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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.
I was hoping for some internal data, I'd really be interested in stanchion and bushing tolerances
https://imgur.com/a/LwLDhkA
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
Stiffness is not moar is betterer, it is about the right amount.
Anywho, there were questions about bushing distance/overlap and stiffness. There's no difference in this regard.
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
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.
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.
https://podcasts.google.com/feed/aHR0cHM6Ly9yc3MuYWNhc3QuY29tL2Jpa2VyYW…
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.
EDIT: oooooooh, he's the guy with the really short rear suspension linkage.
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.
The transmission loss you're mentioning is effectively the damping of the structure and/or material if I'm not mistaken. As for racing, you're mentioning road bikes but mentioned that you raced motocross - they are a bit different, don't you think? More on that later.
As for spacing, it doesn't make that much sense to go wider on an MTB fork, even though it would of course help. As for axle diameter, we've had these kind of discussions with Jeff Brines in one of the threads on this forum (sadly he hasn't commented on anything over here, due to some comments he has decided to become a bit of a ghost :/ ) and he mentioned that MX bikes have very similar axle diameters to MTBs, roughly 20 mm. Or in any case, not THAT much thicker. As far as I've just searched, it's between 20 and 25 mm mostly these days, for both the 450 cc MX and 1000 cc superbikes. Even going from a 20 mm, let alone a 15 mm, axle to a 25 mm axle is a big difference in stiffness, but still, it's not as drastic as it might seem.
As for stiffness, yeah it's a benefit for USD forks, but the torsional stiffness takes an impact. How will that influence precision in MotoGP and WSBK? Or will the added compliance help by making the ride more forgivable (though you were mentioning that you'd want all the information)? And how does road vs. dirt make a difference? Smooth surfaces where the range of movements is much different and the high speed events are much less pronounced, the suspension does a lot more work in the low speed, 'handling' mode of operation, which is the opposite for dirt bikes and mountain bikes, there the majority of work done by the suspension is the high speed, bump absorption as opposed to handling operation.
When you think you will get some answers, but then end up with even more new questions, eh? :D
Firstly, transmission loss is a broad measurement that results from the cumulative effects of stiffness, inertia, and damping. Damping alone tends to decrease transmission loss in the frequency region shared by a system's modal behavior. That's a complicated way of saying that damping keeps things from ringing, but doesn't really do much else. By contrast, stiffness and inertia tend to have effects spanning a much larger frequency range. For engineers, damping is a really powerful tool because we can use it to target problems occurring at a specific frequency. Case-in-point, the Gen 1 Yamaha R6 used rubber blocks bonded into the frame spars to provide damping for frame vibration.
If it would help torsional stiffness to increase fork spacing, then why not do it? I measured the spacing on my RS Pike and my RS Boxxer last night. The pike is further apart, almost like they needed to increase in order to claw back some of the torsional stiffness lost by going to a single-crown arrangement.
As for axle stiffness, I concur that the diameters are only slightly larger for motorcycles. However, like you said your model used simplified contact conditions (bonded) that basically simulate a perfectly rigid clamp. Motorcycles have done a much better job at approaching this ideal by increasing the diameter and width of the axle clamping surfaces. The clamping diameter of a GSX-R600 axle is somewhere around 30 mm, while the main shaft is 25 mm. The problem lies not in the bending stiffness of the axle, but how well we can constrain the ends.
Basically, you do not want any torsional compliance in the fork of a WSBK or MotoGP. None. In fact all the work done for the last 100 years has been to increase torsional stiffness as much as possible.
https://www.cycleworld.com/story/blogs/ask-kevin/how-structural-stiffne…
Lateral compliance is somewhat desirable (hold that thought), but torsional compliance, like fore-aft compliance, leads to too many instability issues.
Back in the day, motorcycle frames (both dirt and road, though the distinction was moot early on) were so flexible that their vibrational modes were extremely low frequency. This wasn't a big issue until tires got better, and engines got more powerful. Grip increased, and with it the loads that could be put into the chassis. This resulted in vibrational modes getting excited. Coupled with the torsional mode of the fork, many early motorcycles would develop "speed wobbles". Frame designers would then increase the stiffness of the frame, and suspension designers would design stiffer forks, and all would be well for a time. Then the engines would get more powerful, and the tires grippier, and the cycle would repeat.
Fast forward to the 90's and frames started getting MUCH stiffer. Now the modal behavior was occurring at frequencies above 100 Hz, rather than at say 10 Hz. Gone was the "speed wobble", now replaced by "chatter". This is where dirtbikes and streetbikes start to diverge since the former is usually not able to generate enough grip to excite high frequency modes in the chassis. It was discovered that the chatter could be addressed by adding back in some flex to the frames, particularly in the lateral direction. This leads us to the story of Ducati MotoGP in the late 2000's.
Ducati decided it'd be great to bolt the head tube directly to the cylinder heads of their engine. This was awesome for weight and stiffness, but the short beam created by the arrangement was un-able to flex like a twin spar frame (the arrangement still common today in both dirtbikes and streetbikes). Riders would lose confidence in the bike under cornering as it had a habit of suddenly breaking grip at high lean angles. What was happening was the tire would experience surface variations at high lean angles (upwards of 45 deg). These forces could not be reacted by the suspension since it was at such an extreme angle to the track. In a traditional frame, the lateral flex could soak up some of the hit, allowing the tire to maintain grip. The stiff Ducati frame was not able to do, this and so grip suffered.
I say all this because if we could, we would design a multi-axis suspension system where we could control wheel movement relative to the chassis is many directions. However, such a mechanical arrangement has not been invented (and may never). Therefore, we use chassis flex as a band-aid. I say band-aid (and I'm not the only one) because it is an un-tuneable parameter. Once you build the frame, its fixed. Worse yet, you can only engineer stiffness, not damping. This means that frame compliance behaves like a shock with no damper, and I don't think I need to dive into the disadvantages of that!
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Or will the added compliance help by making the ride more forgivable (though you were mentioning that you'd want all the information)? And how does road vs. dirt make a difference?
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It is true that compliance can be used as isolation (another engineering term that must be properly explained). As I discussed before, torsional compliance is not really going to help here as it makes the motorcycle or bicycle harder to control. What is actually needed here is a bit of controlled stiffness between the rider and the bicycle. Its a case of "what's good for the vehicle isn't always good for the operator". Handlebar flex is a good example of a way to decrease fatigue without compromising grip and stability. And as a good segue into the second question, this is also where road and dirt differ.
In road racing, there is no handlebar to flex. In fact you are basically holding on to the fork tubes! This is done for ergonomic reasons mainly, but it also helps provide the rider with as much feedback from the front tire as possible. Motocross has a lot more issues with impact than road racing. Therefore, more effort is made to cushion the rider's wrists from the repeated blows via handlebar flex, and even rubber mounted risers.
Lastly, I think there was in implication in one of your questions that may not be 100% correct. It is true that due to a lack of a fork crown, USD forks suffer from a decrease in torsional stiffness. That said, motorcycles never had fork crowns so there likely never was a decrease brought forth by the change to the USD arrangement. Even given the situation in mountain bikes (where we are coming from crowned RSU forks), as an engineer I still prefer the USD arrangement. By increasing clamping area and the fork spacing, it should be possible to claw back the torsional stiffness lost, and end up with a package that has much higher fore-aft stiffness.
To bring back a reference made earlier, Fox did say that they could improve torsional stiffness albeit at a higher weight. I think this is part of the problem with mountain biking. We have been constantly caught between our road biking heritage, and our tendency to look more and more like dirtbikes. In enduro and DH, where you aren't relying on a puny human to carry the machine around (road biking), weight probably has less of an impact on the stopwatch than we are giving it credit for (dirtbikes)! I'll be willing to bet that shifting our engineering focus towards controlling flexibility, even at the expense of weight, will bear sweeter fruits.
Yeah, if it would help it would make sense to increase the spacing, of course. Maybe even throw in a new front hub standard?
I did think that the Boxxer shared at least most of the design for the lowers with the Pike/Lyrik, including the stanchion spacing. At least the old 26" Boxxers and Lyriks shared the lowers if I'm not mistaken?
True on the clamping part. I mean, look at the Maxle, the spacing n the axle in the dropouts is very loose in order to be able to insert it, the threads are very coarse and then you preload it with a lever (which, going off Peak Torque's YouTube video might not be that bad compared to a stealth axle) axially only. Then of course you do also have bushing clearances, which will all decrease stiffness too.
Interestingly lateral compliance, on the fork itself, is the hardest thing to achieve given the construction and will mostly even results in torsion due to instability of the structure in that plane
Well, you can engineer a bit of damping if you go composite (layup, fibre and resin choice all play a role here), but given how this industry works in most cases, I don't think we can count on that. Plus you're still in the area you've mentioned, it's not adjustable. And it would need to be given the LARGE spread in requirements mountain bikes need to deal with (under 50 kg to well over 100 kg riders with the system weight easily being varied by over 100 % between the two extremes - unprecedented for more or less any other vehicle on the market), let alone the complexities it would bring to the table if all of it was even possible to be made adjustable. Some people have a lot of problems even with spring tuning on their bikes, let alone suspension damping.
Interestingly a few people in this thread commented that they like USD forks seemingly because of the lower torsional stiffness that makes them more forgiving and less tiresome, kind of like a carbon vs. aluminium rim situation. On the other hand supposedly Gwin didn't like the Fox USD fork because of lack of precision due to a lack of torsional stiffness.
I guess you meant the bridge instead of the crown in the last two paragraphs of the second post? Either way it might make sense to investigate the stanchion thickness too, it was a factor I didn't consider, as, you know, 'the hub is yay wide'. Thinking squarely within the box.
I have put some work in for the single crown/dual crown RSU/USD comparison, but I'm having a few problems with the model (doesn't play nice with parameters, it kinda builds up to a lefty in some cases :D ), but I'm hoping to put some work into it this weekend to finally add that to the data too. Might add stanchion spacing at some point as well then.
Stanchion spacing wouldn't cause a big weight penalty anyway, the crowns would be a bit heavier and a wider hub could be used (instead of bringing the dropouts inwards), so yeah...
(also found out I had a vitalmtb profile)
Thanks for your work.
I saw someone writing about Crconception. I'm a big fanboy of his products so maybe biased.
I can translate some things for those interested, just ask me. (but I don't know if my english is better than "google translate")
Never rode the fork, as I stopped riding MTB before it was available for purchase.
But I rode three of the open bath cartridges that he put now in the fork. And I followed as closely as I can the development of the fork. (even touched one early prototype
And, as far as I know, if you want to put this budget in a fork, there is not a reason to buy another one. It checkes all the boxes: low friction, stiff (torsionally stiff as a pike), awesome spring and damping. The damping is another level compare to foo-the shelf forks. support, "smoothness", control... well it's taylored for your needs, and low friction so it's kind of expected
Now, for the USD discussion.
When talking about motorsport, don't forget there is... a motor. A powerful one.
With great power comes great kinetic energy, so the front stiffness of the fork has to follow.
But, the torsional stiffness, it only have to support the rider arms, and the geometry. The extra weight and speed of the bike add a small extra requirement.
so on motorbikes, the torsional to frontal stifness ratio isn't the same as in a mountain bike. The main drawback of an USD fork kinda disappear here, and advantages can shine way more.
On a mountain bike, both loads are mainly from rider's input, so the requirement in torsional to frontal stiffness isn't the same. And as we don't have "more HP" as a magical solution, weight does matter a lot more, too.
So an USD fork on a MTB can only work if the deigner find ways to keep the balance everywhere.
Now, I tried a dorado a few days in 2014. chassis was OKish, good in roots (not really better than the boxxer, just different), but hard cornering was a no-go. The lag of the front wheel response made the bike nearly stop in the sharpest corners, while the boxxer could keep my momentum.
Hydraulic and air spring didn't even came close to a coil-sprung crconception eqquiped boxxer (or domain SC), but he hydraulic bottom out is a seller when you hit landings hard.
I also have a shiver SC (not DC), can't ride it now because dead bushings and seals, but that thing is a joke. wet noodle with the old 'zocchi euivalent of a moco. (fork twist when braking!) But there is no one part that have been optimised for USD construction so that is also expected. It's a beauty tho. and it have a 20mm boost hub spacing from 2003.
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