Building new short stroke engine

Hey Steve.
I will be using the motorcycle on the road for most of the time, sometimes a day at the track.
Here in Norway we have 98 octane gasoline at most gas stations, so I will now try to achieve 10:1. But I do not achieve this with a volume of 46 cm3 in combustion chamber, plus .020" head gasket. Then comes volume of the piston in deduction.
As mentioned earlier that I consider using Fullauto head and preferably after Jim has fitted larger intake valves.
But can anyone tell me about the volume of original and Fullauto head.

When it comes to cam. I have chosen to buy JS Stage1 with complete set of valve system; tabs, push rods, springs and valves. Believe it or not so will this reduces valve system with almost 200g total for all four valves system.

Kvinnhering said:

Hey Steve.
I will be using the motorcycle on the road for most of the time, sometimes a day at the track.
Here in Norway we have 98 octane gasoline at most gas stations, so I will now try to achieve 10:1. But I do not achieve this with a volume of 46 cm3 in combustion chamber, plus .020" head gasket. Then comes volume of the piston in deduction.
As mentioned earlier that I consider using Fullauto head and preferably after Jim has fitted larger intake valves.
But can anyone tell me about the volume of original and Fullauto head.

When it comes to cam. I have chosen to buy JS Stage1 with complete set of valve system; tabs, push rods, springs and valves. Believe it or not so will this reduces valve system with almost 200g total for all four valves system.

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The 98 should be fine up to 10:1, but not much beyond


I will probably go withthe JS stage 2 set up, seems to make a lot of sense to use the light valve gear if you plan on using all the available revs.

I think it is difficult getting the compression with the short stroke, actually Jim's dome pistons were not available when I bought mine or I would have gone for those. With a fully sphered head it may be even harder, my original motor way back used a short stroke hemisphered head with the works omega cast pistons for around 10.25:1 in an 850, when i dropped a valve Mick Hemmings recovered the head and we used hepolite flat tops, taking 1.5mm of the barrell and 1.0mm of the head, the pistons were then just chamfered at the edges to clear the head, but no squish band to worry about....there has to be a solution in there somewhere, but I am a long way from starting this build.

Are you considering replacing the valve seats on the head you have first to see what you get then?.....bearing in mind that this will also reduce valve stem length above the guide....

Fullauto is a good way to go, and Jim (Comnoz) will tell you about your options for a short stroke I am sure, but it looks like you aren't far away with the head you have, you could complete it as a first build and see where you need to go, an original 850 head well modified will have a sale value/be a good spare anyway....

When you think about it, why would Norton compromise engine durability once they sorted out a vibration isolation mechanism. Why would they not just leave it at 83% if it were ideal for the engine since the whole thing was now rubber mounted.

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I'm not trying to be adversarial just realistic as I know. Sorry if you like smearing my meaning, the freguency part I mention is easier to take aligned longwise with bike rather than vertical. As to the isolastic need of some BF, I believe Norton simply trial/errored till settling on the simpler cheaper mid 50's BF as more material and maybe even heavy metal slugs needed to rise BF into the sold mount range. i live in loose conditons and push into loose conditons on tarmac so have sense the isolastics also dampen the tire grip hits and my main wonder on BF is how it affects traction on edges on tarmac and climbs on the shitty loose stuff. BF only concerns pilot and frame protection and only reason to go solid mount is its more stable than un-tammed rubber mount but when the rubber and thin tubes no longer rebound just comply to pass loads through, OH LA LA! To investigate the traction affect of balance factor orbital on the power unit's longitudinal tire hysteria hits I will even try like 110% BF.

This short stroke would seem to me to have a wicked potential but needs some CR to compensate of decent over lap duration cam. Bath tub cambers like Sir Eddy has might be another approach but going in stages is the most educational.

Dances with Shrapnel said:

When you think about it, why would Norton compromise engine durability once they sorted out a vibration isolation mechanism. Why would they not just leave it at 83% if it were ideal for the engine since the whole thing was now rubber mounted.

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hobot said:

I'm not trying to be adversarial just realistic as I know. Sorry if you like smearing my meaning, the freguency part I mention is easier to take aligned longwise with bike rather than vertical. As to the isolastic need of some BF, I believe Norton simply trial/errored till settling on the simpler cheaper mid 50's BF as more material and maybe even heavy metal slugs needed to rise BF into the sold mount range.

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"Just realistic" and "Smearing of meaning" are a good ones; I'm hands off on those remarks. :lol:

I accept that Norton may have wanted (needed) to shift the vibrations fore and aft for comfort on the solid mount frames as I can attest to what a featherbed feels like with a Combat engine at 53% BF.

So what savings do you imagine Norton would realize by having to mill off or drill more material from the flywheel opposite the rod journals to make a 53% BF (wet) for Commandos? I am trying to understand why they felt this needed to be done as the whole thing is in rubber now. Why would they compromise a design if 83% or 75% or 63% is superior?!?!?!? :roll:

Work a moment diagram across the crankshaft and see what 50% and what 83% does with respect to amplitude of the bending load reversals.

There's also the fact that the solid hook up Norton engines were somewhat constrained by the mass of the frames whereas the Commando Isolastics allowed the engine to literally float when vibrating. In my opinion, this in itself is a benefit to the durability of crankshafts and crankcases.

For the subject Shortstroke build the owner should consult Steve Maney for specifics on BF as there are some subtle differences between a stock stroke and short stroke.

Start a new thread on BF as this is getting a bit off topic.

The balance factor is not irrelevant to your project. There is a bloody great big hole bored in the standard 850 flywheel opposite the journals. If you tap and plug it with steel the BF moves to about 70% which is perfect for high revs about 7000 rpm. If you do those revs with a balance factor of 58% you will probably shag the cases , regardless of the bloody stupid isolastics. Short stroke is a good idea, but it commits you to higher revs, and all the rest of he do-daddys, to get top end. A standard stroke billet crank might be a much better option in a commando engine.. I've read a bit about what Peter Williams did with the works commandos. Some of it wasn't so smart. They even sent one of their guys to buy some racing triumph cams, so they could copy them - wrong characteristics for such a long stroke. E3134 Triumph racing cams, used with separate pipes come on with a bang at 4,000 revs and go to 8,500, then blow up pre-unit motors. However that is what your short stroke motor should be using. Your balance factor should be between 75% and 80% (not critical but if it's too low, the motor won't be smooth when revved ) The Commando crankshaft design is intended to only go to about 6,500, and I'm sill waiting for mine to explode at 7,000. Some of us have played with big British twins all of our lives - take note of what is said to you it could save you a dollar.
I built my Seeley Commando 850 in about 1980, but never ra ced until 2003 it because I did not believe in it because of the stupid motor design. These days I concentrate on getting even more torque. That is what the motor is about. Developing more top end is silly - better to start with a nourish weslake engine in the first place, it would be cheaper !

For what it is worth, I just chatted with Steve Maney and he said for an isolastic application it should be per factory at 52% BF (dry).

If I were building a short stroke application for a Commando isolastic system 52% BF is where I would start. To me, it all seems to make sense.

For what it is worth, I just chatted with Steve Maney and he said for an isolastic application it should be per factory at 52% BF (dry).

If I were building a short stroke application for a Commando isolastic system 52% BF is where I would start. To me, it all seems to make sense.

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Acotrel has the experience to follow on solid mount balance factor regardless of the stroke length, its to protect pilot and engine cases and frame at hi rpm. Now being adversarial to Dances, I've quizzed the whole world for 12 yr+, [minus Dances] on what BF to use in isolastic big block Peel to find reports of successful isolastic racer with 93% BF. So as long as isolastic mounted BF is over 50% then its good enough to protect bike and pilot and not a danger to raise BF by as much as ya care to spend on heavy metal slugs. The oil mass in sludge trap raises BF a couple-3 points over the dry 52% BF like Steve Maney example above. Sorry Dances but you are missing some data points to compute economic or production reasons for factory lower BF's as there is no other reasons not to raise BF otherwise. If ya think hobot is mis-leading you again, Jim Schimdt has come to same understanding too - testing my claim with his own witness marks proof in solid mount.


Low BF make orbitals more vertical while hi BF makes em more horizontal. Isolastics can take up the vertical vibes of low BF but not the crank jerking vertically on cases so if meaning to use the short stroke rpm potential then upping BF over minimalist= cheapest factory range is a good thing across the rpm range.

In Peel JSM light pistons/rod shot her previous 77% BF with lightened Cosworth pistons/Al rod into upper 90's % so Ken Canaga drilled out 2 big threaded holes to bring back into 50's BF but makes it fairly easy to alter BF above this to see what I can feel, especially in the rear grip department as both the crank throw and power hits shove the power unit fro and aft as various frequencies. Vibration is not an issue with isolastics so not my reason to mess with it.

We've had discussion on short stroke torque prior to see the many above listed reasons it should be as good or better than long stroke down low. I suspect my P!! was short stroke as did not decrease in power over 7000 rpm to 9000, just got meaner faster while we have read Jim Comstock's summary on long stroke friction nullifying power over 7000-ish rpm. I've never had a more down low lugging off dead idle responsive engine in top gear as that OH SHIT quick P!! shop had set to 600 idle rpm, like slow kettle drums felt in gut/chest more than hear w/o throttle.

Readings to entertain on this.
http://www.rehermorrison.com/blog/?p=220

http://www.google.com/#hl=en&sclient=ps ... 31&bih=593

Not disputing rationale for higher balance to accomodate rider comfort on solid frames; this is also from my first hand experience. I recognize acotrel has much experience with solid hook up as does S. Maney. S. Maney is a very successful manufacturer of Norton twin speed equipment as well as a top notch builder and I am sure he has learned vicariously through his clients.

Not seeing eye to eye with you and maybe a few others regarding best BF for the isolastic application.

Circles or ellipsoids you plotted, although interesting, have as much to do with the center of gravity of the engine/trans/cradle/swingarm/rear wheel and its relationship to the source of the vibration (crankshaft journals) as it does with the balance factor.

I think you have the wet versus dry concept backwards. If 52% BF (dry), then lower balance factor when wet. Adding mass on the journals reduces BF whereas adding mass opposite the journals (as suggested by acotrel) increases balance factor. So we are really looking at a wet balance factor of around 50% - think that is a coincidence?

It would be interesting to hear what the Peter Williams and Norman Whites used for their isolastic based racers. In fact I am not aware of a racer nor street rider who has deviated from the recommended 52% BF (dry). I suspect the fact that this thread is about a short stroke probably does not change the recommended BF for an isolastic application.

Again, why would the factory miss the mark on the proper BF for an isolastic application; especially since the motor was now mounted in rubber? I say it has more to do with reducing the dynamic bending moment reversals (reduced stress amplitudes) in the crankshaft for a better service life for the crankshaft and cases.

BUT don't take my word for it. Best if Kvinnhering asks other experts what they would use, have used or recommend being used.

Anyone else care to chime in on this for the benefit of this build?

Ok Dances good hobot correction who flipped sludge trap oil effect which is near center so does lower the dry BF by 10-12% > 4-5 points, in our size cranks and traps. The mid 50's BF gives the least load forces on crank it seems but not the best BF for hi rpm racing it also seems. The way Norton iso's mounted the engine bounces up/dn at front on rear pivot with only half as much fro/aft motion. Peel's iso observation reveal throttle ups bounce engine up and back some in an arc but most the fro/aft motion comes from tire thrust and road loads. So we run into conflicting wisdoms, some say clac's show that mid 50's BF protects the engine cases most while others say they find higher BF's protect hi rpm race engine best. Implies Norton settled on optimal load forces
BF rather than smoothest for rubber or solid mounts.


Open URL to see the graphs but here's the pertinent text.
http://www.audioworld.net/BSA/forum/ind ... pic=3195.0

I decided to use a balance factor of 58%. Let me explain what the considerations for this very number were:
Assuming the weight of the original con-rods and pistons the crank must have been balanced to exactly 54%, which is the original factor used by BSA (as stated in Eddie Dow's tuning sheet).
For an engine with 42mm crank throw radius and a rod lenght of 165mm, using 54% results in the lowest average value for the resulting force over the crank angle (Fmittel/Fmax/% in the spread sheet, sorry it is in German). The polar diagram bottom right shows the force progression in vertical and horizontal direction, the red graph representing 54% and the green one 58%.
The average resulting force is slightly better with 54%, but the peak values are smaller at 58% (graph not visible on the screenshot, sorry), furthermore the amplitude is smaller in vertical direction, while the bigger amplitude in horizontal direction isn't felt that badly than vertical vibration.
I found 58% to be the best compromise between peak and mean amplitude, experience will show how well it is going to perform in the BSA frame. I will let you know once I've covered a few miles.

FYI: 70% as recommended for racing compared to the original 54%. The average and peak forces are worse, resulting in higher bearing load, but the deflection in vertical direction is rather small.

When you did the traces of the engine movement were you stationary on side/center stand or were you on the road under power. I would expect that if on the road you would see different traces for different engine loads.

Your reference above is bending moments that I have been referring to and it looks like the gentlemen in the reference has done some homework on the subject. The question is whether he factored in combustion forces or used a simple analysis; my hunch is the latter.

According to the reference, BSA used a 54% BF. Assuming this is a dry number then a wet BF would move closer to 50%.

The bending moment diagrams seem to support a lower rather than a higher BF when you compare the red and green traces (peak to peak or min to max). I can only speculate that the Eddie Dowd tuning sheet recommendation for a higher BF is for comfort for the racer. This is, after all, a solid mounted engine application.

I recall Kvinnhering stating that this short stroke will be for the street, and not for racing; at least initially.

Interesting stuff.

The BSA balance factor is irrelevant. Their motors were rubbish, they only ever had one really decent high performance twin - the Super Rocket. The frame does not dictate balance factor, except that if it has rigid engine mounts it can crack if it is used too long in the wrong rev range. Rubber mounting the engine usually results in a loss of power. The balance factor of 58% used in the commando was obviously intened to smooth the motor at low revs, and the isolastics were used for the same purpose. If you fang a standard commandeo, you will shag the crankcases. The shorter stroke will make this marginally better. But what you are building is the equivalent of a Triumph 750 motor with a squishband head. It will produce more power than the Triumph, but you will have to grind your own cams to match the Triumph E3134 (at least). By selecting the short stroke crank, you have chosen to go up the rev range a bit. But a Triumph motor can still be made to produce high levels of torque. So you need to decide what you intend the main use of the bike to be. If it is for r oad use, use the lower balance factor, and if you thrash it, you might do damage, but it is less likely with that crank. If you want to race it, use the higher balance factor (around 75%). It is possible to have replaceable plugs in the crank, but I've loctited mine in place, and punched the end of the threads. If it comes out it could kill.

About the BSA frame - if it is from a Gold Flash, the fork yokes probably have too much offset. Yokes from a unit construction Triumph 650 work really well with that frame, and make it handle better than a featherbed. From what you have written, you give me the impression that you believe that the British factories knew what they were doing. BSA rarely raced, and most production from British factories was intended for commuters. The commando had great potential but selling it with it's full performance would have meant it would be dreadful in traffic. At one stage they built some with good steering geometry, but some of the novices hit the cat's eyes in the centre of the UK roads and got chucked up the bitumen, so Norton backed off their design a bit. A normal expert rider would have had no problem, but the lowest common denominator dictated what was sold t o the public. I believe the steering geometry is a common problem in historic racing these days. Most of our Australian bikes are built from road bikes, and the geometry is often not the optimum, so the guys simply ride around their handling problems, and go a bit slower.

acotrel said:

The BSA balance factor is irrelevant. Their motors were rubbish, they only ever had one really decent high performance twin - the Super Rocket. The frame does not dictate balance factor, except that if it has rigid engine mounts it can crack if it is used too long in the wrong rev range. Rubber mounting the engine usually results in a loss of power. The balance factor of 58% used in the commando was obviously intened to smooth the motor at low revs, and the isolastics were used for the same purpose. If you fang a standard commandeo, you will shag the crankcases. ...................................................................... So you need to decide what you intend the main use of the bike to be. If it is for r oad use, use the lower balance factor, and if you thrash it, you might do damage, but it is less likely with that crank. If you want to race it, use the higher balance factor (around 75%). It is possible to have replaceable plugs in the crank, but I've loctited mine in place, and punched the end of the threads. If it comes out it could kill.

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I see the BSA balance factor as relevant as it is a 360 crankshaft balanced at 54% dry. The author of hobot's reference above states that this is optimal for reduction of bending moments on the crank. This is my position all along and is nicely illustrated in hobot's reference. Furthermore, a bending moment is a bending moment is a bending moment, regardless of whether it is a BSA, Norton, Triumph or Volkswagen. The proof is in the analysis.

Having ridden various Norton twins (Featherbeds, Seeley's & Commandos) I can assure you that the isolastic system smooths out the vibration to the pegs, seat and handlebars across the whole spectrum of rpm in a very BIG way.

Kvinnhering stated he is building a streetable Norton shortstroke with a Nourish crankshaft so I suppose one could say this discussion of crankshaft & crankcase durability is maybe academic as the crankshaft will likely live forever and a day with something more than the recommended 53% BF (dry). The crankshaft is a bit heavier than stock so that should be kinder on the cases.

Herb Becker, a highly successful and respected Norton race engine and bike builder uses the 53% BF (dry) on his Commandos. Herb's Norton builds are the only two instances of a 750 Norton Twin pulling a first place at Daytona International Speedway in Formula 750.

As a real world data point, Herb did mention having put an ATLAS engine into a Commando Isolastic frame and said it shook more at idle and low revs but no difference in feel at higher revs.

You stated "If you fang a standard commando, you will shag the crankcases". I assume that "fang" means solid mount an engine in a Commando isolastic frame; well the only instance I have seen of that is a fellow from Louisiana several years ago and the frame broke, not the cases. I have run isolastics too tight and broke the frame behind the gusset to the rear isolastic mounts but no cases............well other than when I grenaded a shaved down cast iron flywheel - what was I thinking.

If you study the bending moment charts that hobot references the 54% BF (dry) results in the lower max to min bending moment (stress reversals) which translates to less stress on the cases. The author even states the same. Greater bending moments result in greater reaction forces at the main bearings which translates to greater vibration loading on the crankcases.

So why would anyone want to increase stress on the cases, the crankshaft and increase lower rpm vibration by raising the balance factor above the 53% that the factory selected for the Commando? I would say Norton got this right.

acotrel said:

............................................ The commando had great potential but selling it with it's full performance would have meant it would be dreadful in traffic. At one stage they built some with good steering geometry, but some of the novices hit the cat's eyes in the centre of the UK roads and got chucked up the bitumen, so Norton backed off their design a bit. A normal expert rider would have had no problem, but the lowest common denominator dictated what was sold t o the public. I believe the steering geometry is a common problem in historic racing these days. Most of our Australian bikes are built from road bikes, and the geometry is often not the optimum, so the guys simply ride around their handling problems, and go a bit slower.

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I Clymers Manual, the 750 Commandos had a 27 degree head angle whereas the 850 Commandos went to a 28 degree head angle, thus the 850's turn in was a bit slower. One trick to improve this is to increase the rear shock lengths by say 1". You can see how this is achieved on Doug McRae's Commando racer by Herb Becker (pictures posted elsewhere on this forum).

I hope in yo'alls considering the forces on cases to consider the crank jump rope shape at high rpm that has ton's of leverage at ends in bearings to lift top of cases apart. MId 50's BF may give over all better even force on the cases but higher BF will put less force to pry cases apart vertically and more horizontally. Is this a real factor or not i don't know but as solid mount racers are well known to exceed 50's BF so frames and nerves break less, so maybe cases are better able to tolerate rpm with higher BF too?

So question remains if solid mount 360's singles and twins are smoother with over 50's BF why did Norton stop there? Why did Norton bump up 850 BF over 750, as using same BF would just mean adding enough mass to make up for more piston mass to remain in optimal circular load range, but they exceeded that some, hm?

From what little i know if my short stroke in rubber i'd aim for 77% solution, if solid then 88%. Sluggish commuter bikes got away with 80's in traffic so why not a hi rev power house just loafing between kicking up heels?

hobot said:

...........................................So question remains if solid mount 360's singles and twins are smoother with over 50's BF why did Norton stop there? Why did Norton bump up 850 BF over 750, as using same BF would just mean adding enough mass to make up for more piston mass to remain in optimal circular load range, but they exceeded that some, hm?

From what little i know if my short stroke in rubber i'd aim for 77% solution, if solid then 88%. Sluggish commuter bikes got away with 80's in traffic so why not a hi rev power house just loafing between kicking up heels?

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I am not aware of any significant difference in BF between the 750 and 850 Commandos, both 53% BF (dry) - am I missing something?

Why is a BF of near 50% on a Commado correct and appropriate?

1.) Moment diagrams tell the story - red trace versus green trace. Lower stress with a balance factor near 50%

2.) From a highly respected successful builder - Herb Becker - the higher BF of an ATLAS motor in a Commando frame shook more at the lower RPM but was comparable at higher RPM.

So the question is why engineer more stress and more vibration into a Commando? Just does not make sense.

~~~~~~~~~~~~~~~~

Regarding the BF of solid mount engines, I am saying it has everything to do with finding the sweet spot of the "engine frame system" as a whole where you want the least vibration for rider comfort. If racing then you want it smoother at higher RPM whereas if you are street riding and motoring then more towards the cruising speeds. This is all dependent upon things including frame type, engine mass, BF.

Again, the moment diagrams do not lie. It is the min to max load net difference is the killer and the moment diagram makes a solid case for the low BF around 50% wet

Raising the BF in solid mounted Norton engines/frames is a mechanical comprimise. I know from personal experience how nasty it is to try and ride a Featherbed with a Combat engine at 53% BF (dry).

To suggest a 77% BF for a Commando will:

1.) introduce vibration that can easily be avoided and
2.) introduce more stress on the crankshaft and crankcases (see moment diagrams) which can easily be reduced.

As for your "optimal circular load range", it is meaningless, not to also meantion it is meaningless as it is a function of BF and the center of mass of the Commando drive train with respect to the crankshaft axis. Did it dawn on you that cirlces are bad and certain ellipsoids are good. :lol:

My bad I thought 750 got 52 and 850 54 dry.

By circular load evening I meant the moments of inertia is more even all around with 50's BF rather than more intense above average in some vectors. But that don't mean its any easier on the way crank flexes or cases break if cases are not as strong in all its radial directions loaded.

I'm assuming this short stroke will not be run to absolute max to test crank and case blow ups, so just seeks a tolerable BF for solid or rubber mounted install. Reading accurately what you quote from Herb, reads that a hi BF ATLAS vibrates more that 52 BF isolastic Cdo at low rpm and smoothes out similar to Cdo at hi rpm. Duh no doubt but not evidence that the Atlas would vibrate everything worse with mid 50's BF eh.

My orbitals observations come from 3 sources,
1. on tires in shop at front iso only,
2. by THE Grit dust grime road going witness marks both front and back
3. by on road sway plate heady steady witness marks.

I wasn't sure if I was thinking right on my crude calc of BF to get more circular orbital in Peel [going by 1.2.3.] than the more vertical oblong I saw with 54 dry BF, till Schimdt pinged me on his logic and witness marks of Featherbed mid engine area. Peel had no vibration sense as a 750 but I was pensive going with bigger heaver piston engine, till jMS kit. Now just a traction mystery detail.

D/t having first to learn, then teach Zint at Linskogs about Cdo crank BF, for most a year I followed up on every contact around the world, [minus you]. The most interesting was a shop in Dallas which kept records of each crank BF per engine configuration and type frame it went in across many Brit brands. Best I could glean before beginning cut off with - sorry this is secert builder detail on specifics, that most BF landed in mid 80's zone. Impression was they did a lot of Big D's cranks.

So we run again into conflict - of graphed least moments BF vs almost universal practice in solid mounts. As for why Norton picked mid 50's - I'm coming into your camp, that they calc'd the best load BF - Then found they could get away with it in isolastics, after they cut em in half, twice, to get isolation near 2000.

Ms Peel is fairly easy test bed some day as to make her act like a solid mount all's I gotta do is snug down the 3 radius links and/or jam big nails between iso and crash cage mount with crank holes empty to over filled.

Dances with Shrapnel said:

Raising the BF in solid mounted Norton engines/frames is a mechanical comprimise. I know from personal experience how nasty it is to try and ride a Featherbed with a Combat engine at 53% BF (dry).

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I'm with you on this one. Years ago I tried taking the 920 engine out of my PR, which was balanced at 62% wet, and worked great with the stiffer PR isolastics, and putting it in Martin Adams Seeley copy Vendetta frame after his engine fragged at Laguna Seca. It was unridable. Fred Eiker was riding, and he came in after a couple laps and said it was so bad he could no longer feel the hand controls. Martin's 872 cc engine was balanced closer to 80%, and worked fine.

Ken

I guess the OP got more than he bargained for with this thread..

Entering the fray.

I ran my 750 360deg Weslake engine in a standard isolastic Commando frame for a number of years. I specifically had it balanced to 50% because it was going on the Commando frame and it survived many years with a life of high horsepower and high rpm (9000) without killing its crankcases.
I took it out of that frame and put it in a Seeley Mk2 but kept the 50% BF. The vibration didn't feel too bad and the engine was OK but the frame wasn't. It shook loose the rear engine plates/frame bolts and cracked the mounts. I had a heck of a time keeping the mounting bolts tight until I switched it over to a 77%ish BF.
This engine is never ridden at less than 4500rpm.

So I guess my particular experience isn't really that helpful to the OP other than to suggest 50% is still the way to go with hot rodded engines in isolastic frames.

To paraphrase Phil Irving in Tuning for Speed, the point of any particular BF is to keep the big blob of jelly sitting on top of the m/c (the rider) comfortable for the particular rev range the machine is going to be most used at. And that is mainly determined by the resonant characteristics of the particular frame/engine mounting combination.

In an attempt to herniate Hobot's heavily conceptualizing grey matter I would ask him to consider the ideal BF for a solid mount 90 deg offset crank ( answer =54%)

For the non Australians in the audience "fang" = wring its neck or at least it did when I was growing up in Oz

oh Seelyweely, I thought i'd posted enough but BF was my first major obstacle in my late great Ms Peel that took me all over world and back into time. Peel sent me past all this nonsense of offset cranks and BF just for the tolerance of it to not flop like a fish off the deck. There's 3 more higher turning orbitals and scads of offroad stuff I think BF can assist Peel in and will have fun the rest of my life finding out. Oh yeah 90' cranks and 850 54%dry>66%wet BF coincidence and Peel's 77% Tyre Solution lookie here.

http://www.nortonownersclub.org/noc-cha ... /412892838
http://www.nortonownersclub.org/support ... -balancing



Here's a short stroke teaser from Old Brits himself

The third engine, a short stroke 750cc engine, was an experiment. We used a used Triumph crank, with Triumph conrods, Norton 850 pistons and cylinder. We only had this engine on a dyno once, but the power curve was beautiful. The engine was designed to run 11 to 1 compression, but went to the track with only around 8 to 1. We never got the chance to thoroughly tune this engine before the crank went on it. We reinforced the crankcases so they did not break in half, just pushed the main bearing out the drive side. I do believe this would have been a screamer and will persue this idea again with a billet crank and new cases. I ran a magneto on this engine for the experience and liked the simplicity of it.