There has been discussion on the board about suspension, valving, spring rates, etc. I know that for many of us there is still a lot of mystery about how it all works. I think it is very interesting and thought I would talk about suspension fundamentals from an engineer's perspective. I know some of you have more tuning experience and I
don't claim to be a tuning expert. This is more from a standpoint of unshrouding the mystery of how suspension works.

First off, it is helpful to introduce the differential equation that describes the time-varying force that keeps the tire pressed against the pavement through all of the variations in bumps, rises, dropoffs, cornering, braking, etc. This equation has the form Kx + Dv + Ma = F(t). As you can see, the suspension force F(t) is made up of three components, a spring force, Kx, a damping force Dv and an inertia force Ma. The spring force is proportional to suspension position (where it is in the range of travel). The damping force is proportional to suspension velocity (how fast it is moving, or rate of change of position). The inertia force is proportional to how fast the suspension is accelerating (rate of change of velocity). The proportionality constants are spring rate K, damping rate D, and unsprung mass M, all of which are tunable parameters.

The spring rate is chosen for a given bike and suited rider weight. Again the spring's job is to provide higher force at higher travel (we all know this, but these are the fundamentals, broken down to their components). It is interesting to note that, since the bike and rider sit on top of the springs with some amount of sag, changing preload does not compress the springs more or less, it only changes where the bike sits in its range of travel and can be used to adjust the bike "attitude" or front/rear ride heights.

Unsprung mass is all of the stuff that does not rest on the suspension (wheels, brakes, etc.). Changing the weight of these components changes the inertia force. Lighter components will accelerate faster for a given input force.

This brings us to damping rate. Again damping controls suspension force according to how fast it is moving. The goal of the damper is to provide an approximately linear variation in damping force throughout the range of velocities encountered by the suspension. In modern motorcycle suspension systems this is done with a combination of orifice passages and deflecting discs (shims). Basically there are three different, highly nonlinear flow controls, which, placed in series allow for approximately linear damping over the desired range of velocities.

At very low suspension velocities (low-speed damping), oil flows through an orifice, with a needle that controls the size of the orifice. This is the external damping adjustment. On VTR forks the low-speed compression damping orifice has a fixed diameter. Rebound damping is adjustable. Compression and rebound damping are separate circuits. As suspension velocity increases, the force required to push oil through the orifice increases rapidly. In simple orifice damping systems, to have good low-speed damping you end up with hydraulic lock at higher suspension velocities, due to the stiffening effect of increased flow resistance.

Hence the need for the next stage. As the velocity continues to increase, the pressure in the oil becomes high enough to start bending the shim stack on the valve body. The flow is then a combination of the flow through the needle/orifice and through the clearance opened up by the deflecting shims. The orifice body has several passages that supply the back side of the shims. If you look at a plot of damping force vs. supsension velocity, you will see a "knee" where the shims start to open, bringing the curve back to a nearly straight line. This is the mid/high
speed damping regime. Where the shims start to open and how fast they open is changed by modifying the shim stack.

At still higher suspension rates, the damping provided by resistance to flow between the shims and the valve body increases at a lower rate because the gap becomes so wide that small changes in gap have less effect on resistance. At this point, going back to the damping force/velocity curve, the line starts curving back to horizontal and
damping no longer increases with velocity. This is where the third stage starts to take effects. At these very high suspension velocities, the size and number of orifices in the valve body control flow resistance/damping rate. Honda/Showa and Ohlins forks have relatively small orifices. The RC51 body uses five orifices, I believe the VTR body has four. Suzuki/Showa forks and RaceTech Gold Valves use five large orifices (Gold Valve orifices are huge).

I have heard several cases where a bike will run wide when pushed hard in bumpy corners. This has been attributed to too little high-speed compression damping allowing the wheel to overtravel and lose contact pressure with the track. The fix is to change to a valve body with smaller orifices, to keep the mid-speed compliance of a given shim
stack and increase high-speed damping. For normal street riding this would not generally be an issue.

For more information with photos and graphs, go to www.peterverdonedesigns.com. He does a good job of illustrating
these differences.

I hope this takes some of the mystery out of suspension tuning. Sorry if I got a little long-winded. I look forward to other people's comments.
Happy tuning!

RC

 

I tried to capture the "essence" of the derivative, using rates of change in a form that non math types could have a feel for.

regarding the port size in the valve body. Yes, I'm sure that orifice size does effect when the shim stack opens, and I believe that is the reasoning that Race-Tech uses. However, the shim stack stiffness is tunable for a given valve body, allowing smaller orifices to handle the very high-speed stuff, for a longer linear damping range. Also, I think the smaller orifice allows for a smoother, more gradual opening. It's part of the magic that makes Ohlins suspension work so well.

That's how I understand it anyway..