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Are Upside-Down forks really flexy?

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1/13/2021 8:51 AM

hamncheez2003 wrote:

I still wish they had some RSU dual crowns and an inverted dual crown in there for us to see....

In the PB video?

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1/14/2021 5:11 AM

@Primoz. I checked the FOX drawings, and they as well spec tolerance +-5mm for axle-to-crown distance, the same as RS on Lyrik from 2009. Wheel axle offset tolerances with +-2.5mm :-D. By any means do I consider such variances worth of premium price.
FOX 38 2021 spec

As for Ocho. Yes, it's shorter chasis when compared to FOX 38 (but only by the lower travel) but still, it has only one leg and handles the impact MUCH better than 38mm wide two stanchions, eventhough from lower height.

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1/14/2021 5:19 AM

Ah, +/-5 mm tolerance on the ATC isn't something i'd give a lot of value to. The -2 mm tolerance on the hub width is... surprising, but I understand the +2 mm. I'm fairly certain (hopeful?) that their internal drawings have much more stringent tolerances than what is show nere, this is for the general public. Their actual tolerances wouldn't give them any leeway when dealing with warranties (this way they can say even faulty products according to internal spec are within public spec...). And this is probably meant for frame manufacturers, so you want to secure some space (maybe to account for any flex in the system) to prevent any frame impact.

I was hoping for some internal data, I'd really be interested in stanchion and bushing tolerances smile

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1/14/2021 9:51 AM

Primoz wrote:

URL for the RS tolerances.

I never said it's not possible to make the inner surface of the outer tube appropriate to have a bushing slide on it. I said it's more expensive, as, opposed to castings, it requires finer control over the surface quality and hardness. And that it adds another component that requires this besides the stanchion. And it requires it on the inner tube surface, which logic dictates is harder to work on than the outer surface.

And I specifically said that even such a good idea (even a Lefty with keyed stanchions) isn't worth much if it's really hard to make it quickly, cheaply and with good reliability. We are bemoaning bike prices in the current state, would you be happy to pay 2000+ USD/EUR/etc. for a 150 mm singlecrown fork from the major manufacturers?

Regarding the Ocho vs. 38, the Ocho was on a 29er, the 38 was on a 27,5" bike. The same movement will appear larger on a 27,5" bike because of the difference in size of the reference geometry (wheel size). Plus the 38 has a longer travel, which, if you look at the last graph in my first post, plays a huge role as well. And the rider does appear to handle the huck a bit worse on the Nomad than on the Cannondale, insinuating a higher jump height on the Nomad compared to the Cannondale. Which would of course mean higher forces...

hamncheez2003 wrote:

I still wish they had some RSU dual crowns and an inverted dual crown in there for us to see....

Primoz wrote:

In the PB video?

Ya, the pinkbike video. It seems that RSU dual crown forks aren't necessarily any better than the latest crop of thick crowned 38s and Zebs

https://imgur.com/a/LwLDhkA

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1/14/2021 11:06 AM

We'll get there. Don't know when, I'm thinking about how, but I can assure you, if I made this, I'll make dual crowns too!

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1/25/2021 6:07 AM

https://www.bikeradar.com/news/bikeradar-podcast-does-stiffness-matter/

I was linked this podcast a few days ago and had a listen to most if it yesterday. There are some interesting topics about the forks and the stiffness in it, I think it might be worth a listen for anyone that took part in this topic smile

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1/26/2021 9:21 AM

Primoz wrote:

https://www.bikeradar.com/news/bikeradar-podcast-does-stiffness-matter/

I was linked this podcast a few days ago and had a listen to most if it yesterday. There are some interesting topics about the forks and the stiffness in it, I think it might be worth a listen for anyone that took part in this topic smile

I had two Lefties. I replaced a TALAS 32 140mm with a Lefty Max Carbon with Fox RLC in 2010, and the first thing I noticed was the handlebar feedback from riding rocky terrain, where the wheel tracked between the rocks, which comes from the incredible torsional stiffness. When I upgraded a year later, I got a Yeti SB66 with a BOS Deville fork. And the first thing I noticed going from the Lefty to the Deville was the fore-aft stiffness, which was not desirable. I have since longed after an upside down fork, but I really hated the lack of effective mudguards for Lefty and upside down forks (and the prices of USD forks).

Stiffness is not moar is betterer, it is about the right amount.

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1/26/2021 10:02 AM

The question is what is the right amount?

Anywho, there were questions about bushing distance/overlap and stiffness. There's no difference in this regard.
[img]https://imgur.com/tM4e5wJ[/img]
[img]https://imgur.com/pQAPSKA[/img]
[img]https://imgur.com/ayFC7SA[/img]
[img]https://imgur.com/odoIwsf[/img]

The results were within 1 % in all cases.

One thing to note is that we are again dealing with idealised results with ideal fit between the bushings and stanchions. In practice a longer distance between bushings will lower the forces in the bushings, decreasing friction. Then, because you have some clearance in the bushings (otherwise it's hard to assemble the forks), a longer overlap will lessen the movement, making it appear as the fork is less wobbly and will also aid to prevent binding and the like.

So yeah... Maybe a bit unexpected, but here we are smile

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1/26/2021 10:11 PM

But you already know what has lower friction compared to sliding friction of bushings. Rolling friction of rollers.Or you could design a super-sealed and pressuried hydraulic chamber in the outer legs where innner stanchions would move. Just like turbocharger shafts spin within the oil bath instead of on rolling bearings, because rolling bearings can't whitstand the high rpm and temperature.

Tell me why has FOX come up with universe blasting invention of floating axle? For poor tolerances of the hubs like they say in their marketing? Because from some weird reason their super 38mm chassis cannot have aligned drop outs. They just dont' give a fvck about slightly more costly but proper manufacturing process. So they introduced workaround letting the hub axle slide within the sleeve.

Bushings are just cheapiest solution. They solve economics, not performance.

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1/27/2021 12:45 AM

Yeah, it's clear why they have the floating axle, to take away the effects of tolerances of hubs (which are a real thing, even among the same hub models, let alone different models and different manufacturers) to prevent leg binding. And it's not if the chassis can or can not have dropouts aligned, they could have a tight tolerance, but that flies out the window once you get a hub that has a width that is 0,5 mm under the specification (for example). And the axle does not slide withing the sleeve, the sleeve is positioned to the hub and then locked in place there with the axle axially preloading the left dropout, hub and the sleeve. With the right leg having the ability to be adjusted to it's position.

Of course bushings are there for economics, but because the performance is good enough at low costs, while the alternative (rolling bearings) is obscenely expensive (bearing surface machining, steel surfaces are required instead of aluminium, assembly and sealing can be an issue, etc.).

As for the turbo part of your post, it's just wrong. Turbos use fluid bearings because it's cheap - you just need a round axle and a properly dimensioned hole to provide the correct thickness of oil in the gap between the two. The turbo shaft then floats on the oil because of oil viscosity and because of it spinning (due to viscous resistance in the fluid). Use the incorrect oil (with the viscosity too low) or overheat it and the shaft can hit the housing (which is not good of course).

You said turbos can't use bearings, which is not true as well. Bearings are expensive, because you need the rolling elements (balls) and, most importantly, the races, that need to be manufactured more precisely than the surfaces of a fluid bearing. But the load bearing capabilities of such a bearing are much higher. Plus due to the speeds of turbos (100.000 rpm is normal, for small turbos it's even over 200.000 rpm) more or less only ceramic balls are used 8another factor for making them more expensive). But ball bearing turbos do in fact exist, they are used in high boost applications, where axial loads are too high for fluid bearings (ball bearing turbos use angled contact races). But yeah, cost.

As for using fluid bearings, they are in most cases used in shaft-bore interfaces with another option of using them as an axial bearing. In that case you need a pump flowing oil into the interface at the adequate pressure to ensure the axle floating on a layer of oil instead of rubbing on the base structure. We were told in college that this is used in electric generation (hydro electric plants - turbine-generator assemblies, which are vertical as opposed to horizontal for most other turbine-generator assemblies).

If you wanted to use a fluid bearing (which does in fact have very low friction) in a linear motion, you would of course require a pump to pump the oil into the interface to make it work. So yeah, not an option. Plus friction doesn't have much to do with stiffness, which is the topic of this thread. Friction is a whole another world of hurt when it comes to designing and, much more importantly, manufacturing a fork due to all the tolerances in play.

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2/13/2021 9:50 AM

Listened to another BikeRadar podcast, tech that never quite delivered:
https://podcasts.google.com/feed/aHR0cHM6Ly9yc3MuYWNhc3QuY29tL2Jpa2VyYWRhcnBvZGNhc3Q/episode/MDhlZTA4YWUtNmIyNC00MmVlLTkxZWItOWVlYTBiYzkxNjg5?sa=X&ved=0CAUQkfYCahcKEwiImoPYtefuAhUAAAAAHQAAAAAQHQ

USD forks are a part of this and a good point was mentioned both for USD forks AND for linkage forks - horizontal(ish) torsion. With the bridge absent (in both USD and linkage forks) the left and right dropout can move along the travel path independently. This can be caused either by external factors (side loading the front wheel), or, more importantly, by different forces from within the system. Which is very common with the damper in one leg and the spring in the other leg.

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2/15/2021 8:36 AM

Primoz wrote:

Listened to another BikeRadar podcast, tech that never quite delivered:
https://podcasts.google.com/feed/aHR0cHM6Ly9yc3MuYWNhc3QuY29tL2Jpa2VyYWRhcnBvZGNhc3Q/episode/MDhlZTA4YWUtNmIyNC00MmVlLTkxZWItOWVlYTBiYzkxNjg5?sa=X&ved=0CAUQkfYCahcKEwiImoPYtefuAhUAAAAAHQAAAAAQHQ

USD forks are a part of this and a good point was mentioned both for USD forks AND for linkage forks - horizontal(ish) torsion. With the bridge absent (in both USD and linkage forks) the left and right dropout can move along the travel path independently. This can be caused either by external factors (side loading the front wheel), or, more importantly, by different forces from within the system. Which is very common with the damper in one leg and the spring in the other leg.

I've been wondering about a single sided linkage fork, like a one night stand between a lefty and a trust fork. I'll see if I have time to fire up fusion 360 this week.

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2/15/2021 8:52 AM

hamncheez2003 wrote:

I've been wondering about a single sided linkage fork, like a one night stand between a lefty and a trust fork. I'll see if I have time to fire up fusion 360 this week.

https://www.pinkbike.com/news/the-single-sided-carbon-fiber-diy-linkage-fork.html

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2/15/2021 9:24 AM

Primoz wrote:

Listened to another BikeRadar podcast, tech that never quite delivered:
https://podcasts.google.com/feed/aHR0cHM6Ly9yc3MuYWNhc3QuY29tL2Jpa2VyYWRhcnBvZGNhc3Q/episode/MDhlZTA4YWUtNmIyNC00MmVlLTkxZWItOWVlYTBiYzkxNjg5?sa=X&ved=0CAUQkfYCahcKEwiImoPYtefuAhUAAAAAHQAAAAAQHQ

USD forks are a part of this and a good point was mentioned both for USD forks AND for linkage forks - horizontal(ish) torsion. With the bridge absent (in both USD and linkage forks) the left and right dropout can move along the travel path independently. This can be caused either by external factors (side loading the front wheel), or, more importantly, by different forces from within the system. Which is very common with the damper in one leg and the spring in the other leg.

hamncheez2003 wrote:

I've been wondering about a single sided linkage fork, like a one night stand between a lefty and a trust fork. I'll see if I have time to fire up fusion 360 this week.

He abandoned the singlesided approach.

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2/15/2021 11:29 AM
Edited Date/Time: 2/15/2021 11:30 AM

Any reasons why?

EDIT: oooooooh, he's the guy with the really short rear suspension linkage.

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3/4/2021 2:54 PM

I'm so glad to see some independent engineering work going into this. Doing FEA is second-best to getting an independent lab to actually MEASURE these things. As an engi-nerd I'd really love to see that data.

I used to race sportbikes and grew up riding motocross, so this has always been a quandary to me. Especially in the world of sportbikes, front end stiffness is critically important. Keep in mind you hear a lot these days about chassis flex in WSBK and MotoGP, but that is different from front end stiffness. In that regard...more stiff, more better. The reason has to do with something called transmission loss. In vibration this is a metric that we use to tell how much energy is lost as a strain wave travels through a medium. Really low modulus materials (rubber, polymers, etc...) have excellent transmission loss in comparison to really high modulus materials. In simple terms, imagine a cooked spaghetti noodle. Wiggle one end. Feel anything in the other end? Now try it with an uncooked spaghetti noodle.

This notion that you want to filter out some of the vibrations is simply not true. The more intimate the connection between rider and riding surface, the more confidence inspiring the machine will be. That being established, why then do motorcycles seemingly give up some torsional stiffness by sticking with USD forks? I think the answer is that they don't!

For one, motorcycles never had bridges back in the RSU era. So the change to USD meant that there was an initial increase in relative torsional stiffness. As someone who has gone from a 1st gen YZF R6 (RSU), to a 2nd gen (USD) all in the same day, I can tell you there is a palpable difference!

But there are other things that motorcycles have done to make the arrangement work. For example a fork spacing on a RS Boxxer is about 5.5 in roughly. A modern sportbike will use closer to 12 in! Of course this is coupled with a wider hub spacing, and much larger axle. Other areas of improvement are clamping widths at the crowns and axle. Nowadays the lower triple clamp is often thick enough to accommodate 3 bolts per side.

Could you simply engineer a RSU fork to take advantage of these same factors? Sure. But why bother, when you would be taking a hit in fore-aft stiffness. This is the really big reason why USD forks tend to win in high-speed applications. The steering geometry responsible for giving you stability, and control, must be preserved at all costs. Dynamics in rake, and trail caused by fore-aft flex are disastrous at speed. This was part of the reason why the old superbikes of the 80's were so deadly! As mountain bikes get faster, I see the demand for more fore-aft stiffness increasing.

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