reducing st1300 fork oil level - has anyone done this

[DavidP]

Hi all,

I am trying to reduce the front end harshness and have been doing a few changes to my 2008 ST1300.

So far I have put a 2.5mm thick washer in the forks to reduce static sag (5mm more to be added, as static sag is still 42mm with a mostly empty tank), and used thinner fork oil (6.7wt - really iso20), which has made it a bit smoother.  (both Shell and Castrol use the correct iso weights for their fork oils    )

I have read that Honda seems to have compensated for the softer springs by using quite a small air gap above the oil (62mm).  After market spring manufacturers actually recommend 120-130mm fork oil level, and that is supposed to make for a smoother ride.

I know one of the reasons the fork oil level/air gap is there to act like a progressive spring and stop bottoming.  I only use 92mm travel out of 108mm total. This was with the zip tie method and 1500km of my normal running.  I weigh 85kg and if I put in another 5mm of spacers in the forks that should drop to 87mm (giving me 21mm of travel I don't use).  7 clicks  on the rear is about the correct preload for me (38mm sag without bike gear on, I have topped up the pre-loader & replaced copper washers)

My question to others, have you reduced your fork oil level, and did that improve the front end harshness? 

Guy at (ignore the part about fork brace) http://www.st-owners.com/forums/showthread.php?25399-ST1300-Forks  reduced oil volume by 50cc per leg (so should be 3-4cm less oil in each fork), but didn't really say anything about harshness.

[DavidP]

anyone?
 

going to try it on the weekend

[DavidP]

Well, here are the results after removing 40cc of oil from each leg (amazing how dirty it is after only 1 year/15,000km  ), and adding an extra 5mm of spacers (now 10mm all up, forgot I had 5mm in there already).  Fuel level in tank is about the same as before I did the changes.

Static sag is now 35mm (not quite level surface, so prob closer to 40mm), max travel used is 95mm (inc. on front brakes while hitting a speed bump, and quite a few rough roads).  That leaves 13mm for even bigger bumps.
I haven't detected any bottoming, but will know more after the QLD/NSW sulk's ride 2 up this weekend.

The front suspension seems to be a bit smoother and follows the road a bit better.   
Harshness is reduced and I'm tempted to try even more air gap at the next fork oil change.  40cc of oil should be around 3 1/2cm increase in air gap, so current gap should be just under 100mm.

[DavidP]

Update-
After sulks rte, 700km 2up (over 200kg) on various roads (Inc lots of potholes to Lennox heads, and  hard braking on highway) used max 102mm of travel.  That leaves 6mm for REALLY bad stuff. 

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[warren]

I would like to know if you managed a sweet spot for your ST!300 front fork oil level. With the springs still in and attached to your bike what is the oil level you recommend? I really only want to top up or reduce the le el I have. I am unsure of what the level should be with the spings in. Does anyone know. my ride is very harsh and the oil measurement on the bike is 62mm. Help!

[warren]

I would like to know if you managed a sweet spot for your ST1300 front fork oil level. With the springs still in and forks attached to your bike what is the oil level you recommend? I really only want to top up or reduce the level I have. I am unsure of what the level should be with the spings in. Does anyone know. my ride is very harsh and the oil measurement on the bike is 62mm. Help!
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[DavidP]

Hi Warren,
reduced level is better than stock.  I plan to get stronger springs, and drop the level even further (to 120mm) Dropping level to 100mm-ish took away a lot of the harshness.  I have 6.6wt oil in there at the moment,  but will bump it up to 7.5wt for a tiny bit more damping when I do the springs.  Stock are .86kg/mm, but for my weight 0.95 is recommended. 
With springs in it is hard to measure oil level, but if you use a syringe and piece of hose you can take the same amount out of each fork.  For proper measurement you really need to remove springs, as oil also sticks to them.
Cheers
David

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[warren]

Thankyou very much for your reply. I have place Castrol 5w oil and 10w oil mix in forks but still too harsh.
The springs will displace oil. If I remove the springs the oil level will drop somewhat. With the springs in I have topped the oil to 62mm as manual states. Even at this level the forks are exceedingly harsh. I want to know if anybody knows what their oil level is with the springs in place. Surely the oil level would be higher that 62mm. Does anyone  know what level it is. Those who have experienced harshness on the front end have you reduced your oil level and if so by how much.
I want to take my partner for a ride but the suspension is uncomfortable for her I do need to fix this.

[DavidP]

Remember,  the manual says to measure the oil level with the forks fully compressed and WITHOUT the springs in.  No idea how much the springs increase the oil level by though.  Most of what the pillion feels will be rear shock as they sit directly above it.  I have rear damping set a little bit softer than what the manual suggest (about 1/4 turn more), and wind the rear preloaded to max.

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[warren]

I have finally mad a science of this. I drained the forks again and filled the forks by volume capacity rather than measurement to top of cylinder. I found there was a considerable difference when re measuring the values. So conclusion. I put a mixture of 50/50 5w and 10w Castrol oil, in the forks. The final measurement in situ that is with the bike on it's centre stand the front wheel of the floor the measurement of oil fluid in the in the forks is 150mm from the lower edge, I have placed a little extra in the forks to 160mm and this is perfect. There are no harsh rebounds , no softness or wobbles. So if any of you want to do a short cut and just drain the forks by removing the bottom forks screw it is possible to do it, in situ. I have done it both ways, that is taking the forks completely out and up ending them and just removing bottom fork screws. Unless there is an extreme issue, I would just drain in situ and measure 150 from the top. I used a 20ml syringe and attached a length of plastic tubing that was 15cm long poured the oil in and extracted extra overfill. The suspension is the best it has been!