crankshaft weights
Turbo Mini Forums

No that I know of. I agree that it screws up the valve geometry and may cause some pushrod issues. But again, my ex valves are offset 31 mm ones and intakes 36, rockers were 1,5:1 so valve geometry needs attention anyway. With bigger valves the short side radius is actually quite good. I did not have a choice since the engine is 1380 and getting the CR down to reasonable level is not that easy figures required.

With current piston options the package would be quite different, and next head will have std exhaust valve size with hardened seat. I am so fed up with valves sinking in the head. I am getting old since I no longer enjoy the porting... BTW Vegard, are you coming to Roskilde in July, would be really nice to meet face to face.

I would love to fit the BMW head on my 1380 but with 20 cc in the head the CR would be in astronomical figures and may not allow any positive pressure...

So, just to get back to the crankshaft weight. Is it worth doing all the work to save 1.5 Kg (3 lb)?
As this weight reduction is acting at a radius of say 4 cm (1.6") it seems relativley insignificant compared to the effort.
Am I missing something?
Regards
Dave

Think it in terms of momentum and the speed at which the crank rotates.

1.5kgs is then significant i'd say. cant be arsed to work it out! lol!

Think of shaking 1.5kgs side to side by 4". quite a bit of force is needed to change its direction. all that is going into the main bearings at high RPM and the extra needed energy to do so.

No Iain, The crank is not shaking from side to side It's rotating.
The con rods shake side to side and weight reduction on these is essential.
I am just trying to evaluate if the cost/effort/performance increase is worth while.
Regards
Dave

Edited by danboy on 8th Feb, 2007.

Yeah i know its rotating but i was trying to use it as an analogy, not a good one! lol! infact it was a pretty poor attempt.


Brain in gear before writing...

Meant to use it in terms of acceleration and decelleration of the crank and the forces involved.

Edited by iain on 8th Feb, 2007.

I know you did Iain
Come on Carl tell me
Regards
dave

two class records and one of only three tin top saloon cars to ever get up baiting dam in under 30 secs Dave,lol.

i like to think of it as doing the best i can with the facilities i have,

i could never afford to have my engines proffesionally built in the past with having a young family, but as you know i could beat many many minis in my class who had there engines built,

it would seen a waste of effort if you were to hang anything but the lightest[ read inertia] flywheel on the end of it though,

you know the score ,lol.

carl

WWW.FORCE-RACING.CO.UK PLEASE CLICK HERE

which is why i did this,

http://i24.photobucket.com/albums/c2/carl1964/DSCI0033.jpg

plus its 1.5 kgs off the ovewrall weight of the car, which is reason enough ,

WWW.FORCE-RACING.CO.UK PLEASE CLICK HERE

"The weight of the crankshaft, rods, and pistons is a major factor in the rpm-producing potential of the powerplant. The lighter this reciprocating mass is, the easier it is to move and rev. This frees up rpm potential by lessening the parasitic weight the engine would normally have to overcome. Naturally, this weight increases dramatically with rpm, so the final goal is a lightweight grouping of rods, pistons, and crankshaft that are still durable enough to deal with the rigors of power production within the target range of the engine's design."

OK guys you are both right and I know it.
Just having a look at the increase in performance / £ for my project. I ain't a millionare
Regards
Dave

where did you copy and paste that from,lol.

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t'internet!



Depends what rev's etc you want to run as to how effect it is. Higher the rev's the more effective as the forces exponential with angular acceleration (i think!)

If your maxing out at 6500rpm and arent looking at the very edge of the performance then i wouldnt bother.

Every little helps though not sure what the cost of it is.

about £30.00 in indexable inserts and a days work for a proper job.

On 8th of Feb, 2007 at 08:02pm iain said:
t'internet!



Depends what rev's etc you want to run as to how effect it is. Higher the rev's the more effective as the forces exponential with angular acceleration (i think!)

If your maxing out at 6500rpm and arent looking at the very edge of the performance then i wouldnt bother.

Every little helps though not sure what the cost of it is.

WWW.FORCE-RACING.CO.UK PLEASE CLICK HERE

I'd say that the weight of the crankshaft won't affect rpm potential at all. Rods, pistons yes, but not crank. Also, what you need is a heavy crank. Light cranks wears out the main bearings. WHen this is said, the reason to wedge the Mini crank is not to lighten it. It's to move the centre of the weight to where it belongs. A Mini crank is far too heavy on the big end side, and too light at the counterweight side. The best one from the factory is infact the 850 one with proper counterweights.

Have a look at Swiftune's ultimate cranks:

This crank is hardly light, and will of course spin up later than a severely lightened crank. However, what this crank does, is to make the mains survive.

Heavy crank is good, but it has got to be heavy in the right place. This means that Blading is rubbish. Wedging is good and backdrilling is good!

Swifty told me this crank is designed to rev better.
Paul

I seriously doubt it!

Compared to what? A seriously lightened crank will rev better, ie. spin up faster. What this will do, is rev better ie. not wear out mains with sustained high rpm race after race. For Carl and his drag-racing, this crank is not the best option. For Le Mans 24h, this crank is the best option.

Rev better...Get them to explain this a bit more. What is better? Faster, smoother, longer etc?

Swifty believes it will have the inertia to spin up quicker than the usual worked A+ crank and sustain it as you have said which will preserve bearings.
Having said that though, apart from them being so expensive I dont have one because Im not sure I believe it.
Paul

Edited by paul wiginton on 9th Feb, 2007.

I seriously doubt it!

What? "the intertia to spin up quicker"

Is that a stupid as it sounds?

I think he meant "it has a lower polar moment of inertia which will enable it to spin up quicker"
At least I know what he meant!
Regards
Dave

i'm sure youv'e got it wrong dave, a low polar moment of inertia meens it had most of its mass near the centre. which is not the case with that crank, tell me i'm right .,

the debate continues, lol,

this one could run and run,

i think what paul is saying is that swifty is bullshitting alledgedly, as if hes told Paul he " beleives" it suggests he has ne test data to prove otherwise,

WWW.FORCE-RACING.CO.UK PLEASE CLICK HERE

Yes carl you are RIGHT!, I didn't read the post properly,I was talking about your lightened crank.
Regarding the Swift crank I belive it will improve the Secondary out of balance forces, thus giving the bearings an easier time at high RPM
Regards
Dave

Edited by danboy on 9th Feb, 2007.

A rotating crank has a mass, it remains the same, its weight increases as the centrafugal force increases with RPM ( force goes up by the square of the velocity). Same principle why it's so difficult to get a motorcycle to lean over at high speed.

Making a crank light yet still retain the balance of the crank throws and strength to survive the abuse it gets is a difficult one as far as i can see and a comprimise is ultimately required in the end. How would you comprimise?

The reciprocating mass balancies itself. two pistons go up as two go down.

Lightening the reciprocating mass increases the reliability of the crank as well, its not being flexed as much by the riciprocating mass so reduces metal fatigue.

Reciprocating mass is also subject to increased forces similar to the centrafugal forces. Higher the speed of the reciprocating mass the heavier it becomes (force goes up by the square of the velocity) as it reaches TDC and BDC where it has to decelerate rapidly. Reducing the reciprocating mass reduces the forces excerted on the con rod bolts and its cap, as well as reduce its decelaration forces upon the crank. Another reason why short stroke engines are happy at high RPM, the velocity of the reciprocating mass is reduced for any given RPM compared to long stroke.

How far you go with weight saving will determine reliability either way. Make it to light that you are sacrificing strength, leave it to heavy, and the forces inside the engine will tear it apart, eventualy

Back drilling not only makes the crank throws lighter but it also makes the crank stronger as the surface area has become larger. I read that in CCC many years ago when back drilling was a new fangled thing.

lightening things does release absorbed power and allows quicker spin up, but is it a noticable amount? as far as im concerned, no. However, lightening serves its purpose in high reving engines to increase the survivebility of the reciprocatong and rotating assemblies.

Thats how I understand it, and no, i do not have a degree, so its all probibly bollox any way so please ignore me

Edited by Sprocket on 9th Feb, 2007.

Just got my new crank for 998 project.
What do you think?
Anyone care to guess who made it?
Regards
Dave

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turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

i think what paul is saying is that swifty is bullshitting alledgedly, as if hes told Paul he " beleives" it suggests he has ne test data to prove otherwise,

I probably wouldnt say bullshitting, he does have the knowledge and the dyno to test it, and he is putting one in his own new historic racer. But for 1/4 mile which as everyone knows is all Im interested in, I dont believe its the way to go.
Paul

I seriously doubt it!