Tire vertical damping

[Jersey Tom]

.. how much is it worth sweating the fine details? What's their level of relevance to begin with?

You sound like someone talking himself out of a job....

[...]

That could be called sweating over the fine details, but the more successful teams are generally those that pay attention to such details.

I don't think I'm talking myself out of a job quite yet. I'll elaborate on my earlier bit - and I think this applies fairly broadly through all levels of engineering and problem solving:

There's sweating details for details' sake... there's chasing relevant details... and then there's chasing relevant details in the right order.

The first is a mark of poor / inexperienced engineering, IMO.. and the last is the mark of experience. That's the value in hiring experienced and practical experts, to avoid getting lost in minutiae which isn't going to contribute to better performance. I know of organizations who have gone that route, investing immense time, money, and effort into some super high fidelity modeling of "X" - and then it ultimately having no impact on how they set the car up or go to a race. It amazes me how many high profile organizations have tales of such things.

It screws you over doubly. I'd say it's rare to be sitting around having to really noodle on the question, "Well what more can we do to get performance?" More often you have a list of potential things that you couldn't possibly all get done in several years, and all you have is several months of off season to pursue them. So if you pick the wrong thing to work on, not only is it money out the window but it's time you could have been spending on something with more ROI.

In any event, first question that needs to be asked is, "How is understanding/pursuing this going to make the car go faster?" Next (or going along with it), "Is this really going to change my decision making process?" Then, "What's the level of significance?" Only then can you decide whether to jump down the rabbit hole now, or shelve it and look at it in the near future, or box it up and say it isn't anywhere on the radar yet.

So in this case maybe the first step would be to play around with arbitrary numbers of damping in your model first. See how big it has to be to be of real consequence. Maybe it winds up being on the same order of what you think are ballpark numbers for tires. Maybe it winds up being inconsequential.

[silente]

JT,

i think you misunderstood my scope here.

Your reasoning makes absolutely sense and i agree with you that things with high priority on performance should be tackled first.

But who told you that i am looking for a real gain in performance, in this particular case?

I only would like to understand a bit better a topic that is often ignored in literature.
Moreover, i am just working on models that require such a value to be entered and where, although small, you can see differences in results depending on the value you use. So why not trying to get a feel about (at least) the order of magnitude of such a parameter?

So ok, Tire damping is not the most important parameter to improve car performance, but since that was not my aim (i am not working on any specific race car right now) and since there are people that are kind enough to share experience and ideas in this forum, why not?

Sometimes you could also want to investigate a topic for the sake of knowing more about it, not because it has any special importance for your work.

Anyway, back on topic, i hope somebody can confirm/not confirm my conclusions in my previous post.

[yzfr7]

silente,

Here goes a link to an interesting paper on measurement of tyre daping, with a brief dicussion of the importance of not neglecting it. The groups in Nottingham and Cambridge (http://www2.eng.cam.ac.uk/~djc13/vehicl ... intro.html) have plenty of material on this topic.

http://www.sciencedirect.com/science/ar ... 0307000495

If you have difficulty accessing the papers through your work or university network, send me a pm and I can give you directions.

In some models I used at work for medium passenger cars, the value of c1 from Ciro's blurred picture (I can give you a better one, Ciro!) was around 20 Ns/m. Obviously it varies, clearly it depends on an enormous amount of variables, but I guess you asked for the order of magnitude, so there it is.

[DaveW]

I'll elaborate on my earlier bit.

Excellent, well worth a read, as ever, even if I don't always agree.

To summarise and check if i understood correctly, a good approximation of tire damping for these particular tires is about 0.75 N/mm/s, with a stiffness around 265 N/mm at the front and 300 N/mm at the rear.

Thats what I would assume.

A quick & incomplete check suggested that race tyre damping varies between 0.1 and 1.0 N/mm/sec. The lower values I found in (some) LMP rear tyres. They are also the tyres with the highest vertical stiffness (up to 600 N/mm), so the phase angles are very low.

In some models I used at work for medium passenger cars, the value of c1 from Ciro's blurred picture (I can give you a better one, Ciro!) was around 20 Ns/m. Obviously it varies, clearly it depends on an enormous amount of variables, but I guess you asked for the order of magnitude, so there it is.

I'd be interested to know what car, tyre size. Also the measurement and analysis technique used to estimate parameter values.

[Jersey Tom]

They are also the tyres with the highest vertical stiffness (up to 600 N/mm), so the phase angles are very low

That's a very interestingly high number. Much more than I would have expected for what I'm guessing are moderate inflation pressures.

[humble sabot]

I could see that making sense, though i wouldn't have guessed such a number. Running a few hundred N of downforce for hour long stints, and probably expecting the tyres to last more than one stint, it makes sense to minimise total deformation to reduce excess heat gain and wear.

[DaveW]

yzfr7: There appears to be big difference between our quoted values for tyre damping. You quoted 0.02 N/mm/sec, whereas my estimates for tyre damping for installed road car tyres are around 0.68 N/mm/sec (averaged over 500+ samples). Despite JT's comments I am genuinely interested in why that should be - but not the extent a paying for access (to work that my taxes have probably paid for already) only to prove that the necessary detail was lacking. Hence, I would be grateful if you could address the questions I posed in my previous post.

[Greg Locock]

In my work I usually find tire damping has little effect. The one exception is in wheel lift off events such as the margin for untripped rollovers, where the recovery of a wheel as it falls back to earth and loads up is very dependent on the damping value.

[Tim.Wright]

How about for ride evaluation Greg?

Also, Dave, I don't know if you do much in the way of 4 post simulation, but I imagine the tyre damping if pretty critical in getting the right contact patch forces at higher frequencies in a 4 post sim as compared to the rig?

[Greg Locock]

No, not even on the horizon as an important parameter for primary ride, and for secondary ride I am so far from getting good correlation that I haven't even tried it.

[Tim.Wright]

Fair enough, cheers

[Jersey Tom]

In my work I usually find tire damping has little effect. The one exception is in wheel lift off events such as the margin for untripped rollovers, where the recovery of a wheel as it falls back to earth and loads up is very dependent on the damping value.

I go with this.

One of the papers mentioned above.. I perused it.. they're charting stuff out to 200 Hz. 200 is an awfully big number for primary handling, IMO.

That and if you're getting road inputs at that high a frequency you're probably going to need a much more elaborate mass distribution and stiffness (including enveloping stiffness) model than many would have appropriate data to characterize. Enveloping stiffness which you will not be able to characterize on a platen rig...

[Greg Locock]

I've got two basic tire models. A bastardized Pacejka model, which is fine for everyday stuff, including limit handling, and FTIRE, which is supposed to be good for 60 hz. Ftire is great for impact strips and the like, although I'm not convinced it really does enveloping properly. I can't get the graphics to work on this machine so I'll never know!

[DaveW]

Dave, I don't know if you do much in the way of 4 post simulation, but I imagine the tyre damping if pretty critical in getting the right contact patch forces at higher frequencies in a 4 post sim as compared to the rig?

I do, actually. My analysis routines were developed and validated using simulation, and I now also use simulation regularly to answer "what if" questions. Experience has taught me that is always better better to work with a real vehicle, so I only rarely use simulation as a predictive tool.

However, as a simple check, I compared the eigenvalues estimated from a simulation (of a Mondeo that I once owned) with those predicted by setting tyre damping coefficients to zero. Essentially, Greg is right. Heave & Pitch mode damping ratios changed by no more than 0.6 percent, but the hub mode damping ratios changed by 36% (front) & 27% (rear). The latter would be affected by top mount's, which allow the hub to move (at 16 Hz.) without overly troubling the dampers. In theory I could compute the effect on "Comfort Rating", but the answer would be complicated by the power train modes (which, like Greg I guess, I have difficulty estimating reliably).

It would be fair, I guess, to assume that contact patch load variations would be improved by the presence of tyre damping, particularly at frequencies in the region of the hub mode, but the "Comfort Rating" would probably not be affected significantly.

[DaveW]

One of the papers mentioned above.. I perused it.. they're charting stuff out to 200 Hz. 200 is an awfully big number for primary handling, IMO.

It would seem that yzfr7 is not going to add to his post..

Hence I would like to hazard a very wild guess and suggest that the paper he refers to describes a tyre model that includes both viscous and hysteric damping and value he quotes is just for the viscous component.

It seems to be the case that as the frequency range used to identify a simple tyre model of the form shown here is increased (upwards), the deficiencies of the model cause the hysteretic component to increase and the viscous component to reduce. If a frequency range of >100 Hz. is used, then a viscous component <0.1 would be possible.

I suppose the trick is to use an identification range that is appropriate to the frequencies of interest (in my case, the sprung & unsprung modes of the vehicle).