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-   -   Crankshaft counterweight question (http://www.rcuniverse.com/forum/gas-engines-142/8107130-crankshaft-counterweight-question.html)

borna 11-02-2008 08:48 AM

Crankshaft counterweight question
 
Hi Guys,
I'm in process of designing and building a 2 cylinder in line 4 cycle engine. The bore and stroke is going to be 1”. I want to know when building the crankshaft, how much counterweight I need to add. Basically what is the formula to determine the counterweight?

Thanks
Borna

khodges 11-02-2008 10:47 AM

RE: Crankshaft counterweight question
 
I don't know the formula, but it will depend on whether the crank is a 180 degree or 360 degree throw. Power pulses are equally spaced on a 360 crank (one each revolution, or 360 degrees, and the pistons travel together); on a 180, the pulses come irregularly spaced, 180 degrees, or half a rev, then a 540 degree space, the again at 180 degrees. The pistons travel opposite each other. If you flatten out the engine (opposed twin), the 180 crank becomes a classic boxer and the pulses become 360 degrees apart again (the advantage of the flat, or opposed engine is even firing pulses and smoother power delivery with less vibration and the pistons/rods tend to counterbalance each other). On either the flat twin or 360 crank in-line, the counterbalance is essentially the same mass as both pistons and rods. The distribution of the counterbalance mass is basically opposite the pistons on the 360 inline and at 90 degrees to the opposed twin pistons, equally divided. There is a lot more involved, as there are harmonics to consider.

soarrich 11-02-2008 12:05 PM

RE: Crankshaft counterweight question
 
IMHO the straight twin is the worst layout you could pick. If you use a 360 crank it shakes like a single, if you use a 180 crank you have uneven power pulses from a very buzzy engine.

Scar 11-02-2008 12:17 PM

RE: Crankshaft counterweight question
 

Quote:

ORIGINAL: borna

Hi Guys,
I'm in process of designing and building a 2 cylinder in line 4 cycle engine. The bore and stroke is going to be 1”. I want to know when building the crankshaft, how much counterweight I need to add. Basically what is the formula to determine the counterweight?

Thanks
Borna

The rule of thumb is, you're balancing all the rotating weight plus half the reciprocating weight. Some would "overbalance", at 60% or more of the reciprocating weight.

Good luck,
Dave Olson

rcdude7 11-02-2008 12:30 PM

RE: Crankshaft counterweight question
 
Right, an inline twin is a shaker, in modern motorcycles a 180 degree crank plus a balance shaft is used to tame the beast. This is how my ninja 250 engine is designed.

Now, a 90 degree V twin on a single crankpin type crankshaft can be balanced very nicely, and makes a smooth running twin.

I know that's not what you asked, but something to think about.

borna 11-02-2008 02:26 PM

RE: Crankshaft counterweight question
 
Thanks for the comments. The crank is 360 throw. The reason I want to use 360, is no ignition distributor needed.
I was searching the net and find a formula but not sure how true it is
The formula says

A - Weight of Piston, with ring and gudgeon-pin
B - Small-end of connecting-rod (Not sure what it means by this? Is this the diameter of pin hole on the rod?
C - Big-end of connecting-rod (Not sure what it is also)

((A + B) /2) + C = Counterweight


Thanks
Borna

pe reivers 11-02-2008 05:39 PM

RE: Crankshaft counterweight question
 

Quote:

ORIGINAL: borna

Hi Guys,
I'm in process of designing and building a 2 cylinder in line 4 cycle engine. The bore and stroke is going to be 1”. I want to know when building the crankshaft, how much counterweight I need to add. Basically what is the formula to determine the counterweight?

Thanks
Borna

rotating mass should and can be balanced. Then half the reciprocating mass. (+ - 10%) It is generally better err on the heavier balance weight side, though if the inline twin is at 180°, it offers a bit more freedom.
Lower half of the conrod is deemed to be rotating mass, the upper half is reciprocating.

borna 11-02-2008 08:13 PM

RE: Crankshaft counterweight question
 
Still not clear, can you please give me an example?

Thanks
Borna

tkg 11-02-2008 11:28 PM

RE: Crankshaft counterweight question
 
Pe's example is your formula in words.
You need a very good set of scales, Put one end of the connecting rod on each scale. Find the weight of the small end (piston end) of the rod, find the weight of the big end of the rod (crankshaft end).
Weigh the piston. ring and wrist pin, add to the weight of small end of the rod, divide by 2, add the weight of the big end of the rod and that's your counter weight.

d7 11-02-2008 11:55 PM

RE: Crankshaft counterweight question
 
Take a connecting rod by itself and lay it on a table horizontal. Lift the small end of the rod and set it on a scale calibrated in grams. Now lift the big end of the rod until it is horizontal and level again. The scale will show the weight of the small end of the rod. Record this number. Now swap the rod end for end, level it again and the scale will show the weight of the big end. Record this number also. Now place the entire rod on the scale to get the gross weight. The little end and big end added together should equal the gross.

borna 11-03-2008 07:42 PM

RE: Crankshaft counterweight question
 
1 Attachment(s)
For example lets consider the following drawing. Lets assume this is the connecting rod. You can see both crank side hole and piston side hole is .28 diameter. The shape is fairly symmetric. Can I simply assume both crank side and piston side are equal?

Thanks
Borna


tkg 11-03-2008 08:40 PM

RE: Crankshaft counterweight question
 
OK with that rod shape it should be symmetric. Half the weight is rotary (C) and half is recpt (B).

soarrich 11-03-2008 09:06 PM

RE: Crankshaft counterweight question
 
Just to stir the pot some.

You know that the British twins, BSA and Triumph used this layout and over-balanced their twins so that rather than just shaking up and down some of the shake was side to side, it made it feel like they shook less.

Tired Old Man 11-03-2008 09:19 PM

RE: Crankshaft counterweight question
 
Yep, and both of them were great machines for getting you there. Getting back was another story because you had to walk back to pick up all the parts that shook off on the way there.

Been there, done that, with a few of them.......... Norton was a little better but you could never find parts.

Scar 11-04-2008 01:06 PM

RE: Crankshaft counterweight question
 

Quote:

ORIGINAL: Tired Old Man

Yep, and both of them were great machines for getting you there. Getting back was another story because you had to walk back to pick up all the parts that shook off on the way there.

Been there, done that, with a few of them.......... Norton was a little better but you could never find parts.
Was that a Norton "Hardtail"? We used to say they were paid under the counter by dentists to sell them...

Dave Olson

soarrich 11-04-2008 02:45 PM

RE: Crankshaft counterweight question
 
Nortons had featherbed frames and the best front forks made if I remember correctly. My dream bike as a Dunstill 750 Norton Domaracer.

pe reivers 11-04-2008 03:51 PM

RE: Crankshaft counterweight question
 
...
but they lost big time to the slick Japanese bikes.
I had a Douglass dragonfly boxer at the time, and even succeeded to eliminate the perpetual oil puddle. The English were dead on their feet without knowing it. I had a Honda as well at the time (remember the sixties), slick as could be, steering properties like a moped. The two were seperate worlds. Guess who survived?
I did for sure to tell the tale [8D]

borna 11-05-2008 12:24 PM

RE: Crankshaft counterweight question
 
1 Attachment(s)
I also need to make the valve
I want the valve face to be 45 degree.
Question is do I need to set the cross slid to 22.5 degree or set it at
45 degree?

Also can I use a boring bar to cut the face? Can someone give me the
procedure of how to do it?

here is a simple drawing of how I want to set it up in the lathe

Thanks
Borna

pe reivers 11-05-2008 01:25 PM

RE: Crankshaft counterweight question
 
wow Borna,
If you ask this kind of questions about using a lathe, you have a long way to go.
You set the cross sled at 45° and use a pointed chisel with rounded nose (R=0.05mm) for final turning. If set up right, you could use the boring bar, but it is not the best tool for the job.

borna 11-05-2008 01:53 PM

RE: Crankshaft counterweight question
 
Hi Pe,
Thanks for clarification. Well yes I got a long way to go for sure but I'm not that bad at machining Once I know what I need to do. I usually get the job done [8D]

Hey have you ever looked at my GMS conversion from glow to GAS

http://www.rcuniverse.com/forum/m_61...tm.htm#6165393

pe reivers 11-05-2008 05:26 PM

RE: Crankshaft counterweight question
 
Borna, I see that you paid your dues, but I was not aware of that. I take back my previous comment.
BTW, I use the piston stopper system to find TDC and set the timing. The spark should fire when I apply some pressure, and then use the strobe light to check it with engine running. It mostly is spot-on.

borna 11-09-2008 09:17 PM

RE: Crankshaft counterweight question
 
1 Attachment(s)
Today I made 4 valve guide, spring retainer and only got chance to make one valve.
The valve hasn't been lapped to the seat yet, but it seals very nicely already. I tested the seal by filling the valve guide with acetone and place the valve in with no leak.


Borna

pe reivers 11-10-2008 11:24 AM

RE: Crankshaft counterweight question
 
Nice work.

captinjohn 11-10-2008 11:40 AM

RE: Crankshaft counterweight question
 
1 Attachment(s)
Borna, Very nice work. I got to make some valves for my "Holt' 4 cyclinder engine. I may be asking a few questions as I go. I will post a couple of photos. Best Regards, Capt,n

borna 11-10-2008 04:30 PM

RE: Crankshaft counterweight question
 
Hi Captin,
I'll be happy tp help if I can. That is is nice engine. Did you make it?

Borna


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