Earlier we published a story explaining the function and benefit of the tuned mass damper to the racer. This device received much of the credit for winning the 2005 Formula One World Championship for Renault. So successful was its implementation it was banned.
Still, the technology remains relevant and incorporating the device would benefit any racer who is obliged to carry ballast. Equally interesting is the story’s reference to the use of lead shot for ballast—a technique that damps all frequencies, but with only half the effectiveness of a tuned mass damper of the same weight. Here is the story by its chief protagonist, Dave Hamer.
By Dave Hamer:
In my previous article, I covered the successful use of the tuned mass damper on F1 race cars. The principle of the TMD is similar to that of a harmonic damper on an engine, which is used to reduce crankshaft torsional modes. Similarly, some convertible sports cars use weights in the front bumper, which are tuned to reduce scuttle shake. Triumph TR6 and TR7 used them, I believe.
Our TMD story goes back to the late 1980s where a 7-post rig was the principal tool for tuning the suspension on our active-suspension cars. This rig uses hydraulic actuators to introduce four vertical movements to the tires to mimic bumps and three forces into the chassis to mimic aero and inertial loads.
Before we went to the complexity of track replay we would use a swept sine input. A sine wave played into a 7-post rig produces a smooth up-and-down movement (like the piston movement in an engine). The swept part means you start at a low frequency, say, 2 cycles-per-second and over a period of time (say one minute) the frequency is smoothly increased until 30 cycles-per-second is reached.
During this time and thanks to the information at the Nevada car accident homepage we found that by using sandbags or a crash-test dummy to mimic the driver triggered a very different car response when compared to that of a real person seated in the car on the rig.
A real person damped out a significant amount of the car’s movement. In fact, having sat in the car on many occasions during swept sine runs (2 to 30Hz), I felt various parts of my body go into resonance at different frequencies. Resonance occurs when the input frequency is the same as the item’s natural frequency, which is determined by stiffness and mass. It is why a bell makes a ringing sound. Frequency refers to the number of cycles-per-second, in this case up-and-down oscillations.
One of the more uncontrolled involved the human belly, which would adopt a life of its own, vibrating at a similar frequency as one of the car’s vertical modes.
It was these flexibilities of a real human body that was affecting the car’s vertical response. To mimic this we devised an artificial driver that had a large part of its mass resting on air springs and tuned to match a human in the car. To my knowledge most other teams still use a crash-test dummy or sandbags in the car, so they are missing a trick. However the problem is that if they use a real person, they run the risk of being sued by a worker premises liability lawyer in order to compensate said person.
To finish this part of the story, it’s important to note that the effect was so marked that on one occasion we had four or five different people sit in the car and each resulted in a slightly different car response. Each person could be identified from their data trace.
The tuned mass driver that we always used on the rig prompted the question, could we use the driver’s mass to even better effect by having a seat with some compliance that was tuned to better dampen the car’s movement? The answer was yes, but we didn’t know if the driver would feel so detached from the car and not be able to drive by the seat of his pants, as it were.
Also, by this time we were conducting track-reply simulations and invited numerous drivers to sit in the car while the rig was operating.
In the early days, Oliver Gavin, five-time class winner of the legendary 24 Hours of Le Mans, sat in the car for a Silverstone replay and announced it was “far too tame”! As a result, we altered our technique to capture a more realistic replay. When perfected, Fernando Alonso could watch the car on the rig, identify the track, and even make it known if he had been driving when the data was collected! Top drivers have a uncanny ability for recognizing the differing sequence of bumps on each circuit.
Returning to the topic of the compliant seat for a moment, one of our young trainee drivers completed numerous runs in the car on the rig and was of the opinion that the small amount of seat movement would be fine, saying he would be happy to run it on track. Sadly, this never happened as during this time TMDs were banned and we concluded we were pushing our luck, so we will never know.
The lead shot experiment
At the same time we tried replacing our TMD, which resided in a round canister in the nose of the car, with the same weight of lead shot. This still helped dampen the car’s response but was only half as effective as the tuned device. The movement and rubbing of the lead shot in the canister gave a damping force which helped calm any oscillation. Though less effective than a TMD it has the advantage of working over a broad range of frequencies, so tuning is unnecessary.
I should add that for motor sport you wouldn’t want to carry the extra weight of a TMD (some of ours weighed 10kg and placed in the nose), unless you have a light car and need the extra mass as ballast to reach the minimum weight limit for your class. However, if you need to carry ballast, why not take advantage from it?
Finally, as circuits become smoother you would think the TMD offers less benefit but it is often during curb riding where it demonstrates its greatest strengths. Double F1 world champion Fernando Alonso’s first comment, “I can take a lot more curb now; I want it for the next race.”
The tuned mass damper reminds me that someone told me a few years ago that a Russian inventor had worked out how to make a frequency-discriminating suspension damper. It was based on the observation that vehicles need damping mainly for their major resonances – otherwise, damping just slows suspension movement and reduces mechanical grip.
Ben is correct, though he is talking about a separate issue. Conventional damping is needed to deter two very different frequencies. The first is the body movement with a resonance which operates at a relatively low frequency (say 4Hz or 4 cycles-per-second on an F1 car) and the second is the wheel movement which has a higher resonance (say 12Hz on an F1 car). Outside these frequencies the damping is just interfering with the bump absorption capability of the suspension. This is why frequency-sensitive dampers were produced. We tested them on our rig and found no benefit, though we did have the hydraulically connected front to rear suspension at the time, which confuses these dampers.
The conventional damper, as you probably know, is anchored near a wheel at one end and to the body at the other and works on the relative movement of the two. The TMD works on what is called “skyhook” damping. In other words, it is like having a damper with one end connected to the body and the other to a hook in the sky, which is fixed in space. Hoping this answers your question and hasn’t caused more confusion