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?

[Dangerous Dave]

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...

[Saint Matt]

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]

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

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?

[ThEGr33k]

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

[yorkie_chris]

Oily hands, probably a busted knuckle or two, maybe even a running engine :-P

'Busa pistons are lighter, so reduce stress on crank, but drilling the crank web compensates for change in mass. Hopefully avoiding any destructive vibrations.

The 'busa pistons do work as-is, but motor is coming apart anyway, so I might as well. (gotta check crank is worth using!)

[ThEGr33k]

Oily hands, probably a busted knuckle or two, maybe even a running engine :-P

'Busa pistons are lighter, so reduce stress on crank, but drilling the crank web compensates for change in mass. Hopefully avoiding any destructive vibrations.

The 'busa pistons do work as-is, but motor is coming apart anyway, so I might as well. (gotta check crank is worth using!)


Fair enough. Why not get some con rods and do it all right, that way you can lighten the whole thing. Itll spin up really fast :D

[yorkie_chris]

Expense.

[ThEGr33k]

Expense.


Aye... :(

[yorkie_chris]

c£500 for some carillo rods, more than the whole motor, busa pistons, ported heads, gaskets and JHS stage 2 cams owes me.

[ThEGr33k]

Who ported the heads? How long till you can derestrict the bike and rag it good? :cool:

[yorkie_chris]

Me. Just a general clean up of casting flash and carp on inlets, polished exhaust ports. No major port shape changes, then again port shape isn't bad, very downdraft, just needs parts tidying up slightly. Then again last head I ported was a CVH escort engine copying off a puma racing design. Must have done something right, about 15bhp gain on that one.

June when I'm officially derestricted, this engine be aimed at the private road/track.

Hoping to get about 85bhp.

[embee]

Just found this (http://www.timberwoof.com/motorcycle/V-TWIN-7.html)which convinces me that the philosophy is for 100% reciprocating balance factor.

The way it's presented in the flash player is actually the total reciprocating mass (for 2 cylinders) at approximately half the crank throw radius, in effect the same as 100% of one cylinder recip mass at the crank throw rad. It's a very good way of representing the system I think.

So, yes, your (17gms x 31.3mm) mass reduction correction needs to be opposite the crankpin. Assuming it was balanced correctly beforehand, all you'll end up with is the unavoidable resultant secondary out of balance due to con-rod rotation, which should be be fractionally less than it was originally due to the lighter components.

[ThEGr33k]

Interesting read embee! Explains the ballancing shafts in my engine. :)

COOL.

[barwel1992]

olllllddddd thread ! but you lot would moan if i did a new one

YC did you do this in the end ? or not worth it if the cranks not coming out any way

[SV650Racer]

I can get it lightened and balanced for you, cost though is about £400 for lightening and balancing and £200 for balancing. We have had great success with this and the SV crank can safely loose 1kg which makes a big difference in terms of reliability and how the motor makes its power.

[yorkie_chris]

No I didn't bother as the crank I was going to use was chowed up to death.

Might try hard welding it and machining it back... maybe. But that is a while off

[ice]

coming from the automotive engineering field, i can tell u how its done. it's called blue printing, and u need a machine to do it. u would do the crank with the flywheel and cluch assambley.but not with the rods or pistons they will be done one at a time, the pistons and rods u can do yourself with a very acurate scale. if u need any help on how to do the rods and pistons drop me a msg and i will talk u threw it, thanks jarrod

[yorkie_chris]

You wouldn't do it with clutch fitted to SV. Flywheel can also be lightened a lot or changed for a trick one.

[yorkie_chris]

I can get it lightened and balanced for you, cost though is about £400 for lightening and balancing and £200 for balancing. We have had great success with this and the SV crank can safely loose 1kg which makes a big difference in terms of reliability and how the motor makes its power.

How does it increase reliability lightening the crank itself?

[barwel1992]

at a guess less stress on the bearings ?

YC how much did the busa pistons cost ? suzuki GB are coming up at £119 each but in the us there are $100 each