Could someone explain USD forks to me?

[cowboyup3371]

So a lot of folks talk about putting USD forks on their bikes and I just figured out it means Upside Down where the bodies are at the top and the tubes are on the bottom. According to Wikipedia, it improves handling.

But I don't understand how. What makes them so much better this way and is there a reason to change to that setup?*





*I'm not changing mine right now just asking the question.

[80GS1000]

Less unsprung weight for starters, not to mention most USD forks are three way adjustable - preload, dampening, and rebound.

[TheCafeKid]

Preload, compression and rebound damping are all adjustable. But even some right side up had that as well. A true cartridge fork like the ones on my ZRX are fully adjustable. Many modern USD forks actually have a step further and have high a low speed damping valves that are adjustable independently of each other. The biggest factor however is like GS1K said, less unsprung weight, and generally shorter suspension travel meaning the bike can be lower to the ground to keep the CG lower for more lightweight and neutral handling feel.

[tkent02]

And the whole thing is more rigid, as most of the length is a thick cast outer tube, not a springy little pipe.

[bwringer]

And the whole thing is more rigid, as most of the length is a thick cast outer tube, not a springy little pipe.

That's the main benefit. All else being equal (which it never is), I think the fork assembly also ends up a bit lighter if it's designed right.


Like any technology, there are trade-offs -- my take on it is that USD forks are a right royal pain in the arse to work on, which you end up doing more often, since the sealing surfaces are down closer to the road and thus quite prone to damage.

I have a whole separate rant on the idiocy of naked fork tubes -- no idea why this has remained fashionable for 40 years.

[1980GS1000E]

Much more rigid. The triple clamp area, where all the force is concentrated, is stronger due to the fat outer tubes. There is much less twisting force at the wheel so the thinner inner tubes are fine.

[cowboyup3371]

Thanks folks. I think I understand but probably will need to experience the difference to fully get it.

[TheCafeKid]

That's the main benefit. All else being equal (which it never is), I think the fork assembly also ends up a bit lighter if it's designed right.


Like any technology, there are trade-offs -- my take on it is that USD forks are a right royal pain in the arse to work on, which you end up doing more often, since the sealing surfaces are down closer to the road and thus quite prone to damage.

I have a whole separate rant on the idiocy of naked fork tubes -- no idea why this has remained fashionable for 40 years.

+1. Full cartridge forks in general are a larger PITA to work on but the adjustability/tuneability IMO is well worth the trade off.
As to stiffness, most modern USD forks are absolutely massive compared to our spindly little GS forks. But with some exceptions, the resistance to lateral flex isn't that much greater. It helps tremendously that basically what would be the lower of our forks is at the top on USD. But the walls of the tubes themselves often aren't that much thicker. But they're also generally quite a bit shorter, so there's less "leverage" to cause flex. Think of it kinda like this. If you had a length of steel rod that was 12" long and you had one end anchored to a fixed point and then pushed on the very tip of the un-anchored end, it would flex. But if you had a rod of the same diameter that was only 9" long and did the same thing it would take more effort to push the rod to get the same amount of flex out of it. Then up the gauge of the second rod, even slightly and it takes more effort still. It's a little goofy I guess but things like that make more sense to me in simple terms..cause I'm simple

However, greater rigidity isn't always better once you get to a certain point. Granted its at the most extreme level but Ducati has found that point with their last GP bike. One of the major complaints from Nicky Hayden and Valentino Rossi about their bikes were that both the frame and fork set were so stiff, when cornering the bike had no "feel" in the front end. Basically they'd press the bike to its ragged edge and not be able to "feel" when they were at the edge of traction often times resulting in them washing out and lowsiding..

When cornering, the suspension doesn't work as effectively because the fork is designed to move up and down when acted on by forces on a relatively vertical plane. When heeled over, the fork is no longer vertical, but many of the input forces are still attacking the wheel from a vertical plane. The suspension won't as effectively absorb that input, and thus it is transferred to the frame. If the frame is overly stiff, it will not comply either, and traction can be broken. At the speeds and G's those guys are pulling, a millisecond of broken traction can be the difference between that really fast tight line and winding up in the sand and hay bales on your head.
Truly the engineers, techs and designers of these components amaze me, and while I do love an engine that puts out ridiculous torque and HP, I've found that I'm much more interested in learning the art of suspension tuning and the relationship of those components to the frame and such....

[cowboyup3371]

Thanks Josh. How did you learn a lot of this stuff? I'm sure the answer is reading it, but maybe I'm not using the right search terms or something. That, and I'm a gotta see, touch, and experience it kind of person when it comes to anything with vehicles. Since I don't have the background behind me, I feel lost with this stuff.

[TheCafeKid]

Thanks Josh. How did you learn a lot of this stuff? I'm sure the answer is reading it, but maybe I'm not using the right search terms or something. That, and I'm a gotta see, touch, and experience it kind of person when it comes to anything with vehicles. Since I don't have the background behind me, I feel lost with this stuff.

Well reading for sure, but also listening to guys who know a whole hell of a lot more about it than I do, and have probably forgotten more about it than I'll ever know. My buddy Dave is one of those sources. He built race bikes down in south Africa way back in the hay day of these GSes and their counterparts. I could sit and listen to his tales from their "Breakfast Run" (which often times amounted to illegal road racing in canyons and mountains) rides and such. Those guys had balls of steel and air for brains but a lot of em were racers that were just in it for the fun/sport and worked day jobs during the week.

[bwringer]

However, greater rigidity isn't always better once you get to a certain point. Granted its at the most extreme level but Ducati has found that point with their last GP bike. One of the major complaints from Nicky Hayden and Valentino Rossi about their bikes were that both the frame and fork set were so stiff, when cornering the bike had no "feel" in the front end. Basically they'd press the bike to its ragged edge and not be able to "feel" when they were at the edge of traction often times resulting in them washing out and lowsiding..

I'm not saying I'm any sort of rotund Rossi or have any inkling of the forces he deals with... but when I installed a Tarozzi fork brace on my GS, I found that I really missed all the feedback I was getting from the fork flexing and wiggling all over the place. The front end just feels sort of numb with the fork brace installed.

Objectively, it's more stable and planted with the fork brace, but near the limits, it's harder to feel what's going on without all that activity. Very strange, but I've gone back and forth a few times, and the brace is currently sitting on my workbench.

[TheCafeKid]

I'm not saying I'm any sort of rotund Rossi or have any inkling of the forces he deals with... but when I installed a Tarozzi fork brace on my GS, I found that I really missed all the feedback I was getting from the fork flexing and wiggling all over the place. The front end just feels sort of numb with the fork brace installed.

Objectively, it's more stable and planted with the fork brace, but near the limits, it's harder to feel what's going on without all that activity. Very strange, but I've gone back and forth a few times, and the brace is currently sitting on my workbench.

Oh I totally understand you Brian. When I bought my ZRX it was quite a different feel in corners from a GS. Not just in the suspension bits but the frame. It's still a tube frame similar to our GSes but much larger tubing, and it's gusseted and braced to hell and back. I was used to a GSs frame acting a bit like another shock absorber when pushed pretty hard (and like you, I am definitely NOT Rossi or even a 70th cousin twice removed) and you get used to the feeling of the bike moving like that under you. You learn the feeling of when it's just some normal flex and when it's "Im wallowing horribly because you're riding like an idiot and if you keep it up I'm gonna dump you on your ass" kinda flex. The bike sorta talks to you. And you don't have to necessarily be going retarded fast to "hear" it.

The ZRX may talk to you too, but so far she's been awful quiet in that respect. It took me a good while to get used to how stiff it was. I almost counted on the frame of my GSes to eat up some of the imperfections in a road surface when cornering hard.
The Rex translates all of those into sharp sometimes jarring exclaimation points.
Before I really started to school myself of suspension set up and was just spinning dials and turning adjusters at random, the bike sucked. It chopped in the back on washboard corners, and it plowed in front and just overall acted like a stubborn bitch. In a long corner it wanted to stand up and lay back down and stand up etc. It fought you the entire time and it was both because I didn't know wtf I was doing with the damping adjusters and because the frame wasn't eating up any of that choppy road surface.
She's still not perfect, but probably as good as I'm going to get her with the stock valves and shocks... A damn sight better than she was at first.
Still haven't heard that frame talk to me, but I have a feeling that it wouldn't start yapping unless it was at speeds and g forces I'm simply not willing to explore on the street.

[Baatfam]

Oh I totally understand you Brian. When I bought my ZRX it was quite a different feel in corners from a GS. Not just in the suspension bits but the frame. It's still a tube frame similar to our GSes but much larger tubing, and it's gusseted and braced to hell and back. I was used to a GSs frame acting a bit like another shock absorber when pushed pretty hard (and like you, I am definitely NOT Rossi or even a 70th cousin twice removed) and you get used to the feeling of the bike moving like that under you. You learn the feeling of when it's just some normal flex and when it's "Im wallowing horribly because you're riding like an idiot and if you keep it up I'm gonna dump you on your ass" kinda flex. The bike sorta talks to you. And you don't have to necessarily be going retarded fast to "hear" it.

Yes, I get this completely...
My GS is like riding a living thing in the corners, especially corners that weren't level...it wasn't scary or unpredictable, just there...
With the Triumph, I don't get that feeling...

[1980GS1000E]

Interesting points, here. Personally, I think that less flex in the frame and suspension is a good thing. I can feel the front tire slip quite readily on my 2nd gen FZ1 and I like knowing that there is no "extra" feeling thrown in. That's on Michelin Pilot Powers prior to full warm-up. Complete feedback from the tire, very little guesswork as to what is fork flex and tire traction. On the GS, things feel more vague and I end up not knowing exactly what is going on down at the contact patch. That is with Bridgestones.

[Flyboy]

Interesting you guys should bring this up, and I tend to agree 100%, it is maybe not faster on the stopwatch but that feel of a "living thing" as Baatfam so aptly describes it, twiching and warning you when it has had enough of your idiocy is a lot more confidence inspiring and tends to encourage one to push the bike further to the limits as you know when it is getting close to letting go, unlike a rigid inert piece of machinery where there is no two way communication, so to speak, and you never really know when it is goig to dump you unceremoniously on your ass without warning, when it has had enough of ones Tom Foolery.
Interesting story, my mate and myself used to take the bikes down to a local track on closed days and sweet talk the guards to let us in with a cold six pack, and we used to thrash around the track to our hearts content.
He was riding a GS750ES at the time, and it was a short circuit track, lots of twistys, only two moderate straights so cc was never an issue.
Point is, I used to put up faster times on Jennifer than either he or I could on the 750ES, despite it being newer, more advanced and better suited.
But it just did not have that feedback, it just never "talked" to you like the G, thus I was reluctent to push it as far as I did the big G.
For the same reason, I also always ran Avon Road Runners on the back, although they were not as high performance as the Metzlers or Pirrellis of the day, they would let go earlier, but gradually, all the while you knew where the limit was, rear wheel slides with a touch of opposite lock were the norm, and quite safe and controllable, whereas the performance rubber would stick to stupid and insane angles, but when they let go, they let go with a vengeance and no warning.