Position sensitive damping/ non linear spring rates

[autodoctor911]

Can anyone refer me to any info on any attempts at making dampers for f1 or other on track racing that have damping rates that change based on the displacement of the unit?

Or, any discussion of it's merits or drawbacks, or method of implementation is also welcome.

I could envision a dual rate setup with either an external bypass or an internal rod that would close off a piston valve at a set displacement, or even a tapered rod that could vary the flow through the pistons progressively over the Pistons travel.

As to any benefits, I could imagine a reduced overall spring/damper rate at near normal ride height for low speed turns, and stiffer for high downforce conditions.

[Greg Locock]

As usual track lags behind production vehicles, position sensitive damping is known technology, Fox in particular. Typically used in off roaders.

[Greg Locock]

Oh, and non linear springs have been around since whenever. Subjective problem is coil clash, objective problems are fatigue life.

[autodoctor911]

The non-linear spring rate is often used on road cars, or I guess off road racers, but not really with most race cars. I think that could change if position sensitive damping is used to compliment it.

I suppose a lot of the same kind of effect is built into the changing motion ratio on a race car, especially with push/pull rods and rockers. Maybe this is why position sensitive damping is not very useful. When the motion ratio changes throughout the travel, both spring rate and damper rate are changed simultaneously.

[DaveW]

.....When the motion ratio changes throughout the travel, both spring rate and damper rate are changed simultaneously.

Forgive me, but I think that statement is misleading.

For a simple mass, spring & damper system, changing the motion ratio of the spring & damper combination will require a change in damper settings - if the modal damping ratio is to be maintained.

For non-linear systems (e.g. rising rate) the set-up task is even more complex...

[Tim.Wright]

I think he was referring to the effective wheel rates, not the actual rate at the spring/damper.

[autodoctor911]

I know my use of the words Spring and damper rate were incorrect, but would not the effective wheel rate for the spring and damper force change at the same rate, keeping them close to optimal, well at least compared to just having a progressive spring with a damper that has a fixed damping in relation to travel? If the damping is less than optimal during certain ranges of a rising rate motion ratio systems travel, due to the change in motion ratio, then that would be a possible reason to explore the use of position sensitive dampers.

Anyway, I would like to see if there are any uses in on pavement racing for a damper that can bypass for a softer force during part of it's travel, or get more firm by closing off or restricting a passage of fluid for part of it's travel, as has been seen in off road racing.

[holeindalip]

Check this out


http://www.f1technical.net/forum/viewto ... &start=825

[DaveW]

.... would not the effective wheel rate for the spring and damper force change at the same rate, keeping them close to optimal, well at least compared to just having a progressive spring with a damper that has a fixed damping in relation to travel?

I think that a vehicle set-up that is optimal, increasing motion ratio - defined as (Spring_Rate/Wheel_Rate) - will require the damping coefficient to be increased in order for (optimal) damping ratio to be maintained. Grinding through the algebra should confirm that (hopefully). Doubling motion ratio will require a doubling of damper coefficient- which is a good reason for limiting motion ratio...

"Rising Rate" geometry complicates things because peak force is usually generated at maximum velocity for the dampers, and at extremes of travel by springs.

You asked about displacement dependent damping coefficients. Patent US6049766 sets out the Lord Corporation's ideas at 2000 on stroke limiting , which was remarkably similar (hence saving me the task of describing it) to the algorithm we used in the Lotus Active Suspension (of 1987, but was implemented some time before that). I believe that a version of it is incorporated in the Magnaride system (of Delphi, now BWI). So far as I was concerned, the idea came from aircraft undercarriage oleos. These have bean around for a while (for example Patent 2077934, dated 1937).

In motor racing, designers have been using rising rate geometries, displacement sensitive springs, and dampers, certainly since the 1970's, often with less than ideal results. The adage worth remembering is KISS.

[bill shoe]

Doubling motion ratio will require a doubling of damper coefficient- which is a good reason for limiting motion ratio...

Am I understanding correctly?... For a simple example--

A car has suspension with motion ratio 1, spring rate (at the spring) of K, damping coefficient (at the damper) of C.

Then the motion ratio is changed to 0.5. If the installation is ideal (linear, perfect linkage stiffness, lack of pivot friction, etc.) then equivalent performance at the wheel will be achieved with spring rate (at the spring) of 4K and damping coefficient (at the damper) of 2C?

[DaveW]

Am I understanding correctly?...

I was hoping nobody was going to ask that...

This contains a comprehensive analysis of a second order system. In it are two equivalent differential equations, this:



and


I then obtained the following:

[Greg Locock]

The way i was thinking of it, the shock would be moving at half the vertical velocity of the wheel, and by levers, the force at the wheel would be half the shock force, so it needs a factor of 4, like a spring. But I can't see the final image on your post!

[DaveW]

The way i was thinking of it, the shock would be moving at half the vertical velocity of the wheel, and by levers, the force at the wheel would be half the shock force, so it needs a factor of 4, like a spring. But I can't see the final image on your post!

Agreed... but the image is a PDF, and can be downloading by selecting this (hopefully).

[bill shoe]

DaveW, thanks for response. Example again -

A suspension starts with motion ratio 1, then the motion ratio is changed to 0.5. There are at least a couple different ways of looking at this change and determining equivalent performance.

1. What I was thinking-
Keep k (stiffness at wheel) constant, which requires increasing K (stiffness at spring) by a factor of 4. If damping ratio is same before and after, then the “after C” (damping rate at damper) must be 4 times as big as the “before C”.

2. What DaveW actually described-
Keep K (stiffness at spring) constant, in which case “after k” (stiffness at wheel) goes down to 0.25 of the “before k”. If damping ratio is same before and after, then the “after C” (damping rate at damper) must be 2 times as big as the “before C”.

If you simply change the motion ratio of a linear system, then both springs & dampers must change by motion ratio squared to keep equivalent system performance. However, if you make system non-linear (such as rising rate rockers, etc.) then damper will only give desired damping ratio at one point in the travel.

[Jersey Tom]

The non-linear spring rate is often used on road cars, or I guess off road racers, but not really with most race cars. I think that could change if position sensitive damping is used to compliment it.

On what are you basing this statement? Slew of ways of having nonlinear wheel rates - be it through just putting a bump stop on a shock, or letting a spring coil bind, or a nonlinear wound spring, a spring rubber, etc. Lots of racecars with nonlinear wheel rates.

Of course there are some race series which straight up deny the use of nonlinear wound coil springs by rule. But in any event, I find it hard to imagine a car engineer or crew chief saying, "Oh no no we can't run this spring, we don't have the right shock for it!" Even though as DaveW correctly states, racing tends to lag behind the production world, there's a very wide array of shock options as well, depending on series.

There are series / applications where you will happily piss away mechanical grip and let a spring bind solid, and/or throw away any sense of an ideal 'damping ratio' and use a shock that doesn't even start to move until you pull it with hundreds of pounds of force, because the aero advantage just outweighs it all.