front fork question

[Guest]

I have a 1980 gs750e, It had about 9400 miles on it when I bought it. Went through carbs (thanks to GS carb tutorial) also re vamped fuses to new type. I also replaced rectifier / regulator with the series style Polaris unit as told in the stator papers.

The bike runs good and I drive it pretty hard though the corners (I live in the Ozarks USA and there are a LOT of corners) so I figured that at 12,500 miles I would change fork oil. I did, and used 15w fork oil and the front suspension is now very soft (or seems to be) that it really dives when I brake. The old oil that came out looked reddish like ATF and had a bit of black in it. But the original stiffness was much better than it is now. I weigh in the 200 lbs range. I drained fork oil and replaced with amount manual says I should use.

Any pointers... Do I need to go to a 20W?

Thanks

[Grimly]

I wonder how much oil was actually in them - could be the PO had raised the level more than standard. Nothing to stop you adding some.

[Steve]

Thicker oil only means that you will dive slower.

A better solution is to upgrade your springs, then the appropriate oil.

My bike and I are heavier than your bike and you, respectively. I have Sonic 1.1 rate springs and 10w fork oil, and have a WONDERFUL ride with full control. You might want to go with the 1.0 rate springs, but use the calculator on Sonic's website to find out for yourself.

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[maintenanceman]

I wonder how much oil was actually in them - could be the PO had raised the level more than standard. Nothing to stop you adding some.

Actually, I tried to measure the amount of oil that I drained... Given the minimal spillage and the fact that I did not flush, it still seemed like there was less out than the amount the manual called for.

[maintenanceman]

Thicker oil only means that you will dive slower.

A better solution is to upgrade your springs, then the appropriate oil.

My bike and I are heavier than your bike and you, respectively. I have Sonic 1.1 rate springs and 10w fork oil, and have a WONDERFUL ride with full control. You might want to go with the 1.0 rate springs, but use the calculator on Sonic's website to find out for yourself.

.

I understand but the issue is that it seems much softer than before I changed the oil... I assume that changing oil does not change spring tension, only dampening? Please let me know if I have this wrong!

Thanks

[Steve]

You are correct in understanding that oil only affects damping. However, how do you know that the previous oil wasn't 30w or 40w?

In one of my previous bikes, I noticed that the forks were wonderful on smooth road, but were rather harsh when the road got a little rougher. I discovered that the oil level was just below the damper piston. On smooth road, it was just floating in air. When the suspension started moving around a bit, it essentially did a "belly flop" into the oil, which made it feel REALLY harsh. I flushed the forks and filled with the proper amount of oil (much more than what came out) and found that it rode even better, and on all kinds of roads.

.

[maintenanceman]

You are correct in understanding that oil only affects damping. However, how do you know that the previous oil wasn't 30w or 40w?

In one of my previous bikes, I noticed that the forks were wonderful on smooth road, but were rather harsh when the road got a little rougher. I discovered that the oil level was just below the damper piston. On smooth road, it was just floating in air. When the suspension started moving around a bit, it essentially did a "belly flop" into the oil, which made it feel REALLY harsh. I flushed the forks and filled with the proper amount of oil (much more than what came out) and found that it rode even better, and on all kinds of roads.

.

You are right, the oil could very well have been any weight oil... (though it looked a lot like ATF) but my question is that the bike behaved very differently after I changed the oil and put the recommended amount in the forks. Do I need to put heavier oil in or maybe more oil... and will I run the risk of blowing out a seal if I add more oil than is recommended?

Any input is welcome!!

Thanks

[mmattockx]

I did, and used 15w fork oil and the front suspension is now very soft (or seems to be) that it really dives when I brake.

Did the forks previously have air pressure in them and are now at atmospheric pressure? Air acts as a supplementary spring and going from some pressure to none would result in pretty much what you describe. You should never have to go thicker than 15W for these bikes to get them to work decently well. Best answer is a set of Sonic springs in the correct rate and no air pressure.

Altering the oil height does affect the fork feel somewhat, but only deep in the travel near bottoming, not through most of the stroke. Raising the oil height too much will result in blown fork seals eventually.


Mark

[maintenanceman]

Did the forks previously have air pressure in them and are now at atmospheric pressure? Air acts as a supplementary spring and going from some pressure to none would result in pretty much what you describe. You should never have to go thicker than 15W for these bikes to get them to work decently well. Best answer is a set of Sonic springs in the correct rate and no air pressure.

Altering the oil height does affect the fork feel somewhat, but only deep in the travel near bottoming, not through most of the stroke. Raising the oil height too much will result in blown fork seals eventually.


Mark

Hi Mark

I don't think that air pressure was involved... There are no air valves and there was no sound of escaping air When I changed the oil. FWIW, the ride is VERY smooth now, and it seems to corner okay, but it really dives in front when you brake hard. The only thing I can think of is that the oil that was in there was much heavier, and beings that is how I've always known the bike to be, what I have now might be the 'intended' way the fork is supposed to act and feel. I don't know?

[mmattockx]

Hi Mark

I don't think that air pressure was involved... There are no air valves and there was no sound of escaping air When I changed the oil. FWIW, the ride is VERY smooth now, and it seems to corner okay, but it really dives in front when you brake hard. The only thing I can think of is that the oil that was in there was much heavier, and beings that is how I've always known the bike to be, what I have now might be the 'intended' way the fork is supposed to act and feel. I don't know?

If there was no air pressure and everything is the same in terms of seals, springs, etc. then your new oil must quite a bit lower viscosity than the old oil. If those are the OEM fork springs they will be very soft rate and should be upgraded to a set of proper rate Sonic springs. Combine those springs with tweaking the oil height and you can tune the behavior under braking to be a better compromise than you have now. If you really want to go crazy, install a set of cartridge emulators and you will get the best of all worlds. FWIW, you want to use the lowest viscosity oil that you can make work because the lower the viscosity, the less the viscosity changes with temperature. You shouldn't have to go heavier than 15W no matter what else you have going on.

I am deeply jealous of you having lots of twisty roads, Alberta is severely lacking in that department.


Mark

[salty_monk]

If it was a 30w or something your compression damping would have been affected, maybe "propping up" the fork. The rebound would have been quite slow though. If you decrease the oil height (put in more oil) you will also stiffen the fork a bit on its initial stroke. Good springs & cartridge emulators make a world of difference.

[Steve]

Good springs & cartridge emulators make a world of difference.

I have "good springs" in my bike (Sonic, 1.1 rate). I have good oil in my bike (stock level of 10w).

Have yet to try the emulators, but there is already a world of difference.

Need to get the Konis on the rear to match what's up front before I do anything else to the forks.

.

[maintenanceman]

If there was no air pressure and everything is the same in terms of seals, springs, etc. then your new oil must quite a bit lower viscosity than the old oil. If those are the OEM fork springs they will be very soft rate and should be upgraded to a set of proper rate Sonic springs. Combine those springs with tweaking the oil height and you can tune the behavior under braking to be a better compromise than you have now. If you really want to go crazy, install a set of cartridge emulators and you will get the best of all worlds. FWIW, you want to use the lowest viscosity oil that you can make work because the lower the viscosity, the less the viscosity changes with temperature. You shouldn't have to go heavier than 15W no matter what else you have going on.

I am deeply jealous of you having lots of twisty roads, Alberta is severely lacking in that department.


Mark

Thanks for the info, you have given some good advice... I will consider springs and maybe some emulators.

Yes , the Ozarks were created for motorcycles. Just have to watch for deer and dogs and gravel... Makes for an exhilarating ride!!

Appreciate your knowledge!

Chad

[maintenanceman]

If it was a 30w or something your compression damping would have been affected, maybe "propping up" the fork. The rebound would have been quite slow though. If you decrease the oil height (put in more oil) you will also stiffen the fork a bit on its initial stroke. Good springs & cartridge emulators make a world of difference.

I am not sure what the difference was (is ) exactly... the big difference is how the bike dives when I brake hard. It is diving like a submarine now. But the bike is smoooooth. Gotta give it that!

[Steve]

I am not sure what the difference was (is ) exactly... the big difference is how the bike dives when I brake hard. It is diving like a submarine now. But the bike is smoooooth. Gotta give it that!

During the life of this thread, you could have ordered your new Sonic springs and gotten them installed.

Go for the 1.0 or 1.1 rate, install the proper amount of 10w or 15w oil, then go ride.

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