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.

211301211302

[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.

[Mom]

Who was it on here that opened up some forks and there was $2 worth of old 20c peices under each cap?

Not sure, but failing the trusty 20 cents pieces, I got a spacer milled up for my front forks. Shaun Harris did some measurements for me and worked out how much Pre-load I needed and away I went.

[F5 Dave]

Which may have adjusted your preload & ride height correspondingly. I assume the spring rate was ok 'cause this cannot change the spring rate.

[flyingcrocodile46]

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.

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.

Cheers. I have checked the spring winding count and free movement and calculated that without cutting the spring to remove coils, I can add another two inches of solid spacer without bottoming out. However am aware that adding a spacer alone is very limited in effect (other than raising ride height).

Yes I have concerns about the coil folding/tipping to one side (and binding on the inside of the tube) where the cut spring gives no support to the washer and spacer. I raised those concerns with the guys doing the mod at the GSR forum but they reckon they have been doing the mod successfully for more than 4 years and when checked see no evidence of the spring wearing on the inside of the fork tube. I don't have access to a gas axe to heat and reform the end of the spring where cut so will try the mod to see how it works without re-forming the cut end.

[flyingcrocodile46]

Not sure, but failing the trusty 20 cents pieces, I got a spacer milled up for my front forks. Shaun Harris did some measurements for me and worked out how much Pre-load I needed and away I went.

Dadpole and RDJase were discussing the use of twenty cent pieces as spacers in the BFTP ride thread (which doesn't seem to be used for tracking ride days anymore).

[Mom]

Dadpole and RDJase were discussing the use of twenty cent pieces as spacers in the BFTP ride thread (which doesn't seem to be used for tracking ride days anymore).

Yeah they were just the right size :D

[Robert Taylor]

Yeah they were just the right size :D

If you preload any front fork springs in road going motorcycles beyond around 25 mm then the rate is too light and you need firmer springs.
What F5 Dave says is correct, all you are doing is adding spring force at the top of the stroke ( fully extended ) You are not increasing the rate, they are still oversoft springs. Latter stages of stoke there will be a ''crossover'' point where irrespective of how much the soft springs have been ( over ) preloaded the spring force will be less than a correctly preloaded spring of correct rate.
Over preloaded too soft fork springs will often have more spring force at the top of their stroke than much less preloaded firmer rate springs. The firmer springs will because of that deliver a plusher ride at extended strokes but because of their firmer rate will have much more spring force at the latter stages of their stroke. This gives benefit of much improved chassis pitch control, especially under sudden braking.
One of the worst recent examples of a bike with woefully weak fork springs is the Chinese made GN250s, woeful to the point of being dangerous and I wonder aloud how many riders lost the front end on these when they had to stop in a hurry.
70s ''solutions'' are not a solution and just dont cut the mustard.

[flyingcrocodile46]

70s ''solutions'' are not a solution and just dont cut the mustard.
Certainly true if you have a 21st century budget. Though a lot of old bikes sell for well less than an average set of new shock absorbers. If a cost free (cept for owner labour) modification can be made that makes the old beasts easier and safer to ride then the mods have value and do cut the mustard when weighed by value for money.

[Robert Taylor]

Certainly true if you have a 21st century budget. Though a lot of old bikes sell for well less than an average set of new shock absorbers. If a cost free (cept for owner labour) modification can be made that makes the old beasts easier and safer to ride then the mods have value and do cut the mustard when weighed by value for money.

Fork springs are relatively affordable but cutting of fork springs( where options are unavailable ) if done properly and retaining stroke integrity( as previously detailed in this thread ) can ''cut the mustard''. If its not done properly ( and this happens all too often ) then its not value for money, so to speak.
What I was referring mostly to was the ''old wives tale'' of adding what is excessive preload. We all used to do that in the 70s but 40 years later there is a lot more knowledge of the correct way to sort front end springing issues. The most modern bikes are also a lot less forgiving of bodge fixes.

[F5 Dave]

. . .
One of the worst recent examples of a bike with woefully weak fork springs is the Chinese made GN250s, woeful to the point of being dangerous and I wonder aloud how many riders lost the front end on these when they had to stop in a hurry. . . .
Meh. I think you'll find they were just copying the original design. When I used to help out instructing back in the day when GNs were new we quickly formed that opinion that the 'dynamite' front brakes were more a case of the front forks diving & bottoming the front. Small Kawasakis of the era (GPX250s, BR250s etc) seemed to have had (fish) oil added merely for lubrication. Maybe they saved a few 100l on the production run? Worked a bit better with correct amount of fork oil.

[Robert Taylor]

Meh. I think you'll find they were just copying the original design. When I used to help out instructing back in the day when GNs were new we quickly formed that opinion that the 'dynamite' front brakes were more a case of the front forks diving & bottoming the front. Small Kawasakis of the era (GPX250s, BR250s etc) seemed to have had (fish) oil added merely for lubrication. Maybe they saved a few 100l on the production run? Worked a bit better with correct amount of fork oil.

Approximately 4 or so years back I lined up a brand new out of the crate Chinese made GN against a mint 85 model and bounced on the front forks. While neither were stunning the old Japanese built bike had fork action that was light years better than Chairman Maos legacy. It was clear that the spring rate was indeed nastily less and damping control was also woeful. Id bet that checkplate clearance around the damper rod in the bottom of the sliding tube was extra generous and Id also bet that damper rod centreline didnt coincide with main fork tube centreline causing lots of uncontrolled bleed.
It struck me then that approximately 20 years later the world has gone backwards with many products in terms of quality and function.