Are they really necessary?

I know lots of small engines have balance shafts. But could the crankshaft be balanced in such a way that they are not needed?

Just wondering

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U.S. Army Retired

In my opinion they arent. I was gonna pull my motor out and install new bearing this winter. Then i was going to have the rotating assembly balanced so that i could then delete the balance shafts. I think they are a cheap and easy way for the factory to create a balanced and smooth engine without spending alot of time on it. Unfortunately for us its extra mass to spin and suck up some power.

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05 Liberty Limited CRD, converted to KPA 2863 ball bearing and billet turbo, 50hp injectorsl, complete egr delete, cooling fan delete, weeks intake kit, cummins in tank lift pump, ARP studs, 3" turbo back exhaust, samcos, etecno plugs, GDE trans tune, custom GDE engine tune.

To the best of my memory, I was always taught that there is something innate to the harmonic vibration generated in inline 4 and inline 6 engines that require it. Inline 3 and 5 do not, nor do V-config engines or flat opposed, which helps them to be lighter, among other advantages. I may be wrong though, and welcome anyone to correct me.

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A inline 6 doesnt need a balance shaft but a V-6 does.
Even still, not every V-6 has a balance shaft and neither does every I-4.

I know that some industrial I-4 diesel engines use balance shafts to reduce vibration.
This is mainly to counter the up/down motion of the heavy diesel pistons. Especially in small engines where there is not enough room to have large counterweights on the crankshaft.

But I really have no idea what removing the CRD balance shaft would do.
Or what it would take to balance the crank.
Although, apparently the rods are only sold as a matched and balanced set.

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U.S. Army Retired

Correct, technically EVERY engine should have balance shafts EXCEPT inline 6's and boxer engines, but the industry has figured how to get around that along time ago.

The rods/pistons come equally balanced so that the balance shafts can work. Balance shafts dont remove vibration caused by unequal weights being thrown around in an uneven pattern, rather they cancel out vibrations caused by the SAME weights being thrown around in an even pattern. Think of it like a wave pool with a wave generator on each end of the pool. If you started one end making the same sized waves(lets call this one the engine), and you started the other one making the exact same waves just at the opposite time the first one did (balance shafts), you would have a calm spot in the middle of the pool where the waves cancel each other out. However if one side got water in the control box and started shorting out making random sized waves (engine with unequal weight rods), the other side (balance shafts) that keeps making the same sized waves cant cancel it out.

Inline 6's and boxer engines (subaru uses these currently) dont need balance shafts because their design cancels themselves out.

I believe the balance shafts are used mostly for automotive use where the drivers dont/wont buy something that rattles and shakes.

You can definitely take the crank/rods/pistons/flywheel and get them balanced. It will be just as smooth (if not smoother) and is that much less parts to break/wear out/ rob hp, balance shafts typically spin 2x engine speed opposite of the crank rotation.

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05 Liberty Limited CRD, converted to KPA 2863 ball bearing and billet turbo, 50hp injectorsl, complete egr delete, cooling fan delete, weeks intake kit, cummins in tank lift pump, ARP studs, 3" turbo back exhaust, samcos, etecno plugs, GDE trans tune, custom GDE engine tune.