Smoothing Out The Engine

[cosworth]

These arent spin balanced at the factory, I guarantee you.
The size of the flywheel holes should be a percentage of the weight of the top half of the connecting rod plus piston/rings weight plus wrist pin weight.
You can increase the hole size or you can use a lighter wrist pin. Both affect balancing in a positive way on this engine. But there is no getting around the need for a 2 stroke CDI to replace the 4 stroke one that comes with it because it has too advanced an ignition timing at high rpm.

Once again a wealth of very useful information. Thanks very much for your time.
Cosworth

 

[Fabian]

Fabian, your non-understandable video (because of teachers heavy accent) doesnt prove what you are saying. But anyway your statement of 2 strokes not being able to be balanced is too broad. You need to give details and specify.

I don't need to give any details because all the detail is in the maths.
Since when does maths written on a whiteboard become unintelligible due to a lecturers accent. A complete nonsensical argument on your behalf - i expected better from you.
Yes he does have a heavy accent, but i didn't have any issues understanding him, although i had to listen carefully and everything he talks about is written up on the whiteboard.

The maths still stands - a single cylinder engine cannot be balanced, unless using balance shafts.

 

[Deleted member 12676]

The non-linear forces in a 2 stroke make it impossible to balance it perfectly. But a moderately good balance is possible.

If all engine engineers believed that it cant be balanced then they would put forth no effort to do so. But the fact that they do a fairly good job of balancing them totally destroys your all-or-nothing point of view.
I agree that it cant be perfectly balanced.
I disagree with the viewpoint that it cant be balanced.

I have owned at least 7 different street and dirt bikes and not a one of them had anywhere near the imbalance that these Grubee engines have. Surely that was done on purpose to limit top rpm. No one can be that incompetent, can they?

 

[Fabian]

a good portion of the reason why these engines feel so unbalanced is the lack of mass that surrounds them, which in most cases only adds up to around 10 kilos (for the bike). Considering the engine weighs around 10 kilos, you already have issues, given that there isn't enough frame weight to dampen out vibration; not helping the situation is most aluminium frames are made from thin wall material, allowing the engine to effectively bounce around on the frame tubes.

This simple fact is why rubberised engine mounts make little if any difference to vibration transmitted through a bicycle frame and that concept has been worked to death.
If looking for the the best option to quell engine vibration, the solution is a heavy wall steel frame, with lead shot inserted into the handle bar ends.

 

[Fabian]

I too have owned more than a few single cylinder dirt bikes and can say that they all had unacceptable levels of engine vibration - but when the manufacture only gives you the option of a single cylinder engine, you only have the option of purchasing a dirt bike with a single cylinder engine.

 

[Fabian]

I agree that it cant be perfectly balanced.
I disagree with the viewpoint that it cant be balanced.

That's a bit like saying a woman can be half pregnant. Either a woman is pregnant or she isn't - there is no half way option of only being "a little bit pregnant", no matter how much you try and alter the viewing perspective.
It's a bad analogy with respect to a motorized bicycle, but still a valid analogy.

The best results you can achieve are to alter the balance factor to minimise vibration at the chosen rpm zone of most frequent engine operation - anything outside of that rpm zone will have the engine giving ever escalating vibration, until the rpm drops to a point that inertia forces significantly fall away, which wouldn't be far from idle rpm.

 

[Fabian]

You just can't argue with the maths !!!

 

[Deleted member 12676]

Math is just a tool. You can pick and choose aspects of the whole truth to present to reinforce your "imbalanced" point of view. Math shows that a bumblebee can't fly. get my point? Let me restate my opinion:
It can't be perfectly balanced.
It can be moderately balanced.
The Grubee engine isn't even moderately balanced.

 

[Fabian]

I place my trust in maths, and the maths says that a single cylinder engine cannot be balanced, except to minimise vibration at a chosen rpm zone by the adjusting the balance factor.
I suggest you watch the video again, and if you don't understand the maths the first time, just watch the video a second or third time.

Doesn't matter which way you look at things, the crankshaft becomes a massive out of balance force when at 90 degrees to the cylinder centre line, because there isn't a piston and connecting rod to balance out the counterweight force.

That's the reason why a 90 degree V-twin is a logical solution for perfect primary balance.

 

[Fabian]

Since when can't bumble bees fly - there is a bumble bee spammer on here that flies from one motorized bicycle forum to the other.

I rest my case