Looks like Seismic Skate has a steel bearing version with built in spacers that is much more reasonably priced, $40 for a set of 8 is not terrible.
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The bearing spacing in urethane wheels is a standard at 0.400", or 10.16mm nominal. 10mm spacers will be sliightly too short. That 0.16 makes a difference, itās the difference between preloading the bearings and not. I made a batch of 10.2mm spacers if needed.
6900-2RS bearings will inherently not last as long as 608-2RS because theyāre smaller bearings and have a smaller load rating. Only way to mitigate this is to source quality brands and to really avoid preloading them when you tighten your axle nut.
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https://www.amazon.com/gp/aw/d/B086VYC3GK?psc=1&ref=ppx_pop_mob_b_asin_title
These are what I have and they are the hardest steel that is corrosion resistant and sealed, my machinists buddy picked them out for me because I donāt know metallurgy.
He warned me against the ceramic ones as the ceramic are not intended to be longer lasting or smoother rolling or even corrosion resistant as the races are often steel that isnāt corrosion resistantā¦ but are meant to prevent electrical damage to the race in specific special cases where transient currents and high static can arc across all metal bearings and are only sold outside of this as a sort of idiot tax. Not my words but it made sense ymmv.
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Some bearings just donāt like to be cracked on either. Just make sure there isnāt any movement on the axel and it should be okay as long as you have full contact between everything.
I forgot about the seismic options. They have been around for some time but I have never once actually seen the 10mm built-ins in stock anywhere. The website does have a spring 2023 availability date tho. I donāt trust ceramics as I think they are over priced and not worth the safety risk but the steel version cost $40 still. A bit steep for my blood, especially since Iād have to clean and grease out the box for better performance.
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Another point against ceramics is the failure mode is way worse than steel. Ceramics can explode and bind where as steel just gets crunchy.
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Take this advice, i had a bunch of 10.16mm spacers cut for me because i kept killing bearings and the problem evaporated, i had gotten some 10mm spacers from Amazon (bike axle parts i think) and the little bit of side load from being too short was making them blow out quick. Good luck
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if you see any water then removing inner seal and packing grease into the core is good
Boardnamics, doh, this will make a huge difference. I couldnāt find a good source of what the spacer should be and wrongly assumed a spacer sold for skateboards by a skateboard website would be correct. Do you have any spacers you could sell?
If not I guess I could use a 1.5mm and 0.1mm shim off McMaster to add to my incorrect 10mm long shim.
Hoyt sells some 10.5mm long spacers which Iāve found work well in the Hollow Wheels:
@jack.luis told me they chose the slightly oversized 10.5mm specifically so you could crank down on the axle nut without compressing the bearings.
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If the spacer is too large the bearing can slide in and out of the wheel on both sides as the bearing is not up against the stops in the wheel (one side will go in while the other pushes out). This could over time bore out the wheel and put weird loading on the bearing. I donāt know if 10.5mm is too big though?
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I believe the bearing to bearing distance isnāt very precise on the Hollow Wheels due to the two piece core. When Iāve used them on 8mm axles, 0.400ā spacers did not allow me to crank down the axle nut without affecting freespin.
I think 10.5mm will be fine for these wheels in particular, and Iām sure someone from Hoyt can chime in about the effects on other wheels. You make a good point that if the spacer is too large, it also puts stress on the bearings and the bores.
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Okay digging into this some more. Another issue I found is the tolerance stackup becomes a real problem on a two part wheel (Momentum Hollow Wheels). Here are my two front wheels, they have a ~0.2mm difference which is not much but enough that I probably need to build a custom shim set for each wheel. To make things worse the bearing seat on the wheel is tapered so the gap is larger than what is measured. I will need to put a 22mm OD washer in to get an accurate measurement then minus out the thickness of the washer.
Wheel 1
Wheel 2
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Hey, im the guy that ordered the BN M1ās for savage1 TKPās recently, can i have a set of those 10.2mm spacers added into the order lol? Those would definitely be helpful.
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Why donāt you guys just chuck a speed ring in there?
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Lol my axles always end up a mish mash of washers, aluminium/3dp spacers and speed rings.
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Seems like a simple fix to me 
I donāt work for fse anymore, couldnāt give a fuck 
3dp spacers take a couple of minutes to design/print and have sorted me out every time.
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can you pull one out that has been used for a while and show us how its holding up?