A guide to internal and external crank balancing.

G'day all

I decided i might give out my views on some good simple easy to achieve ways to balance your crank.

Warning: You will require more tooling than just a dremel and a blancer.

First off it is a good idea to take your bike for a spin. You really need to test your engine for balance at full throttle and load, as load on the engine will change the balance. So jump on floor it a couple of times and feel the vibration through the rev ranges.

now your ready to start balancing.

External balancing:

External balancing is a way to balance your crank assembly with out modifying the crank itself. This is very easy to acchieve by adding or removing weght in certain positions around the fly wheel to counteract the crank shafts out of balance.

The simplest way to externally balance your enngine is to drill and tap 2 holes in your flywheel 180' appart in the line of TDC and BDC (Top Dead Centre and Bottom Dead Centre) at the same PCD (Pitch Circle Diamater) and use these holes to bolt counterweights (like small washers) to the fly wheel.

Once you have your flywheel drilled and tapped insert a bolt and your starting guess of washer weight into the hole in your flywheel you choose to weight first. Start your engine and run it at idle, did it vibrate more or less. Give your engine a few more RPM (say 50%) and note if the vibration is worse or better. Take your engine to almost full RPM (say 90%) and note once again if vibration had improved or not. If you are aiming for balance at a specifc RPM range then only use your notes on that specific range.

If you feel the vibration is better at your desired RPM running range than before then you may well be on the right track. Try adding a slight more weight and repeat the test.

If you feel the vibration is worse at your desired RPM running range than previously you will probably be heading the wrong way. Remove some washers or possibly swap sides and test for vibration again.

Continue these processes until you have found your "happy spot" . Now you have a couple of choices, remove weight or add weight. If you choose to add the weight the simplest way is to just loctite your bolt and washers in place. This is best if you have your hole parrallel to the crankshaft axis as centrifigual force will not have an effect on the bolt coming loose.

However if you choose to remove weight from your flywheel this is a bit of a harder trick. You need to balance your flywheel. Set your flywheel up in a balancer of your choice and notice if the flywheel falls to your weight end or the non weight end.

If the flywheel falls to your weight end then you need to add weight to the opposite side of your current weighted end until it balances ok. Once you have your flywheel balanced you can now remove your original balance set and lighten the flywheel until it balances again. Remove all weights from the flywheel and you are done.

If the flywheel falls opposite the side of your weights you need to add further weight to your weighted side until your fly wheel balances. Remove your original weights but retain your "added balance" weight and lighten the flywheel until it balances again. Remove all the added balance weights and its off to the race track.

Bingo now you should have your own externally balanced flywheel.

Internal balancing:

This one is a bit tricky and a fair bit more time consuming. You need to start with a perfect balanced flywheel and clutch (if you cannot balance your clutch you can do a semi external balance which can be a pain though. You need to match mark your clutch and if you ever replace your clutch you need to rebalance your engine). You also need to have 2 holes drilled and tapped in your fly wheel similar to the one in external balancing but you should have your PCD for your holes the same as the crank journal PCD.

Repeat the same steps as in the external balance. Run engine, take notes, add remove weights until you find your "happy spot". Once you are happy with your engine balance dissasemble your engine. Remove your bolt and washers from the flywheel and weigh them (you will need good quality digital scales that can weigh 0.1g minimum).

If your added weight was on the same side as the crank journal this is quite simple to do from here. Take a roll of lead solder and cut a long peice then weigh your solder. Trim solder until the solder weighs the same as bolt and washers and wrap around the crank journal. You may now begin to add or remove weight from your crank to achieve balance.

If your added weight was opposite the crank journal this can be a bit trickier. Take your weighed solder the same as in the first step, roll it into a ball and sticky tape this ball at the same PCD as the crank journal on the opposite side (180' away from crank journal). You may have to take the weight of the tape into account. You may now begin to add or remove weight to achieve your desired balance.

Ok so that should be pretty much it. A nice simple way to balance your engine and the hardest work you have to do is remove and replace the starter cowling everytime you adjust your flywheel weight.

I hope this has been helpfull to at least some one.



Additional:

I appologise for the lack of information some people may find but i feel if you need a step by step guide for every motor i am affraid i cannot do this. This is just a guide to make life easy.

Also, this is theory, I have never actually done this before and cannot gaurentee it works but i will be giving it a run in a few months. So try it if you want, i cannot see why it wont work or you could just wait until i finally try it and i can review my works.

or a eiser way if you own a water cooled get a bzm or polini crank

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barnsey
aka bj#5

i think you will find neither the polini or bzm to be perfectly balanced. if they where you wouldnt get your nuts shaken off riding

but i just statically balanced my gp3s crank and it picks up the revs much much faster. so for a $20 crank its definitely worth trying it. dunno if id take a drill to a bzm gold tho

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www.demon50s.com

- BZM 6 port Banshee SHO
- Polini 911 - pro chinese for sale! PM for details.
- Pro SQ Cag

Has anyone tried external balancing on the flywheel?
I've got to do something because my little cag vibrates so much
i think that is my fuel problem.
Mabye the fuel is being atomised in the float bowl with the vibration?

doubt it, but if you have a lathe just put the crank assembly (flywheel clutch and all) in between centers then you can see the static balance and adjust it if neccessary.

im sure you could true up the flywheel alone but doing the whole assembly is much better

what problem are you having?

__________________
www.demon50s.com

- BZM 6 port Banshee SHO
- Polini 911 - pro chinese for sale! PM for details.
- Pro SQ Cag

A single cylinder engine will always vibrate as there is nothing to cancel out the secondary balance of the crankshaft. The only way this can be achieved is through a balance shaft turning at engine speed in the opposite direction to the crank as is used in the TZ125 gp bike and large 4-stroke singles. Multi-cylinder engines use the other cylinders to provide this function.

I am not saying that this is a futile excercise, espesially with chinese bikes. I think the external balancing is a good idea to minimize excessive vibration through trial and error. What i am saying is that single-cylinder crank balancing is always a compromise and proves to be more of an art form than an exercise in science.

The other thing to consider with the external balancing is the idea of a heavy or light crank. Moto GP bikes have a flywheel which is divided into sections like a cake. In wet conditions, or for a higher top speed they will put more dense material into the slots. Or if they want quicker acceleration they will remove slices or use lighter materials. 125 gp bikes also run heavy cranks to prevent the rear wheel from breaking loose and for higher top speeds (also called inertia cranks).

The pocket bike flywheel could be drilled so that extra weight could be bolted to it. This has three applications:

1. To change the crank balance as discussed
2. To add weight to the crankshaft by using extra bolts close to the edge of the flywheel
3. To reduce the rotating mass by removing all bolts and weights, leaving holes where aluminium used to be.

Just another tuning tool

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Trying to get more people to pocket bike events held around Tasmania

Whats happened is a mate and myself brought 2 chinese pocket bikes new and his dosen't vibrate any where near as much as mine and he is quicker than me down the straight because his bike seems to pull just those few more revs.
I'll give this external balanceing a go toomorow and see what happens.
Be a good experiment anyway.(both bikes are stock and same gearing)

check the clutch and flywheel first. They have larger rotating masses due to larger diameters and would be the first port-of-call in finding large vibrations. Also check wheels for balance.

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Trying to get more people to pocket bike events held around Tasmania

I Think i just found the fuel problem,(gaskets between carb and reed block have fallen apart) so i'll adress that problem first then have a crack at balanceing the flywheel.

hey how do ya change the crank do ya have to take the hole engine apart lik into 2 pieces???