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#834991 12/31/20 1:16 pm
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Hi guys,

sorry for pestering you with the zillionth question around the balance factor for an A65.

I am about to resurrect my early A65 Star puzzle. As the conrods are really old and do also seem to have been mis-treaten significantly in the past new steel conrods will find their way into the engine.

[Linked Image from up.picr.de]

That said, dynamic balancing of the crank is mandatory.
In the past I have done this with the standard balance factor of 70% according to the workshop manual. The bikes are quite okay vibration-wise up to 4k rpm or a tad above until vibration sets in.

I have done a forum search and found recommended figures of around 70% or slightly higher.

What puzzles me a bit is the statement in Pete Crawford's wonderful book the BSA guys would have recommended 55% to 60% to smoothen out the engines at about 6k rpm.

Being well aware that my intended rev-range for a smooth engine will be more in the region of 4.5k to 5k rpm what do you recommend?
Move towards more than 70% or go for less?

Any of your input is highly appreciated.

Cheers!

Phil


Best regards
Phil
Duesseldorf/Germany
'62 A 65 Star (disassembled)
'69 A 65 Lightning
'71 A 65 Firebird
'84 Yamaha SR 500
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I’d have to double check but my Lightning is set to 62%, I didn’t give a value, just asked for it to be smooth around 4500 revs.

It’s been noted that it has more vibes lower in the range but smooth at highway speeds, that’s more because I still haven’t got the carbs in tune properly with the exhaust, when I was using stock setup it was really smooth all the way through.


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68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

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Hi Allan,

many thanks for your swift reply.

Could you check the balance factor if you had some notes about it?

Cheers!

Phil


Best regards
Phil
Duesseldorf/Germany
'62 A 65 Star (disassembled)
'69 A 65 Lightning
'71 A 65 Firebird
'84 Yamaha SR 500
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Just found the carb Phil,

Sadly I was 10% out, it is infact 72%

[Linked Image]

Crucial thing I think is sorting the rocking couple.

Interestingly I have all the data for my A10 cranked/B44 piston’d A65 but no factor listed and I don’t know how to calculate it.

C553F708-0127-417B-9B17-E75D0415119E.jpeg

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68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

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I dont know what my bike is balanced % wise, i do know the B44 pistons fitted are lighter than stock A65 pistons, I think it has the stock 70 % , its smooth from 3.5 - 5.5 K, gets ruffer after that, for road use this is fine. I seldom run at over 5.5 K for any length of time.
I weighed my MAP rods, they are approx 30 grammes heavier than stock BSA rods, whether this is at the big end or small end I dunno.
If I was you I would weigh my old rods small end, and compare to the MAP small end weight. To do this support the rods at the big end so the small ends are weighed with the rod horizontal, if its within a few grammes I would leave well alone and just dynamically balance, I suspect most of the extra map weight is from the steel around the big end.


71 Devimead, John Hill, John Holmes A65 750
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Thanks for your input, mates.

Also after a forum search it looks like common sense is to stay around the 70% factor according to the manual or go a tad higher for a dry frame A65.

I do also prefer the rev-range between 4k and 5k rpm, so BSA's Nigel Rollason's comment to go for a balance factor of around 60% for a smooth engine at 6k rpm will not be in line with the bike's use.

I do have a dynamic balancing shop nearby. In the past, I had dropped the crank, the rods and pistons, provided the requested balance factor of 70%, and they sorted it out including weighing the rods. As stated before, the bikes are reasonably smooth until 4.3k rpm when vibration starts to kick in.

After all, there are an awful lot of factors to consider, starting with the frame and the engine fixings, resonance figures and further more.

I'll let you know how it turns out once the bike runs.

Cheers!

Phil


Best regards
Phil
Duesseldorf/Germany
'62 A 65 Star (disassembled)
'69 A 65 Lightning
'71 A 65 Firebird
'84 Yamaha SR 500
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A parallel twin will never be vibration free without a balance shaft.
The higher the RPM the more powerful the vibration. There will be engine speeds when the engine vibrations are in harmony with the resonant frequency of the frame


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Originally Posted by Andy Higham
A parallel twin will never be vibration free without a balance shaft.
The higher the RPM the more powerful the vibration. There will be engine speeds when the engine vibrations are in harmony with the resonant frequency of the frame

But there’s vibration which I’m sure is there but you can hardly feel if at all. Or vibration where you can’t keep your feet on the pegs.


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68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

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My Polish friend weighted steel MAP rods small ends and find out they are nearly the same in weight as originals.
So most of a difference in weight will be included in a big end of the rods.

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Allan
The can't keep feet on the pegs type vibration is usually when the harmonics of engine and frame coincide. If they are out of phase the vibration is less noticeable
Handlebar vibration can be tamed by heavy end weights or filling the ends of the bars with lead.
Changing the balance factor is exchanging up/down vibration for fore/aft vibration or vice versa.
My GM 500 powered bike isn't a "vibe monster" even though it is a single cylinder engine without balance shaft and revving to 11,500-12,000


BSA B31 500 "Stargazer"
Greeves 200 "Blue Meanie"
Greeves 350
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Suzuki GSX1100 EFE "Sorcerers Apprentice"
GM500 sprint/LSR bike "Deofol"
Jawa 500 "Llareggub"
Aprilia RSV Mille "Lo Stregone"
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Originally Posted by Adam M.
My Polish friend weighted steel MAP rods small ends and find out they are nearly the same in weight as originals.
So most of a difference in weight will be included in a big end of the rods.

I noticed yesterday that Marino has the little and big end weights for all the rods in the Map catalogue if that helps anyone. The weights are the same as the late 70’ on rods (the early rods are about 20grams lighter)


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

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Originally Posted by Andy Higham
Allan
The can't keep feet on the pegs type vibration is usually when the harmonics of engine and frame coincide. If they are out of phase the vibration is less noticeable
Handlebar vibration can be tamed by heavy end weights or filling the ends of the bars with lead.
Changing the balance factor is exchanging up/down vibration for fore/aft vibration or vice versa.

As I said earlier
Originally Posted by Allan G
Crucial thing I think is sorting the rocking couple.

That will effect any engine severely. Following that, I agree with you.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

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Hi all,

surely, dynamic balancing the crank will sort the rocking couple and even out left and right pot. I am also perfectly aware that a parallel twin with a 360°-crank without balancer shafts will always shake significantly.

What has me left puzzled is Pete's book with Nigel Rollason's comment and the indicated 55% to 60% figure.

After all it is trial and error but for obvious reasons I am not really keen to dis-assemble the engine several times until having found a decent balance factor that suits my preferred rpm-range in the dry frame bike.

Surely resonance and the frames play a role as my dry frame Lightning behaves different to my oif-Scrambler although both cranks are dynamically balanced to 70%.

Thanks your your valuable input.

Cheers!

Phil


Best regards
Phil
Duesseldorf/Germany
'62 A 65 Star (disassembled)
'69 A 65 Lightning
'71 A 65 Firebird
'84 Yamaha SR 500
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AFAIUI the 56 % BF is for racing when the motor will spend most of its life in the upper rev range. Fine if you ride around flat out all the time.

As for dynamic balancing, my motor was smooth before, its smoother after the dynamic balance, my footrests dont have rubbers and I feel little or no vibration through them no matter what the revs. When it gets rougher over 5.5 K I feel it through my crotch. Awaiting salacious comments.

I can only really comment for the OIF, most stuff is rubber mounted , however in the past I have had vibration damage, the coil mount was cracked, and it eats rear number plates, most of the shakes same to be amplified at the tail end. possibly the reason for the beefy grab rail mudguard support, without out this the rear mudguard has a very short life.

Last edited by gavin eisler; 01/02/21 3:21 pm.

71 Devimead, John Hill, John Holmes A65 750
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Surely, the 56% BF is intended for race use ways beyond my ususal riding style.
The bike is old, I am old as well and the bike is also too noisy to ride at 6k rpm.

As stated by Andy, vibration is (rider-) felt differently up and down/ fore and aft. The latter seems to be considered less disturbing.

From a gut feeling I would have expected the BF to go up for smoother ride with higher rpm but have learnt this is not the case.

The nasty thing about all this is that vibration is witch craft/rocket science and either requires very very detailed mathematical models or trial and error-based experience. And even then the frame and engine fixings do have significant impact.

So after having knowledge of the BSA race shop figures I was tempted to go for the middle, anything about 63% to achieve any comfortable zone between 4k and 6k rpm, but unfortunately, vibration behaviour is complex and not linear. Sigh....

Many thanks for your patience.

Cheers!

Phil


Best regards
Phil
Duesseldorf/Germany
'62 A 65 Star (disassembled)
'69 A 65 Lightning
'71 A 65 Firebird
'84 Yamaha SR 500
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Aside from the rocking couple, the problems a parallel twin has with balance are the same as a single-cylinder engine. The only way to significantly reduce vibration in either is to have the lightest piston and rod small-end that is possible, and the longest rod possible.

The less your piston and rod small-end weight, the LOWER the balance factor can be, which is an advantage because the crank counterweight has less effect when it is at 90-degrees to TDC and NOT being used to counter the weight of the pistons and rod small-ends.

Dynamic balancing will take care of the rocking-couple and other details that are difficult to solve with static balancing, but aside from that it will not be as helpful as getting light pistons and rods.

For the Triumph and Norton there are special long-rod kits that use a piston with a piston pin set as high as possible, and this is a great advantage because the longer the rod the slower the acceleration of the piston before and after TDC, so with no other changes the longer rod alone makes a big difference in reducing engine vibration and lets the balance factor go ever lower thus reducing vibration even more.

If I was going to spend a lot of money on new engine parts, I would certainly go for the long rods and lighter than stock pistons and a lower balance factor than stock. With longer rods some have chopped 15% off the balance factor with amazing results in vibration reduction.

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I trusted my '69 A65 crank to a local shop, along with + 0.020 pistons, pins, clips, rods, and bearings. We talked about the factory recommended factor 70% factor. He consulted the balance machine manufacturer and went with their recommendations. Here is the paperwork he provided.
[Linked Image from i.postimg.cc]

My crank looked like freaking swiss cheese!!! I probably should have taken pictures, but I was so appalled I didn't. He assured me that when first set up the balance machine shook like crazy! He got it as close as he could but assured me it would be much better. (Sigh, ok) I paid the man for services and took the crank with a wait and see attitude. I don't have a tach, and the speedometer has failed as well. Subjectively: The engine is smoothest and happiest at speeds from 50 to 60 MPH. Vibration increases considerably at higher speeds. True highway speed is not much fun. Riding around, the BSA is quick through the gears and has plenty of torque. It is smoother than the '79 Bonneville I had. With a 5th gear and bigger lungs the Bonne was a faster bike, quite capable at highway speeds, but it was more buzzy across a wider rpm range. The BSA is not as smooth as the '74 Norton 850 Commando I had, for obvious reasons, but it may be as quick, and the clutch hasn't slipped once. Slipping clutch was an ongoing nuisance with the Norton.

As a neophyte engine builder, I'm just happy it runs, so far is relatively oil tight, and I can ride it around.

Come on warmer weather......

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Thanks for the paperwork, Tracey. I do struggle a bit but will do the maths tomorrow and hope to achieve figuring out the actual balance factor Buddy Rice applied.

A low vibration level at 50 to 60 mph sounds good if the bike has a 20 tooth engine sprocket as I'll habe a 21 tooth one. This would raise the usuable speed.

Many thanks again and cheers!

Phil


Best regards
Phil
Duesseldorf/Germany
'62 A 65 Star (disassembled)
'69 A 65 Lightning
'71 A 65 Firebird
'84 Yamaha SR 500

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