Fork Oil #15?

[Rich82GS750TZ]

I know, another fork oil thread. I'm confused and probably overthinking this. My forks are apart, thoroughly cleaned, lowers polished. I got new OEM seals, cap o-rings, and copper washers for the bottom bolts and drain plugs from a local Suzuki shop. I wasn't initially going to replace the teflon rings, but think I will, for piece of mind. They look good to me, but what do I know? Anyway, my manual says to use fork oil #15. The bike and the manual are 37 years old. So what is the modern conventional wisdom on fork oil? Does #15 mean 15W? I asked the "kids" working the parts counter at the Suzuki shop about what's the modern thing to use. They said 10W fork oil, but they were out of it. Someone please set me straight.

[RichDesmond]

10w for all-around normal riding, 15w if you're a pretty fast sport rider and willing to give up a little compliance for better control and stability when learned over.

[Steve]

Rich is the resident expert here, but I will also vouch for 10w oil in the forks.

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[bwringer]

10w for all-around normal riding, 15w if you're a pretty fast sport rider and willing to give up a little compliance for better control and stability when learned over.

I am firmly in that "sport rider" category, and I found that straight rate springs from Rich ( sonicsprings.com ), in an appropriate rate for my bike and weight, with 10W oil to be the best combination. You really lose a little too much compliance with 15W, I think, at least on crappy public roads.

You get better front end traction under hard braking and accelerating hard out of corners with 10W as opposed to the heavier stuff. Then again, if your local roads are perfect and/or you do track days at a track where cars haven't washboarded the pavement all to hell, you might enjoy the firmer control of 15W.

When all I could find was progressive fork springs and I needed monster preload just to get sag in the ballpark, 15W was needed to keep things under control. Once I was able to get proper springing, I was able to move to 10W and enjoy better compliance.

In the end, it's somewhat a matter of personal preference as well.

[Rich82GS750TZ]

I will go with the 10W fork oil, at least to establish a base line for myself. Anything will be an improvement over the few ounces of probably 37 yr old sludge that I emptied out of the forks. I'm quite certain there wasn't enough oil in them, though I didn't measure what came out. One seal has been weeping for years, I'm just finally getting to do the full tear down and new seals. The stock springs are well within their service limits and I'm not going to change them. I want to get a good feel for what the bike should ride like with everything stock before I start to change anything up. Thanks all.

[Steve]

The stock springs are well within their service limits and I'm not going to change them. I want to get a good feel for what the bike should ride like with everything stock before I start to change anything up. Thanks all.

In my opinion, that is not a good move.

Although the "service limits" (length) are apparently "good", those springs were not of sufficient strength when they were new, hence the air caps, so you could add air. Over the years, those inadequate springs have not gotten any stronger, so you will not be getting a good feel for what the ride should be like.

It is highly-recommended that, while you have everything torn apart, to just bite the bullet, get a new set of springs (cost is less than $100), then put it all together ONE TIME and enjoy what the ride is REALLY supposed to be like.

Go to the Sonic Springs website, use their rate calculator to determine which one might be best for you, make your purchase.

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[Rich82GS750TZ]

Steve, I value opinion greatly. I'm sure you're right. However, after adding up all the costs of parts I've bought and still need to buy to overhaul the caliper, MC, new brake line, fork seals and other bits, new valve cover gasket after I do the valve adjustment, upgrading the springs is not is the cards for this year's winter tear down. I'm assuming(dangerous, I know), that I would be able to swap to progressives in the future without taking the forks apart, maybe even leaving them on the bike?

[Steve]

Steve, I value opinion greatly. I'm sure you're right. However, after adding up all the costs of parts I've bought and still need to buy to overhaul the caliper, MC, new brake line, fork seals and other bits, new valve cover gasket after I do the valve adjustment, upgrading the springs is not is the cards for this year's winter tear down. I'm assuming(dangerous, I know), that I would be able to swap to progressives in the future without taking the forks apart, maybe even leaving them on the bike?

Yes, you could swap to "progressives", but for the same money, you could go to straight-rate Sonic Springs and get an even BETTER ride. (See bwringer's post #4.) The only difficulty in leaving the forks on the bike is in the accuracy of setting the oil level. The book calls for the measurement to be done with the springs removed and the tubes vertical and fully-collapsed. Easiest done when the forks are removed. It's not all that hard to remove them one at a time, which keeps the triples engaged, along with the headlight bucket.

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[Rich82GS750TZ]

Thanks Steve. I misunderstood the distinction between progressive vs. straight rate. I clearly have a lot to learn. Going to leave as is for now.

[Tom R]

A little insight on different fork oils.

http://www.peterverdone.com/wiki/index.php?title=Suspension_Fluid

[Rich82GS750TZ]

Wow
Who knew there was so much to know?! Thanks for the info.

[Steve]

A little insight on different fork oils.

http://www.peterverdone.com/wiki/index.php?title=Suspension_Fluid

If you look at that long list, you will see some interesting stuff. Some 5w oils are thicker than other 7w oils, which can make tuning "interesting". Best suggestion would be to find a brand that is readily available in your area or from your favorite online store. Stick with that brand if you decide to to any viscosity changes.

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[Rich82GS750TZ]

If you look at that long list, you will see some interesting stuff. Some 5w oils are thicker than other 7w oils, which can make tuning "interesting". Best suggestion would be to find a brand that is readily available in your area or from your favorite online store. Stick with that brand if you decide to to any viscosity changes.

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Will do Steve, thanks. And thanks TomR for the great article. You guys are the best.