20mm thru-axle hub bearings are too weak...

[Overclocker]

so my bearings went bad again after just a few months! these are 6805-2rs. 25 x 37 x 7mm. the bearing balls are just too small for heavy-duty ebike applications

so now i'm thinking of upgrading these hubs to bigger bearings. 6904 (20 x 37 x 9mm). same 37mm O.D. but 2mm wider so would stick out 2mm on both sides. 20mm I.D. so bearing would sit directly on the thru-axle (some hubs are actually designed this way)

any thoughts?

 

[MadRhino]

Buy a pro DH hub, even a used one. My favorites are Sun Ringlé Bubba or Abbah, DT Swiss 440 or DTFR, and the likes...

 

[Chalo]

The bearings are plenty strong; you're just not giving them the conditions they need.

It's important that radial groove cartridge bearings like these do not have an axial preload on them. Ideally you will know they don't because they have a miniscule amount of free side play. If you don't squeeze them laterally when installing, they can last for decades. If you do squeeze them, they can crap out in no time.

Also, the least amount of water or dirt intrusion will kill these bearings very quickly.

https://www.mcmaster.com/5972K281/

Static load rating 660 pounds each! You are not putting >1300 pounds on your front wheel.

6904 bearings only have about 25% higher load rating, which won't make the difference if you're squeezing your bearings on install. The hub assembly will be 4mm wider, which may cause your fork to bind or prematurely wear the bushings.

 

[Overclocker]

The bearings are plenty strong; you're just not giving them the conditions they need.

It's important that radial groove cartridge bearings like these do not have an axial preload on them. Ideally you will know they don't because they have a miniscule amount of free side play. If you don't squeeze them laterally when installing, they can last for decades. If you do squeeze them, they can crap out in no time.

Also, the least amount of water or dirt intrusion will kill these bearings very quickly.

https://www.mcmaster.com/5972K281/

Static load rating 660 pounds each! You are not putting >1300 pounds on your front wheel.

6904 bearings only have about 25% higher load rating, which won't make the difference if you're squeezing your bearings on install. The hub assembly will be 4mm wider, which may cause your fork to bind or prematurely wear the bushings.

just for some context my rear hub uses 6201 bearings (690lbs static load rating but uses bigger bearing balls). never failed despite getting dunked in water a lot more often due to low axle height (it's a smallish 17" moto wheel). and i'm sure it's getting a lot of axial preload due to the whole stack getting compressed by the 12mm bolt axle which has to be torqued really tight to prevent it from sliding forward/back on the swingarm

6805 bearing, 660lbs static load
6904 bearing, 830lbs

my front hub bearings were installed properly. it's easy to feel for any binding

i think i'll go ahead w/ the upgrade to 6904. seems like bigger balls do make a difference :lol: will just have to shave 2mm from each endcap

 

[Overclocker]

Buy a pro DH hub, even a used one. My favorites are Sun Ringlé Bubba or Abbah, DT Swiss 440 or DTFR, and the likes...

abbah uses 6805 bearings as well

 

[serious_sam]

so my bearings went bad ... any thoughts?

What do you mean "went bad" ? Did you disassemble to inspect ? What is the condition of the grease? Got photos ?

If you have corrosion, try stainless bearings, and more regular cleaning.

abbah uses 6805 bearings as well

6805 is common in HD/quality hubs. Even in FR/DH usage, generally no major problems.

 

[Overclocker]

so my bearings went bad ... any thoughts?

What do you mean "went bad" ? Did you disassemble to inspect ? What is the condition of the grease? Got photos ?

If you have corrosion, try stainless bearings, and more regular cleaning.

abbah uses 6805 bearings as well

6805 is common in HD/quality hubs. Even in FR/DH usage, generally no major problems.

bearing seized and started spinning around the 20mm axle, turning the whole setup into a sleeve bearing :lol:

 

[Chalo]

just for some context my rear hub uses 6201 bearings (690lbs static load rating but uses bigger bearing balls). never failed despite getting dunked in water a lot more often due to low axle height (it's a smallish 17" moto wheel). and i'm sure it's getting a lot of axial preload due to the whole stack getting compressed by the 12mm bolt axle which has to be torqued really tight to prevent it from sliding forward/back on the swingarm

I think you may misunderstand the source of axial preload that wrecks bearings. It's almost always a mismatch of the distance across the bearing seats versus the width of the inner race spacer. If those dimensions are in agreement, you can tighten the axle as much as you please and it's fine. If they're in disagreement, you can barely tighten the axle and it will trash the bearing in a short time.

Correct spacing sometimes means the bearings are in a bind when the wheel isn't installed and tightened, so that they free up when the axle is tightened and the spacer is compressed.

 

[Overclocker]

I think you may misunderstand the source of axial preload that wrecks bearings. It's almost always a mismatch of the distance across the bearing seats versus the width of the inner race spacer. If those dimensions are in agreement, you can tighten the axle as much as you please and it's fine. If they're in disagreement, you can barely tighten the axle and it will trash the bearing in a short time.

Correct spacing sometimes means the bearings are in a bind when the wheel isn't installed and tightened, so that they free up when the axle is tightened and the spacer is compressed.

the tolerances seem to be fine. i don't think that's what's causing my premature bearing failures

i think it's the small bearing balls that are less tolerant of contamination as well as simply being less durable. check this out

http://faqload.com/faqs/bicycle-components/wheels-and-tyres/wtb-super-duty-front-hub

WTB Super Duty Front Hub apparently uses 6904. so i'm on the right track

PS these hubs were never designed for light motorcycle use

 

[serious_sam]

bearing seized and started spinning around the 20mm axle, turning the whole setup into a sleeve bearing :lol:

I'd say that there's a fundamental problem that is not related to the bearing size.

Need more data.

Suggest that you disassemble the bearing and take some photos.

i think it's the small bearing balls that are less tolerant of contamination as well as simply being less durable.

Not really. Once you have contamination that can cause damage to the surfaces, it's already too late. One size up isn't going to make any difference. You're already frocked.

If you've got contamination, that's your fundamental problem. Nothing to do with bearing size.

 

[Overclocker]

If you've got contamination, that's your fundamental problem. Nothing to do with bearing size.

well there's nothing that could be done to prevent water getting in. so if a bit larger bearing can extend MTBF somewhat then that's good enough for me

btw i define failure as totally seized up :lol: i just checked my rear hub bearings (6201), they feel a bit rough but haven't seized up yet. so empirically my theory holds

 

[Chalo]

i just checked my rear hub bearings (6201), they feel a bit rough but haven't seized up yet. so empirically my theory holds

Cartridge bearings that display any roughness or noise at all have already failed. Your 6201 bearings still turn because they don't have nearly as many balls as your 6805 bearings, so not as many places to rust solid.

If you contaminate 6904 bearings, they'll crap out too. It seems me to that you could use hubs with some additional sealing over the bearings.

 

[Overclocker]

http://www.ckit.co.za/secure/conveyor/troughed/bearings/bearings_skf.html

The seize-resistant concept

SKF reluctantly accepted the inevitable penetration of grit, dirt and other abrasive contaminants into the raceways and rolling elements and designed seize-resistant ball bearings.

Its special internal design enables larger size balls to simply roll over the contaminants, crushing, milling and ejecting them in the process.

So, even when operating at moderate speeds, the bearings will continue to function.


Design characteristics

The largest possible ball size is used, while still adhering to internationally established inner ring bore and outside diameters.

Bearing capacity is increased, and more importantly, actual life expectancy extended.

i'm not a bearing engineer but my intuition about larger balls is likely correct

 

[Overclocker]

Cartridge bearings that display any roughness or noise at all have already failed. Your 6201 bearings still turn because they don't have nearly as many balls as your 6805 bearings, so not as many places to rust solid.

If you contaminate 6904 bearings, they'll crap out too. It seems me to that you could use hubs with some additional sealing over the bearings.

doesn't matter to me if my bearings are rough as long as the wheels turn w/o excessive play :lol:

my beater car is a 2005 honda fit. the rear wheel bearings hum at 70kph :lol: