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Discussion Starter #1
So, first time changing the fork oil on this bike and have one leg disassembled, getting ready to add oil and put it back together. On page G2-14 of the 2004 ST4s ABS Ducati factory service manual it says to check the free length of the spring and it gives the spring service length at 270mm. I also have the 2005 Ducati factory service manual ST4s ABS supplement but it only covers the delta between the '04 and '05, and it does not indicate that the forks are any different between the two years. Are they the same?

My bike is non-ABS. Do the ABS and non-ABS bikes use different fork springs?

The first oddity is that the spring on my bike measures 400mm instead of 270mm. The second oddity is that on page G2-17 it says to put the end of the spring with the tighter coils toward the bottom of the fork leg when reassembling. The coils on my spring are not progressive (dual rate); they are all the same spacing.

If the springs are the same on the ABS and non-ABS models, the latter leads me to believe that my springs are not stock. The length discrepancy doesn't make sense because it would seem that a spring that much longer would be very difficult to assemble. Using the Traxxion Dynamics tool to compress the spring wasn't any more difficult than I would have expected.

The original owner apparently had some mods done on this bike; it has the open airbox, carbon fiber cans and Helibars installed but since he is deceased I can't ask him about the forks and since his service paperwork is not available (estate sale) I don't know who the out of state dealer was (there are none in my state) . There were two other local owners before me but none of them did any mods or changed the fork oil.

Can anyone shed any light on this?

TIA
 

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If you know his name and where he had service done on the ST(if he didn't do it himself) they might have the specs from a service record.
 

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400 mm is the length of the OEM spring that I took out of my non-adjustable ST3 when I made them into a fully adjustable hybrid. Maybe that's what you have in your forks - springs from a non-adjustable fork. Maybe a previous owner needed that spring rate for their weight.
 

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Discussion Starter #4
@flynnmon - I know his name but have no idea what state or dealer he would have bought the bike from. I am assuming Ducati dealers don't have a national database where one of them could look that up.

@Yorik - could be, but might also be aftermarket. The spring wire diameter is of this bike is .107" according to my calipers which is heavier than than the .095 size I have seen mentioned in other threads on aftermarket springs. I am searching in Google to see if I can find out more. If the manual is correct about the 270mm length it would appear that I have aftermarket springs and the spacer tube that goes on top of the springs might be the one from the ST3.
 

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Picture a 1 foot long fishing pole, now imagine a 6 foot long fishing pole. Which one is going to bend easier? Now imagine them coiled up. The point is, all other dimensions being equal, the longer spring is going to be softer.

I'm not sure if you have the correct spring or not but I wanted to share some info I learned from a suspension guru years ago.
 

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If the spring was changed, many times a spacer needs to be made to account for the length....these are usually custom jobs but some folks can really do a professional looking job.

That being said, does the current setup give you the proper sag? If not, you just may be buying some springs rated for your weight and weight of your riding gear.


Sent from my iPhone using Tapatalk
 

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There is really so much bad info about fork springs out there ( by well intentioned people)
that you should just do yourself a favor: Try to set the sag correctly with what you have. If you can’t, the means the rate isn’t correct. Buy the correct rate springs from a reputable suspension guy. You will save yourself time and money.
 

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Discussion Starter #9
Thanks for the responses.

By posting I was trying to establish whether or not the spring was factory or not due to the discrepancy between the FSM and the spring length. Assuming the manual is correct for my bike (i.e. both ABS and non-ABS versions use the same spring) the spring in my bike is non-factory. FWIW, I also have a copy of the 2001 ST4s FSM in pdf form and it too lists the fork spring length as 270mm. Offhand, I don't see any reason that the ABS and non-ABS versions would need to use different springs.

Given the fact that Yorik's bike also had the 400mm spring it would appear that Showa makes a shorter top spacer tube to accommodate that spring length, because the one on my bike looks like a Showa factory part. If I have to swap springs that spacer won't work unless the chosen aftermarket springs are 400mm.

I haven't put very many miles on this bike because I quickly discovered that the front end had the well known lack of compression damping and with my weight (155lbs w/o riding gear) the ride was pretty harsh. I checked the compression damping adjusters and found they were already all the way out. I am now changing out the oil for 5wt and found out about the spring when I pulled it apart. My plan was/is to finish the job with the existing springs, see if the correct range of static/laden sag can be obtained and if the compression damping can be adjusted to the sweet spot. If not, re-valving and possibly swapping springs will need to be done.
 

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i've measured the st series springs at approx 370mm and 400mm. don't think i've seen any as short as 270mm. all the st series springs are linear, and all about 0.8- 0.83kg/mm. the number of coils affects the stiffness, not the actual free length.

all of the ss, ssie and monsters have dual rate springs, and manuals can certainly have recycled previous manual material or just plain bullshit in them.

as long as the preload spacers match the springs giving an appropriate preload (10 - 20mm usually) then that's fine.
 

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Discussion Starter #11
Ok, an update to the saga...got the fork oil changed to 5 wt Belray, filled to 109 mm down from the top of the collapsed fork, sans springs (about 500 ml) per the FSM. Put it all back together, compression damping all the way out, rebound now at 3 clicks out and preload all the way out (more on that, below). Change to 5 wt did soften the hits on sharp edged bumps but the front end is still too stiff and wants to ride over bumps instead of absorbing them.

For example, my test loop contains a set of railroad tracks and a manhole cover in the middle of one lane of a 25 mph curve (industrial area) that is set into a concrete square that sticks up about 1.5 inches from the surface of the road. When I deliberately run over the manhole cover (while leaned over) with my 97 Guzzi Sport 1100i (WP 41 mm USD forks) it soaks it up such that I don't even feel it. When I do the same with the ST4s, it makes the bike hobby horse. Running over the railroad tracks, which are set level with the pavement is taken in stride by the Guzzi but the ST gets a little whoopy. Riding the loop back to back, the Guzzi front end feels planted and confidence inspiring, while the ST front never really feels planted.

In general, the ST doesn't do anything evil but there is an imbalance between the hard front and the compliant Ohlins in the rear of the bike, which gets tiring even on a moderate length ride.

On the bike, wearing the gear/helmet, 14.2 lb tank bag and the preload adjusters all the way out I get 23 mm of laden sag, which seems a little shy of the 25-30 mm I've seen recommended on Ducati MS and other articles on suspension setup. Braking hard with just the front brake got the zip tie within 30 mm of bottom of the fork axle boss, which seems to be a lot of travel left.

Although it now appears the 400 mm fork springs are stock length I am thinking they are aftermarket ones that are stiffer. I don't know anything about the original owner, who had the other mods done, but when I got the bike the preload adjusters were screwed all the way in. Assuming this was deliberate, he may have been a chunky lad, as even with preload all the way out, the bike seems stiffly sprung to me. It certainly does not match the reputation the ST4s has for being softly sprung in the front end and it is way stiffer than the Guzzi.

In case you are wondering I did do the FSM front end jostle prior to tightening the front axle pinch bolts, after making sure the axle went smoothly into both fork legs, prior to fitting the wheel.

At this point I am thinking of contacting Rick of Cogent Dynamics to discuss a spring change and revalve work to get fully independent compression damping..
 

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When you say rebound is 3 clicks out, are you saying out from fully closed? I didn't read the whole thread, if this was already covered, ignore.

If you have rebound out from closed 3 clicks, you are also significantly effecting compression. The ST forks, like the adjustable forks on Monster and SS of the era, have a flaw. The compression circuit and rebound circuit are not separate. Rebound has the biggest effect on the forks performance as a whole. 3 clicks out from closed is way too stiff regardless. Try backing off the rebound a good bit, like go in 3 clicks from fully open and see what you get.

Regarding fork oil level. It's anyone's guess what the correct oil level should be once you put in other than original for the bike springs. The manual is out the window once you've changed to different springs. Coil wire mass, spacers etc change how much oil is displaced once the springs are installed and therefore changes the air gap you expected to have as measured with the springs out. I've adopted Race Tech's method of measuring with the springs installed and shooting for 110mm. I've done it on ST forks and Superbike forks. Different length forks and several different spring sets. Hasn't failed me yet. I get full stroke with no harsh bottoming out.
 

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you can calculate the spring rate if you measure the spring wire diameter, coil od and count the coils. maybe they are just way too hard, or maybe they're an sbk spring someone has dropped in.

oil height only influences the air spring effect and that only really has an effect in the last third of travel. but higher is crappier than lower. 110 mm springs in is fairly low, but that's not a bad thing.

getting rick to valve and spring them properly is a better idea.
 

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Discussion Starter #14
@SS904 - yes, from fully closed. At 4 clicks out the ride becomes bouncier. If I keep going out from there, I can then feel the front wheel skittering when cornering on bumpy road and if a straight line bump is big enough the front wheel briefly leaves the ground when the suspension rebounds. When I first took it for a test run I started with rebound damping backed all the way out but pulled over after a couple blocks to add it back in to try to settle the pogoing front. The fact that the rebound damping has to be set so high is an indication that the spring is too stiff for that damping circuit, at least with a properly designed fork with independent damping circuits.

I am aware from reading a number of posts on DMS (used the search function) of the poor design linking compression and rebound damping. I had been initially leaning towards sending the forks off to get them working the way they should have been from the factory but first wanted to see what could be done with what I had to work with. Since the bike shows no sign of being too softly sprung in the front I thought I might be able to get it good enough but not so.

The FSM calls for the oil level to be checked sans springs, which is what I did. I mentioned the oil level only for sake of completeness. I can't see what the wheel is doing while riding but I'm sure it isn't isn't compressing the fork enough for the air spring to come into play when I am just riding along under mostly steady throttle.

@belter - from all evidence, including the fact that I cannot get the recommended 30 mm of laden sage (for street) even with the preload backed all the way out, the springs appear to be too stiff, aggravating the marginal compression damping situation. I next plan on talking to Rick about it to see what he says.

Thanks to all who have responded!
 

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Yeas, too stiff. That 30mm figure is not enough sag, even for a race bike. Shoot more towards 35 to 40.

A agree with talking to Rick. He's very good.
 
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