Front forks?

[ants67]

can anybody tell me how the variable damper system works on the front forks of an 85 yamaha fz400 . i find that when i ride the bike pressure builds up in the forks which i can then release via the air valves at the top, i have also been told that i should be putting 4 to 6 psi in them before i ride it is this correct , i have found that whatever i do makes no difference at all that i can notice to the handling. i have always been stumped as to what i am supposed to be doing ,or is it some factory gimmick that i shouldnt even worry about ? any help would be much appreciated.

[ants67]

sorry i a meant front shocks

[F5 Dave]

Variable damper system is a sticker on the forks (my RZ of similar era had the same sticker). Don't read any more into it than that. Maybe you could order a new sticker perhaps.

Adding air will make them bouncier. Yam forks of this era will be v.softly sprung. Some stiffer fork springs would make it considerably nicer along with a good clean & some 10w fork oil + check bushes when apart. Beyond that you are into gold valve emulators (do a search) but that may be the point where you are spending a lot on an old bike.

[ants67]

if that is the case all good then ,cheers for that. but it rasies another question how often should i replace the fork oil it was done maybe 7000kms ago

[schrodingers cat]

The air/gas acts as an additional spring. To prevent pressure build up, try some nitrogen in there (Firestone have it) and make sure the pressure in each fork leg is the same.
The increased spring rate (mechanical and gas pressure) will increase the rate of rebound somewhat.
Probably more to be had playing with tyre pressures than playing with those old forks .

Just ride the thing!

[F5 Dave]

Presumably you fully cleaned it out, not just drained & filled? Think you'd be pressed to notice until 20k. I would stretch to stiffer springs though if you like the bike, otherwise you will be riding almost compressed a lot of the time braking & cornering unless you are the size of one of those mats gymnast gurlies.

[F5 Dave]

The air/gas acts as an additional spring. . .

Yeah but adding pressure always sucks. Air isn't a good spring in that application. Coiled wire unadjustable as it is without physical changes is always better.

[ants67]

i comptley stripped them down on the bench to clean them out , who would i approach about stiffer springs as i have had a bit of problem with the shocks bottoming out at times and i do like the bike as i intend to hang on to it for a long time yet, and seen as i ate all the pies it may be a good mod to attemp

[HenryDorsetCase]

what about the period modification of " a bunch of washers on top of each spring? Who was it on here that opened up some forks and there was $2 worth of old 20c peices under each cap?

[F5 Dave]

yer local bike shop should be able to source some stiffer springs, or CKT who advertise on this site. I can't tell you what rate to run, but I'd guess std is probably something silly like .5kg & you'd be better with between .75 & .85 depending on your weight.

[F5 Dave]

what about the period modification of " a bunch of washers on top of each spring? Who was it on here that opened up some forks and there was $2 worth of old 20c peices under each cap?

sigh. Do some reading. This does nothing useful, really - it's bollocks. Appealing 'cause it's easy, almost convincing 'cause it feels stiffer by hand. But doesn't alter the spring rate so no use.

[ants67]

thanks for the advice , i wont be tempted to use the washer trick, been there done that in an old v8 holden oil pump with not much success , funny how we learn. any way will check out a few bike shops for heaver springs cheers you have been a great help

[Robert Taylor]

F5 Dave makes total sense. All of these old forks are severely undersprung and relied on addition of air pressure. Thats a largely discredited way of doing it as the higher the pressure the more the seal lips are pressed onto the chrome sliders, creating lots of friction and prematurely failed seals.
Linear wind springs in an appropriate rate for your personal stats will make a huge difference to that bike, if you are not averse to going one better, combine them with RACE TECH Emulators ( not the nasty Asian copies ) Progressively wound springs are also something to stay away from as they usually ramp up in rate too aggressively and that really hurts abrupt bump compliance.

Send me a PM and Ill reply with a menu of options

[flyingcrocodile46]

I had a look at some photos of another mod which supposedly alters the spring rate of springs. The claim is that by cutting two or three inches off the more tightly wound section then replacing with a equal length spacer will alter the spring rate at is removes the softer section. I am considering taking 3" off (my GSX1100EF springs) so the cut remains in the tightly wound section which means for a bigger cut/bearing surface on the separation washer.

I have two of these machines and am pulling one of them down. I currently have a 1" spacer at the moment and static sag of 1 &1/2" (in the better set of forks on the dismantled bike). The set on the new old beast (which I suspect don't yet have spacers) have static sag of 2" front and back.

When I sit on him another 3/4" goes at the front and 1&1/2" at the rear. It looks like fork oil is blowing back up past the top screw adjuster (prolly over filled)

I have a ducati Monster 900 shock to swap into the existing links which I have been told works quite well in the GSX1100EF with a stiffer ride and half inch less travel.

The bike has quite literally been ridden into the ground. Lots of spark plug, engine and fork oil changes but other than that it has only been fixed when broken. No other apparent maintenance in the last 10 years. Quite strange as the PO appears to have been proud enough of the bike that he has spent a truck load of time putting touch up paint all over the bike (using a tooth brush by the looks of it...incl graphics) and had the levers and side stand and other bits chromed.

[F5 Dave]

I had a look at some photos of another mod which supposedly alters the spring rate of springs. The claim is that by cutting two or three inches off the more tightly wound section then replacing with a equal length spacer will alter the spring rate at is removes the softer section. I am considering taking 3" off (my GSX1100EF springs) so the cut remains in the tightly wound section which means for a bigger cut/bearing surface on the separation washer.

Well more it affects the free length of the spring. A coil spring is just a conveninet packaging of a bit of spring steel. Imagine a plastic ruler & hold the ends; it bends easily. Move hands close together; Harder to bend. So adding a 'booster' valvespring will still reduce spring rate as it adds more active coils. Removing active coils increases rate.

Things to consider as it is possible to make something dangerous or rubbish: if you remove too much length the spring could coil-bind before full travel (& the hydralic bottom is met). you need to measure the spring when it is as compressed in the fork & then the length it would be at full travel.
Older bikes & their longer springs are less prone.

So then you need to reheat & bend & then grind flat the top of the spring. You don't want to make the bend brittle & getting it flat is important so the spring isn't being forced heavily against the side of the tube. That one shown in the picture is particularly badly done.

+ you need to get both springs the same as those forks aren't very convincingly attached to each other.

on my old 1100G budget bike I threw in some progressive brand front springs & progressive rear shocks. It was a considerable improvement on worn std stuff. But the shocks were miserable quality, had seemingly more compression that rebound damping. They started 'going off' after about 10,000k. bloody mexicans.

. . . It looks like fork oil is blowing back up past the top screw adjuster (prolly over filled)

That sounds like a fault worthy of fixing, there will be an o-ring that is munted on the circumference of the adjuster. It will reduce any std air spring effect to sod all.