Fork oil and air gap.

[Hornstar]

Hi all,

I am hopefully going to re-fill my forks with oil and replace the springs for Hagon Progressive ones today.

I was just wondering whether the quantity of fork oil needed is the same as standard?

This is the first time I have done this but I assume that the air gap is basically the gap from the top of the fork leg down to the level of the oil when the stanchion is fully compressed into the lower?

If I adjust the air gap (by adding/removing oil) how would that change the suspension? Does it change the compression or damping or something else?

Thanks guys

[yorkie_chris]

The air gap is basically another spring, it makes the fork stiffer especially in the lower part of its travel as an air spring is very progressive in nature.

You can use it to resist bottoming on braking. It does not have an effect on the damping.

Don't go on quantity for it, measure the gap. It's measured from the top of the leg to the oil level, with the leg fully compressed without springs or spacers fitted.

[Hornstar]

Thanks yorkie. Thats very helpful. So the bigger the air gap the "stiffer" the suspension?

Have you got any kind of guidance as to what I should be having as an air gap? I tried to contact Hagon but have not yet received a reply.

[DaveW_42]

not totoally sure but from memory with my hyperpro progressives, I think it was a 140mm air gap and 20 weight fluid. Correct me if I'm wrong though - my memory isn't what it used to be!

[Berlin]

Opposite. The smaller the air gap the stronger it is. (less air to compress.)

The curvy is 109mm if I remember correctly. Dunno about the pointy

10 weight oil if you're a featherweight. 15 weight for normal riding and 20 weight for the track.

C

[Stig]

Opposite. The smaller the air gap the stronger it is. (less air to compress.)

The curvy is 109mm if I remember correctly. Dunno about the pointy

10 weight oil if you're a featherweight. 15 weight for normal riding and 20 weight for the track.

C

Mine was 104mm from factory. But the air gap is as much an individual choice as the make and type of springs put in. Not enough air gap and the forks won't compress properly and can judder going over dips in the road. Too big a gap and the forks will bottom out going over bumps. I would stick to the same level as is already in the forks and just see what difference the springs and new oil do before worrying about making changes with regards to the air gap. If it still feels 'soggy' when riding after changing the springs and oil, it's possible to lessen the air gap by adding spacers (washers) rather than adding oil.

[yorkie_chris]

it's possible to lessen the air gap by adding spacers (washers) rather than adding oil.

no it isn't, that's preload. The airgap will only change in that case by the volume of the washer you added, which is small.

[Stig]

no it isn't, that's preload. The airgap will only change in that case by the volume of the washer you added, which is small.

Agreed. Another bit of carp came out of my mouth :roll:. I need to listen to myself sometimes.

[jambo]

Last time I was fitting a set of Hagon springs they supplied a recommended air gap and fork oil viscocity with the springs.

I would suggest a call to Hagon Engineering would be a good place to start and then alter settings to taste having started at a recommended base.

Jambo

[yorkie_chris]

I need to listen to myself sometimes.

Looking at your forks... Yes, yes you do :smt003

[barwel1992]

i used a 120 air gap on my pointy

[Preston]

Opposite. The smaller the air gap the stronger it is. (less air to compress.)

The curvy is 109mm if I remember correctly. Dunno about the pointy

10 weight oil if you're a featherweight. 15 weight for normal riding and 20 weight for the track.

C


stock Oil is 15W but as the forks suck balls i would advise 20W with 100-98mm gap.

you put 10w in stock sv forks and they WILL! bottom out! alot

[barwel1992]

^ wrong ... i had 10w oil with 9.kg/mm springs and worked fine, i coudent bottom them out even loaded up with near 25 stone (including rider and pillion)

[yorkie_chris]

No way is stock oil 15w, it is closer to 10w by most manufacturers numbering scales.

Oil weight will not help bottoming, you don't get any comp. damping anyway. It mostly controls rebound. If you go too heavy with oil it will pack down and bottom! You need to match oil viscosity to spring.

Don't worry about 2mm on air gap, you won't notice couple of mm either way. I would try 120 and go from there if you have right springs.

[Berlin]

... as an asside to this I fitted air valves to the fork tops of my FZR400RR in an attempt to have adjustable pressure in the air gap.

It blew the fork seals out on a ride :) Oil all over the front brakes.. etc. etc.

Don't try this at home! :)

[yorkie_chris]

CBX550 came with those as standard. Complete load of sh*t! They would make it too hard at bottom and still mushy at top.

You want spring to be linear and correct for you, pretty much anything else is bodge.

[Stig]

Looking at your forks... Yes, yes you do :smt003

Looking at the outside or the inside? :)

They were freshly powder coated and only went on the road once. The inside has Maxton springs in. Did they possibly need new oil? Or was it my very well cut spacers I put in. :razz:

[Preston]

No way is stock oil 15w, it is closer to 10w by most manufacturers numbering scales.

Oil weight will not help bottoming, you don't get any comp. damping anyway. It mostly controls rebound. If you go too heavy with oil it will pack down and bottom! You need to match oil viscosity to spring.

Don't worry about 2mm on air gap, you won't notice couple of mm either way. I would try 120 and go from there if you have right springs.

99-02 oil stock is 15w always has been.. Oil weight DOS! affect bottoming as what do you think gos in the damper? air lol? if the oil is to light the amount of damping resistance is redused meaning the damping speed is reduced puttin gmore force on the spring and air pocket. the amount of air gap and to force put on the spring will not have enuff force to stop the forks from bottoming out. you forget that each part of the fork is made to work with the other forces aplyed by the other parts ie spring and air compretion + any preload force applyed to the spring.

i would also guess you have had to replace your fork seals once maybe more in a year....

10w oil really should only be used in forks with preload addjusters/ progresive comption springs. 15w is best for forks with no reload addjusters but have progresive springs 20w with none of the above.

[yorkie_chris]

10w oil really should only be used in forks with preload addjusters/ progresive comption springs. 15w is best for forks with no reload addjusters but have progresive springs 20w with none of the above.

Stock oil is showa ss-8, no?

Care to explain why to any of the above points?
What the devil is a comption spring?

How much compression damping does the standard damper rod assembly give you?
Granted, a lack of comp. damping can allow bottoming. However, the behaviour of the stock forks is such that if you put such a heavy oil in to gain compression control, the rebound will be ridiculously slow.

Why do you think I have to replace my fork seals more than once a year?
The current seals in my are at 45000 miles, with 5w oil. What's your point?

[Hornstar]

I have had a response from Hago now who recommend a gap of 95mm using SAE10W oil.

I am quite a bit heavier than the average guy (18st) and so I was thinking of going for a 15W oil. Would the 95mm gap sound ok?

Should I just go with that and if it is bottoming out then reduce the air gap?

[Dicky Ticker]

I have watched this with interest but surely one of the main considerations is the weight of the rider and type of riding.
It is all very well using different oil grades and types of springs and spacers with variable size air gaps but a bike set up for track use will be like riding a board on the road.
Surely it is finding what suits the individual within the parameters of suspension set-up being used

[Stig]

I have watched this with interest but surely one of the main considerations is the weight of the rider and type of riding.
It is all very well using different oil grades and types of springs and spacers with variable size air gaps but a bike set up for track use will be like riding a board on the road.
Surely it is finding what suits the individual within the parameters of suspension set-up being used

Indeed. When I ordered my springs I was asked several different questions.

How heavy was I?
How often would I carry pillion?
What sort of riding I do?
What type of roads I ride?
What motorcycle the springs were for?
With this information I was provided the appropriate springs, oil and spacers and also given advice on the air gap.

Given advice not a specific measure.

Setting up suspension is always a compromise not an ideal. One setting for the better is always going to have an adverse effect on the opposite. Especially when we are talking about SV front suspension. That's just the way it is and why it's always a compromise.

Better quality and more control over damping, compression and rebound will lessen the compromise but it still is a compromise.

[Aidan]

Haynes give an air gap of 104mm with the springs removed and the leg fully compressed. I'm a bit concerned about compressing my suspension fully as there are a number of pits in the crome which are above the level the seals normally run, but which could damage the seals.

Is it possible to measure the air gap with the form leg not compressed - I assume it would be 104mm + the length of extension of the fork inner (which I can't measure due to the likelhood of wrecking the seals)?