CRANKSHAFT balancing Australia

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hobot said:

Btw I Jim goofed a bit, O BF gives most vertical vector while 100% give most horizontal vector but one can exceed 100%. 50's factor BF gives like 2/3's vertical motion at front iso, 1/3rd horizontal,

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Yes this is correct:

Lower balance factors shake up & down more and higher balance factors shake front to back more.

And I edited my previous post so it agrees.

A motor in a solid frame with the crank balanced at 72% wet vibrates in a perfect circle at 4000 RPM. The circle is about .040" in diameter. This is with lightweight JS pistons. Stock pistons vibrate much worse.

At 6000 RPM the circle elongates to an ellipse stretching frontwards & backwards - vertical distance stays at about .040" but horizontal distance increases to about .060" to .080". This means that 72% is too high. I estimate that 65% to 68% wet BF is better. The 50% BF given above gives more vertical motion than horizontal motion and that feels worse because you really sense the vertical motion since you are sitting on your butt.

Note that 70% wet balance factor (accounting for oil in the crank) equals about 77% dry balance factor. People should state wet or dry.

Also - installing lightweight JS pistons in a Commando without modifying the crank increases the wet BF to about 65%. All Commandos without exception RUN MUCH SMOOTHER with this set up because the vertical shaking is reduced while the horizontal shaking stays the same.

Note that Hobot measured more vertical shaking than horizontal shaking with a stock Commando. The lighter pistons correct that problem. You want the vertical and horizontal motion to be about the same at higher RPM where the most stress occurs. But you also need to consider at what RPM the motor is used most and check the motion at that RPM.

Oh cool Jim thanks for refining my sense of what's doing what about when. So - in upper range mid 60's wet BF works out about sweetest on isolastics and crank loads, cool. At least by your light piston relief if not by adding to bob weights too. Would be interesting to see how you finally got such good trace measures and very revealing there that much jostle in a Featherbed Norton.

Peel's crank was initially balanced to Norton rods and lightened Cosworth pistons to 77%, but gave too hi CR, so jumped on Jim's NACAR kit to freak out it shot BF into upper 90's, dry. Canaga bored out flywheel to bring back into upper 50's. Do ya think a ring of holes in the light pistons would move BF into the mid 60's as your witness marks imply? If Peel could reliably rev to 8000 I may not want/need Drouin clutter to do most of what I want on leaning. Rocking chair bench building is easiest fun part of hobby.

@Dances with Shrapnel I'm using 45deg cams

Thanks. My confusion again. Somehow I got it in my head you were building a 45 degree offset crankshaft.

The crankshaft is a nice looking piece of work.

Who did your cam work?

jseng1 said:

A motor in a solid frame with the crank balanced at 72% wet vibrates in a perfect circle at 4000 RPM. The circle is about .040" in diameter.

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This is all interesting and nice because it is a direct measurement but a word of caution here. For a given set up if one were to change the motor from a solid mount to an iso mount, the circle/ellipsoid phenomena would change.

Furthermore, for a given bike and holding everything equal, changing from cast iron to alloy barrel will change the circle/ellipsoid phenomena. The reason is as follows:

The change is a result of the resultant forces from the crankshaft and how they react upon the mass of the engine and what the engine is attached to (distance and direction to the Center of Mass (CoM)). With a Commando, it is CoM of the engine, trans, cradle, iso units, oil filter (if attached), sing arm spindle and a portion of the swing arm as a solid assembly. For a soild mount, it is the CoM of nearly the complete bike. Big difference, not only in total mass but also in distance and vector from the crankshaft to the CoM.

The bottom line is for each very specific application, unless someone has done the homework (trial and error) the only practical way to get the results you want are through trial and error. In my instance, Steve Maney has done his homework for me in recommending a BF for a Sealy Mk2 with alloy barrels.

This is why I suggest to the OP that he seek input from someone who has actually balanced and run a 90 degree offset crank. My BF experience is limited to singles and 360 degree twins so when BF numbers get thrown around, I am not sure if they mean or render a similar result to that of a single or 360 crank.

jseng1 is spot on about being careful to specify wet or dry. I recall having to run through some additional calculations since I was using a billet crank with an oil gallery of about 3/8" diameter so my dry to wet change in BF was a bit different from that of a Steve Maney race crankshaft which has a considerably larger oil reservoir in the rod journals (similar to an OEM crank).

that cranks is not mine its Lee Kernich's he was mitshubishi design manager in adelaide.. he build that 76 deg norton cranks for his triumph..

here is mine still in the making, 4340 for 90deg


Keith Jon make the cams for me.. i'm still struggling on the total weight for the flywheel, stock norton cranks weight 22-24pounds, should i try to reach the same weight, heavier or lighter?? will this also effect the balance factor?

about BF i read that it also depends on RPM range that we requested.. for my application i'm told to go anywhere from 45-55%

Who TF knows what you may like in BF. The racer survey on Crankshaft Porn thread had by far most saying they were faster with cranks on heavy side, Yet Ms Peel 360' Combat had like 5 lb lighter flywheel cut down 1/2" smaller OD and had pleasing-useful throttle tire spin response, yet prefect slow idle and tamed off idle response to hi throttle w/o stall out or spin out on loose steep climbs on gravel or grass or mud, yet would leap forward w/o delay dicing paved chiances. Had a number of rev up tests with the inline 4 hot rods up to the 1300 cc Hyuabrusas and beat em all in unloaded 5000-6000 rpm increases to redline events. The inlines were so sluggish just off throtle I had to spot them 4000 rpm to Peels 2000 just to be competive engine rev response wise. varrrRRRRPOOMM vs BLATTT!

With offset cranks allowing one piston to help the other past TDC or BDC, maybe lighter crank would be more effective? A call to Geoff Ed Collins might give some insights to guide you. http://www.offsetcrank.com/norton.htm

Personally my opinion is that too loose or too rigid a chassis on tire vector conflicts needs more crank mass to tame their upset tendency on leaning on power and traction edges.

I have a penchant for lighter on the road use. Gives me better drive out of corners (better acceleration) which infers more useable torque. Been told it does nothing for overall peak horsepower (maybe even diminishes it a bit) and at least one reputable fellow on this list can back this up with dyno data.

A BF of between 45% and 55% is quite a range. I believe there's someone near Toronto Canada who is doing weld up cranks at 90 degrees; maybe he can chime in on best BF for this particular application.

I don't recall if you ever stated whether this was for a Commando or a Featherbed frame. You showed a Triumph cam lobe picture above; is it safe to assume this crankshaft is going into a Norton?

I need to ask the obvious but are the cam lobes properly phased to the crankshaft?

Good looking stuff!

John, on the better dyno power with more crank mass, was this 360' or offset crank? About how much difference measured?

Lightening the rods/pistons or increasing crank mass has similar BF effects. The fastest badest reving Norton I've experienced was the P!! dragster and for sure it had a ligther crank and for sure vibed the fillings out of me so definitely no offset crank. My wonder in BF in isolastics no longer concerns vibration but tire pulse bite on various surfaces and lean angles.

First of all - when people suggest a BF how do they qualify it? By the sensitivity of their butt and how well they remember how a Nort vibrated one day and they how it vibrated a few days later after they changed the balance factor?

The first clue is that Nortons are balanced anywhere from 52% to 80% and that 67% is the midpoint average.

Then you hear that different frames, isolastics and tilt of the motor effects everything.

And the science is confusing. If you switch a heavy rod cap to a lighter rod cap on the same rod you get difference weight measurements on the small end of the rod even though in fact the small end of the rod did not change weight (because of the way that each end of a rod is weighed horizontally). This means that OEM alum rods with heavy steel caps will give different BFs than all steel or all alum aftermarket rods - so things are screwy, complicated and messy.

There is a way to actually verify correct BF. This is the only way I know of to figure out which BFs are correct and which are just regurgitated BS. NOTE THAT THE TEST BELOW CHALLENGES THE NOTION THAT HIGHER BALANCE FACTORS WORK BETTER AT HIGHER RPMS (I USED TO BELIEVE THIS BUT NO MORE).

Here is a crude example below:

The scratch tool is a piece of 1/32 ID brass tubing with a 1/32 piano wire sharpened to a needle point (hobby shop stuff) . The brass tubing is taped to a heavy steel block and the block mounted on something solid like a car jack or a stack of bricks. The needle should not extend more than 1/8" from the brass tube or the steel block or it will flex and not give a accurate reading. The opposite end of the wire should extend out of the brass tube so you can push it at the right moment. A piece of polished steel sheet metal is taped to the timing cover NEAR THE AXIS CENTER-LINE OF THE CRANKSHAFT (not on the frame).



Start the bike, hold the front brake.

Rev it the the RPM you want to test (4000 or 6000 etc) and have a helper tap the end of the wire as quickly as possible against the sheet metal. Move the bike forward 1/2 inch and repeat the process.

This test is very difficult to get right and it needs refining. I could only get a good witness mark about 2 times out of 10 tries. And you have to have a microscope to see the elliptical scratch clearly. It was not possible to get a good photograph so I drew an image (below) of the scratch test of my featherbed with a 72% wet balance factor with JS lightweight pistons & longer rods. The ellipse measured about .020" or so in dimension. It will probably measure a lot more in an isolastic frame or in any bike with heavy stock pistons. As you can see the 72% is too high because the motor shakes horizontally more than vertically at 6000RPM. A lower balance factor would shake less horizontally but more vertically. More tests need to be made and a good time to do it is when the bike is on the Dyno with various frames - isolastics etc. I'm guessing that tests will show the best BFs will be in the 60 to 70% range. Note that 65% wet BF(accounting for oil in the crank) = about 72% dry BF

Commando engines are not Triumph engines, but if you remember - the Triumph Saint 650 had a much lighter crank than a bonneville or thunderbird, and did not perform as well. Many of us have tried to use the lighter Triumph crank for racing - not good. Like a lot of things - great in theory, bad in practice. If the standard commando crank is balanced to 72%, the response acceleration is excellent, and the bike is smooth at 7000 rpm. My bike rocks when idling, but you don't ride it at those revs. Even at 3000 rpm, it is pretty good.
It is easy to tap the existing big hole in the flywheel and fit a threaded steel plug with a slot for a screwdriver and use a spot of RED loctite. You can punch the end of the thread later if the new balance factor agrees with you.

hobot said:

John, on the better dyno power with more crank mass, was this 360' or offset crank? About how much difference measured?

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This is what I was told (or read) by a respected engine builder on this forum. I think the context is 360 degree crank. The conversation included going with a heavier flywheel and improving lap times. I can get my head around why improving lap times but less peak power is one I am struggling with.

I have only one experience where I started with a stock flywheel and raced it, then went to a lightened flywheel and raced it and eventually blew it up catastrophically. My take on it was that it had significantly better drive out of the turns and when launching at the start. I liked it.

Unfortunaetly I do not have a before and after dyno results.

Ok - the two 360' Nortons I've had with lighter cranks via the flywheel both pleased me with their throttle response and no issues at all for idle or stalling on let offs. But hey some like fatter tires while I don't so maybe we both operate cycles differently than others when going gets tough. Lightest crank I've seen listed is a 18 lb billet while factory cranks can top over 25 lb.

Geoff suggest to go with 50% balance factor, and about weight here is what he thought

"Ultra-light weight crankshafts is another area that should be introduced. Racing Nortons are being built using 15 pound, light-weight cranks using exotic steel alloy billets. I have only seen images of these cranks, on the web, but have an idea how they are made. Since my cranks are made for street use, and primarily for smoothness, I’m not looking to build anything less then 16 pounds for a BSA or Triumph, or 18.5 pounds for a Norton. That weight reduction, plus reduced inertial weight, gives an immediately noticeable result, even using a mild cam and stock bore and pistons. Anything lighter will bring back the vibration problems at some point in the RPM range that you are trying to eliminate with an offset crank."

that is a really cool method on measuring the vibrartion..

have you tried free stuff like this (vibration meter start at 0:30)
[video]http://www.youtube.com/watch?v=d7Cqfmr-lwU[/video]

Wow Martin thanks for pointing that out. I sure want an objective way to pass around for vibration pecking order. Regardless of how else a Commando may perform, smoothness tops the list for me. I assume a mid bar mount would be best for baseline. With enough data bases say including just laying on engine cases we be eventually get a sonic diagnosistic sytem as reports of various faults show up and measured again or finally solved a mystery of a unique graphic.

If the crank is 360 de g. the question is always 'smoothness at what rev range?'.
DWS, About pea k power . The major objective of a race bike is to g et maximum acceleration out of corners and top speed towards the ends of the straights. If you build a top end motor and set trhe gearing so you rev the tits off it, you get a similar result to a low end motor with loads of torque giving you more acceleration out of corners by pulling higher gearing harder, and winding up towards the end of the straights. The difference is the top end motor has all the problems due to high revs. So if you are going there - big ports and valve s, short stroke billet crank, huge overlap cams, high balance factor - you will end up building a 920cc Nourish Weslake, not a commando if you want to use a big British twin. I suggest that if you want to build a fast commando engine - think differently to the old 'big is better, point and squirt'. If the bike is all top end, it can be a real bastard to ride. - 'Torque wins races' !

You can have your cake and eat it too and I have a few Nortons that demonstrate that just fine; plenty of mid range torque/power and silly great peak power. It is all in the state of tune and design.

BUT

What we are talk about at this point in the thread is an apparent loss of peak power if going to a lighter flywheel. I still cannot get my head around that one.

Dances with Shrapnel said:

You can have your cake and eat it too and I have a few Nortons that demonstrate that just fine; plenty of mid range torque/power and silly great peak power. It is all in the state of tune and design.

BUT

What we are talk about at this point in the thread is an apparent loss of peak power if going to a lighter flywheel. I still cannot get my head around that one.

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It has to do with the ability to efficiently store the energy produced during the power stroke and release it during the next three strokes. Power can be wasted when you are speeding up the crankshaft and everything connected to it and then slowing it back down over the next three strokes. Too much velocity change on a per revolution basis wastes power. Jim

Crank shaft inertia will also affect how power is measured via rolling drum or water or electric brake dyno's. I of course don't know how it'd show up on meters though, just how it feels on the fly, if not hardly ever backing off of throttle, weedoggiedoo. Light no no flywheel in hot rods makes slow steady idle dicier too so they tend to keep blipping to keep turning over on low rpm power. More pistons out of synch with others also tends to need less flywheel mass.

I am not so sure a dyno of any kind is a good way to measure the effect of changing crankshaft weight.
I will say that anytime I have made a crank lighter it made the bike feel faster but it didn't end up any faster in a drag race from one corner to the next. Jim