There are some interesting discussions going on regarding air spring, and dynamic forces associated with the air spring, and oil inside a TC fork. Many points have been made about what would happen, and how it would feel with various elements modified. It’s clear that to continue the discussions some real test data would be a benefit the understanding of some of the basic points.
Using our test equipment we where able to measure the actual total spring force or characteristic. The following numbers represent the last 7inches of stroke in Showa TC used on a 08 RMZ250. The effects are similar regardless of TC the fork, although you will see less change of the overall spring force in a KYB or MXT E7 fork because the rod charge is not affected by the pressure increase. The data represent the last seven because we can only test 7, and the first 5 as you can see from the graphs don’t really show much happening (as expected). The fork was mounted in JC’s RE Spring tester, and run through its stroke while measuring the force. You can make the following oberservations about spring rate and air spring based on these measurements.
At 250cc the only real force is the total of the springs rates (ICS and Main spring), but notice right at the end the force increases.
Notice the relatively large increase in forces as the oil volume increases, the point at which it increases is not impacted so much as the rate at which it increases.
This factors nothing in terms of speed. It is only a series of static (100’s) tests added together to create the plot.
I think this data clearly demonstrates that oil level is not an effective way to hold a fork up in the stroke. The concept of up in the stroke is generally assumed to mean the top of the stroke. Clearly nothing involving the oil level deals with the top 50 % of the stroke and the major rate of change in the force occurs in the last 75% of the stroke. This precludes the concept of “up in the stroke”.
In regards to spring seats our testing has shown they have a significant effect on overall fork damping rates. John Curea and I had dinner with Ross Maeda in 02 and he mentioned to us about how he struggled with the Showa TC spring perch when using sub tanks and how this effect impacted the fork negatively with use of higher oil levels required by the sub tanks. I was surprised by his comments. I continued to dismiss it as fork lore, but by 2005 and lots of my own testing I was also starting to sees the design implications. Subsequently we discontinued using sub tanks on TC forks, and it lead me to further investigate the use of tertiary viscous damping systems and how they where implemented in different forks.
Many of the designs are not intended to create exponential damping but fluid drag or viscous style which is linear relative to speed, while still some others are designed to be more progressive in nature. Either way the design has limitations that are often paradoxically related. We wish for smoother mid stroke, but we have good travel bottoming, we wish for better bottoming control but have a great mid stroke. With the TC forks riders and some tuners tend to prefer lower oil volumes for midstride compliance but sacrifice bottoming control. They are left with adding compression damping but that makes the fork less ride able, often being too stiff for optimal feel and traction. Another typical solution is to increase the effect of the stock oil lock system with a larger piston. This will prevent low speed bottoms efficiently but will often leave the fork with a harsh feel in slap down landing situations or breaking bumps with chuck holes at the bottom. (Think bottom of Yamaha hill at Glen Helen.)
This compromise has lead me to investigate other types of bottoming systems for forks, Needle style, Ohlin’s style 1.5 designs and finally a completely new design that I have sought patent protection for. My new design uses an open style spring seat creating little to no viscous damping and an external speed sensitive bottoming system that fits in the bottom of the fork. Low oil levels can be utilized (no progression) with out the lose of bottoming control. In low speed bottoms we have a smooth controlled action in high-speed we have potential area created by a speed sensitive digressive valve system. What’s best is that the system can respond mid cycle adjusting damping rates as the fork speed changes.
I will be releasing more information at a later point. With my design we can take advantage of the best linear characteristics with great control.
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