Jacking force

[flavorPacket]

Originally posted by Warpspeed:
Discussion is always worthwhile, if it helps to clarify the thinking.
And sometimes you think you understand something, until you try to explain it to someone else, then begin to realise the complexities go a bit deeper.

Some of the more abstract ideas can be difficult to get ones head around, and I don't see discussion as being a waste of time.

You could argue that any one single aspect of vehicle dynamics (taken in isolation) as not being that significant in itself to lap times.
But the final combination of compromises sure are.

Understanding the interactions between inputs is the mark of a good engineer. Being able to rank those interactions in order of importance is the mark of a better engineer.

[STRETCH]

Originally posted by Warpspeed:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by exFSAE:
I'd argue that 2-D childish gibberish is just fine when the problem or specific answer you're trying to address is frequently, almost entirely two dimensional.

Absolutely ! </div></BLOCKQUOTE>

Thank you, I thought the explanation was entirely appropriate given the question being asked... I should apologise though; this approach is only adequate for a championship winning F1 team, I should have known it wasn't good enough for Z.

[Z]

Originally posted by exFSAE:
... this is all effectively meaningless if you're not using the actual tire forces and moments
... What are your jacking coefficients really telling you without it?

exFSAE,

Your "jacking coefficient" (= n-line slope, in radians) tells you that a larger coefficient = a larger jacking force, for whatever lateral force you happen to have at the wheel. For example, an n-line-slope = 0.10 radians (roughly 10cm rise for 100cm run) has twice as much jacking as n-line-slope = 0.05. Or if n-line-slope = ~0 (ie. horizontal wrt car body), then NO jacking force. Or if n-line-slope = negative, then downward jacking.
~~~o0o~~~

Mark Ortiz has used the concept of "force line slope" (= n-line-slope), in preference to "IC position", for at least ten years. (Perhaps a better, albeit longer-winded, expression would be "control-arm force line slope", to distinguish it from spring-damper forces.) Bill Mitchell switched from "IC-position" to "slope only" in this 2007 paper.

Unfortunately, the majority of discussion these days is still in terms of IC-position (eg. Lee Stretch, above, "Just remember, all the tyre forces feed into the chassis at the IC for that wheel."). In the long run this misleads the next generation of students.

So, once again....

A force acts along a "Line-of-Action"! To suggest that any one point on this line is more important than any other point is misleading. This very quickly leads to misunderstanding, and then big mistakes.
~~~o0o~~~

One such mistake (very common) is to suggest that IC height is an indicator of relative jacking forces. Wrong!

For example, an IC height of 10 kilometres (yes!) is not unusual, and it can give less jacking than an IC height of 0.1 metres. Very well behaved suspensions (eg. with negligible jacking) often have astronomically high IC heights (ie. over the moon!). Hint: Think about the horizontal position of the IC.

Z

[exFSAE]

Originally posted by Z:
Your "jacking coefficient" (= n-line slope, in radians) tells you that a larger coefficient = a larger jacking force, for whatever lateral force you happen to have at the wheel.

But if you don't know what value those lateral forces are, then it's all for naught. Tells you no end answer. At the end of the day you want to know what the net force and moment on the body amounts to.. and what that will do to body control and load distribution. So to go on about 3D being better than 2D is purely academic IMO. Might as well get at what we're really trying to solve and what you need to do it.

In any event I'd argue that in my experience, a simple 2D approach works just fine for a lot of things - particularly explaining concepts. To do any meaningful work you might as well wrap it all up in a sim and not worry about it.

I'd also argue that one very common mistake is to think jacking forces are bad and something to be avoided (ie that well behaved suspensions have negligible jacking). It's a tool in the toolbox, and sometimes high jacking forces (or coefficients) are your best friend.

[ChassisSim]

Varun,

You might find the following really helpful,

http://www.youtube.com/watch?v...ndex=38&feature=plcp

All the Best

Danny Nowlan
Director
ChassisSim Technologies

[Z]

I will keep on at this.... Someone might benefit...

Originally posted by exFSAE:
But if you don't know what value those lateral forces are, then it's all for naught. Tells you no end answer. At the end of the day you want to know what the net force and moment on the body amounts to.. and what that will do to body control and load distribution. So to go on about 3D being better than 2D is purely academic IMO.

exFSAE,

A lot of this has got to do with "tolerances", or knowing how accurate your results are.

The (correct) 3-D approach to kinematics gives results which are "perfectly" accurate, so that only numerical roundoff (or graphical pencil) errors in the calcs, and manufacturing errors and compliance in the parts, remain (and can be estimated). This 3-D approach is also very easy and can be done graphically on a drawing board. The currently popular 2 x 2-D method often has 10% error. But, most importantly, almost no one is aware that this method can give the wrong results. No one knows when they are wrong, or how wrong they are!

Regarding the lateral forces, I am reasonably sure that in FSAE they lie between -0.2 tons (outward) and +0.5 tons (inward), per tyre, give or take a bit. Those extremes require a good aero package, which no team in FSAE yet has, so typical forces are quite a bit less.

The n-line-slopes, or jacking coefficients, lie between -0.2 and 0.5, give or take a bit.

If a car has n-line-slopes at the extreme ends of this range, AND large horizontal tyre forces (Fx or Fy), AND long travel, soft, independent suspension, then it will very likely "misbehave". Softly sprung beam-axles can have steep n-lines without this "surprising" behaviour from lateral forces. So also any car with very stiff springs ("Any suspension will work if you don't let it."), which includes most circuit racecars.

My earlier comment regarding well-behaved cars having small n-line slopes referred mainly to softly sprung cars. In such cases n-line slopes between -0.1 and 0.1 should not cause too many surprises.
~~~o0o~~~

Lee,

Can you explain why you think that "all the tyre forces feed into the chassis at the IC for that wheel"?

Is this the only point they can act?

If not, then why is this point preferable to some other point?

(BTW, I can think of another point that is much easier to use in calculations, but would like to hear your views first.)
~~~o0o~~~

Danny,

In your link you suggest that the tyre (horizontal only?) force should act at the "Force Application Point" (this is similar to Mitchell's thinking, referenced above).

Why?

Along with the questions above for Lee, do you think your different choice of "special point" might be why students new to this subject, like Varun, might be getting confused?

(BTW, I am reminded of "Gulliver's Travels", and the argument between the Big and Little Endians over which end of the egg should be opened!)
~~~o0o~~~

Varun,

This subject is really quite simple. Hopefully the next few posts might help....

Z

[Moop]

Z,

None of us can change the fact that the correct approach to 3D kinematics isn't taught in engineering curriculum. Where would you suggest we learn it from?

Our suspension points are laid out in a spreadsheet with 2 x 2D kinematics to pick RCH, FVSAL etc. The points are then brought into ADAMS and the various suspension curves are produced. I've noticed that the swing arm lengths and roll centre heights calculated by ADAMS are always a little bit different than what was calculated in the spreadsheet, but reasonably close. At first I thought it was an issue with ADAMS, but now it's clear that the issue has to do with 2 x 2D vs 3D.

Now, if you use the FVSAL and IC heights calculated by ADAMS in your FBDs to calculate the forces transmitted to the sprung mass by the suspension, how is this wrong or any different?

Yes, when the unsprung mass is zero, the jacking force ends up being the lateral force multiplied by IC height over FVSAL, which ends up always pointing at the IC, or along the n-line or whatever. With the IC approach, you actually only need the IC inclination angle. But what happens when the unsprung mass is no longer zero, the unsprung mass has an acceleration greater than zero and you have an overturning moment?(or couple, to make you happy, haha)

The mechanism will still move the tire contact point in the same manner, and so have the same n-lines, but the lateral and vertical forces transmitted to the sprung mass are different. Sticking a vertical force and lateral force at the IC and summing forces and moments allows me to calculate the lateral and vertical forces transferred to the sprung mass. And in this case, I actually do need both the IC height and FVSAL, at least to determine the jacking force caused by Mx and the unsprung mass.

I was only talking about front view above, but the exact same thing applies in side view.

[kcapitano]

Normally I don't like starting an off-topic rant, but today I feel it necessary.

None of us can change the fact that the correct approach to 3D kinematics isn't taught in engineering curriculum

This is absolutely the wrong attitude. Have you tried talking to your professors? I can't speak for how engineering is taught everywhere, but I know at my school the professors have a fair bit of flexibility. I've seen some add/drop sections to a course, and another who completely ignored the textbook and taught us mostly calculus.

Where would you suggest we learn it from?

Where did you learn about vehicle dynamics from? We are living in the age of information! We have the internet, libraries, and disgruntled elders who think kids these days are morons (I'm going to group Z in this category ). Use your resources and you can learn anything you want.

Apologies for the rant.

[exFSAE]

Originally posted by Z:
Regarding the lateral forces, I am reasonably sure that in FSAE they lie between -0.2 tons (outward) and +0.5 tons (inward), per tyre, give or take a bit.

Well I could just as easily say that the 2D approach is accurate "give or take a bit." Actually I'd say the forces are quite a bit lower than those estimates. ~640 lb car, ~1.7G lat let's say, even with 100% lateral load transfer you're more in the order of 0.2-0.3 ton per tire.

The tire lateral forces are going to be changing immensely going around a circuit. With how stiff FSAE teams seem to like running their suspensions, might as well call the kinematics non-existent with so little travel. If we say the jacking coefficients are relatively steady ("give or take a bit") with minimal travel, then your dynamic tire forces and lateral distribution thereof will have immensely more impact on your actual jacking effects then the kinematic aspect.

Yet when the topic of jacking forces and roll centers and all this malarkey comes up, everyone loves to go bananas for the kinematics and completely ignore the tire (or actual FORCE) aspect.

[Warpspeed]

I really cannot see the value of calculating suspension jacking down to thousandths of an inch, even if that were even possible.

It is probably enough to understand the effect, and have some respect for it.
It only really starts to becomes a significant (and scary) problem with an unusually high roll centre, narrow track, and very low wheel rates.

With a competent purpose built race car with low cg and high roll stiffness, jacking effect is about the very last thing you need worry about.

As with many things in engineering, if you can keep the effect negligible by design, you can probably ignore it, and focus your efforts on much more important aspects.