fork antidive removal

[Klavdy]

Andy, you are WAAAY under-gunned in this one mate.
You are getting advice (not mine) from a professional suspension guru.

[Arnie]

Hey Guys!  Lets not start (or continue) tearing strips off each other. :-)

Sure the FJ is way undersprung and needs stronger springs, but unless you put RT emulators or F3 cartridges in, changing the oil viscosity is one of the main ways to adjust damping in these forks.  Using 15W oil instead of 10W oil will increase the damping.  It will increase the damping in BOTH directions which you may not want.  And, using more oil (less trapped air) will affect the "progressiveness" of the spring rate as the fork approaches bottoming.
There are a number of ways to improve the FJ's fork action.  Some will be cheaper, some will work somewhat better, but has been said before - servicing the forks with new oil and seals (and maybe bushes) will make a huge improvement for most stock forks.

Cheers,
Arnie

[andyb]

Thank you Arnie.

My point is that if you've got a stock fork (or motor, or chain, or whatever) you're nearly always best served by taking it and assessing what you actually have first.  In my own case, I took ownership of a stockish FJ that had no signs of leaking forks, other than some strange wear on the tires, and it used the suspension travel up very quickly.  Instead of buying springs, I took it apart and found it was totally devoid of oil on one side and had perhaps a cup of sludge on the other.  After fixing that and returning it to a spec that I felt would give me a bit better ride (15w and 0.5" less airgap), I found it to ride quite well and to be relatively plush.... with the stock springs.

That doesn't mean for a moment that I thought it was worth much when leaned over hard at 60mph on a bumpy corner.  It does mean that it would do fine in the conditions that the original poster described, which is the 30-0mph braking in traffic.  

Putting in a stiffer spring and doing a full service to the fork will definitely improve things, and make the bike handle rather better when you're playing in the corners, as well as reducing diving in town.  It's not an absolute requirement, and may not be the best answer depending on how the bike is used; someone who sport rides their FJ hard every weekend undoubtedly is best served with a full swap at both ends to rather better hardware.  Someone who wants a more comfortable and controlled ride for a touring role is likely going to be pleased with the stock equipment, assuming it works well.

That being said, it strikes me that if the brakes are giving an uneven feel it's translating to a rough effect at the ground under braking, and that is causing excessive inputs to the suspension, which is going to make nearly anything handle badly.  So rather than throwing parts at it to start, finding the problem first is the wiser way to go.  Disabling the antidive (read this and then read this) seems to be a great start, and a servicing is not a bad idea (fork oil has a hard life and gets icky pretty fast!).

But the answer of "throw parts at it" without taking things and finding out what's happening?  If my doctor recommended a surgery before even an examination or assessment, I'd find a different doc, wouldn't you?

[racerman_27410]

Andy,

i dont recall the previous owner saying he wanted to half ass the job..... if the springs are not replaced then that is exactly what is happening.

IT IS NOT THROWING PARTS AT A PROBLEM........ it is documented time and time again cure for the excessive diving issue with positive results each and every single time the advice has been followed.... this aint brain surgury its a set of forks on a motorcycle.

ask anyone who has replaced their springs if it screwed up the ride....and you dont have to be Racerman to appreciate the difference


a normal fork service is always a good thing..... i change my fork oil every year..... but its not "the cure" for a diving front end nor will it give a smooth plush ride like replacing the springs and using the proper weight fork oil will (unless you ride on racetrack smooth pavement ALL the time)

the OP never mentioned anything weird happening when applying the brakes other than excessive front end dive......

the best you have ridden is all you know.....and put some WD40 on your wallet.... you're squeaking   

to the OP.....do the job right the first time and then enjoy.


Kookaloo!

Frank

[the fan]

wait for it....

[Marsh White]

I'll chime in for a second:  Andy - you seem like you have a chip on your shoulder during this whole discussion - it is a side of you that I haven't seem before.

The 87 and earlier OEM FJ Anti-dive forks suck.  We all know that.  I concur with the recommendation made to put new springs in and (obviously) change the fork oil / and seals if needed.  Also to disable the anti dive circuit.

I had stock forks on my 89 that were regularly serviced.  They dove pretty hard during braking - but I never really realized it until I rode a friends nearly identical 89 with YZF 600 cartridge forks on it.  HOLY SHIT - in comparison there was literally no brake dive!  I was sold instantly - I actually ended up buying the front end off of that friend (Gary Mastro) and I couldn't be happier!  So I also concur with Frank's statement that "the best you have ridden is all you know".  

[rktmanfj]

Marsh, where's the popcorn thingy?

Randy T
Indy

[andyb]

Instead of going back and forth, let's just have a single answer.

The FJ has some deficiencies in the front end department!  There's a few issues here, and one of them I think is a lack of a standard set of terms, so let's start there.

Excessive dive is the condition that I'm most familiar with.  You hit the front brake and the front immediately slams downwards.  It's an unpleasant effect!  This is pretty common on bikes with twenty or more years under their belts, and indicates a lack of damping force.  Old oil or no oil within the fork is the normal culprit.  The fix is to drain and clean the forks internally.  An ultrasonic tank is the best way to get them spotless inside, but in a pinch you can flush them by adding oil and working them back and forth, drain and repeat until it comes out clean.  If you find that your forks had less than the right amount of oil in them, it's important to figure out why, and the answer is nearly always a fork seal leak.  Easy enough fix while you're doing the oil.

Fork diving can also happen in a (more) controlled fashion.  Under hard braking from say 60-0, you can put the forks down on the stops, but it takes a moment for them to get fully down.  This is where increased spring rate comes into play.  There's two options you can go with here, the first is progressively wound springs (like the OEM) and the second is straight rate springs.  Straight rate is typically considered to be superior, and there's companies that will happily sell you a set of springs based on your weight and riding style.  Adjusting preload via the adjusters is not a substitute for using the right spring.

Adjusting the air gap (the amount of oil within the fork) is another way to help it resist bottoming.  More oil means you have less air within the fork, and as the fork compresses you get a non-linear additional spring effect right at the bottom, helping keep it off the stops.

The anti-dive units that were used on the early FJ forks helped, but were not the right answer.  The major problem that they have is they contribute to numb, weak brake feel.  They can be removed and/or disabled as linked above.  To further improve your front end feel, braided brake lines are highly recommended (some manufacturers spec that the rubber lines be replaced as a routine maintenance item every few years!).  To further improve braking feel and power, monobloc calipers from the late 90's through the early 00's from the YZF series can be used.  And don't forget the pads themselves!  


I use my FJ as a street/dragbike, and can definitely bottom the forks out under emergency braking.  I also don't take it and beat through twisty roads with it, for that I have a ZX-9R with the big inverted cartridges  .  The stock fork works nicely for commuting and for touring when it's working as the factory intended.  Depending on the type of riding you do, it may or may not work well for you in stock form with everything serviced well.  Because of this, it's best to do a full service on the forks to find out if there's something wrong internally as well as learning what parts you already have.  It's entirely possible that there's aftermarket bits already within the units, but that something else isn't working (or there's a broken spring, something binding up, etc).  Doing a service will give you an idea of what your starting point is.  

(At this point I could insert a sad story of trying to get more gearing on my FJ, bought a 42t sprocket and was happily wrenching away when I realized that I had wasted $40... it already had a 42t on it!  Should have taken a closer exam before spending money....)

Depending on your budget, it may be worth saying hell with the entire assembly and going directly up to a better setup.  Options include a full front end swap (watch for swaps that retain the speedo drive if that matters to you), the later model lowers/wheel/brakes, the lowers from a CBR600F3, or full aftermarket Ohlins race spec forks.  It depends on how you use the bike, and what you expect from it.  In extreme situations, it's likely best to simply replace the entire bike with something more appropriate (the FJ is a terrible platform for motorcross racing, for example).

[racerman_27410]

write all you want Andy.... the lack of the proper weight springs  cannot be covered up with oil weight.... you can slow the dive down but it only makes the ride harsher than they were orignially.


why do you think they make all the different weight springs and different weight oils.....sheesh


my experience with plenty of stock FJ's says the weight of the motorcycle alone is enough to use up almost half of the available travel..... let's see proof of your heavier oil ,stock sprung forks holding the bike up in it's suspension stroke with you on it and i'll buy into your theory.

all you need is a tape measure, a zip tie and a camera

up until that point all you have is a lot of words that dont mean anything to me and are not guiding the original poster correctly..... you cannot skip step one and go directly to step two.....   

this whistle never gets tired

KOokaloo!

Frank

[rktmanfj]

To further improve braking feel and power, monobloc calipers from the late 90's through the early 00's from the YZF series can be used.

...but not on the on the old anti-dive equipped forks.

the lowers from a CBR600F3  

The F3 cartridges can be adapted to the FJ tubes... never have seen F3 lowers on the FJ though.

Randy T
Indy

[SlowOldGuy]

Excessive dive is the condition that I'm most familiar with.  You hit the front brake and the front immediately slams downwards.  It's an unpleasant effect!  This is pretty common on bikes with twenty or more years under their belts, and indicates a lack of damping force.  Old oil or no oil within the fork is the normal culprit.  

Fork diving can also happen in a (more) controlled fashion.  Under hard braking from say 60-0, you can put the forks down on the stops, but it takes a moment for them to get fully down.  This is where increased spring rate comes into play.

Andy, sorry to pile on, but

Everything I know about how a spring works and how a shock works is exactly backwards of what's stated above.

Spring controls position.  If you're bottoming out, then the spring is too weak and is letting the bike position move too much.  

Oil controls velocity.  How fast the fork moves through it's stroke either up or down is controlled by the weight of the oil.  The oil can become a pseudo-spring when it's too thick to move through the orafice and stops the fork movement.  This is not a desireable option.  The amount of oil can influence the "air spring rate" but that is just due to the quantity of oil, not the properties of the oil itself.

Here are extreme examples of each:

Suspend the bike so it's not resting on the suspension.  Take the spring out of the fork and fill it with water.  Release the bike and it will bottom out rather quickly.  Replace the water with 90 weight gear oil.  The bike will still reach the bottom of the stroke, but it will take longer to get there.  The weight of the damping oil affects the velocity of the movement.  It does not hold the bike up.

Now, put a spring in, but leave out the oil.  With an appropriate spring, the bike will not bottom out.  However, it will pogo up and down at an uncontrolled velocity.  Very annoying.  Add oil to control the velocity of the movement.

Spring is position, oil is velocity.

DavidR.

[andyb]

Yes, Randy.  But that's up in the changing parts area, be silly to not do a bunch if you're going to bother at all, yes?

David:  it's possible I wasn't as clear as I'd like to have been.  You're correct with how you're describing things for the most part.  It's possible for an underdamped assembly to have enough inertia to move past where the spring would balance the load at, and it'll bounce around that point (which is why they need damping, natch!)

The oil's action controls how quickly the fork uses the travel that it has.  If you're in stop and go traffic (as the original poster) and using all of the fork's travel in that short of a time, I have to think that it's using the travel far too quickly, leading to bottoming.  Alternately, stiffer springs will prevent bottoming in that situation, but the bike will handle... strangely if there's still a lack of damping.  Still agreeing?

A bike's suspension is not a question of right or wrong, it's a question of if it's working for the rider, and that may change depending on the road condition, riding style, and weight of the load (rider/passenger/luggage/fuel).  My point is very simply that with the original springs on my 90, the ride is good enough for the riding that I do that spending $100 on springs would be a poor investment.  That's not an arguable point, really.  It could certainly be better, but it's not going to significantly improve from where it is for what its being used for.

Seems that if you're doing the springs, you also do the service.  Doing the springs after doing the service takes what, 15 minutes at most?  Just seems like very much the common sense way to approach things.  

[Klavdy]

[racerman_27410]

Seems that if you're doing the springs, you also do the service.  Doing the springs after doing the service takes what, 15 minutes at most?  Just seems like very much the common sense way to approach things.  

Mr Engineer man,



it's called suspension "TUNING" and it takes time and possibly multiple oil changes to fine tune it for the particualr rider weight and riding style....... particulars i know but everything starts with the right springs that are KNOWN to be suited to you and your bike's weight.



  get a book ....read it !

then we can have a conversation  



KOokaloo!


Frank


PS you will never baffle me (or others) with your BS.. and yes you are still wrong ! 

[Dan Filetti]

All hail Frank, the king of all things suspension related.  This is why the AMA racers line up at Franks door for advice. The rest of us are simply not worthy.