Damper tuning for roll vs bump (or pitch)

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Jersey Tom
Jersey Tom
166
Joined: 29 May 2006, 20:49
Location: Huntersville, NC

Damper tuning for roll vs bump (or pitch)

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A thought I had the other day..

Assume you have a car with a 'conventional' double a-arm suspension.. ie single spring and damper at each corner of the car, plus ARBs front and rear.

Your dampers are active in bump, roll, and pitch. Its all coupled. Traditional thinking is to select your damper curves for slight underdamping (at least as a baseline setup) for the sprung mass in bump. Afterwards, you can worry about roll dampers if theyre allowed in your racing series.

Take the typical coned autocross course though, instead of a more open circuit. Very short straights, lots of corners, slaloms, chicanes. High emphasis on response rather than flat out peak grip. Suppose the track surface is very smooth. If your car is well-damped in bump but heavily (damping ratio of 3-4) damped in roll, it would make sense to me to select and tune your dampers for roll rather than bump, relying mosty on the tires to damp out most high speed road undulations. Much quicker, snappier transient response.

One might think you could almost 'decouple' bump and roll damping in this scenario by having extremely progressive or digressive damping curves.. for low speed (roll) and high speed (small bumps).

Thoughts?
Grip is a four letter word. All opinions are my own and not those of current or previous employers.

ReubenG
ReubenG
0
Joined: 21 Apr 2004, 15:31

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If your damper bounce / rebound plot looks like the graph below then the effect of the damper during roll should be minimised. I remember chatting to someone who built custom dampers for a living, and he said that most dampers have a response like this anyway (but they try to minimise the non-linearity) - from your description I understand that you would want minimal damping in the low velocity range and more damping for high speed bounce.

I doubt this would work in a longer track setup (where a car settles into an "equilibrium position" for a larger portion of a corner, but for an auto cross track, with very rapid changes of direction I think this would be beneficial.
Image

Jersey Tom
Jersey Tom
166
Joined: 29 May 2006, 20:49
Location: Huntersville, NC

Post

Yea a highly progressive curve like that would be nice. However, most that I find are linear to digressive. Maybe I can rework the internals
Grip is a four letter word. All opinions are my own and not those of current or previous employers.

Carlos
Carlos
11
Joined: 02 Sep 2006, 19:43
Location: Canada

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For a new perspective on this subject:
Vehicle Evaluation of the Performance of Magneto Rheological Dampers for Heavy Truck Suspensions ASME,Journal of Vibration and Acoustics,Vol 123,NO 3, July 2001.pp 365-375

This system is also used as a recoiless system foe small arms, where transients are extreme .

Carlos
Carlos
11
Joined: 02 Sep 2006, 19:43
Location: Canada

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Another way to compensate for roll vs bump , although unrelated to dampers is consideration of another suspension arrangement. I don't have a link but it is the"X Link Suspension System"

Carlos
Carlos
11
Joined: 02 Sep 2006, 19:43
Location: Canada

Post

An article on the X Link Suspension : Automotive Design and Production, April 1, 2002, published by Garner Publications

A double wishbone system---- the lower wishbone attaches to the chassis
above the upper wishbone---the lower wishbone is a "wider" fabrication
than the upper wishbone---thus it is called X Link--- in a "roll" attitude the outside wheel is loaded by the motion of the chassis. Originally intended as a measure to prevent SUV's with high centre of gravity from (eliminates the need for an antiroll bar and improves on the function of an ARB--- leaving each wheel truely independent of the action of the opposite wheel)
"roll- over" an unfortunate habit of a poor design philosophy and poor drivers--- never actually adopted for street or track--- an interesting functional innovation---well worth exploring
Patented by Todd Wagner:
Wagner Engineering, Wallington CT USA

Lukin
Lukin
0
Joined: 21 Oct 2004, 17:34

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ReubenG wrote: from your description I understand that you would want minimal damping in the low velocity range and more damping for high speed bounce.
That seems like the exact opposite of what you want. For the lower frequency inputs (pitch and roll modes) you want more damping to increase the transmissibility of the suspension (which makes it more responsive by transmitting road forces to the sprung mass) while for high frequency inputs (bumps) you want less damping to reduce the transmissibility of the bumps to the sprung and unsprung mass which increases variations in contact patch loads.

Every circuit racer I have seen has had a digressive damper curve. In most situations the high frequency inputs are high speed (kerbs, bumps) while the low frequency inputs are low speed (less than 50 mm/s).