Jump to content
Sign in to follow this  

Why do 4strokes use roller bearings?

Recommended Posts

Seeing as 4ts have the same oil pumping abilities as any other automotive type engine, I don't understand why they are still designing them like a 2t with roller crank/rod bearings. Why not plain shell bearings which are lighter and simpler to maintain. Using rod caps, main saddles with caps and split bearing shells with pressurized oiling would make so much more sense I think. In my mind it appears that although the 4ts are an advanced design, the manufacturers are somehow stuck on using 2 stroke mentality in their design.

Share this post


Link to post
Share on other sites

Not 2T mentality, there are pros and cons between the two bearing types based on performance issues and manufacturing costs.

Ball bearings are referred to as anti-friction, and shell bearings are journal bearings. While shell bearings may be easy to change that is only one of many considerations for an application. Your list of component parts for shell bearings compared to ball bearings illustrates one of the manufacturing cost issues.

Share this post


Link to post
Share on other sites

Roller bearings can tolerate poor lubrication much better than a journal bearing. Momentary lapses in oil floe due to a crash, whip, etc would let plain bearings have physical sliding contact, which will cause damage eventually. Roller bearings can also survive happily with just splash lubrcation, simplifying oil passages necessary, lowering pump pressure and flow requirements, all reducing cost to manufacture.

Share this post


Link to post
Share on other sites

I understand your points but since 4ts already use dry or semi dry sumps and scavenge pumps I don't really see problems with lubrication under adverse conditions.

The manufacturers seem to have gone with regular type 4cycle journal oiling for the cam journals and even the wrist pin has eliminated the needle cage. I still don't see why they didn't continue with the crank, rod and trans? It almost seems as if they just added a 4 stroke head to a 2 stroke engine and then threw on an oiling system for it.

Share this post


Link to post
Share on other sites

The new KTMs have plain shell bearings. The part about surviving longer under poor lubrication is accurate. That said, once a motor is pressured up and the film is there, it won't go metal-to-metal if the pump cavitates for a couple of seconds.

Share this post


Link to post
Share on other sites

Now that many of the new 4T bikes with separate engine and gearbox oil, how much of an issue is dirt and debris? I don't at all mean that in a smart-ass way, I'm genuinely curious. Are there unfiltered breathers or other means for introducing dirt?

Share this post


Link to post
Share on other sites

Name 2 that have separate engine and trans oil volumes other than a CRF. I'm not aware of them.

As to unfiltered breathers, definitely. Not that big a deal under way, because the time to pull air in on a given upstroke is too short to pull the engine's whole displacement through the tube, and there is a net outflow, anyway.

Share this post


Link to post
Share on other sites

Oh I see

I thought more had gone that way by now.

I know that the shared oil makes it fairly likely to have filings or at least clutch material contaminating the oil but the original V-Max had plain bearings, in a 100hp/litre V4 with shared oil. The transmissions and clutches on them were fairly expendable but the motors seemed to hold up.

If there's not a lot of dirt ingested through the breather, maybe it's the hammering of the big single that makes them want rollerized bottom end bearings?

Share this post


Link to post
Share on other sites

There shouldn't be dirt in the engine either way.Bearings aren't designed to run in dirt and besides, plain bearings are designed to embed debris.

Plenty of street bikes with shared oil and plain bearings so I'm not buying in on that theory.

Share this post


Link to post
Share on other sites

rpm's play into the factor of plain bearings as well...also eng cases swell up in high heat conditions seen in relativly small motorcycle engines...ktm was one of the only factories to use two part roller bearings in thier 2 stroke engines...as the eng heats up the roller brg gets tighter and causes more friction...the roller brg also requires less supporting material when designing engine cases and also lowers the cost of line boring the crank journals during manufacturing...cant buy a cam cap for your cylinder head because they are a matched set...roller brg's also are more forgiving like others have posted and have less friction...the needle brgs used on crank are cheaper to use and make than a plain brg type crank and lower cost to make .

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×
×
  • Create New...