Too much vibration.

[FurryOnTheInside]

How many of you all would rather suffer vibration (and nuts flying off) for hundreds of hours instead of working a couple hours to make the crank right?

ps- lightening the wrist pin is not enough

A couple of hours? Hmm *looks sceptical* -_- okay I am in no rush while I'm saving up for shift kit bits so I am considering reconsidering splitting the case.
I am worried about getting it back together again without making it worse, though. I think most people who have read earlier posts about it would be too.

I have read that much of the vibration problem is solved by reducing the pin and piston weight (easy if you can get access to a drill press), and correctly timing the ignition after the engine is built and running. (But how much this helps seems to be difficult to quantify)

What tools are required to make the modification to the crank and to reassemble it correctly?

 

[45u]

How many of you all would rather suffer vibration (and nuts flying off) for hundreds of hours instead of working a couple hours to make the crank right?

ps- lightening the wrist pin is not enough

And how many people have the knowledge or the know how to balance the crank? Some more then likely would only make things worse or mess up something. I know some folks when they change a spark pug I get nervous. This is why many make s living working on them for those that do not know. But thread lock and nylock nuts will help keep things from falling off and most can do this. Yes it dose not stop the vibration but at least you can ride without have to stop so much or at all.

 

[crassius]

seems easier to assure the crank is balanced before ordering that motor

 

[Deleted member 12676]

the lighter the piston assembly (including the pin) is, the closer you can get the balance to perfection.
I'm just about finished with my balance calculator and can tell exactly what size holes to drill. I've got someone who is willing to do the work as a test case (69cc) so others can follow his example. I successfully balanced my 55cc (upgraded from 48cc) but it had non-stock piston and wrist pin. It needed two 9mm holes but that size requirement may be different for a stock engine.
tools that are sometimes helpful: rubber mallet, impact driver

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

 

[45u]

seems easier to assure the crank is balanced before ordering that motor

You can if you order from a place that sells motors and balance cranks for another $100 to $150 that will make the price of the kit go up that much or more so the kit will cost you $250 to $300 maybe more. None of these cranks are balanced well some are worse then others. Now why you are balancing the crank do not drop it or hit it put too much presser on it when drilling holes you you can get the crank out of true and a balanced crank out of true is just as bad sometime worse than a out of balanced crank. So want do it your self make sure you know what you are doing!!!!!!!!!! If not dig in to the pocket and shell out MORE money as it might be cheaper.

 

[45u]

the lighter the piston assembly (including the pin) is, the closer you can get the balance to perfection.
I'm just about finished with my balance calculator and can tell exactly what size holes to drill. I've got someone who is willing to do the work as a test case (69cc) so others can follow his example. I successfully balanced my 55cc (upgraded from 48cc) but it had non-stock piston and wrist pin. It needed two 9mm holes but that size requirement may be different for a stock engine.
tools that are sometimes helpful: rubber mallet, impact driver

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

Jaguar looking forward to the experiment and seeing how it works. Will have my eye out here and on your site.
Thanks so much for your input and time.

 

[Deleted member 12676]

I think it was Fabian that ordered a “balanced crank” (or engine with balanced crank) and he sent it back because it vibrated so much. If you want something done right you have to do it yourself. The internet has been void of what I’m fixing to offer for free- a crank balancing program. No wonder dealers fail to properly balance a crank when all they have to go on is the old wives tales of how to.

 

[45u]

I think it was Fabian that ordered a “balanced crank” (or engine with balanced crank) and he sent it back because it vibrated so much. If you want something done right you have to do it yourself. The internet has been void of what I’m fixing to offer for free- a crank balancing program. No wonder dealers fail to properly balance a crank when all they have to go on is the old wives tales of how to.

This is why YEARS ago I started doing my own work as I trust my work but not others. This was way back when I was 15.

Jaguar do you know who the company was? I called and talked to a shop about balancing cranks and from the way they talked I sure would not let them put a spark plug in my motor if it was frozen up and they are suppose to be a big race motor builder.

 

[Timbone]

My hypothesis is that any kind of vibration problem relates directly to the bicycle frame. Bicycle frames work best the stiffer they are. That's the idea: a stiff frame transmits the pedaling force as efficiently as possible to the wheels. Frame flex is a no no for a racing bike. You strap a motor in there (a one cylinder engine at that!) and vibrations have to get worked out somewhere.

Once I was descending a big hill with a bike racer buddy and I watched as he experienced a speed wobble at about 50mph. He very quickly lost control and slammed directly into a cliff face. Yes, every rigid bicycle frame has a natural vibration resonance point.

I would think that creating a frame with flexibility could be a way to seriously relieve vibrations. Instead of all tubing connected via unyielding welds, maybe a key joint held by something strong but flexible (like steel cabling) could work as a kind of shock absorber while not affecting safety or performance.

A rear swingarm linking the rear wheel to the frame via shock absorbers would help, too.

 

[Deleted member 12676]

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.