Define Instant Center

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SB 377
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Define Instant Center

Post by SB 377 »

Would someone please explain this term. Thanks
hotrod
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suspension geometry

Post by hotrod »

Its the point where the upper and lower suspension links would meet if they were long enough. Its height above the ground and postion front to rear changes the way the suspension behaves during hard acceleration and braking.

This link gives a quick discussion of it as applied to the GM 4 link rear suspension.

http://www.chevyhiperformance.com/techa ... ndex3.html

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Post by OldSStroker »

Instant Center (IC):

A point about which the axle centerline (assuming solid rear axle) rotates or "swings" for any given axle-to-chassis condition.

Imagine a 1 link suspension with the link attached to the axle and to a pivot someplace, usually forward, on the frame. Looking at the side view, as the wheel moved up or down with respect to the chassis, the axle would describe an arc with the radius being the length of the link or from the pivot center to the axle housing center. This length would remain constant and the pivot point, which is the Instant Center (IC), would remain constant. Think of it as the "Instant Center of Rotation". In the side view, a Nextel Cup rear suspension is pretty much this idea.

If there are multiple links, the previous answers and the Chevy HighPerformance article explains it a little better. Looking only at the side view, draw the axle, the links and pivot points. Draw a line thru the upper control arm mounting points on the axle and the chassis and extend it forward. Do the same for the lower control arm mounting points. The point where the extended lines cross is the IC for that axle/chassis relationship.

As the axle moves up (compression or jounce) or down (rebound) with respect to the frame, those extended lines will meet at different points because the upper and lower arms are not the same length. Therefore the IC moves around. As the axle moves, it is only at one point at a given "instant", and in the next instant it is somewhere else. Hence, "Instant Center".

All suspensions have ICs. For typical double A-arm independent front suspensions, you look at a front view, and the IC of each wheel is usually on the opposite side of the vehicle which causes the tire to lean in at the top as the suspension compresses (negative camber change or "gain").
A notable exception is GM A-body cars (Chevelle, GTO) in the '64-'77 era. They had positive camber change during some of the compression travel. That's a bad thing for cornering with wide tires, but the OEM tire tread width in '64 was about 4-4.5 inches. This can be corrected by adapting taller knluckles from big chevy (Caprice, Impala, etc) and shorter upper control arms which will result in negative camber gain. Sometimes the modified suspension is said to "roll negative" because when the body/chassis rolls, the outside wheel goes into compression.and you get negative camber gain.
More than you wanted to know, SB 377?
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Post by BillyShope »

It's also important to remember that parallel lines meet at infinity. Consider the trailing links of a rear suspension, as viewed from the side. If the links are parallel to each other and, at the same time, parallel to the no squat/no rise line, the "instant center" is considered to be on the no squat/no rise line. If the links are parallel to each other and at a steeper angle than the no squat/no rise line, the rear of the car will rise; less steep, squat.
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