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Dynamic ballance spec's. Does this look right

10209 Views 32 Replies 12 Participants Last post by  Snakeoil
I likely misread something some where. But to be sure...
Crank was ballanced to 50% Should that be correct?

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Triumph 650 crank balance

In my experience, Triumph got the balance factor pretty much nailed for the general population but dropped the ball when it came to dynamic balancing. They actually could have reversed their reputation for buzzy bikes if they had spent the extra time on each crank assembly. Dynamic balancing makes so much more difference in perceived smoothness than static balancing that they could easily have ignored the static balancing without detriment. I have sometimes sent my cranks to the balancer with instruction not to bother with static balancing.
However when I have, this is the result I got.
50% bf is fine on low compression bikes and 500 pre units. However, the higher compression bikes seem to vibrate ALL the time at the same amplitude but with different frequencies depending on the RPM's(naturally) With dynamic balancing this is redered not very severe but to me it is annoying that there is no RPM range where the bike is perfectly smooth.
58-62%bfOn the recommendations from my balancer(THE hot-rod drag racer balancer in SoCal at the time) this range would serve me best for my type of riding. Bearing in mind that I specialize in pre units, I don't run these bikes too hard, although I have been known to ride them for daily transportation in the greater L.A. area and that means 70mph on the freeway or die. I suspect that these engines would have been quite buzzy at over 6k rpm but I wouldn't know since I never rode them that way. Some of my bikes were electric motor smooth throughout the entire rpm range below 6k.
85%bfThis is what I normally use these days as it serves to not confuse my current balancer. I always upgrade to unit 650cranks for my pre units and they are all 85%bf. The bikes are smooth from start-up but gradually gain vibration as the engine climbs in rpm. There might be a smooth-out after 6k, but there again I don't ride these bikes that way. Tootling along in top gear at 40-45 mph there is no perceptable vibration at all.
Hope this helps.
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I'll be running Morgo 750. 9.5:1 compression.
So I think I'll build it with this crank. If worse comes to worse
I suppose I can always start over with an unmolested crank.:rolleyes:
I wouldn't be worried about the strength of the crank. Most of the weight was taken from the flywheel which is where the factory did it. What little was taken off the throws isn't going to affect the strength. You might like the quicker response of the lighter flywheel because of the extra weight removed. If the engine viberates too much for your tastes at 50% you can just change out the flywheel and rebalance using the original core and a different flywheel.
here are the specs for my 72 tiger 650 with a morgo 750 kit, i had it balanced during the rebuild but i dont really know what it all means except the 76% part is supposedly good
Ok, so I've gone ahead and got a brand new flywheel from Britishonly.
Couple of question marks here.
The first and most evident. It doesn't look like my flywheel.:confused:
It looks like the earlier version.
The part number that's printed on the sticker is 71-2425. As far as I can
tell, this matches the 64 flywheel. The correct number 70-9687 has
been hand written. Now, I don't mind using this one. So long as I'm not
taking a step backwards with it. Just not sure if I should return it and
hope for the correct one. Or just run with it.
After further investigation. It also appears to fit the 71-80 650 and 750 part numbers.:rolleyes:
Don't know if that's better or worse.

Looks like this T140

Instead of this 1970 T120
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This flywheel is an explosion going somewhere top happen. Never mind the "off the wall" balance factor, the balancing holes in this flywheel should never be bigger than 1/2."

The typical factors used on these motors varies between the low 60% to as high as 85%. But what do I know?
After further investigation. It also appears to fit the 71-80 650 and 750 part numbers.:rolleyes:
Don't know if that's better or worse.

Looks like this T140
HOLD EVERYTHING!Luckily you'll have to hold because the later flywheel is too big to fit a pre-'71 crank.
THERE IS SOME GROSS MISUNDERSTANDING OF BALANCE-FACTOR,like what can be used,when it should be used,and what Triumph used.
*You would not use 50% balance-factor on a Triumph twin (most agree on this).On a 90 degree twin (or a Chevy,etc.) with a 2:1 rod/stroke ratio,you would counterweight for 50% of the weight of ONE piston and small-end;which is actually 25% balance-factor.The rotating big-eng weight is 100% counterweighted.
*Triumph cranks are balanced dry,but they are used wet (full of oil).The weight of oil effectively reduces the balance-factor about 2%.With a rod/stroke ratio of 2.0134,a 650 Triumph has least imbalance forces with 62.4% (running wet).This works out to 64.4% dry balance.This would explain EddyJ and the 65% balance-factor.The bike was raced,and they only wanted to reduce stress on the crank and main bearings.
*Vertical imbalance is more noticable to the rider,so Triumph used a higher balance factor at the expense of putting higher imbalance loads on the crank in the horizontal direction.They quoted 85%,but in reality it was more like 80%-81%,on a new bike.They later reduced this to 74% on the T140,which actually required more balance factor (1.05% more) because of the different rod/stroke ratio.
*So,a long-rod engine could use 73% balance-factor (dry) and run as smoothly as a T140 engine (in the same frame).
*With a lighter flywheel,you and your engine will feel more imbalance and vibration.A heavy flywheel at 70% can feel smoother than a light flywheel at 85%,and the lower balance factor puts less stress on the crank.

*You will certainly need to dynamically balance the new flywheel to eliminate any rocking couple.This is more important than changing the balance-factor.
*The final balance-factor you use depends on your frame and your intentions.A Rickman frame runs smoothly with 65%,a Triumph frame feels smoother with more.If you end up with 70%- 75%,it could be a good compromise.
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Yep, that's why I ask and ask and ask.:)
So if I'm straight here. I should send back the new flywheel and hope
they have the correct one. Or start looking for a proper used one.

And NO I'm not in the least bit happy with what the shop did to my crank.
Many years ago I was a tool maker for Boeing aircraft. We maintained a machine tolerance of .001". If one wing was within tolerance of .001+ and the other wing was within tolerance of .001- the wings could be as much as 8 feet longer on one side than the other!
I am not a mathemetician, but the above seems to calculate to 8,000 foot-long wings.
I think he is talking about tolerance stack up. By that I mean if you take a stack of 100 pennies and each penny is +.001 of the design dimenion and another stack with each being -.001. When you compared the two stacks, there would be a .200 inch difference between their heights.

So from this, it would appear that there are 4000 parts that are assembled in a wing that determine the overall length of the wing and if all are .001 over called for size on the dwg. It would make the wing 8 feet longer.

My guess here is that is probably a bit exaggerated over the actual stack up. It is probably a story they used to drive home a point. When you look at a wing, I'm doubtful that there are 4000 parts required to determine the length. It should be several spare members and several pieces of alum skin. Everything else fits in between with sufficient space to not interfere with each other.

We had similar stories when our locomotive business was here. The stories went that if you built everything to the max and min tolerances, nothing would go together. Bores would be smaller than pistons and so forth.

Urban legend existed long before the internet.

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I was going to get my TR7 crank dynamically balanced but I've decided against it. I once had a '66 T120 which had had a single carb head fitted before I bought it. It was incredibly smooth, like an electric motor at all speeds above very slow. That year had the light flywheel (about 2.5lbs lighter than earlier and later models). I put the smoothness down to this at the time.

I've owned several T140s and none vibrated badly. I owned a '72 T120 and that thing shuddered like a pneumatic drill above 5000rpm - it was literally painful to ride and I was glad to get rid of it.

I imagined that if I had my TR7 crank dynamically balanced it would be as smooth as that '66 T120, and I was on the verge of sending my crank away. But then I happened across a website called 'Goffy's A10' (don't have address handy but should find ok by googling - its worth a look, quite interesting). Goffy has done well over 100,000 miles on his A10. He's had the crank balanced 3 times. 1st time, like me, he hoped to get a really smooth engine. Turned out much worse than before. He got it balanced again by a different firm: slight improvement but still worse than before. He finally got it balanced by Dave Nourish - this time it turned out well, that is, just about how it was before he started!

So I'm sticking to the moral of: "If it aint broke, don't fix it" (and saving myself a hundred quid or so!).
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Perfect timing, John. I was just lamenting not having my crank balanced during my rebuild. Guess I'll go find something else to lament about, like not having all my parts to start the reassembly yet.

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