Guide to crank rebuilding

Making it easier to wade through the data :-)
Brakes, Suspension, Carburetor, Bottom end, Top end, Chassis, Discontinued parts, Wheels, Plastic, Tools
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81cr450
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Joined: Sun Jan 13, 2013 6:08 am

Guide to crank rebuilding

Post by 81cr450 » Sat Mar 02, 2013 5:53 pm

I've been looking for detailed instruction on putting a new rod on the RMX crank I picked up @ OEM-Cycle a little while back & came across this very informative article.

http://homepage.ntlworld.com/david.gibb ... ioning.pdf

I found others but this is really clear & concise.

This one was great for the views of his hand built tools, that & I have been curious how the rd, rz, banshee cranks come apart. After seeing one in pieces they dont look scary at all. I didnt realize the center was splined for alignment.

http://www.rrracing.co.nz/crank/index.html

Gotta love the internet for all the ideas you can steal

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Sandblaster
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Location: Eugene, OR
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Re: Guide to crank rebuilding

Post by Sandblaster » Sun Mar 03, 2013 11:17 pm

Yes, stealing ideas and making them better so someone else can steal from you is how I got through College :D
Thanks for the articles.
It will be a bit before I can get to them as I have a bit of catching up to do from being gone :shock:
If bikes are for kids I'll never grow up.

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Sandblaster
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Re: Guide to crank rebuilding

Post by Sandblaster » Sun Mar 31, 2013 3:44 pm

Very good info and worth a read.
Thanks!
If bikes are for kids I'll never grow up.

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Sandblaster
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Re: Guide to crank rebuilding

Post by Sandblaster » Mon Aug 10, 2015 4:51 am

Here, Dutch K5 Fan over at KXriders wrote some info on crank Balance factors:

I'm trying to explane what Balance Factor means. Ad least what I 've read/ learned about it.
I'm not trying to have a discussion here. Just a simple easy post about this.

You weight your Bigend side of a conrod, complete with bigend pin, cage and pins.
Lets say 373gr

Then you weight the smallend side and piston with rings, pin, circlips and smallend bearing.
Lets say Smallend 114gr and complete piston 510gr. Totall 624gr

With a Balance Factor of 50% you take the piston/smallend weight. 624gr
You take 50% of the 624gr. Thats 312gr.
Than at 312 with 373. Thats 685gr. This is your bobweight.

You replace this bobweight were your bigend should be. now when you spin the crank around. It should stop
every were, else if it stops at the same point. The bottom side is to heavy.

I'm gone balance mine crank each half by them self. I'll take half bobweight,(685 split it an halve).
Press this 342.5gr into one half, spin it around. Remove weight were it's to heavy.
The picture is a rig I've made were I hope to do this.

Next time more about this. Dutch
**************************************

Here is some info posted by Motorrad over at KXriders regarding crank balance factors and in response to Dutchies post.

This also works great if you would like to work backwards to see what your doing to a motor...

Just work backwards and solve for the % as an unknown (X)

So. for example...
(Ill use your numbers to start for normal)
((114g + 510) * .50) + 373 = 685

So lets say you think you can shave 30G off the piston!! (by giving Kaw Rider a call).. hah shamless plug.... and your Balance factor is already set at 50%... what are you doing to the motor!? Is it going to vibe like crazy? still be within the (what I say is acceptable 50-60% balance factor)

So you would use
((114G+480)* X)+373=685
(Note your bob weight didnt change... because you are trying to see what just a piston weight change will do to the balance factor...)
So thats
(594g * X) + 373=685

subtract 373 from both sides

you get
594G*X=312
Divide both sides by 594

X= .5252

So you now have changed the balance factor to 52.5%

Simple algebra....

Nifty little program I just stumbled upon from a Norton website..
http://tonyfoale.com/progs/BF.exe

as well as a article to go along with it...

http://www.tonyfoale.com/Articles/Engin ... alance.pdf


Also note that the KX has a rod to stroke ratio of 1.686. for calculating secondary forces into the equation

Very interesting that the Lowest resulting forces (with secondary) are generated at 61% not 50% like one would think..

(this doesn't mean that balancing at that factor is better (direction of forces)... just the general forces produced is lower)

Now if they only made a program that made me spell better.... :roll:
If bikes are for kids I'll never grow up.

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