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-   -   question about bearing speed ratings (http://www.horizonsunlimited.com/hubb/bmw-tech/question-about-bearing-speed-ratings-35692)

desert dweller 5 Jun 2008 04:04

question about bearing speed ratings
 
g'day.

i just bought a spare final drive bearing for my r11gs. i was able to obtain the right size, but without the 'C3' rating for rpm.

as i understand it, high-speed rated bearings have a little more space between race and balls, i assume to allow for heating at speed. can anyone add to this?

the bearing i bought is NTN, should be good. though not C3, it is rated to 6300rpm, and my maths tells me that at 120km/h, my rear wheel is turning at about 1000rpm. according to the following maths:

c (circumference) = 2 x pi x r,
where r = .33m,
c = 2.1m

right, why should C3 speed rating be required at all when there is such a wide margin (6300 - 1000 = 5300) between real-world rotation speed and the rated tolerance?

i have not yet fitted the bearing. can anyone suggest why i would not? any input welcome and appreciated.

cheers,
andy.

Margus 5 Jun 2008 06:46

Not a bearing specialist myself, but didn't BMW changed the ball numbers inside the main FD bearing? First it was something like a 17 balls and later 19 (not sure if the numbers are correct, but the difference was 1-2 balls) because of better tolerance capability.

My mind says it's all a compromise between a high speed and huge load. For huge load more balls seem to be a better option, since there's more support area in the bearing to take that load, while it'll be limited for speed (more drag=heat and wear!). Less balls mean better speed tolerance, but probably also less load tolerance (less "support" area in between the bearing race and balls).

R11xxGS main bevel box bearing is C3 rated in stock - 'C3' means there should be increased radial clearance between the inner race and outer race, usually used for bearings that are going to run fast and hot.

So I can't see how speed can be a "problem" (R11xxGSes are rated to 200km/h speeds), I rather see the load (big bike, 2 up, lot of luggage) and potholes/harsh road conditions as a threat to the bearing longetivity.

Anyways, make sure you shim it correctly on puting it on (=correct preload) - wrong shimming will cause it broke down after just few hundred miles to few thousand, since it'll start to wear out one side of the race coz it's preloaded (by shims) wrong. If you preload it precisely right, then I've seen them lasting for 150,000+miles! So it can be kind of "lottery" between the precision of the work versus the real-life road conditions and your riding style. Some last 10K miles, others last even stunning (for the bike that has been riding offroad too, not just smooth tarmac like Goldwings etc do) 200+K miles I've heard in UKGSer forum.

Always, being a bit paranoid person muyself not to be involved into the "lottery" game - I carry a spare main bearing, main seal and few different sizes of shims with me on my travels. They're so called "GSers spare chain kit" :) - but they cost less than a good chain/sprocket kit, take much less room and weight less too. I bought mine from BMW since the same bearing from bearing shop was more expensive than from BMW.

Ride safe, Margus

henryuk 5 Jun 2008 10:24

Wheel bearings are bearings are bearings. Based on my (limited) experience the only bearings that need special construction are things like cam roller bearings, as these rotate at several times engine rpm.

Whenever I have to replace a bearing I just order the same bearing based on the number stamped on the side (- Simply Bearings Ltd), which is generic across all jap bearing manufacturers. The only non-generic bearing I have used was a ceramic bearing from a skateboard (used the wheel as a chain roller, works GREAT)

Bin Ridin 24 Jun 2008 18:17

What are C3 bearings?
 
What are C3 bearings?

C3 bearings are bearings that have an additional internal radial clearance to cope with high speed environments where excess heat is generated.
They are not suited to environments where critical alignment is required from the onset e.g. Where blades and cutters are set up to each other.
Applications like motorcycle wheels or bicycle wheels do not require C3 rated bearings.
Unless the bearing you are replacing specifically says C3 on it then you should not replace it with a C3 bearing, C3 Bearings are loose to the feel from new and often people complain at the quality of the bearing being poor and having more movement than the one they are replacing.
This is the characteristic of a C3 bearing.

Brian E 24 Jun 2008 19:28

Quote:

Originally Posted by henryuk (Post 192998)
only bearings that need special construction are things like cam roller bearings, as these rotate at several times engine rpm.

Cam's actually run at half the RPM speed of the engine.

Cactus Bob 25 Jun 2008 03:42

Just to weigh in on this "C3" issue ...
it is correct that C3 indicates a "larger than standard" internal clearance but this suggests a tolerance "range". There is more than 50% overlap in the internal clearance "probability" between a standard and a C3 ball bearing.
Heat expansion of the shaft (especially on an electric motor) would tend to suggest a C3 bearing. At the speeds you're talking about, I wouldn't worry about C3 or not ... but C3 is quickly becoming the default here in North America.

oldbmw 25 Jun 2008 22:47

This is interesting because Later Triumphs (post 1981) went from a C3 main bearing to a C2. But I do not know what the constructional difference is in actuality.. Are the C2 bearings capable of carry a heavier load ?

Cactus Bob 26 Jun 2008 01:57

Ironically, C2 has an even "tighter" than "standard" (or C0) internal clearance.
Two steps tighter than C3.
Definitely won't provide a higher load capacity but will give better radial and axial running accuracy.
The entire bearing will tend to heat and cool at the same rate so a tight internal clearance running in an oil bath is a pretty stable application.

oldbmw 26 Jun 2008 23:09

It is the crankshaft main bearing on the 750 twins. prior to that and on the 650's they ran C3's the 650's revved much higher.


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