Explain to me crank bearings/cranks

[Larry D]

From what I've read, my '81 GS750L has plain or shelled crank bearings. Plz stop me if I'm wrong. I'm not a engine guy by any stretch. I also understand that these bearings may or may not be as dependable as roller types. So....will someone explain what the crank is and how it relates to pistons,valves and such.
I'm not wanting to argue the virtues or downsides of the different types. I also understand this is a vague kind of question. Just want to get a general understanding of what this component does.
Thought it was a rather slow night in the forums, so I thought I'd liven thing up a bit!

 

[TheCafeKid]

From what I've read, my '81 GS750L has plain or shelled crank bearings. Plz stop me if I'm wrong. I'm not a engine guy by any stretch. I also understand that these bearings may or may not be as dependable as roller types. So....will someone explain what the crank is and how it relates to pistons,valves and such.
I'm not wanting to argue the virtues or downsides of the different types. I also understand this is a vague kind of question. Just want to get a general understanding of what this component does.
Thought it was a rather slow night in the forums, so I thought I'd liven thing up a bit!

Your crank is what is driven by the explosion in the cylinder. The pistons are connected to it by connecting rods, and they are offset so that its always being driven by one combustion stroke. The resulting explosion in the cylinder bore is what drives the piston down, which turns the crank, which turns the tranny which turns the output shaft which turns the sprocket which drives the chain which drives the rear wheel. TO put it simplisticly...

 

[Larry D]

Your crank is what is driven by the explosion in the cylinder. The pistons are connected to it by connecting rods, and they are offset so that its always being driven by one combustion stroke. The resulting explosion in the cylinder bore is what drives the piston down, which turns the crank, which turns the tranny which turns the output shaft which turns the sprocket which drives the chain which drives the rear wheel. TO put it simplisticly...

Okay, understand that. So the crank bearings are in the connecting rods, opposite the pistons and attach around the crank?
I'm really starting to like this stuff!!

 

[Baatfam]

Okay, understand that. So the crank bearings are in the connecting rods, opposite the pistons and attach around the crank?
I'm really starting to like this stuff!!

No...those are the connecting rod bearings....the crank bearings support the crank in the engine case...
To put it simply....

 

[salty_monk]

and roller bearing ones are better than shells purely because they will last longer as there is less wear (shell bearing is essentially like a fancy bushing).

Downside is roller bearings are bigger & heavier & likely more expensive...

Dan

 

[Nessism]

The crank runs on bearings in the cases, and there are bearings between the rods and crank as well. Plane bearings don’t actually touch the crank except for when there is no oil pressure – the crank runs on a film of oil.

To put things into perspective, about 99% of all internal combustion engines use plane bearings since they are both robust and inexpensive. It’s the GS roller cranks that are the oddball.

 

[Larry D]

No...those are the connecting rod bearings....the crank bearings support the crank in the engine case...
To put it simply....

Great, I understand that! I'm guessing that there are 4 connecting rod bearings, 1 for each connecting rod/piston. How many crank bearings are there in my '81 GS750L? And there is only 1 crank vs. 2 cams, which I'm sure is a completely different animal...right?
I know....alot of you guys are laughing, but it's learning to me.

 

[salty_monk]

Cams control the intake & exhaust Air/gas flow. Yes the GS has two but you can also get a SOHC engine which would have one but would have twice as many lobes on it as it's working the same number of valves. The 16v variant also has two cams working twice as many valves on each cam.

Dan

 

[Larry D]

Cams control the intake & exhaust Air/gas flow. Yes the GS has two but you can also get a SOHC engine which would have one but would have twice as many lobes on it as it's working the same number of valves. The 16v variant also has two cams working twice as many valves on each cam.

Kinda lost me, but that's okay.
The reason for my inquiries is that my bike had just under 12,000 miles. I as thinking about the wear on these bearings already. I know that's not even beginning to touch the miles these engines can acheive. Just got me thinking, is all.
I search and read on this site much more than post questions. I've read alot of posts that beat up the 16v 750 for oil delivery and crank bearing problems. Although being different size displacement engines, don't other models have the same design?:-k

 

[TheCafeKid]

Cams control the intake & exhaust Air/gas flow. Yes the GS has two but you can also get a SOHC engine which would have one but would have twice as many lobes on it as it's working the same number of valves. The 16v variant also has two cams working twice as many valves on each cam.

Kinda lost me, but that's okay.
The reason for my inquiries is that my bike had just under 12,000 miles. I as thinking about the wear on these bearings already. I know that's not even beginning to touch the miles these engines can acheive. Just got me thinking, is all.
I search and read on this site much more than post questions. I've read alot of posts that beat up the 16v 750 for oil delivery and crank bearing problems. Although being different size displacement engines, don't other models have the same design?:-k

All of the 8v GS motors (your 750 is a 16v variant as you know, but they made an 8v from 77-79) EXCEPT to my knowlege the 650, used roller bearing cranks. The 16v 550s and 750s used, to the best of my knowlege, plain bearing cranks. The 1100 16 valve AND the 1100 8v also used a roller bearing crank. I am not sure about the smaller than 550 GS motors, as ive never really done alot of reading about them.

 

[76FJ55]

most other internal combustion engines use plain bearing cranks including Diesel engines which have significantly higher loads than gas. there is no problem with plain bearing cranks.

The most comon issue with the 16V engines is oil starvation at the cam journals usualy due to neglect of oil level or service frequency.

Using plain bearing crancks greatly simplifies the production of the crank and can develope a stronger crank assembly. With plain bearings it allows the crank to be made of a single forging or casting.

In order to assemble a crank with roller bearing mains and rods the crank has to be made of many parts that are pressed together to form the crank shaft assembly, this is neccessary to allow the installation of the roller bearing. in order to keep the roller bearing cranks from twisting at the pressed together joints as modifications are made to generate more torque each individual pressed joint needs to be tack welded to increase tortional tigidity. this increases the cost and complexity of manufacturing and servicing the roller bearing cranks and has driven the industry to move to plain bearing cranks.

 

[Larry D]

Thanks for the answers, everyone.You've answered my questions.
Larry D

 

[76FJ55]

here are some good pics of a welded roller bearing crank.
http://mywebpage.netscape.com/oldkawman/okndcranks.html

 

[Guest]

If you google "alphasports",you'll find exploded parts diagrams of the whole bike.They may make it easier to understand how it all goes together and how it works.It'll give you part numbers,availability and price.I ran off a complete set for my bike and refer to them often.Cheers,Simon.

 

[Nessism]

Yes Ed. But the roller bottom ends are using the two stroke technology and I have never seen one wear out no matter what the mileage. The 99% use shell bearings because of cheapness to manufacture and repair. The typical roller bottom end in the Suzuki 4 cylinders has 10 parts to it that have to be pressed together on assembly and then balanced and that does not include the conrods, bearings, shims and seals that also make up parts of the bottom end. The shell bearing crank is one piece. So it can be seen that for ease of manufacture (Read CHEAP) they have moved away from roller bearing (superior) bottom ends.

Anyone that has worked on a roller crank and seen the construction and how the oil is injected inside of each bearing and directly onto the rollers will have an appreciation of why these bottom ends do not wear. Also less friction as rollers have less surface area compared with shells. As someone said earler there is no direct contact and there is always a film of oil in there anyway. But the large drag area of the shell bearing even if covered in oil is excessive compared to the rollers in a roller bearing.

My views only, but then I was brought up on two strokes.

Hey Don,

I like the GS roller cranks and think it is one of unique features of the engines. I don't think it is a superior feature though from a technical standpoint.

Regarding friction, where are you getting your data that says a roller crank has less friction? It's been a long time since engineering school but I don't recall that being the case.

Don't mean to sound disagreeable. As I said, I like the roller crank. Once you become accustomed to the sound of a GS engine, you can actually hear the rollers on the crank.

 

[Larry D]

The most comon issue with the 16V engines is oil starvation at the cam journals usualy due to neglect of oil level or service frequency.

So as long as we're still going....Is there something about the oil pump on my model ('81GS750L) that makes it inferior to other models? Don't recall exactly, but seems like I read that somewhere.

 

[Suzuki_Don]

Hey Don,

I like the GS roller cranks and think it is one of unique features of the engines. I don't think it is a superior feature though from a technical standpoint.

Regarding friction, where are you getting your data that says a roller crank has less friction? It's been a long time since engineering school but I don't recall that being the case.

Don't mean to sound disagreeable. As I said, I like the roller crank. Once you become accustomed to the sound of a GS engine, you can actually hear the rollers on the crank.

But Ed they were still using horse and cart when you were at school weren't they. Seriously though they are just my views as I said. I deleted the post because I thought it was probably a bit controversial. My thinking be it right or wrong was that comparing the surface area of the shell bearing against a roller bearing which only has a contact patch of a number of very thin lines where the rollers contact the bearing surface results in less friction. As I said I am probably wrong.

Anyway in the end we are all individuals and we each have our favorite engines and bikes for that matter with their different design features. If nothing else a discussion like this enables us to formulate our thinking on how our motors work and if we (I) get it wrong then there are many people more expert than me than can gently correct me and lead me back on to the correct GS path.

ROLLER CRANKS RULE

 

[salty_monk]

There's a reason these GS engines go for so many miles... I don't see a plain bearing crank going for that many miles but maybe I'm wrong

 

[Ed Vrolijk]

There's a reason these GS engines go for so many miles... I don't see a plain bearing crank going for that many miles but maybe I'm wrong

Yes, you are. Over here in Europe a motorcycle magazine once tested a BMW K1000, and pulled it apart after 250.000 km, (About 150.000 miles) and were surprised to find that al the crank bearings were within spec. It was a long term test bike, driven year round, and was serviced as BMW advised.
Not bad now, is it?
Ed

 

[journerr]

This was a great learning thread for me too - thanks everyone !