ST90 Fork Oil - The Manual says automatic transmission fluid ??

[DAX-IT!!!]

Hi All
I've just rebuilt the fork legs on my ST90 and am ready to fill em up with oil, checked manual and it says use automatic transmission oil - anybody had any experience with this scenario ?? Do you really have to use automatic transmission oil or should I use something like Castrol fork oil ??

If you think something like Castrol fork oil will do OK, what grade should you use?? If you dont think something like Castrol fork oil will do - what do you guys on here use/recommend ??

Cheers

 

[69ST]

Automatic transmission fluid is okay. Problem is, it's ~SAE 7W viscosity. Dunno how that plays out with the bigger ST90 fork assembly. With a CT70, it's too thin...resulting in weak fork action. Your bike's smaller cousin works best with 15W fork oil. Just know that it's a balancing act...between internal valving, spring rate, fill quantity (amount of trapped "air spring" inside the fork legs) and viscosity. With the valving matched to thinner oil, 7W tranny juice or fork oil could deliver the same result as 20W fork oil.

I don't want to go too far down into the rabbit warren with this. Suffice it to say that refilling with 7W transmission fluid or 10W fork oil should deliver similar results. If fork action is where you like it, leave well enough alone. If it's too soft, try increasing fill volume or/and heavier fork oil...which acts like installing stronger, progressive-rate, springs.

 

[DAX-IT!!!]

Automatic transmission fluid is okay. Problem is, it's ~SAE 7W viscosity. Dunno how that plays out with the bigger ST90 fork assembly. With a CT70, it's too thin...resulting in weak fork action. Your bike's smaller cousin works best with 15W fork oil. Just know that it's a balancing act...between internal valving, spring rate, fill quantity (amount of trapped "air spring" inside the fork legs) and viscosity. With the valving matched to thinner oil, 7W tranny juice or fork oil could deliver the same result as 20W fork oil.

I don't want to go too far down into the rabbit warren with this. Suffice it to say that refilling with 7W transmission fluid or 10W fork oil should deliver similar results. If fork action is where you like it, leave well enough alone. If it's too soft, try increasing fill volume or/and heavier fork oil...which acts like installing stronger, progressive-rate, springs.

Thanks very much for replying. I dont want it too soft so, I think I'll go for something a bit thicker like SAE 20 OR 30 (correct me if I got the SAE numbers wrong). Never done this job before so I'm a bit in the dark with it

 

[Tripod]

St90 front forks are already prone to sagging. I put 15w in mine.

 

[Gary]

You will have to experiment, 30 sounds way too high - you do want the suspension to work after all. Back in the day when they were new I had a '74 CR 250M. ATF was what was recommended, and really that's only what we had. I did however remove the springs and used DG Air Caps which allowed you to use air pressure to adjust "spring" rates. Now if your riding that ST as hard as a 20 year old rode an MX bike by all means use that 30w

 

[DAX-IT!!!]

You will have to experiment, 30 sounds way too high - you do want the suspension to work after all. Back in the day when they were new I had a '74 CR 250M. ATF was what was recommended, and really that's only what we had. I did however remove the springs and used DG Air Caps which allowed you to use air pressure to adjust "spring" rates. Now if your riding that ST as hard as a 20 year old rode an MX bike by all means use that 30w

Hi Gary................. at my age now, I'll just be pottering down the road at about 30mph so I dont need it to be tough settings, just middle of the road I guess . If I were still young.............

 

[DAX-IT!!!]

St90 front forks are already prone to sagging. I put 15w in mine.

From the replies I've had, this sounds like the right viscosity I think .

Thanks for all the info guys - I bought middle of the road Castrol Grand Prix 15W Mineral Fork Oil today so fingers crossed it'll be OK