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63ImportBeater
11-27-2005, 10:09 PM
Could someone please explain the physics of ANTI-SQUAT? I am a Mechanical Engineering major so I can appreciate a lot of very technical information, so throw it at me. I am interested in designing my own 3 or 4 link for my 63 nova and need to know the physics of 'Anti-Squat' so I can decide how to best use it in my application.

MrQuick
11-27-2005, 11:18 PM
anti squat is a term used with rear wheel drive cars and is the amount of compression that is applied to the drive wheels under acceleration.a suspension with 100% anti squat will exibit no compression of the rear suspension during acceleration. the chassis and suspension links take 100% of the load.
A car with 50% anti squat will plant the tires harder than a car with 100% anti squat.wait... a car with 100% anti squat will squat under acceleration so the rear of the car will drop. I believe this squat is action that goes into the suspension movement instead of tire traction. I think I just confused myself? :candle: were's my books? :rtfm:
(EDIT): more info not from memory...
Heres a section from Herb Adams. Memory is bad so books are good...

Anti-squat is not difficult to calculate, however certain values on the vehicle must be known, such as the center of gravity between the front and rear of the car, and the height of the center of gravity . With this information, a scaled drawing of the tires and this information can be made. Once a drawing is made, then the rods of the 4 bar system can be plotted onto the drawing. According to Adams, “The instant center is that point around which the linkage can be assumed to react” . To determine the instant center, the paths of the 4 bar rods are extended forward to find their intersection point. This is the point around which the rear axle would rotate if the 4 bar connected to it.


The anti-squat value is based on where the instant center falls. Using the same drawing as before, a line is drawn from where the rear tire touches the ground; to a point at which a vertical line is drawn through the front axle and the center of gravity meet. If the instant center is on this line, then there is 100% anti-squat in the rear suspension. Remember from earlier, more or less than 100% and the vehicle will rise or squat, respectively. This amount of rise or squat is relative to the instant center’s height in relation to the “100% line”. hope this helps and hope someone eles jumps in here.

Norm Peterson
11-28-2005, 05:56 AM
Figures 17.13, 17.14, and 17.15 in Race Car Vehicle Dynamics are sketches showing all of the "antis". The percentage can be graphically determining pretty much as Vince has already described. But if you're more comfortable with just numbers, "anti" percentages represent the relation between longitudinal load transfer and vertical loading through the suspension linkage (rather than through the springs/shocks).

Longitudinal load transfer is a function of h (the vehicle CG height) and L (wheelbase). Graphically, that's the intersection of the "vertical line is drawn through the front axle and a horizontal line drawn through the center of gravity". Or, h/L, and this represents a vertical:longitudinal force ratio.

Load carried through the suspension linkage is based on the SVIC* location. Knowing the vertical and longitudinal coordinates of the link pivots means that equations for the lines between them can be written, and their virtual intersection (the SVIC) determined. The suspension (in side view) can now be considered as a virtual swing arm pivoted at the SVIC. Since there is (obviously) zero moment about the SVIC, the relation between the vertical and longitudinal force components depends on the vertical and longitudinal offsets between the IC and the point of application of the basic fore/aft force**. Using the xyz = longitudinal/lateral/vertical convention with the origin at the contact patch directly under the axle (for a stick axle), the vertical:longitudinal force ratio for the suspension linkage becomes [z(SVIC)]/([x(SVIC)]. RCVD discusses this in terms of the tangent of angle theta, for reasons related to the ** note.

Anyway, since all of the longitudinal acceleration load is resisted by the longitudinal linkage load, the anti-squat percentage ultimately boils down to being the ratio of vertical force through the linkage to the longitudinal load transfer. Or the ratio of the tangents of the two angles.

* Side View Instant Center

** this is at ground level for a stick axle, but at axle height for an independently sprung drive "axle". Specifically, it's the point where the traction force only is applied (zero moment).


Using it best in your application kind of depends just what that application is. Understand that it's entirely possible to have too much A/S, and that one downside is the possibility of brake hop occurring, or occurring too early. If you're only going to drag race and drive no harder than moderately on the street, that probably isn't going to be an issue, and 100% or slightly more A/S may be the ticket. But for any driving that includes hard braking it isn't. Compromises to tame brake hop are to either lower the A/S% and ignore or drive around a somewhat poorer launch or reduce the rear braking proportion (which increases the front braking effort as well as stopping distances relative to what they'd be with ideal brake bias and no hop). A/S% figures closer to 50% are generally more road-race friendly, and I've heard of lower values being used in a very specific non-streetable application for a well thought-out reason (and with more than a little success).

There's more, actually, as the longitudinal location of the SVIC has its own influence even with the A/S% held constant. Where do you want (need) to apply that vertical force?

Don't get so carried away with A/S that other aspects such as axle rollsteer get compromised too much for your application. Playing games with the inclination of the lowers will affect that, or force you to raise the rear RC (which throws you into another bunch of iterations).

Suspension engineering is just that - finding the right balance for a given situation, which involves some compromises. It's why you won't find any "fits all" solutions but will see lots of discussion.

Late edit for clarity. Thanks, wendell.

Norm

wendell
11-28-2005, 06:04 AM
It's a lot easier to explain when you can use pencils for links and beer cans for tires but here goes...

Newton said, that drinking enought mercury will cure anything. He also said that for every action there will be an equal and opposite reaction. We should focus on the later.

When the average car takes off it transfers weight rearward (because objests at rest try to stay at rest). The rear weight transfer causes the back of the car to squat DOWN. The equal and oposite reaction to that is tire force pointing UP. That's bad.

A properly designed suspension can reverse these properties. As weight is transfered back, tire force is pointed DOWN, loading the tires with the newly transfered weight. 100% anti squat is neutral. The weight transfered to the back of the car is the same as the force on the tires. Visually the back of the car stays level. At a value greater than 100% the force on the tires is greater than the weight transfer and the back of the car can be seen lifting up. At less than 100% the opposite is seen.

Anti squat is calculated by extending a line from the rear tire contact patch forward through the rear IC untill it intersects the vertical plane of the front axle. The ratio of the height of this intesection divided by the height of the COG X 100% is the % anti squat. Example: COG is 17 inches (a fairly well developed 1st gen) and the line from the contact patch intersects the front axle line at 15 inches, you have 88% anti squat.

That's the best I can do for you. I'm going to put on my helmet and mouth guard and get ready. The best thing you can do is get some books. The herb adams book is good, so is Paved Track Stock Car Technology.

Or just read the above post!

parsonsj
11-28-2005, 06:12 AM
Anti-squat is a term that indicates how the vehicle reacts under straight-ahead acceleration. The value (say 50%) is the amount of the force that isn't translated into body squat.

It's kind of a backward term: you could just as easily call it "body lower" or some such and turn the value around.

The calculation itself is based on the lever length of the control arms divided by the lever length of the force generated by the tires.

One other thing to consider is that under vehicle braking, the forces reverse. In this case too much anti-squat is harmful: it will cause wheel hop. That's why 50% anti-squat is a good starting place for road racing and street cars: it's a good compromise between forward acceleration and braking (from a physics point of view: positive and negative acceleration). Finally, since most of the vehicle's braking is done by the front brakes, many racers find some success by increasing the anti-squat beyond 50% to give their cars more forward bite and giving up some rear braking effectiveness.

jp

nutley
11-30-2005, 05:19 PM
Hey guys i'm a 4x4 guy, and my cousin that I live with wants me to delvelop and suspension for his 68 camero. I thought I would share with you guys what we have developed in the 4x4 world for link calculations for suspention geometry. What are your recomendations for anit squat values and roll steer? 50% anti squat seams to be a recuring number in this forum for what little I have read. Should be an easy value to hit with such a low car :)

here is the link to the forum to the link calculator that has been developed.

http://www.pirate4x4.com/forum/showthread.php?t=204893&highlight=4+link+calculator

and here is the newest version of the calculator itself.

http://mysite.verizon.net/triaged/files/4BarLinkV3.0.zip

Here is a good explaination of anti-squat and diagrams

"When a vehicle accelerates, the weight shifts rearward. The higher the weight is above the ground, the greater the shift off the front and onto the rear. This extra weight will compress the rear suspension, make it SQUAT. But there is also a torque on the rear axle (http://www.pirate4x4.com/forum/showthread.php?t=212765&highlight=antisquat#), equal and opposite of the tire torque. The pinion will try to rotate upward. The links resist this torque by putting forces into the frame. These forces may try to lift upward. The upward force will cancel some of the squat from the weight shift, ANTI-SQUAT. The easiest suspension to understand is a ladder bar/radius arm setup. if the axle trys to rotate backwards, the end of the arm will push upwards. The shorter the arm, the more upward force it will create. The greater the force, the more anti-squat it has. For understanding anti-squat, a 4 link can be visualized as a ladder bar, wherever the links would cross is the theoretical length of the arm. This picture is a ladder arm and 4 link with the same anti-squat geometry

https://static1.pt-content.com/images/noimg.gif

For more antisquat, the 4 link should have more angle, making the intersection shorter, for less anti-squat the links should have less angle. The height of the intersection is also important, the higher it is the more antisquat it has. Another pic, the red setup has more than 100% anti-squat, the blue has less. The drawing has a horizontal line through the CG, and a vertical line through the front wheel. Then a line is drawn from the rear tire contact patch through the 4 link intersection, till it crosses the front axle (http://www.pirate4x4.com/forum/showthread.php?t=212765&highlight=antisquat#). In the drawing the CG happens to be 38" the red suspension crosses the front axle at 48" 48/38 = 1.26. Thats 126% anti-squat."

https://static1.pt-content.com/images/noimg.gif

It should also be noted that the COG measurment is of the unsprung weight

BRIAN
11-30-2005, 06:07 PM
Nut, What about a parallel 4 bar set up where the bars do not crate an intersect point?

nutley
11-30-2005, 08:01 PM
That is a good question. It largely depends on the angles of the links not just the intersection. If the links are parallel as well as parallel with the ground, the antisquat at that instant would be 0. This is bad because when you get on it the suspension will compress and this changes your values to a negative anti-squat number, continuing the squat to progress. However if the links are parallel and angled up to the front of the vehicle you end up with a positive anti-squat number. Remember that length of the links will effect their angle toward each other as the suspension cycles. Also an intersection behind the rig is negative antisquat, mean that the axle will actually compress the suspension when power is applied.

This may not apply to short travel vehicles like slammed race cars but hang on for a wild ride into choosing a suspension that works consistently for suspension travel. To determine link length and location, plot all the critical values: WB w/ proper tire size, frame location, unsprung COG height and then the anti squat projection line you are shooting for, and link placement at the axle. From there you can plot the links so they intersect with this line. Then plot your axle end mounting points at a different Z (up or down) as the suspension cycles, then hit your antisquat line again at a different location farther up on the vehicle. Where your upper and lower links at these different projections intersect gives you your location and link length to achieve constant anti-squat for that suspension cycle. I'll try and find the link where a genius derived this with pix.

check out the link calculator, it has way more info than you could want, just play with the numbers and you'll see how seperation and angle effects squat

I'm not an expert but trying to study and get a good grasp on suspension design, and try and help my cuz out.

nutley
11-30-2005, 08:09 PM
found it!

http://www.pirate4x4.com/forum/showthread.php?t=208915&page=3&highlight=AntiSquat

posts by strange rover, towards the bottem. Cool stuff

Norm Peterson
12-01-2005, 03:32 AM
When the links are truly parallel, their "intersection" is assumed to be at infinity. That makes the anti-squat construction line parallel to the links, as it now has to intersect them "at infinity". If you are somewhat uncomfortable with the use of infinite distances, or if your software/spreadsheet pukes at the concept, you could make the links converge at some huge distance (100 yds? 1 mile?). The error in A/S% would be so small as to have no meaning, and the consequences of realistic fabrication tolerances would probably have a greater effect.

On edit, I'm getting a
There seems to have been a problem with the database.
Please try again by clicking the Refresh button in your web browser.error with the pirate4x4 link. Is it simply a case of the site being down for maintenance or is membership required to access the message board?

Norm

nutley
12-01-2005, 08:25 AM
Try this

http://www.pirate4x4.com/forum/showthread.php?t=208915

3rd page

I noticed there was an antisquat in the url, maybe adds a seach fuction which you would have to be registered for.

Mean 69
12-01-2005, 08:58 AM
Some outstanding explanations, nice work guys. I'd add one thing, which we feel is very important, and is a big influence in how we design our stuff.

Anti-squat is derived, as stated using several parameters, one of which is the "instant center." This is important, dynamically, because what the A/S value is at one instant, for instance static ride height, it will change as the suspension bumps, droops, and rolls. The IC can move up, back, forward, down, you name it, it depends upon how the links are oriented, and is pretty sensitive to link lengths. The general case is that the longer the links, the less the IC will migrate dynamically, and the resulting A/S migration will be "quiet" relative to a similar system with short links.

Once you start modelling this stuff out, you will learn pretty quickly that there are several things in tension with a (coupled) link suspension. For instance, the enemies of high A/S are axle (roll) steer, and the potential for brake hop. Suspension design is kind of like poking a balloon, you nudge one area, and another area pops out, you have to pick your battles for you individual application.

Mark

BRIAN
12-01-2005, 05:58 PM
Great info. I have set up several 4 bar parallel bar set ups and every manufacturer has said to set them up parallel to the ground at ride height. The last set up was a S&W kit that had short (approx 17") bars. I think I remember being told that to leave the bars parallel or for a harder launch to raise them both approx 1-2" each the same measurement. They had 4 equally spaced holes on the front brackets.

I kind of understand the above but will take a little to sink in.

I have to say I always stick with a system and because of all the complex design elements that go into a suspension design you have to be scared with all the cars being constructed where guys are just making parts with no thought of how they actually effect the vehicles handling characteristics. Even worse they think they have improved upon them.

parsonsj
12-02-2005, 10:09 AM
Brian,

The reason parallel 4 bars need to be level at ride height is to minimize roll steer. As the body rolls, one side of the parallel bars goes up, and the other side goes down. If they start level, then the amount of "shortening" that occurs as the bars go up/down will be same on both sides, keeping the rear housing perpendicular to the vehicle center line.

jp

andrewb70
12-18-2005, 07:56 PM
Well this info is fine and dandy for solid axle cars, now what about IRS?

I am specifically interested in my RX7. I would like to optimize the rear suspension for drag racing. What info is needed to determine what I can chenge in order to increase the rear bite?

Right now when the car is launched hard with sticky tires, the rear of the car squats down very hard. From the info posted above this would indicate that the tires are being pulled up, since for all actions there is a reaction.

Attached is a picture of the junk that I have to work with.

Video of the car launching (http://www.speakeasy.org/~andrewb/rx7/RX7_11_40.mp4)

Andrew

MrQuick
12-18-2005, 10:39 PM
Ok start, not too bad.
Beef up the pumpkin mounting abit. Maybe fabricated rear suspension cradle that uses longer link style control arms. Is that a spare or did you start already?

Norm Peterson
12-19-2005, 04:04 AM
Well this info is fine and dandy for solid axle cars, now what about IRS?

I am specifically interested in my RX7. I would like to optimize the rear suspension for drag racing. What info is needed to determine what I can chenge in order to increase the rear bite?

Right now when the car is launched hard with sticky tires, the rear of the car squats down very hard. From the info posted above this would indicate that the tires are being pulled up, since for all actions there is a reaction.

Attached is a picture of the junk that I have to work with.

Video of the car launching (http://www.speakeasy.org/~andrewb/rx7/RX7_11_40.mp4)

AndrewThe geometric construction is only slightly different from what's been posted above, but it generally represents a huge difference in launch behavior (as you already know). The anti-squat line for IRS passes through the rear axle rather than the rear contact patch.

See the ** note in the November 28 post in this thread. Redraw the constructions to suit and you'll probably find the A/S line is at a fairly shallow slope. Perhaps somebody has C5/C6 rear suspension data; I suspect that it runs a bit more A/S than the RX7s did.

Other than doing things to make a big change in SVIC location - maybe some rear shocks with lots of bump damping will help?

Norm

andrewb70
12-19-2005, 06:39 AM
The geometric construction is only slightly different from what's been posted above, but it generally represents a huge difference in launch behavior (as you already know). The anti-squat line for IRS passes through the rear axle rather than the rear contact patch.

See the ** note in the November 28 post in this thread. Redraw the constructions to suit and you'll probably find the A/S line is at a fairly shallow slope. Perhaps somebody has C5/C6 rear suspension data; I suspect that it runs a bit more A/S than the RX7s did.

Other than doing things to make a big change in SVIC location - maybe some rear shocks with lots of bump damping will help?

Norm


The launch in the video had a 1.685 60' time. I should list the things that I have already done.

Tokico adjustable shocks. The rebound and compression are always 50/50, but I can stiffen or soften them. The stiffest setting works the best.

The stock non-adjustable links that control the camber have been replaced with adjustable ones. Most of the negative camber has been dialed out.

There is a pinion snubber installed to control the upward rotation of the rear housing. The stock rubber front diff mount is notorious for breaking.



Ok start, not too bad.
Beef up the pumpkin mounting abit. Maybe fabricated rear suspension cradle that uses longer link style control arms. Is that a spare or did you start already?

So how does one change the SVIC location? How do I calculate it given my current configuration?

I am not at all opposed to fabricating various control arms that will transfer the load of the rotating diff assembly to a different point on the chassis.

Thanks,
Andrew

Norm Peterson
12-19-2005, 05:11 PM
Heavy bump damping keeps the chassis from dropping as quickly, and sort of mitigates the lower value of the A/S associated with most IRS arrangements. More specifically, I'd think heavy low speed bump damping. High speed bump damping would be probably best left relatively soft, so that the occasional bump in the track isn't quite so successful at unloading the tire. You want the relative [vertical] positions of the chassis and axle to remain relatively constant under body inertia motions (launch) but have minimum force response from wheel displacement effects.

In the case of IRS, increasing A/S typically involves relocating one or more of the chassis side pickups and perhaps an upright pickup (per side, natch). It's a little more difficult to visualize this than moving the SVIC for a stick axle with a linkage - with IRS you have to think in terms of the planes containing the control arms and their intersection within the vertical plane containing the rear wheel. More A/S means either the LCA plane needs to slope upward more (think side view), the UCA needs to slope downward (ditto), or a little of each, all the while trying not to disturb or having to correct other aspects such as toe steer. Figures 17.15, 17.21 and 17.34 in RCVD give a pretty good picture of this, BTW.

Norm

andrewb70
12-19-2005, 05:48 PM
Heavy bump damping keeps the chassis from dropping as quickly, and sort of mitigates the lower value of the A/S associated with most IRS arrangements. More specifically, I'd think heavy low speed bump damping. High speed bump damping would be probably best left relatively soft, so that the occasional bump in the track isn't quite so successful at unloading the tire. You want the relative [vertical] positions of the chassis and axle to remain relatively constant under body inertia motions (launch) but have minimum force response from wheel displacement effects.

In the case of IRS, increasing A/S typically involves relocating one or more of the chassis side pickups and perhaps an upright pickup (per side, natch). It's a little more difficult to visualize this than moving the SVIC for a stick axle with a linkage - with IRS you have to think in terms of the planes containing the control arms and their intersection within the vertical plane containing the rear wheel. More A/S means either the LCA plane needs to slope upward more (think side view), the UCA needs to slope downward (ditto), or a little of each, all the while trying not to disturb or having to correct other aspects such as toe steer. Figures 17.15, 17.21 and 17.34 in RCVD give a pretty good picture of this, BTW.

Norm

Norm,

Thanks for the help, but I am having a little trouble visualising. The RX7 suspension is a pretty simple swing arm design. There are no upper or lower control arms. Please see the picture above.

At one point I was thinking about fabricating a torque arm that would attach to the center housing and extend forward to the rear crossmember. Is that something that might help?

Andrew

Norm Peterson
12-19-2005, 06:26 PM
That looks like semi-trailing arms. You'd still have to relocate at least one of the chassis side pivots, maybe all of them. This one is Figure 17.25 in RCVD. Last I knew for under 2-liter Trans-Am racing, teams running cars equipped as OE with this arrangement simply stiffened everything up (Datsun 510, BMW) just to keep suspension movements small.

Geometrically, you have just one plane to consider, but since the hub carrier is firmly fixed to the semitrailing arm's plane, when the arm rotates in 3-D about its skewed chassis pivots, so does the wheel (meaning that changes will occur in both toe and camber that you more than likely don't want for either drag or road course performance).

Norm

MrQuick
12-19-2005, 11:18 PM
he's right, your link or arm design would have to be built to the spindle end and not the pumpkin since its just about fixed in place. something in this nature....
https://static1.pt-content.com/images/noimg.gif

todd8541
12-20-2005, 03:34 PM
Another view on anti-squat. Same information just stated slightly different.

Anti-squat is a measure of how much of the wt being transfered during acceleration is supported by the suspension linkages. The rest is supported by the springs.

Ie if you have 50 percent anti-squat your linkages will support 50 percent of the wt being transfered and 50 percent is being supported by the rear springs.


The rear suspension inputs a force at the SVIC. anything less than 100 percent the linkages are only able to hold up the that percentage and the springs the rest. When it is over a hundred percent the linkages are able to input enough force to actually over come the wt transfer.

IE at a 110 percent the suspension inputs 10percent more force than what is being transfered during acceleration resulting in the rearend of the car lift.

Also keep this in mind. The higher the anti-squat the quicker the force is supplied to the rear tires. This can in some situations cause the car to get upset. Because tires are basically rubber springs (think basketball) the FASTER they are slammed into the ground the more they will rebound. When this happens the tires will lose traction.

So remember you need to pay attention to the rate of force being applied not just the magnatude.



Todd

David Pozzi
12-20-2005, 04:43 PM
On a high horsepower road course car the above is very important. High HP cars need a rear suspension that can still absorb a few bumps or dips while accelerating out of a corner. Anti-Squat is great as long as you don't get too much. You don't want to lift the whole car much when you accelerate out of a turn.

alcino
12-22-2005, 09:56 AM
Was wondering on a Leaf spring car how the anti squat % changes with the addition of liftbars or calTracs/slide-A-link. A diagram would be nice showing me the SVSA. I got an idea, but wanted to confirm it. I already know the 3/8 of the leaf thingy to figure the SVSA w/o a traction aid.

Also is pinion angle related with anti squat? I feel no, but seems the Drag racers talk more about pinion angle for traction than anti squat while we "roadracers" talk about AS. Is one better for straight line? Am I delirious?

Alcino

todd8541
01-03-2006, 06:29 AM
Alcino

The anti-squat is determined by the theoretical location of force application from the rear suspension. I've never really thought about analysising (I think thats how you spell it haha).

The way I understand the system to work is that it pivots at the front spring Eye and applies a force to the top of the spring to reduce it's spring wrap during launch.

If I am correct then your Instance Center would be the front spring eye location. This is because once your suspension is locked (lack of a better term) meaning the spring and cal tracks are reacting with each other that there is no longer any more pivot the force is then applied directly at the mounting point. It then becomes a psuedo ladder bar with a softening characteristic if you will. Because it's movement is determined by the spring.

If you were using bolt on traction bars then the contact point of the traction bars to the chassis woulc be the instant center (usually the front spring eye). You could change it be changing the location in which the traction bars contact the chassis. Either lower , and/ or lower.

If

alcino
01-03-2006, 09:39 AM
Todd,
If I understand you correctly. Does that mean that the different hole positions on the Cal-Tracs have no effect on anti squat? They must be there to adjust something.

Alcino

todd8541
01-03-2006, 10:16 AM
Alcino

The additional holes do have a reason. It's to adjust the severity of the initial hit of the launch. The further away the more leverage applied to the spring.

This system is difficult to decifer because it has a flexible member (the spring). I tell you what now you've got me really interested in this. Give me a few days and let me do a more indepth view of this thing and see what I come up with. I think the answer will still be the same but let me put some pen and paper to it with some numbers.

Todd

johnny rockett
04-11-2007, 04:25 PM
I have been drag racing for 35 years and have national event wins and mutiple records.......never could even come close to some of the knowledge and understanding you guys have of A/S. I am new to this forum and most impressed by the interest and sharing of knowledge you guys do on here.
Here is one for you....what would a typical 4 link setting in a good suspension drag car like pro stock be in reference to anti squat? This is what I drove for years and I just know what would work and what would not. We would start at a setting of anywhere from 48 to 54 in an imanginary connection point and about 4 to 5 inchs off the ground. Smaller HP cars could work at 36 to 44 in and about the same height. The bottom bar would work with a very small downward angle. Not level but maybe 2 degrees. These cars also have to brake well at 200mph.could not afford any wheel hop. I was curious how much anti squat these cars have. I am not a chassis builder but there is a big difference in the way these cars were built over the years. My first serious 4 link car I built in 1981 and the links were not of equal length. Probably more for package purposes. The intersect point would change more radically going down the track so I dont think that is a good thing in any car. Since then of course 4 links have evolved into equal length bars. If you looked at the car during a launch you would see the car drop as it plants the tire but the body to tire reference should be the same and not move. I was curious as to how much A/s that would represent? My guess would be 100% but how difficult is that to figure out? I am sure ride height c/g has alot to do with this.

With that being said...you might want to look a new bar called the Max-trac and how the adjustments work with that style of bar...www.smithracecraft.com (http://www.smithracecraft.com) Those bars are not 4 links but work pretty good. Can an A/S be figured out with this style of traction device?
Thanks

Norm Peterson
04-27-2007, 09:54 AM
The CG at ride height and wheelbase need to be known, since they both are direct parts of what determines how much load transfers rearward. Perhaps more specifically, the weight and CG height of the chassis and the weight and CG height of the front unsprung weight. I'm pretty sure that A-S increases as the nose rises, at least as long as the rear tire to body reference remains reasonably constant.

FWIW, I get ~60% A-S using numbers in the middle of the ranges you gave for the bigger-HP cars and an 18" CG height and 120" wheelbase. More like 75% for a similar car with the little motor.

I'm not at all sure what's happening with the Smithracecraft piece, at least not without seeing how it's installed (does the leaf run between the bolts shown in that front piece that does not have the adjustment holes?).


Norm

johnny rockett
04-28-2007, 04:57 AM
Hey Norm.....thanks for the reply.....yes the spring runs between the two rollers much like a cal trac. Smith did get permission to apply for a patent so something must be different. The lower piece actually contacts the spring eye which is different than the Cal Trac.......the three holes in the front and the two holes in the rear of course enables you to change the bar angle which does affect how hard the tires are hit. I like the contact much better on the sping eye than the Cal Trac method. I never have backed to back them so dont know of any real difference in performance but being able to adjust the bar at least gives you a chance to learn something if there is even anything to be learned?

A good four link car in a prostock will maintain the body to tire line as it leaves the starting line with the front wheels off the ground only a couple of inchs through first gear. Of course at that point anycar with the wheels off the ground have all the weight on them and this has to be a big factor in anit-squat I would think. I always thought they had closer to 100 % but I dont really understand this. I just knew what worked. If you had 100% A/S would those cars even be able to brake from 200 mph?

BillyShope
05-02-2007, 05:40 AM
Hi, guys. Just found this forum (StatCounter told me someone referenced my blog from here.) I see my friend, Norm, is here, so I'm sure this is a good place to post.

For this discussion, I would only add that the 100% anti-squat line (at Chrysler, we called it the "no squat/no rise line," which I consider to be more descriptive) changes slope...very slightly...as the percentage of rear axle assembly weight (or, with an IRS, the unsprung weight) changes. In other words, it passes through the rear tire patch only if the rear axle assembly weight equals zero. With any finite weight, it passes below at a vertical distance equal to the height of the rear axle assembly cnter of gravity times the ratio of the weight of the rear axle assembly to a value equal to the total weight less the weight of the rear axle assembly. (It takes a lot of words when I'm not there to wave my hands in the air, but I hope that's clear.) This is normally between one and one and a half inches, so its omission...in your calculations or sketch...does not introduce a large error.

I lied: I'm going to add something else. Any discussion of this sort of thing, and its effect on what the dragracer calls "launch," should include the effects of driveshaft torque on rear tire loading and methods of driveshaft torque cancellation. The maximum tractive effort from a tire pair is achieved when they're equally loaded and this is only possible...with a beam axle car...when some form of asymmetry is present. A static cancellation is effetive, but, since this achieves equal rear tire loading only at a specific level of driveshaft torque, a dynamic method...one which would assure equal tire loading at all levels of driveshaft torque...is to be preferred.

My blog discusses various forms of dynamic cancellation and provides spreadsheets for specific applications:

http://home.earthlink.net/~whshope

73-TYPE-LT-LS1
11-07-2007, 02:13 PM
Sorry to dig up and old thread, but I could use some help (yes I know, again :) )

By putting in my truck arm suspension info I'm coming up with a A/S number of 13%

Front point 11.25" off the ground
Rear point 10" off the ground
52" arm from front mount to axle centerline
Using 20" as my ICG

But playing with it, and putting a 30" ladder bar design in the program I get 185% based on

UCA Front 10"
UCA Rear 18"
Length 30"

LCA Front 10"
LCA Rear 10"
Length 30"


Now my question is, is the program not figuring A/S correctly due to there not being an upperlink on the truck arm set up?


If I use my truck arm math and add the upperlink in as:

52" long
Front point 11.25
Rear point 0

It draws a line front the bottom of the rear tire through the front LCA point and gives me 120%.. Is that figure right?

Beige
11-07-2007, 04:45 PM
The anti-squat line for IRS passes through the rear axle rather than the rear contact patch.


Could you explain why?

I'm guessing it's because the contact patch on the opposite tire is the other point where force is transferred on a stick axle car. While on an IRS the inner pivots on the transverse links are that point.
(I can't think of the proper technical terms at the moment, sorry.)

Beige
11-07-2007, 04:52 PM
Sorry to dig up and old thread, but I could use some help (yes I know, again :) )

But playing with it, and putting a 30" ladder bar design in the program I get 185% based on

UCA Front 10"
UCA Rear 18"
Length 30"

LCA Front 10"
LCA Rear 10"
Length 30"


The upper arm might have to be longer
try
31.0483494

or so

73-TYPE-LT-LS1
11-07-2007, 05:17 PM
I'm not too concerned about the A/S with the ladder bar. I more than likely won't go that direction, my issue is more with the A/S being 13% .. I don't think 13% is right. Adding the upper link clued me in to that. I think based on my numbers I put in my A/S would come out to the 120%.. I'm just trying to confirm that.

Norm Peterson
11-08-2007, 09:23 AM
Could you explain why?

I'm guessing it's because the contact patch on the opposite tire is the other point where force is transferred on a stick axle car. While on an IRS the inner pivots on the transverse links are that point.
(I can't think of the proper technical terms at the moment, sorry.)With a stick axle, the suspension linkage resists the axle torque reaction in addition to the forward thrust, with both taken at axle height. Since axle torque reaction and forward thrust are related via tire radius, it's convenient to consider that only the forward thrust acts at ground level (the moment there being zero).

With IRS, the suspension linkage cannot resist any amount of axle torque reaction, as the half-shafts cannot transmit any moment across the bearings and there is no connection between the pumpkin and the uprights. So you're left with only the forward thrust at axle height and nothing else to resolve it with to get a force with zero moment at some different elevation.


Norm

Beige
11-08-2007, 06:29 PM
I'm not too concerned about the A/S with the ladder bar. I more than likely won't go that direction, my issue is more with the A/S being 13% .. I don't think 13% is right. Adding the upper link clued me in to that. I think based on my numbers I put in my A/S would come out to the 120%.. I'm just trying to confirm that.

I rushed my first response and didn't explain any of this, sorry.
What I was getting at is that the program might not see your ladder bar as a ladder bar.

Since the front pivots are at the same height, but the upper rear one is 8 inches higher, the upper bar would have to be longer to intersect the front one at the same point, like a ladder bar would.

That could be throwing off the A/S numbers.

The same thing could be happening to your truck arm figures when you added a 2nd link.

Try making the length of the 2nd link
52.9528092

and see if the A/S goes back to 13%

Beige
11-08-2007, 06:31 PM
With IRS, the suspension linkage cannot resist any amount of axle torque reaction, as the half-shafts cannot transmit any moment across the bearings and there is no connection between the pumpkin and the uprights.

Norm

Thanks Norm.
That makes sense now.

BillyShope
11-13-2007, 06:56 AM
Could you explain why?

I'm guessing it's because the contact patch on the opposite tire is the other point where force is transferred on a stick axle car. While on an IRS the inner pivots on the transverse links are that point.
(I can't think of the proper technical terms at the moment, sorry.)

While Norm's succinct response was excellent, I'm sure Norm would agree that some readers appreciate pictures and diagrams. So, I just finished adding a short section...on the force and moment balances involved...to the "Getting Started" section on the first page of my site. Hope it will help someone.

http://home.earthlink.net/~whshope

rq375
02-15-2008, 08:01 AM
does anyone know of a similar calculator for front suspensions?

Chewy72ss
02-17-2008, 02:31 PM
I don't think there is any "anti-squat" persay, for the front suspension. Because the rear suspension gets that characteristic from the rotation of the rear axle. The front would gain "anti-squat" from trick springs, or a stiffer shock rebound.

If you are looking for a term to describe the relation between the angle of the front control arms, "Roll Center Height" is what you are looking for.

Norm Peterson
02-17-2008, 04:36 PM
Actually, what happens in a front suspension might be termed 'anti-rise'. The math would generally follow the same constructions as anti-squat, i.e. the side view 'anti' line being referenced to axle height for IRS situations (most common) or from the ground (if you've got a live axle up front as well as in the back). How much you'd want, or if you even want any at all, is a separate (but pertinent) question. Off the top of my head, I'd guess that rapid initial load transfer off the front wheels from geometric effects in a FWD application would not be the hot tip. Best to remember that body/chassis front and rear ride heights are at best the visible effects of load transfer, not the causes of them.

Roll center height is associated with front view geometry and is a separate animal.


Norm

BillyShope
04-13-2009, 05:05 AM
Since then of course 4 links have evolved into equal length bars.
I've been reading through some old threads (boy, do I need a life!) and ran across this comment.

I would consider this a DEvolution. As the front of the car rises on launch, the antisquat changes. As the front end rises, the IC location rises with the car, but the array of constant percent antisquat lines lags behind. Norm mentions this effect in the next post. So, there should be an ideal ratio of upper link length to lower link length which...for a given car...retains the original antisquat. I'm certain there is a satisfactory compromise ratio which the manufacturers could provide. Is it worth looking into? Probably not.
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