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The sprung/unsprung mass is more of a factor in motorcycles than in bicycle forks. As you pointed out, the die-cast lower legs found in MTB forks are very light already.
As for lubrication, there is a massive advantage there. With a conventional fork, ideally, the bushings are "sized" to provide the tightest fit to the the stnachion tube without binding. Suspension tuners often refer to this as "burnishing" the bushings. The reason this process exists is because of how difficult it is to keep fluid "splashing" between the stanchion tube and fork bushing.
Our NINE ONE is different in the fact that we actually want a gap between the stanchion tube and the fork bushing. This gap is taken up by the chassis fluid(oil), which is incompressible. With this design, the stanchion tube runs on a fluid bearing rather than the bushing itself, and only runs on the bushing in temporary high load situations such as heavy braking into a square-edged impact. This eliminates binding friction, and reduces heat, which both improve fork sensitivity.
The same would be true for seal lubrication. The performance gains from a seal that is continuously submerged in fluid, rather than being fed fluid from a foam ring is massive.
And that wouldn't even be possible on a conventional fork because of the location of the fluid at its resting state? I know it's hard but can you speak at all to someone like Intend or Fox with their podium? Is there any merit to an inverted design (and lubrication) if you aren't using the fluid as an active bushing in their case? Is it noticeable smoother if you aren't using a more relaxed bushing tolerance like the NINE ONE and just the oil sitting on the seals?
Would love to hear any thoughts you had on my third point of bushing overlap as well?
I think I missed this video. Appreciate the insight.
If memory servers me correctly (big if), the original 2005 era Fox 40 bushing/stanchion interface was claimed to be designed this way. They felt loose if you locked the front brake and rocked back and forth but felt solid when riding and fluid filled the gap. Given that this was over 20 years ago it may have been a positive way to spin what were really just loose bushing tolerances. (Edit: see page 13 of the 2005 owners manual)
To your second point, anything that helps reduces friction is going to be beneficial, even if it's just a constant coating of fluid on the inner edge of the dust wiper. Given how relatively light mountain bikes are the effects of any friction in the system can feel significant in ways it would not if our frames weighed 50 kilos. Lighter riders in particular feel the effect of constant system friction compared to heavier riders.
A bit of a tangent, but QuarterHP recently did an interesting test where they added system weight directly over a Fox 38 to see if it could match the feel of the heavier Podium. Adding about 300-400g seemed to be the ideal. I imagine much of what Remi might have been feeling was the decreased effect of friction due to the increase in sprung mass. 400g doesn't seem like much, but considering how much-or little-weight is over the front fork it's a decent % increase. Same goes for DH racers adding mass to their frames.
For MTB forks any small reduction in friction, especially stick-slip and reduced breakaway force, is going to have a large performance benefit, especially for lighter weight riders.
Oh interesting. Had no idea this was already done. @PUSH Industries I'm interested to what tolerance an effective "oil bushing" is to? Isn't that a great way to develop premature bushing play? Also what this is effectively doing is having a "burnished bushing" and filling the void with oil. In many cases burnished bushing can create more friction on heavy compression since the station has more wiggle room in the lowers/bushing and can go off axis easier. Would this not be the same for an oil bushing since it's easily movable?
Yeah enjoyed that video. I really like what Henry and Remi are doing at Quarter HP. What I mean is that the 38s unsprung mass is relatively light off the bat since magnesium lowers really don't weigh all that much.
The hypothesis-yet unproven-would be that while the difference in unsprung mass is too small to matter, especially when factoring in the weight of the wheel and tire, a targeted increase in sprung mass might be beneficial. Cycling in general has always been adverse to any weight gain, and as a result maybe we’ve overlooked where it might actually be of benefit. But again, it’s all a bit anecdotal at this point.
I’d be curious what the actual difference in weight is between a 38 casting and axle vs. Podium lowers and 20mm steel axel.
And now we’re way off topic
Hold on, isn’t the term “Enginerding” straight from the Department of Redundancy Department?
re: USD forks are „too heavy“, you got that wrong, only american designed ones are
It's definitely already been done....every inverted motorcycle fork is designed to function this way. It's a very proven technology.
As a company, we prioritize performance and durability over weight.
We could offer a fork that would be competitive in weight, or lighter than the European version you're referring to. I've said many times that they make a good product....it's just a different category than what we're building for. If we wanted to reduce weight and make an equivalent product to what they're building, we would have to remove the following items:
- Coil Spring based spring system
- Independent Hydraulic Bottom Out Damper
- Sub Chamber with external adjustment
- IFP based sealed damper
- High Speed Compression with external adjustment
- Torque Cap Compatibility
- Tool-free external Chassis Bleeders
- Speed Service Bleed Ports
- Modular Axle Lugs
- Integrated "Low Mount" Fender
- Fluid Bearing Fork Bushings
Since we're banging the inverted fork drum, I'll throw in something that hasn't been mention yet: Alignment
Here's list of the dimensions you need to control for optimal chassis alignment.
Typical RSU Fork:
- Width between the axis of the stanchions pressed into the CSU
- Parallelism of the two stanchion axis
- Concentricity between the upper and lower bushings in each leg
- Width between the axis of the bushings pressed into each leg of the lower
- Parallelism of the bushings axis between the two legs
- Concentricity of the axle bore between each leg (in two vectors "x & y")
- Width of the hub mounting surface (if not using a floating axle, but also somewhat hub dependent)
Nine.One Fork:
- Parallelism of the axis of the bushings between the upper tubes
- Concentricity between the upper and lower bushings in each upper tube
- Concentricity of the axle bore between each dropout (only on one vector "x")
I just called them and that's not what they told me, at all.
Digging into all of the hubs I only see the King as having solid axles, front and rear, with preload adjusters. And on the King website they specifically talk about accuracy as a primary advantage of their product.
I also spoke with Project 321 and they are in the process of converting all of their front axles to solid thru axles with preload adjusters and the rears are already like this.
Pretty sure I'd only buy King hubs at this moment, in spite of hating the 'angry bee' sound. Maybe I'd combine the King front hub with a more pleasant-sounding rear like the E13 or the P321.
What did they tell you?
If you wanna go "all the stiffness I can get" then maybe Chris King is suboptimal, since it seems like their Torque Caps don't have a lot of surface:
They told me swappable endcaps insert into the bearings then slipfit into the axle. So that would be: Internal Spacer & Internal Nested End Caps. The fact that the endcaps are swappable between 15 or 20 indicates to me that the center axle is oversized and it's all floating anyways. I see in your photo that even the axle isn't solid, but it doesn't touch a 15mm front axle anyways. I don't think this is a good choice if your goal is stiffness.
I described exactly what I was looking for and he said: 'The previous Hadley axle was built like this with threaded on endcaps that made it solid, but that was discontinued about 2 years ago.'
I also contacted my acquaintance Jake that used to own P321 and is now a consultant for them. He told me (paraphrasing here): 'we already have solid rear axles with preload adjusters (I already knew this as I have 2) and are in the process of swapping the front axles to this style. The gravel hub is already done in fact.'
My suggestion was an axle with the Torque Cap machined right in on one side and the preload adjuster would have it on the other side.
If I had a mid-high pivot e-bike on the way (I don't) I'd likely go King front and P321 rear, but I don't want 288 POE on my new Wild even though I love the fast rolling and relative silence of the P321 product. I noticed many years ago that high engagement hubs increased feedback, fine on a trail bike but not ideal on a 170mm e-bike. Could maybe just remove some pawls and reduce the engagement perhaps. For the Wild I'd drive myself crazy with the angry bee sound and buy the King or possibly the E13 Sidekick in the rear.
Maybe Push can comment but looks to me like the surface area is exactly where it matters.
The way I read the article from the first post, this type is what's a good TC design (DT):
I might be wrong, though, so yeah.
It’s not a ratio thing for me. What I am saying that a magnesium lower is very light. When factoring in the weight of the drop out, steel axle and stanchions you get pretty much the same unsprung weight. Not only that but in the case of the Podium the air spring is connected to the stanchion (unsprung mass) so if you weighed it all out you could get a very different result.
I feel like this photo is TC for a 15mm axle which is why both ends are so flared where-as the King photo is for a 20mm axle.
But I dunno.
US designed? The Swedes already tried USD forks years ago and they scrapped them because of the weight and price.
@PUSH Industries had no idea about that either. Would like to know tolerances? How can a fluid really act as a structural piece in a fork? I’m no engineer I would love to know more!
Paul Component make an awesome solid axle plus collar set up.
If only the was some form of communication that Hadley could use where they publish all of their products and information, which everyone can access at any time?
Do they at least let you fax them a request to get a catalogue mailed out?
This is the closest I’ve ever found besides calling them directly.
https://www.balleracing.com
I’d be curious if the hub designs prone to axle flex also cut through bearings fast. Not an USD fork specific question, but an interesting extra benefit of using a better hub in USD applications.
If only...
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