Balanced Engine

[B Murr]

I've seen references here and there to having an engine 'balanced' and its not referring to carbs. Anyone here know what that entails and whether it does anything useful? Would you get a very smooth engine if it were 'balanced'?

 

[Greg Masters]

It means taking all the components and ensuring they match for deadweight and are dynamically even.

So each piston should weigh exactly the same; the rods weigh the same - also when measured at each end of the rod; the crank goes onto a dynamic balance machine, the flywheel is then added and dynamically balanced.

It all adds up to a smoother engine that maybe could be revved a little higher.

 

[Rob Farmer]

I've had mine done. The conrods are balanced against each other (end to end weight as well as total weight), pistons and rods are then weighed and the crank is dynamically balanced to match the weight. Flywheel and clutch is also balanced.

I'd heard ( no idea if it's true or not) that all of the later cranks were balanced for 800cc bikes which is why the 800 is smoother. Anyway! With everything balanced the 1000 engine is smoother than an 800 with a lot more umph. At 70 mph I pulled the clutch in and hit the kill switch, it felt exactly the same as with the engine running. The downside is with the high comp Pistons and the lightened crank I get a lot more rattle at tickover.

 

[B Murr]

800cc smoothness in a 1000cc engine sounds like a sensible mod to do especially if keeping the bike long term. How much does all of this cost to do, presumably the cranks, rods and pistons are sent off to a specialist who can do this or do they need the whole engine?

 

[airhead_eire]

Rob, I would have thought (dangerous thing for me to do !) that the 800cc motor's smoothness would have been a by-product of
being a smaller capacity ? I would think an R60 would be smoother again. I always found my airheads to be much smoother than
the oilhead bikes I had for, what I presumed, was this reason. No ?

 

[Rob Farmer]

800cc smoothness in a 1000cc engine sounds like a sensible mod to do especially if keeping the bike long term. How much does all of this cost to do, presumably the cranks, rods and pistons are sent off to a specialist who can do this or do they need the whole engine?

I shipped the striped bottom end off to moorespeed and bought the Pistons and other parts at the same time. The cases were vapour blasted at the same time. I'd have to look at receipts to see what it cost but £600 springs to mind for the balancing work, new mains, vapour blasting and bottom end rebuild.

 

[Rob Farmer]

Rob, I would have thought (dangerous thing for me to do !) that the 800cc motor's smoothness would have been a by-product of
being a smaller capacity ? I would think an R60 would be smoother again. I always found my airheads to be much smoother than
the oilhead bikes I had for, what I presumed, was this reason. No ?

Clearly it isn't if my engines anything to go by. I'm running moorespeed Pistons which are around 11:1 I believe. You'd expect the engine to be vibey but it's quite the opposite. It is ported, flowed etc and all works very well.

 

[Packer]

I'd hazard a guess that if an engine is being put together at the factory from parts bins full of bits made to tolerances that the larger the capacity the more the potential for vibration. The bigger capacity needs bigger heavier pistons, conrods, crankshaft etc. so the weight variance within tolerance would also be greater.

Balanced or blueprinted the tolerance issue evaporates and the extra capacity doesn't matter. Maybe

 

[fayeslane]

Last time I pulled a 100 apart I weighed the standard pistons - exactly the same.

 

[Sylvester]

Clearly it isn't if my engines anything to go by. I'm running moorespeed Pistons which are around 11:1 I believe. You'd expect the engine to be vibey but it's quite the opposite. It is ported, flowed etc and all works very well.

How much did you have to change the carb jetting after the rebuild?

 

[Canuck]

Last time I pulled a 100 apart I weighed the standard pistons - exactly the same.

I think it more about balancing the 'actuation' parts.

 

[Rob Farmer]

How much did you have to change the carb jetting after the rebuild?

I fitted dellortos with r90s jetting but has a persistent light pinking at 2-3k. I've since refitted the bings and the pinking has disappeared (proving its a jetting issue) it's been too cold and there's far too much salt on the ground for me to evaluate the standard jetting in the bings properly. I'll tell you in March

i bought a honda to ride through the shitty weather

 

[PW Cymru]

Last time I pulled a 100 apart I weighed the standard pistons - exactly the same.

I used to balance commercial jet engine compressors with small blades and the blades were rarely the same weight.

It'll possibly depend on the limits you use.

 

[B Murr]

I used to balance commercial jet engine compressors with small blades and the blades were rarely the same weight.

It'll possibly depend on the limits you use.

I've seen online references to the use of piezoelectric sensors when balancing aircraft turbine blades. Would such a thing be feasible for an airhead engine, thereby allowing you to balance the crankshaft without having to remove it from the engine? Installing balanced pistons and con rods is fairly straightforward, but for someone like me taking the engine down to crankshaft removal level seems like an awful lot of hassle for potentially small gains if a balanced set of pistons and co rods are fitted. Plus, as located in Ireland there's the hassle of packing and shipping the crankshaft. Obviously if using a sensor method there would have to be some means of adding removing weight to the crankshaft, not sure if that's a possibility with the R100 engine.

 

[Rob Farmer]

On some critical plant we used to have a specialist come in with vibration analysis equipment. Worked a treat and gave an incredible amount of detail on the balance of electric motors. As you built up a profile for each piece of equipment you could see individual components degenerating over time. Of course years ago the maintenance guys would know their plant like the back of their hands and be able to tell a difference just by touching the machines. These skills have mostly been lost.

The problem with using it on an airhead engine is having to strip it down, make a change to build of a profile of what component relates to which vibration.

 

[Dogbytes]

...for someone like me taking the engine down to crankshaft removal level seems like an awful lot of hassle for potentially small gains if a balanced set of pistons and conrods are fitted.

It's nice to have a balanced motor but for reliability at sustained high revs, as in racing, you need a balanced motor. So for normal use a lot depends how much you value smoothness. It's probably fair to say that it is most often, if not always, done as part of a more extensive package of work. In those cases the cost of the balancing can pale into insignificance! Believe me, I know. as soon as the electrics are done I'll be able to let you know whether it's smooth or not.

 

[chasbmw]

I have had two engines built for me by Jim Cray and as part of a full rebuild they were balanced, very nice and smooth, especially the 90/6 with lightened and balanced flywheel

 

[Dogbytes]

It's nice to have a balanced motor but for reliability at sustained high revs, as in racing, you need a balanced motor. So for normal use a lot depends how much you value smoothness. It's probably fair to say that it is most often, if not always, done as part of a more extensive package of work. In those cases the cost of the balancing can pale into insignificance! Believe me, I know. as soon as the electrics are done I'll be able to let you know whether it's smooth or not.

I'm now in a position to confirm that my Richie Moore engine is very smooth indeed

 

[Myke Rocks]

I've seen references here and there to having an engine 'balanced' and its not referring to carbs. Anyone here know what that entails and whether it does anything useful? Would you get a very smooth engine if it were 'balanced'?

Won't make a great deal of difference on a BMW horizontally opposed engine. Perfect Primary balance, Perfect secondary balance.
The main components are already balanced properly, but the rocking couple is a mess.
Look down on motorcycle from above.
Note that the cylinders are not in line.
Up to and including 1150s no compensation. 1200 on, a single balance shaft but 2 are needed to do the job properly.
The single shaft quarters the amplitude, but multiplies the frequency by 4.
What is a rocking couple?
To see it in action, take a push bike & turn it upside down.
Take off chain.
Crank is in perfect balance.
Give the crank a hard spin.
Bike just flew all over the place.
This is a rocking couple.
Myke

 

[Rob Farmer]

I'm now in a position to confirm that my Richie Moore engine is very smooth indeed

So is mine. Pull the clutch in at 70 and kill the engine and the whole lot feels the same as it did with the engine running.