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knife edge crank 202

Started by basicgreen , Jun 26 2015 03:53 PM

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91 replies to this topic

#51 _oldjohnno_

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Posted 30 June 2015 - 08:01 AM

I like to think of the whole assembly mass as the issue, crank, rods, pistons, balancer and flywheel... I guess its a balance (pardon the pun) between weights of assembly... I would not hesitate to use a knife edge crank from someone who has plenty of experience in machining them (like Mick), as the weight is still more than a red crank anyway (from memory). Also, I think the whole package has to be considered- cam duration, final gearing, transmission etc etc are important as the light set-up may not suit some packages.-

I think you hit the nail on the head - it's the whole assembly that counts. The total weight is a matter of personal preference but I think how the weight is distributed matters.

Obviously the pistons and rods should be as light as they possibly can be; weight here causes a whole bunch of problems.

The next thing then is how much weight do you have in the crank and how much in the flywheel and balancer? The trouble here is that we have conflicting requirements. Every time we take weight off the counterweights we make life harder for the block, and this might be an issue for a very high revving engine.

But if we leave the crank at full weight and use a super-light flywheel then we get clutch failures and so on from the torsional vibrations being channelled towards the back. Whatever we do is going to be a compromise.

Probably the best thing to do would be use the lightest pistons and rods that are affordable along with a moderately light flywheel, and then if more weight still needs to come off then trim it from the crank, ideally in a way that doesn't reduce the effective bobweight too much.

Edited by oldjohnno, 30 June 2015 - 08:03 AM.

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#52 ls2lxhatch

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Posted 30 June 2015 - 01:09 PM

Something else to keep in mind - the mass of the rods and pistons behaves in exactly the same way as the rotating mass. In other words heavier pistons and rods have the same effect as a heavier flywheel.

Maybe I have misunderstood what you have said. It appears to me that you are saying that increasing the piston/rod weight by 300 grams would have the same effect as increasing the flywheel weight by 300 grams.

Wouldn't the pistons and to a lesser extent the rods loose a significant amount of their inertia every time they change direction?

A flywheel never changes direction so it would not loose its inertia in the same way as pistons and rods.

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#53 LC-GTR-1969

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Posted 30 June 2015 - 02:30 PM

Maybe I have misunderstood what you have said. It appears to me that you are saying that increasing the piston/rod weight by 300 grams would have the same effect as increasing the flywheel weight by 300 grams.

Wouldn't the pistons and to a lesser extent the rods loose a significant amount of their inertia every time they change direction?

A flywheel never changes direction so it would not loose its inertia in the same way as pistons and rods.

I cannot speak for OldJohhno, but I am not sure that this is what he meant...

I think he just meant that by increasing the weight of the pistons/ rods, the 'observed' effect is the same as when we increase the weight of a flywheel. Not sure that he was suggesting that an equivalent increase in rod/piston weight to flywheel weight would be the 'same' level of effect.

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#54 _oldjohnno_

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Posted 30 June 2015 - 02:32 PM

Maybe I have misunderstood what you have said. It appears to me that you are saying that increasing the piston/rod weight by 300 grams would have the same effect as increasing the flywheel weight by 300 grams.

Wouldn't the pistons and to a lesser extent the rods loose a significant amount of their inertia every time they change direction?

A flywheel never changes direction so it would not loose its inertia in the same way as pistons and rods.

Nope. You can't "lose" inertia, it has to go somewhere. Every time the piston approaches TDC or BDC it decelerates and that inertia or energy is picked up by the crank and flywheel. As it departs TDC or BDC it's accelerated again and that energy is swapped back from the flywheel to the piston and rod. In effect the piston and rod is just more flywheel mass. Adding 300gm to the piston would be roughly equivalent to adding 300gm to the flywheel, at a diameter somewhat less than the stroke length. If you know the reciprocating weight and the average piston speed then you can work out the exact equivalent in flywheel weight/diameter.

A side effect of this is that speed of crank rotation isn't strictly uniform, with the speed increasing and decreasing as the piston passes TDC and BDC. The forces that cause this can be high at high rpms, greater than those produced by combustion pressures. They are also a major cause of torsional vibration of the crank, so you can see why it's so important to keep the recip weights down as the rpm range rises.

Edited by oldjohnno, 30 June 2015 - 02:35 PM.

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#55 ls2lxhatch

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Posted 30 June 2015 - 05:16 PM

Thanks for explanation.

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#56 _STRAIGHTLINEMICK_

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Posted 30 June 2015 - 05:44 PM

Yup the reciprocating weight needs to be kept down in a performance engine because of the effects on the crank apart from slower response .The worst effect is tortional vibration and if you can imagine a whipping effect on the crank from the weight of the middle cylinders at top and bottom dead centre .

And for those who havn't built a lightweight crank/rod/piston engine the difference in response is like night and day . But like Johnno said the whole package must be a compromise between how much response is required ,how much inertia is required and how much reliability is required . I think i have found a good compromise through trial and error with these cranks ,the engine in my car was built in 2006 and has survived countless dyno runs ,around 60 1/4 mile passes and about 20000 hard road miles .

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#57 _STRAIGHTLINEMICK_

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Posted 30 June 2015 - 05:58 PM

The combo you describe here is close to mine, only it's 7000rpm then off the clutch, it would be nice to have a "slipper" clutch.

I believe the flywheel was about 30lbs, a little birdy told me.

7000 is even more fun , what type of clutch do you use Warren ?

Wow 30lb is pretty heavy , I also heard it from a bloke who got under it to have a look .

I think with that much weight you would have to do anything possible to not let the revs drop .if you held about 8000 and dropped the clutch ,maybe let it slip a little and flat changed every gear to make use of all that inertia it would work .

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#58 warrenm

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Posted 01 July 2015 - 12:45 AM

Button clutch, so no slip. The combo at the moment seems to be a good balance. After reading oldjohnno's replies, I wouldn't bother touching the crank again, what he's saying makes sense.

I've tried various combos, all with a manual trans. The red crank with the 6kg flywheel was the worst setup for drag racing, but would suit a circuit engine, like LC-GTR-1969, it was a pig on the road, revved great when you got it on the boil, but it eventually came loose on the crank, so that flywheel now sits on the shelf collecting dust.

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#59 _Agent 34_

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Posted 10 July 2015 - 08:42 PM

This discussion has been very helpful for me at the moment as I am planning a new engine build. The question has been for me to lighten or not to lighten . I did get in contact with the original owner of my car and asked him about the motor and to my surprise it's running a RED crank with a full weight fly wheel and a romac up front. It was still running standard rods till he changed them to " star fire rods". which it currently has now.

I will say that the current engine is very responsive but at the moment i'm only reving it to 6800 rpm on the tacho due to SU 1 3/4 being on the motor .

Vibration wise - it seems smooth with no physical feel BUT the bloody nuts and bolts come loose all the time and i wonder if this is a symptom of the lighter red crank " and increase in torsional frequency or increase in severity in the wave due to less dampening" or poor assembly preparation " sump especially " .by not cleaning the threats.

The question with lightening a crank is how much is TOO much or what total end weight is acceptable.

lets say the rods and pistons are lighter than standard offerings

I'm trying to find some facts to use as a gauge moving forward in the area.

G

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#60 SA EH

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Posted 10 July 2015 - 11:06 PM

Grant, just for your reference...

I'm running a standard red crank with no real lightening except for getting the bmw rods to suit, rods weigh in at 479g, pistons at 437g, so still pretty light but no flywheel obviously as it's an auto. With the stud girdle in place I haven't yet found any loose bolts or had any bad vibes...

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#61 _duggan208_

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Posted 10 July 2015 - 11:24 PM

Grant, take a look at the SCAT crankshaft website. They explain the weight of their cranks and what applications they suit, may be useful info.

regards

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#62 _duggan208_

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Posted 10 July 2015 - 11:33 PM

My super light flywheel has a steel insert added to the inside, I hope it works. With the light flywheel I had to slip the clutch a lot every time I started off, It really seemed to struggle with any standing start.

Regards

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#63 _oldjohnno_

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Posted 11 July 2015 - 10:57 AM

The real question is how much are you prepared to spend on this?

Think about this:

A straight six has perfect primary balance no matter what counterweights the crank has. But even so forces originating from the pistons and rods produce bending loads in the block. In the little six they can be severe enough to crack and break the block. Making the piston and rod lighter reduces these forces.

These forces are in proportion to the square of the rpms, so a small rise in revs produces a big increase in the harmful forces. As rpms rise it becomes increasingly critical to reduce the piston and rod mass.

The secondary imbalance forces are in proportion to rod length - an infinitely long rod has zero secondary imbalance. In other words as far as "balance" is concerned the longer the better. I use quotes around the word balance because it is a largely imaginary condition.

The little six has issues with torsional vibration that have all sorts of nasty effects. The TV is largely a byproduct of forces produced by the mass of the piston and rods. Can you guess what you can do to reduce these?

Managing the combined rotating mass of the crank, flywheel and balancer is always going to be a juggling act between reducing the rotating mass and maintaining enough bobweight to keep the block in one piece as well as keeping a lid on torsional vibrations. Making the pistons and rods as light as possible reduces the flywheel effect and allows you to keep more mass in the crank.

Do you see the common thread in all these issues? Step one is save up your money and buy the lightest pistons you can afford along with the lightest, longest rods you can afford. Stock rods are too short and way too lard-arsed. Once you've got these parts you can think about the rest. But if you aren't prepared to spend the money then your chances of success at high rpms are slim to none.

There was a somewhat similar thread some time ago - based on a Starfire I think - that turned into a train wreck. I wasted a lot of time on it and it's made me wary of getting involved in another.

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#64 _STRAIGHTLINEMICK_

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Posted 11 July 2015 - 02:13 PM

Yep weve all put our two cents in ,here it is in simple terms . and the outcome is lighter = faster but it is a balancing act between light ,fast ,reliability and intertia required to get off the line . Lower the gears the less inertia required . A red crank has enough weight to be reliable but its all in the centre . A lightened blue crank has about the same weight but spread across the length of it . The flywheel will give the required inertia ,the choice is red or blue motor weights or a custom unit.A decent balancer will help reduce vibrations and 7/16" flywheel bolts will help keep the other end together , Long rods will be a big help as well.

Every engine builder has a combination that works for him and changing one element because a mate said to can make it a dog ,so it is wrong to bag any combo with proven results . Only change a combo if you are ready to for lots of trial and error testing which can be frustrating ,costly and really piss u off.

At the start of this thread there was mention of a long time torana group nc racer named Warren who had built one of these long rod nissan piston combos and said it was the best he has had or words to that effect .

Like Johno said all we can do is put the info out there and in the end the choice will depend on your budget and your application.

Mick

Edited by STRAIGHTLINEMICK, 11 July 2015 - 02:16 PM.

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#65 N/A-PWR

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Posted 11 July 2015 - 02:38 PM

A Question:-

Is the Red or Blue Crank balance weights,

equal to the Standard Piston and Rod weights?

if so,

knife edging the crank a bit to match the lighter Piston and Rod combination.

Edit:-

just did a rough measurement of the Red Crank excess lobe,

of about 90x20x40mm = 550grams approx

So may have answered my own question,

where the weight is equal to half the Piston and Rod assembly, hmm. :furious:

Edited by NA-PWR, 11 July 2015 - 02:51 PM.

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#66 _STRAIGHTLINEMICK_

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Posted 11 July 2015 - 03:25 PM

A Question:-

Is the Red or Blue Crank balance weights,

equal to the Standard Piston and Rod weights?

if so,

knife edging the crank a bit to match the lighter Piston and Rod combination.

Edit:-

just did a rough measurement of the Red Crank excess lobe,

of about 90x20x40mm = 550grams approx

So may have answered my own question,

where the weight is equal to half the Piston and Rod assembly, hmm.

Those calculations mostly apply to V type engines . In an inline engine the crank is balanced to itself as if it were a round metal bar without any added weights . The counterweights are mostly there to offset the crankpin on the opposite side and no additional weight is required for smooth running . The piston and rod sets are usually balanced to within a gram or so of each other and their weights cancel each other out as they travel up and down in pairs .

So as you can see the requirement is for lighter reciprocating parts as rpms rise ,and that includes valvetrain as well.

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#67 N/A-PWR

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Posted 11 July 2015 - 03:41 PM

OK thank's Mick,

makes a lot of sense :spoton: .

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#68 _oldjohnno_

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Posted 11 July 2015 - 03:57 PM

Those calculations mostly apply to V type engines . In an inline engine the crank is balanced to itself as if it were a round metal bar without any added weights . The counterweights are mostly there to offset the crankpin on the opposite side and no additional weight is required for smooth running . The piston and rod sets are usually balanced to within a gram or so of each other and their weights cancel each other out as they travel up and down in pairs .

So as you can see the requirement is for lighter reciprocating parts as rpms rise ,and that includes valvetrain as well.

This isn't true. It's correct that the individual cylinders cancel each other out and that no additional weight is required for smooth running...

But - all those cancelling forces are transmitted through the crankshaft and the block, and they are the reason that the blocks are jerked into pieces at high rpms. They are also hard on main bearings and journals as well as contributing massively to torsional vibration.

The same sort of calculations that are used to balance V8s are used to design the crank counterweights on any modern straight six and this counterweighting can reduce the destructive forces greatly. It's not possible to measure the bobweight on a conventional balancing machine so most of the monkeys on balancing machines just ignore it saying it's too hard/not needed. Which of course is pure horseshit. Any decent design software will give an accurate bobweight number based off counterweight dimensions.

As for your question Dave, even the blue/black crank is significantly underbalanced (around 30% from memory) for any of the stock piston rod assemblies. And at any rate balance isn't the main problem, it's excessive rod/piston mass.

I'm out of this thread, I can hear the trains coming already...

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#69 N/A-PWR

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Posted 11 July 2015 - 04:24 PM

Thank's Oldjohnno,

Found this short Video:-

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#70 _STRAIGHTLINEMICK_

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Posted 11 July 2015 - 04:25 PM

This isn't true. It's correct that the individual cylinders cancel each other out and that no additional weight is required for smooth running...

But - all those cancelling forces are transmitted through the crankshaft and the block, and they are the reason that the blocks are jerked into pieces at high rpms. They are also hard on main bearings and journals as well as contributing massively to torsional vibration.

The same sort of calculations that are used to balance V8s are used to design the crank counterweights on any modern straight six and this counterweighting can reduce the destructive forces greatly. It's not possible to measure the bobweight on a conventional balancing machine so most of the monkeys on balancing machines just ignore it saying it's too hard/not needed. Which of course is pure horseshit. Any decent design software will give an accurate bobweight number based off counterweight dimensions.

As for your question Dave, even the blue/black crank is significantly underbalanced (around 30% from memory) for any of the stock piston rod assemblies. And at any rate balance isn't the main problem, it's excessive rod/piston mass.

I'm out of this thread, I can hear the trains coming already...

Its dead true Johnno ,

i just called the engine balancer to confirm this and he was taught by Bert Jones in the late sixties and still balancing engines now V8 and six cyls .

Next time you get a crank balanced just watch the guy and talk to him . Im not here to argue the point or be a smart ass but these are the facts .Its been done this way since the dawn of time .

We'll have to agree to disagree here Johnno, just talk to an engine balancer yourselves ,preferably one that has years of experience with both types of engines .

Edited by STRAIGHTLINEMICK, 11 July 2015 - 04:29 PM.

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#71 _oldjohnno_

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Posted 12 July 2015 - 04:13 PM

Come on Mick, it doesn't matter how many cranks they've done the laws of physics are the laws of physics. It's not rocket science either; you have a beam (engine block) with forces at each end going in one direction being cancelled out by forces near the middle of the beam in the opposite direction. Sure the beam/block won't shake but the forces are trying to flex the block repeatedly, and eventually it breaks. Besides, every modern straight six is designed with counterweights. Or does some old bloke that was taught by Bert Jones know more than the OEM crank designers?

Like you said, we'll have to agree to disagree on this one, and I'm sticking with the laws that have been known for hundreds of years, not some grub on a pedestal drill...

Edited by oldjohnno, 12 July 2015 - 04:16 PM.

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#72 _Muzzy_

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Posted 12 July 2015 - 04:44 PM

Come on Mick, it doesn't matter how many cranks they've done the laws of physics are the laws of physics. It's not rocket science either; you have a beam (engine block) with forces at each end going in one direction being cancelled out by forces near the middle of the beam in the opposite direction. Sure the beam/block won't shake but the forces are trying to flex the block repeatedly, and eventually it breaks. Besides, every modern straight six is designed with counterweights. Or does some old bloke that was taught by Bert Jones know more than the OEM crank designers?

Like you said, we'll have to agree to disagree on this one, and I'm sticking with the laws that have been known for hundreds of years, not some grub on a pedestal drill...

This being my first post on this one, I'm totally with you on this. Oldjonno . I'm surprised at how much miss information is out there on balance, and how many different views, one trouble being most engine recon shops employ semi skill labour who really know how to work a machine, to grind or machine, but have no idea on mechanical engineering concepts and wouldn't know an engineering handbook if it bit them. Let alone static and dynamic balancing principles

Edited by Muzzy, 12 July 2015 - 04:46 PM.

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#73 _oldjohnno_

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Posted 12 July 2015 - 05:49 PM

Yes, I'm sure there are a lot of balancers out there doing a perfectly fine job that are ignorant of the maths behind what they do. And to be fair to them there's normally no need for them to know it. As well, there's nothing their machines can do to determine the balance factor of a straight six crank, all they can do is make sure that the crank itself is balanced overall without any regard at all to the piston/rod and bobweight relationship.

Still, it's easy to work it out by other means these days, and any 2nd year mechanical engineering student can pretty easily calculate exactly what and where the forces are for any given speed, crank position and piston/rod/counterweight mass.