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Page 40
3LINK SETUP
(also: Triangulated 4link and Torque Arm Suspensions)

This is something which is probably of more interest to the dragracer, but, since Jaguar used this on their early C-Types, I would urge those who are running a RWD beam axle car on road courses to also consider this information.
 
This is an asymmetric trailing 3 link arrangement which, during forward acceleration, completely cancels driveshaft torque, thereby providing equal rear tire loading for maximum acceleration. Commonly, there are 2 symmetrically situated links (in plan view) and a third "odd" link which can be offset from the car's centerline (to the right). It is not necessary, however, that 2 of the links be located symmetrically about the car's centerline, In addition, the "odd" link can be either above or below the other 2 links. It is only necessary that there be enough vertical and lateral spread between links to adequately satisfy strength considerations. Consequently, the 3 links are named simply "first," "second," and "third." The default numbers indicate that I have chosen the "first" link as the middle link, but this is entirely arbitrary. All links, in plan view, are assumed to be parallel to the long axis of the car (i.e., parallel to the SAE X-axis).

TRIANGULATED 4LINK:

An exception to the above paragraph is the triangulated 4link, where the triangulated pair can be treated as an "odd" link which is centrally located (zero offset). In other words, by "combining" the triangulated pair into a single link, the triangulated 4link can be treated as a 3link in this spreadsheet.


TORQUE ARM:

Setup numbers for the torque arm suspension can also be obtained from the spreadsheet. Select a "link" for the torque arm and insert a value of zero for the link length. For "distance forward from rear axle centerline," insert a value for the distance forward to the torque arm contact point. Insert the same value for "distance forward to IC." A value must be entered for the percentage of weight transfer to be carried by the torque arm. (Percentage values inserted for other links will not be used.) Different values for this percentage and the torque arm offset can then be inserted until the locations of the other 2 links are acceptable. For instance, with 100% of the weight transfer carried by the torque arm and the proper torque arm offset, the remaining links can be horizontal. If the torque arm is not simply a sliding "slapper" arrangement (in other words, if it has a short link at its forward end), it is assumed that the short link is positioned vertically.


In the dragracing application, it is generally desirable to have the antisquat at or near 100%. In a road racing application, however, this high value of antisquat commonly causes wheel hop during braking. The spreadsheet allows the user to specify the percent antisquat. The default value is 100%.

Jaguar placed the odd link above the axle, but it can be either above or below. When placed below, the situation becomes more favorable with a "tubbed" car. The drawback is that the single lower link is carrying a very large compressive load. Care should be taken to use tubing with sufficient wall thickness and diameter for safety.

This spreadsheet...as opposed to the earlier spreadsheet...takes advantage of odd link offset AND an asymmetrical adjustment of the other two links. In other words, the side view will show the other two links having different angles. In addition, the user has the freedom to use different rear pivot locations for the links.

The spreadsheet assumes the rear pivots to be in essentially the same side view location. If, however, the locations of the front pivots are to be controlled, it is only necessary that the appropriate values be input as "rear" values and a negative sign be placed before the link length value.

Another difference (from the earlier spreadsheet) is that the user is allowed to locate the forward position of the instant center. While it is the percent antisquat that is significant...and NOT the location of the instant center...this input is necessary in order to select a unique answer set from the infinite number of sets that would also satisfy the other input requirements. It is therefore recommended that you try different values until the front mounting points fall within an acceptable range. This, in conjunction with the "trick" described in the previous paragraph, should satisfy all your packaging constraints.

The spreadsheet provides the user with a great deal of freedom in the design of a 3link. A 3link is commonly thought to have 2 symmetrically positioned links (in plan view) and a third link more centrally located. The designer is now free to use any 3 points, so long as "LEFT" and "RIGHT" link offsets have different values. Offsets are to be considered positive from the centerline of the car. No 2 link lengths need be the same. This design freedom might prove useful if packaging problems exist.

distance forward to IC =

effective rear tire radius =

axle ratio =

weight of car with driver =

weight of rear axle assembly =

center of gravity height =

wheelbase =

desired % antisquat =

     

FIRST LINK REAR MOUNTING POINT

distance forward from rear axle centerline =

vertical distance from track surface =

link offset (see instructions) =

link length =

percentage of weight transfer
(torque arm suspension only) =

SECOND LINK REAR MOUNTING POINT

distance forward from rear axle centerline =

vertical distance from track surface =

link offset (see instructions) =

link length =

percentage of weight transfer
(torque arm suspension only) =

THIRD LINK REAR MOUNTING POINT

distance forward from rear axle centerline =

vertical distance from track surface =

link offset (see instructions) =

link length =

percentage of weight transfer
(torque arm suspension only) =

ANSWERS:

VERTICAL DISTANCES FROM SHOP FLOOR TO FRONT OF LINK
First Link Second Link Third Link

INSTANT CENTER HEIGHT

LINK LOADS AT 1G ACCELERATION
First Link
Second Link
Third Link
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