seat vibration

[Fabian]

and what happens to the crankshaft counter balance weight force vector when at 90 degrees to the piston or perpendicular to the reciprocating plane of the piston or the 9 O'clock position and the 3 O'clock position?
Where is the counter force to balance the rotating crankshaft counter weight in a single cylinder engine?

 

[grinningremlin]

welding the motor mounts on solid works for me. seems to make a loose rattly motor into a solid little runner. let the frame absorb the vibrations!
putting anything squishy between clamps and frame is pointless, if not counter productive from my way of thinking.

I don't know if you're familiar with RC aircraft/ 2-stroke glow engines.There are ALWAYS rubber isolaters put between the engine and firewall lest you vibro your plane to bits.All motorcycles have rubber motor mounts.Look at a VW Sirocco's motor mounts,I want that type of small scale motor mount for my GEBE, no pieces rattling loose.

 

[grinningremlin]

As far as balancing a 2-stroke (granted not the engines you're using, but same idea) check out a guy named Ronald Valentine, and his modded Cox engines, quite amazing work done to an already amazing engine.I personally don't like the idea of the cheap china-girl and having to mod it before you use it (not to mention the look) but they obviously can be made to run well.

 

[Deleted member 12676]

Fabian you are referring to secondary vibrations which are allowable usually. Emphasizing the lightest weight possible of piston/wrist-pin/upper-conrod allows less drilling of flywheels which allow less secondary vibrations.

 

[Fabian]

Jaguar, best if you carefully watch this video and pay attention to his analysis of force vectors, with the "Y" force being at the centre of our discussion, hence the reason why i throw my hands in the air at the notion of eliminating or even significantly reducing the inherit imbalance forces generated in a single cylinder engine.
All you can do is moderate the vibration at a chosen rpm by altering the balance factor, but it's just a lot easier to maintain similar engine power as standard, but at lower rpms; reducing vibration intensity, which has side benefits of prolonging engine life.

 

[Fabian]

I am referring to primary forces which are otherwise known as "simple harmonic motion"

When the crankshaft counterweight is perpendicular to the reciprocating line of the piston, the weight of the piston and wrist pin becomes somewhat irrelevant in having any significant influence on the counterweight force vector.

Again i ask, how does the piston and wrist pin have any significant influence on the "Y" force, so what is the point of lightening the piston and wrist pin for it will only change the change the balance sweet spot to an altered rpm zone from standard, but it won't make the vibration go away.

 

[Fabian]

Rubber isolators only work if the surrounding mass is significantly heavier than the engine and the surrounding mass has enough rigidity to resist bending forces. In the case of thin wall aluminium frames, rubber isolators can make vibration worse.

 

[Deleted member 12676]

Fabian I'm not sure how to answer your valid question. As a test you should buy the lighted wrist pin I recommend and then use JBWeld to glue into the standard wrist pin a steel bolt. Then you can do test runs with a heavy wrist pin and a light one to see how much of a difference it makes. Then you can put your thinking cap on and answer your own questions. (of course share your insight with us)

 

[grinningremlin]

Rubber isolators only work if the surrounding mass is significantly heavier than the engine and the surrounding mass has enough rigidity to resist bending forces. In the case of thin wall aluminium frames, rubber isolators can make vibration worse.

I may be comparing apples/oranges here, and I'll state for the record you guys know more than I, but in RC aircraft (probably full scale as well) rubber isolators are vital, and the engine has close to the mass of the fuselage/firewall.

 

[MikeJ]

I applied my own solution a few years back for this very same problem. It worked for me. Everyone agrees the engine is inherently unbalanced and one writer pointed out that the cylinder head swings forward and backward. The causes the somewhat-flexible downtube and seat tube to flex forward and backward. Vibrations travel throughout the frame and cause undue handlebar vibrations and seat vibration. Attachment bolts were loosening and backing out. Lost a few on the road.

I thought, "Stop the cylinder head from swinging back and forth, and the vibrations stop." But how?

Use a heavy steel wire one-eighth inch in diameter and a small turnbuckle. Loop a length of wire starting at the side of the engine, around the cylinder head between the first and second horizontal fin, to around the seat post, high up where the horizontal tube intersects the seat tube but cannot slide downward, back to side of the engine. Open the turnbuckle wide and form eyelets on the ends of the wire threaded through the turnbuckle ends. Twist the tunbuckle tight by fingers only; you don't want to break the cylinder off the crankcase. (The wire now becomes a "noose".) This will pull the cylinder head backward, toward the seat post. Twist the turnbuckle until you cannot rock the cylinder forward or backward by hand. The tight wire harness will greatly limit cylinder rocking forward and backward.

When I did this move to harness the cylinder head, my 4500 RPM upper limit at which I could not tolerate the vibration shifted upward to over 5800 RPM as measured by a tach. At that point, I was traveling too fast for comfort and did not push the engine for more speed.

I tried the isolators mentioned in the postings. Tried deadweights on the frame tubes. Rejected all of them. Only after locking the engine in place, did high-rev vibrations become insignificant. Though I moved into 4-stroke engines, that original 2-stroke engine still has my wire harness in place and intact just like it was over three years ago.