Balance crank

[yorkie_chris]

For you people who have built motors.

Any idea how much it would be to get a crank and pistons assembly balanced up?

Haven't a clue really, it takes a lot of work I believe. I cheated and went for a billet crank when I modded further.

I'll see if I can dig out the name of a guy that does this sort of stuff...

I built the motor in my old CRX and was looking at that, the advice I got was get some mags like Max Power and look in the back at the tuning companies, give them a ring. I couldn't find a local engineering company that would balance my crank so left it

[embee]

Coming from the engine design/development industry I'll offer my thoughts, but stand to be corrected by others.

Normally I would expect the crank to be balanced on its own, without rods/pistons. For a single throw crank I would expect a suitable mass to be added to the crankpin and a dynamic balance to be done (rather than static which doesn't sort out where along the axis the out-of-balance is).

The mass depends on the configuration, for a single cylinder the empirical starting point is usually 100% rotating+50% reciprocating mass. The rods are weighed at the big and small ends, the rod big end plus bearing shells make up the rotating mass, the rod small end plus piston+rings+pin are the reciprocating. You need to know these masses accurately so the balancer can add the correct amount, and the 2 cyls need to be matched.

However for a 90deg V-twin on a common crank-pin it's not that simple, I think you'd normally use the total rotating mass (i.e. both big ends) but only one "50% reciprocating" because it works for each cylinder separately in effect, being at 90deg. (Need to check this)

The only engines I've been involved with where any balance was done in an assembled state was with large V-8 luxury car engines which were "trim" balanced as a short engine after all the components were done individually to avoid undesirable stacked out-of-balances (i.e. all tolerances in the same direction adding up).

If it's a stock crank and you're not changing the rods/piston masses significantly I'd leave well alone, Suzuki know what they are doing.

If you want it lightened that's a different ballgame, start with the gen rotor.

I did a balance design for the original Triumph Trident engine, they were never smooth and I calculated that the balance factor was only about 35% reciprocating. A friend used to build engines for club racers, and we did half a dozen Tridents with the balance corrected to almost 50% (plus about 30% of the rotating inertia removed) and they were silky smooth, relatively. Different designs were done depending on the rods/pistons used (Carillo/Omega etc) and the capacity (bore/stroke). The cranks were machined by a workshop in Shrewsbury and balanced by a guy down in Wiltshire IIRC, can't remember who though.

[yorkie_chris]

Pistons are losing 17g apiece.

[embee]

just wrote a lengthy reply and it vapourised in the ether!!

take 2................

I've been pondering this and come to teh conclusion that there's no reason not to use 100% reciprocating balance Factor (BF), or close to it, on a 90deg V-twin.

On a single or in-line arrangement the recip BF is typically 50% since it works as a shaking force in the transverse plane (at 90deg to the bore axis) so any more and you just trade off "vertical" balance for "horizontal" shaking.

With a V-twin the recip BF from one cyl works in favour of the other cyl when the crank turns 90deg so it could be increased to 100% and not produce the shaking forces. In fact it would seem wrong not to do this.

It may not be exactly 100% since piston acceleration is different at TDC and BDC due to rod rotation and they might well split the difference, but it would be close to it.

If I were you, I'd be happy to simply calculate the drilling size for 17gms and just do it. Providing you're not affecting the overall balance strategy by serious lightening of the crank/rods and just making small changes, then you're not going to put it out by much whatever you do. Take the density of steel as 7.8, and share the mass between 2 drillings, one each web, and the mass factored by whatever the radius of the web is compared to the throw (half the stroke), for example 17gms at 31.3mm is the same balance as 10.6gms at 50mm radius. 17gms would be about 2.18cm3, so you'll be looking at 2 holes say 10mm dia and around 10mm deep once they're out at the web radius, just to check they'll be sensible sizes. Do the sums.

Remember that the reciprocating BF is arbitrary anyway, there's no "right" answer just a good compromise.

[yorkie_chris]

From a rough measure (got a knackered crank here to look at), I get the web radius to be about 70mm. 31.3/70 = 0.4471 ..... 0.4471 x 17 = 7.6g

0.97cm3, which would be a 10mm hole about 12mm deep, or 2 of half that.

[embee]

That sounds very realistic. Post up what you decide to do.

I'll see if I can find out anything definitive about the balance philosophy for a V-twin. I must confess I never really got the better of V-8 balancing, the cruciform cranks introduce strange characteristics and they are not normally fully weighted so it all looks confusing, in-line engines are much easier to understand.

[yorkie_chris]

And just to check, that drilling needs to be opposite the crankpin?

Interesting read. What will you actually gain from doing this?