GtoProject

iTrader: (2)

I recently purchased a new rear and while I go about the switch I finally want to get my rear suspension really dialed in. The car has always hooked well from a dig (probably based on luck), but as always, there is room for improvement. I have looked around at plenty of different forums and have come to a few conclusions, but I just want to make sure that what I am assuming is correct before I go trying to tune my rear suspension with false information. For me, teaching someone else has always been a good indicator for how well I know the subject, so I did a quick write-up of what I have found and hopefully someone smarter than me can do a quick read-through and see if what I have been finding online is correct.

This all pertains to the standard f-fody torque arm 3-link rear suspension.

• To start, in a standard street/strip car you need about 45% of the cars full weight on the rear wheels for the suspension to work properly upon launch. In higher powered vehicles (1000+ whp) a 60/40 distribution, bias to the front, is recommended to keep the front wheels on the ground. Any less and you will just be spinning the tires unable to put the power down to the ground.
• To start tuning the rear suspension we must first find our vehicles instant center. What’s instant center you may ask? Instant center is an imaginary point in the vehicle about which the chassis rotates in a given position. In a 3-link TA car, this is found using the front TA mount position as well as the angle of the lower control arms. While this is not an exact formula, the majority of TA style 3-link suspensions will loosely follow this interpretation. The length of the TA establishes the instant center from front to rear. Draw an imaginary vertical line about the front mount point of the TA, this is the front to back position of the IC. Now to determine the height of the IC about this vertical line, you must draw another imaginary line following the angle of the LCA to the existing line. The intersection of these two lines yields the IC. Note that the only way to change the front to back position of the IC is to change the overall length of the TA. Also note that the height of the IC is solely influenced by the angle of the LCA’s; this is where relocation brackets come into play, especially on lowered cars. Here is a useful graphic showing this relationship.

• Now that we can find the IC, we need to figure out where to place the IC to achieve our desired launch characteristics. The placement of the IC is nominally dependent on the “100% Anti-squat-line”(ASL). In theory, if the IC lies directly on this ASL, the car with neither squat nor lift upon launch. All of your vehicles power will be utilized to push the car forward (in a 3-link TA car, the LCA’s, and the LCA’s alone, push the vehicle forward). This ASL is generally found using the center of gravity’s (CG’s) height and the front and rear tire contact patches. Generally the CG’s height in a vehicle can be estimated using the height of the center of the camshaft in the engine (this is a rough estimate but from what I have seen it tends to be a good estimate). Now we will draw another imaginary line from the middle of the rear tire contact patch on the ground, and extend it to the point at which the center of gravity intersects with a vertical line taken from the middle of the front tire. Here is another useful graphic showing this relationship.

• So now we have our imaginary ASL as well as an idea of how to adjust/move our IC, let’s now make sense of their relationship. Generally, if your IC lies below the ASL you will have less than 100% anti-squat (AS<100%). The rear will squat, your suspension will lift, and you will be essentially applying less downward force to the tires. On the other hand, if your IC lies above the ASL you will have more than 100% anti-squat (AS>100%). The rear will lift, your suspensions will squat, and you will be essentially applying more force to the tires.
• With all of this in mind we can make some general assertions

1. Lowering the rearmost point of the LCA’s will raise the IC, making the tires hit harder, while raising the rear of them will lower the IC, making it hit not as hard.
2. Shorter TA's raise the IC (hit harder), and longer TA's lower the IC (hit less). Higher horsepower cars (700+) tend to fair better with a longer arm (with more power a longer lever to control the torque can be useful), although you can change other geometry to make a short arm work similarly.
3. Lowering the rear ride-height will lower the IC, making it not hit as hard. While raising the ride height in the rear will raise the IC, making it hit harder.
4. Pinion angle has nothing to do with how hard the tires will hit. However if you are smoking the tires for no apparent reason, the pinion angle may be crossing 0* under load. You must adjust the pinion angle farther from 0* (generally more negative) to compensate.
5. The height adjustment on the front bracket of the torque arm is up for debate. Some say it is mostly useless, minus allowing for slight pinion angle adjustment, although some have argued that in relation to the centerline of the axle, when mounted above the centerline the tires will hit harder, while mounting below the line will make the tires hit less.
6. If your car is hitting hard then losing traction, your best bet is to employ one of the above methods in order to lower the IC. If your car is just spinning off the line your best bet is to employ one of the above methods in order to raise the IC.
7. When tuning suspension adjust one thing at a time and test accordingly. If adjusting to much at once you are allowing change in too many variables and not attaining useful and practical data.

Is this a correct way of thinking about this? Or am I making false assumptions?

gesto

I'm not the expert and am learning like you are but I think your comments are generally correct. There is similar info in the sticky in the drag racing section but I think your post is a pretty good primer. You also have to think about the suspension components and set up (bushings, shocks, springs, LCA type, drag bars and even tire selection).

I'm sure there are some drag whizzes on here that can really set up a car properly.

BMR Sales2

iTrader: (40)

I do agree with most of your post. However there are a couple of points I will pull out here that stood out to me that I have some dialogue for

I have found this to be true in most low powered or street/no-prep applications. High powered radial cars (1000+ hp) seem to work best with around 60/40 weight distribution. 40% being on the rear. Any more and we tend to have trouble keeping the nose down and they try to go on the bumper too easily, even with tight shocks and travel limiters. The actual weight on the rear of the car loading the tire is more than made up for by the leverage action of the high amount of anti-squat we put into them. So there is a bit of a sliding scale on the effectiveness of weight distribution

The first part of your statement is correct, however I disagree with the second part. usually above 700 I will not tell someone to run a long arm unless it is either a stick shift car, or a dedicated street/no prep setup. A higher power auto car on a radial with a long arm will live on the back bumper on even a halfway decent track.

This is a topic I have hotly debated with many, including some within the industry. While the diagrams and such show no real effect of this, I have had too many real world experiences with it making differences to say that it plays no role. In my experience, you can fine tune the instant center with torque arm height to make incremental IC or AS adjustments rather than changing LCA position.
From what I have seen, the mounting height of the torque arm in relation to the centerline of the axle is an important factor. Typically, if the TA mounting height is lower than the axle centerline, it will take some hit away from the tire. Higher, will make it hit harder.

The torque arm suspension is one of the trickiest to plot and predict. Since it combines elements of a 4 link, and that of a ladder bar in how it reacts and responds. So it has taken much trial and error over the years to learn what these suspensions like and how they react to certain changes

GtoProject

iTrader: (2)

Thanks for the reply. Added this info to the original post in addition to what was there originally.

This is where I think most of my confusion is contained. Thinking about it from a very basic physics standpoint, (and I am butchering an attempt at simplifying a 3 link system to a very simple system here) I would liken a torque arm to a lever. This lever is attached to the axle (point of rotation) at one end and to the body (an object of great mass) at the other end. This force required to "rotate" or attempt to rotate this lever at one end will be effected by the length of the lever. A longer lever will house the mass father away from the point of rotation which I would tend to think would create a greater rotational inertia (creating a greater resistance to rotation) while a shorter arm would do the opposite (having a slightly less resistance to rotation). Its like opening a door from an area close to the hinge, the more mass there is at the handle end of the door, the more force there would be required to rotate the door at the same point. Likening this to a torque arm we can assume that a shorter arm will in fact hit harder and will raise the front end of the vehicle faster and easier than a longer arm.

I have found that everyone is in agreement that shorter arms will hit harder, and have more of a tendency to pull the wheels, which is inline with this simple system. But what you are saying is that in higher power situations a longer arm tends to put a car on its bumper more often than a shorter arm will (again to make it simple we will ignore the auto/manual/tire-type information. unless of course this is critical to the understanding of the physics of the actual system), and you advise these vehicles to run a short arm to battle this front end lift. I just don't see the correlation here, I am in no way saying that you are wrong, you obviously have a much greater level of experience in this matter, but I am just trying to understand why this is true.

I am just trying to get a handle on how these torque arm suspensions actually function, and it seems as though there isn't a real go-to anywhere with all the correct information. From what I have found there are two different ways most people find the IC, the one I thought made more physical sense is the one I attempted to explain (wherein an extrapolated line from the LCA's intersect with a fixed vertical line at the location of the front TA mount point). The other method I found takes into account following the center line of the torque arm and finding the intersection of the same LCA angle and this new line.


From my preliminary understanding (mentioned in the original post) I would question how the IC is changed from simply raising or lowering the front mount of the TA. As you are keeping the relative front-to-back distance the same, the IC location "shouldn't" be affected, but I suppose the real way to obtain the IC is probably a combination of both methods I have found online, so the angle of the TA must have some influence on the IC location. However, the "exact" location of the IC isn't all that critical correct? Having the knowledge to estimate the IC location should be enough to roughly set up the preliminary location of suspension components, but small individual changes and testing are all that will really get a car dialed in (i.e the math really doesn't matter). Thanks again for the info, and I'll update the original post to reflect what I had gotten wrong initially.

BMR Sales2

iTrader: (40)

The reason I mention the transmission and tire choice is they are absolute key factors in understanding how the car should be setup and how hard you should be able to hit the tire. A bias ply slick tire works better with a stick shift car. Bias ply tires cannot take as much load or hit as a radial as the sidewalls are too soft and will wad up. I have literally had guys with way too much anti-squat that has driven the tire down so hard their rim bounced off the track surface. The tire completely deforms and does no good at that point. A radial tire with this kind of setup will work well because the stiffer sidewall can handle the load and will translate it into downward pressure onto the track. However, this generates separation between the body and tire. This is why shock rebound control is so crucial on a radial tire car, to control this separation. A bias ply car will be nearly opposite. I tend to run a very neutral or a slight amount of squat which softens the initial hit on the tire, then I use shock compression and spring pressure to keep constant load to the tire.

The 2 theories here are a long, low instant center for a more neutral anti-squat or even squat. For a radial car I like to work with a short and high IC to hit the tire hard as possible. The torque arm height plays into this as it doe seem to change the relative height of the IC. Not in dramatic amounts, but enough to make tune-able difference at the track.

The short arm adds more anti-squat by moving the lifting point farther back and lifting more of the rear of the car, rather than the front. This adds leverage against the tire driving it downwards while lifting the body upwards