Fork oil
 

Hi all

Thinking of changing the oil in my forks to a heavy grade. The drain plug is at the bottom, so do I take the cap off the top to fill the forks?

I have a 87 model with 20k on the clock

Thanks

RE: Fork oil
 

Yes.
Interestingly, I've always thought that to stiffen the front you put in heavier oil. Last time I did my forks the shop owner said it's a big no no. The reason is that on older bikes the springs are usually soft & saggy, sogoing to a heavieroil only causes the forks to sludge & react slower. The result is then loss of road contact over bumpy roads. Sorry don't mean to thread jack but I thought it may be an opportune time to discuss the idea.

RE: Fork oil
 

Mate I am open to any input.

The reason I posted the question is that I hit a hard bump on the weekend and the forks bottomed out. Under hard braking the nose does dive a bit also.

From what I have read the forks on the CBR are a bit soft.

Thought that I might be able to stiffen the forks a bit without going to the extreme of replacing the springs.

Thanks again.

RE: Fork oil
 

The ideas I've put forward may not be correct. It's what I did & it seems ok to me but I'm no pro. You may find that just replacing your oil improves things anyway. I had an ex race rider once tell me to put spacers inside the forks to compress the springs. I think that only lifts the front & reduces the travel??? See what some of the others on the site think. Good luck.

RE: Fork oil
 

I would go for the new oil first and see if it helps (not "heavier" oil)...probably you have to replace the forksprings, as bluebeard says "they are usually soft & saggy" on these old bikes.
I resently bought me some new forksprings ( Hyperpro)i plan to change the coming winter.
My bike also drops down a lot when breaking, so with new springs and oil it should be solved...hopefully.

RE: Fork oil
 

To change the oil, open those plugs at the bottom and bounce the front end with the brake applied. To refill, you need an engine hoist or some such device to pull up the front and and completely decompress the springs, otherwise you will have a big nasty problem on your hands when those springs fire out of the tubes. Try some progressive rate springs if the new CORRECT fluid doesn't seem to make a difference. The springs may indeed be sagging. New OEM=High dollar.

RE: Fork oil
 

Ok I will replace the old oil with the correct oil and see what happens.
So the springs will fire out the top of the tubes.
If I lift the front wheel off the ground are the springs still under pressure when you take the top caps off?

thanks

RE: Fork oil
 

They are probably under some pressure but not much. There is always some degree of preload, if not, there would be a lash in the fork and that would be dangerous.

RE: Fork oil
 

Release weight from your front wheel first (you may jack up the front part of engine) then remove top end cap of forks. You may use right size of box wrench and hold the wrench as you are revolving the top end cap because fork spring give force to the cap. One time, I did not hold the end cap and the cap flew off and hit my fresh repainted fuel tank. Now you may remove the drain bolts on the bottom of the fork and drain old oil. I suggest you to remove front wheel, so that you can pump the forks easily. In order to drain oil completely, you need to pump the forks 5 to 6 times. If you really want to drain completely, you may need to remove the forks from your bike and remove springs from the fork and hold the fork up side down and pump it several times. Stock fork oil is 5 weight, and I am using 10 weight. Forks work fine and more stability on front. I also dropped front end about 15mm. More stability on hi-speed turn.

RE: Fork oil
 

I put 15weight oil in my forks. Put a floor jack under the engine when it is on the centre stand to lift the front . There should be preload when you take the top caps off . Bleed out the air before you undo the drain plug .measurementof oil thatyou take out of each fork i think the left fork holds more oil. To check the sage in the front the forks theyshould be at the top with out you on the bike and only dropwhen you get on. You can put spacers in the forks to reset them. I thing theoil amount is 450 to 480ml .

RE: Fork oil
 

I got a tip from a guy who's been racing, he told me that the best is to have the forklegs loose.
When you've filled up the oil into the forkleg you push the leg all the way inSLOWLY then pull it all the way out alsoSLOWLY about 5-6 times then if you have the time let it "rest" over the night. Cause if there are bubbles they will have time to dissapearand then finally measure from the upper edge to see if the level of oil is correct...if not...fill up to correct level.

RE: Fork oil
 

Thanks Guys for the replies,
I drained the oil and removed the caps, they were under slight pressure.
After reading the manual and seeing that the recommended oil for the forks is fork oil or transmission fluid and seeing what some of you guys had to say and talking to the guy at the bike shop, I decided to try 10w fork oil. 483ml R & 494ml L
This has certainly stopped the bottom out problem and has stiffened up the forks slightly but noticeable, I have only test riden around local streets so will take it for a good ride on the weekend and see how it goes.
This site is a great source of information thanks again.

RE: Fork oil
 

If your forks are bottoming out you need new heavier springs. They are 20 years old and their spring rate deterioates.

RE: Fork oil
 

Also don't forget that you can reduce the air gap above the oil to help reduce bottoming out.

I have a 1990 model and had the same diving feeling under brakes. I replaced the oil with 10wt as per the book but
it still didn't feel right.

Next step was to add more oil and reduce the air gap above the oil ( The gap on mine is now 150mm (5.91")

bike feels much better now because the air compresses quicker under dive and acts like a spring assister.

rgds Dean

RE: Fork oil
 

Heavier oil is better because the anti dive is on the left fork. I tried 2.5 oil in mine and it was just asponge to ride when you hit the brakes it just bottomed out and jumped over the road on the ruff.

RE: Fork oil
 

Trans fluid equates to aprox 8 weight. That means 10 works fine. Beware that you don't reduce the air gap too much or go too heavy. Rare but the down side is it can cause the forks to lock.

RE: Fork oil
 

The 15 weight works good with spaces.