Could use some thoughts...Not fj, but xj

[ApriliaBill]

I have a 900 Seca, the forks needed new seals, I put them back together today, dropped 10 oz.(recommended is 9.67oz) of 10w belray fork oil in, went to the bottom of the driveway and the forks bottomed out. Now... they didn't bottom out before the new seals, and I haven't added air yet, but felt they shouldn't bottom out, and air should be for fine tuning. What do you gentlemen think?

[Arnie]

Did you put the springs back in?   How about the preload spacers?
Most "air-assisted" forks start by having full extension and equalizing the air pressure before you add more air.
9.67 oz of oil doesn't sound like much. I assume that is for EACH leg, right?
What was the height of the oil compared to the top of the fork tube with springs out and fork fully compressed?
What I think is that you (or the manual you used)  fu*ked up, but I'm not sure exactly how.

Cheers,
Arnie

[ApriliaBill]

9.67 per fork, I put everything back in the same way it came out, no extra or missing parts. Everything I can find on the bike says 9.67 per fork...Even did the conversion 286 c.c.'s per fork. This has me stumped...

[SlowOldGuy]

Springs control the position (height, bottoming, etc) of the fork/bike. 

The oil controls the velocity of the fork travel and even new oil will not (can not) prevent bottoming caused by weak springs.

DavidR.

[ApriliaBill]

Springs control the position (height, bottoming, etc) of the fork/bike. 

The oil controls the velocity of the fork travel and even new oil will not (can not) prevent bottoming caused by weak springs.

DavidR.

But that's the odd thing, the front end was fine (except leaky seals) before I tore them apart, how could they lose three to four inches sitting in the garage for a few days?

[RichBaker]

Air space is a variable-rate spring that assists with bottoming resistance.....  1st thing I'd do is check the oil level. Should be around 130mm, no matter what bike it's on; springs removed, tubes fully compressed.

Before that, check preload. Should be 10 - 20mm under the bikes weight alone.  If there's too much, look for missing spacers....

[flips]

Hi there.

Its been a long while since I played with air assisted forks (20 yrs ago on an rz250) but the first thing that springs  to mind besides oil level is... are the top caps/valves sealed properly?....do the forks hold air pressure?

Hope this helps

Cheers

Jeff P

[WestOzXJR]

Here is some info from Ohlins which graphically demonstrates the impact of oil height on the compressibility of the air in the space contained in the upper fork tube...

[ApriliaBill]

I somehow found a XJ900 micro fische on dvd. I found the blow up of the forks, and it appears I didn't miss anything. Everything is correct. I just can't see letting the springs sit on a bench for a few day, they would lose enough spring to drop the front end down...

Before the rebuild the bike rode fine. I'm obviuosly going to have to take them apart again. The air spec is between 0-15lbs. This would tell me I should be able to set static height before I add air...

I guess if worse comes to worse I'll just order new springs...

[Arnie]

The springs will not have changed, unless you grabbed the wrong set of springs to re-install. 
You're missing something, probably the preload spacers.

Arnie

[andyb]

If you blow through your travel too quickly, you'll bottom.  Spring rate is not the only thing that controls bottoming... unless you live in phsyics experiments land, where cows are spherical and ice is frictionless.

If you live in the real world, you won't instantly bottom, but there will be some time while the damping works.  A sufficently stiff damping will prevent bottoming, but it'll be unridable on roads with bumps and such. 

Like anything else, it's a series of compromises.  But address the air pressure, because that will make quite a difference (until the fork seals let go, which was always a soft spot on air-assisted forks).

[fj1289]

How much air did you have in them before?

Keeping it simple - try adding 5 or 10 lbs of air...

[ApriliaBill]

Maybe I'm going about this wrong. Let me know what you guys think. Manu. Specs say 0-15lbs. To me this says the bike should be rideable at 0 pounds. I understand the bike, could use air, to tune the ride, which I will use, but the static height on the bike seems to be compromised.

Of course yesterday as I tried to add air, the valve snapped off in the fork, I was able to retrieve it, but need a new one to further the project. I love it when a minor cheap part holds up the whole project.

I guess what I'm saying is, I'm confident this bike should be able to ride and not bottom out even without air. I'm wondering if the p.o. knew the springs were weak and used air to carry the front end till he could sell it....

I'm going to pull this back apart, and use the blow up to reassemble, if the problem persists I'll get some progressive's to replace the original's.

Let me ask, the springs in there are not uniform springs, they get tighter spung as they get towards the bottom (I don't have them upside down do I?) does this sound correct???

[Arnie]

The variable spacing on the coils of the spring make it a "progressive rate" spring.  The close wound end is lower spring rate than the more widely wound end. It is a terrible shame that a company has called itself "Progressive Spring", thereby confusing many.
Obviously, it is impossible to know what a PO has done.  Does your manual give a spec for "free length" of the fork spring?  To be the OEM spings and as soft from sacking as you're describing, they'd be 5-10% shorter than spec.
15 PSI is only 1 atmosphere of pressure.  That's not a lot.  Once you repair the valve, try them at 15psi and then at 30psi to see if that makes much difference.  They likely won't bottom, but they may become pretty harsh.  If more air pressure improves them, I think I'd add more fork oil (in 10mm increments) and run with less (at rest) air pressure.
I still think you are missing preload spacers.

Arnie
 

[JCainFJ]

 I've just been to the Race Tech site and it looks like the stock fork springs for an XJ900 are only .440kg/mm (very soft). Race Tech recommends .984kg/mm fork springs for that bike and a 220lb rider. I've used RT recommended spring rates on 11 or 12 fork rebuilds and they have always been correct. So my recommendation on this is to order a set of propper rate springs (.95kg/mm is the closest they make for your XJ900)  part number FRSP S2938095. Set the fork fluid level at 130mm (to start) with the springs removed and fork collapsed. Set the preload so that you get 30 - 35 mm of static sag - that's the bike with a full tank of gas and you sitting on it. If you feel that you are gitting too much fork dive when braking hard, add 10mm more fork fluid at a time till you are happy. You should also look into a fresh set of shocks too. I have 2 XJ650RH Seca's, so I like those old bikes.