Benifits of USD forks?

[TDC]

Can you explain why they would be less rigid for the same diameter? Personally I don't understand why there would be a difference either way

Also, I think the reason the unsprung weight is reduced is because the oil reservoir is shifted above the triple clamp and has hence become part of the sprung weight. So I don't think it's just to do with materials.

The USD fork has two bushes mounted at a fixed length of about 250mm apart in the upper tube, the lower bush is at the bottom of the upper tube and is quite poorly supported, the upper bush is at least at about the same place as the lower triple clamp. Under brakes the upper alloy tube bends quite readily in the areas between the clamps and below the lower clamp. Quite often now USD forks are common it usual for the upper tube to be bent but the lowers to be fine! Go push your USD fork equipped bike up against a wall and at a fast idle give it a little clutch and observe, with just a small amount of load the forks deflect quite easily.

With conventional forks the lower bush is at the bottom of the upper tube and the upper bush is at the top of the lower tube (slider) As braking load is applied the bushes move further apart and the lower bush moves closer to the source of load (axle) both actions making a more rigid assembly.

As for the oil reservoir both forks hold their oil in the same place, at the bottom of the fork.

In a USD for the brake mounting is elaborate and heavy compared to the conventional forks requirement to have a couple of small brackets on the lower tube, and thus negatively impacts unsprung weight. the balance of the fork assy tubes, internals, oil weighs similar amounts, hence its usually just modern design / materials / manufacturing methods that give USD forks any weight advantage if any.

[Cruisin' Craig]

I guess I'll have to take your word for it then. Thanks for the explanation.

[Cruisin' Craig]

Although I must say that I have now gone back to being quite confused as to why manufacturers are bothering with USD forks. One could be forgiven for coming to the conclusion that it's nothing more than the fashionable way to make sport-bikes.

[TDC]

Although I must say that I have now gone back to being quite confused as to why manufacturers are bothering with USD forks. One could be forgiven for coming to the conclusion that it's nothing more than the fashionable way to make sport-bikes.

Sort of but don't underestimate how much the freedom to put larger more capable cartridges has done to front end performance! For this reason alone the other trade offs are well worth it!

[Cruisin' Craig]

Sort of but don't underestimate how much the freedom to put larger more capable cartridges has done to front end performance! For this reason alone the other trade offs are well worth it!

I don't doubt their worth. Judging by the list of benefits given by that wikipedia link above, it sounds like exactly what my bike would need to get rid of it's handling woes.

[TDC]

I don't doubt their worth. Judging by the list of benefits given by that wikipedia link above, it sounds like exactly what my bike would need to get rid of it's handling woes.

If you mean for the very tidy looking GSXR would you not be best to make the stock units work better and keep such a classic looking right? As its very possible to get much improved performance out of them.

[HDTboy]

Go push your USD fork equipped bike up against a wall and at a fast idle give it a little clutch and observe, with just a small amount of load the forks deflect quite easily.

Funny, when I watched Rossi Gal's bike (K3 GSXR600 with right side up forks) being Dyno'ed, there seemed to be a lot more flex in the front end than when I saw Two Smoker's bike (2007 ZXR600 USD) doing the same thing.

With conventional forks the lower bush is at the bottom of the upper tube and the upper bush is at the top of the lower tube (slider) As braking load is applied the bushes move further apart and the lower bush moves closer to the source of load (axle) both actions making a more rigid assembly.

Other than manufacturing costs, why are the top bushes not mounted on the lower tube?

In a USD for the brake mounting is elaborate and heavy compared to the conventional forks requirement to have a couple of small brackets on the lower tube, and thus negatively impacts unsprung weight. the balance of the fork assy tubes, internals, oil weighs similar amounts, hence its usually just modern design / materials / manufacturing methods that give USD forks any weight advantage if any.

Here I was thinking aluminium tube was lighter than steel tube

[Cruisin' Craig]

If you mean for the very tidy looking GSXR would you not be best to make the stock units work better and keep such a classic looking right? As its very possible to get much improved performance out of them.

I would absolutely love to get much improved handling out of them! And thanks for the compliment.

I know that the 85 GSX-R750 was reputedly a little dodgy on the handling side, so I've been uncertain as to whether what I'm experiencing is the result of worn, under-performing suspension, or whether it's just the way the bike is.
If you have suggestions I'd be glad to hear them!

[R6_kid:Me]

I would absolutely love to get much improved handling out of them! And thanks for the compliment.

I know that the 85 GSX-R750 was reputedly a little dodgy on the handling side, so I've been uncertain as to whether what I'm experiencing is the result of worn, under-performing suspension, or whether it's just the way the bike is.
If you have suggestions I'd be glad to hear them!

Use it as a trade on a k4+ GSXR750 and you'll know what you're missing out on by not having USD forks

[Cruisin' Craig]

Use it as a trade on a k4+ GSXR750 and you'll know what you're missing out on by not having USD forks

Oooh! Them's fightin' words!

Besides, my bike is far too pretty to trade :-)

[TDC]

I would absolutely love to get much improved handling out of them! And thanks for the compliment.

I know that the 85 GSX-R750 was reputedly a little dodgy on the handling side, so I've been uncertain as to whether what I'm experiencing is the result of worn, under-performing suspension, or whether it's just the way the bike is.
If you have suggestions I'd be glad to hear them!

In 85 it was a fine thing when compared to what else was round. With 20 ish years of progress and 20 ish years of wear on the stock items improvement is not hard to find. The front responds very well to an upgrade, the rear responds well to replacing the damper and modernizing the linkage ratios.

PM me for pricing etc

[TDC]

Funny, when I watched Rossi Gal's bike (K3 GSXR600 with right side up forks) being Dyno'ed, there seemed to be a lot more flex in the front end than when I saw Two Smoker's bike (2007 ZXR600 USD) doing the same thing.

Other than manufacturing costs, why are the top bushes not mounted on the lower tube?

Here I was thinking aluminium tube was lighter than steel tube

"Seemed to be" did you measure it? Were you looking for flex in both cases? Which bike made more power? had one bike been bent before? etc etc My point was simply the design lends itself to lower levels of rigidity, not that all usd forks are inferior to conventional.

The manuifacturing costs would be significant, and would result in a heavier lower tube (thicker wall to hold the upper bush) which would negatively impact unsprung weight even more. It would be getting so high in the fork that a rider who tended not to "exercise" the suspension might find the top bush getting a little dry as well.

Aluminum is lighter than steel volume for volume, or about the same strength for weight. In alloy tubular sections compared to steel its wall thickness is superior which in well design applications can lead to a weight reduction over a steel part while exhibiting similar rigidity. As we were discussing the comparison of the steel tube and large alloy brake fitments of an USD fork compared to the alloy tube and simple brake fitment of a conventional fork and their relative effects on unsprung weight you were agreeing then

[FilthyLuka]

In 85 it was a fine thing when compared to what else was round. With 20 ish years of progress and 20 ish years of wear on the stock items improvement is not hard to find. The front responds very well to an upgrade, the rear responds well to replacing the damper and modernizing the linkage ratios.

PM me for pricing etc

If it was me (and it is, im buying a 86 GSXR750 this week) i'd just throw in a WP shock in the rear and be done with it...

[Biggles08]

two pages and no comments from skidmark???

[Robert Taylor]

Sort of but don't underestimate how much the freedom to put larger more capable cartridges has done to front end performance! For this reason alone the other trade offs are well worth it!

But the shameful thing is that with most of these forks they are still fitting only 20mm cartridges, most probably for cost reasons. Recent experience with road and track bikes is that 25mm gives much better benefits in tolerancing, braking performance and shim response. That is why most manufacturers of aftermarket cartridges have adopted 25mm and we have found that in our own switch from 20 to 25mm the benefits are very real.
A negative of USD forks is that because of the bushing positions in the upper tubes ( one at the bottom of the upper tubes and one above the lower triple clamp ) they deflect off centre due to the tubes being distorted under braking loads or abrupt bump deflection. That causes friction so it is not all a bed of roses......