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Other Honda Mini Trails
CT90, CT110, ST70, ST90 Discussion
ST90 Fork Oil - The Manual says automatic transmission fluid ??
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<blockquote data-quote="69ST" data-source="post: 191884" data-attributes="member: 5"><p>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.</p><p></p><p>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.</p></blockquote><p></p>
[QUOTE="69ST, post: 191884, member: 5"] 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. [/QUOTE]
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Other Honda Mini Trails
CT90, CT110, ST70, ST90 Discussion
ST90 Fork Oil - The Manual says automatic transmission fluid ??
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