[engineer1229]

Originally Posted by WildWill But neither should they lift UP the rear end. That doesn't make any sense. That probably would mean something is wrong, because no matter what when you are accelerating weight is shifted to the rear wheels, and adding weight to the rear of the car should cause it to "squat" somewhat. Just thinking with common sense man.

Its a little more complicated than common sense. The car is lifting mainly because of its changed instant center.

Originally Posted by JROD Chassis engineers talk about "anti-squat" at the rear on acceleration and use a traditional diagram to calculate "percentage of anti-squat" (see illustration next page). Because the weight (centre of gravity) of a typical road car is supported by the front and rear wheels, engineers draw a "100-percent anti-squat" force line from the rear tyre contact patch to the front wheel vertical centre-line, at the height of the centre of gravity. If the actual "line of force" for the car – the line from the rear tyre contact patch through the instant centre of the rear suspension – coincides with the 100-percent anti-squat line, then theoretically the rear suspension will neither lift nor drop, and the rear of the car will not rise or squat, as the car accelerates. That is, if the instant centre of the rear suspension lies anywhere on this line, the car will have 100-percent anti-squat. If the instant centre is anywhere below the line, the rear will squat (or, have a certain percentage – a fraction – of anti-squat). If the instant centre is above this line, it will have more than 100-percent anti-squat, which means the rear of the car will be pushed upwards by the suspension links as the car accelerates. Look at this another way. If the suspension is trying to push the back of the car up, that's the same thing as saying it's trying to push the rear wheels and tyres down, against the track surface, which obviously increases bite. That's what this rear suspension geometry is all about. Chassis builders and tuners call this situation (over 100-percent anti-squat) "separation." That is, the rear axle "separates" from the chassis as it swings down and/or the body lifts up, and the springs and shocks extend. The point, of course, is to have the axle swing down (or at least try to as much as possible), thus "planting" or "shocking" the rear tyres on the ground. This is a dynamic situation, with many variables acting at once. Of primary importance is that there be enough weight in the back of the car to make it work. This is a real concern in most "early" street rods. If the centre of gravity in the car is too far forward, a four-link or ladder bar adjusted to plant the rear tyres will simply lift the rear of the body instead. Most builders say you need at least 45-percent of the car's weight on the rear wheels to make a four-link work properly. Spring and shock rates will also influence the effects of the suspension "tune." Particularly important is the extension stiffness of the shocks. Increasing this stiffness decreases the effect of the separation, and vice versa. The same is true of spring rates. The main focus here is to understand that where a builder locates the instant centre of the rear suspension can have a positive or negative effect on how the car launches. A fully-adjustable four-link allows a chassis tuner to move the instant centre not only up or down, but also forward or back (by increasing or decreasing the angle between the bars) at the track. The obvious question is "Where is the right point to set it?" I don't think there is any way to accurately calculate such a point in a drag car. There are too many dynamic variables. For one thing, when a high-powered race car launches, its front wheels are usually off the ground, so the standard engineering diagram for 100-percent anti-squat doesn't apply. Trying to calculate the "correct" instant centre for the four-link in your car is not the point. In fact, it's probably impossible, because as the chassis lifts in the front, the location of the centre of gravity changes; and, more significantly, as the rear suspension moves up or down, the instant centre of the four-link also changes. The shorter the bars, or the more angled they are, the more dramatic this change will be (unequal length bars also give the same effect), and thus the harsher the shock on the rear tyres will (usually) be.

Shit can get pretty complicated, but this should help you get an understanding. That lifting is the tires getting pushed into the ground.

[WildWill]

I need to read that again when I'm not so dog tired... but thank you for that explanation. I'll get back to you tomorrow with my thoughts... It makes a lot of sense tho...

[engineer1229]

And there are slight variations on what works for different suspension systems, 4 links, torque arms, leaf springs etc. But the basic concepts, instant center, center of gravity, pinion angle, can be applied to all of them.

[WildWill]

So what I take from this is that the optimum set up would be for the rear suspension to neither "lift" or "squat." You want it to remain stationary as the weight of the car is transferred to the rear wheels on takeoff. I understand that as the rear lifts, it acts to "plant" the rear axle and the tires to the ground. That makes sense. I'm not trying to make an argument, but wouldn't the springs compressing ultimately do the same thing? As they compress, they store the kinetic energy needed to decompress themselves in return - as the force of decomression acts against that of compression, wouldn't that as well as the combined weight of the car plant the rear axle to the ground in the same way?

I'm not an engineer (as I would guess you are... lol), so if you can put that concept into the perspective I just looked at, can you try and clarify that for me?

[89Domindart]

It was a while ago, but I think I remember I had a friend whos stang would lift slightly up on all four corners when he got on it, while cruising... but I think it was normal. He had subframe connectors and stuff done to it. a 3.73 rear gear too. was a nice stang.

thought Id add- his was a 92 I think. fox body.

[engineer1229]

Originally Posted by WildWill I'm not an engineer (as I would guess you are... lol), so if you can put that concept into the perspective I just looked at, can you try and clarify that for me?

Its wasted energy compressing the springs when that force could be pressing down on the tires. A slower car might not even run any different if it set up on complete opposite sides of the spectrum (one side being it raises and the other squatting) but if your going max effort with something its all the little things that will put you ahead at the finish line.

Yeah, yeah def not an arguement. I just wanted to help clear up the confusion so next time you see the car lift, or the car squat, you might know what causes it and a little bit on how to adjust for it.

I'm not an engineer yet, I've still got a couple years of HELL left.

[03_StangGuy]

I just read that info on this "anti-squat", while its all tru, like the info it self says that is all ideal condition. Most of the guys on here have stangs running slightly modded suspension not race set ups. Ill admit I dont know much about suspension but I do know engineering. Also theres a reason you always see drag cars squating. When a car squats it transfers weight to the rear wheels to help with traction. If the rear is lifting and your suspension isnt designed for that you MAY be losing some traction. Very little in the world of engineering works as it does in theory.

[Traidmarx]

alot of lowered cars are gonna "raise the rear end" its not actually lifting the rear end up like alot of you are taking it to mean....while the car normally would squat from transferring weight when its lowered the suspension is gonna be alot more stiff, so when torque is applied to the springs in the back the suspension is pushing back hard from being more stiff, thus making the body appear to lift up, its just the suspension pushing hard thats all.

[engineer1229]

Originally Posted by 03_StangGuy Ill admit I dont know much about suspension but I do know engineering. Also theres a reason you always see drag cars squating. Very little in the world of engineering works as it does in theory.

ha ha The quote didn't come out of a statics book. Dam near every 9-10 second car at the drag strip has suspension tuning.

It came from here.


Downloads - Race Glides Australia

[engineer1229]

Originally Posted by Traidmarx while the car normally would squat from transferring weight when its lowered the suspension is gonna be alot more stiff, so when torque is applied to the springs in the back the suspension is pushing back hard from being more stiff, thus making the body appear to lift up, its just the suspension pushing hard thats all.

Thats not whats making the car lift. thats like saying if you put a stiffer stock ride heighth spring on the car its going to lift. Read the link I posted above.

As the axle and ds both spin it will try to rotate the axle out from under the car. The different angles will all affect this. This rotation causes the force, if large enough it lifts the cars ass end up, if its small enough it will be over taken by the weight transfer and squat the car.

Springs and shocks are a related but thats not what they do. Springs can make the car lift up in the rear when they rebound, but when a spring rebounds it won't stay extended. When it comes back down it will unload the tires. Shocks smooth this motion out.