Crankshaft counterweights

[evelrod]

This question has cropped up on another forum and since I have intimate knowledge from a practical point of view... I've been sponsored, off and on, by a balance shop for nearly 30 years and have learned to do much of the actual balance work...
The question is, how much added counterweight is necessary to stabalize a three main crank as in my 1380cc Mini? Currently I am using a "name brand" EN40B design as in the UK race series with a friction damper and a <8 lb. flywheel/clutch, 14:1 CR, 7600 rpm rev limiter and, so far, doing just fine with it. (Knock wood)!
Several racers have questioned the counterweight design and have submitted some rather odd designs as "better". One such design is the Graham Russell ($5000 Australian) billet crank with eight counterweights (as opposed to my four)!
My question is---Will adding additional counterweights aid my little the three main crank at speed...Better? Worse? It certainly will effect my bank account!

Rod

 

[NakedTestMonkey]

In racing form I have seen good amounts of reduced bearing wear by running an 8 counter weight crankshaft versus a 4 counter weight crankshaft. Naturally the 8cw crank has the side effect of increased windage and weight, but those can be addressed rather cost effectively.

This is on an inline 4 running 5 mains turning 8500 RPM.

 

[GregLocock]

It's a very complex subject.

On the one hand, more counterweights can give you reduced main bearing loads.

But on the other hand the extra mass will reduce the frequency of the torsion (in particular) and bending modes.

We found only a slight benefit in going to a 12 cwt crak for an I6, from an 8 cwt crank, and it weighed a fair bit more.

So, given you only have 3 mains, I can't really see much benefit in having more than 4 cwts.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[evelrod]

NTM---I did do some test runs on a five main Lotus twincam years ago...4 CW- 19 lb., 4 CW-22 lb. and, 8CW- ~28 lb. cranks. Bottom line...the 19 made most power accelerating and was equal to the others at steady state. The 28 (which is currently in our newest engine) was by far the smoothest. I chose to use this crank as this engine is for a restored vintage Lotus Cortina and not a full blown racer. My original question pertains to a THREE main eight CW crank...whole 'nuther' game!

Thanks, Greg. That's pretty much my position. However there is a certain group that NEEDS the ultimate high tech gadgets to 'go fast'. In my experience spending that $5,000 on chassis and track time would lead to larger overall improvement in lap times. JMO!

I did weld some pretty big counterweights on my Model A crank and it made a huge difference in the primary inertial imbalance. She doesn't try to 'hop' out of the chassis at anything over 45 mph any longer!!!

Rod

 

[NickE]

evilrod -- isn't it always better to spend money on the "nut" behind the wheel?

 

[NakedTestMonkey]

NickE -- True, but I am willing to bet he can't push the car as fast as the engine can propel it.

The car must first finish, then the driver can win.

 

[NakedTestMonkey]

Evelrod -- I know, I was just throwing out my experience with CW's in a racing environment.

 

[evelrod]

Nick, we call it the 'spacer'. The semi intelligent biomass that fills the space between the 'headrest' and the 'steering wheel'. ;o)

NTM, I never had any bearing concerns with the 5 bearing 105 series Fords (Kent) or their derivitive Lotus twincam in 28 years of racing them...no matter what crank I used. Out tests (experiments in more power?) were just something young guys do when they have too much time on their hands.
This little Mini Cooper's 1380 three main, five port head deal is something else. Bearings have a lifespan of about 3 to 5 races as I see it now. With this new EN40B crank (wedged and bladed) the bearngs look 'new' after two weekends, that's 4 races, Laguna Seca and Willow Springs---a 4th and a 2nd (by just 0.182 seconds)---not too bad for an old guy amongst them thar whippersnappers and their BMW 2002's, hey? Go to the AMB site, Mylaps.com and check out the VARA Big Bore Bash at Willow Springs...it should not take too much effort to find my times.

Rod

 

[patprimmer]

Rod

Good to see you back. A few of us started to worry a bit about you

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Majik]

Us young whippersnapers like our bmw 2002's.. if you ever come out east coast side I'd definitly like to race ya not to get off topic

 

[evelrod]

I'm still alive and kicking, Pat. Winter coming on and dark is early...new FIOS connection for internet...It's likely y'all will see me a bit more often (especially since I haven't broken anything in the race car in a while...knock wood)!

Majik...The VARA C/S class is dominated by BMW's and Datsun 510's...when a Mini does well against them at a high speed track like Willow it's unusual (as Scott P. -winning BMW- said after the race, "...looked in the mirror and said, oh s*** there's that old man again..."!)
At some of the other tracks like Buttonwillow and Laguna Seca, the big cars are not so dominant...I am NOT the fastest Mini here in Socal!!!
Oh yeah---The unofficial lap record for vintage CS is a 1:40.396 (BMW 2002) and my time was 1:40.413. Not too shabby for a 1963 Austin Cooper and a 67 year old fat guy, hey?

Rod

 

[Greenlight]

There is good infomation on this subject in this technical paper:

A Characteristic Parameter to Estimate the Optimum Counterweight Mass of Symmetric In-Line Engines

Journal of Engineering for Gas Turbines and Power -- July 2004 -- Volume 126, Issue 3, pp. 645-655

 

[kenre]

evelrod

not sure windage would be much of an issue in a Mini, with the gearbox throwing oil every which way under the crank. Unless you have a shield fitted. Extra counterweights surely must affect acceleration for the worse assuming the whole crankshaft is heavier.

Only 7600Rpm ??? my old mini used to do 9500rpm.. had one of those 643 cams, factory race grind.

http://www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

 

[panelman]

Pete Baldwin in the uk used to be a guru on A series internals, he used to be associated with Marshalls in Cambridge, might be worth seeing if you can get in touch with him.

A quick search comes up with

http://www.wilshers-garages.co.uk/location_map.htm

He’s been winning in minis since the 70s

 

[evelrod]

Ken, I do have a windage tray fitted to stop some of the 'spray'. I built the engine as 'vintage'(presumably as it would have been in 1972) and use a Kent SP310 for a powerband of 5000/7500. (110.49 hp @ 7000 and 84 to 89 ft/lb torque from 4750 to 7400 at the wheels) many of the other guys are turning 9500...I just cannot afford those engine costs and I do seem to be able to 'keep up'.

Our Mini racing forum does not lack for expertise in 'Minis' as it has such members as Keith Calver, David Wells, Nick Swift, Bill Solis, etc.etc. My question was to give me expert opinions from engineers outside the immidiate Mini fraternity. Often one can get 'too close' to a project, hey?

Thanks guys.

Rod