Australian Design??

[dolly3900]

Not sure if this a Workshop thread or for here, so will post here and await to be moved if need be.

Why Upside Down (USB) Forks?

Now I know what they are ( I think), Instead of the upper part of the fork travelling into the lower part to give the suspension travel, with the compression parts in said lower part, the way I understand it is that the upper part houses the compression systems and the lower part travels into this instead.

Assuming I have this right, what is the advantage of this USD system?
I see a lot of bikes boasting about USB forks and such, saw a bit on a new 125 race replica (KTM I think), but I fail to understand the benefits.

OK, so I am assuming that the un-sprung mass of the wheel/tyre/lower fork would be less, giving better rebound response, but with the weightier part of the shocks and dampers being now higher up, the centre of gravity would change to the detriment, so is there that much of a difference, or is it just bragging rights?

I am genuinely Interested in your input here and Tef's inevitable wall of text response

Cheers

Mike
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2002 Rieju RS2-50 (Sold, spares or repair), 2010 Suzuki DR-125SM (Traded for ->), 2007 Honda CBF 600 (Sold), 2010 Suzuki V-Strom 1000 (Briefly, now returned), 2007 Suzuki Intruder 1800m (Even More Briefly, traded in for ->), 2014 Suzuki V-Strom 650

[Northern Monkey]

As you have already identified, it is to reduce the unsprung mass.

Less unsprung mass gives improved grip.
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[c_dug]

He did make an interesting point about the heavier bits now being up top though, is it enough to counter this change in CoG?
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[daemonoid]

[nowhere.elysium]

[dolly3900]

[P.addy]

USD = sexual looking.
RWU = you peasant.

[lihp]

[stinkwheel]

It's to keep spare parts sales up. WIth RWU forks they were hardly shifting anything near enough stanchions and oil seals. Some unscrupulous motorcycle owners were fitting dust deflectors or even gaiters!

So they started putting the bits that go rusty nearest the road and have all the oil resting against the seal so a small leak very rapidly becomes a large leak.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Teflon-Mike]

USD has the steel stanchion at the bottom & the alloy slider full of oil at the top.
It's all rather muteable whether turning them upside down has ANY effect on unsprung mass or center of gravity, even less handlng.
Origially developed for MX bikes where hugely long forks gave a lot of bending leverage over the slider bushes, they allowed a longer slider, without it reducing suspension travel, hence allowed slider bushed to be spaced further apart, hence offer greater leverage against bending.
Migration to sports bikes, came in the early '90's; Fashion & Marketing played a significant part. Benefts of higher response from increasing sprung to unsprung mass ratio was much vaunted; along with original advantage of longer slider relieving loadings in bending, that also reduces 'sticktion' tendency for slider to resist sliding when under bending load.
But, '93 Fire-Blade that set new standard for sports-bike handling, used RWU forks.... and vaunted THEM as 'superior'... using larger diameter tubes to make forks stiffer, instead of a more complex design, so thier RWU forks could be as light and have better Sprung/unsprung mass ratio and overall, offer same performance with less weight.
So bottom line is.... makes little odds, what's important is fork quality, not construction; would you rather have the 'trendy' USD forks from a 2010 SKY-Jet or the untrendy RWU forks from a 1986 Honda NS125? Built down to a price USD's with crappy damping mechanism and less than precice slider bushes ent gonna be as good as a well made RWU fork with decent damping mech and close tolerenced slider bushes, are they?

Personally I rather liked the Britten Neuvo-Girder... shame all we got for a proddy bike was the Saxon Swing-Arm rip-off BMW called 'Tele-Lever', but there you go.
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[dolly3900]

[lihp]

Also, seal change on a RWU fork can be done without seperating the fork leg.

Try doing that with an USD fork

Never mind being able to change springs etc without removing the fork from the bike, or even just the hassle of trying to get an USD fork back together without a spring compressor.

RWU forks are far quicker and easier to repair than USD
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[map]

Sort of topic related my lad wanted to rebuild a bike with USD forks.
He then saw the cost against RWU forks and questioned why.

I gave him the weight reduction spiel and that it's more performance/sports bike orientated. So now he's getting standard forks as easier to deal with.
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[weasley]

Don't forget that USD forks have the wider, and stiffer, part of the assembly at the point where the bending forces are highest... around the yokes.
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[MCN.]

Stiffness and you can accommodate bigger cartridges.

[Itchy]

[Kickstart]

Hi

They were originally claimed to reduce unsprung weight, but not convinced. With a conventional fork the lower part might be thicker walled but is relatively light cast alloy. While with a USD fork the lower is the heavier and longer steel tube. Add that to hassle to brace it (can't use a conventional fork brace) and the odd mounts needed for the brake calipers and I suspect they are worse for unsprung weight.

Big advantage seems to be that with conventional forks the point where stress is at its highest is just under the bottom yoke. With USD forks this also applies, but at this point the forks have both the fork inner and outer providing strength.

All the best

Keith
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[daemonoid]

[daemonoid]