Can you have to long of an Instant Center? rear suspension.

[Bryce]

Im trying to get my head around this. What would happen if the IC was in front of the front wheel; therefore, longer than your wheelbase.

I know long is good for reducing geometry changes during suspension travel. But how long is too long?

[LowBuckX]

woulld like to know also.

Just a thought wouldnt a parallel 4 bar have an infinate IC length........ Herb adams said That a SVsA length of at least 42 inches is desired but didnt specify an outer limit... Shorter equals more bite.

[ArtosDracon]

The biggest issue you're going to run into with that long, far forward of an IC is going to be balancing AS, unless your COG is quite high.

[Bryce]

Cg will be around 18" to 20". A/S will be adjustable from 45-150%. This is a multi-purpose application. The reason I ask, I want to know the maximum limit for the IC. I can keep it as long as possible with my packaging, obviously I cant have a 100% A/S with 109.5" IC (my cars WB).

Is there any rules for link length to IC length ratios?

[ArtosDracon]

Not really, link length is usually dictated by packaging first on street cars, and geometry first on race cars. And you could have 100% AS with your IC and wheelbase at the same length, just probably not at your ride height. Your IC is going to be limited by your AS, so if you want 45-150%AS in your adjustments, then start at a mid point, say 75% AS, and move out from there, checking your roll axis, AS and IC, you'll find that keeping your AS to set paramaters and keeping your roll axis in check will severely limit your IC and you won't really have to worry about having it be too far forward.

[Bryce]

So simple answer is "No". There is no issues with too long of an IC? Is there any reasoning behind this?

I will be building a multipurpose car. For drag racing I dont car about roll axis and I will have a upward slopping LCA. For handling I will have a horizontal LCA and all adjustment will come from the UCA slopping down. I will keep my IC as long as possible.

[ArtosDracon]

Sounds fine to me. though a completley horizontal LCA wit a downward sloping UCA will result in low anti-squat which will push much more of the work for preventing the rear from dropping under acceleration to your springs, which will in turn force you to run hard springs, which will make your car much harsher in the rear and less capable of handling washboards or fast changes in elevation. I would highly consider some elevation change on the rear LCA, 50% anti-squat is the lowest I would personally go, though others have gone as low as 40% that I know of. Also, a level LCA will likely result in AS going DOWN as you accellerate which is the dead opposite of what you want, evena 1" elevation change pointing up is going to make a huge difference for you.

[Bryce]

It will only result in zero A/S if the LCA is on the ground. Mine we be about 7 or 8 inches of the ground. This will result in a 45% A/S.

[ArtosDracon]

It will only result in zero A/S if the LCA is on the ground. Mine we be about 7 or 8 inches of the ground. This will result in a 45% A/S.

Nice low COG! I wasn't meaning literally 0, but not enough. a 45% AS with your LCA at 8" puts your COG below 17". I will update my post more appropriately.

[BillyShope]

woulld like to know also.

Just a thought wouldnt a parallel 4 bar have an infinate IC length........ Herb adams said That a SVsA length of at least 42 inches is desired but didnt specify an outer limit... Shorter equals more bite.
Yes, a 4bar (upper link parallel to lower link) places the instant center at infinity. And, no, a shorter IC location doesn't necessarily mean more bite (whatever "bite" means).

In side view, the lines of constant percent antisquat all pass through the rear tire patch. The greater the slope, the higher the percentage of antisquat. The location of the IC is determined by the intersection of 2 lines; one drawn through the pivot points of the upper link and the other drawn through the pivot points of the lower link. When the lower link is kept horizontal (or nearly so) and IC adjustments are made with the upper link, a raising of the front of the upper link causes the intersection of the link lines to move forward and fall on a line of less percent antisquat; a lowering of the front of the upper link causes the intersection to move backward and fall on a line of greater percent antisquat.

So, a shorter IC...UNDER THE CONDITIONS JUST DESCRIBED...would move the antisquat value closer to...or perhaps in excess of...100%. At values over 100%, the car is said to have "separation" or "hit," which I assume is the "bite" to which you refer.

With a longer IC, there is less antisquat and the rear of the car is seen to squat on launch. The front of the car will always rise, of course, but that visible squat at the rear is accompanied by additional rise at the front. This additional rise is the result of a low antisquat value and NOT the result of the long IC.

And, this is the reason for the upper case letters a couple of paragraphs ago. If the lower link is NOT kept horizontal, but, instead, is used as a part of the adjustment, the link intersection could be placed on any value of percent antisquat with EITHER a short or long IC.

But, if you're going to move that lower link, you'd better have some 4link software handy to keep track of antisquat.

Returning to the 4bar for a minute: You might wonder how you can determine the link line intersection when it's out at infinity. In this case you fall back on the knowledge that parallel lines meet at infinity. So, if you want 100% antisquat, you'd have the 4bar links parallel to the 100% antisquat line (which happens to have a slope equal to the center of gravity height divided by the wheelbase).
http://www.racetec.cc/shope

[LowBuckX]

Bite refering to accelerating harder out of the corner...

[Marcus SC&C]

Everything in suspension design is a compromise. IC location is no different. There is no "too much is just right". Practically speaking having the IC outside of the length of the car (giving it a very long SVSA) typically results in a car with poor anti squat and poor off the line traction. Cars with the IC closer to the rear axle (think ladder bars) tend to have much better off the line traction. Go to the drags and look at the fast cars, 99% of them will have a very short SVSA. Can you work around it? Sure,to some extent but why fight it. Very short SVSA lengths result in more abrupt pinion angle changes and can unload the rear tires under heavy braking (although I`ve found this to be grossly over blown by some) etc. so there`s no panacea. Middle of the road IC locations producting SVSA lengths between say 36"-72" usually work better on an multi purpose cars than more extreme lengths IMO. Mark SC&C

[exwestracer]

We get this question a lot in class, and the instant center length effect can be compared (loosely) to an arm wrestling match. Remember, our goal is to use lift to counteract weight transfer and plant the rear tires....

Let's say your IC length is directly under your C/G location; this would be like the beginning of the arm wrestling match. You and your opponent both have an equal chance. Moving the IC forward of the C/G location is like tilting your arm back and giving your opponent a "head start". It's now much harder to gain lift (hit, bite, separation, etc.) at the rear due to the angle between the C/G and the IC, no matter how close you are to the neutral (100%) line.

As Billy noted, inclining the lower links is tricky, because now you are introducing anti-squat from thrust, as opposed to lift, not to mention increased roll steer. Thrust anti-squat can result in more erratic low-speed acceleration (launch) handling.

IMO, the short IC braking issue is not overblown...it all depends on how hard you want to be able use the rear brakes and maintain control. I've found that most drivers that encounter this problem solve it by simply dialing brake away from the rear and giving up a lot of potential trail braking ability.

[BillyShope]

Hit (or bite or separation or whatever you want to call it) has absolutely nothing to do with the IC length BY ITSELF. It is the RATIO of IC height to IC length that is important. 8 inches up and 50 out is exactly the same as 8000 inches up and 50,000 inches out.

The ratio is proportional to the percent antisquat. Hit occurs when the percent antisquat is above 100%.
http://www.racetec.cc/shope

[exwestracer]

Hit (or bite or separation or whatever you want to call it) has absolutely nothing to do with the IC length BY ITSELF. It is the RATIO of IC height to IC length that is important. 8 inches up and 50 out is exactly the same as 8000 inches up and 50,000 inches out.

The ratio is proportional to the percent antisquat. Hit occurs when the percent antisquat is above 100%.
http://www.racetec.cc/shope

I am not disagreeing with the principle of the neutral line. What I was getting at is the relationship between the acceleration vertical force and the C/G, as the IC moves up the neutral line.

From page 11 of your website: "Note, also, that (L - a) becomes negative when "a" is greater than the wheelbase. This would be expected as the vertical force, acting at the IC, is attempting to rotate the car CCW when the IC is forward of the front tires."

I believe we are saying the same thing...? Once the vertical vector force gets in front of the C/G location a significant distance, doesn't the leverage against the C/G diminish until it crosses the front axle C/L?

[BillyShope]

From page 11 of your website: "Note, also, that (L - a) becomes negative when "a" is greater than the wheelbase. This would be expected as the vertical force, acting at the IC, is attempting to rotate the car CCW when the IC is forward of the front tires."


No, we're definitely not saying the same thing. (First, that's from Page 12.) The CCW to which you refer is with reference to an arbitrarily chosen point for a moment balance. I could just as well have chosen a point 1000 inches ahead of the front axle for the moment balance calculations.

Again, the "out" of the "up" and "out" designations for instant center locations is, by itself, absolutely meaningless. The direction of the rear tire patch force vector is determined by the RATIO of up to out.
http://www.racetec.cc/shope

[MonzaRacer]

Threads like this make me wonder how my rear suspension on my Monza will work out till the 3 or 4 link is done.
I have swapped in a swing arm suspension, made from cut down S10 rear springs and S10 2wd Rear end.
The car would lift the rear end, pretty much violently and hook like it was stuck with epoxy on velcro.
The center of the front piviot bolt to the rear end centering pin from the S10 rear springs is all the control arm I had, and were 12.5 in long springs were about 13 or 14 total, and I used leaf spring clamps and all of the leaf section except one maybe, my only issue was in not remounting the spring pads and rotating the pinion down better.
I simply welded on the spring pads cut from the old Monza Rear and Ubolted it together.
The car hooked awesome and I keep wondering if rotating the pinion back down like it should be to a down angle will mess up things (but still be better for pinion bearing!).
And now I am building a bolt in cross member for upper control arms to do away with the torque arm because it takes up too much room.