1936 Suspension Video

[mep]

Dave, I don't really get what that plot should tell us?
The points are all over the place and we don't know for what car the individual points are.

[DaveW]

Dave, I don't really get what that plot should tell us?
The points are all over the place and we don't know for what car the individual points are.

Mmm... You are right of course. But maybe that is the point. What I was trying to do is to relate what is actually produced with the "ideal" world presented by Olly and, latterly, by Sharp (without pointing a finger at individual vehicles). More later, perhaps.

[mep]

Well I don’t know what Olley and Sharp claimed because I did not took the time to rear all the previous stuff in detail but I actually struggle to see why there should be a clear relationship between damping ratio and rear/front natural frequency. Actually, from the plot I could read out exactly this because the points are spread everywhere.

[Blanchimont]

Maybe the plot would tell us more if there where separate plots for different types of cars (sports and race cars, pickup trucks, suv, family cars...) and for weight classes (<1000kg, 1000-1500kg, >1500kg).

But all data in one plot without knowing which point belongs to which car doesn't really tell us much.

Dave, maybe you could add some info which cars where used for the plot?

[WilO]

I suppose what you see depends somewhat on what you're looking for....personally I find that plot very interesting, particularly the data along the trend line.

Thanks for that Dave.

Wil

[DaveW]

But all data in one plot without knowing which point belongs to which car doesn't really tell us much.

When I started rig testing road cars they were almost always set-up subjectively (still are, actually). I decided that a good starting point (in order to try to be useful) was to have a "look" at as many cars as I could get my hands on (good, bad or indifferent). The cars covered the range of saloon cars (Roller's to mini's), estates (station wagons), sportscars, some SUV, the odd "pickup truck"; no race cars, however. I learned a lot from the exercise, and the results have remained useful. Inevitably, the scatter is huge, but I managed to extract useful trends, and even managed to postulate reasons to explain some outliers.

You asked about separating the results in some way (e.g. weight classes). In my view (& you are going to have to trust that), a better way of dividing the results would by country (first), and by the badge on the front (second). Cars for countries that are characterized by long distances between corners & dubious road surfaces tend to concentrate on ride, whilst cars for countries whose roads follow small field boundaries tend to concentrate on control. The badge on the front is important because the same people set-up the product range (largely subjectively).

You comment that the results don't tell you much. I disagree. The fact that there isn't an obvious relationship is important, I think. As is the fact that the dispersion increases as the damping increases. It is interesting (to me) that the first information I am asked for by a development driver are the "Quarter Car" natural frequencies (despite the fact that they don't exist, as such).

[Blanchimont]

Maybe i better should have said that the data could tell us more if they were separated in some way.
In no way i wanted to express your data is useless, i only wanted to show a possible method for maybe getting better results/relationships between the natural frequency and the damping ratio.

If your data isn't top secrect, you maybe could publish them on your site in the format "car - damping - frequency" without publishing the whole test results?

[bill shoe]

Without doing too much thinking (which I should), here is a plot extracted from a series of road car rig tests.

What an awesome trove of data. Initial thoughts--

There is the main “mid” group of vehicles with freq ratios from 1.0 to ~1.25. This group seems to have no significant trend in freq ratio vs damping ratio.

There is a “high” group with freq ratios from 1.25 to 1.5. They string out diagonally with a clear positive correlation between freq ratio and damping ratio.

There is the <1 “low” group that DaveW has trended with the black line. This has no trend or perhaps a small negative correlation between freq ratio and damping ratio.

The low group blatantly violates Olley/Sharp rules for rear/front freq ratios. This is the rear/mid engine group plus some generally high performance cars. The vehicle type suggests more emphasis on handling and less on ride, so this seems like weak evidence against the golden rules for primary ride. Or perhaps primary ride perception at high speed changes in a way that makes <1 feel more comfortable in that more safety-critical environment.

Overall, but especially for the large mid group, I don’t see much trend for freq ratio vs damping ratio. I think this supports the Sharp perspective that primary ride characteristics are relatively insensitive to damping (as long as there is some reasonable damping?).

I have two hypotheses for why the high group exists. 1. Those vehicles are for countries with low speeds, and therefore according to conventional Olley/Sharp wisdom there needs to be a larger rear/front ratio due to the longer front-rear phase lag. This doesn’t explain the increasing damping. 2. These vehicles are pickups, SUV’s, or minivans that have a large difference in rear mass for empty vs loaded condition. Therefore, the empty rear stiffness has to be extra high so they will continue to provide good ride when loaded. This could also explain the increasing damping.

I think F1 ratios < 1 are dictated (or are at least perceived to be dictated) by aero desires to run the tea-tray close to the ground. This leads to rock hard front and therefore softer rear.

[gato azul]

I suppose, one could say, that with enough damping, you can make any frequency ratio work, and that the "golden ratio" becomes less important.
Or taking the trend line into account and the, to an more extreme extend, F1 values, Frequency ratios < 1 seem to need more heave damping

DaveW, just a question, so that I don't get this wrong.
The number given for Heave mode damping is fractions of critical? (0.5 would mean 50% of critical damping) for the sprung mass, or is there another metrics associated with it?
(like ratio between sprung mass/unsprung mass damping or bump/rebound etc.)
Thanks

[DaveW]

The reason I plotted the results as a function of heave mode damping ratio (as a fraction of critical, by the way) was because it was explained to me that a higher rear natural frequency allowed the (delayed) rear axle response to "catch up" with that of the front axle, hence minimizing in some way the vertical response at the centre of gravity (or, possibly, the drivers seat). That seems to make some kind of sense for the under-damped "boulevard cruisers" that Olley was concerned with, but possibly not so important when a vehicle is more critically damped.

I used heave mode damping ratio as an indicator of the overall level of damping, gato azul, nothing more than that.

Bill is quite right to point out the groupings. His high group did include an Explorer & Landrover, but it also included a range of well respected German saloons and (again interestingly) a couple of Corvettes & a couple of Lexus' (Toyota thus appearing in both high & low groupings).

Averaging out all results gave a mean ratio of slightly over 1.1.

[olefud]

I've often wondered if Indycars on superspeedways would benefit from placing ballast in the nose and tail rather than the middle. This would make pitch slower and more stable (closer to bounce freq) and the resulting slower yaw freq would do little harm (maybe??) in an environment where the yaw velocity is very low.

An interesting thought. While the freq may not be troublesome, the response or phase delay might be the butt biter in an event unanticipated by the driver.

[gato azul]

I used heave mode damping ratio as an indicator of the overall level of damping, gato azul, nothing more than that.

Thanks Dave, I thought that's the case, and I agree, that for a road car, or car with a conventional suspension layout,
it's a good & valid representation for the overall damping level, so perfectly fine to be used as indicator.
Just wanted to be sure, hence the question.

[DaveW]

I believe that Sharp’s argument to justify “Olley-tuning” uses frequency response functions to estimate the way individual axle responses can be combined to cancel pitch response.

Strictly, this approach can be justified in general (leaving aside a special case) only if the following applies:
a) the vehicle is linear,
b) Individual axle responses have perfect coherence.

The latter will be true only if:
i) the off-diagonal modal damping terms can be neglected (generally implying very light damping).
ii) and/or, the individual axle inputs have high coherence (generally implying a time delayed, mainly sinusoidal, input).

None of the above can be used as an argument to dismiss the case for “Olley-tuning”. It simply claims, I think, that Sharp’s argument is flawed.

Arguably, it might be used to explain why some modern road vehicles are lightly damped, and exhibit some degree of "Olley-tuning".

[bill shoe]

The Flat Ride by Maurice Olley, Road & Track, October 1968. This article has a fantastic amount of history and engineering in 4 pages.

http://imageshack.us/a/img59/5907/theflatridep01.jpg
http://imageshack.us/a/img594/8196/theflatridep02.jpg
http://imageshack.us/a/img854/4103/theflatridep03.jpg

I think I'm understanding the "DaveW-tuning" primary ride philosophy a bit better-- If the car is stongly damped (close to critical) then there is not enough primary ride oscillation to require good primary ride motion. If damping is weak then Olley-tuning is probably important.

Olley favored little, or perhaps no, damping. His primary ride work was done before decent mass-produced hydraulic dampers were available for cars. He took what he had and made the ride vastly better.

[flynfrog]

thanks for the scans Bill