Too much vibration.

[FurryOnTheInside]

Awesome! Thanks Jaguar!
I will get myself an impact driver for the primary gear and case screws, and I already have a rubber mallet.
If I can get (access to) a drill press to do the piston and pin then I may as well try this too (**). After all these engines are the perfect place to learn!
(**Along with correctly timed ignition, of course)

Hmm, the O.P. hasn't posted since the first post, maybe expected a less involved solution to this problem.

Better read up on clutch shaft disassembly..

 

[Nanonevol]

I missed something. So where on the crank do I drill those holes? Can someone post a link to the aforementioned article?

 

[Deleted member 12676]

drill the holes in the crank wheels between the two existing 15mm diameter holes.
It's not easy but you can drill holes using a new carbide bit even without a drill press. Start with a smaller drill bit (carbide) so that the final hole is not so hard to drill. Their center should be 36mm from the center of the crankshaft.

 

[Nanonevol]

Alright, thanks for that. Now, what size hole? And what does this accomplish - less rotating mass? I know, I have lots of questions. Never mind, I found your article. Good stuff.

 

[butre]

nothing helps with vibration more than a suspension seatpost. it's rare that I build a bike without one. that takes care of the vibrations in the butt, and if necessary you can fill the handlebars with lead shot to take care of those vibrations.

It's easier to drill two holes in the crank than to reinvent the frame.

I just finished the program and figured that for a Grubee with near stock compression and a weight of 95 grams for piston and pin and bearing that two 3/8" holes would be perfect at 5000 RPM and would be at the limit of the yummy zone at 6500 RPM. If the piston assembly is only around 85 grams then those same holes would keep the engine balanced up to 10,000 RPM. I'm not sure what the stock piston weighs. These figures are for the cranks with four 15mm diameter holes. If you take yours apart and it is different then let me know the specs and I'll run the program for you. Best drill bits are carbide. Home Depot has them for around $6.

my home depot doesn't carry carbide other than stone drilling bits, but those are junk for doing metal. I have to go online to buy carbide drills.

 

[Steve Best]

<SOME SNIPPED>
Does anyone know of a video showing how to split the cases and remove the crank?

Fred has a good one here:


If it doesn't come apart this easy, slow down and think about it.
No steel hammers! (I know you know this Jag, its for others)

Steve

 

[Deleted member 12676]

Grainger carbide drill bit set $13
use 5/32” to make pilot hole then 3/8” for final hole.

McMaster-Carr has many carbide drill bits. #2750A28 is 3/8” diameter split point drill bit for $22. It is of higher quality and can be used without a pilot hole.
www.mcmaster.com

 

[FurryOnTheInside]

This is all great info. Thanks guys!!

I was very surprised that the clutch spring and shaft etc doesn't come into it at all!

 

[Deleted member 12676]

I have new recommendations now after delving deeper into the theory of vibration. This refines my ideas even further. Now I go by a factor that is dependent on both the average radial force and the variation of forces (from lowest to highest). The idea is that the closer to a pure circular pattern on the 360 degree radial graph, the less vibration one will feel because the vertical and horizontal movement transitions will be more gradual. So using this refined method I've come up with the values below for Grubee engines using an 85 gram piston assembly (piston, rings, bearing, wrist pin).
RPM..extra hole diameter
Max..100psi (cranking pressure)
6000 6.0mm
7000 6.7mm
8000 7.4mm
9000 7.7mm

RPM..165psi
6000 5.0mm
7000 5.2mm
8000 6.1mm
9000 6.8mm

100psi is basically stock with the head surface planed flat on sandpaper on glass and using a fiber gasket. 165psi is about the average for the high compression heads sold.
To change millimeters to inches just divide it by 25.4 so 5mm/25.4=.197 which if multiplied by 16 gives how many 16ths of an inch so .197 x 16= 3.15/16 which is just a tad more than 3/16" which is just right if you expect the drill bit to wallow a tad if you are using a hand drill.

 

[Deleted member 12676]

It may be possible to just take the top end off and drill into the exposed crank wheels. The difficulty lies in keeping metal shavings from entering the crankcase and keeping the crank set at TDC. Maybe if the top of the crank was cleaned good with alcohol and then duct tape used to tape onto the top of the crank wheels then that may be a good barrier.
Maybe channel locks could be used to clamp onto the primary gear to keep the crank set in one position.
I need to calculate what bit size to use and how deep to drill.
The advantage to this method is the ease of making the hole larger later on if the hole you make still allows some vibration.
I have to admit that the program still needs some refinements. I am still guessing at how much of the cranking pressure to put into the formula. Also the vertical and horizontal forces from the conrod shaft need to be calculated using trig. Right now the basis of those calculations are derived from a larger than life example drawn on paper and measured.
I don't have a working engine now. When I moved to Paraguay I just took the top end, reed valve, carburetor, CDI, and torque pipe. I get around on a Suzuki 100 now.
When I used a 9mm bit on my crank the results were good. I don't know what "perfect" would of been since I didn't have the time to try different sizes.
Now the program is saying that 6mm would of been perfect. That is basically half the area of 9mm which is a big difference. So if the above recommendations aren't right then use the area calculator at http://math.about.com/library/blcirclecalculator.htm to figure out what diameter bit to use to increase the area by 50% and then by 100% (doubling the area). That way you could try 50% bigger first to see if that gives really good results.
So in effect I need someone to verify the needed size.