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Old 03-06-2013, 09:15 PM   #19
Phreaky Phil
Beastly Adventurer
Joined: Mar 2006
Oddometer: 1,922

Originally Posted by ontic View Post
Sorry for the thread drift R-dubb, just stirring the pot a little until your next post
A little bit of USD vs RSU ponderings.

Around the net in various discussions I've read of this being challenged in many examples, and in both claims- ie some USD's have been shown to have just as much flex as the conventionals they replaced- probably less twist, but this can be remedied with a fork brace on a conventional. Where they most certainly are stiffer is up top where they are clamped... however when they flex here (as they still do) you get stiction because of the bushes, whereas conventionals when they flex, even if they do flex more, don't suffer this the same flex-stiction problem.
Also, in a crash or big front impact, rather than the conventional fork routinely bending the tubes at the bottom of the triple clamp (tubes being the easiest to replace or even straiten), the USD often bends at the sliders in the triple clamp (destroying the fork) while also often bending at the tubes where they meet the sliders, and/or might be more likely to damage the frame/headstock because of all this stiffness... If you get lucky and just slightly bend the tubes where they met the sliders, then the USD fork is still buggered and gets stiction until the tubes/lowers are replaced, whereas a conventional fork bent up top there will often keep you riding and get you home or round the other side of the world... maybe without you even realising.

The less unsprung weight claim is also often in dispute (and often less emphasised as the main advantage- stiffness is)- I've seen a few broken down weight by weight comparisons of conventional and USD forks (including clamps) of the same tube diameter, and in these cases the USD was often heavier in total, and heavier in unsprung mass- however, of the few cases that went into something approaching adequate detail, I didn't see any inclusion of oil weights within this break down (correct me if I am wrong, but USD oil is sprung weight and conventional are unsprung?- on a WP50 fork, not a small omission!).

So, advantages IMO:
'unsprung weight' depends on a case by case basis, but potentially not a huge advantage either way. (btw, it would be interesting to get some actual weights on these, I can pull apart and weight my spare WP50's when I return home in a month or so).
Stiffness yes, definitely, however if flexing when it does occur causes stiction then this needs to be weighed up against the stiffness 'advantage'- and this and all the other positives and negatives (of both kinds) speaks to the 'just a better design' claim- it depends on the details and the application.

I'd emphasise a third advantage as has often been stated here and elsewhere, and I think it is the main one (when talking RSU and USD apples and apples- ie both good forks), modern USD's are modern. They are pretty much all that is being made on the top end, will be newer and will be the home of ongoing tech improvements, have more parts for longer, etc, etc.
Using the newest best forks are probably the best forks to use...

However, I question the performance advantage in principle and specifically for a heavyweight bike like an airhead- well set up I don't think there would be much in it to the point it doesn't matter at all.
Even if I do expect a fine set of 4860's properly set up to (slightly?) outperform (at least in some types of use) my WP50's once they are dialed in, there are other considerations- if I blow a seal in the middle of nowhere in my country or somewhere else, I like it that I keep my oil in my fork for longer and have a much smaller chance to dump it over my caliper and rotor- and I'm prepared to accept positives and negatives and compromises along these lines because of how and where I intend to use it.

Regardless of all this, I do think it comes down to Aesthetics and Parts.
Good modern USD's win for parts and seem a more sensible option I rekon.
Aesthetically either look good to my eye but conventional look a little better for me.

At the moment I plan on keeping and using my G/S until the WP5060 and WP4354 and WP4860's forks are ALL considered too old to service, so just as long as I can stockpile enough parts for a decade or two of use it doesn't matter much to me- I'll probably have to change forks again someday (and by this time they might be making conventionals again)
I was speraking with Gary Emig of Emig Racing about clamps and offsets for my DR as I would like more stability on sand tracks and his opinion is USD's are the go. He reckons there is to much flex at the wheel with any RSU forks and the front wheel goes where it wants in the sand. He reckons they change all the XR650R's to USD. They have a USD clamp kit for DR650s. I quized him on the conventionals being smoother, he says they have no problems with the Showa's they use.
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