Definition of Anti Squat 3

[Milanoguy]

Okay, first a dumb question and then hopefully a less dumb question, both about anti squat.

First question: Lots of references are made to anti squat in terms of percentage like 40% or 100%. But what does the percentage refer to? The total weight of the car? just the weight carried by the back wheels? If we have a 1,000 kg car with equal weight on each wheel(250 Kg's) and a 100% anti squat what is the weight on each rear wheel when the car is accelerating?

Next question: how does anti squat relate to a de Dion rear suspension, specifically the kind found in Alfa Romeo's cars like the Alfetta and it's successors the GTV6 and the 75/ Milano? Does the usual rule that limits the available anti squat in a I.R.S. suspension to about 25%, apply to a de dion suspension. How do you calculate anti squat in de Dion suspension?

To see a drawing of the de Dion suspension used in the Alfa, go to this link for some scans

Http://photos.yahoo.com/digestingtheinternet

 

[BillyShope]

MoreWing says, "Assume the spring in this scenario is a link with pin joints, infinitely stiff. If this were the case, would you know [sic] say that the trailing link is taking none of the load?"

No, but then I never said that in the first place. I did say that there was nothing extra "available" in the link forces, which is just another way of saying that a force and moment balance exists and there are no Z direction inertial forces.

Look, you have changed your position and now recognize that the suspension is not locked with 100% anti-squat. Obviously, you just got a little careless in your first statements. That's nothing to be defensive about. I've certainly done it enough times. Seems I spent a good portion of my time, as a working engineer, explaining to my boss that I had made a silly mistake and would like my report back so that I could correct it.

 

[MoreWing]

Billy,

It's easy to come across wrong on the internet. Please understand this is not a flame, but a series of calm statements.

1. Please change 'know' to 'now'. There doesn't seem to be an edit function for posts. The reason that I pointed the loads out is to show that suspension links do indeed take vertical loads, even when they are sprung and have the ability to travel up and down. I thought it would be easiest to understand this if the 'spring' were solid. Perhaps you are way ahead of me, but I wanted to point out that in most cases, bump loads are shared between the suspension arms and the suspension spring. This is especially true in the scenarios you are presenting in which the suspension arms are steeply inclined.

2. I stand by _all_ my statements. I used the term 'essentially' in my sentence about the suspension being 'locked'. I was writing about how it works in practice (first sentence of the paragraph). This _is_ how it works in practice. I have not changed my position. If you have different experiences, please share them. I am genuinely interested.

Don't confuse book world and real world. In book world something might be the best thing since sliced bread, but if doesn't work well in practical application then it's all for naught. I think this is what you were trying to explain to Greg.

3. I love to discuss these types of topics, but I will not argue about them. I'm just some random guy on the internet, if you think I'm lying or talking out of turn, please feel free to ignore me.

 

[BillyShope]

MoreWing:"The reason that I pointed the loads out is to show that suspension links do indeed take vertical loads, even when they are sprung and have the ability to travel up and down."

Indeed, but a spring deflection is necessary to balance an additional vertical force. This will always be the case, no matter the amount of anti-squat (disregarding transients and the effects of loading on bushing "stickiness").

MoreWing:"Perhaps you are way ahead of me, but I wanted to point out that in most cases, bump loads are shared between the suspension arms and the suspension spring. This is especially true in the scenarios you are presenting in which the suspension arms are steeply inclined."

The parallel arm arrangement I suggested does, indeed, require more angle than I would like. Some would complain about roll oversteer. But, it would still be less than 15 degrees, not the 45 degrees you suggest. And, even that can be eliminated if the IC is placed on the no squat/no rise line directly ahead of the axle. With this arrangement, virtually all the added force would be carried by the spring.

MoreWing:"2. I stand by _all_ my statements. I used the term 'essentially' in my sentence about the suspension being 'locked'."

Ah, but it is the word "locked" which catches the eye, not "essentially." You seem to believe that the presence of certain forces changes the way in which the suspension absorbs vertical loads. This simply doesn't "jibe" with a force-moment analysis, which brings me to the next troublesome statement:

MoreWing:"Don't confuse book world and real world. In book world something might be the best thing since sliced bread, but if doesn't work well in practical application then it's all for naught. I think this is what you were trying to explain to Greg."

No, this certainly had nothing to do with what I was explaining to Greg. Greg's understanding of engineering principles appears to be in perfect agreement with my own. I was merely saying that I am of the "sliderule" generation and, therefore, occasionally use different analytical tools.

An engineer doesn't deal with theories. That's the realm of the scientist. The engineer applies established scientific laws and rules, which he finds in the world of books. When empirical evidence appears to contradict the information in books, he knows that either his observations are incorrect or that he has incorrectly applied the scientific information found in the books. He would be choosing to reject the principles upon which his profession is based if he were to deliberately se aside the "book world" and base his analysis on that which he perceives to be the "real world."

It would appear this philosophical difference makes our complete agreement impossible.

 

[patprimmer]

MoreWing

I am not a suspension man but I have been around many cars for a long time as a hobbyist, and I have dealt with automotive design engineers on a professional level for many years, and I have learnt to sort out the useless boffins from good practical engineers.

Good practical engineers do know and understand the theory as well as understanding the problem. Their ability to observe and understand and relate is what makes the good.

I from my observations, both Greg and Billy show both the theoretical knowledge and the understanding to apply it.

Also, many books were written by people who actual did things in the real world, then wrote it down so others could benifit from their findings

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MoreWing]

From BillyShope, "When empirical evidence appears to contradict the information in books, he knows that either his observations are incorrect or that he has incorrectly applied the scientific information found in the books. He would be choosing to reject the principles upon which his profession is based if he were to deliberately se aside the "book world" and base his analysis on that which he perceives to be the 'real world.'"

I whole-heartedly agree with your statements. There are at least 2 other possibilities to consider, however.

1. The 'book' is wrong. All of us, Billy, Greg, Warps, myself, etc. disagree with what Herb Adams wrote in his book. This is what prompted Milano's questions in the first place, and what prompted me to do what I did 10 years ago.

2. The simplifications that we have chosen to allow us to use the engineering principles and equations we have taken from our books makes our model invalid. Keep in mind, we aren't talking about a simple problem. We are talking about a mechanical system with many degrees of freedom, a couple interconnected mass/spring/dampers, various linkages with assorted compliances, and a couple inter-related force inputs. Greg says CarSim isn't up to the task of figuring it all out. I didn't correctly predict what was going to happen, and I'm willing to bet that Billy can work his slide rule 'till the cows come home and he won't have a definitive answer either.

When the problem is complicated enough, sometimes the quickest, simplest, and most cost effective approach is to simply try it. That's what I did. I'm not alone.

Every auto manufacturer in the world spends considerable time, effort, and money on vehicle testing. They have access to the most sophisticated simulation systems available and have entire departments who spend their days running these simulations. Even with all of that ability to get the 'book' answer, they still find it mandatory to physically test their vehicles. If anything, that should be a pretty good clue as to the complexity of the issues we are dealing with.

Does that mean that an engineer that draws conclusions based on empirical findings has abandoned "the priciples upon which his profession is based"? Or, does it mean that the engineer is smart enough to realize that there are things going on that he doesn't fully understand. He is willing to accept the fact that he cannot fully quantify everything that is going on even though he understands that this indeed is a mechanism that must be governed by the laws of physics.

I reckon it's the latter.

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Pat writes that "many books were written by people who actual did things in the real world, then wrote it down so others could benifit from their findings "

Smith set an anti-squat limit of "about 20% seems to be the maximum before we get into tire compliance problems".

Van Valkenburgh doesn't give a number, but allows, "An excess, however, can cause wheel chatter under hard acceleration."

Milliken doesn't seem to have an opinion on the matter; he just shows how to calculate the percentages.

I'd be interested at to what Gillespie and Dixon say on the matter, I don't have those books handy. Staniforth and Puhn could probably chime in as well.

My point is this, there are people besides me that have had the same basic experience. No one has an equation that shows how much is enough or how much is too much. I designed this into a racecar, but I did it to a lesser extent to my street car around the same time as well. It exhibited the same bad characteristics. I can report what I felt by having my butt in the seat(s). By saying that 100% anti-squat is not going to effect the car over bumps, BillyShope is absolutely sending Milanoguy down the wrong road. I have no question in my mind about this. So we end up with 'a good practical engineer' that apparently has never played with this particular tuning feature, but is, in as many words, championing it's use. Sorry, that's poor form where I come from.

 

[Warpspeed]

My experiences pretty much parallel those of MoreWing, I have experimented with anti-squat designs only slightly, but with anti-dive rather a lot more. The results convinced me that beyond a certain point more is definitely not better. The results always produce harsh suspension feel, and sudden wheel locking/wheel-spin on less than perfect surfaces. Roadholding seems definitely to deteriorate.

Suspensions are complex things, and it is often difficult to carry out tests that isolate only one factor. But my own experience and subjective testing backs up the findings of MoreWing.

There are a lot of people on this Forum with a very great deal more theoretical knowledge and experience than I. All I can offer my own experience, and an opinion. I certainly am not going to argue. I may very well be completely wrong, but my own testing suggests otherwise.

There seem to be quite a few of us that have had a similar experience, which also convinces me that there is something here to be seriously considered, and not to be lightly dismissed.

 

[NormPeterson]

MoreWing - Do you have 'before' and 'after' values for the roll steer on that car you ran with 100% and 0% anti-squat? I suspect that rollsteer (mentioned only by BillyShope thus far) was not particularly important in Billy's competition experience but is much more so in yours. That might go a ways toward resolving the variety in observations and directions that tuning is "pushed". As might the thought initially raised by Greg (see bushing effects) that anti's are not constants.

It seems that Herb Adams has always favored relatively more A/S, at least as far back as his late 70's road-racing Firebird (I think that car was named the "Silverbird", if that rings any bells). He liked rear leaf springs at that time "because he could build a lot [of A/S] in".

Norm

 

[BillyShope]

First, I would make it clear that an engineer should not dismiss, out of hand, anecdotal reports which appear to disagree with established truths. Rather, the engineer is often called upon to identify those parameters which are affecting the observations.

Specifically, what could be causing a car to react differently to a vertical rear tire loading merely because of different link loads before load application? It has been suggested that this was somehow connected with anti-squat, but, if we consider this as a simple kinematics problem involving links and springs, terms like "anti-squat" become meaningless. We are dealing, rather, with the emergence of pivot moments which didn't exist with different link loads.

Is there a way of isolating and simplifying our examination? Yes, we can simulate the inertial load with a static force. By selecting two transmission gears to prevent engine rotation and then pulling backwards...at CG height...on the roll cage with a "come-along," we simulate the forces incurred during acceleration and make it possible to visually determine if structural deflections are the cause. We can then impose vertical (and transverse, if you like) loads and observe the results.

This is a relatively simple kinematics problem, with the only complications involving the matters of observation and measurement. Many of these complications would be eased with the suggested setup.

 

[GregLocock]

We measure 'antis' in several ways.

One way is to push the wheel back, and measure the change in vertical force at the contact patch.

Another way is to lift the wheel up, and measure the rearward displacement.

Then we also measure the pitch gradients for the full vehicle in real life manouevres.

The agreement between my ADAMS models and the the real world measurements is very good for the latter, rather less good for the first two, because the experimental procedure is susceptible to things like friction and so on.

For the kinematic measures the agreement between an ADAMS model with rigid bushes where appropriate and hand calculations is also very good.

Therefore the only areas of weakness I have seen from 3 fully correlated vehicle programs, from hand calculations, via computer models, to lab tests to road work, is the PRACTICAL effect in the LAB of friction and so on, which on the road is irrelevant due to the much higher forces that can be generated.

Our basic suspension geometry is set up using a spreadsheet model based on the hand calcs, these locations are then refined to take account of bushes (and frame deflection) in ADAMS. There is no intuition, no hand waving.

Tha Car Sim model showed almost exactly what I'd have expected - an instantaneous benefit to the antisquat car compared to the optimum as the torque is applied, followed by an oscillating wave of advantage and disadvantage as the pitch damps out to a mean value. Given real world tyre characteristics, an oscillation around a mean vertical load will always reduce the average level of grip available.

The difference in terminal speed is slight in this case, but the optimum solution in terms of pitch control /is/ the fastest of the three.



Cheers

Greg Locock

 

[BillyShope]

"One way is to push the wheel back, and measure the change in vertical force at the contact patch."

Brilliant! This accomplishes the same thing as my come-along and is certainly much more practical for a production car. Do you just stick the selector in "Park," or do you have another way to lock the engine?

 

[MoreWing]

Lord, I'm a glutton.

Greg,

What are the ranges of A.S. that you have measured? Any ideas why the designers chose these values?

While I agree there is no 'hand-waving' involved, there are a series of compromises when designing nearly anything. It would be intersting to see the compromises that others chose and their justifications for such compromises.

In regards to your CarSim model, am I to understand that the higher anti-squat model had a higher amplitude in the vertical load on the tire (higher highs & lower lows), even though the average loading on all the models was the same? What do you mean by 'optimum'?

 

[GregLocock]

Billy - for realistic ranges of antidive (in particular) the experimental data gets very noisy. Yes, we do it in Park for A/S on the rear suspension, are you implying that is incorrect (wouldn't surprise me)

MoreWing


A/S for normal production cars (say 0-60 of 9 secs or greater) is set primarily by the the vehicle pitch gradient spec, which is primarily set for comfort/refinement. There may be some interaction with impact harshness, governed by wheel recession in jounce, but that's not come up in my work, it is the anti dive that is critical there. Extreme values of a/s are known to be detrimental for ride quality.

So, the A/S value is chosen to meet the pitch gradient target.



Cheers

Greg Locock

 

[BillyShope]

Greg: "Yes, we do it in Park for A/S on the rear suspension, are you implying that is incorrect (wouldn't surprise me)"

No, not at all! It's the common sense thing to do. It's just that I've been hesitant to suggest it for fear that somebody would break something and then blame me for it. I think I'd still be a little cautious before suggesting it for someone planning on using a come-along to pull the front wheels of a heavy sixties era car with wrinkle walls off the ground. Although I worked in Transmission Design for a while, I was stuck in a corner doing computer matching of torque converter components. I know nothing of the design requirements for a parking sprag.

 

[GregLocock]

Now I see what you are getting at. The body is bolted to the floor for this test.

Interesting point about the sprag, we haven't broken one yet, when we do I'll let you know! Our test force is only 2 kN.

Cheers

Greg Locock