb also. is that ok?
there is a company called "cranktech" in cottage st. blackburn. they are mainly a car place but they do old bikes and know their stuff (they did a great job on my B50 head).

steve

PM. there is a guy by the name of Les Bowen who is very fast on anything, but he does race a B25 and it too is fast. he races in the events put on by "the classic scramble club" they have a web site:
http://www.sportingpulse.com/assoc_page ... sID=218575
you might be able to contact him by ringing their ph# or just go to one of their meets.

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Re: B25 Crank shaft balancing

Post by roadplough » Fri Jul 27, 2012 10:32 am

Partly with a view to you maybe recouping some money on the flywheels, exactly how much lighter than standard are they now?

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Re: B25 Crank shaft balancing

Post by koncretekid » Sat Jul 28, 2012 2:07 am

For what it's worth, I AM running a B25 crankshaft, I can turn 9000 RPM's, and I DO race it. I also know what my balance factor is and how it was balanced.

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Re: B25 Crank shaft balancing

Post by Ian Hingley » Sat Jul 28, 2012 7:18 pm

:thumb

Ian

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Re: B25 Crank shaft balancing

Post by Graham » Fri Oct 03, 2014 6:20 pm

Well, I for one found this thread very useful. Thanks konkrete kid and others for the advise.

I've read enough on balancing and balance factors in my life to turn me blue. I can spout off nonsense from half a dozen books but first hand experience and a known good balance factor is very particular to a single engine.

that said, what numbers have you guys used, and in the B25 engine (assume roughly standard components) does going much over or under 60% shift vibrations up or down the powerband?

say, does 65% make the bike smoother at high speed and rough at low speed, or 55% ... etc?

I know its an impossible to answer question, but for the sake of discussion anyone have a decent guesstimate?

now let me get on to reading (and bumping) some of those oil threads.... ;-)

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Re: B25 Crank shaft balancing

Post by HPbyStan » Fri Oct 03, 2014 7:21 pm

http://s296.photobucket.com/user/HPbySt ... ort=6&o=97
Notes under the pics. Clicking on the pic takes you to the same pic on Photobucket and then the left arrow takes you to more pics on the B25 bottom end.
Image
@#$%^&*( Photobucket, nearly every time I try to post a pic it takes a different plan.

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