[Mk2] [Turbo] Broken a crank? I now know why

[bobhatton]

For anyone that has broken a crank I now know why.

The counter weight on the crank opposite the piston and rod is way too light to balance out the piston and rod weight.

We have had a crank cut up to just leave the one counter weight and the big end with the weight of a piston and rod added to it. By doing this we could see how the two balance out, but they do not by a large amount.

This is the way the balance works on a normal 4 cylinder engine
The number 1 piston and rod is balanced by number 2 piston and rod and the same goes for the counter weight but they are across the crank not opposite and what this does is bend the crank when the revs are taken higher.

What this means is never have any weight taken out of the crank, no knife edging or lightening.



I will have more info when I get the full report.

[stephano]

i was thinking earlier i hope they cut the right crank up!
The one with the spun bearing and not my one.

[bobhatton]

Well they cut one up thats for sure.
The young chap is on his way to Cornwall over Easter and may bring all your parts down with him, save me a drive

[gavinda]

Intresting stuff bob as always. is that a stock 3s crank , hav u compared with a 5s is it any better?

[bobhatton]

This is the stock 3s but the 5s will be no better.

I will get some side by side photos over the weekend of all 3 cranks, the other being my Jun crank and then you can see how much better the Jun design is

[timlowthian]

Thats some really interesting findings there! Will be good to see the pictures of the difference

[dawesy]

should be interesting to see. to be fair though the only broken cranks ive ever seen have been on cars that have had lightweight crank pulleys fitted and as such have lost the harmonic damper removed.

[bobhatton]

This is the info I was given from the tests. I do not understand all of it yet but will find our more.

This is the company that did the tests.
http://vibrationfree.co.uk/


3SGTE stock crank, stock rod and Wossner piston


Piston weight total 466.9g
Shell bearing 38.5g
Big end 520.3g
Little end 204.0g


Counter weight required 894.2g

Counter weight on crank 419.70g

Shortfall on crank 474.55g = (20078 gmm)

F(kg) @ 8500 rpm = 1285kgf

[Peter Gidden]

Would be interested to learn more, since initial info. suggests Toyota are near enough 100% out on their calculation of counterweight requirement, which is difficult to believe.

[bobhatton]

Some good reading if you like this type of thing...............lol

http://www.vibrationfree.com/resources/ ... per.10.pdf

[bobhatton]

Would be interested to learn more, since initial info. suggests Toyota are near enough 100% out on their calculation of counterweight requirement, which is difficult to believe.

Yes they are but the strength of the crank will stop it bending if the revs and power output are no more than stock. Its when light weight flywheels and pulleys are fitted together with higher revs that then show up the problem.

[bobhatton]

Stock 3SGTE crank 86mm stroke





Stock 5SFE crank 90mm stroke





Jun designed racing billet crank for high revs 91mm stroke






You can see on this crank where all the weight has been added to stop the crank bending. A much heavier crank that is better balanced is far better than a light weight crank.

[rs007]

I find it very hard to believe the Japs of all folk would get this wrong - not saying you are wrong Bob, and I find this very interesting myself - just saying it must have been deliberate IMO

All real world engineering is a compromise, like you say, within stock limits it should never be an issue, so maybe they have done it to keep the rotating mass down/aid engine spool up :just thinking out loud:

[bobhatton]

I find it very hard to believe the Japs of all folk would get this wrong - not saying you are wrong Bob, and I find this very interesting myself - just saying it must have been deliberate IMO

All real world engineering is a compromise, like you say, within stock limits it should never be an issue, so maybe they have done it to keep the rotating mass down/aid engine spool up :just thinking out loud:

They all do it to keep the cost down. The less metal they can use the cheaper it is to build the cars and engines.
This is also why I say not to use lighter flywheels, if it could have been done Toyota would have done it.

[rs007]

I find it very hard to believe the Japs of all folk would get this wrong - not saying you are wrong Bob, and I find this very interesting myself - just saying it must have been deliberate IMO

All real world engineering is a compromise, like you say, within stock limits it should never be an issue, so maybe they have done it to keep the rotating mass down/aid engine spool up :just thinking out loud:

They all do it to keep the cost down. The less metal they can use the cheaper it is to build the cars and engines.
This is also why I say not to use lighter flywheels, if it could have been done Toyota would have done it.

Ahh I forgot the most obvious about commercial engineering; cost - makes more sense now

[Ethan21]

If Toyota used fibre head gaskets to begin with to keep costs down then like bob has said I can defiantly believe they've done the same thing here

[planted1]

I asked my engine builder to check my crank for wear and balance.
The crank was balanced as an assembly,
with flywheel,clutch,and harmonic balancer attached,and matchmarked.
It showed on his screen three lobes (iirc)needed material removed to
get true balance.
These cranks were mass produced,and their will allways be slight variations in quality of casting and machining.

[bobhatton]

The thing about 4 cylinder cranks is that they cannot be balanced with the counter weights matching the weight of the piston and rods on the one pin.
This is because as number one piston is going up number two is coming down so the two balance each other out, but that is across one main bearing.

So what you get is the crank bending. That is ok at stock power and revs but take the power and revs up and in the end the crank will snap.

The weight of the flywheel and harmonic balancer help to stop the bending, but as soon as they are changes say for lighter ones then the crank will be under greater strain.

[planted1]

Aluminium front pulley=bad.
The blitz flywheel on mine is 20% lighter,but,because it has a seperator on the twin clutch,i would imagine its as heavy,or a little heavier than stock,hopefully.
As for balancing,all 3sgte cranks were balanced,to an extent,but as i saw for myself,not perfectly.

[synXero]

Does this mean then that if you fit lighter rods (Eagle) you should change the crank for an item that is perfectly balanced to suit?