Post rigs

[Belatti]

Can it be that the compliance effects are not important in rigid F1 monocoques? They use 7 post rigs....

[Smokes]

Dumb question, How do extract the actual track input data i.e curbs bumps from the setup ie (damper spring rate tyre pressure and tyre temp rollbar rate) of the racecar that recorded the data. Also will different tyres and /or pressure on the same car affect recorded data.

Do also have to take average of the data due to variances of the line the drivers take lap to lap? Or do you go by fastest sector times?

Thinking about doing a masters in racecar dynamics but i guess i am too old/dumb to do the maths.

[DaveW]

Dumb question, How do extract the actual track input data i.e curbs bumps from the setup ie (damper spring rate tyre pressure and tyre temp rollbar rate) of the racecar that recorded the data.... i guess i am too old/dumb to do the maths.

It isn't a dumb question, & you are wise not to attempt the Maths. The process is complex, iterative, time consuming, & not necessarily convergent. Fortunately, the manufacturers of multi-post rigs that offer the capability also sell software for the purpose. I suppose MTS's Remote Parameter Control (RPC) is the most widely known but both Servotest & Instron-Schenk offer equivalent capabilities. To reveal my prejudices a little, I have not seen a track simulation that appears to be totally convincing.

... Also will different tyres and /or pressure on the same car affect recorded data.

Yes.

Do also have to take average of the data due to variances of the line the drivers take lap to lap? Or do you go by fastest sector times?

The process attempts to match rig vehicle responses to previously recorded track data. Normally, one lap of track data is selected for the purpose. Technically, making set-up changes that might result in a different driving line, or different sector times will make the drive files obsolescent, but it is probably realistic to ignore that fact.

[Jersey Tom]

Can it be that the compliance effects are not important in rigid F1 monocoques? They use 7 post rigs....

I'm sure they use K&C rigs plenty too.

[Belatti]

Well, guess what? I didnt know anything about K&C rigs till now!

http://www.morsemeasurements.com/kc7post.html

The AB Dynamics K&C test machine at Morse Measurements is not a 7-post shaker rig. We don’t “shake” cars or report results in the frequency domain. We don’t use hydraulics, and the results are not difficult to understand.

7-post shaker rigs are used primarily for tuning suspension dampers. The typical goals are to (1) Minimize contact patch load variation over known road/track profiles, and (2) Improve vehicle pitch behavior for improved aerodynamics.

K&C rigs are used for tuning everything else in the suspension, beyond the dampers. The typical goals are to (1) Optimize the loaded and unloaded suspension geometry over known road/track profiles, and (2) Maximize the mechanical grip of each tire by optimizing longitudinal and lateral weight transfer.

K&C testing and 7-post testing actually compliment each other in many ways because, as shown in the table below, K&C tests capture information that is just not available from shaker rig testing:

Key Suspension Items K&C 7-post
Apply braking, cornering, and traction forces at all tires
Simulate true trim with steer, tire loads, roll, pitch, & heave
Measure ride and roll rates with scrub relief
Find roll center heights, instant centers, and jacking ratios
Measure toe, camber, and caster (geometry and stiffness)
Locate center of pressure for each tire contact patch
Identify Ackermann and loaded steering geometry
Measure steering system friction & stiffness characteristics
Measure suspension friction levels
Measure local chassis deflections from real tire loads

K&C and 7-post testing can be used together to create comprehensive tuning maps for a vehicle suspension, particularly for pitch and aerodynamic stability. Be cautioned, however: 7-post rigs cannot introduce cornering or braking/tractive loads. Without the inclusion of jacking forces, 7-post rig-tested tire loads will not be correct. (This is dicussed in more detail and illustrated with some examples on the page comparing K&C vs. Pull-Down rigs.

Very nice graphs and explanation, but still I think a post rig is a faster performance enhacer. The leap you can achieve with the K&C seems to be more for the mega bucks level than for the modest car I work with.

[Mystery Steve]

Don't forget that a collection of forces on a body can be combined into a triad of forces and moments about the center of gravity. You can then equate this collection of forces and moments into three forces placed strategically on the body.

I am quite aware of this, but these 'ideas' are strictly for rigid bodies. If one of the aims of the rigs is to check for effects of compliance, then this might not be very accurate. However on second thought, may be the estimate is not too bad away from the points of application of the load.

Yes, in theory it applies to rigid bodies, but as with any form of simulation you will have some inaccuracies. The key is identifying your limitations and accounting for them in your analysis. But I can't imagine the 7-post setup is that far off since most of the bending moments in the chassis are due to the wings because the underbody loading is distributed and the wings are cantilevered.

[DaveW]

Well, guess what? I didnt know anything about K&C rigs till now!

http://www.morsemeasurements.com/kc7post.html

Interesting link... K&C rigs certainly contribute significantly to the database of knowledge about a vehicle, & they complement (& not compliment!) other types of rig test.

The web site did nothing to allay my concerns over "dynamic" K&C tests, however. "Dynamic" is any test for which velocity &/or acceleration is significant.

[Jersey Tom]

Very nice graphs and explanation, but still I think a post rig is a faster performance enhacer. The leap you can achieve with the K&C seems to be more for the mega bucks level than for the modest car I work with.

I'd say the opposite actually.

[Belatti]

Very nice graphs and explanation, but still I think a post rig is a faster performance enhacer. The leap you can achieve with the K&C seems to be more for the mega bucks level than for the modest car I work with.

I'd say the opposite actually.

Why do you always try to leave a halo of mystery? Explain why would you say the opposite my friend !!!! [-o<



I think that if you have a modest team with,lets say USD 7K budget per race where you carefully built a steel frame chasis with good materials, a known suspension geometry, etc... using a 4 shaker postrig only for a day you can find 1 sec or 2 plus driveability instantly by just tunning a couple of dampers.

With the K&C rig you may need to modify chasis, change wishbones, uprights, geometries (susp & steering) and more. Try to tell a guy who has paid USD 30K for a new racing chasis that it may need an additional 10K to make modifications to it because the rig test say so.

But maybe Tom the secondary effects of compliance are bigger than Im actually thinking of...

[DaveW]

I'd say the opposite actually.

How so, JT?

I think that a K&C test is very useful (& can yield estimates of many static vehicle properties that are "invisible" to a multi-post rig test), but can't contribute significantly either to spring selections or to damper settings for a given race vehicle. Neither is a K&C test able, for example, to identify the source of "ride" issues for a road vehicle.

I think the two types of test are complementary, & not "either/or".

[Jersey Tom]

If I were designing a new race chassis, or had bought one and was about to do a race season... and I had the funding to do either K&C testing or 7-Post testing, I'd probably take K&C. When it comes to chassis setup, steady state comes before transient and dynamic.

For me, as a vehicle dynamics engineer, K&C data tells you a wealth about the cars and is pretty straightforward to analyze. 7-Post data, not so much.

If on a 7-post I see that I've reduced my RMS tire load variation by say 200 lbf... what does that tell me? Without a way to correlate high frequency tire load input to pure mechanical grip (good luck w/ that) .. you don't know if that's significant or meaningless. You can make the argument that any reduction is "good" .. but engineering and test tuning does come down to significance levels.

Or if I see I've reduced my peak to peak front ride height range from X to Y.. what does that tell me without an aero map? What if I'm racing some stock sedan which is relatively aero insensitive to begin with?

K&C data on the other hand...
I can map out roll stiffness distribution with any combination of springs, bars, and tire pressures I have available. When you get to the track and need to tune, you know exactly what each tuning tool is going to do and to what extent. If you don't know that.. you're don't know --- going into a test session. Car comes in.. driver is complaining of understeer, and you only have 30 minutes left in the session. How much does it need to change? Are the bars going to even do anything? Can it be done purely with inflation? When time is precious you don't want to be guessing.

I can measure my real kinematic rates and be able to determine what my mid-corner tire loads, cambers, and steer angles are. If you don't even know these, you can be throwing heaps of grip out the window. With even the most basic of tire data it's real easy to show how much grip you're gaining or losing.

If I know a degree of camber compliance is killing me (which it would), it's also pretty straightforward to sort out how much of that is coming from one wheel versus another.. bearing compliance.. suspension mounts.. you name it.

Plus you can measure "real" roll center heights (force based) through any range of ride height, roll, pitch, etc... suspension friction (grip killer), any number of things.

You're not going to get any of those things off a standard 7-post, and there's a good chance you're going to be getting a realistic answer through kinematic software either.

In the grand scheme of things if I were preparing for a race season, the order of data I'd want would be...

1. Force & moment
2. K&C
3. Aero map (particularly open wheel)
4. 7-post

But that's just me.

[DaveW]

But that's just me.

My apologies, JT. Your post explains your views completely.

Suppose, on the other hand, you could use a multi-post rig test to:

- derive a complete (vertical) dynamic model of your vehicle with no a priori input, including installation stiffness
- compute rigid body eigenvalues
- extract sensitivity parameters for springs, dampers, tyres & ballast
- measure axle-based roll stiffness contributions of springs & bars
- estimate torsional stiffness of the chassis alone & overall spindle-spindle
- extract dynamic ride height variations
- extract peak & minimum load variations
- evaluate the rate of heat input to the tyres, dampers & the vehicle
- examine (for an open-wheeler) up to 50 different set-ups in a day.

Might that modify your priorities especially if, as Belatti has suggested, the scope for implementing changes that might be suggested by K&C test results is limited?

[Tim.Wright]

Jersey Tom,

I'm interested in how you go about investigating force based roll centres from KnC data as I'm currently using some KnC for a project at work. The data we have is from an MTS system and that post processor gives geometric instant centers. I guess you do some further posting of your own to get the force-based centres?

I might need to shoot you an email since you've said a fair bit of stuff which is interesting to me at the moment and Im a bit of a rookie in the real world.

-Tim

PS I agree the KnC measurements would be a first step. Especially if its not your own suspension design.

[Jersey Tom]

I hadn't thought of using a 7-post to determine roll stiffness split, but it makes sense. I'd be down with that.. particularly looking at the contribution of dampers to transient roll stiffness.

But still... ride height variations aren't going to tell me much without an aero map. Maybe a 1" change in FRH is huge.. maybe it's nothing. And someone would have to show me pretty convincing data to sell me on the tire force variation bit. I don't know of any machine in the world that can show the impact of high frequency load variation on tire grip, at least at 'racing conditions.' Without that sort of lab data you don't know if 500 lbf of tire load variation means anything significant or not.

With regard to the tire temp thing as well.. I could see that being somewhat useful.. but it's still a static tire. A tire rolling at 1mph isn't the same as one that's static.. and at 150mph further different. I'd be curious to see how well heat generation lines up with force variation.. probably fairly well.

In any event if I had a car where the only parameters I can change are spring rates, bars, tire pressure, static camber, and static toe... K&C rig info would be immensely valuable.

My rank order stays the same. Tire data first, K&C, wind tunnel, shaker rig last.

[DaveW]

I suspect a few F1 teams would like reliable aero maps...

Tyre load variations... like many parameters (including some aspects of aero), it isn't necessary to know absolutes, just whether a set-up change has made an improvement or otherwise. Tyre heat rate is interesting. I have computed it for several years without being entirely convinced of its value, but then this year I received direct feedback for one vehicle, at any rate, that rig predictions were reflected reasonably accurately (on a relative basis) during track tests. BTW, there is a little more to tyre heat rate than CPL variation - tyres are non-linear, as you well know & changing damping "style" can increase heat rate, reduce CPL variation & increase minimum CPL values.

I find it interesting that you don't appear to consider dampers important in setting up a race vehicle, except to manipulate steering transients. That would explain why I was unable to change your mind (why was I not surprised?). Belatti is correct, however. Optimizing damper settings can & usually will improve lap times significantly. Very simplistically, dampers provide mechanical performance, whilst springs, bars & damping style help a driver turn mechanical performance into lap time.

p.s. It hasn't been my intention to denigrate K&C tests. Far from it...