Definition of Anti Squat
Definition of Anti Squat
(OP)
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





RE: Definition of Anti Squat
%antisquat=100*a/b/(height of cg above ground/wheelbase)
antisquat does not affect the load transfer during uniform acceleration (actually there is a tiny effect)
I got that from Milliken.
What rule limits it to 25%?
Cheers
Greg Locock
RE: Definition of Anti Squat
There is a far too prevalent myth that anti-squat pushes the wheels down increasing traction.
There is another prevalent myth that lots of rear squat pushes the wheels down increasing traction. hmmm....
Antisquat only offers some advantage in reducing suspension vertical travel (squat) thus sometimes improving the rear suspension geometry.
With an IRS that is set up for huge negative camber gain in bump, under acceleration, squat can cause massive camber change and even some track change at the contact patch. With wide flat tyres, this can lose a lot of traction.
With a di Dion rear end, there will be no geometry related problems, so I cannot really see the advantage of changing the original anti-squat geometry.
RE: Definition of Anti Squat
Thanks for the response. Okay here are my responses and some new questions
Responses
“What rule limits it to 25%?”
“…but a value of about 25% is about the practical limit. This compares to a well designed live rear axle rear suspension which often has over 100% anti-squat.” P.74 of Chassis Engineering by Herb Adams HP Books. Also “About 20% seems to be the maximum before we get into tire compliance problems.” P. 36 Tune to Win by Carroll Smith.
Questions
(A.) I have been operating under the assumption of “Squat good, Anti-Squat Better” meaning that squat increased the weight on the rear tires giving better traction and anti squat was better still, placing even more weight on the rear tires. I got this idea from the Adams book at p.64 he states
“Anti-Squat can counteract the squat force and it can be made strong enough to actually raise the rear of the car during acceleration…Because any force that can raise the rear of the car will need to have an equal and opposite force pushing against the pavement you can use anti squat to increase tire loading during acceleration.”
Warpspeed, am I understanding you correctly that the above quote is incorrect and that there is no relation between traction/grip and squat or anti squat.?
(b.) Greg, you stated “If the SVSA IC is…” I’m guessing that the IC term stands for Instant Center but what does SVSA stand for.?
Bye for now
RE: Definition of Anti Squat
Nothing you can do to the spring rates is going to change the static weight of the car.
Likewise changing spring rates is only going to change total dynamic weight transfer if the centre of gravity height changes as well (which it probably will).
Anti dive and anti squat use torque reaction to effective increase spring rates. It cannot alter the mass of the vehicle.
If you take an extreme case of a wheel-stand, with the front wheels completely off the ground. Total vehicle mass will then be on the rear tyres. Changing rear spring rates, or suspension geometry is not going to further increase the weight on the rear tyres beyond total vehicle mass.
RE: Definition of Anti Squat
In a steady acceleration the increase in vertical load on the rear axle is always given by (m*a/g)CG height/wheelbase (or something like that). Note antisquat does not come into that equation, you don't even need to know which axle is driving.
Therefore we can assume that squat or antisquat provides a dynamic modifier. What goes up must come down. The instantaneous increase in vertical load will be paid for, some time later, by a corresponding decrease. Unless you are very cunning you will probably lose more than you gain, due to the non linear relationship between grip and vertical loading.
Hmm, OK, that is the theory. I'll put together a CarSim model and see what the effect actually is.
I agree there are practical limits to antisquat. But as someone once remarked (paraphrased) "building a succesful suspension is not just a process of applying rules of thumb to every aspect, the reasons behind the rules of thumb need to be understood"
Cheers
Greg Locock
RE: Definition of Anti Squat
Look in the gallery, first two links.
cg height /wheelbase is about 0.203
Cheers
Greg Locock
RE: Definition of Anti Squat
(Actually, if you want to be picky and take the rear suspension unsprung weight into consideration, "a" would be measured from a point slightly below the wheel centerline for an IRS and slightly below...about an inch and a half...the tire patch for a live axle. And, the CG height would be the CG height for all but the unsprung weight at the rear.)
Because the no squat/no rise line, for an IRS, is displaced upwards by a distance equal to the effective radius of the rear tire, it is difficult...but certainly not impossible...to sufficiently angle the trailing links to achieve 100% anti-squat. Much easier with a live axle. Chrysler dragrace cars of the sixties, having leaf springs with a relatively short distance between axle and front eye, would "jump" off the line like frogs. Lots of favorable transient loading, but, as Greg correctly points out, what goes up must come down.
RE: Definition of Anti Squat
Cheers
Greg Locock
RE: Definition of Anti Squat
Let me take a shot at your question.
A.) I got the exact some idea from Herb Adams book when it first came out. Unfortunately, he's a quack. Normally, I wouldn't be some offensive, but when trying to talk to him about the issue he treated me like I had leprosy. So I don't feel too badly about it.
Warpspeed said that longitudinal load transfer is a function of wheelbase length and C.G. height. He's right on. There are 2 basic ways that one can choose to accept that load transfer. It can either be in the springs or in the control arms. When you accept the load in the springs, there is a certain time lag before the motion of the body can sort itself out. It tends to load the tire 'slowly'. Antisquat lets the suspension arms take the longitudinal load transfer. Suspension arms are made specifically not to defect. The load transfer happens instanteous. It loads the tire very quickly.
Here's how it works in practice. If you apply 100% throttle to a car that has no antisquat it will accelerate, pitch back, and then load the contact patch of the tires. It will all happen rather slowly. If you accelerate with a car that has 100% antisquat it will accelerate and at the same time load the rear tires. The front will lift, but the rear will not squat. The rear has essentially been locked solid. If you go over a small irregularity in the road, the rear tires will not follow the road and will probably get wheelspin. On an exceptionally grippy and smooth piece of pavement (launching pad on a drag strip) it might be fastest because it reacts so quickly, but no where else is it good.
Incidently, after reading Adams' book, I built a car that had a 100% anti-squat option. His book makes you think it would be great for rear grip on throttle. If you were turning the steering wheel at all it was impossible to apply any throttle. When you would apply the throttle the car would lock the rear suspension. That would not only take longitudinal load transfer, but it would take lateral load transfer. After the huge change in rear stiffness the car would either get very sideways or spin....almost on cue. To make the rear work well I ended up going to nearly 0% antisquat. It was much more controllable and had significantly better grip.
For my money, all 'anti's' should be used sparingly. It's probably reasonable to use a little antidive and a little antisquat, but think of them as spice in the soup. A little goes a long way. Concentrate on good camber and instant center control. Make sure bumpsteer is eliminated. Make sure caster doesn't change much. Then, if you want to use some anti to allow you to run softer springs, knock yourself out. Just don't build a car around it, and don't believe Herb Adams about it.
RE: Definition of Anti Squat
"If you accelerate with a car that has 100% antisquat it will accelerate and at the same time load the rear tires. The front will lift, but the rear will not squat."
I'm with you to this point. All of what the dragracers call "weight transfer" is being carried through the links and there is no change in spring loading.
Continuing: "The rear has essentially been locked solid. If you go over a small irregularity in the road, the rear tires will not follow the road and will probably get wheelspin."
But, I can't reach this conclusion. The situation, as far as the links are concerned, is essentially no different than when the car was sitting in the pits with the engine off. A force and moment balance exists in both cases. The only difference, during launch, is that an inertial force has been added to the mix. But, a force is a force. The suspension is no more "locked solid" during launch than it was in the pits. Other forces are simply additive.
As for an explanation of your experiences, I would have to look elsewhere. Specifically, I would strongly suspect that the additional loading resulted in a "stickiness" at certain suspension pivot points.
RE: Definition of Anti Squat
If you have 100% anti squat, that suggests that there is zero suspension movement with a considerable rear weight transfer. That must mean the suspension rate has actually risen to infinity. In other words zero suspension movement with considerable extra weight applied.
A spring with infinite rate might be thought of as going solid. He is also right saying that with the suspension effectively non functional (locked solid), traction is going to be less than wonderful with normal road tyres.
Try making a triangular structure with needle roller bearings at the joints. It will be solid. Why ? because all movement is restricted by resisting forces in the links, even if the pivot points are free. Likewise torque reaction in your anti-squat linkage opposes all vertical movement.
If a wheel tries to move upward, that generates higher downward anti squat forces, and the whole thing effectively locks solid.
RE: Definition of Anti Squat
For instance, let's use a twist beam De Dion. That is, a single trailing arm at each side of the car, joined by a torsional beam typically behind the wheelcentre. It is a perfectly sensible suspension, not that I've ever seen one.
The anstisquat is controlled by the SV angle of the trailing arms, yet there is no mechanism there to lock up. The wheel will always move up and down in response to bumps, won't it?
Cheers
Greg Locock
RE: Definition of Anti Squat
To have 100% anti-squat those arms would need to be at a very steep upward angle indeed, probably not far off vertical !
If that were true, bump loads would be trying to compress those arms as much as swing them around the torsion bar axis. A pretty harsh setup.
If it was set up with the arms nearly horizontal with little or no built in anti-squat, it would probably work fine. If you could get the driveshaft past the torsion bar that is.
I have not seen one of those either Greg, but I have a good imagination.
RE: Definition of Anti Squat
From Greg's long. load transfer (I hate the term weight transfer) equation let's say we have a 3000# car with a 110 inch wheelbase and a 24 CG height. It's accelerating at 0.5 G. We transfer 327 pounds off the front axle and the rear has to accept it.
If we have 100% AS, the suspension links are pushing vertically with 327# of force to counteract that load transfer. In a perfect world, if you were to run over washboard pavement at this point, the car would soak up all the bumps in the springs, but the rear wouldn't have any more (or less) squat in it due to constant long. acceleration. We don't live in this world.
If you are anywhere near the limit of tire grip and go over rough pavement, the tires will see small bits of wheelspin due to the transient loadings. Even on a simple city street, these transients can be very large. The load in the suspension arms that are reacting to the load transfer are directly related to the longitudinal force on the tire contact patches (i.e. your AS force ultimately comes from the force of accelerating the car). As soon as you get a touch of wheelspin, the anti-squat force goes away. When the tire hooks back up, the AS force is restored. All this loading and unloading acts in a very short time period and is pretty severe, and it's a lot of force we're talking about 300 some odd pound going on and off the tires not to mention the loads from the bumps. When faced with rapid loading in pretty much any direction, a tire's first inclination is to slide or spin. They just don't react well to rapid inputs.
This is why I said that on a perfect launch pad, it might work, but in any environment I've ever worked in, high levels of anti-squat were a nightmare.
I only did the 100% anti-squat on one car. That was an option, but there were several other combinations available. It was a racecar and the links were monoball bearings. There wasn't excess friction in the system. Reducing the AS to 0 was the next step. 100% didn't work, so instead of screwing around with all sorts of other options, we just took it all out. This probably was not a perfect test. It was years ago, and I don't remember the exact numbers of the car. It did change motion ratios very slightly. It was as close to being a straight back-to-back test as I could do, though. No spring/shock/bar/setup changes. The rear of the car was massively better to work with. Yes, it had some squat. That didn't seem to matter. It would hook up nicely and was very docile to drive after that change. Mind you, it wasn't like a shade of gray change...I'm talking black and white.
One of the things I worried about on that car was braking. Excess AS will make the car prone to brake hop due to short SVSA lengths. Because of that, I had the brake system decoupled from the drive. There were seperate leading links that the calipers transmitted their forces through (with very little anti-lift ~10% or less). Braking of that car was always pretty good. I guess that was something I didn't overlook, but even at that, it was competely different from what I expected. In that respect, it was a very good learning experience even if not a particularly successful one.
RE: Definition of Anti Squat
Hi Morewing
Thanks for your post. While I like the Adams book I have always had my doubts about it, for instance on page(p.35) he recommends cutting off one coil of a coil spring. All the other books, articles, about suspension modification I have read, condemn this practice.
Are you familiar with “How to Make Your Car Handle” by Fred Puhn ? If so, what do you think of it?
Bye for now
RE: Definition of Anti Squat
No, it simply means that a force and moment balance exists without deflection of a spring. Any new forces...such as those which would occur if an irregularity in the road surface was encountered...would be additive, requiring a change in spring loading.
Greg's post should be read more closely. He explains that the locating links are trailing. Perhaps a more common design would be clearer. Consider two trailing links on each side. Each link pair would be in a plane parallel to the XZ plane. And, the members of each link pair would be parallel to each other. Finally, all links, in side view, are angled upwards, from the axle, at an angle with a tangent equal to Greg's "a" over "b." Since parallel lines meet at infinity, this would mean 100% anti-squat. This also means that all inertial loads, transmitted back through the links, are at the same angle. Any additional load, parallel to the Z axis and received at the tire patch, would require a change in spring loading, since there is no component of vector forces in the links available to balance it.
(Obviously, there is also nothing available to overcome the moment generated by the inertial force of the unsprung mass of the rear axle assembly, which is why I made the earlier comment about the no squat/no rise line actually passing a short distance below the rear tire patch...or axle centerline for an IRS or DeDion.)
RE: Definition of Anti Squat
I'd have liked to extend the CarSim model to include an acceleration over bump, sadly that program seems to be able to cope with bumpy roads, or horizontally accelerating vehicles, but not both at once.
Cheers
Greg Locock
RE: Definition of Anti Squat
(Incidentally, I see a danger in the blind trust of a comprehensive model. I have, for instance, asked if the engine modeling software takes into account the "water hammer" principle in the modeling of the manifolding. Nobody seems to know. Everybody seems to be placing their faith in the consideration of flux at an increment of flow passage length, but I'm not at all certain that this adequately "captures" the water hammer phenomenon. In other words, once the model goes beyond the initial development phase, basic assumptions are no longer questioned. This could be very dangerous.)
RE: Definition of Anti Squat
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Billy,
Regardless, If we have trailing links angled up at something like 45 degrees and a spring coming off them, you have to see that the trailing links are going to take a meaningful portion of a jounce load even if the car is coasting (no AS force). Since the tire is going to naturally move backwards (make the wheelbase longer) to accept the load it might not be as bad as I think it would be, but it's still going to be pretty severe.
I know what you're thinking in terms of any change in tire loading would require a change in spring loading, but let's do a thought experiment.
Assume the spring in this scenario is a link with pin joints, infinitely stiff. If this were the case, would you know say that the trailing link is taking none of the load? Of course not, a simple bit of statics shows you that is taking a verical load that corresponds to the longitudinal load it is being asked to take. This change depending on the angle of the arms, spring, loading, etc. but it's very real. The trailing link will take a portion of the verical load. I suppose if the load were directly in line with the spring, then you could argue that the spring takes all the load, but again, we don't live in this world.
I'm not trying to rain on a parade here, but what I'm saying is that large amounts of AS just do not work well in practical application. If someone wants to build one, more power to them. Just do yourself a favor and make sure that's not the only scenario you give yourself.
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As far as books go, I've seen Fred Puhn's book, but I don't own it. There are a bunch of books that cover pretty much the same material, but in slightly different manners. I think Greg has a list of good books to read, you might want to check that out. Van Valkenburg's book is pretty good. Smith's are pretty good. Puhn's has been in cirulation long enough that it must be pretty good. A fair bet is to read them all. Start with the oldest and easiest to read and progress into the more scientific. You'll get something from all of the them.
RE: Definition of Anti Squat
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.
RE: Definition of Anti Squat
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.
RE: Definition of Anti Squat
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.
RE: Definition of Anti Squat
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.
RE: Definition of Anti Squat
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.
RE: Definition of Anti Squat
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.
RE: Definition of Anti Squat
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
RE: Definition of Anti Squat
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.
RE: Definition of Anti Squat
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
RE: Definition of Anti Squat
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?
RE: Definition of Anti Squat
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'?
RE: Definition of Anti Squat
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
RE: Definition of Anti Squat
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.
RE: Definition of Anti Squat
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