Punsun crank failure theory

[sno-bro]

Putting hours and hours into my diesel bike build (Dnepr + Punsun V-2 + manual gearbox), reading about the Punsun crank failures keeps me worrying. After all the hard work and countless days am I going to drive around the block and push back a bike with a crank failure The thought sickens me.

I just came up with a (hopeful) theory for the root cause: Could it be that the crank PTO end bearing is not designed to take the sideloads imposed by a belt drive or CVT? To my best understanding it is a single row ball bearing only, so it will transfer bending loads to the crank big end effectively twisting it while the crank turns. Does anyone know if ALL the failed cranks were on a belt/chain driven builds? All the industrial applications (generators, water pumps etc) where the engine is used do not have any side loads.

This of course with the hope that my application would not be at risk

sno-bro

[pietenpol2002]

It is only those tapered shaft engines (principally generators) that are limited to rotational loads. Most straight cranks have side loading imposed by belts or chains. Even so, a reasonable hypothesis worthy of further research.

[skoleskibe]

most likely the bearing in the PTO side is a bronze bearing as on the L100 type engines. -they have the ball bearing in the flywheel side.
Even though the V twin is an other engine, its derived from the L100 type engine, just beefed up with an other crankcasecasting and an extra cylinder and so on.

[thundercougarfalconbird]

how heavy is a cvt cluth at speed?????
its a big heavy chunk s@@t spinning on the end, with a belt pulling not only back but causing a shifting side load as it shifts up and down , i have been beating the crap out of my punsun 836cc with turbo and havent broken it yet

[sno-bro]

I guess I was just being hopeful... If you have a turbo Punsun with belt drive and no problems it sounds more like it has nothing to do with the mounting direction.

What is interesting is that everyone seems to know that a lot of the Punsun cranks have failed, but no-one seems to actually knows how many or can point to a person or bike that has failed. I wonder how many ACTUALLY have failed besides the one or two in Germany? It does seem like there are quite a few Punsuns happily motoring along too. I'd LOVE to know about the failed crank(cranks?) bikes, what kind of tranny, rubber/no rubber mounting etc. It would be very human to lets say skimp on the tranny alignment, run the bike, brake a crank and then blame crappy chinese parts.

Oh well, I'm so far in my build I'll have to use my Punsun anyway and hope it works. Keep your fingers crossed!

sno-bro

[thundercougarfalconbird]

its true, no one ACTUALLY knows the number of failures, it sounds like the rumors and bad rap that the GM 6.2l and 6.5l engines got from failed cranks, the problem with those engines was cheap owners, not replacing the failed ballancer, and about 6 months or so after the ballancer fails the crank fails.

never blame the machine, blame the human that owns it

[pietenpol2002]

Below are some interesting comments by Dennis Franz over on DieselMotorcycle · Diesel Bio Veggie/PVO/SVO Motorcycles. He seems to be suggesting something similar to your hypothesis Sno-bro. I would also wonder what, if any effect the absence of a balance shaft has on the V-twin. The singles were directly cloned of course from the original Yanmar and thus copied Yanmar research and technology. The V-twin appears to be a Chinese departure that dispensed with the balance shaft. I wonder how much research went into the redesign before production began.

"The V twin punsun and for that matter all small diesel engines will not
break a crankshaft if it is properly installed. The engine is mainly
designed for use as a generator which has in reality a flywheel on each
end of the crankshaft.This double flywheel is in reality the same
attachment as a harmonic balancer on a auto of truck engine.These
balancers take the shock of combustion and spread it out to both ends
of the crankshaft. A car or truck engine can and will suffer a crank
failure if the balancer is removed. Motor cycle engines have most of
the weight of there flywheels internally and not hanging on one end of
the crankshaft. Ever notice a motorcycle engine has an alternator on
the end of the crankshaft, this alternator acts as another flywheel
minimizing the twisting of the crankshaft. If you are considering
building a diesel motorcycle make certain that the driven parts are
absolutely rigid with no play on the pto shaft. A loose attachment to
the pto shaft will certainly cause a crankshaft failure. Better yet is
to have a solid and heavy drive member on the output shaft, this will
smooth out the power pulses and minimize the strain on the
crankshaft.Also be sure to use vibration isolators to keep main
bearings and frame breakage to zero."

I notice also that the new production V-twins have a re-designed pto end nose casting. Whereas the original was simply bolted to the crankcase by the flange, the newer castings utilize carry-through bolts that would introduce greater rigidity to better support the crank. The question remains, how much research and testing was done?

Original casting
http://imgs.inkfrog.com/pix/alzubaidy/VT17.JPG

Newer production casting
http://www.vtwinengine.com/uploads/alli ... 2939_1.JPG

[sno-bro]

Interesting write up! I have a flywheel at both ends, hope it helps.

Thinking (and worrying ) yesterday while working on my bike made me more and more suspicious about the installation and usage on the person who had the three failures: I mean even if the crank quality is crap what are the odds that one person would get three of them? After all a lot of the Punsuns ARE working fine. It would sound a whole lot more probable that there is something in the way the engine is used and or installed in his application.

The new casting is different and looks a whole lot more rigid. Interestingly also the new casting is different in the bottom: The mounting bolt holes are up-down direction while the old engine had them horizontally and there is a separate L-piece for up down. In the new version the L-piece is cast in the block already. Let's hope my current engine works as I couldn't use the new engine block, I use the horizontal mounting bolts

sno-bro

[zarquon]

Punsun Harley von bigzark auf Flickr

This one is broken. Bike is still on Diesel, using a Lombardini three-in-line.


Punsun Kurbelwelle von bigzark auf Flickr

A broken crank itself


Big Knock 2011 - 35 von bigzark auf Flickr

Whiskers Punsun is also broken


09-dieselmotorradtreffen_hamm_2009-beckedorf_v2 von bigzark auf Flickr

But this is still running, as far as I know

[zarquon]

Punsun Sportster von bigzark auf Flickr

Not sure what happened to this punsun build?



Punsun Raptor von bigzark auf Flickr

This one is not in regular use. It runs once or twice every year.

[zarquon]

But keep in mind

[Henk]

It seems to me as a fatigue failure. It started at the drilled hole. It doesn't need a heavy force, only an force with an alternating direction wil do the job.
Just a tiny deficiency wil start the proces.

[old clunker]

[quote="zarquon"]
Punsun Sportster von bigzark auf Flickr

Not sure what happened to this punsun build?


This is what happened with the Punsun Sporster, a post from the past.....
The motorbike was also put up for sale later on, for spares or repair.

Punsun Kaput!!!

Post by Fiddler » Mon Sep 15, 2008 7:13 pm
Guys,

Loads of talk of the reliability or otherwise of the Punsun engine. Seems only fair to share my experiences with others thinking of investing in this engine.

Just got back from the Hamm Rally courtesy of a recovery lorry. One minor problem on the way over when an injection pipe fractured at the olive. No great shakes though and back on the road in an hour or so after some very helpful dutch guys in a truck garage found a similar line from a refrigeration unit that was bent to fit.

Trundling back home though, enjoying the ride and the sun, when I became aware of an unfamiliar noise. Lived with it for a short while as the bike was still running well, before deciding to pull in to check it out.
It was immediatly obvious all was not well by the amount of engine oil over the crankcases behind the rear pot. A quick wipe around revealed a crack in the crank cases roughly in the middle above the engine mounts spreading across the case to the side cover on the crank side, then carrying on the other side of the gasket up to the fuel pump.
Incredibly I managed to limp the last 60km back to the Ferry, certainly spreading the crack further and covering rear tyre and brakes in oil in the process.

One possible theory is the engine mounts offered no flex, and the engine gave way. I was fully prepared for all manor of issues with a budget engine but certainly did not expect a catastrophic failure.
Add this to the crank issues and my advice would be to hold off buying one of these until Chinese manufacturing quality has caught up to modern standards.

This is directly from Fiddler's experience several years ago, in the "Punsun Kaput" post, and there have been other Punsun failures reported.
They maybe more reliably manufactured now. You pay your money and take your chances!

[Stuart]

I dont think Whiskers bike ever broke a crankshaft. He has ongoing seal and small leakage problems but as far as I know it's still running.

Saw similar engine in Hamm had suffered this and owners said they had altered flywheel.

[thundercougarfalconbird]

these engines are very poorly timed, the 2 pumps can be altered individually, fighting forces from in correct timing could also cause failure

[DieselFly]

I second the timing being critical. I had to change my engine do to a rod bearing failure that was probably my own fault. It can be repaired the crank is okay. But the engine I installed vibrated so badly it broke my intake into three pieces. I check the timing which was set by the factory and found one cylinder at 27 degrees and the other at 20 where it belongs. I had to use the shims from the other engine to time it now it is running smoother than ever. My engines say Kingpoo on the outside but have Punsun cast on the inside. The long and the short of it is time your engine before you ride. It seems if it starts and runs they ship them

[Sphere]

Still can't believe some Chinese Marketing Master Mind decided to brand engines Kingpoo.

[Rhynri]

For all we know sphere, that means AWESOME RUNNING THING in Mandarin.

[espe]

Well, maybe he drunk some diesel en it just happened to be the first word that came to mind (or the first thing he needed real badly)