Balancing

Under 'Heavy Twins' there's currently a note relating how rumour has it that balancing practices and AMC were perhaps not very satisfactory.Someone on a Norton site wrote rude remarks about inadequate balancing a couple of years back. The critic complained that only about 1 in 10 were individually balanced.But in mass production where every item is theoretically identical it is perfectly normal and reasonable to test a selected fraction of output - and to tighten up testing as soon as things start going wrong.And all this stuff about 'dynamic balancing' curing all ills: surely all that does is to reduce the rocking couple - which must be the least of the problems with a parallel twin? The same goes for matching pistons.And there are frequent discussions and no agreement about balance factors anyway. What with that and frame interactions, I'm would not be at all surprised if AMC didn't treat the issue as seriously as some would say they should!And every change will make a motor different - and if you didn't like it before and have spent a lot of money, different = better. But I wonder how often it really is?

Well as for balancing the best balance in the Steam locomotive wheel are Balanced at 90 degrees so the Twin crankshafts have a Better balance factor is with a 90 degrees type crankshaft and firing order , But the Big twins 750 plus had this unbalanced trouble but Did anyone try and find out the problem was it the pistons out of balance or the crank its self , have a look at JSmotorsport.com and there light weight pistons and carrillo con rods and PW3 camshaft and light weight cam followers and dynamic balanced crankshaft all make for a smooth running motor that as less trouble and is more reliable, But someone going to come up with the 270 degrees argument but this has a train of balance weights added to it. even the 961 motor as a balance weight fitted where as a 90 Degrees you can run this without balance weights and its more smoother than any yours Anna J

The Atlas comes in for a lot of slagging - rough, vibratory and worst of all made in Plumstead. However, in my limited experience - i.e. one Atlas over 15 years or so, it's fine. Vibration is acceptable (no worse than some 99s, better than others), heaps of torque and plenty of power on tap. There is no reason that it should be any worse than a 650 or, dare I say it, Manxman. Same stroke crank and the pistons much the same weight. I have no idea what the balance factor is. Plumstead seemed to get it right with this one!

Hi David, you will only get a rocking couple if the crank was a 180 degree item. (turn your bicycle upside down and wind the pedals round, that will give you a rocking couple) the 360 crank will give you vibration as both pistons go up and down together, but not a rocking couple. 180 cranks give good dynamic balance but two close together power pulses. 360 cranks give poor dynamic balance but even power pulses. 270 cranks give poor dynamic balance. Because two pistons are not at the end of their stroke at the same time, one piston tends to smooth out the change in inertia of the other. The British twin manufacturers favoured the even power balance of the 360 crank and the dynamic balancing of the crank. (probably only done on the development engines) Most Japanese parallel twins opted for the 180 crank.

Not quite sure of the difference between a 90 degree crank and a 270 degree crank. Depends which way you look at it

Hi David, you will only get a rocking couple if the crank was a 180 degree item. (turn your bicycle upside down and wind the pedals round, that will give you a rocking couple) the 360 crank will give you vibration as both pistons go up and down together, but not a rocking couple. 180 cranks give good dynamic balance but two close together power pulses. 360 cranks give poor dynamic balance but even power pulses. 270 cranks give poor dynamic balance. Because two pistons are not at the end of their stroke at the same time, one piston tends to smooth out the change in inertia of the other. The British twin manufacturers favoured the even power balance of the 360 crank and the dynamic balancing of the crank. (probably only done on the development engines) Most Japanese parallel twins opted for the 180 crank.

Not quite sure of the difference between a 90 degree crank and a 270 degree crank. Depends which way you look at it

yours anna j

The Atlas comes in for a lot of slagging - rough, vibratory and worst of all made in Plumstead. However, in my limited experience - i.e. one Atlas over 15 years or so, it's fine. Vibration is acceptable (no worse than some 99s, better than others), heaps of torque and plenty of power on tap. There is no reason that it should be any worse than a 650 or, dare I say it, Manxman. Same stroke crank and the pistons much the same weight. I have no idea what the balance factor is. Plumstead seemed to get it right with this one!

Well as you never ridden a Norton Manxman you will not no about weather it vibrates on not well been a 650 the vibes are there but its only can just be felt at around 60 mph but other than a tingle in the foot rest that it its run smoother than you think our Chairman say they run more smoother than his ES2 and hes just had his crank done too he says his bike get over 60 then it get teeth chattering so he thinks the crank not balanced right

Going back to the top, dynamic balancing does only address the rocking couple, seldom a major problem in a parallel twin. Dynamic wheel balancing does the same. Our old bikes with skinny tyres get by with static balancing, but vehicles with wide wheels do benefit from dynamic balancing. So as long as your pistons weigh pretty much the same you are unlikely to see much benefit from dynamic balancing.

Balance factors do see to very fairly drastically. The Velocette Club has a long discussion running regarding balance factors and people swear by wildly varying ones. So it would seem that it's a case of trial and error with no holy grail.

Anna, I have never ridden a Manxman, only 88, 99, 650 and Atlas Norton twins plus an interesting Model 50 and have clocked up well over 100,000 miles on them so I have some vague idea of what they are like. I once even rode a Norton twin with high bars - great up to 50mph but dangerous as I approached three figure speeds. No for me ta.

Going back to the top and talking about Norton twins, not steam engines or ES2's, dynamic balance can only be done for a certain RPM. bin 270 degree cranks and 180 degree cranks and deal with the question. The only smooth Norton is a Rotary. The Manxman is a parts bin special designed to get rid of surplus bits and dump them on the American market. It ain't anything special.

3 2 1 incoming. Taking cover.

Going back to the top and talking about Norton twins, not steam engines or ES2's, dynamic balance can only be done for a certain RPM. bin 270 degree cranks and 180 degree cranks and deal with the question. The only smooth Norton is a Rotary. The Manxman is a parts bin special designed to get rid of surplus bits and dump them on the American market. It ain't anything special.

I am affrayed you have no idea of what your writing about, The Manxman was the first 650 to be built so it not a part bin special as you say , Part were made made specially for this machine and only fitted to its one motorcycle model , where a part bin Special is a machine made up of old stock part to be used up as the machine model is the last of there line, hence the word parts bin special so before going on about these motorcycle just get your facts right first, I am sorry you feel this way , Yours Anna J

Going back to the top, dynamic balancing does only address the rocking couple, seldom a major problem in a parallel twin. Dynamic wheel balancing does the same. Our old bikes with skinny tyres get by with static balancing, but vehicles with wide wheels do benefit from dynamic balancing. So as long as your pistons weigh pretty much the same you are unlikely to see much benefit from dynamic balancing.

Balance factors do see to very fairly drastically. The Velocette Club has a long discussion running regarding balance factors and people swear by wildly varying ones. So it would seem that it's a case of trial and error with no holy grail.

Anna, I have never ridden a Manxman, only 88, 99, 650 and Atlas Norton twins plus an interesting Model 50 and have clocked up well over 100,000 miles on them so I have some vague idea of what they are like. I once even rode a Norton twin with high bars - great up to 50mph but dangerous as I approached three figure speeds. No for me ta.

well that depends how high the bar was where standing up on the foot pegs

now that really dangerous and just as bad has having clip-ons ,near the bottom yokes you cannot stear the thing , where as my bar are just right at all speeds so cheer up on-ones dead yet ! you can bring the cat in now, yours Anna J

Previously anna jeannette Dixon wrote:

Previously david_evans wrote:

Not quite sure of the difference between a 90 degree crank and a 270 degree crank. Depends which way you look at it

Well its like I said before the 90 degrees crank comes from the Steam locomotive days As there engine all run at 90 degrees and run nice and smooth , and there is lots of engine tuners that built Race engine with a 90 degrees crank see Nourish engineering

yours anna j

Iâd been waiting for Anna to give a detailed explanation of the difference between a 90 degree crank engine & a 270 degree crank engine but it seems she is either unable or unwilling to do so & the best we are going to get is that it comes from the days of steam. This is totally irrelevant as your average steam engine injects steam to both ends of the piston & doesnât have induction, compression, power & exhaust strokes, instead, each stroke of the piston is a power stroke, regardless of direction, so the balance issues are completely different.

Now, you may think that a 90 degree crank & a 270 degree crank are one in the same, & mathematically of course, they are, it just depends on which crank pin you start from. You may have noticed I used the term 90/270 degree crank ENGINE, as it isnât until you start to add in camshafts & valve timing that there is any difference, when you factor this in, you will find it results in power strokes that are separated by either 270/450 degrees, or 90/630 degrees, Iâll try to explain why:

The true 270 degree crank engine defines power strokes that are separated from each other by 270, then 450 degrees. Imagine the left hand piston, which weâll call #1, at TDC at the beginning of itâs power stroke, while the right hand piston, which weâll call #2, is 270 degrees behind it, halfway through it's own intake stroke. Follow this around, and 270 degrees later, the #2 piston is at TDC at the beginning of it's power stroke. Following the progression of the #1 piston, we find it is now halfway through it's exhaust stroke, a full 450 degrees away from the beginning of it's next power stroke.

Now take the same crank but this time change the valve timing set-up so that when the #1 piston is at TDC at the beginning of itâs power stroke, the #2 piston is 90 degrees ahead, half way through itâs own power stroke, meaning the power strokes overlap. The #2 piston finishes itâs power stroke, completes itâs exhaust & induction strokes & is back at TDC ready for itâs next power stroke 630 degrees later, with the #1 piston following 90 degrees behind.

As for the balance question, with a 360 degree crank, both piston/conrod combinations achieve maximum velocity together twice per revolution (once on the way up and once on the way down). In addition, they both come to a complete stop together twice per revolution (once at TDC and once at BDC). As you can imagine, as RPM increases, so does the vibration coming from this Primary Force Imbalance (PFI). This phenomenon is made worse by the laws of Physics, which dictate that doubling the RPM quadruples the forces (and thus the associated vibrations) involved. Flywheel weighting added opposite the throw of the crankpin minimizes PFI, but a compromise has to be achieved between cancelling out PFI and exacerbating a Secondary Force Imbalance (SFI). This is the centrifugal force of the weighted portion of the flywheel trying to move the entire engine fore and aft as it spins.

The primary balance in a 180 degree crank engine is perfect, but at the cost of a violently malevolent rocking couple, torquing the engine first to the left, then to the right, as separate pistons reach TDC and BDC simultaneously, again both stopping together twice per revolution.

In the 90 or 270 degree crank configuration, you can maintain higher inertial momentum (flywheel effect) about the crankshaft by having one piston always at or near max velocity while the other one comes to a complete stop and then changes direction. They are never both stationary at the same time. Since secondary imbalance is at a minimum, additional counterweighting on the crankshaft can be reduced, allowing a better compromise of Primary/Secondary imbalance percentages, while reducing rotational weight. Also, the flywheel effect allows the use of smaller actual flywheels, further reducing rotational weight.

I hope that makes sense & helps to clarify a few things,

Regards, Tim

I hope that makes sense & helps to clarify a few things,

It does make sense, excellent post Tim.

Are there production twins being built with 90 or 270 cranks ?

What are the disadvantages compared to the 360 I know and love in my smooth commando.

I presume the 360 degree PFI /SFI is easily hidden by the isolastics but any rocking couple would not be.

Maybe this should be a new thread ...

Regards

Tony

> Also, the flywheel effect allows the use of smaller actual flywheels, >further reducing reciprocating weight.

> I hope that makes sense & helps to clarify a few things,

> Regards, Tim

You might want to rephrase that. No part of the flywheel reciprocates.

JS

The more recent Triumph twins are 270 degree engines and so are the 961 Commando's, also the TRX and TDM Yamaha's.

Thanks Tim I wondered about the differences in 90 and 270 but couldn't be bothered to work it out. So if I read it corrctly the second scenario above gives two power pulses close together, similar in effect to the "big bang Honda V4 race engines.

Jonathan, thanks for pointing out my error, the word reciprocating should read rotational, Iâll edit the post to correct it.

Tony, as David has mentioned, there are a few 270 degree parallel twins in production, most notably for us is the 961 Commando. Iâm not sure about any advantages/disadvantages, just differences, the Yamaha TRX850 started out with a 360 degree crank, then changed to a 270 degree item, the bikes feel & sound different, & there is a difference in the torque curve but I wouldnât say one is better than the other, same with the Triumphs, they worked perfectly well with a 360 degree crank & still seem to work perfectly well with the 270 degree crank, but do feel a bit smoother. With a 270 degree crank, you reduce both primary & secondary force imbalance, but at the expense of introducing a rocking couple which you donât get with a 360 degree crank, this can be dialled out with dynamic balancing &/or the introduction of balancer shafts & you do get better throttle response with a 270 degree engine due to the weight reduction. (A lighter flywheel to accelerate so less power absorbed)

David, yes you have read correctly, the two power pulses are only 90 degrees apart, similar indeed to the V4 Honda racing engine.

One other thing you may find interesting, at least I did, is that the original idea for a 270 deg crank in parallel twin came from an Australian chap named Phil Irving, co-designer of the pre-war Vincent engine, in a series of articles he wrote in the 1940's.

Regards, Tim

Previously tim_gostling wrote:

Jonathan, thanks for pointing out my error, the word reciprocating should read rotational, Iâll edit the post to correct it.

Tony, as David has mentioned, there are a few 270 degree parallel twins in production, most notably for us is the 961 Commando. Iâm not sure about any advantages/disadvantages, just differences, the Yamaha TRX850 started out with a 360 degree crank, then changed to a 270 degree item, the bikes feel & sound different, & there is a difference in the torque curve but I wouldnât say one is better than the other, same with the Triumphs, they worked perfectly well with a 360 degree crank & still seem to work perfectly well with the 270 degree crank, but do feel a bit smoother. With a 270 degree crank, you reduce both primary & secondary force imbalance, but at the expense of introducing a rocking couple which you donât get with a 360 degree crank, this can be dialled out with dynamic balancing &/or the introduction of balancer shafts & you do get better throttle response with a 270 degree engine due to the weight reduction. (A lighter flywheel to accelerate so less power absorbed)

David, yes you have read correctly, the two power pulses are only 90 degrees apart, similar indeed to the V4 Honda racing engine.

One other thing you may find interesting, at least I did, is that the original idea for a 270 deg crank in parallel twin came from an Australian chap named Phil Irving, co-designer of the pre-war Vincent engine, in a series of articles he wrote in the 1940's.

Regards, Tim

Very interesting and informative Tim. I notice no response from Ann, it probably went over her head.

JMc