Front shocks too stiff after fork oil change with after market fork springs

[Longhorn]

Long ago I bought after market springs to solve the front fork dive problem.  The shocks were ridicuously soft from the factory so folks change the spring and the fork oil and all was well.  I finally got the earl change at the stealer (mistake-shite) and the new oil has to be thicker or something.  Anyhow the red oil used by the stealer was 10w.  If I want to reduce the stiffness do i go to 5w or 15w? 

[hubcapsc]

I think (a lot of us think) that 10wt is good... perhaps the wrong amounts were put in?

I put "the right amount" (per the manual) in while I had the forks off and the springs out... I used
the "measure from the top" method outlined in the manual.... perhaps you don't want to take the
forks apart enough to get the springs out... I can imagine taking the forks off, and the caps off the forks,
and dumping them upside-down and leaving them for a day (or however long) to drain all the way,
and then use the "by volume" method outlined in the manual to refill them... the manual specifies
a different amount for each side...

Perhaps there's an easier way?

-Mike

[fudgie]

I did my own on the bike and guess how much oil I needed to add. It was stiffer then I would like. Lived with it a year or so till I had the seals replaced by a independent service guy. Rode alot better.

I believe non oem springs are stiffer which helps out with the big bumps. You can try and take out a oz of fliud and see how it rides.

[Bone]

My 98 Tourer had a seal leaking so I replaced both seals, bushings and oil. I didn't have much dive so I stayed with the stock springs. Using 10 weight oil and measuring the level I was happy with the ride after the change, it was more firm.  Used Bel Ray oil which is clear and the old oil (OEM I guess) was clear.

Maybe removing the same amount from both tubes with a cattle syringe would soften it up. Some guys mess with the volume used.

[Ken Tarver]

depending on the model of bike and springs, progressive recommends less oil

[Longhorn]

Sounds like I'll suck out an ounce and see how that works first.  Could be the stealer put in too much earl.  I guess it's a experiment till ya get it right sort of thing.  THanks for the input guys.
 

[PAVALKER]

I think you are headed in the right direction by removing a small amount of oil from the forks, but don't
 remove too much at a time.  I believe most use 10w.  I use AMSOIL suspension fluid, which I also believe is 10w.  What I noticed in my Valk and VTX when changing fork springs, was that one fork had a thick oil like gear or motor oil and the other a red fluid, which looked and smelled like automatic transmission fluid.  If you have other fluid in besides the brown thick oil and red trans fluid stuff..... it's been changed before at some point.

[Rio Wil]

I am not sure how these things work, but I can't imagine that removing a few ounces of oil will change the compression or rebound characteristics.  Wouldn't the quantity of oil be determined by the travel of the piston  so as to not introduce air bubbles and change the "viscosity" of the oil and alter dampening/rebound . So if you want to effect the shock function, you would alter the weight of the oil and not the quantity.????

[John U.]

There have been reports from several of our members over the years that have reduced the amount oil to soften the
ride. They reported that it worked.
Longhorn, if you remove some oil, please let us know how much you removed and how it affected the ride.

[Ricky-D]

I agree with both Rio Wil and John U.

Assuming that a few ounce change in oil volume will affect the function of the shock would mean that the shock is not only oil tight but air tight to boot.  I don't think it was designed to be that way.

I feel the only way to definitively change the performance of the shock is to use a different weight oil, changing springs not withstanding.

Remember that there is a damper in the right side shock and none in the left side so that changes how each will react to the different weight oil.  But that really doesn't make much difference as long as the front axle is tight and the binders on the triple tree are tight. The whole assemble acts as a unit.

It would be interesting to drill a vent in the top caps on the shocks and see what the result would be.

***

[hubcapsc]

Assuming that a few ounce change in oil volume will affect the function of the shock would mean that the shock is not only oil tight but air tight to boot.  I don't think it was designed to be that way.

What do you speculate that the designers were getting at in the manual with their assertions
(22.7 oz for the right, 25.2 oz for the left) about how much oil should go in each leg?

-Mike  ???

[Ricky-D]

Assuming that a few ounce change in oil volume will affect the function of the shock would mean that the shock is not only oil tight but air tight to boot.  I don't think it was designed to be that way.

What do you speculate that the designers were getting at in the manual with their assertions
(22.7 oz for the right, 25.2 oz for the left) about how much oil should go in each leg?

-Mike  ???

Like I had previously stated:

Remember that there is a damper in the right side shock and none in the left side

Seeing that the shocks are different makes it easy to understand the varying amount of oil for each.

***

[Gryphon Rider]

I am not sure how these things work, but I can't imagine that removing a few ounces of oil will change the compression or rebound characteristics.  Wouldn't the quantity of oil be determined by the travel of the piston  so as to not introduce air bubbles and change the "viscosity" of the oil and alter dampening/rebound . So if you want to effect the shock function, you would alter the weight of the oil and not the quantity.

I agree with both Rio Wil and John U.
Assuming that a few ounce change in oil volume will affect the function of the shock would mean that the shock is not only oil tight but air tight to boot.  I don't think it was designed to be that way.
I feel the only way to definitively change the performance of the shock is to use a different weight oil, changing springs not withstanding.
Remember that there is a damper in the right side shock and none in the left side so that changes how each will react to the different weight oil.  But that really doesn't make much difference as long as the front axle is tight and the binders on the triple tree are tight. The whole assemble acts as a unit.
It would be interesting to drill a vent in the top caps on the shocks and see what the result would be.

Of course the forks are air tight.  The only place for a fluid to get in or out of the fork is at the seal, and if air could get in or out there, it would carry oil out with the air.  Increasing the oil level reduces the air volume, and thus the air gets compressed to a higher pressure as the forks are compressed.  Higher pressure requires more force to produce it, thus it takes more force to compress the forks with more oil in them.  There is also the possibility of adding enough oil that the air is completely compressed into a liquid, and thus the forks would have reduced travel.  I'm guessing that it would be virtually impossible to bottom out your forks in those conditions, because you would have increased your effective spring rate way over stock.

My 1981 Honda CM400T had a schrader valve to adjust the air pressure in the forks, which is way easier than playing with the oil level to adjust suspension stiffness.

In short, adjusting the oil level changes the effective spring rate (higher level = stiffer spring), while changing oil weight changes the damping characteristics.

[Rio Wil]

But these shocks are not "air/gas shocks" where  a compressed air is the primary dampening medium. Isn't the air compression simply a side effect of a decreasing volume as the shock is depressed (don't laugh yet) with the primary dampening done by a viscous fluid being squeezed through a port in a piston and assisted/resisted by a spring as well. The seal in this shock is not (in my mind) designed as a pressure seal. The size of the port, the viscosity of the oil and the tension of the spring are the determining dampening rate components.....

[Rio Wil]

An interesting comment:

Front Forks

Front forks on Harleys are very simple. They are just like emulsion shock absorbers with straight wound springs, except the springs are on the inside instead of the outside. Wide Glide forks look very different than FXR forks from the outside, but underneath the chrome covers, they're almost identical.

Since the forks get shorter when you hit a bump, all forks are effectively air-assisted. Because of this, it is important to get the oil level set correctly when you change your fork oil, or the spring rate will be wrong. In extreme cases, if you use too much oil, the pressure in your forks when you hit a large bump can blow out your fork seals. This is a real mess - you'll be cleaning your bike for days. Also, you'll be rebuilding your forks.

[Gryphon Rider]

But these shocks are not "air/gas shocks" where  a compressed air is the primary dampening medium. Isn't the air compression simply a side effect of a decreasing volume as the shock is depressed (don't laugh yet) with the primary dampening done by a viscous fluid being squeezed through a port in a piston and assisted/resisted by a spring as well. The seal in this shock is not (in my mind) designed as a pressure seal. The size of the port, the viscosity of the oil and the tension of the spring are the determining dampening rate components.....

Air (or gas in gas shocks) in vehicle suspension is never used as a damping mechanism, whereas oil is.  Damping is required to keep the spring (and the vehicle) from pogoing up and down after the bump is passed.  When pressurized gas is used in a shock, it is simply to raise the pressure of the oil so that when oil is being sucked through the orifice(s), the oil is prevented from cavitating.
http://dictionary.reference.com/browse/damping

[04strider]

Forks are supposed to be airtight.  The level of the oil is directly related to the amount of air in the cavity. As the shock piston moves, it also compresses the air above the oil as well as forcing the oil through the orifice.  Granted the weight or viscosity of the oil controls how fast the fluid moves through the orifices, but the amount of air volume also aids in the dampening.  The lower the oil level increases the air volume.  This larger volume of air will compress less thereby making the fork feel softer, whereas a higher level of oil will stiffen the fork due to the fact the air volume is reduced and will compress faster forcing the fork to rely more on the rate of oil flowing through the orifice to provide the dampening.  The air volume acts as a rising rate spring installed above the oil. More air, softer spring, less air, harder spring. 

[Gryphon Rider]

...the amount of air volume also aids in the dampening.  The lower the oil level increases the air volume.  This larger volume of air will compress less thereby making the fork feel softer, whereas a higher level of oil will stiffen the fork due to the fact the air volume is reduced and will compress faster forcing the fork to rely more on the rate of oil flowing through the orifice to provide the dampening.  The air volume acts as a rising rate spring installed above the oil. More air, softer spring, less air, harder spring. 

I can't see how a stiffer fork will compress faster.  Less air doesn't mean less resistance to compression.

Two things affect compression speed and thus stiffness of the fork.
1. Spring rate - a higher spring rate means it takes more force to compress the spring a certain amount, and if the force of the bump is less than the force required to compress the spring by the height of the bump, the upwards force of the bump will be applied to the the bike and rider.  A stiffer spring rate will more more quickly reduce the speed of compression of the fork.
2. Damping rate - when more force is required to allow the fork oil to pass through the damping rod orifice at a particular rate, this indicates a higher damping rate.  More viscous oil or a smaller hole means a higher damping rate.  A higher damping rate will also more quickly reduce the speed of compression of the fork, and may even reduce the initial speed of compression of the fork by instantly transmitting some of the initial speed of the bump directly to bike.

These two things are independent of each other (i.e. increasing spring rate by adding oil or changing springs does not affect damping, and changing oil weight or orifice size does not affect static ride height) but both affect perceived stiffness.  They also affect handling characteristics, but in different ways.

This is as detailed as my explanation is going to get right now, as I need to direct my attention more to my engineering studies.

http://www.motorcyclistonline.com/howto/122_0509_motorcycle_suspension_damping/index.html

[lucky_1_chris]

I agree with both Rio Wil and John U.

Assuming that a few ounce change in oil volume will affect the function of the shock would mean that the shock is not only oil tight but air tight to boot.  I don't think it was designed to be that way.

I feel the only way to definitively change the performance of the shock is to use a different weight oil, changing springs not withstanding.

Remember that there is a damper in the right side shock and none in the left side so that changes how each will react to the different weight oil.  But that really doesn't make much difference as long as the front axle is tight and the binders on the triple tree are tight. The whole assemble acts as a unit.

It would be interesting to drill a vent in the top caps on the shocks and see what the result would be.

***

 

C'mon Ricky....

[R J]

I have progressive front springs in MGM.   Have had them since the VRCC had a Rally type meeting in Custer, SD during bike week.

Progressive had a tent real close to where Lamont was changing trigger wheels for customers.

Progressive, told me to use Bel-Ray 10W fork oil and to leave out 1 OUNCE in each fork for each change.     At the time I had 113K on the bike and it now has 242K.   No fork seal problems as of this writing, so they must know what they are talking about.    I've changed the fork oil about 8 or 9 times in that last 129K.

The Chevy Trike has a Harley Wide Glide front end on it and it also has Progressive front springs to hold that big V8 up.   Progressive also said to do the same with it, 1 ounce less per oil change.    Just changed fork seals and put in a 7 degree tree, drives like it has power steering and a beautiful ride.  I can now drive it in the twisties with one hand.     If I should by another Trike, it will also get a degree tree in it.   I got my tree from Daryl at the XF 6 Store.

As Forrest Gump would say:
“That's all I have to say about that.”