Seen on multiple bike at Sea Otter the Fox RAD prototype USD fork:
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Fox Podium inverted fork
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Rick Draper
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Now I'm curious about busing overlap on USD SC vs DC. So if you have the Push at 170mm and the Dorado at 170mm, how far apart are the bushings?
I've no idea how far apart they are on my right side up forks. I could probably guess from the casting bulges.
I've no idea how far apart they are on my right side up forks. I could probably guess from the casting bulges.
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326 Posts
The difference in unsprung mass between USD and right side up in mtb forks is very small. Infact the bigger difference in unsprung mass is by getting the damper body and oil into the uppers. That is regardless of whether it is USD or right side up.
I feel like there's some mis-comm, in that Jayem is right that bushing overlap stays same, but Push/Darren brings up the effect of bushing location with respect to the axle, which for me is something new and learnt.
Unfortunate case of semantics, maybe, was a good learning bit. Also I suppose bushings in modern forks are no longer bound on stanchions (which would mimic the USD advantage) due to packaging/diameter issues?
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Which makes me wonder if on a right side up fork friction is at it's lowest at full extension and increases, and therefore a USD fork friction is at it's highest at full extension and reduces?
Not sure which a I prefer to be honest!
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yeah I'm in your boat too, i thought A on the pedantic of the english language, Darren brings a perspective I didn't think of which makes sense. It however means people are debating different things and then complaining the other person is wrong. I love forum banter though, where's Dougal when you need him 🙃
Sorry for crude paint diagram, but my rough understanding is that on RSU forks, the leverage effect (blue line rotating about the red bushings) stays similar regardless of travel.
Whereas on the USD, the blue line decreases as travel is used. Feels like an incredible benefit under dynamic sag and through mid travel cruising. I'm no expert though so ready for errors to be pointed out.
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My opinion is it is usually due to cost/manufacturing expenses.
There are two issues here. One is that a bush in the larger tube is easier to contain, and two it only requires one sliding bearing surface (the stanchions).
To mount a bush on the smaller tube makes it much harder to keep in place, and you now require two sliding bearing surfaces: both the smaller and larger tubes, and you also require additonal manufacturing to facilitate a way to install and remove the bush on the smaller tube.
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A good point. I was going to attempt my own drawing. I however know nothing about physics I'm afraid. Is there leverage provided in the other direction, IE the rider is resisting that impact so are they applying leverage from the other end, bars to top bushing?
I'm still curious how many mm the overlap difference is between the Push and a right side up fork
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Could be nice to have confirmation but from my view this was the point Darren was making.
I don’t want to speak for Darren, but I think the point he was making was referring to the moment arm that would applied to the bushings.
“Bushing overlap” could be defined as the distance between bushings. But like Darren is pointing out, this doesn’t mean a whole lot if you don’t look at the moment arms from axle to bushings.
On a RSU fork, this moment arm is constant. The distance between the bushings and axle does not change. Whereas on a USD fork, this distance decreases as the fork moves through its travel, which is beneficial since the deeper in travel you go, the more force is acting on the fork. On USD forks with floating bushings this is two-fold because you have increasing distance between bushings, and decreasing distance from axle to bushings.
So when comparing a RSU to USD fork, even if the distance between bushings is identical, the USD fork will have a shorter moment arm between bushings and axle, which means it has less force acting on the bushings and trying to cause binding (assuming same travel, of course).
“Bushing overlap” could be defined as the distance between bushings. But like Darren is pointing out, this doesn’t mean a whole lot if you don’t look at the moment arms from axle to bushings.
On a RSU fork, this moment arm is constant. The distance between the bushings and axle does not change. Whereas on a USD fork, this distance decreases as the fork moves through its travel, which is beneficial since the deeper in travel you go, the more force is acting on the fork. On USD forks with floating bushings this is two-fold because you have increasing distance between bushings, and decreasing distance from axle to bushings.
So when comparing a RSU to USD fork, even if the distance between bushings is identical, the USD fork will have a shorter moment arm between bushings and axle, which means it has less force acting on the bushings and trying to cause binding (assuming same travel, of course).
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I'm describing both "A" and "B". I would add a "C" distance to your diagram, which would also be the bushing length. Distances "A", "B", and "C" all contribute to the overall performance and "feel" that you're able to tune with an inverted design.
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This is correct. In reading the comments, it looks like most people are switched on to the point that I was making.
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@Jayem you really need to get a modern USD single crown fork. I know you have the means. 
Basing your opinions and advice on technology and products that are 25+ years old is really not adding anything to the current conversation.
You're like the old man standing in his yard yelling about how climate change is not real and that his 1970's V8 truck that gets 5mpg is the best vehicle ever made.
Basing your opinions and advice on technology and products that are 25+ years old is really not adding anything to the current conversation.
You're like the old man standing in his yard yelling about how climate change is not real and that his 1970's V8 truck that gets 5mpg is the best vehicle ever made.
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I asked before, but is there not a moment ärm from the other direction too? I understand the wheel wants to deflect backwards at an object but surely that's also causing a force at the other end IE from the bars back to the top bushing?
So does the new Fox and Push fork have floating bushings?
Best case the top bushing on a SC USD fork will be 180mm from the top cap and presumably at the bottom of the upper. Obviously the benefit of dual crown USD forks is you can stick the top bushing higher up.
I’m not 100% sure I understand what you’re asking, but in this example the steerer tube/headtube would be the “fixed” portion of the lever relative to the axle. There are definitely other forces as play in the system (i.e. headset bearings, flex through the uppers/stancions). But the moment arm created from the from the axle to the bushings is then opposed by the upper and lower bushings. Thats why decreasing the lever arm or increasing the distance between bushing will improve friction, as it decreases the force acting on the bushings.
Without increasing A2C (or making a dual crown, or having axle drop) you’re limited by the same variables for the distance between bushings in a single crown fork. In a RSU fork, there would be ~180mm from axle to lower bushing in the casting. And in a USD fork, there would be ~180mm of stanchion between the axle and lower bushing.
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He was absolutely correct that neither the USD or RSU layout is more conducive to a greater distance between bushings for a give axle-to-crown length. This is assuming single crown and fixed bushings. Starting with a given axle-to-crown length and travel requirement, the shape of the axle clamps, the crown, and height of the chassis seals dictate the remaining space for bushings.@Jayem you really need to get a modern USD single crown fork. I know you have the means.
Basing your opinions and advice on technology and products that are 25+ years old is really not adding anything to the current conversation.
You're like the old man standing in his yard yelling about how climate change is not real and that his 1970's V8 truck that gets 5mpg is the best vehicle ever made.
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6,134 Posts
Yes, it is the distance from the upper bushing to the handlebar. My tall handlebared DH & Enduro bikes put a lot more load on my forks than my low handlebared XC bike.
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and absolutely wrong for not reading the point regarding leverage relative to axle position
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Why are you claiming I didnt read this? Did you read the original/total conversion?
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I did. The most notable piece was where you told the expert suspension manufacturer how their "entire premise was wrong".
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