Use of multi-able mass dampers

[horse]

I think one of the consequences of the mass damper is that it will keep the ride height more constant. This therefore has a (positive) aerodynamic influence.

There is an interesting piece about them here.

[Ciro Pabón]

...

The Citroen 2CV used such a system.

I would like to add that this happened (initially) because the Citröen had outboard brakes, so they needed the system in the rear wheels (initially, again) to dampen the extra unsprung mass, which would have forced the rear wheels to bump. I once drove a Citröen with a broken damper and it was horrible.

Reynolds, in his book about Citröens, wrote that "later models, with no outboard brakes, had tuned mass dampers at the front because the leading arm had more inertia and 'bump/thump' than the trailing arm, with hydraulic telescopic dampers / shock absorbers front and rear. The uprated hydraulic damping obviated the need for the rear inertia dampers".

[bill shoe]

Your responses are fantastic.

Any knowledge or guesses regarding what frequency the Citreon device was tuned for? Ciro's post implies the hop frequency (typically 10-15 Hz?) but this mode already has some damping from the conventional suspension because it involves the upright moving relative to the body. The mode with no damping from the conventional shock is tire bounce (conventional wisdom on internet says 2-6 hz for street car), so this seems like a plausible candidate too.

Hmmm, now that I think about it, the wheel hop mode is the result of parallel compliance from both the (damped) conventional suspension and the (undamped) tire stiffness.

Tire bouncing of course results from tire compliance alone.

So the undamped tire contributes to both modes, but the modes respond at different frequencies.

[timbo]

Rubbish, if I may say so.... The ban had much to do with Ferrari's claim that they didn't work with Bridgestone tyres. That was also rubbish. The most likely reason was that Ferrari failed to tune them to the correct frequency.

I heard it came from other team than Ferrari.
Most likely Toyota or Macca.

[747heavy]

The mode with no damping from the conventional shock is tire bounce (conventional wisdom on internet says 2-6 hz for street car), so this seems like a plausible candidate too.

Hmmm, now that I think about it, the wheel hop mode is the result of parallel compliance from both the (damped) conventional suspension and the (undamped) tire stiffness.

Tire bouncing of course results from tire compliance alone.

not 100% sure I can follow you here Bill.
what is the base of this "conventional wisdom"?
If you see the tire in isolation, and neglect it´s internal damping, you end with
and mass of the tire/rim and the spring rate of the tire.
Most likely this will result in higher freguencies then 2-6 Hz, don´t you think?

And do we talk about road/race cars in general, or specific F1, if the former why to you think that this fequency is not damped by the "normal" suspension?
As long as the suspension moves, the dampers normally dissipate energy out of this movement.

The mass in the 2CV suspension is said to have been ~3.5 kg

[PlatinumZealot]

Ok.. let's get on thing straight here.. The mass dampers are there to absorb vibrations that the regular shocks cannot. riiight? From what I can scrape up from my memory is that the frequency of vibration depends on the Spring constant of the system, the mass of the system, the damping constant - and the initial displacement (or a if you want to work in terms of forces you can) right? And we can break up the F1 car into multiple vibrating systems that are connected to each other... right?

If we look on the mass damper as one system, the mass damper has no damping unto itself OR ELSE it would just be another regular damper. The mass damper has one small range of frequencies that it can work at. A viscous damper can work at a large range of frequencies. Correct or Incorrect?

[bill shoe]

not 100% sure I can follow you here Bill.
what is the base of this "conventional wisdom"?
If you see the tire in isolation, and neglect it´s internal damping, you end with
and mass of the tire/rim and the spring rate of the tire.
Most likely this will result in higher freguencies then 2-6 Hz, don´t you think?

And do we talk about road/race cars in general, or specific F1, if the former why to you think that this fequency is not damped by the "normal" suspension?
As long as the suspension moves, the dampers normally dissipate energy out of this movement.

The mass in the 2CV suspension is said to have been ~3.5 kg

My previous rambling was probably not as clear as it could have been.

When I talk about the 2-6 Hz mode I am referring to "boulevard ride" and I believe you also gave some common names for this mode recently. This mode involves little or no suspension movement (caused by any number of reasons) which results in the entire car mass bouncing up and down on the spring rate of the tires. The 2-6 Hz figure applies to street cars and is the result of a brief internet search I did for "boulevard ride" (BR).

If a car was operating in pure BR mode then a TMD would clearly help, and it wouldn't much matter if the TMD was located on the upright or the main body (since BR means no relative motion between them).

If the main suspension was operating normally then the suspension stiffness and tire stiffness act as springs in series between the road and the sprung mass. The conventional shock is good at taking energy out of the conventional suspension, but the tire does not have much effective damping to get rid of its energy. If you crank up the damping on the conventional shock too much then you actually make the underdamped tire energy worse (you get closer to pure BR mode). I believe DaveW has given general examples where he was able to improve overall energy dissipation at a corner by reducing shock stiffness.

I think the purpose of a TMD on the unsprung mass is to directly damp the tire energy so the conventional shock absorber does not have to be compromised (softened) to accomodate the underdamped tire.

[timbo]

The mass dampers are there to absorb vibrations that the regular shocks cannot. riiight?

If we look on the mass damper as one system, the mass damper has no damping unto itself OR ELSE it would just be another regular damper. The mass damper has one small range of frequencies that it can work at. A viscous damper can work at a large range of frequencies. Correct or Incorrect?

The two statements above contradict each other IMO.
Yes, regular dampers "absorb" vibrations, or more correctly they dissipate energy. A mass on a spring would not dissipate energy so they are very different things, and it is misleading that both are called "dampers" IMO.
The wiki article is quite good
http://en.wikipedia.org/wiki/Tuned_mass_damper

[747heavy]

O.K. Bill,
I´m with you now.
I will freely admit, that this is not my field of expertice.
So yes, if there are road cars/tuning cars out there which choose to "lock out"
their suspension, for one reason or the other, either by excessive spring rates or low speed damping, then I agree with you.
you will end up bouncing on your tires, if you excite the system at the "right" frequency.

The ratio of suspension stiffness/tire stiffness plays it´s part here.
If this is the case, you probably can improve the situation in the from you suggested way. (TMD at the upright)
But increasing the unspung weight, has it´s downsides too.
If you have "locked out" all for suspensions this way, you may need to tune for bounce and pitch frequencies individual.

As long as your suspension is in "lock out" mode, you can place the TMD at the sprung mass ala F1 Renault front/nosecone.

If you have a workable suspension, it will move and an inboard TMD, is maybe not as effective.
I think DaveW made a good point in regards to their use at the rear of an F1 car.

[747heavy]

Ok.. let's get on thing straight here.. The mass dampers are there to absorb vibrations that the regular shocks cannot. riiight? ....

a TMD does not disipate energy, that is correct, it changes the frequency response of an given spring/mass system, by shifting the resonance frequency.

In simple terms you can look at it, as an oscillation with an 180° phaseshift in relation to your base oscillation, similar to the concept used in noise cancellation headphones.

At the correct frequency (tuned), the mass in the TMD will move down, while the mass in the base spring/damper systems tries to move up and vis versa.
Imagine a small hammer which will hit onto your tire every time it tries to move up, forcing it down again

a bit idealized like this:



A+B would be the load variation at the tire contact patch.

you try to shift the resonance frequency of the base system, out of the normal working range.



http://www.pump-zone.com/index2.php?opt ... orm&id=213

[timbo]

I wonder whether the area under the FR curve is the same with TMD and without.

[PlatinumZealot]

The mass dampers are there to absorb vibrations that the regular shocks cannot. riiight?

If we look on the mass damper as one system, the mass damper has no damping unto itself OR ELSE it would just be another regular damper. The mass damper has one small range of frequencies that it can work at. A viscous damper can work at a large range of frequencies. Correct or Incorrect?

The two statements above contradict each other IMO.
Yes, regular dampers "absorb" vibrations, or more correctly they dissipate energy. A mass on a spring would not dissipate energy so they are very different things, and it is misleading that both are called "dampers" IMO.
The wiki article is quite good
http://en.wikipedia.org/wiki/Tuned_mass_damper

That is EXACTLY what I said before. I just went along with everybody else calling them "dampers." check my previous posts in this topic.

My real intention in the above post though, is to get a sense of what the design targets are (Real numerical targets)

[PlatinumZealot]

Ok.. let's get on thing straight here.. The mass dampers are there to absorb vibrations that the regular shocks cannot. riiight? ....

a TMD does not disipate energy, that is correct, it changes the frequency response of an given spring/mass system, by shifting the resonance frequency.

In simple terms you can look at it, as an oscillation with an 180° phaseshift in relation to your base oscillation, similar to the concept used in noise cancellation headphones.

At the correct frequency (tuned), the mass in the TMD will move down, while the mass in the base spring/damper systems tries to move up and vis versa.
Imagine a small hammer which will hit onto your tire every time it tries to move up, forcing it down again

a bit idealized like this:



A+B would be the load variation at the tire contact patch.

you try to shift the resonance frequency of the base system, out of the normal working range.



http://www.pump-zone.com/index2.php?opt ... orm&id=213

I like this explanation.

[747heavy]

My real intention in the above post though, is to get a sense of what the design targets are (Real numerical targets)

nothing earth shaking, but maybe you find this worth a quick read.

http://www.roush.com/Portals/1/Download ... VC-217.pdf

one of the most common automotive applications, are probably crankshaft dampers.
replacing the "spring" element with an elastomere, adds some "real" damping (energy dissipation)- as seen with the M3 gear box mount.

[bill shoe]

If I'm clarifying this then we're in trouble. I think this is correct--

A vibration absorber modifies the resonant frequency of a system. It re-allocates resonant sensitivity to other (hopefully less harmful) frequencies. A perfect vibration absorber with no friction has no damping.

A tuned mass damper is a spring-mass system with an actual damper on it. This is sometimes a conventional hydraulic damper. The TMD therefore has some ability to directly remove energy from the system as the spring-mass system vibrates. The effective damping rate may be low, but the difference between a little damping and no damping can be important.

Wikipedia seems to have good articles about VA's and TMD's.