Fork oil height level and weight suggestions.

Joined
Aug 10, 2008
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New York
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1993 ST1100
Looking for suggestions for oil level other than standard specs from the shop manual for my '93 Non-Abs..
I'm about 225lbs with full gear on and ride the twisties semi-aggressive but also use for commuting so don't want it too stiff for rough roads of NYC area.
Thanks
Michael
 
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Oil level, unless you run without it will not change the stiffness of the ride. The viscosity of the oil will have more effect on that. You could go from a 7.5 weight to a 10 weight and make the suspension a little stiffer. I would stick to the recommended levels as they are calculated to be enough to work with the valving in the tubes and lube everything else.
I rode with 15 weight in my forks for several years and thought I had something. When I had the forks professionally built by Traxxion... the valve changes and the springs they used made all the difference in the world... and BTW, they used 7.5 weight in the forks.

So there you have it... the valving, springs, and the viscosity will be what you need to work on.
 
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Changing the oil height in the forks does add some bottoming resistance as the air volume is smaller and so reaches higher pressures when the fork is compressed, than when a lower oil height is used. If bottoming or ride-height is something you want to change, you might be better off buying stiffer springs.

Changing oil viscosity affects both compression and rebound damping, so a higher weight oil will make for a firmer and possibly less-forgiving ride.
 
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I/we can offer you many suggestions, but could you please give a bit more information about what it is with the stock settings that aren't working for you.

I've run 15wt and found it to be a bit on the harsh side, but I'm only 170 lbs and ride fairly aggressively (former sport biker). 10wt was OK, but a little softer than I'd like. I'm now running 12.5 wt (15/10 mixed equally) and find that to be the best compromise between too soft and too harsh.

I haven't experimented with level, but agree with TerryS that the airspace is a secondary spring, so more/less air changes the compression/rebound characteristics in theory, but I can't say by how much because I've never tried it.
 

wjbertrand

Ventura Highway
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I've found the best solutions with both the ST1100 & 1300 is to stiffen the spring and then drop the oil level a bit. Often the new springs will come with a recommendation for oil level that's lower than what the manual spec is. I think Honda sets these bikes up too soft and then uses excess compression damping and high oil levels to reduce bottoming.


-Jeff
 
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The extra oil doesn't affect the compression/rebound characteristics, just the bottoming resistance. Oil viscosity will affect both the compression and rebound damping.

wjb, not sure I agree with your comment about the oil level, but you are dead on with the the low spring rate + high compression damping to reduce bottoming. It is a common trait across all my Hondas, the VTR1000 and VFR800 are worse due to very restrictive ports in the HMAS compression valve body; at least the ST11 uses the freer-flowing Showa 3-port pistons in it's lonely cartridge.

I now have a second right-leg cartridge inserted in the left leg, I've re-shimmed the cartridges and have just installed 1.1 kg/mm straight rate springs. I guess I'm trying to make my ST behave like a more sporting bike but with a rational ride position and luggage capacity, but without degrading the ride comfort too much. So far, so good.
 
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The extra oil doesn't affect the compression/rebound characteristics, just the bottoming resistance.
wouldn't "bottoming resistance" be similar to "compression characteristics", the fork has to compress to bottom out? Maybe splitting hairs, but sounds like the same thing to me.
 

wjbertrand

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I think the air space above the forks helps resist bottoming but it also contributes to a non-linear compression to varying degrees, depending on the trapped volume, during other parts of the stroke as well. Even without re-valving the cartridges, the stiffer springs (straight rate Sonic) and OEM fork oil, but at reduced level, resulted in a plusher, but not perfect, ride on my ST13. Later I installed gold valves and now I'm pretty happy with the action. Both Race Tech and Sonic recommended lower oil levels than the Honda spec.
 
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I think the air space above the forks helps resist bottoming but it also contributes to a non-linear compression to varying degrees, depending on the trapped volume, during other parts of the stroke as well. Even without re-valving the cartridges, the stiffer springs (straight rate Sonic) and OEM fork oil, but at reduced level, resulted in a plusher, but not perfect, ride on my ST13. Later I installed gold valves and now I'm pretty happy with the action. Both Race Tech and Sonic recommended lower oil levels than the Honda spec.
Thanks for all the science about the oil levels... hell, I just thought raising the oil levels would just blow the seals out!
 
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wouldn't "bottoming resistance" be similar to "compression characteristics", the fork has to compress to bottom out? Maybe splitting hairs, but sounds like the same thing to me.
You're right, I was being a bit pedantic there. But to me, the terms compression and rebound are normally used to describe the damping forces generated by the passage of oil through orifices or shims, and those don't change with different oil height.
 
Joined
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2005 ST1300
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8901
I think the air space above the forks helps resist bottoming but it also contributes to a non-linear compression to varying degrees, depending on the trapped volume, during other parts of the stroke as well. Even without re-valving the cartridges, the stiffer springs (straight rate Sonic) and OEM fork oil, but at reduced level, resulted in a plusher, but not perfect, ride on my ST13. Later I installed gold valves and now I'm pretty happy with the action. Both Race Tech and Sonic recommended lower oil levels than the Honda spec.
My understanding of the oil heights specified by Sonic and RaceTech is that they use higher oil/smaller air-gaps to enhance the progressive air spring effect for their straight-rate springs, where Honda use a progressive spring and lower oil height.

Ride comfort/control is a combination of springing and damping, and your experience with your ST13 suspension mirrors my own experience with my VFR800 which uses the same damper components. Most of the ride harshness arises from the compression valve body and the very small fluid ports there. Switching to valve bodies with larger ports (e.g. Gold Valves or similar) takes away much of that harshness and allows the shims to do more of the work.
 
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I'm about 250 lbs kitted out, I went with Hyperpro progressive fork springs,motul 10 wt oil at the original recommended level/air gap and the bike is perfect for me,sag is just right, of course a new wilbers rear shock with the sag set right was needed as well, she is like a different bike,handles the rougher roads fine too, but the real difference is how well she corners and holds a line.
 

wjbertrand

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Thanks for all the science about the oil levels... hell, I just thought raising the oil levels would just blow the seals out!
Well I suppose you could cause that if the oil were so grossly over-full that the fork hydraulically locked before reaching the end of it's travel. Can't compress a fluid so something's got to give!
 
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RaceTech claims the OEM forks have springs that are way too soft with way too much damping. In fact, their spring rate calculator says the springs are perfect for a rider that weighs zero pounds !!

I installed proper straight-wound springs for my weight and a RaceTech Gold Valve ( right side ) & Emulator ( left side ) kit. Now my ride is firm, but plush as RaceTech claims it would.

Go to the RaceTech website to learn more.
 
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mznyc
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Aug 10, 2008
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167
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New York
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1993 ST1100
RaceTech claims the OEM forks have springs that are way too soft with way too much damping. In fact, their spring rate calculator says the springs are perfect for a rider that weighs zero pounds !!

I installed proper straight-wound springs for my weight and a RaceTech Gold Valve ( right side ) & Emulator ( left side ) kit. Now my ride is firm, but plush as RaceTech claims it would.

Go to the RaceTech website to learn more.
Thanks Jim,I had the fork springs replaced with Progressive springs about 10 years ago,about 15K on them so not time to replace and happy with how they work with me for good all-round performance.I didn't do the change as I didn't have a garage then so local shop did it,don't have the spring rate,oil weight or air gap info.Will measure what I remove old oil to get a ballpark # of where I was at.
 
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