balancing big changes in rotating masses

tadd · 06-05-2006, 05:59 PM

This is from an old classified thread... but... would anyone be willing to explain how one would go about deciding how much material to remove to duplicate the crank that OliveR posted pics of here?

http://www.pelicanbbs.com/showthread.php?s=&threadid=164117

The mechanics seem easy enough by jigging on opposite pairs of rod journals, one can mill down the crank webs. Fip over and do the 'other side'. Repeat for the other two pairs of journals.

I am ordering up a set of Ti rods so I will have about 240g difference between the stock 2.4 rods and the Ti. I assume that this is more than traditional knife edging will cover... so how does one balancing a crank decide how much metal to remove to get into the 'ball park' for the person doing the actual dynamic balancing?

I would like something better than using photoshop to measure pixels to get a close measurment.

tadd

chris_seven · 06-05-2006, 11:46 PM

Tadd,

Very interesting crank and probably a significant reduction in inertia.

By reducing countreweights it will of course be more difficult to acheive the highest levels of lateral balance and the engine may not be as 'smooth' as with a standard S crank.

I think you will also need to consider rod and piston weights in the overall balancing equation.

I have a contact that could do the correct math here in the UK if there is a drawing of the crank available and data concerning rod weights, overall and end to end, piston and pin weights.

It would then be possible to prepare a 'balancing method statement' that any good balancing shop could follow.

You will also be changing the mass elastics of the rotating assembly and this could have some impact on torsional vibration and it would be interesting to have a quick look at the effect these changes have on the defections that will be produced.

The problem with a torsional is that it will not cause any roughness in the engine and the first sign of problems will be when the end falls off the crank.

It would be interesting to know if these cranks had a 'life' specified by the factory race department and the frequency with which they were changes.

Clealry reducing the inertia will allow the engine to spin up more quickly but it will also slow much more quickly due to engine friction when shifting gear and with the speed of shift of a 901 box it may be interesting.

We are currently developing a 'dog' gear kit for the 901 and this may be an important part of the performance package when you have low inertia engines.

David · David's Garage

I would imagine any of the top shops like Ollies or EBS could handle the lightening for your application. I just want to hear a sound clip of it when it's done.

A very light flywheel, like a Tilton clutch setup might help with torsional stresses. The light rods and pistons should lower the stress too.

chris_seven · 06-06-2006, 05:27 AM

My concern with the torsional vibration is not knowing where the critical speed is as if there is a resonance close to the running range it could be a problem.

I would imagine that replicating a 906 should be OK but if the original had a short life so would the 'replica'

tadd · 06-06-2006, 05:44 AM

Chris:
Thanks for chiming in. I am way out of my leauge here as I only play at physics. If you wade thru that post, there are some good tid bits. It was surmised that the crank started life as a sprint piece, but then moved to 'general status' as reliability was recorded. I also note in that posting that there was mention of later RSR cranks that had that general counterweight shape as well, so the racing department must have considered it 'acceptable'.

Now into the fully non-scientific... So the material they removed was in the 'ears' of the CWs. Another note is that the 'points' of the triangle that form the CW are just outside the main journal if not tangent to it. I guess I am speculating that that material is far enough out from the centerline that it would provide very little stiffness or strength, just mass. But I am truly guessing...

tadd

David · David's Garage

interesting read:
http://www.indiacar.com/index2.asp?pagename=http://www.indiacar.com/infobank/torsional_vibration.htm

tadd · 06-06-2006, 08:16 AM

Stuff that I kinda knew and have thought about, but all tied up in a nice neat package. Cool.

Thanks,

tadd

chris_seven · 06-06-2006, 08:39 AM

Great article.

davidppp · 06-06-2006, 01:30 PM

Chris is right about light inertia motors...

I learned this lesson in a very light Ford |Anglia!

The engine was always being sold and another built..I put one in which I had done quite a bit of lightening flywheel and crank etc..and a very high CR, with water injection at full throttle etc..

Very first time out, changed down to second for a roundabout, double declutched let clutch out with no throttle, instantly lthe rears seemed almost locked..!

You soon learn to always have some throttle in on a downchange..

But the combination of a light-spinning motor and dog box is electric...

Kind regards
David