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Old 03-31-2013, 08:29 PM   #1
Hootowl OP
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Location: Bend Oregon
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Fork Springs Question

The forks on the 990 I bought late last year as too harsh.
I've changed all the settings to "comfort" via the owner's manual. Still too harsh.
The seller said that stiffer springs were installed. The purchase came with the OEM springs as well.

With a couple of days of semi-free time I decided to change the fork oil and swap the springs.
The stiffer springs are 55MM shorter than OEM plus there was 160MM of fork oil in the tubes.

Here's my question: Do I go with the OEM springs or would the stiffer springs work well with 100MM of oil?
I could try both sets of springs, vary the oil levels and play with the fork adjustments but its spring & I'd rather be riding.
Looking for a reasonably compliant front end until I can afford to have the valving redone professionally

richard
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Old 03-31-2013, 10:45 PM   #2
Head2Wind
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I'm no suspension guy, but what I do know about it I suggest that you try to explain the "too harsh" with more details before anyone would be able to supply any useful information.

Things that are good to know:
1. weight of rider (you)
2. speeds ridden at (are you a AAA desert rider or ??)
3. terrain ridden (soft desert or hard surfaces and rocks?)

these are a start, but honestly there are way too many variables to suggest much as far as spring rates and damping changes.

What I can offer is that in general, unless you are a 160 lbs rider, the OEM springs will most likely be too soft. 100 to 110 mm air gap running 5w Redline suspension fluid is where I'd start. Its easier to start at 110mm and ADD oil through the bleed screw hole to help resolve harsh bottoming on G-outs or big hits/jumps. You can measure the static and race sag and calculate the necessary spring to get the right numbers. RaceTech has a online calculator that will get you very close when choosing a correct spring.

In my experience, a too soft of a spring that has a bunch of preload cranked in to get the race sag in the ball park will tend to not be "plush" on hard fast sharp hits, will bottom too frequently. It seems counter intuitive, but a stiffer spring can be more "plush" than a softer spring.

One question, did the previous installer of the aftermarket spring make a take-up spacer to make up for the free length difference? If not, then this could be part of the problem as the forks will be down in their stroke early. Combine that with the stated 160mm air gap and you would be bouncing into the anti-bottoming cups frequently

If its a early bike, then disassemble and remove the mid bushing. This will make a big difference.

For any of the aftermarket springs, make sure that the spring guide fits inside the spring without sticking. Most of the time the stiffer springs have smaller ID and therefore require the spring guide to be machined/turned down to accommodate.

Pull the dust seals and clean/lube the seals to help reduce sticktion, or replace with KYB or your choice of super slippery seals.

Prop the bike up with the front tire off the ground, loosen all but the top clamp bolts to include the lower triple clamp, lower axle clamps and the axle nut, then drop the front back onto the ground and pounce on it to compress the suspension as best as possible a couple of times. Prop back up and then torque all fasteners to KTM specs.

Make sure that the rebound needles move. Frequently the spring will become rusty from water intrusion, plus it adds to the debris in the area to make the rebound needle stick at full on. If this is the case, the forks will stay down in their travel and cause the forks to feel harsh. It also effects the compression slightly (fluid flows through the rebound needle in both directions of travel)...

Fun stuff for sure. Not necessarily in this order, I recommend making sure that things are lined up, get the right preload in there and set the static then race sag, make sure that the spring guides are not sticking, change the oil out for fresh, set at 110mm, make sure the adjusters are working correctly and ride it.
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Old 04-01-2013, 03:51 AM   #3
rossguzzi
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For not a suspension guy you sure have some good tips there.

Can I ask (as I think its related) If you want to pull a spring and check it, can it be done with the forks on the bike? And to change oil too?
I am guessing they need to be removed.

Cheers.
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Old 04-01-2013, 10:41 AM   #4
Hootowl OP
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[QUOTE=Head2Wind;21080382]I'm no suspension guy, but what I do know about it I suggest that you try to explain the "too harsh" with more details before anyone would be able to supply any useful information.

Things that are good to know:
1. weight of rider (you)
2. speeds ridden at (are you a AAA desert rider or ??)
3. terrain ridden (soft desert or hard surfaces and rocks?)

Incredibly useful and instructive post. THANK YOU!
Your questions:
1. 175-180 plus gear
2. 70 yr old old fart, slow but not too slow
3. Mostly hard terrain

Too harsh is hitting a rock on dirt or a ripple/ridge on a paved road and feeling a strong jolt

One question, did the previous installer of the aftermarket spring make a take-up spacer to make up for the free length difference?

No spacer


If its a early bike, then disassemble and remove the mid bushing. This will make a big difference.

'07 990

For any of the aftermarket springs, make sure that the spring guide fits inside the spring without sticking. Most of the time the stiffer springs have smaller ID and therefore require the spring guide to be machined/turned down to accommodate.

The stiffer springs are the same ID as OEM

Richard
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Hootowl screwed with this post 04-01-2013 at 10:49 AM
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Old 04-01-2013, 10:42 AM   #5
Hootowl OP
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[QUOTE=rossguzzi;21080982]

Can I ask (as I think its related) If you want to pull a spring and check it, can it be done with the forks on the bike?

Yes

And to change oil too?
I am guessing they need to be removed.

Correct
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Old 04-01-2013, 10:34 PM   #6
Head2Wind
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+1, what he said
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Old 04-01-2013, 10:47 PM   #7
Head2Wind
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I recommend making a spacer to make up for the length difference. I use PVC pipe, I think that 1" or maybe 1.5"... I don't remember off hand, but the OD of standard size PVC (.75,1 or 1.5 inch) works very well for this application.

I'd pull the forks, drain and flush, refill with Redline 5w fluid, fill them like this Slavens video: http://www.youtube.com/watch?v=glG3gUjxjEU Set at 110mm, then go from there.

While in there you can remove the rebound needles and make sure that they are clean and lube the o-ring on the needle. When assembling, make sure to get the rod lined back up with the needle and the cap adjuster! Oh, and while filling with oil DO NOT pour it into the center section where the rebound needle goes... it does not drain into the fork internals.

put it all back together and check/adjust the ride heights, then ride it.

[QUOTE=Hootowl;21083543]
Quote:
Originally Posted by Head2Wind View Post
I'm no suspension guy, but what I do know about it I suggest that you try to explain the "too harsh" with more details before anyone would be able to supply any useful information.

Things that are good to know:
1. weight of rider (you)
2. speeds ridden at (are you a AAA desert rider or ??)
3. terrain ridden (soft desert or hard surfaces and rocks?)

Incredibly useful and instructive post. THANK YOU!
Your questions:
1. 175-180 plus gear
2. 70 yr old old fart, slow but not too slow
3. Mostly hard terrain

Too harsh is hitting a rock on dirt or a ripple/ridge on a paved road and feeling a strong jolt

One question, did the previous installer of the aftermarket spring make a take-up spacer to make up for the free length difference?

No spacer


If its a early bike, then disassemble and remove the mid bushing. This will make a big difference.

'07 990

For any of the aftermarket springs, make sure that the spring guide fits inside the spring without sticking. Most of the time the stiffer springs have smaller ID and therefore require the spring guide to be machined/turned down to accommodate.

The stiffer springs are the same ID as OEM

Richard
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Old 04-02-2013, 05:35 AM   #8
Hootowl OP
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[QUOTE=Head2Wind;21088460]I recommend making a spacer to make up for the length difference.

Thanks for the tips
I intend to make that spacer today and re install the forks
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Old 04-02-2013, 06:54 AM   #9
geometrician
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Hmmm...

Quote:
Originally Posted by Head2Wind View Post
Prop the bike up with the front tire off the ground, loosen all but the top clamp bolts to include the lower triple clamp, lower axle clamps and the axle nut, then drop the front back onto the ground and pounce on it to compress the suspension as best as possible a couple of times. Prop back up and then torque all fasteners to KTM specs.
I agree with everything except what not to loosen...

you want to keep the lower triple clamp pinch bolts tight and loosen the top triple clamp pinch bolts. The lower clamp doesn't ever move in relation to the tubes- it's the upper which twists.

Also keep in mind the upper steering stem nut is responsible for keeping the steering bearings tensioned. If he loosens it and starts bouncing it he's going to change the bearing preload. All the upper pinch bolts must be loose for the bearing tension to be set correctly. I see KTM steering bearings (dirt & street) loose all the time from this- they aren't like most other manufacturers where there is a ring nut under the top triple clamp to capture/set bearing tension.
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