Benifits of USD forks? [Archive]

Winter

28th January 2008, 17:40

My new bike has them, don't think they are standard with the model but interested in the advantages/disadvantages of them.

In amongst all this USD hilarity - I'm not certain i know the true advantages either.

I always guessed it was to help keep road spooge off the forks - to help improve fork seal life?

I wouldn't mind a sensible, honest answer to this question.
Dont know why I'm asking here then!

bane

28th January 2008, 17:43

my (admittedly uninformed) opinion is USD forks reduce unsprung weight and probably something else important....

GSVR

28th January 2008, 17:57

I think Rigidity is the major gain. But someone like Robert Taylor will have accurate info on the subject.

You could always type a few searches into Google like USD + Advantages + Forks

Drum

28th January 2008, 18:00

My understanding was that they are more rigid and therefore less deformation under heavy braking. Never had them tho, so shouldn't really comment.

GSVR

28th January 2008, 18:02

http://en.wikipedia.org/wiki/Motorcycle_fork

Skunk

28th January 2008, 18:05

Cheers Garry.
On many sport bikes, this system is inverted, with the spring/damper unit clamped to the yoke while the sliders are at the bottom of the assembly. This is done for two reasons: to reduce unsprung weight by having the heavier components be suspended, and to improve the strength and rigidity of the assembly by having the bulkier and stronger component being directly supported by the pivot. Such a system is referred to by many as upside-down forks or USD for short.

R6_kid

28th January 2008, 18:14

skidmarks answer: the zxr has them and the cbr doesnt, hondas are gay therefore USD for the win.

TDC

28th January 2008, 18:17

My new bike has them, don't think they are standard with the model but interested in the advantages/disadvantages of them.

The whole USD fork question is quite interesting. They are probably not quite as rigid (bending wise) as a conventional fork size for size and as a result tend to bigger in diameter (and as a result more rigid). More difficult to mount brakes on. The unsprung weight in some instances can be a little lighter, but only due to quality of design / materials. The clamping area on the triple clamps is much improved which tends to improve the resistance to a head shake twisting the front end out of alignment. The real benefit is when fitted a cartridge damper it does not suffer the same undesirable critical flow problems that conventional forks do when fitted with a cartridge damper. Simply the oil on the bottom of a conventional fork has to pass between the cartridge and the fork tube in the conventional design in comparatively large volumes, this imparts undesirable damping "artifacts" in to the action of the fork and limits the maximum size of the cartridge that can be fitted for a given fork tube diameter. The USD fork does not have these problems and as such was used in racing, them became fashionable, now its probably more expensive to make a conventional fork due to the numbers game.....

Cruisin' Craig

28th January 2008, 18:44

The next question that occured to me was: why do we necessarily want to reduce the unsprung weight?

I found that this wikipedia article explaned the reasons very well.

http://en.wikipedia.org/wiki/Unsprung_weight

Fastchick

28th January 2008, 18:47

they look cool:sunny:

Cruisin' Craig

28th January 2008, 18:52

..... They are probably not quite as rigid (bending wise) as a conventional fork size for size and as a result tend to bigger in diameter (and as a result more rigid). More difficult to mount brakes on.

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

The unsprung weight in some instances can be a little lighter, but only due to quality of design / materials.....

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.

HDTboy

28th January 2008, 19:33

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.

Oil reservoir? Please explain.

Cruisin' Craig

28th January 2008, 19:42

Oil reservoir? Please explain.

I mean that with UPS forks the part where all the damping oil is stored is up top above the clamp instead of down the bottom where it would otherwise contribut to the unsprung weight.

TDC

28th January 2008, 19:50

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

28th January 2008, 20:07

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

Cruisin' Craig

28th January 2008, 20:13

TDC

28th January 2008, 20:18

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

28th January 2008, 20:23

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

28th January 2008, 20:40

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

28th January 2008, 20:43

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:stupid:

Cruisin' Craig

28th January 2008, 20:47

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

28th January 2008, 21:25

Cruisin' Craig

28th January 2008, 21:41

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

Oooh! Them's fightin' words!:2guns:

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

TDC

29th January 2008, 04:15

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

29th January 2008, 04:32

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:stupid:

"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 :bash: 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 :yes:

FilthyLuka

29th January 2008, 08:49

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

29th January 2008, 09:27

two pages and no comments from skidmark??? :blink:

Robert Taylor

29th January 2008, 17:12

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......

HDTboy

29th January 2008, 17:34

Would an extra bush between the upper and lower ones fix it?

TDC

29th January 2008, 19:54

Would an extra bush between the upper and lower ones fix it?

Maybe from a rigidity perspective, but three bushes on a pair of disimilar tubes in a bending moment environment spells lots a stiction, so sadly no....

HDTboy

29th January 2008, 19:57

I should've known that

TDC

29th January 2008, 20:18

I should've known that

Don't beat yourself up! Most of the time I enjoy your questions, except for the one that require lotsa typing late at night :bleh:

HDTboy

29th January 2008, 20:55

Is it late yet?

I understand that cartridge forks are better than damper rod forks, but I don't understand why. And where do I mount a fork oil reservoir on my bike?

Pussy

29th January 2008, 21:03

Is it late yet?

I understand that cartridge forks are better than damper rod forks, but I don't understand why. And where do I mount a fork oil reservoir on my bike?

You're a cruel bugger, Gav!! :bleh:

TDC

29th January 2008, 21:20

You're a cruel bugger, Gav!! :bleh:

No Puss he's a count!

TDC

29th January 2008, 21:20

You're a cruel bugger, Gav!! :bleh:

Actually there is no 'o' in that is there.....

JD Racing

29th January 2008, 22:56

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.

Not all USD Showa forks have fixed bushes, on most of their forks the upper bush is mounted on the stanchion so the bush moves up the slider as the forks compress. KYB, Soki and Ohlins use the fixed bush, Marzocchi use a sliding bush as do WP on the Roma fork.

JD Racing

29th January 2008, 22:58

JD Racing

29th January 2008, 23:04

The real benefit is when fitted a cartridge damper it does not suffer the same undesirable critical flow problems that conventional forks do when fitted with a cartridge damper. Simply the oil on the bottom of a conventional fork has to pass between the cartridge and the fork tube in the conventional design in comparatively large volumes, this imparts undesirable damping "artifacts" in to the action of the fork and limits the maximum size of the cartridge that can be fitted for a given fork tube diameter. The USD fork does not have these problems and as such was used in racing, them became fashionable, now its probably more expensive to make a conventional fork due to the numbers game.....

Why is the oil flowing into the fork from the cartridge comparitively large compared to a USD fork, how much damping does this create, is it measurable on a dyno?

JD Racing

29th January 2008, 23:17

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......

True, but all forks flex under braking and turning whether they are USD or conventional, it's not ideal from a stiction point of view but is a necessary evil for feel, if you made a set of forks that didn't flex at all nobody would like them.

skidMark

29th January 2008, 23:56

skidmarks answer: the zxr has them and the cbr doesnt, hondas are gay therefore USD for the win.

Pretty much.

TDC

30th January 2008, 05:18

Not all USD Showa forks have fixed bushes, on most of their forks the upper bush is mounted on the stanchion so the bush moves up the slider as the forks compress. KYB, Soki and Ohlins use the fixed bush, Marzocchi use a sliding bush as do WP on the Roma fork.

Agreed, it was the majority I was representing for the purposes of discussion.

TDC

30th January 2008, 05:19

Has it never been possible to fit 25mm cartridges in conventional forks?????

Technically yes but practically not so easy see next

TDC

30th January 2008, 05:26

Why is the oil flowing into the fork from the cartridge comparitively large compared to a USD fork, how much damping does this create, is it measurable on a dyno?

I didn't say the oil flowing from the cartridge is any different on either solution.

The lower slider on a conventional fork is largely full of oil that is displaced by the entry of the upper tube when the fork compresses, this very thing thing is what a damper rod fork relies on to work. However when you put a cartridge in there unless you have a lot of clearance round the cartridge and inside wall of the upper tube the large volume that has to make it way in and out of the lower tube via the upper tube has to squeeze past the cartridge. If this space becomes even slightly restrictive the dyno will show it.

Robert Taylor

30th January 2008, 07:27

True, but all forks flex under braking and turning whether they are USD or conventional, it's not ideal from a stiction point of view but is a necessary evil for feel, if you made a set of forks that didn't flex at all nobody would like them.

It was a negative by product of fork flex itself that I was pointing out, not the very neccessity to have flex, and indeed the can of worms that it presents designers.

Shaun

30th January 2008, 09:41

I didn't say the oil flowing from the cartridge is any different on either solution.

The lower slider on a conventional fork is largely full of oil that is displaced by the entry of the upper tube when the fork compresses, this very thing thing is what a damper rod fork relies on to work. However when you put a cartridge in there unless you have a lot of clearance round the cartridge and inside wall of the upper tube the large volume that has to make it way in and out of the lower tube via the upper tube has to squeeze past the cartridge. If this space becomes even slightly restrictive the dyno will show it.

So, are you saying that, if a Bigger Cartridge O/d was fitted, this would restrict oil flow?

jrandom

30th January 2008, 09:44

Point.

Counterpoint.

Stir.

My, what an odd direction for a suspension thread to take.

:corn:

TDC

30th January 2008, 19:32

I think he's saying that there is that potential if you don't know what you're doing, aren't a competent machinist, and only know enough about suspension to be dangerous.

Yup that works for me.

We started off by discussing why USD forks are better, which is the reason they made it to the uber buck teams, then to fashion<_<, and finally to mas production.

The reason is IMHO not unsprung weight, or rigidity, it is oil flow in the bottom of the inner tube in a conventional fork limiting how much of the tube can be occupied by a cartridge. In a USD fork if you wanted to you could use 100% of the ID for the cartridge, you simply cant with the conventional design.

Pussy

30th January 2008, 19:52

Yup that works for me.

We started off by discussing why USD forks are better, which is the reason they made it to the uber buck teams, then to fashion<_<, and finally to mas production.

The reason is IMHO not unsprung weight, or rigidity, it is oil flow in the bottom of the inner tube in a conventional fork limiting how much of the tube can be occupied by a cartridge. In a USD fork if you wanted to you could use 100% of the ID for the cartridge, you simply cant with the conventional design.

That's a great explanation, TDC. Even Stevie Wonder should be able to see that

JD Racing

30th January 2008, 21:21

Would an extra bush between the upper and lower ones fix it?

WP have done this on some of their MX forks, I think it started life as a mod that was being done by supermotard riders.
As long as you make sure it's getting plenty of lubrication it doesn't cause a great deal of stiction.

JD Racing

30th January 2008, 21:28

So, are you saying that, if a Bigger Cartridge O/d was fitted, this would restrict oil flow?

If you fit say a 20mm cartridge in a 35mm fork then it does start to act like a damper rod.
I've fitted plenty of 25mm cartridges in 43mm conventional forks and not noticed any damping resulting from doing so, I'll have to dyno a set with no damper rod connected to detect this significant amount of damping that needed the whole set up turning upside down.

TDC

30th January 2008, 21:38

If you fit say a 20mm cartridge in a 35mm fork then it does start to act like a damper rod.
I've fitted plenty of 25mm cartridges in 43mm conventional forks and not noticed any damping resulting from doing so, I'll have to dyno a set with no damper rod connected to detect this significant amount of damping that needed the whole set up turning upside down.

What brand of cartridges? Most modern 25mm cartridges have a usually much larger than the diameter of the cartridge fitments for holding the top bush and allowing for holding the cartridge during fitting with some form of tool. In this instance many brands of cartridge would need their top fitments adjusted significantly to ensure no stray damping. Your average 25mm cartridge only has on average 4.8 ish mm per side clearance in a 43mm tube or 3.8 ish mm per side on a 41mm tube, given the length of the cartridge tube the velocity of the oil etc it doesn't take much hanging over the top of the cartridge tube to cause problems....

Mental Trousers

30th January 2008, 21:40

WP have done this on some of their MX forks, I think it started life as a mod that was being done by supermotard riders.
As long as you make sure it's getting plenty of lubrication it doesn't cause a great deal of stiction.
I would've thought there'd be more stiction because the tube is trying to bend (as all fork tubes do) and you've got more bushes trying to deal with that. Shouldn't stiction be significantly higher with 3 bushes in this case??

JD Racing

31st January 2008, 00:28

I would've thought there'd be more stiction because the tube is trying to bend (as all fork tubes do) and you've got more bushes trying to deal with that. Shouldn't stiction be significantly higher with 3 bushes in this case??

On a supermotard fork you've got a fork that's extremely long even when shortened. Add a super sticky tyre and huge disc, jam the brake on under those circumstances you get a significant bend in the fork, this can bind the fork up completely, if you add a 3rd bush it can give enough support to limit that bend and allow the fork to move. If you had zero side load then it would have more stiction in the circumstances above it has less stiction.

JD Racing

31st January 2008, 00:30

What brand of cartridges? Most modern 25mm cartridges have a usually much larger than the diameter of the cartridge fitments for holding the top bush and allowing for holding the cartridge during fitting with some form of tool. In this instance many brands of cartridge would need their top fitments adjusted significantly to ensure no stray damping. Your average 25mm cartridge only has on average 4.8 ish mm per side clearance in a 43mm tube or 3.8 ish mm per side on a 41mm tube, given the length of the cartridge tube the velocity of the oil etc it doesn't take much hanging over the top of the cartridge tube to cause problems....

Do a calculation of the displaced volume and the area available for flow and see how high a velocity you would need to generate damping.

SixPackBack

31st January 2008, 06:11

Stiction is obviously an issue and so why do manufactures not utilise linear roller bearings? this would completely eliminate stiction.

Mental Trousers

31st January 2008, 08:15

At a guess I'd say cost and the fork tube still needs to bend a little otherwise you lose feel.

FzerozeroT

31st January 2008, 08:47

I was going to say that a roller bearing would only touch at one point and make it vunerable to damaging the stanchion, but I suppose you could use a curved one like boats use, but once you have your magnificent $300 worth of bearings you have added a kilo of weight, made the fork outers cost $10,000 to build (needs to contain the bearings with very little excess space, created a large pool for fluid to circulate through AND you now have the play in the bearings, no matter how infitesimally small to deal with, hmm, I think I'll be searching for lower friction bushes :P

SixPackBack

31st January 2008, 09:01

Ocean1

31st January 2008, 09:32

Stiction is obviously an issue and so why do manufactures not utilise linear roller bearings? this would completely eliminate stiction.

At a guess I'd say cost and the fork tube still needs to bend a little otherwise you lose feel.

I was going to say that a roller bearing would only touch at one point and make it vunerable to damaging the stanchion, but I suppose you could use a curved one like boats use, but once you have your magnificent $300 worth of bearings you have added a kilo of weight, made the fork outers cost $10,000 to build (needs to contain the bearings with very little excess space, created a large pool for fluid to circulate through AND you now have the play in the bearings, no matter how infitesimally small to deal with, hmm, I think I'll be searching for lower friction bushes :P

For general engineering considerations the types of linear bearing that can work with a cylindrical shaft do place a hugh point contact load on the shaft surface. The bearings themselves aren't that much heavier than a conventional bush but the shafts tend to be large, solid or very heavy wall, hardened and prone to damage. You can get reciprocating ball bearings that run in a grooved or non-cylindrical shafts, but then you've got seal issues.

There's some new bearing materials that may be quite good for such applications, scintered ceramics is one.

I'm not sure what would feel different if there was zero flex, or if that difference would be beneficial or detrimental to handling. I'm guessing eliminating an uncontroled lateral variable would be a good thing, same with less stiction.

These problems are much less evident with leading/trailing arm arangements, which can have a wonderful lack of initial friction, it's just that there's structural issues there too.

JD Racing

31st January 2008, 19:55

I didn't say the oil flowing from the cartridge is any different on either solution.

The lower slider on a conventional fork is largely full of oil that is displaced by the entry of the upper tube when the fork compresses, this very thing thing is what a damper rod fork relies on to work. However when you put a cartridge in there unless you have a lot of clearance round the cartridge and inside wall of the upper tube the large volume that has to make it way in and out of the lower tube via the upper tube has to squeeze past the cartridge. If this space becomes even slightly restrictive the dyno will show it.

Having done the calculation I can assure you that the cartridge top generates absolutely zero damping.