Balancing Happy Time style 70cc Crankshaft

[clay]

From what i understand, a balance factor of 50% favours higher rpm and a 60% balance factor favours lower rpm.


Fabian

you got it backwards. more weight is needed at higher rpms because there is a greater force being applied to the piston at max rpm than idle.

on a side note, my 10mm stroker crank motor made 5.36 hp at the rear wheel today, with max rpm being just under 8800. and 47mph

 

[Fabian]

Clay, you are completely correct.

I have stuffed up the way i look at the balance factor percentages.

Looking at my modified crankshaft, in it's current form with a 70% balance factor, it would only be achieve balance at very high rpms.

The standard crankshaft is best balanced for low rpms.
This makes sense as both the engines (the original and the replacement) started to vibrate heavilly from 3500 rpm.
Below that figure they were both very smooth in operation.

If the same crankshaft is used for the 48cc engine, it would have good balance at higher rpms - not surprisingly, people comment on the fact that the small engines are much smoother to ride.

Crikey - you've got a full 5.36 horsepower to play with - your bike would have more than enough oomph to keep you happy.

 

[210061741]

I would agree about the 48cc crank.

My idle and low end is a little rough.
Once i'm goin 15 MPH + she really kicks in.
Climbing RPM in the power band very quickly and cruising at 40 MPH all day long without a hicup.
I rode about 40 mi yesterday down a 40 Mph speed limit road and kept up with the cars.
At one point wile traveling down a steep hill for about a mile. I relly let her loose and at WOT. I had to have hit 50 MPH. = very high rpm.

The only vibration i felt was from the road and tires.


Good for the motor ??? "Prolly Not"
But i want to see how far i can push it before it breaks

 

[keystone]

fabian, have you checked out freestudy.couk/dynamics/balancing pdf file interesting,but it's over my head. If you can find someone to decipher this info. might be useful

 

[happycheapskate]

I'm no genius, lol, still putting together my grubee bomber bike with crappy dept store hand tools from a garage sale.

But here's a good question:
If you balance a crank and piston system under no compression, will it still be "balanced" during operation? I ask because the rings will drag, the oil or fuel mix will drag on it, and the compression and combustion will resist it.

Your assessment is spot on Keystone.

With a single cylinder engine, you can only set up the balance for an rpm zone where the engine least vibrates.

I am sure there's a mathematical formula allowing you to calculate balance factor for the desired rpm zone.
I think it's important because if making changes to the power band, like cylinder porting and a tuned exhaust system that raises rpm 2000 revs from the previous optimal balance zone, engine vibration will be outside of the previous sweet spot.
Smoothness can be restored for the new commonly used rpm range by adjusting balance factor.

For the life of me, i can't find any information on how to do this simple task.
I refuse to accept that it's just guesswork.
I'll have to get in contact with an automotive engineer from one of the major car companies to sort this issue out, once and for all.

Fabian

 

[210061741]

Sometimes i think rough running is caused by bad bearings and the motor not being tuned right and also imperfect mounting.

I haven't had this balance issue.

 

[Rconce01]

One of my bikes runs perfectly smooth through all rpms , and the other has always vibrated heavily near top end. Both are the same engine the 80cc which is really 69cc with a 47 mm bore. Usually these little motors will smooth out after 500 mikes break in. Its really weird because the idle is smooth and the ride all the way up until about 29 mph is really smooth (this is on a 26" rim with 36 tooth sprocket) from 30 to 36 mph is shakes uncontrollably.

It's interesting how how you guys are trying to resolve this balancing issue. Any idea if these symptoms point to balance issues. I really can give you rpms because i have no tach, but i would say the final 1,000 rpms are the biggest problem.

I read somehwhere on this forum that some of the counterbalances on these engines have been assembled backwards from the factory. What are your thoughts?

 

[gothicguy64]

hi just a few words .
try tony at rock solid engines
his factory is in adelaide .
i think theres a link on left ..
i have his hp1 48cc motor but made to hp3
it revs to the moon
an i cant feel any virb at idle


brad

 

[happycheapskate]

re: What are your thoughts?

I'd see if the vibration was coming from a loose hub, an unbalanced/poorly aligned drive cog on the wheel, a bent chain link, or the motor itself.

If its the motor, send it back and exchange or take a refund.

 

[Rconce01]

yeah its the motor itself the bike has always been like that its over 4 years old, and i felt i should address it. tony from rock solid engines was nice enough to return my email. Apparently the crank needs to be statically balanced and trued. He says sometimes the cranks are machined wrong or assembled improperly and it causes major vibrations which are increased by higher rpms. I am going to try some hard rubber strips where the engine mount meets the frame.

re: What are your thoughts?

I'd see if the vibration was coming from a loose hub, an unbalanced/poorly aligned drive cog on the wheel, a bent chain link, or the motor itself.

If its the motor, send it back and exchange or take a refund.