new front fork problem

[jerryz]

My S4 forks are now dangerously hard at speeds above 70mph on anything more than a racetrack smooth surface ,below that speed they are usable but harsh. They have lost all compliance since fitting the recommended Hyperpro springs  with Hyperpro 10w fork oil ,  the old Ducati springs could be compliant with 5w oil but handling was mushy even with many setups of compression damping and rebound tried. The Hypepro springs have made the handling accurate but ride comfort and safety at speed is now bad.

The oil is 10w and air gap is 150mm . compression is set at 2 clicks with 8 on 7 rebound ,sag is 24mm but bike is not using full suspension travel so i suspect ,springs are too hard or oil level to high and 10w oil to stiff.

Dont say fit OHLINS forks its not an option here and as to suspension gurus in Thailand I am on my own , the forks have been to race shop in UK for overhaul recentlly and are in perfect condition 

[jerryz]

Well today I set preload on forks to zero to see where the tie wrap went down to ,bumped up the rebound damping to 10 leaving compression at 2 ......it was a bit better but the tie wrap does not go nearer than 25mm to the bottom of the fork even under extreme braking so i reckon the new springs are too hard or most likely the air gap is not enough so when I get back from Xmas familt holiday to Chiangmai in the car I will drain 10mm oil out and see what happens ... then try a change back to 5w oil after that .
Handling of front end is very accurate now ,just ride quality thats nasty
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MONSTERMAN

[rockaduc]

You never told us the spring rate of the springs you put in or what you weigh w/ riding gear.  Sounds like the springs are way too stiff for you (I said stiff...).

[Howie]

A description of road conditions would be good also since most of us never ride where you live.  You might also consider adding Race Tech valves to your shopping list.

[ducpainter]

Did you ever check sag after fitting the springs?

[jerryz]

The springs are set for 180lb rider with full kit  sag was set as per Hyperpro recommendations  i then tried max sag today= zero preloada and still too stiff . a zip tie attached to forks does not get nearer tha 25mm from bottom.
Roads here vary between smooth tarmac to concrete roads to tarmac that is very bumpy from rain striations or slowly undulating from semisubsidance of the underlay all types are common .add in pot holes and ruts at random.

I got 2 friends who have 916 and a M696 to ride the bike today and both were terrified once the bike went over 70mph .

The 916 owner has his setup for trackdays and his forks are similar to mine and standard apart from 5w oil and ohlins spring his forks are very compliant ,and accurate ,he reckons mine have too much oil in (oil lock) but I set airgap as directed by hyperpro at 150mm. He liked the accuracy of the steering and the ohlins rear but the front end is way to firm .

[hypurone]

10w sounds like way to much to me. When I went thru my Z1000 front end I used Bel Ray US1 synthetic. Which I believe correlates to 5w and it was FIRM. Nothing near what you are describing, but firm. 

[Speeddog]

What rate are the hyperpro springs?

Try opening the compression adjusters up.
I don't think that'll fix it, but it may give you confirmation that less compression damping is the proper direction.

With the info so far, I'd say go to 7wt oil.

OEM S4 valving is pretty harsh.

[BK_856er]

It would be good to know the actual spring rate and your actual geared weight.

Also, the rider sag (24mm initially?) is one of two sag numbers that tell the spring story.  The relationship between the free sag and the rider sag is important.

Is your stiction value high?  Make sure the axle and forks are correctly aligned and installed.  Lots of stiction could point to a binding issue, and that'll make things feel harsh.

For oil level, after you have the other things squared away you can put a zip tie on and do some 60-0mph hard stops on a smooth surface to check travel.  Remove a few mm of oil at a time to use most of the travel.  You don't want to bottom it out and this is fine-tuning if you are starting out at an appropriate oil level.

Thinner oil should be the right direction to go in, but the effects on high-speed compression might be minimal.  Might need to be revalved to achieve nirvana.

BK

[scott_araujo]

10w sounds like way to much to me. When I went thru my Z1000 front end I used Bel Ray US1 synthetic. Which I believe correlates to 5w and it was FIRM. Nothing near what you are describing, but firm. 

+1.  When I resprung my Marzocchis I went with (stiff) .95 RaceTech springs and DOWN from standard 7.5w oil to 5w.  It feels stiff now but not jarring.  My test run was running over 2" drop sewer grates at 50mph.  Handled very well with none of the mushy dive and jarring handlebar feel I had on softer stock springs with heavier oil.

The fact that you notice them being too stiff at speed seems to indicate that the damping is what's to blame.  If it was too stiff a spring they would feel too stiff at all speeds.

If your springs are correct you should be able to get away with less damping, as long as it doesn't pogo you're fine.  Some folks don't care for the feel but set your sag properly and then swap out the oil for a lighter weight, set compression and damping to stock recommended, and see how you like it. 

Also, go with stock air space whatever that is for your forks.  Reduced air space is only really needed if you're undersprung and/or bottoming excessively.

Also, oil is quick to swap out and cheaper than another set of springs

Scott

[jerryz]

It will have to wait until New year now as  am taking family on a car trip over the holidays but I plan to do all the sensible suggestions and will post again on results .

Cheers Jerry

[ducpainter]

I agree it's more than likely a damping issue...over stiff springs will not make things as drastic as you describe.

Oil weight or height will help.

[scott_araujo]

Note too that whatever the manual says about oil volume has little to do with the resulting oil level/height! It should, but it doesn't and I've been caught out by that one! You can forget volume and just go by air gap/oil height!

Big +1.

[scott_araujo]

And try to vary only one thing at a time.  More than one thing at a time and you can't tell if any single change was good or bad.

[jerryz]

I have been working on the damn forks now for 2 years I have a notebook full of each adjustment running to 61 pages .I am now being told by other suspension gurus that progressive springs are what is causing my problem and to go back to the rubbish ducati linears springs ,.I test each mod on same road  . This aint science  its a black art ...hell i can write a powercommander map that works  without a dyno, rebuild scrap 8v heads that rock but these forks are something else?