Balancing individual steel flywheels

[Roger Gwynn]

Shawn, The Jawa rod is robust and short, uses a needle roller race and is reasonably easily available. I built it into a short stroke crank machined from a billet and use it with a Yamaha high comp piston that is about 9:1 in a shortened Ariel ally cylinder. I beat a SQ4 up to 60 mph on it but transpired that the valve timing was out ... on the SQ4. The performance has improved since then though.

[SEDoan]

Thank you Roger - that sounds like an interesting engine!

[SEDoan]

The holes are now filled and I re-drilled 3 holes where there was evidence of factory holes. There was evidence of one factory balance hole on either side of the oil purifier plug so balance should now be close, but it's not so something is fishy...

I built a little roller fixture for the 1/2 crank that allows me to determine the balance of each flywheel without pressing out the mainshafts. It will determine the balance to within about 1% of the reciprocating mass of 649g. My goal at this point is to confirm that the balance is close and improve it if possible before pressing the flywheels together.

Below are the relevant numbers...
The 1/2 crank is missing the crank pin & nuts, rollers and the big end of the conrod - all rotating mass.
crank pin = 330g
nuts = 66g
rollers = 56g
rod big end = 350g
Rotating mass total = 802g
The missing rotating mass is balanced at 100% so a 1/2 crank needs to balance 401g.

The reciprocating mass includes the piston assembly and the small end of the rod.
small end with bushing = 201g
piston with pin, clips and rings = 448g
total reciprocating mass = 649g
The reciprocating mass should be balanced at 65% = 422g. The 1/2 crank should balance 211g.

If the above calculations are correct a weight of 612g will balance each half of the crank.
Picture showing the balance weight in the pin hole.

The problem is the counter balance on the timing side balanced only 562g and drive side only 557g. (plus or minus 5 grams each)
This is a total balance weight of 1119g. 802g of this needs to balance the rotating mass of rod and crankpin leaving 317g to balance the reciprocating mass.
This means that 105g (= 422 - 317) needs to be removed from the crankpin side of the flywheel which seems too much. However, since the rim of the flywheels is about twice as far from the center as the crankpin, that 105g can be halved to 52 grams if the holes are drilled in the rim. It still seems like a lot of mass needs to be removed - 5 more holes each 21/64" dia. x 7/8" deep. This will give 4 balance holes in each flywheel.

If you've gotten this far - congratulations! Now, can you tell me if it makes sense to anyone but me?
Thank you!

[david.anderson]

Further to Rogers comments, the normal set up for using a Jawa pin is to use flywheels made from 4140 steel, generally nitrided. The press fit normally used here in Australia is .004”, and a greater fitment length in the flywheel than with the standard Ariel crankpin is achieved as there are no nut recesses in the flywheel. However for use on the racetrack I was advised to replace the Jawa rod every race season (race use not road use). Further advice to me was to use a longer rod stroke ratio than the already long 2 – 1 ratio of the VH. The reason for that relates to our vintage race rules and carburettor sizes. Because the carburettor size is limited then the use of a longer rod stroke ratio means there is more dwell time around tdc with the longer rod stroke ratio, which results in better cylinder filling with the smaller inlet port and carburettor. A hot short stroke VH easily puts out about 40hp, which is similar to the 4, but in a lighter bike.
David

[Roger Gwynn]

Are your wheels a pair? Sorry can't do the maths at the moment as Covid has addled me brain. There is a list of flywheels on Drags site

[alan.moore]

Might be an idea to do a sanity check. If you have an old crankpin which you can adjust to be a sliding fit (to save having to press it in) assemble the crank and check overall balance factor.

[nevhunter]

It IS a bit mind numbing doing them individually but unless you are sure they are an original matched pair it's a "Do it once, do it right' thing. Check the Assembly when finished for reassurance. Flywheel run out will cause vibration. Nev

[SEDoan]

Thanks for the help guys. Yes, the flywheels came to me as a set so should be a matched set.
Roger, get well soon!
Alan, I do have an old pin that I could shave down - great idea!
Thanks Nev. It looks like both flywheels will end up with 4 factory-like balancing holes which seems like a lot - just got to do more checking before drilling.
And David - that is a clever way to maximize the intake charge - using valve overlap and momentum of the intake charge - correct? And getting greater than 2:1 by using a 350 crank and a 500 cylinder. I used to have a 500 cylinder with a base flange cut down to 3/4" and it was Roger that explained how it could be used with a Goldstar piston. Traded it away tho... Another lost opportunity.

[pappleton]

viewtopic.php?p=47801#p47801
Picture of the shaft I made for balancing flywheels. There's a lot of info on the forum
BR, Paul

[SEDoan]

Thank you Paul. I do have a piece of 1" stock so may do that.

Trying to get at the balance showed the drive side flywheel was too wobbly so forced to press that side apart. Should have expected...

The appeared to be a bit of shim in the keyway and the nut was distressed so presumably it was taken apart and then reassembled without the proper torque.

Anyway a bunch of work with bluing, rotary burr, grinding paste and a large wooden block and the runout at the flywheel edge is less than 0.004". Hope to get it a little better still, but tired of working on it for now.