Moparts

instant center ?

Posted By: Stroker Scamp

instant center ? - 10/30/09 04:54 AM

i have new clue on chassis tuning, so how would you find IC on a car ive got stock front susp. cal trac mono leaf rear,motor is in stock location, and how do you use this info to tune the chassis, and what do you look for?? ignorant mind wants to know

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Posted By: Gumbydammit

Re: instant center ? - 10/30/09 11:52 AM

with a leaf spring car, the front eye of the spring is your instant center. Just like a ladder bar car's IC is the front mounting point of the bar. On a four link, the top and bottom bar do not intersect at a single mounting point. Therefore an "imaginary" line is drawn out from each bar to follow the angle out to a point where they would intersect.

You can lower or raise the front spring eye mounting point on a leaf car to lower or raisr the IC. Even with Caltracs, the IC does not change. By raising or lowering the bar height on the caltrac bar, you are increasing or decreasing the "hit" that the bar puts on the front of the spring.

Many of the radial racers I know of that are running monos and caltracs find that a good set of double adjustable shocks are essential to getting down the track. Monte Smith knows a ton about this. He would be a good person to talk to if he has the time.
Posted By: Bob_Coomer

Re: instant center ? - 10/30/09 03:30 PM

Took me a good 5 minutes to find this for you...So this is a good read posted a few weeks ago or so..
Pretty much in a nut shell, you can not change your IC on leaf spring cars, unless you have more holes up front for the front eye of the springs..
https://board.moparts.org/ubbthreads/show...part=1&vc=1
Posted By: demon440

Re: instant center ? - 10/30/09 05:03 PM

That is one of the reasons I
put 3 different mounting holes in
my relocation kit for my car.
The other is so I can raise or lower
the rear of the car.
I have Caltracs and Mono's on it.

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Posted By: Monte_Smith

Re: instant center ? - 10/30/09 05:08 PM

As said, you can only move the IC up or down with leafs. Where the spring attaches to the car, is the IC, end of story, no matter where you set the lower bar..............Now one thing that gets greatly overlooked in all the suspension debates, is the FRONT of the car. The front spring rate and front shocks are MORE IMPORTANT than the rear. The trickest rear suspension in the world will not work if the front does not. We see people all the time spend hundreds or thousands on the rear, only to have junk, $30, 3 way adjustables on the front and wonder why their stuff won't work. And before all you cheap shock proponents start telling me how well your CE 3 ways work, all I can say is, the baddest stock suspension cars in the world, run in the Outlaw Radial classes and Stock Eliminator. Take a look under those cars, see what they have for shocks and get back to me. We personally have Santhuffs front and rear on our radial car and the shock control is so precise, that we run the limiters completely backed off and control the car with the shocks. On a super slow motion, you can see the car carrying the front and the wheels are still slowly dropping out of the fenders, with the fronts in the air. That is shock control.

Monte
Posted By: Chassisman

Re: instant center ? - 10/30/09 05:27 PM

Quote:

As said, you can only move the IC up or down with leafs. Where the spring attaches to the car, is the IC, end of story, no matter where you set the lower bar..............Now one thing that gets greatly overlooked in all the suspension debates, is the FRONT of the car. The front spring rate and front shocks are MORE IMPORTANT than the rear. The trickest rear suspension in the world will not work if the front does not. We see people all the time spend hundreds or thousands on the rear, only to have junk, $30, 3 way adjustables on the front and wonder why their stuff won't work. And before all you cheap shock proponents start telling me how well your CE 3 ways work, all I can say is, the baddest stock suspension cars in the world, run in the Outlaw Radial classes and Stock Eliminator. Take a look under those cars, see what they have for shocks and get back to me. We personally have Santhuffs front and rear on our radial car and the shock control is so precise, that we run the limiters completely backed off and control the car with the shocks. On a super slow motion, you can see the car carrying the front and the wheels are still slowly dropping out of the fenders, with the fronts in the air. That is shock control.

Monte

AMEN BROTHER.....sometimes wornout stock shocks actually work better than cheap 3ways....I had a customer once that didn't believe we were battling a front end issue.So I went to Autozone and got cheap stock replacements...got rid of the CE 3 ways and picked up a tenth on the 60'...of course now he has Strange DA's on the front and is happy as a clam.
Posted By: @#$%&*!

Re: instant center ? - 10/30/09 05:38 PM

Quote:

i have new clue on chassis tuning, so how would you find IC on a car ... cal trac mono leaf rear



That's real easy, you don't have an "instant center" on your rear suspension, really.
The hot-rodder term 'instant center' is a contraction of the much more general term 'instantaneous center of rotation' which is used when describing 4-bar linkages (a 4-link rear suspension is a pair of 4-bar mechanisms in parallel). The reason for the word 'instantaneous' is that with a 4-bar the center of rotation constantly changes with the motion of the links. This is really the purpose of the 4-bar mechanism, you can get them to trace out all kind of motions by changing the lengths, angles and positions of the links.
For a leaf spring or ladder bar all you have is a Center of Rotation.

Posted By: Stroker Scamp

Re: instant center ? - 11/01/09 04:31 PM

thanks guys, kinda an ignorant question but now i know, mike
Posted By: VernMotor

Re: instant center ? - 11/01/09 04:38 PM

OK with all that said how do you figure out what IC you do have in case you wanted to change it?
I mean we could move the front of out leaf spring if we wanted to.
Posted By: BillyShope

Re: instant center ? - 01/01/10 04:21 PM

With a production parallel leaf suspension, "wrap up" of the front of the leaf during launch causes the IC to shift slightly from the front spring eye. With bolt-on attachments to stiffen the spring, the IC moves back toward the front spring eye.

The Mopar pinion snubber presents a different situation. To find the instant center, consider a line perpendicular to a line from the rear tire patch to the front spring eye and passing through that same spring eye. Next, consider a horizontal line passing through the contact point of the snubber. The intersection of these two lines is the IC.

As you can see, this increases the percent antisquat a bunch!! This explains why the rear end of the sixties Mopar dragstrip cars jumped as they did during launch.

Another solution...for dragstrip cars...is to add a torque arm and shackle. This is better than the snubber for two reasons. First, the antisquat can be brought closer to 100 percent, and ,second, the torque arm can be offset to the right to dynamically cancel the driveshaft torque's tendency to unload the right rear.

A horizontal line through the arm end of the vertical shackle is used to find the IC.
http://www.racetec.cc/shope
Posted By: badmoon

Re: instant center ? - 03/07/11 10:08 PM

Everyone here is saying the IC for a leaf spring car is the front spring eye , period.
And there is nothing that can be done about that. True under ordinary circumstances.
BUT for what it is worth,
the guys at Cal-Trac say the the IC shifts FROM the front spring eye TO whatever hole the link is in at the front "belcrank".
So putting the link in the lower hole moves the IC down and forward.
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