fork oil question

[mikedabike64]

I want to increase the spring rate upfront by adding fork oil I added 10 ML per fork this was in addition to what was already in there, I have replaced the fluid earlier in the year with 15W
I am not looking for any drastic returns but my question is 10 mL is that not enough it seems hardly noticeable. I remember reading too much oil and blow the seals.
Thanks

[subxero]

oil doesn't compress, if you put to much oil in the tube then the little bit of air you have in there will get compressed until it can not be squeezed anymore and you still have travel left in the tube if you apply more force sooner or later something has to give and what is left other than to blow a seal.

Some guy has a thread in the ninjettes at speed, a build thread. He cut a few turns of the spring coil off to increase the spring rate. It makes sense if you think about it. He had a formula i believe for determining how much to cut = what rate. You could try that if you don't want to spring for new springs

edit: here it is

https://www.ninjette.org/forums/show...&postcount=110

[mikedabike64]

I will check that out much obliged

[InvisiBill]

You can only increase the spring rate by getting springs with a higher rate.

Changing the density of the fork oil used will change the compression and rebound damping rates. More compression damping will make it harder for the fork tube to compress, similar to a stiffer spring, but it's not exactly the same as having a stiffer spring.

Changing the oil level (and therefore the amount of compressible air in the fork tube) changes the progressiveness of the shock. The more air is compressed, the more pressure is needed to compress it further. Changing the amount of air in the tube changes the total amount of force needed to compress it.

http://www.strappe.com/suspension.html has some pretty good info on how the different parts of a fork work.

[mikedabike64]

Now I understand the why and how ty

[dcj13]

Do you really want to increase the spring rate?

Or are you trying to increase the force the springs exert at minimum load condition?

[mikedabike64]

So I had ninja 500 springs cut 4 inches with 15w oil.

But what happen was I noticed when I took the snap ring out of the top of the forks
The top of the fork that holds everything down would not come out I would have to hit it with a rubber hammer and then it would pop out
And so there was on equal spring rates because of that,
so I took everything out and went back to stock

I did notice better feel in the corners with the stock springs as I weigh 147.

But I think the washer was getting jammed on the 500 Springs compressing it un- equally between the left and right Forks

The reason why I took everything back out is that I suspected unequal pressure from the way it was riding unequal spring rate rather

So rather than cut the stock ninja 250 springs I just wanted to take a little bit of that Pogo effect out

The static sag is fine I just wanted to slightly tighten things up just a little bit and I added 10 mL more
But I was afraid to add more cause I did not want to blow the seals I don't know that much about that .

[flitecontrol]

What about adding some preload to the springs? Anyone tried adding a short length of PVC pipe above the springs? Lots of folks do this on other models.

[mikedabike64]

Lee


I would like to hear from somebody that you tried that on our bikes. See I like experimenting in small increments as long as I can reverse what I did ha ha.

[subxero]

^ lots of people change the pre load spacer with PVC in order to get the desired sag, heck the race tech springs come with PVC just for that. You could adjust the preload to get it in the correct range for you but that won't stiffen things up much and if your springs are still to soft changing the preload to make the bike feel more stiff and out of the recommend sag ranges is going to compromise other aspects of the suspension. In short, it isn't ideal.

[InvisiBill]

Correct, preload is a setting that determines how the suspension operates within the springs' range. It doesn't actually affect the capacity/stiffness of the springs, since they're staying the same rate.

When your springs are too soft, adding preload just feels stiffer because preloading the spring "blocks out" some of the excessive travel by compressing through that range ahead of time. If your springs (with no prelaod) compress 2" with your weight on them, adding 1" of preload will mean it's compressed 1" ahead of time and will compress another 1" when you get on it. Either way, it's still compressed a total of 2" with your weight on it (because the spring rate means that your weight will equal 2" of compression). Your weight only compresses it 1" more, so it feels half as squishy, but the spring is still compressing at the same rate, and the same total amount.

Like subxero said, cranking up the preload on a soft spring enough that it feels firm will inherently result in your sag settings being way off, so the suspension won't operate optimally. There is an acceptable range for these settings, plus personal preference on how it feels, but the physics behind it dictate that you need the proper spring rate if you want everything to work together optimally.

[mikedabike64]

Would you addThose spacers on top of the stock spacers or

Do you just use one piece of PVC rather than the stock spacers AND the PVC .
I would like to take maybe 2 inches of preload
I would like to try it. Stock vs Additional preload.

[subxero]

take out stock spacers, cut two pieces of PVC longer than stock spacers if you want more preload. How much, do some research, and math.

You would use one or the other not both.

[iGodlyReflexes]

Join localgroupride.com new social media site for all riders. Be sure to add MohawkWarrior!!

[mikedabike64]

I appreciate the information thank you very much guys

[subxero]

there is probably a DIY in the, you guessed it, the DIY section of this subforum. Only a few clicks away

[Lemonfresh]

You can get a better understanding of how forks work by starting here.

The spring in the front fork plays very little role in compression damping. It is there return the bike to the same spot after the fork is compressed. The springs set the ride height. That is why clipping springs to "increase" rate and spacing them does not work very well. You may get the height set right, but the ability to return it there is lost because it will not have as much spring compressed to return it (Hookes law). You need to get springs which are rated for your weight and get the proper ride height or "sag" set. Unless you are 70 lbs, the .44 kg/mm stock springs are junk and you should get replacements.

The air gap in the fork of a conventional damper fork provides most of the compression damping or how fast or slow the fork squishes. The oil weight, spring and valving do provide some but not as much as the air gap. Once the air starts to compress, it will provide a much greater force to support the weight on the forks than the spring. That is why getting the air gap set properly is very important. Too large an air gap and the forks will bottom out. Too small an air gap and the fork is not using the full travel of the suspension and the ride is harsh. The factory manual recommends 108±2 mm from the top of the fork to the top of the oil. This is just a guideline. You will have to tweak this value to get it right for your bike and riding style.

The oil viscosity and orifice size play the largest role in rebound damping. Think of driving a Buick vs. a sports car. The Buick is under damped and floats or wallows around when the suspension hits a bump. A sports car should hit a bump and return back to it's rest position (like before the bump was hit) without going past that point (ideally). Changing orifice size is not easy and once you go bigger, its a PITA to go smaller. Changing oil viscosity is the easiest way to change a forks dampening. The factory manual says 10w. Again this is something you have to play around with to get right.

With all that said, I can tell you what I have done to my front end and it feels very good. Not as good as the Daytona, but much better than stock. I am a 200 lb rider (and losing!).

I started by getting .80 kg/mm springs from Race Tech but I hear the Sonic springs are nice as well as a bit cheaper. I cut the spacers to set my "race" sag at 35mm. Your spacer length will be different than mine based on your weight.

I upped the oil viscosity from 10w to 15w but the forks still wallow at the top of the up stroke a little. I need to try 20w but the wallow is not bad enough to justify taking it apart for a little bit of gain. Next time I change my oil, I will try the 20w and see how that works. If its too slow, I will try a mixture of 20w and 15w until I get the top end wallow to go away when still moving up smoothly and quickly after pressing down on the forks..

I set my air gap at 125mm from the top of the fork to the top of the oil with the springs out and the forks compressed. While the forks where still compressed, I put a zip tie around the fork next to the dust seals. Once the fork is uncompressed, that zip tie shows me on the tube where the mechanical bottom of the fork is. After putting the forks back together and reinstalling them, I put another zip tie next to the dust seals again. I then rode and I grabbed brakes as hard as I could to try and bottom the front forks. (BTW, The SBS sintered pads work very well). The zip ties are just under a 1/2 inch apart. That tells me I have the air gap set just about right . I am using almost all of the suspension travel but not bottoming out the forks, even under heavy braking.

I plan on getting the Race Tech Emulators, but it will have to be next year sometime.

I hope this helps answer your question on what weight oil to use and the proper air gap. I also hope you take the time and get your suspension set up properly. It makes worlds of difference in the feel and handling of the bike as well your confidence.