Direct Acting Spring Damper

Ritwik,

Only time for short comments...

1. Excel file has TOO MANY NUMBERS!!! Redo to ONLY SHOW 3 SIGNIFICANT DIGITS per entry ... AT MOST!

2. Your Static-Deflections of Xf = 1.6 cm, and Xr = 1.9 cm look OK. They are at softer end of workable range and could work well if backed up by good bump-rubbers.

3. Consider that if you corner hard enough to just lift inner-wheels, then outer-wheels get double their static load, and hence outer-suspension deflects an additional Xf,r = ~1.6 - 1.9 cm (assuming ~horizontal n-lines, = ground-level-"RCs"). Since half-track = ~57 cm, this gives Roll-Angle = ~1.7 degrees (in Steady-State, but maybe more in "transients" if underdamped...). So, if you only want 1 degree maximum SS-Roll-Angle, then fit springs with 1 cm Static-Deflection. (<- See how easy this is to work out! :))

4. Length adjustable damper-rod-extensions are a good idea, so that you can LOWER YOUR STATIC RIDE-HEIGHT. Down to 4 cm (1.5") is good.

5. Final LLTD will depend on the type of diff you have, and other real-world issues. Generally, start at 50:50, then soften the springs at the end that needs more grip. If spool-diff + difficult-turn-in, then stiffen rear-springs and dampers. But much bigger changes can be made by adjustments to tyre-pressures, toe-angles, camber-angles, and ... driving-style... All this best addressed in testing. MUCH TESTING...

Anyone have other comments? (I haven't had time to check the equations...)

Z

Quick, change your significant figures before Z reads it!!!!

Anyway,
1. Not sure why your chassis design will cause your MR's to be fixed? Just design a different rocker/change S/D direct mounting (sorry, can't remember which)
2. Relates to 1
3. Your LLTD can be decided by you by plucking a number from your head and tuning your car from there. You could start at 0.5 if you wanted. This is then tuned via ARBs/spring rates
4. Your roll gradient (assuming you mean degrees body roll per g of lateral acceleration) is determined by your static wheel deflection expectation at your maximum lateral acceleration
5. -
6. -

There's nothing wrong with using wheel deflection. Frequencies are only really required when you are trying to specify your damping characteristics, but it doesn't look like you're there yet.

Well, Z beat me to it

Ritwik,,

I suggest you make a simple Excel spreadsheet of the variation Vs time of your front and rear chassis ride height variation (or wheel travel Vs the chassis if you want) after a sudden impact of a vertical force at the wheel. You will have a nice sinusoidal output with logarithmic decrements.

Make it simple by using a damping coefficient with is linear (same at low and high speed) and symmetrical (same in bump and rebound)

I suggest 1.5 G of vertical acceleration of initial impact. Ignore the tire deflection in a first calculation and input it in a second one.

Even at the first period, the amplitude will be significantly smaller once you include your damping.

With a damping coefficient of 0.3 the first amplitude variation (without tire) will be about 65 % of your amplitude without damping.

With a damping coefficient of 0.9 the first amplitude variation (without tire) will be about 40 % of your amplitude without damping.

You can then repeat the same calculation in pitch and roll.

Choosing your static ride heights or your springs stiffness or your suspended mass frequency on the simple criteria of suspension deflection without taking into account your damping will lead you to either to too high cars or too stiff suspensions or both.

Claude,

I think you should re-think above comments. Your very long signature at the bottom of your posts implies a responsibility to get these things right.
~o0o~

Quote:

---

With a damping coefficient of 0.3 the first amplitude variation ... will be about 65 % of your amplitude without damping.

---

Much, much less! From memory a Damping-Ratio of 0.3 gives an amplitude reduction, per cycle, of something like x 20%, or less. So, each successive peak is one-fifth as high as the preceding peak (or less?).
~o0o~

Quote:

---

With a damping coefficient of 0.9 the first amplitude variation (without tire) will be about 40 % of your amplitude without damping.

---

Huh!!!??? It should be obvious that 0.9 is quite close to 1.0. And, BY DEFINITION, a system with Critical-Damping (ie. DR = 1.0) NEVER returns to its zero position! It has NO "first amplitude".

In practice there is always some extra friction involved, above that of the dampers themselves, and usually of the "stiction" type, and this is usually enough to also prevent a system with DR = 0.9 from getting back to its zero. So abovequoted "40%" should be more like 0%!

A good rule-of-thumb for fast return to zero with negligible overshoot is DR = ~0.7. Less than 0.7 gives faster return, but with some small overshoot. More than 0.7 gives very sluggish return, and may not actually get back to zero. (Simple, 1 DoF, Spring-Mass systems being considered here.)
~o0o~

Quote:

---

Choosing your static ride heights or your springs stiffness or your suspended mass frequency on the simple criteria of suspension deflection without taking into account your damping will lead you to either ... too high cars or too stiff suspensions or both.

---

This whole sentence makes no sense to me, at all. It seems to be an irrational slur against the use of "suspension/static-deflection" as a simple number used to characterise a simple suspension's stiffness.

So, Claude, can you please explain your thinking here in more detail. Advantages, disadvantages, etc. Please be as clear and rigorous as possible.

Z

"impact" i.e. "impulse"

Quote:

---

Originally Posted by Claude Rouelle

... make a simple Excel spreadsheet of the variation Vs time of your front and rear chassis ride height variation ... after a sudden impact of a vertical force at the wheel. You will have a nice sinusoidal output with logarithmic decrements.

Make it simple by using a damping coefficient with is linear...

Even at the first period, the amplitude will be...

---

Goost,

Your graph confirms that the logarithmic decrements, per period (= cycle), are as in my last post. Damping-Ratio = 0.3 reduces the amplitude to ~15% per period, and DR = 0.9 reduces amplitude to well under 1% per period. Also evident is that DR = 0.7 gets back to zero quickly, with only small overshoot. (FWIW, the "logarithmic decrement per period" = ~ e^-(2 x Pi x DR).)

BUT (!), if Claude's post was suggesting that the heights of the first peaks in your graph (ie. after one-quarter period) somehow imply that DR = 0.9 is better than DR = 0.3, because of the lower peak for 0.9, then I say that is VERY BAD ADVICE!

Why? Firstly, note that your graph is NOT representative of "chassis ride height variation" under Claude's assumed conditions. It should be quite obvious that a suspension with very stiff damping will transmit to the chassis much more of the "sudden impact ... at the wheel", than a more softly damped suspension. The stiffer damping launches the car higher into the air whenever it hits a "sudden vertical impact", and generally makes a real mess of the tyre Fzs. This drastically lowers grip.

Secondly, you might wonder why damper manufacturers started fitting all those expensive blow-off valves to their dampers, all those years ago. In short, on a racetrack with "sudden impacts", linear dampers with DR = 0.9 are a disaster. Softer damping (via blow-off) and good bump-rubbers are a much better solution. Of course, on millpond smooth FS/FSAE tracks you can get away with excessive damping, because "Any suspension will work, if you don't let it ... Although in FS you should dress it up with some pretty (but irrelevant *) graphs, so it looks like you know what you are doing...".

(* Your graph is of an upward "unit impulse" (= finite delta of momentum) applied to the wheel with the chassis rigidly fixed to inertial space (= infinitely massive). This is hardly realistic.)

Z

Z,

I'm pretty sure mass (+ maybe coulomb friction) is the only thing that transmits the 'sudden impact ... at the wheel' if we're being pedantic.
The other effects all require motion.
But, I know what you meant and won't sit and argue about that because it's childish.

"stiffer damping launches the car"
Maybe so, maybe you meant friction? There are plenty of reasons high damping rates are better than lower ones...

~~~

chassis fixed? No. Why do you say that? the mass in the model is the chassis, not the wheel. "Ignore the tire deflection..."
So the impulse is either a force on the chassis, or a displacement at the ground, which have the same response (ratio) for impulse.

Anyway you are of course right that it's "hardly realistic" , though it's certainly not "hardly useful"

If you want to get into a more complex model, how about set up some conditions in a model or provide one?
Here's a one wheel bump (like rolling over a broomstick with one tire) in a 7-dof ride model with nonlinear damping curves and friction to boot.

Now please tell me from this what damping I should change based on what you see here?
(and perhaps to stay on topic, why should I use direct-acting spring dampers instead of whatever this is?)

This picture is near useless unless I provide every spring rate, damping rate, what anti-roll/anti-pitch springs I use
which are of course useless without mentioning track, wheelbase, inertia properties, wheel-set mass
which are of course useless without damper hysteresis since this is small amplitude
which are of course useless without mentioning the input Amplitude since we are now nonlinear.
And of course not forgetting all the assumptions about rigid suspension linkages, no temperature dependent springs and damping, etc. etc.

I think there is much more to see in my previous plot for discussion, even if it is 'hardly realistic'.

~~~

I'm worried the concept of 'static suspension deflection' is going to confuse people if we include friction, because then the term isn't quite right, is it?
Even if we were accounting for preloaded coil-overs the 'static suspension deflection' won't quite be the static suspension deflection.

Sorry for the late reply . Some health issues.
Anyways back to topic

1.Claude, I am attaching the response functions for my Front and Rear Ride Frequency(~3.9 & 3.6 Hz respectively) along with a 2 Hz to see the difference with DR of 0.3 and 0.9 .The amplitudes on the y axis are not the same as Goost because he assumed (Zo=1) while I took the Boundary Conditions as y(0)=0 (or z(0)=0) and y''(0)=1.5g(verified it be around 65% and 40% respectively).Will do in roll and pitch and update this post in a while.
Attachment 641

2.Claude, I know that I have to also consider damping in roll,pitch and heave while calculation of ride and roll rates. So I made an excel sheet and combined them both.This is the excel . The figures in there are not final yet and have to be finalized . These are the initial calcs

https://docs.google.com/spreadsheets...it?usp=sharing

3.Since I already have posted , that we are using a Quarter Midget Damper(http://www.kaztechnologies.com/quarter-midget/). So the knee speed is fixed (not a damping speed as in the excel sheet) and we have to choose among 25 options(5 compression and 5 rebound) for valve code.I am attaching the valve codes again (so that you don't have to go to the first page).
Attachment 642
4.For Roll Gradient a value of around 1.76 is deemed okay.I aim to tune it in testing.
5.For LLTD as per my calcs I am starting at 50:50 and aiming to tune in testing too.
6.To help the above cause I have thought to buy 20% softer and stiffer springs than that calculated both for each corner of the car.
7.Jay the MR is fixed because the points on uprights are also fixed .Although MR can be changed by offsetting the shock point from the LCA upright point but that would only result in additional bending moments in the X direction due to Fz and in the Z direction due to Fx without achieving any significant change in MR.
So therefore MRf=0.766 and MRr=0.5
8.Z you told that the given static deflections/frequencies are on the softer side, but the typical FSAE frequencies are in the range of 2-4 Hz. and mine is 3.5+Hz. So its stiffer right ? with the obvious implication of less mechanical grip but better response in transient and lower ride height and center of gravity.




On another note , Claude you said that you are planning to hold a seminar in India (and also asked us about the price that we maybe willing to pay).I hope you have not forgotten that Sir because it is not included in your seminar calendar for 2015.

Ritwik, I just wanted to point something out to you which might be a slight problem.
You have in your spreadsheet a rear Spring rate of ~180N/mm which is over 1000 lb/in. Do you know if you can buy such stiff springs that will fit on your damper? If not have you tried contacting someone that will make custom springs for you in about 150mm free length with 35mm diameter? I don't think it will be that easy.

Your values look correct to me, just ask yourself it would be easier to change the rear motion ratio than work with what you have. You have all the information to make an initial guess at what damping codes you need as an initial guess. Pick one based on the information you have and go testing. Learn and repeat.

I see that "blind thinking" (cogitatio caeca) is strong here. Just pick an equation, any equation...
~~~o0o~~~

Goost (Austin),

Quote:

---

Here's a one wheel bump (like rolling over a broomstick with one tire) in a 7-dof ride model...
...
This picture is near useless unless I provide every spring rate, damping rate, what anti-roll/anti-pitch springs I use
which are of course useless without mentioning track, wheelbase, inertia properties, wheel-set mass
which are of course useless without damper hysteresis since this is small amplitude
which are of course useless without mentioning the input Amplitude since we are now nonlinear...

---

I agree 100%! Useless!!!

And far too much of that nowadays. Too many vaguely chosen examples, which are specified in sloppily worded sentences and presented in contextless plots, and which are in no way representative of a given problem, yet nevertheless emphatic conclusions are still drawn from their useless results.

Quote:

---

I think there is much more to see in my previous plot for discussion, even if it is 'hardly realistic'.
[with this plot referring to]
... the mass in the model is the chassis, not the wheel. ... So the impulse is either a force on the chassis, or a displacement at the ground, which have the same response (ratio) for impulse.
[and]
... I'm pretty sure mass (+ maybe coulomb friction) is the only thing that transmits the 'sudden impact ... at the wheel'...

---

I still have no idea how your above model/plot is supposed to represent Claude's original problem of "... ride height variation ... after a sudden impact of a vertical force at the wheel."

No, "mass", in itself, CANNOT "transmit the sudden impact".

Rather, any force or motion at the wheel is transmitted to the chassis through the spring-damper (neglecting "jacking-forces" acting through the control-arms). The springs transmit the force as K x extra-spring-deflection, so giving greater "impact" with greater K (and this K includes bump-rubber at end of stroke). The dampers transmit the force as C x damper-velocity, so greater "impact" with greater instantaneous-C (or Damper-Ratio, "zeta").

So, assuming a car travels at a given velocity over a given sized bump, and all-else-equal, it is bleeding obvious that the car with greater DR (eg. = 0.9 vs 0.3) feels the greater "impact". That is, its chassis launches higher into the air, which is the OPPOSITE of your plot. This, of course, is the whole point of fitting suspensions to cars in the first place (ie. softer-suspension = better-bump-absorbancy)!

It is really disappointing to me that so many young "engineers" can produce so many pretty plots without seeing the obviousness of their flaws!

Quote:

---

I'm worried the concept of 'static suspension deflection' is going to confuse people if we include friction, because then the term isn't quite right, is it?

---

The use of "Static-Deflection" to characterise a suspension's "stiffness" was originally brought up as an alternative to "F&R Ride-Frequencies" (<- on a thread of same name).

Both concepts are flawed in that they both rely on many, usually unspecified (!), simplifying assumptions. For example, "SD" implies linear spring-rates, which is usually only approximately the case. But the "RF" concept is even more ridiculous, because its simplifying assumptions are almost NEVER met in practice (eg. CG position and Pitch-MoI must both be "just right", which is very rare, +++).

Neither concept says anything more about the suspension than the other. Well, except that, since SD = (M x Ge)/K, and RF = sqrt(K/M), the SD concept implies that you are on a planet with same gravity "Ge" as Earth. So for "moon-buggies" you must use Gmoon...

But, bottom-line IMO, is that "RF" is used nowadays for no other reason than that it sounds oh-so-clever. By contrast, "SD" is so-stupid-simple that I calculated suitable numbers without even using pen or paper (see earlier post).
~~~o0o~~~

Ritwik,

Quote:

---

Z you told that the given static deflections/frequencies are on the softer side, but the typical FSAE frequencies are in the range of 2-4 Hz, and mine is 3.5+Hz.

---

Despite claims to the contrary, many FSAE cars have run, at the actual competitions, with effectively RIGID suspensions. This is obvious from seeing the cars visibly "bouncing on their tyres", because no suspension movement to damp-out such bouncing. Such cars have, in fact, been outright winners of FSAE competitions. These cars would have been even faster if they had softer suspension.

Your current spring-rates are about right.
~~~o0o~~~

Claude,

I see that Ritwik has included calculations for Pitch and Roll inertias and frequencies based on "the parallel axis theorem". As you may be aware, I believe that approach is deeply flawed.

So, rather than you spending so much time "Politeness Policing", in insisting that Forum-newbies properly introduce themselves, could you please spend some time clarifying why you think the parallel-axis-theorem is necessary? FWIW, I think I know why you use that approach, but it has NOTHING to do with "oscillation frequencies".

After all, isn't this Forum supposed to be about discussing technical engineering issues, rather than enforcing unnecessary social protocols? (For example, I have had countless very polite, very long, and very interesting technical discussions with people at racedays, etc., without ever exchanging any names! Only if future contact is sought is it necessary to exchange names, business-cards, etc.)

Z