Balance factors

All I can tell you about 270 cranks is Dave Watson spent a small fortune developing his! He then told me that Gary was quicker at Brands with the old engine, Cam chain was discussed as causing problems I believe Cosmos mentioned this as well?
Wear & tear is another matter.

acotrel said:

With a balance factor of 54% ,the rods and pistons, and the counterweight are in balance at about 4,000 RPM.

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No, nothing is in balance, not at any engine speed. The bike might feel dead smooth at 4,000 rpm but that is only because the frame and/or engine mounts are flexible and are able to absorb the vibration being produced at that frequency.

If you want to spin the motor higher, you need a higher balance factor,

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I don't think so. A higher balance factor shifts the direction in which the most severe vibration is felt; at all engine speeds. It might be the case that your particular frame and engine combination work well with vibration in a horizontal direction at 7,000 rpm, but I don't know that it is a general rule that higher engine speed requires higher balance factors.

ggryder - your 50% thinking is rational and logical but it doesn't take into account that the "snap" at TDC and BDC when the piston reverses direction is more severe than the counterweight passing through 90 deg at half stroke (the shorter the rod to stroke ratio the worse the "snap" and this is why longer rods run smoother). So more counterweight must be afforded to the more severe shaking forces at TDC and BDC and thats why all singles and 360 twins have a BF higher than 50% - the average being around 66% (or 2/3rds the way towards 100%). I remember this BF principle way back when I was working at a Brit shop as a teenager - now I've come back to it after all these years.

Higher BFs are not for higher RPMS. I used to think so like many otheres. But after trying BFs ranging from 49 to 85% I changed my mind.

Yes the 270 degree crank puts a tremendous load on the cam chain. Look at a 270 cam and you'll see that two lobes are lifting at the same time. Other advantages are up for argument.

I keep pushing the scratch test tool because so far its the only way I've found to actually verify the correct BF. Low 60s may give you a more orbital vibration but I prefere it a little higher in mid to high 60s because then you have slightly less vertical vibes than horizontal vibes which makes life easier on your butt and spine.

As I mentioned when I cracked a frame the vibes got worse in one direction - but the problem when away after the frame was repaired.

Isolastics (like cracked frames) change everything and I don't pretend to understand the math. I would have more faith in some sort of vibration intensity tester.

Photo below shows desparate attempts in my early racing years to keep the bike from shaking apart. I also sharpened the skirt and drilled shallow divits under the piston crown. This was a stock piston before I got Wiseco to make lighter pistons for me. The drilled pistons never broke. Vance & Hines used to do something similar and I've heard of Manx pistons being drilled. But drilling doesn't really take off enough weight.

There was one Triumph 650 which had a light crank and a low balance factor. If you are building a Triumph 650 classic racer, that crank is cheap - but not worth having. A fast Triumph 650 usually has the heavy crank and a balance factor up near 80%, ans revs to 8,000 RPM. The same applies to the 750cc Atlas.
With my bike, it is either stationary or it is doing at least 60 KPH. For that bit in between, I always have to make sure nothing gets inside my braking distance. With the 4 speed close ratio box, first gear is as high as second in the normal Commando. So I rev the tits off the motor to get the bike moving. If I do that, it will leap away in any gear. I just do not like doing it with the Norton box. If it fails, I could be hit from behind.
The bullshit about balance factors is very old technology. I was building hot motors in the 1950s - hot cams, high compression and high balance factors - the one thing we did not usually have was the good gearbox - but every Japanese two-stroke had one. My 500cc short stroke Triton always had 4 gears close ratio but badly needed 6 - it could always stay with my mate's 650 Triton which was very fast - and still is.
If you have a low balance factor and rev the motor high, the vibration is usually horrible
My 500cc Triton was a 1957 motorcycle. I began racing it in 1969 in Allpowrs C grade. I just happened to like it - 'some things are so bad that they are good'. But I copped a severe injury because of the drum front brake, so I sold it to my mate who had built it and crashed it at Bathurst in 1958. He put it on petrol, so it was almost sane. I rarely ever discussed that bike with him.
If you are building a race bike, you need a frame which does not flex, a motor which does not vibrate at high revs, a decent gearbox and a front brake which will not kill you. Most converted road do not have those things.
The 'L' model GSXR 750 Suzuki was the only 80s road bike which had the close ratio 6 speed box as standard. Many guys in Australia now race XR69 Suzuki replicas in 1970's classic racing and use the Katana motor. The XR69 that Crosby raced was much more inspiring. It had the close 6 speed box and race cams. I don't know whether the GSXR gear box fits the Katana motor, but it fits the 1200cc Bandit. Race bikes are much faster if they have close gears. But if you cannot rev the motor high, you might be beaten before you start.

If you fit lighter pistons into a Triumph 650 motor, there is a distinct difference in the way the motor spins up. But that usually creates a problem every time you need to change a piston. I once fitted standard comp. BSA 350 Gold Star pistons into a Triumph motor by reshaping the edges of the piston crown to fit the head. It ended-up with a squish band and no big lump obstructing the gas flow at TDC when both valves are open. The pistons were lighter, but I do not think that was the most important change.
If you look at high comp. pistons out of any Triumph 650, there is usually coke on the side of the crown which is away from the spark plug. High comp, does not help as much as squish. 7 to 1 comp. on methanol is still excellent. Petrol is just a bit more difficult to tune.
One of the things I noticed when I was racing regualarly was - there are a lot of myths. My 850 Norton motor is quick enough with almost nothing done to it. However when I was building the bike, I never believed in it. It has a minor mod to the inlet ports, a rebalanced crank, and a slightly modified cam - the exhaust is 2 into 1 with no restiction. The rest is in the methanol fuel and tuning. Petrol would need a bit more effort, but would not make much difference.
One of my mates told me 'if you have a torquey motor, you do not need a close ratio gearbox' - that shows how much he knows. If you did not know, you might think that. I once rode my Seeley 850 with the standard ratio Norton gearbox - it was absolutlely hopeless everywhere. High overall gearing with close ratios is much better. Jetting the carbs can be very deceptive, you sometimes think you have been getting the best out of your motor, when you have not.

I am not joking when I say the most important thing about my Seeley 850 is it's steering geometry. Accelerating hard from the beginning of a corner and all the way through it, is very difficult to beat. It does not take much to make the motor fast enough, if you have oversteer when you accelerate and the back of the bike squats. But too much power might crash you when you do that. With me,it's no sense-no feeling. I have already crashed everywhere I am ever going to do it - so I don't.
Faster in the corners is faster down the straights - you do not need as much power. You do not need to be Marquez to find out. Just do it slow and careful and keep your wits about you. If you come onto a straight 10 KPH faster, that is 10 KPH you do not have to make up towards the end of the straight. And that is where a more powerful bike can beat you.
One of the advantages of being older is you get to know where you can be beaten. Getting more power out of a Commando engine is subject to the law of diminishing returns. But fast enough to be in the lead bunch is good enough. A Commando will not usually grab you by the throat, and jump on you.

Chris said:

All I can tell you about 270 cranks is Dave Watson spent a small fortune developing his! He then told me that Gary was quicker at Brands with the old engine, Cam chain was discussed as causing problems I believe Cosmos mentioned this as well?
Wear & tear is another matter.

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When I was campaigning my 500 Domi twin years ago several friends suggested I go the 270 route. I didn't. Partly because I had heard and seen problems. Since my last post I also remembered a Matchless 500 twin down here that had been converted to a 270 crank. It was part of a well resourced race team running G50s. The 270 bike came to several meetings but never delivered great results for the significant expense and work that had gone into it. They have two separate camshafts with gear drive so that's probably not the issue with that engine however.

I'm very happy with Jim's longer rods and lighter pistons which have gone onto my Commsndo. Considering the same for my 500 although life is a bit busy and expensive just now with lots of family events, weddings etc coming up.

If I was going to do a performance job on my 850 engine, I would buy Jim Scmidt's long rods and his 12 to 1 comp. pistons. I might get a bit more out of the methanol. When you fit 12 to 1 comp. pistons into a 650 Triumph engine, they usually have very high crowns and no squish. But Jims high comp. pistons probably only have slightly raised crowns. I think they would be excellent. With petrol 12 to 1 is probably a bit high and you might need to retard the ignition a bit. As I said previously, improving performance is usually subject to the law of diminishing returns.
Phil Irving was usually reliable when he said something, however the staggered crank might cause more secondary vibration, due to a rocking couple. The other thing is, when I got the Seeley rolling chassis, I could not get the Laverda 750 motor without paying really big bucks, even though I theoretically owned it. When my mate had the complete bike, he had a 2 into 1 exhaust system on it, whicjh I think did not work. The factory 750SFC Laverda had a cross in its exhaust, under the gearbox. Paul Dunstall did something similar with Norton Twins. The Laverda is a 180 degree twin. With a 270 degree crank, the problem is the same - the way the exhaust system works is important. It resonates and each section of pipe has it's own standing wave. With a 360 degree twin, the 2 into 1 exhaust needs the same length tail pipe as one of the header pipes. The tail pipe resonates at twice the frequency of one of the header pipes. With 270 degree and 180 degree cranks, that gets a bit stuffed up.
I think Paton twins have 180 degree cranks, but they are only 500cc. Extrapolating that to a bigger engine and making the crank 270 degree, might not work well. I get stuffed in the head even thinking about cam timing on a normal twin cylinder motor. With a normal motor, it does not matter if the lobes are 180 degrees out. You just time the motor on one cylinder, and check the other one. With a 270 degree crank in a Norton Twin, the lobes on the cam would not be in the same relationship.
If your motor is rigidly mounted and you can feel savage vibration when it is revving at7000 RPM, you probably should not race it Isolastics do not change what is going on inside the motor. VIbration represents loss of energy which should be going down the chain to the rear tyre. And how that tyre deforms under power is important. Twin cylinder motorcycles are different from single cylinder. A Manx always feels better than a Triton. Smooth power delivery is not always better, when you are trying to be smooth. In the old days when tyres were really shit, when you were on the absolute limit, it would take almost nothing to crash you. These days tyres are better, but if you go fast enough you end up in the same situation.
I had an opportunity in 1973to buy a very good 1961 Manx for $1300. I did not buy it. But later on I did buy a TZ350 Yamaha - that was just as silly. They were both too good to waste on historic racing. The Landsdowne Cup might be different. My Seeley 850 is cheaper - I can afford to blow it up. I think a good A grade rider with a Manx could beat me on the Seeley 850 on most circuits any day. When I was a kid, I used to watch them at Phillip Island. Tom Phillis was the fastest.

I remember Hobit doing something similair and the pictures of the patern the scrib made. Much discussion followed. I can't remember which site he posted them on though, as he contributed to several sites. That guy did love his Nortons.

franko said:

I remember Hobit doing something similair and the pictures of the patern the scrib made. Much discussion followed. I can't remember which site he posted them on though, as he contributed to several sites. That guy did love his Nortons.

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Yes I remember Hobot showing vibration witness marks he made with a pencil or felt pen. If anyone can find that post please repost it. Someone else should try it and post some photos - showing both the vibration of the frame and the motor with isolastics at various RPMs.

One of my friends had a business helping people with engineering for racing motorcycles. He was big on putting two-strokes on methanol. The other thing he did with two strokes was rubber-mount the motors to stop their frames from cracking. He made a comment which might not have ben objective. He said 'two-strokes never seem to perform as well after their motors have been rubber-mounted '. I cannot think why that would be the case. However he was extremely experienced. He is my age and his whole working life has been about racing motorcycles, so he might know something. For me road-racing motorcycles was only ever a part-time hobby.
I think that if rubber mounting the motor reduced performance, there would be evidence of energy dissipation somewhere.

That thing about the smoothness of the power delivery affecting how the tyres perfom, came out of a discussion I had with Steve Oszko. He was the top A grade rider in Victoria on Manx Nortons in the early 1960s. He was not stupid. I have only ever ridden one Manx. It was very different to most other bikes I rode. I could ride it much faster with confidence.

robs ss said:

I prefer the brit-bike sound and feel compared to my mate's "Triumphati"
A 360 twin's vibration can be quite well managed.

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Yes, you can manage it to vibrate at low rpm, or manage it to vibrate at the mid-range, or manage it to vibrate at high rpm. Take your pick. Fantastic.

HopperXL said:

Yes, you can manage it to vibrate at low rpm, or manage it to vibrate at the mid-range, or manage it to vibrate at high rpm. Take your pick. Fantastic.

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You build the motorcycle to suit it's intended purpose. A good motorcycle makes a good rider. So if you ever go road racing, start with something decent and you will crash less. For a race bike the usable rev range is usually high. If I put my Seeley 850 on public roads, it would be horrible. When I race, I only use the bit between 5,500 RPM and 7,500 RPM. The crank is balanced to suit that. A normal Commando is a good compromise between power and tractability. If you want more performance, buy a more modern bike. The value in a Commando probably lies in it's originality. I have seen a couple which were nut and bolt perfect - they are a joy to behold. But I would not own one, because I do not ride to suit a bike like that.

For me, riding on public roads is far too dangerous and frustrating. I drive a Mazda 6 which has a 6 speed close ration gearbox. It is a waste of space. I only use half the gears and it bores me shitless. If I drove it using all the gears, I would not have a licence for long. It is a very nice car, but there is no way I can use it in the way for which it was designed. When you road race on a circuit , there is more freedom. The experiance is much better. When I am in my car, I like my wife to drive so I can look at the scenery. When you are driving, you cannot see much of the world. Road racing is different. If I was 20 years older, I probably have been chasing Focke Wulf 190s with a MK9 Spitfire, to get my jollies.

acotrel said:

For me, riding on public roads is far too dangerous and frustrating. I drive a Mazda 6 which has a 6 speed close ration gearbox. It is a waste of space. I only use half the gears and it bores me shitless. If I drove it using all the gears, I would not have a licence for long. It is a very nice car, but there is no way I can use it in the way for which it was designed. When you road race on a circuit , there is more freedom. The experiance is much better. When I am in my car, I like my wife to drive so I can look at the scenery. When you are driving, you cannot see much of the world. Road racing is different. If I was 20 years older, I probably have been chasing Focke Wulf 190s with a MK9 Spitfire, to get my jollies.

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....and the relevance to balance factor is?

robs ss said:

....and the relevance to balance factor is?

View attachment 102265

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The relevance is this - the Commando engine is designed for use in a road bike, The balance factor is usually too low for high revs. When you talk about improving the performamnce of Commando for use on public roads, you are kidding yourself. You never use a road bike in the way in which you use a race bike. Race bikes which are made by converting road bikes are never very successful.- and vice versa. A modern sports bike is usually not a good road bike.
There are two conversations going on with this topic. If you talk about a race motor, you are not talking about a road motor. If you have a normal Commando, you should be happy with what you have.

What balance factor is used as standard for the 750 motor in the N15 and P11 ? - Do they also have isilastics ?

acotrel said:

The relevance is this - the Commando engine is designed for use in a road bike, The balance factor is usually too low for high revs. When you talk about improving the performamnce of Commando for use on public roads, you are kidding yourself. You never use a road bike in the way in which you use a race bike. Race bikes which are made by converting road bikes are never very successful.- and vice versa. A modern sports bike is usually not a good road bike.
There are two conversations going on with this topic. If you talk about a race motor, you are not talking about a road motor. If you have a normal Commando, you should be happy with what you have.

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WTF…?