Front Hub configuration, style, and bearing size have little influence on the performance that you get out of a conventional or "right side up" MTB suspension fork. This is due to the fact that the lower legs are one-piece, connected by the lower leg fork brace.
The same cannot be said for manufacturers of inverted MTB forks. With an inverted design, the hub ties the two lower legs together. The more robust the hub, the more robust the performance from the chassis and vice versa.
Is it possible that we'll see a shift in front hub designs as Inverted forks continue to gain momentum?
@TEAMROBOT and I spoke about this at length during a recent video call discussing the NINE ONE fork that he recently tested.
Interested in the deep dive? WARNING! It's a bit of a read! 😁
Link below:
Front Hub Influence on Inverted Forks
Darren
How much does the manufacturer's through axle design interact with some of these hub designs? I am, of course, thinking about Fox's use of a 20 mm hollow steel axle for the Podium. They've stated that it was chosen based on ride testing, but could hub "compatibility" (i.e. reducing hub flex) also have been a concern, and would it be a noticeable benefit?
Hadley DH hubs and their new boost hubs with torque caps axle are in the Solid Axle with External Preload Collar bucket.
This is great @PUSH Industries!!! I really enjoyed our conversation about hub designs, but my review was already at 6000 words and I had to make the tough call to leave a lot of this hub talk out of it. Stoked to see you wrote this up yourself, and I hope this info can help hub buyers and manufacturers going forward.
All good! I wish I could take credit, but this one was written up by our Engineering Director Matt White. 👍
this is the kind of content i'm here for. great work folks.
The system as a whole determines the performance. Hub interface, bearings, axle, lugs. Each plays a role in an inverted fork.
Darren
theoretically speaking, what would be the ideal/optimal hub / axle system interface configuration for an inverted fork?
thank you for posting this. I did my 3 of my 4 weeks of testing the 9.1 with a dt350 front hub and standard end caps. The last week, I used a i9 1/1 hub and standard end caps. The 1/1 hub was noticeably better in terms of front end accuracy, specifically in situations where I was doing very large rock rolls into fully loaded 90 degree catch berms. I have to admit, after riding the podium for a month I really want to re-visit the 9.1 with a chris king front hub or a i9 hydra with torque caps.
Certainly my time on the 9.1 highlighted to me the difference in front hub designs and what the implications of that are. I love dt350s and they have been my go to for many years... but I now question their front hub design and I view it as a shortcoming and limitation.
Hey @ebruner I think you live close to me and you're welcome to borrow my XTR M9220 front wheel if you want to use it for back to back comparison**. It has the "Solid Axle with External Preload Collar" hub design (the fourth style described in the Push front hub white paper).
Plus, you live just up the 405 from me and we need to ride together! I still haven't ridden in the Santa Ana's in more than a decade. Or if you wanted a Tour de Rock Smashé at my local in Simi Valley. Either way, would love to connect.
**It's a 28 hole carbon rim than IMO is pretty similar in ride feel to a 32-hole alloy wheel. The only downside is it's a centerlock hub, and I only have a 220mm HS2 rotor or a 203mm RT-MT905 rotor, so you may need to provide your own centerlock rotor for a good apples to apples comparison to whatever you're currently running. I think you said you're running Mavens on all your bikes, and FWIW I also have a six bolt 220mm HS2 and a +20mm Maven Adapter, so you could also switch to 220 on both wheels for your comparison test (DT350 front hub and standard end caps vs. XTR w/ Solid Axle with External Preload Collar). That's what I ended up doing for the Push vs. Podium test, compared to my normal setup with a 200mm front rotor.
For bigger riders who like to go fast on tech, do hubs with bearing preload collars still work loose, or have they become more reliable over the years? Once you were over a certain threshold they required frequent tightening, but for lighter/average riders who charge they were reliable perfection. I'm guessing/hoping the King system was tweaked over the years and the problem went away.
I'm not the biggest or the fastest, but I think I'm a decent helping of both. I'm certainly hard on products.
Haven't ridden a Chris King rear wheel in a while, but when I did, I the rear hub would develop a small amount of play once or twice a year. Never had front hub play. [You probably know this, but] part of the reason King hubs develop play over time is that they're designed to. King hubs run a hybrid cartridge/loose ball angular contact bearing design, and a slightly softer and more durable material on the cups and cones that has a super long lifespan. As the bearings and races bed in over time, the preload ring should be adjusted to take up the slack that develops between the angular contact cups and cones. That's why King hubs can be run forever without replacing the the bearings, because they're designed to "wear in" instead of "wear out." Apparently well-worn King hub bearings roll super crazy fast compared to brand new ones, but I don't know if that's true or not.
Again, I've never developed play on a King front hub. The rear hubs did, but I didn't think it was particularly often or that they were particularly annoying to adjust. I rode King hubs on my primary bikes from 2012 to 2018, and a running change that I loved during that time was that they added a little 3mm hole into the preload color to slip a 3mm allen wrench in to use as a simple lever to reef on to help you twist the preload collar. They also bumped up the allen key size from a 2 to a 2.5mm or maybe from a 2.5 to a 3mm, so it was harder to round out the screw on the preload collar. Both of those updates meant that I could do a solid job tightening the preload collar while the rear wheel was still on the bike, which meant that it was a 30-second problem when it happened once or twice a year, and I could even make the adjustment trailside with no special tools. Contrast that to the old hub which was actually pretty hard to get properly tight, and as a result seemed to come loose all the time.
My 2025 XTR hubs with a bearing preload collar have not come loose in a year of riding, but they also use a standard sealed cartridge bearing design, so I wouldn't expect them to be developing play in that time.
Come on now, you're fast, really fast. Any ex-pro is fast AF by mortal standards.
Nice to hear about the small but meaningful updates King has made since I ran them back in the dark ages. Adjusting 1-2x a year is nothing, even DT350s need grease that often.
Will there be an "Forum-Review" from you, Push 9.1 vs. Fox Podium? Would be nice!
Probably one where the fork manufacturer designed their own proprietary hub to precisely control all tolerances and optimize the hub/dropout/axel as one system rather than separate ones.
As far as I know the closest anyone has come to doing that was Foes with their 30mm axel - and maybe some of Sunn era custom Boss forks - but even then they were probably only looking at axel diameter, not all the other parameters Darren/Matt outlined above.
Would be curious to get someone's take on what the optimal design would be within current standards vs what it might be if the standards didn't matter.
Didn’t the RS-1 use a proprietary hub?
Yes. Totally forgot about that fork. 27mm 'Torque Tube' hub axel and end caps with a 15mm ID to run a standard fork axel. Pics and specs are in a PB review here.
As a former CK tech for the local distro I will also add that if they came loose too often it was because they didn't have quite enough preload - it needs to be a scooch past "no play" so theres a tiny bit of tension in the bearing (this probably applies to most bearings, but its more noticeable with King because angular contact bearings with adjustable preload are much less common)
I dont think I have ever changed the front bearings on any hub on any of my bikes... except the dh bike where I run Dorados. Its just low cost stock hub but I'm sure it would be the same with hope or dtswiss (I know of a different example with flexible swingarm that destroyed few hubs).
In the end its a minnor expense and one I can easily live with considering the fork amazing performance and ease of maintanance but the lower stiffness absolutely impacts the front hub/bearing and in my opinion different/stronger hub design would help.
Spesh Enduro in ~2008 had a 24mm axle on their proprietary dual crown.
I think it was a common error for people to strip the preload collar pinch bolt rather than fractionally overload the collar itself. After a year or so I always found they settled in.
Theoretically speaking, the "Solid Axle with External Preload Collar" is the most optimized. That being said, it still comes down to execution. Bearing sizes, material use, and manufacturing tolerances all play a role. Both the new Shimano XTR and Chris King front hubs feature this layout. The large-format King bearings provide better performance.
I'd like to add that this is no fault of the hub manufacturers. All of the various configurations work excellent with conventional forks. There hasn't been a real need to look at front hubs differently until this point.
I’m still trying to figure out why it’s taken approximately 40 years to get to usd forks on mtb. All the tech has been there for years.
This is super cool nerds, love it. To guess on this question - the ‘gravity’ market is now much bigger with e-bikes. People already decided xc is not the place for inverted forks, and maybe the gravity folks willing to buy an inverted fork before e-bikes was too small a number.
Cost and convenience. Cost is what it is, but there's no denying that removing and reinstalling the wheel on a conventional fork is better for the mass market.
IDK, I find some of the RSU axles significantly shittier to operate than the simple dropouts and pinch bolts on (some) USD forks. And then there's the ease of maintenance of USD forks as a counter point in the convenience debate.
I'm the absolute last person you'll need to convince when it comes to inverted forks and the advantages! 😃I believe springs should be coil, and forks should be inverted! 😁
Tech within a system evolves together. An upside down fork on a bike with bad geo, brakes, tyres, wheels, rear suspension design would have been a disservice in some ways.
We are now at a stage where tyre a wheel tech enable amazing performance, geo works hand in hand with it, and for the most part rear suspension designs are pretty awesome, brakes are fantastic. So the timing is pretty good for an optimised front suspension unit.
once you put a motor on the bike, it’s even more so. Given the characteristics of an USD fork.
Hi Darren, I saw this thread and thought I'd get some insight into this. Please let me know if this was already covered somewhere! This is more inverted fork and less hub related but would love to get some insight. I think inverted forks are a little too heavy and rich for my blood on my enduro bikes but they are cool nonetheless and definitely something I want to see evolve. A friend recently brought up in inverted fork conversation that on a conventional fork the reduced unsprung and to sprung mass is not as much of a talking point as we though since a heavy parts (damper and air spring) are housed in the CSU which is an sprung member of the fork and the magnesium lower is the only thing moving and is actually quite light. He also brought up the fact that once the you get the fork moving the bath oil starts sloshing around so the better lubrication are less apparent.
My sales pitch was always that there are three main advantages to inverted forks two of which I stated above leaving the increased bushing overlap as main advantages as the unsprung and bath oil was already solved with a conventional fork. Would love to learn and hear your opinion on the matter!
I have a 2017 36 factory with pinch bolts. Also 100% spring nerd with smashpot. I find the RS axle with threads at each end on the boxxer
Boxxer more hassle with alignment. USD forks have been on moto since the 80s so all the R & D already done.
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