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Our 4yo is not yet fully comfortable on her next bike, so she's been riding her Chicco Red Bullet to school - until the bearing packed up on her back wheel today (has she been doing stunts or what?). I'm really impressed it's the bearing that gave and none of the plastic parts, because the bearing might actually be the easiest to repair. We might just try and turn this into an opportunity to encourage her to switch to the new bike for good, but let's get to the actual questions:

  1. Can I use this 7-ball bearing as replacement for the damaged 6-ball bearing? The caliper gauge is in a cupboard in the girls' bedroom, so I can't get to it before tomorrow night now, but I took some rough dims:

    • Outside diameter (=internal diameter of socket in grey plastic plate) 20 mm
    • Cone rolling surface diameter range 11 to 14 mm
    • Bearing width 5 to 6 mm - but I doubt this matters as there is plenty of room for adjustment.
    • nuts are 13 mm (must be m8 axle then) So the dims generally match and the difference seems to be only in the number of balls. Is there a reason why the bearing in the link would not be a suitable replacement?
  2. Is there an issue related to the bearing failure that's causing the wheel to be offset?

Please see the photos below, clearly it's not all in the best of conditions, but I guess I didn't appreciate even balance bikes need proper care.

The damaged bearing is on the rider's RHS (highlighted in the photo). I can't remember if it's always been the case that the wheel is off-centre and there is a lot of axle visible on rider's RHS side but none on rider's LHS. I doubt the cones could "travel" gradually (one tightens, the other loosens) in usage without a substantial change to how tight the bearing is. And I can't imagine the bearing failed because it was overtightened: if it was overtightened, I would expect the plastic "plate" to crack. I think the bearing failed due to rider weight (would it be called radial load?). NB I loosened the cap/end nuts before taking this photo but not any of the remaining nuts. wheel condition on inspection Overview of wheel construction Overview of wheel construction Overview of bearing construction: bearing overview Damaged bearing cage (by the way, it's interesting it's called a cage - my impression is that a much more appropriate name would be bearing crown or nest - these are the terms I googled first). I also find it strange how the six "wings" on the damaged side have different shapes and I wonder how much that has to do with the load (they all look even and equal on the good side). bearing crown nest ball holder good bearing

I know it's a lot of photos but I don't want to put a new bearing on, only to find that a stronger new bearing causes the failure of a part which is harder to replace!

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    The main thing is that you want the new balls to be the same diameter as the old ones. If you can't find a caged set that matches, I'd suggest you get 8-9 individual balls from a local bike shop (take the good bearing to match), and use them without cage -- set them in grease. (Of course, you should grease up the bearings, whether caged or uncaged.) – Daniel R Hicks May 21 at 0:12
  • (And yes, it is a "cage", at least in the US.) – Daniel R Hicks May 21 at 0:13
  • And I notice that everything is rusty. I suspect the bearing failed due to lack of lubrication, perhaps from using a pressure nozzle to wash the bike. – Daniel R Hicks May 21 at 0:16
  • Daniel, isn't your comment about balls pretty much an answer? Anyway, I have one reservation: if I use a bearing without a cage, would it not need to be screwed much tighter than the caged bearing, therefore increasing the load on the plastic plate? – pateksan May 21 at 0:17
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    No, with more balls you'd be at almost exactly the same adjustment point with the nuts. You should never screw things "tight" in a bearing, just adjust to the point where there's no play. And if the balls are the same size the adjustment will be the same. – Daniel R Hicks May 21 at 0:20
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Ok, answers are not exactly flooding in, but I can see why my OP might not be the most interesting. However, I will record briefly what I've done in case anyone does face this situation in the future.

  1. Research: I researched the subject of bearing replacements a bit. I failed to find a proper single thorough article dealing with all the considerations, but I got the impression caged bearings must be avoided, so I went for replacing with a loose ball bearing.

  2. Buying: I measured the balls with a caliper gauge and confirmed they were 5 mm. I bought 20 on eBay for £2, and a 20g tube of lithium grease for another £2. I'd love to support an LBS but every penny counts at the moment and I don't even have time to go into one.

  3. Work: I built a loose ball bearing, the process was very much like in the many YouTube videos. I used the surviving caged bearing first, and reassembled the wheel so that the axle stopped the balls falling in. The process was messy: the plastic plate disc doesn't have a trough-shaped race to hold the balls, so they bundled together around the axle instead of forming a circle around the perimeter of the recess. It didn't help I was distracted with my 2 and 4 yo helpers. For example, I did a dry assembly before using any grease to see how many balls I need. I failed to write the number down. With grease, I found the number that seemed right would not allow the cone to go all the way in , so I had to take one ball out, and the remaining gap seemed big. But everything seemed to fit together well, there is no play and the wheel seems to spin ok. I really don't know how tight the cones are supposed to be, I would say they are just a touch more than "finger tight" as I was conscious about snapping the main plastic disc, but it all seems to work, and I have been checking regularly since. Finally, I also greased the other wheel.

Conclusions for the two questions in my OP

  1. I wish I had ignored the superiority of loose ball bearings and gone for a caged bearing. Everything would have been quicker and the size I needed was available on eBay as a pair with a 5 g tub of lithium grease for £2, and it would probably have been fine for a balance bike. I wish I had kept track of which plate and cone went with the surviving caged bearing. And although it was reasonably clear which way the caged bearing needs to face, I did find it reassuring that the other wheel had it facing the way I expected: .

  2. The space between the screws and the frame in my OP was because the nut had worked itself loose, there are no visible parts of the axle after I put it all back together.enter image description hereenter image description hereenter image description here

  • Note that when the old bearings failed they probably forced the cone and lock nuts to unthread themselves, causing the offset axle situation you observed. With that bike the setup should be roughly symmetrical (within 2-3 mm). – Daniel R Hicks Jun 9 at 21:44

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