Built-up cranks versus one-piece

What is the motive for using antifriction bearings on four-stroke
motorcycle engines? On a two-stroke, with limited lubrication, I
understand....ball and roller bearings need less oil. But they also
need what appear to be expensive built-up crankshafts.

With two fairly modern Suzuki four-strokes (drz400s and sv650s) it's
not at all obvious to me why the crank construction is so different.

Thanks for reading, please post any replies: Email responses will get
lost in the spam.

bob prohaska

 

I thought the SV had a plain bearing crank?

If I remember correctly, the big Kawasaki KZ900/1000, Suzuki GS1000, and
SOHC Honda 750s had roller cranks. Evidently something wasn't up to the
task, probably better oil and better manufacturing tolerances have made
plain bearings the way to go.

 

It does, and thus my puzzle: Two reasonably modern machines,
both with single-pin crankshafts. The dirtbike uses ball
bearing mains and rollers at the crankpin. The streetbike
uses journal bearings at both main and crankpin.

Curiously, both machines use journals at the wristpin, which,
because of its limited motion, is the one bearing that would
seem to benefit most obviously from an antifriction design.

Perhaps Larry L can offer some insight...

bob

 

Well, a dirtbike could obviously benefit from a roller crank
since it has a high likelihood of being run at some extreme
angles, including perhaps tipping over on its side occasionally.
When the oil pickup tube is sucking air, ball and roller
bearings can tolerate loss of oil pressure better than plain
bearings.

I'm not at all sure that is the case, but I'm not an expert on
bearings by any stretch of the imagination.

 

The big Kawasaki did, as did all the Suzuki GS two-valve fours. The
Honda was plain bearing, although Honda made plenty of bikes that had
roller bearing cranks.

Old BMW flat twins, up until the R69, had roller bearing cranks too.

Not really. Built-up cranks are much more expensive to manufacture. It's
as simple as that. By their nature, they are much more tolerant of
lubrication shortage/failure (they're still used in all two-strokes,
obviously). Their only flaw is a slight tendency to twist if they're
really, really abused. It is common practice in racing to weld them up
to prevent this.

Plain bearings are cheaper, and with modern lubes they last just as
long. And they're also quieter, which is a major consideration these
days.

 

<snip>

Was it anything to do with the fact that until the production of these
bikes, the manufacturers had made mainly 2 stroke engines so used their
knowledge of existing technology when they moved to 4 strokes? I remember
such reasons bandied about in the early '80's when I had a Suzuki GS550.

Steve H

 

Improvements in manufacturing technology is more likely.

Plain bearing cranks prior to the 1980 GS750 used a chain
and jackshaft to drive the clutch. Suzuki demonstrated that
technology had improved to the point that a lighter gear-driven
primary drive could be used with a plain bearing crank.

 

Not necessarily - cf BMW, who AFAIK never built a two-stroke in their
lives.

And Honda, like I said, used built-up as well as plain bearing cranks,
although the built-ups were used on smaller stuff - singles and twins.

Remember that the Suzuki GS series was a very careful copy of Kawasaki
engine technology. It worked for Kawasaki, so Suzuki reckoned it would
work for them.

Odd historical footnote - after the US 8th Air Force bombed the German
ball-bearing factories in Schweinfurt in 1943, they cut off the supply
of ball races to German industry and the war machine. All that was left,
until production re-started some weeks later, was what was in the
distribution chain.

The Germans improvised and in many cases went over to plain bearings
until output returned to normal.

 

Plain bearings, dude.

Holdover from the 2S days.

 

That certainly makes sense....maybe it's the whole story.
Mark's observation that dirtbikes spend more time tipped
over seems to have some merit as well.

My drz has both dry sump lubrication and a roller big end
bearing...belt and suspenders design? Btw, it's noisy too.

Thanks to all!

bob

 

There *was* the Flink.

 

<snip>

Just out of interest, may I put another twist to the question?
Is there or has there been any thumpers, street or dirt
(preferably not failures or real odd ball ones) with plain
crankshaft bearings?

And apart from the fall over in the dirt explanation, has
anybody got a technical reason?

 

You've got me thinking now. I can't think of any myself, and no, I've
got no alternative explanation either!