Benifits of USD forks?

[TDC]

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]

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]

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]

Point.

Counterpoint.

Stir.

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

[TDC]

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]

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]

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]

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]

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]

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]

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]

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]

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

[Mental Trousers]

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

[FzerozeroT]

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