Crank balancing

A

Al Seim

Well-known member
I'm having a VW 4 cyl crank balanced, it's being sent out by my engine machinist.

For the first time (I have not had a crank balanced for a while, btw) the balance shop wants to know the piston, pin and rod weights. Engine shop guy, who works on mostly V8s but also I4s etc, says that the balance shop only recently started wanting that info on flat crank 4s, apparently their new balance machine wants the info. I think that four-throw V8 cranks have always needed these weights....

By my remembered-from-engineering-school understanding of engine balance, crank and piston weight (if all the same) should make no difference to the balance of the crank on this type of typical flat crank 4. Am I missing something?

Al
 
I recently had a Spridget crank balanced and was asked the same question. When i questioned the shop they acknowledged they didn't need that info on a 4, just used to asking because they did lots of V8's.
 
V8 guys just don't "get it" when it comes to us 4-banger lovers. Their computer balance may well need, or at least request those numbers, but 0s can be entered on the 3 systems I have seen and heard of that make the request. As an experienced race engine builder, I actually found that static balancing worked best for my VW and Porsche engines. There was a difference I found while static balancing AFTER having my cranks computer (spin) balanced. The extra smoothness experienced in my engines caused me to stop wasting time and money sending them to spin balancers. At any rate, all inline and flat engines should be perfeclty balanced, in my opinion (as well as most every other engine builder of flat or inline engines).
 
My engine shop guy - a very sharp fellow and quite aware of inline 4s - was also surprised when his balance "subcontractor" started asking for the data, he had the same impression as the rest of us that it should not matter. He told me that he asked the balance guy and was told that the balance machine manufacturer told them to enter it.

Maybe it's just a software issue, the balance machine software always asks for the data and simply ignores it.... Or someone's idea that always asking for the data saves failing to ask for it when it does matter.

Good to know that physics has not changed since I went to ME school..... 8)

I'm intrigued by the comment re static balancing. Gut feel tells me that something as long as a crankshaft could be WAAAAY out dynamically while still being statically balanced.

OTOH I also suspect the the VW watercooled cranks are good enough as stock that I am probably wasting my money regardless. Of course I could be really paranoid and worry that my crank is being balanced in V8 mode and will shake the engine out of the car this spring.... :cry:
 
Some years back I had a crank balanced by a shop that had done work for National Record setting drag cars. My crank was out by 6 grams and he thought that was close enough! When I told him that I static balanced my rods and pistons to 1/10 of a gram he thought I was nuts. Like many other things, I think you'll find some widely varying ideas as to what is correct.
 
...yeah, it doesn't take very long to get it down to fractions of a gram,
and I always figure it can't HURT, right?

t
 
As for the "static balancing," I would never consider working on a crankshaft until I have cleaned and inspected it, including benchmarking it on an engine lathe, checking the journals for straight and round, then spinning it at 250, 1000, and 1500 RPM to check it's dynamics. After that, any machining happens, including lightening and deburring, prior to balancing and finishing. Any failure in the above steps and the crankshaft becomes a lamp base. I also never use cranks that have ground undersize journals. I'ts easier to find a GOOD crank with good and straight journals, than putzing around with oversize bearings and trying to fit (and hoping and praying for) perfection for racing.

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Balancing a V8 crank requires "bob weights" to be attached to the rod journals. That is the reason for asking the weights of the pistons and rods. Setting up the bob weights takes time so balancing a 4 cylinder crank should cost less labor than a V8 crank since the 4 cylinder does not require the bob weights.
 
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