front suspension oil viscosity

airoh69

Junior Member
Joined
Feb 26, 2012
Messages
123
Reaction score
0
Points
0
Location
Cyprus
Visit site
Hi all,

Being a while to post in the forum. So here is the deal. I have installed hyperpro suspension a while ago. as i learn the bike and did also lvl 1 and lvl 2 from california superbike school, i am pushing the bike harder. i will be doing lvl 3 next week. I did a track day a week ago and i found out after setting the rear suspension to my weight that the front suspension is still smoother from what i wanted causing instability on the bike when hard pressing. So i have the oil that came with the suspension, hyperpro sae 15 and my question is, what oil and what viscosity do you recommend to put so i make it a little more stiffer. i weight around 100kg (220lbs) with full gear.
 

darius

Never stop exploring
Joined
May 26, 2012
Messages
1,155
Reaction score
19
Points
0
Location
Above California
Visit site
Springs are set for rider weight rather than oil. If the oil has broken down, you could refill with Hyperpro's recommended 15 to at least get performance back to when it was freshly installed.

The hyperpro progressive spring upgrade does not change the budget suspension into anything remotely sport, track or race worthy. It lacks decent damping and the progressive springs will feel springy and see-saw/dive a lot with hard riding especially at your weight.

If you're on a budget, at a minimum, ditch your springs for linear springs matched to your weight.
If you are going to keep the bike for the foreseeable future, consider a full suspension upgrade: R6 forks/Andreani conversion/Cartridge emulation front and an Ohlins shock.
 

TownsendsFJR1300

2007 FZ6
Site Supporter
Joined
Feb 7, 2009
Messages
12,516
Reaction score
1,157
Points
113
Location
Cape Coral, Florida, USA
Visit site
You may consider adding "air" to your forks for fine tuning.

http://www.600riders.com/forum/fz6-mods/54803-air-assist-fork-caps.html

A cheap and easy mod, now over three years in testing on my own bike.

With the stock springs/spacers, about 16 lbs in each leg is optimal for myself. Much more, it's too stiff, want it softer, let out some air. Added air twice in that time period, no oil leaks (26,000 miles on the original seals, etc).

Don't like it, let the air out..Simple

As your forks are pretty close (mine are bone stock), it wouldn't take much air to firm yours up.

Shrader valves were from old MX tubes, you can drill the fork caps (I used a lathe) and some epoxy.

 
Last edited:

airoh69

Junior Member
Joined
Feb 26, 2012
Messages
123
Reaction score
0
Points
0
Location
Cyprus
Visit site
Thanks for the suggestions. Actually i am planning on changing the bike to something like an r6 or gsxr. If i put higher viscocity oil or some more oil is it safe and will it make any difference ?
 

darius

Never stop exploring
Joined
May 26, 2012
Messages
1,155
Reaction score
19
Points
0
Location
Above California
Visit site
Thanks for the suggestions. Actually i am planning on changing the bike to something like an r6 or gsxr. If i put higher viscocity oil or some more oil is it safe and will it make any difference ?

Good call on getting a supersport- the right tool for the job.

Higher viscosity oil will firm up the front end by slowing the fork action down overall. It might help but you should first be looking to the springs and preload.

The fork preload isn't easily adjustable on the FZ6- it's just a steel tube. You could try adding a stack of washers to add preload and start at a higher spring rate.

But really, you should be running linear springs for your weight vs progressive springs that are more suited to road use, comfort and variable rider weights.
 

Gary in NJ

Junior Member
Site Supporter
Joined
May 6, 2014
Messages
1,964
Reaction score
1,908
Points
113
Location
Amoungst the Twisty Roads
Visit site
I'll post a techncal answer tomorrow, but in short a higher viscosity oil will result in the forks hydolocking earlier in the stroke during fast ( hard edge) motions.
 

TownsendsFJR1300

2007 FZ6
Site Supporter
Joined
Feb 7, 2009
Messages
12,516
Reaction score
1,157
Points
113
Location
Cape Coral, Florida, USA
Visit site
Adding more oil won't hurt anything but will lessen the amount of air in the top of the tubes.

Basically adding air (much smaller amount of course) and yes it will stiffen up the suspension..
 

Gary in NJ

Junior Member
Site Supporter
Joined
May 6, 2014
Messages
1,964
Reaction score
1,908
Points
113
Location
Amoungst the Twisty Roads
Visit site
I'm very active on the S40 forum, the bike which my cafe racer is built upon. This is a cut-n-paste from a series of suspension upgrade and tuning articles that I have written for that forum. It has been edited/adapted for FZ6 purposes.
_______________________________
Damping Rod forks are a fairly simple design with few parts - that's why they are used on budget bikes. The damping rod is a tube with an opening on the top and a pair of small holes on the lower portion of the tube. The damping rod has a seal that acts as a piston as the slider moves up and down the fork tube. As the fork is compressed the piston moves/forces oil through the damping rod from the top of the tube through the smaller holes on the bottom. This creates resistance inside the fork itself. This resistance to movement is called damping; compression damping as the fork compresses and rebound damping as the spring in the fork seeks its static or neutral position.

How did Yamaha select the rate of the spring and the size of the holes in the damping rod? They estimate an “average rider” across their vast market. In this case a rider who weighs around 150-160 pounds. If this is you, your springs have been designed with you in mind. If this is not you, your bike is most likely under-sprung for you. Also, you may weigh 160 pounds, but you have added equipment and accessories to the bike that increased its weight. If this is the case, the combined weight of the bike, accessories and you are now outside the design limit of the springs.

The limiting item in the FZ6 forks is the damping rod itself. It offers zero tunability. Damping rods are inexpensive to manufacture but have significant limitations. As noted above, to create compression damping, oil is forced through the holes at the bottom of the damping rod. Forcing oil through a hole creates very little resistance to flow at low speed, such as hitting a dip or gully or applying the front brakes. This allows the forks to shoot through their travel fairly easily, diving or bottoming in these situations. On the other hand when the wheel hits something fast or sharp edged (like a pothole) it needs a lot of oil to pass through the damping holes very quickly. Unfortunately the nature of forcing oil through a hole is that as the wheel velocity increases the damping force increases with the square of the velocity. In other words if you double the velocity you get four times the force. This means the hole basically "hydraulic locks" resulting in a harsh or stiff ride. Damping rods give the worst of both worlds; they are both too mushy and too harsh at the same time. The use of progressive rate fork springs further complicate this this because they amplify these undesirable performance characteristic.

There aren’t many “easy” or inexpensive solutions here. The inexpensive solution that many try is to use higher viscosity oil. While this will improve damping during slow-speed events, the forks will hydrolock sooner during high speed events. For this reason I don’t suggest this approach. One could experiment with increasing the size of the holes at the bottom of the damping rod AND using heavy weight oil, but this more than likely would be a zero sum gain. The only real solution to improve the damping of the forks is to use a cartridge emulator that bypasses the holes in the damping rod, replacing their function with a spring actuated valve that provides dynamic damping.
 

airoh69

Junior Member
Joined
Feb 26, 2012
Messages
123
Reaction score
0
Points
0
Location
Cyprus
Visit site
That was very informative thanks a lot. So i guess the only thing left to do to gain some stiffness is just to add some more oil, noticed that actually my mechanic used less oil in the forks (something like 10-20mlml in each form). How much oil do you think is safe to use in each fork or in case i change the oil weight what is the max i can use that wont have risk of leaking or anything that could happen from high viscosity oil

I'm very active on the S40 forum, the bike which my cafe racer is built upon. This is a cut-n-paste from a series of suspension upgrade and tuning articles that I have written for that forum. It has been edited/adapted for FZ6 purposes.
_______________________________
Damping Rod forks are a fairly simple design with few parts - that's why they are used on budget bikes. The damping rod is a tube with an opening on the top and a pair of small holes on the lower portion of the tube. The damping rod has a seal that acts as a piston as the slider moves up and down the fork tube. As the fork is compressed the piston moves/forces oil through the damping rod from the top of the tube through the smaller holes on the bottom. This creates resistance inside the fork itself. This resistance to movement is called damping; compression damping as the fork compresses and rebound damping as the spring in the fork seeks its static or neutral position.

How did Yamaha select the rate of the spring and the size of the holes in the damping rod? They estimate an “average rider” across their vast market. In this case a rider who weighs around 150-160 pounds. If this is you, your springs have been designed with you in mind. If this is not you, your bike is most likely under-sprung for you. Also, you may weigh 160 pounds, but you have added equipment and accessories to the bike that increased its weight. If this is the case, the combined weight of the bike, accessories and you are now outside the design limit of the springs.

The limiting item in the FZ6 forks is the damping rod itself. It offers zero tunability. Damping rods are inexpensive to manufacture but have significant limitations. As noted above, to create compression damping, oil is forced through the holes at the bottom of the damping rod. Forcing oil through a hole creates very little resistance to flow at low speed, such as hitting a dip or gully or applying the front brakes. This allows the forks to shoot through their travel fairly easily, diving or bottoming in these situations. On the other hand when the wheel hits something fast or sharp edged (like a pothole) it needs a lot of oil to pass through the damping holes very quickly. Unfortunately the nature of forcing oil through a hole is that as the wheel velocity increases the damping force increases with the square of the velocity. In other words if you double the velocity you get four times the force. This means the hole basically "hydraulic locks" resulting in a harsh or stiff ride. Damping rods give the worst of both worlds; they are both too mushy and too harsh at the same time. The use of progressive rate fork springs further complicate this this because they amplify these undesirable performance characteristic.

There aren’t many “easy” or inexpensive solutions here. The inexpensive solution that many try is to use higher viscosity oil. While this will improve damping during slow-speed events, the forks will hydrolock sooner during high speed events. For this reason I don’t suggest this approach. One could experiment with increasing the size of the holes at the bottom of the damping rod AND using heavy weight oil, but this more than likely would be a zero sum gain. The only real solution to improve the damping of the forks is to use a cartridge emulator that bypasses the holes in the damping rod, replacing their function with a spring actuated valve that provides dynamic damping.
 

TownsendsFJR1300

2007 FZ6
Site Supporter
Joined
Feb 7, 2009
Messages
12,516
Reaction score
1,157
Points
113
Location
Cape Coral, Florida, USA
Visit site
If adding oil, you want to make sure you have room for fork travel.

Say you have 5" normally of travel. IF, IF you leave 4" of air, your fork will hydraulically LOCK as fluids don't compress.

You obviously can add with the forks on and can remove oil with a hose and small hand held vacuum (save yanking fork legs).
 

Gary in NJ

Junior Member
Site Supporter
Joined
May 6, 2014
Messages
1,964
Reaction score
1,908
Points
113
Location
Amoungst the Twisty Roads
Visit site
That was very informative thanks a lot. So i guess the only thing left to do to gain some stiffness is just to add some more oil, noticed that actually my mechanic used less oil in the forks (something like 10-20mlml in each form). How much oil do you think is safe to use in each fork or in case i change the oil weight what is the max i can use that wont have risk of leaking or anything that could happen from high viscosity oil

The factory spec for oil height (it's actually an air gap you're measuring) is 134mm. In a damping rod fork the oil viscosity effects compression and the oil height effects rebound, so if you are trying to add "stiffness" going to a heavier weight oil would be the way to go. But keep in mind the warning in my last paragraph above; a higher viscosity will improve slow-speed compression, while at the same time make the bike ride more harsh. The least expensive real solution are Race Tech Gold Valve Emulators and a set of linear rate springs.

You mention that you want to add stiffness, but where in the stroke? Are you trying to eliminate brake dive? Is the suspension bottoming? Do you feel as though you can't hold a line with trail braking?
 

airoh69

Junior Member
Joined
Feb 26, 2012
Messages
123
Reaction score
0
Points
0
Location
Cyprus
Visit site
My problem was brake dive and also when accelerating through the exit of the corner i could feel the front causing instability.

The factory spec for oil height (it's actually an air gap you're measuring) is 134mm. In a damping rod fork the oil viscosity effects compression and the oil height effects rebound, so if you are trying to add "stiffness" going to a heavier weight oil would be the way to go. But keep in mind the warning in my last paragraph above; a higher viscosity will improve slow-speed compression, while at the same time make the bike ride more harsh. The least expensive real solution are Race Tech Gold Valve Emulators and a set of linear rate springs.

You mention that you want to add stiffness, but where in the stroke? Are you trying to eliminate brake dive? Is the suspension bottoming? Do you feel as though you can't hold a line with trail braking?
 

Gary in NJ

Junior Member
Site Supporter
Joined
May 6, 2014
Messages
1,964
Reaction score
1,908
Points
113
Location
Amoungst the Twisty Roads
Visit site
For brake drive you can try a 20wt oil. I might suggest a 50:50 blend of 15wt and 20wt (by the same manufacturer). The reason I suggest the same manufacturer is that the variation of actual oil viscosity in fork oil among the manufacturers is wide. If you keep with a specific manufacturer you are more able to "tune" the oil weight.

The front end feeling vague or unstable could be from a shock that is undersprung allowing too much weight transfer to the rear of the bike, or from not having the proper sag or preload set for the forks and shock. Have you taken the time to measure the race sag and the resulting static sag?
 

airoh69

Junior Member
Joined
Feb 26, 2012
Messages
123
Reaction score
0
Points
0
Location
Cyprus
Visit site
Talked with my mechanic. took they bike, he said he would insert some rollers (thats the English word i think) that would make the suspension stiffer. Gonna get it tomorrow check it out and tell you the results.
 

airoh69

Junior Member
Joined
Feb 26, 2012
Messages
123
Reaction score
0
Points
0
Location
Cyprus
Visit site
Tool the bike today and wow what a difference. I asked him and he said he put nuts inside. Unfortunately dont have any pics. Basically what he did was compress the suspension inside emulating the top adjustment that other bikes have
 

darius

Never stop exploring
Joined
May 26, 2012
Messages
1,155
Reaction score
19
Points
0
Location
Above California
Visit site
Tool the bike today and wow what a difference. I asked him and he said he put nuts inside. Unfortunately dont have any pics. Basically what he did was compress the suspension inside emulating the top adjustment that other bikes have

Sounds like a stack of washers was added to preload the spring. Glad it's allowing you to get by until you get the track weapon you want.
 

airoh69

Junior Member
Joined
Feb 26, 2012
Messages
123
Reaction score
0
Points
0
Location
Cyprus
Visit site
Did level 3 of California Riding School last Sunday. I must say that i am really impressed how my front suspension works now. Highly recommended. Pictures of my rear tire below :p

IMG_20180401_161958.jpg

IMG_20180401_162005.jpg
 
Top