Author Topic: How Big Can Displacement Get Without Needing A Counterbalancer?  (Read 703 times)

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nicholastanguma

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Since the 80s the Japanese have been putting at least one counterbalancer on all their thumpers, from 200 up to 800 ccs of displacement, and in some cases the larger sizes even get 2 counterbalancers.  The Europeans do likewise these days, too.

Mass market machines must only meet a certain bare minimum of tolerances to pass quality control, therefore counterbalancer assemblies make sense since each machine's engine is not going to be individually balanced to the minutest degree possible, as though it were being assembled by a master craftsman taking great pains for the utmost in precision.  Pre-1980s thumpers were of course most characterized by their boneshaking vibration, and the appearance of counterbalancing measures was inevitable.  Imagine for instance the tooth shattering vibes that would accompany the DR650-800 single cylinder engines if they had not been counterbalance equipped. 

But by all accounts Tom Lyons hot rod Fireball thumpers are individually balanced so well people are perplexed to outright shocked at the smoothness.  And clearly no RE Bullet has a counterbalancer or rubber engine mounts or other vibration mitigation in place, so I'm left wondering just how big can a thumper get without needing a counterbalancer if a master engine builder such as Tom is the fellow putting it together? 

700cc?  800cc?


« Last Edit: March 24, 2021, 03:27:05 pm by nicholastanguma »


muezler

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I would say 2.000cc

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https://www.youtube.com/watch?v=Ygyk_eCMGzU
https://www.youtube.com/watch?v=rMvZN8FLgIo

does not look like a pleasant ride....but good enough for racing

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ace.cafe

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It's unlimited.
No engine ever needs a counterbalance shaft. The crankshaft has counterweights built into it which balance it.

The purpose for a counterbalance shaft is to counteract the remaining vibration forces for perceived comfort.

Counterbalance shafts are totally unnecessary for engine function.
You might notice that cars and other vehicles do not have them.
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TrianglePete

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Most modern 4 and V-6 engines have balance shafts.  It does wonders for singles or the other
manufactures would not spend the extra $
Balancing is a compromise. I use 65%.
Two thing I learned
   The heavier Bullet crank helps stalling in the 535
   Steel pushrods create bad vibrations


Keef Sparrow

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I think an old British motorcycle manufacturer called Panther used to make a 700cc single many years ago - I doubt it had balance shafts as it was so long ago.
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johno

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I think an old British motorcycle manufacturer called Panther used to make a 700cc single many years ago - I doubt it had balance shafts as it was so long ago.
The biggest Panther was the M120 which was 650cc, not far off though. ;)
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Richard230

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How about a homemade 5 liter V-twin?   ;D
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nicholastanguma

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It's unlimited.
No engine ever needs a counterbalance shaft. The crankshaft has counterweights built into it which balance it.

The purpose for a counterbalance shaft is to counteract the remaining vibration forces for perceived comfort.

Counterbalance shafts are totally unnecessary for engine function.



The solid mounted H-D Evo Sportster is well known for its heavy vibrations, so when Buell lopped off the rear cylinder to create the 500cc Blast single they essentially created an old school big single, as they did not add counterbalancing.

Since an Evo Sporty engine can be bored up anywhere from 88 to 100 cubic inches a Blast can be commensurately bored up, obviously, making somewhere around 750 - 800cc I think. 

Ace, could such a monstrously sized solid mounted thumper actually be made tolerable for daily riding without use of a counterbalance shaft?  I know you've already proven it can be done with 535cc Enfields, but a 750 - 800 cc engine is just so massive...


ace.cafe

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The solid mounted H-D Evo Sportster is well known for its heavy vibrations, so when Buell lopped off the rear cylinder to create the 500cc Blast single they essentially created an old school big single, as they did not add counterbalancing.

Since an Evo Sporty engine can be bored up anywhere from 88 to 100 cubic inches a Blast can be commensurately bored up, obviously, making somewhere around 750 - 800cc I think. 

Ace, could such a monstrously sized solid mounted thumper actually be made tolerable for daily riding without use of a counterbalance shaft?  I know you've already proven it can be done with 535cc Enfields, but a 750 - 800 cc engine is just so massive...
If the crank is made right, no counterbalance shaft is needed.
Now, some people might want one for added comfort, and that's why they exist.
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nicholastanguma

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If the crank is made right, no counterbalance shaft is needed.
Now, some people might want one for added comfort, and that's why they exist.


I suppose then the question more specifically might be: could such a monstrously sized solid mounted thumper actually be made "comfortable enough for daily riding" without use of a counterbalance shaft?

Let's say this enormous single was powering a dedicated hack, so between the bike and the sidecar there would be lots of metal to help absorb vibes, including a heavy flywheel, and the engine itself would be mated to a six speed transmission so there would be plenty of gearing to help things along and keep piston speeds from going astronomical.
« Last Edit: March 24, 2021, 10:50:15 pm by nicholastanguma »


ace.cafe

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I suppose then the question more specifically might be: could such a monstrously sized solid mounted thumper actually be made "comfortable enough for daily riding" without use of a counterbalance shaft?

Let's say this enormous single was powering a dedicated hack, so between the bike and the sidecar there would be lots of metal to help absorb vibes, including a heavy flywheel, and the engine itself would be mated to a six speed transmission so there would be plenty of gearing to help things along and keep piston speeds from going astronomical.
It's based on a personal desire for comfort.

There is no reason that any size engine "needs" a counterbalance shaft, if it is made properly.
The only "need" arises if a designer or the consumers want more vibrational comfort than the basic design provides.

For my opinion, I think counterbalance shafts typically add unnecessary weight and complexity because I have never felt the need for one in any vehicle I have owned, including many "crude" vintage vehicles.
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TrianglePete

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I like my bike as smooth as possible.

The balance shaft was invented by a Brit

named  Lanchester in 1904.

The shaft takes care of the second order .

The manufacturers would not spend $ if they

didn't think it was necessary.


Bullet Whisperer

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Modern singles with balance shafts generally have much lighter cranks than older, traditional machines, which had enough flywheel mass to enable quite effective counterbalancing by removal of material in appropriate areas. Where the more modern cranks are concerned, they may not have enough material in these crucial areas for removal of mass in order to be balanced anywhere near ideally. The counterbalance shafts take care of this instead, but absorb some power while doing so, just as heavier flywheels also do, so it is six of one and half a dozen of the other, to some degree at least.
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nicholastanguma

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Modern singles with balance shafts generally have much lighter cranks than older, traditional machines, which had enough flywheel mass to enable quite effective counterbalancing by removal of material in appropriate areas. Where the more modern cranks are concerned, they may not have enough material in these crucial areas for removal of mass in order to be balanced anywhere near ideally. The counterbalance shafts take care of this instead, but absorb some power while doing so, just as heavier flywheels also do, so it is six of one and half a dozen of the other, to some degree at least.

Good explanation of the differences between two generations of thumper tech, thanks for your insight.

And you're right: engineering is always a compromise somewhere.  Make something better here and you have to make it less better somewhere else.



nicholastanguma

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I like my bike as smooth as possible.

The balance shaft was invented by a Brit

named  Lanchester in 1904.

The shaft takes care of the second order .

The manufacturers would not spend $ if they

didn't think it was necessary.


I like smoothness, as well...but not too much.  I don't want my motorcycles to become dull anonymous modern appliances.  If I wanted dull and anonymous and modern I'd be riding something like a Husqvarna Svartpilen or a BMW F650.

The thought of an 800cc, carbed, air cooled thumper makes my heart sing, but I'm not willing to be naive--while my heart is singing my head is wondering just how much vibration I would in REAL LIFE consider too taxing to enjoy beyond a half hour.

A DR650 engine can be bored up to 790cc, and then stroked up to 900cc if so desired, and of course that engine has been counterbalanced since its inception in 1990, so the potential for huge hot rod power is well documented while still maintaining reasonably low levels of vibration.

But as terrific as the engines of the Suzuki DR650 and Honda XR650 and Yamaha XT660 are, with their overhead cams and four valve heads and compact dimensions and counterbalancer assemblies, none of them quite strum a luddite's heart strings like the raw thunder of an even older school pushrod two valve head Royal Enfield 535 or a Sportster "halfster" 800.