Balancing a engine to match a frame ?

ashman said:

I don't know what he done to my crank

Click to expand...

That doesn't help much then, does it !

That 72% is probably the vital bit though, if thats what he actually did...

I wasn't there when he did it but could see what was done with the drilling and a few other things, but how he did it at the time and all I can do was to take his word for it (how many machine shops will let you watch when they are working), all I know he's done a good job of it as my Featherbed is great to ride with no problems with things falling off and riding it between 70 and 90 mph it will do it all day if you could without being pulled over, its smooth down low I ride in top gear just on 40 mph with out any problems, the cam works good at low revs buts kicks arse when it hit 4,500 rpms and over and has no troubles getting to the ton very quickly and over.
I have learned a lot in the 40 years of owning my Norton, knowing every nut and bolt, I have rebuilt a few motors and gearboxes for mates with Nortons Commandos in the last 20 odd years and haven't had any complaints about my work yet and all machine work is outsourced anyway, all I do is pull them apart, tell them what is needed parts wise or how far they want to go with the rebuild, sometime I get the parts for them or they chase them up, I send the parts out for machining then put it all together when everything is back in my hands.

Ashley

This is true, but I was just commenting about the balance factor - its precisly the what that folks are interested in.
Very few could do it themselves - I certainly wouldn't. The investment in equipment alone ...
But I have had a look afterwards, and measured and compared things.

I remember the sign in the 1st motorcycle workshop I ever saw.
Charges
$10 per hour.
$15 per hour if you watch
$20 per hour if you help.

Rohan said:

This is true, but I was just commenting about the balance factor - its precisly the what that folks are interested in.
Very few could do it themselves - I certainly wouldn't. The investment in equipment alone ...
But I have had a look afterwards, and measured and compared things.

I remember the sign in the 1st motorcycle workshop I ever saw.
Charges
$10 per hour.
$15 per hour if you watch
$20 per hour if you help.

Click to expand...

Thats true, as any machine work you got to put your trust with the trade person doing the work (he told me he did the B/F at 72% so I had to take his word for it), Ivan Tighe, when he did my cam work he was only in a small workshop, but it grew into a big business, but he retired many years ago and his sons took over the business, he wounldn't have put me on the the crank balancer without having knowing his workmanship and that how word gets around whether someone is good at what they do and you soon find out the ones to avoide.

Ashley

ashman said:

and that how word gets around whether someone is good at what they do and you soon find out the ones to avoide.

Click to expand...

If only that was always true. !

I had a crank (single cylinder) rebuilt by someone who sounded to be good.
When I got it back (and paid) he told me he didn't have any new big ends, so it had been fitted with a 'good used one'. !
When I had a close look at it, it had also been trued with a steel hammer = marks on the flywheel.
Cross him off the list. And look for another crank. At least they were plentiful back then.

84ok said:

any idea with triumph or bsa? what they did?

Click to expand...

I'm not familiar enough with them to say for sure. Anyone ?
Some Triumph ones are a one piece crank, with a bolted on flywheel in the middle.
Which, in theory, makes them better than Nortons bolt-up 3 piece design.
Bert Hopwood had a hand in the design of each of them, with others.

Matchie twin cranks were designed/built with a centre main bearing as well, which in theory also makes them a better idea.
But they had a lot of trouble with that centre bearing, and vibration, and cranks breaking.
So good theory doesn't always turn/translate to good outcomes...

Rohan said:

jseng1 said:

To find the best balance factor for certain - see the top post in the thread below.

Click to expand...

Without chewing through all that again, have you applied this to a non-Commando and had a notably smooth motor ?

The difference between BFs, unless they are really wide variations, is only a few ounces of counterweight at most.

Click to expand...

Yes
I raced a featherbed and built a solid frame monoshock. I would revv into the valve float range. I tried every balance factor between 85% down to 48%. With stock pistons and rods vibration was always a problem. Things cracked and broke. If the balance factor was too low or high it just got worse. So I went to lighter pistons and things got better. I got tired of all the different BF numbers and measured the actual shaking motion of the motor. 72% wet was too high. See the scratch test to know whats really going on. And be sure your are talking "wet" or "dry" because a wet BF of 65% = a dry BF of 72%

Longer rods helped even more. The solid frame 750 with 170 gram pistons is smooth enough that you don't even think about vibration. The measured movement of the motor in frame is down to about .020". No need for isolastics.

You want to take the stress off the crank. You don't want the motor shaking one direction more than another. You want the scratch test to result in a round orbit - maybe a little more front to back ellipse than up and down in consideration of your butt sitting on the seat.

Again - read the top post of the thread below.

balance-factor-scratch-test-tool-t15134.html?hilit=BALANCE%20FACTOR%20SCRATCH%20TEST%20TOOL

Spend your time finding out whats real and actual. Make scientific measurements. Then make corrections accordingly.

Earlier in the thread the assumption as made that the pistons,con rods and end caps were matched. Then the poster said it was all assembled and spun up for balancing. I was under the impression that the combined weight of each piston, rod,pin etc. would be made into a similar sized weight, clamped on the big end journal and then spun up. You can't just spin up the whole nine yards in it's raw state.
(the "whole nine yards" is the length of the belt of ammo fitted to a Spitfire machine gun by the way)

Rohan said:

It is variously quoted places that the Atlas balance factor WAS taken out to 84 or 86%, to suit its frame.

It is also quoted various places that it has since been discovered that AMC contracted out the crank balancing,
and that the guy doing it was just randomly (?) drilling holes in cranks - to make it look they were being being balanced.
Or maybe that was an attempt to make all cranks the same, when the basic cranks weren't, so that process was flawed (??).
The mind boggles....

Click to expand...

i guess they never got this far
balancing-engine-match-frame-t23077-15.html#p303174

Rohan said:

Without chewing through all that again, have you applied this to a non-Commando and had a notably smooth motor ?

The difference between BFs, unless they are really wide variations, is only a few ounces of counterweight at most.

Click to expand...

jseng1 said:

Yes
I raced a featherbed and built a solid frame monoshock. I would revv into the valve float range. I tried every balance factor between 85% down to 48%. With stock pistons and rods vibration was always a problem. Things cracked and broke. If the balance factor was too low or high it just got worse. So I went to lighter pistons and things got better. I got tired of all the different BF numbers and measured the actual shaking motion of the motor. 72% wet was too high. See the scratch test to know whats really going on. And be sure your are talking "wet" or "dry" because a wet BF of 65% = a dry BF of 72%

Longer rods helped even more. The solid frame 750 with 170 gram pistons is smooth enough that you don't even think about vibration. The measured movement of the motor in frame is down to about .020". No need for isolastics.

You want to take the stress off the crank. You don't want the motor shaking one direction more than another. You want the scratch test to result in a round orbit - maybe a little more front to back ellipse than up and down in consideration of your butt sitting on the seat.

Click to expand...

Thanks for the summary, I seem to recall most of that.
Does Jim say what balance factor he went with with the lightweight pistons ?
Have they done a significant number of road miles on test with anyone yet ?

About that .020" the engine jumps around.
How much does a less 'balanced' engine move ?!
Thats a new one on me....

Rohan said:

84ok said:

any idea with triumph or bsa? what they did?

Click to expand...

I'm not familiar enough with them to say for sure. Anyone ?
Some Triumph ones are a one piece crank, with a bolted on flywheel in the middle.
Which, in theory, makes them better than Nortons bolt-up 3 piece design.
Bert Hopwood had a hand in the design of each of them, with others.

Matchie twin cranks were designed/built with a centre main bearing as well, which in theory also makes them a better idea.
But they had a lot of trouble with that centre bearing, and vibration, and cranks breaking.
So good theory doesn't always turn/translate to good outcomes...

Click to expand...

turns out triumph BF was ~ covered somewhat in this (prior) post, might be interesting what all was involved in the "spot on" setup
http://www.ngwclub.com/forum/viewtopic.php?f=16&t=42680

Changaroo said:

My Royal Enfield Interceptor has a balance factor of 75%. Interceptors and Commandos both have long stroke motors and I think a similar factor would be suitable for the Commando engine.
When I rebuilt my Interceptor motor I could no longer purchase the original pistons only aftermarket ones, and they were heavier.I took my crank and, when I phoned, 'all items attached to the crank' to a re-balancing place. A lot of ''experts" told me that 75% was ridiculous. The chap who did the re-balancing told me they were wrong. He said all long stroke motors should be balanced to a similar factor. He also said that Triumph twins did not have any quality control over the crank balancing. He said some were way out while others were spot on.We have all heard the stories about getting a dud bike built on a 'Friday' and a beauty that was built on a 'Monday'. Perhaps it was just slack quality control.

David

Click to expand...

Rohan said:

Thanks for the summary, I seem to recall most of that.
Does Jim say what balance factor he went with with the lightweight pistons ?
Have they done a significant number of road miles on test with anyone yet ?

Click to expand...

Yes. A friend of mine in Brussels, Yves, a member of this forum, has the whole shebang in his engine, bolted into a Seeley frame, his bike being known as "The Big Spender".

Well, he has covered thaousands of (high speed) miles with his engine, that thing is incredibly smooth, no vibration to speak of, yet it is a missile on the road or on the track. I wish I had the money to build myself a similar engine.

Although the chap did tell me,I can't remember what the "spot-on" balance factor was for Triumph motors, it was about 2008 when I had my Interceptor crank done.
Now for something off topic a bit but interesting .
The excellent book "Matchless Twin Motorcycles by F.W.Neill published by C.Arthur Pearson Limited in 1957 states ( for the Matchless and AJS 500 twin motors) "The balance factor is 50 per cent of the reciprocating masses as shown below". It then goes on to give the weights of pistons, connecting rod small and big end etc. All in grams. Quite detailed. It also states "The two-throw crankshaft is balanced dynamically by the makers, who also weigh each piston and connecting-rod,which are paired to ensure that the balance factor is correct. This is for the 500 twin motor (also the AJS 500 twin) Also states "this engine has a vibration period which comes in at 60-65 mph in top gear"It goes on to say that if that is your regularly used road speed to change the sprocket to move the vibration to a different road speed. Bore and stroke were 66mm X 72.8mm. This book goes into a lot of detail about balancing the engine.

David

The Triumph Forum guys say that the 650 was 85% and the 750 was ~74%.
That was factory, but everyone has their favourite number....
Jim makes the point about wet and dry numbers. (which is only a matter of an ounce or 2.)

Not that this has much to do with Commandos.
But high factors were factory favoured for solid mounted engines,
- makes the shakes go more fore-and-aft, rather than up-and-down which the rider will feel more.

Common use for rigidly mounted racing Triumph 650 engine in featherbed frame is about 78%. The stroke is a lot shorter than Norton 750/850. I use 72 % in my 850 rigidly mounted in the Mk3 Seeley frame. The whole bike rocks backwards and forwards when the motor is idling, however at 7000 RPM it is super smooth. I would have used about 78% except I couldn't achieve that without drilling dangerous holes in the flywheel. If you tap and fit a steel plug into the hole in the flywheel of the 850 motor, you get a pretty good balance factor for a rigidly mounted motor intended to rev to 7000 RPM. I used blue Loctite and punched the end of the thread when fitting the plug. If it comes out, you are probably dead.
Depends on what you intend to use the bike for. If it is for commuting, use a lower balance factor than 72% or the shake at low speeds will probably be annoying.

Well mind don't shake at idle its pretty much smooth, I have my idle set at 1,000 rpms and my front wheel just move a little like a Iso fitted Commando, its not shaking all over the place, my foot pegs are smooth all the way through the revs range, I have done a lot of miles on this bike in the 35 years its been in the Featherbed frame and was a everyday ride till I brought my new Thruxton 2 1/2 years ago and now find I enjoy riding the Norton more when not riding it all the time.

Ashley

I actually enjoy the mild shake my bike has at low revs - it helps the adrenalin rush as it smooths out. As it smooths is when the power comes on strongest. The footrest position on the Seeley is more of a problem. It takes about 5 laps of our local circuit before my tendons stretch and I become comfortable and relaxed enough to get stuck into it.

it not in the featherbed frame but rather a 65 matchless frame with the roadholder fork assemblies..... I actually sent my crank out to comnoz
to static balance it to go vertical... give him a shout ..... I'm actually back in town to finish this up, so after I sort thru a couple of minor setbacks I will have it up running soon

Jagbruno said:

Rohan said:

Thanks for the summary, I seem to recall most of that.
Does Jim say what balance factor he went with with the lightweight pistons ?
Have they done a significant number of road miles on test with anyone yet ?

Click to expand...

Yes. A friend of mine in Brussels, Yves, a member of this forum, has the whole shebang in his engine, bolted into a Seeley frame, his bike being known as "The Big Spender".

Well, he has covered thaousands of (high speed) miles with his engine, that thing is incredibly smooth, no vibration to speak of, yet it is a missile on the road or on the track. I wish I had the money to build myself a similar engine.

Click to expand...

Important considerations:

The cost of a broken crank and or cases can run from $2600 to $3600 depending on quality. A broken crank or fatigued arum rod can take out your entire engine. The cost of a lightweight piston set and longer rods to help prevent crank and case breakage is under $1200 (and you will eventually need new pistons anyway).

A wet balance factor of 65% equals a dry balance factor of approx 72%.

Cutting a radius at the PTO shaft is the best thing you can do to strengthen a stock crank.



Balancing your crank with too high (or too low) a BF will increase the stress on your crank. The lowest stress balance factors are in the mid 60s range - wet.

Note that Yves switched to lightweight pistons because his crank broke with the previous heavier pistons.

Hi Jim , thanks for the pic of your radius crank cheek.....may be more sketches could be usefull for us! I had tried to do it on my Seeley crank , but not sure if I had done it in correct manner?