Airhead trans bearing

Discussion in 'Airheads' started by Wirespokes, Dec 19, 2011.

  1. Wirespokes

    Wirespokes Beemerholics Anonymous

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    A while back one of our Aussie members who knows bearings recommended a unique bearing for the front of the output shaft. I'm now looking for that data and would love hearing from him to recover that data.
    #1
  2. One Less Harley

    One Less Harley OH.THAT'S GONNA HURT

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    is this what your referring to?
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  3. Wirespokes

    Wirespokes Beemerholics Anonymous

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    This is the post I was looking for - from a thread called: A Tale Of Two Transmissions - Airhead Style, Post #53 03-19-2009, 10:32 PM by Max Headroom

    FWIW, I've gone out of my way to avoid having to resort to paying through the nose for the special bearing at the front of the input shaft. Apart from the cost of the thing, it also requires the use of the unique-to-BMW front oil seal because of the 26.5mm diameter of the extended inner bearing race.

    To get around the special extended inner race, I buy a conventional NJ204 C3 bearing and a separate 20x25mm precision ground and hardened inner race commonly used with needle roller bearings. I trim the length of the 20x25mm inner race so that its length and the NJ204 inner race combined match the length of the original inner race. (The NJ204 has a lip on the inner race, just like the original).

    The result is much cheaper than a genuine bearing, functionally identical, and allows me to use a Viton 25x40mm oil seal which is readily available.

    I also substitute the 6304 C3 bearing at the front of the cluster (or intermediate shaft) with an NJ304 C3 bearing. It is dimensionally identical, allows for easier assembly of the shafts and it has double the dynamic load capacity of a 6304 because it's a cylindrical roller design instead of a ball race. And because it has a lip on the inner race like the front input bearing, it can take axial load.

    The 6403 C3 at the front of the output shaft is still the big challenge, since that's the bearing that usually fails first. For the 'box that's currently in the R90S I was fortunate to find a NOS 6403 with a brass cage. Unfortunately it was the only one available, and I haven't been able to find any more yet.
    __________________
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    #3
  4. Rob Farmer

    Rob Farmer Long timer

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    I'm sure Max is going to love that :lol3
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  5. One Less Harley

    One Less Harley OH.THAT'S GONNA HURT

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    New Zealand/Aussie close enough....:lol3
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  6. Zoef zoef

    Zoef zoef Been here awhile

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    In general: The cylindrical roller design can indeed take some axial load, but usually quite a bit lower than the ball race design. If there are straight cut gears in the transmission, this is not a problem. But, if there are gears with teeth at an angle, the roller bearing may wear out rather quick.
    I would like to know what his experiences are.
    #6
  7. disston

    disston ShadeTreeExpert

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    Also very interested in this idea and have heard the same thing concerning roller and ball bearings. But if this trans was to be perfect, and each of the three shafts has an angle cut gear on it, then you would do all the bearings with tapered roller bearings. Like the wheel bearings and steering stem. I think balls can take more thrust that rollers but tapered roller bearings can take the most. That is with out an extra dedicated thrush bearing.

    I think we used to just say Timken when we wanted them. Sort of like saying Jello and getting generic gelatin. But Timken did invent the tapered roller in the 1890's I think.

    Charlie
    #7
  8. Wirespokes

    Wirespokes Beemerholics Anonymous

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    Aussie, NZ - Same thing, Right? Just two big islands out there in the Pacific.

    :hide
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  9. Box'a'bits

    Box'a'bits In need of repair

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    Much the same as the US (which I am sure consists of Canada, Amercia & Mexico), and, of course Cuba....:D

    All functionally one & the same - someplace over there - And not here.
    #9
  10. Wirespokes

    Wirespokes Beemerholics Anonymous

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    Yeah, that's about it. What I don't understand is why you guys get long-distance touring bikes when you live on a small island.
    #10
  11. Box'a'bits

    Box'a'bits In need of repair

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    'Cause we all dream of taking them on RTW trips....

    Actually a good portion of the population leaves NZ in their 20's & tours around for a couple of years.
    #11
  12. One Less Harley

    One Less Harley OH.THAT'S GONNA HURT

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    CUBA........HA HA HA , communist CUBA...:rofl:rofl:roflIsn't it about the size of New Zealand?

    Autralia is to NZ and the US is to Cuba...at least by size.
    #12
  13. Box'a'bits

    Box'a'bits In need of repair

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    Well Cubans comprise 1/2 the working population of Florida don't they?
    I'm sure if you asked the Australians, they'd classify us as closer to Mexicans - poor immigrant workers...:rofl
    #13
  14. Wirespokes

    Wirespokes Beemerholics Anonymous

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    Ok, now I'm getting the picture! But in that case, why would Max be upset if I called him an Aussie?

    Frankly, I've heard NZ is a paradise and the people are wonderful. I knew a guy who couldn't get enough and every year took a bike holiday there. Some day I'll have to visit.
    #14
  15. Padmei

    Padmei enamoured

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    Ask a canadian why they don't like being called American & you'll see it in their eyes:D

    As for wondering why we have big bikes - It's something a lot of people in the bike industry here have been wondering for years. 600s & 800s are perfect for our roads but for some reason we like our litre bikes.
    #15
  16. disston

    disston ShadeTreeExpert

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    So Wirespokes, How did the NJ304 C3 work out for the lay shaft on this trans? You did use this mod? Any miles yet on the bike? Any word?

    You also mentioned the 6403C3 on the front of the output shaft. I bought the dealer part yesterday and they gave me a plain Jane 6403, no C3. I've been thinking that this may be correct. Everybody wants to use the C3 bearing because it is rated to a higher rpm but this bearing does not see a higher rpm. Therefore the C3 is not needed and the plain Jane bearing may be better?

    I'm rebuilding my second or third (depends on how you count) transmission. The 6403 on the front of the output shaft is always worn. The second bearing to go seems to be the front of the layshaft, 6304.
    #16
  17. chollo9

    chollo9 Screwed the Pooch

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    I thought the difference between NZ and AUS was in the sheep to human ratio?
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  18. supershaft

    supershaft because I can

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    This discussion needs to get back on why what Wirespokes is reporting is not right. Granted, it's not very popular with WS and a few other inmates that are always spreading bad advise but it's a big help to everybody else.
    #18
  19. Kai Ju

    Kai Ju Long timer

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    I've been told that the C3 classification allows for more fore and aft shaft movement (radial clearance, C3 is an internal clearance, or the amount of internal free space between the rolling elements and the raceways of a bearing ) because is has a slightly looser tolerance. So when you have a steel shaft sitting in an aluminum case that will grow at a greater rate that makes sense.
    The inner race is pressed onto the shaft while the outer race is a interference fit in the case.
    So as the case grows longer then the shaft as it heats up, the play in the Class 3 bearing prevents the balls from being pulled against one side of the inner race and the opposite side of the outer race.
    #19
  20. Max Headroom

    Max Headroom lost in the '70s

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    There's nothing mysterious about rolling element bearings and their internal clearances. Bearings with C3 clearance are often specified where there is an operating environment involving heat or vibration, or where greater than usual interference fits can cause the internal clearances to close up during assembly.

    They are usually specified by the OEM bearing supplier either at the design stage or as a result of prototype testing. In automotive gearboxes, it's not unusual to encounter C4 or even C5 clearance bearings.
    #20