Balancing Conrods

[rgh0]

We have been discussing the benefits and approach to doing a primary balance on the components

There are many secondary affects around balancing a reciprocating engine we have not discussed . An inline 4 cylinder engine can never be perfectly balanced as you have secondary out of balance effects as what is going up versus what is going down is not exactly balanced and in line on the same crank journal and this causes vibrations to be introduced. This is why large 4 cylinder engines often have balance shafts rotating at twice engine speeds with out of balance weights on them to counteract the secondary out of balance of the piston crank assembly. Similarly flexing and vibration of components introduces out balance forces and vibrations and why modern engines are so much smoother as the computer aided design of individual components makes them more rigid

The mathematics of this is complex and I have forgotten most of it since I studied it many years ago and is probably beyond the scope of this discussion anyhow.

In the end the simple practical answer is balance everything you can, as good as you can afford time and dollars Getting all the rods ( and each end of the rods) and pistons within 1 gm of match weights as discussed is certainly what I aim for in the engines I build. For a road engine within 2 or 3 gm is probably fine.


cheers
Rohan

[AHM]

Unless you know that the effect of the twisting is negligble compared to the gram or so you're removing from the rod and piston you can't know that this is doing any good.

We do not need to know the effect of the Twist. We have to assume that the designer optimised the crank to certain conditions and would have done this assuming the components were balanced.
We can say that the twist varies with load and cycle so a difference in mass may well be beneficial on the power stroke at full load but on the other 3 strokes and no load it is likely to have a detrimental effect.

There is a good deal of empirical evidence that says balancing to one gram is beneficial.

We are talking about the home builder with V-block and scales. We have the limits of our equipment and diminishing returns that say 1g is the reasonable limit to what we can achieve.

[AlfaLofa]

Could someone explain how the big end of a conrod is weighed using a V block?

Thanks

[c42]

You need to push a gudgeon pin or similar diameter piece of steel through the small end and support this on a pair of vee blocks, you will also need digital scales or similar to weigh the big end. The vee blocks really need to be mounted so that when the big end is on the scale the centre line of the rod needs to be as near horizontal as possible. The position of the vee blocks and scale relative to each other must be the same for all rods. This method can also be used to balance the small ends.

Hope this helps.

John

[AlfaLofa]

I found this which I found very informative:

http://www.youtube.com/watch?v=QLpiF0E0EJU

I couldn't find anything showing V blocks.

[AlfaLofa]

Am I right in assuming that the big end shells (plus bolts) should be fitted to the con rods when the rods are being balanced?

Also I presume that lightening, polishing and peaning are unnecessary if you stick to the roads rather than the track.

[rgh0]

The bearing shells and rod bolts are precisely made and all weigh the same so no absolute need to include them when doing the balancing. However including the bolts and nipping them up helps keep the big ends together properly while doing the weighing.

No need to lighten , peen or polish the standard 125E rods when using them on a 6500 rpm rod engine.

cheers
Rohan

[AlfaLofa]

Hi Rohan,

Thanks for your reply.

If the BE shells are left off - how would this affect the "end to end" balancing?

[rgh0]

What the actual weights are does not matter, so the process is the same with the bearing shells in or not.
The aim is to get the weights of the big ends all the same and the weights of the small ends all the same. You weigh them all and then take mass off until they all weigh the same as the lightest one at each end.

cheers
Rohan

[AlfaLofa]

Rohan - thank you for enlightening me !!

I understand now - I'm a dimwit