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Old 01-26-2004, 08:18 PM   #1
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Default Instant center

I have been asked about what instant center is so here we go.

The instant center of a T/A car is the line you draw from the lower control arm mounting point on the housing through the front mounting point of the lcas. Then draw a line down the center of your T/A from the rearend to the front of the car. Where these lines cross is your instant center.

The farther back and higher the instant center is the faster ther tires plant BUT the quicker they unload. Most fast cars want an instant center of long and low on these cars with T/As so that it will plant the tire and KEEP it planted. I have found that moving the instant center 1 inch up will change my 60 ft by 2 hundredths of a second.

As in the bind on the rear of these T/as, this comes from pinion adjustments. On any T/a you have a set measurement from the lower bolt hole on the rear to the upper blot on the rear. When ever you adjust pinion angle by making the lower rod end longer or shorter then you change to measurement from top to bottom. This will cause a bind. This is what tears up rod ends and mounting brackets.
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Old 01-26-2004, 08:30 PM   #2
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can you draw a pic of this?

i sorta understand where your going, but cant visualize it too good.
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Old 01-26-2004, 08:44 PM   #3
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I havent figured that out on computer screens with my pencil yet. Let me see if I can draw it out and get my wife to scan it.
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Old 01-26-2004, 08:50 PM   #4
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ok so are you saying that the LCA's angle inward?? and that the longer a torq arm the better?? so what about the T/As that no longer mount on the transmission, aren't they usually a bit shorter??
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Old 01-27-2004, 12:55 AM   #5
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Quote:
Originally Posted by MADMAN
I have been asked about what instant center is so here we go.

The instant center of a T/A car is the line you draw from the lower control arm mounting point on the housing through the front mounting point of the lcas. Then draw a line down the center of your T/A from the rearend to the front of the car. Where these lines cross is your instant center.

The farther back and higher the instant center is the faster ther tires plant BUT the quicker they unload. Most fast cars want an instant center of long and low on these cars with T/As so that it will plant the tire and KEEP it planted. I have found that moving the instant center 1 inch up will change my 60 ft by 2 hundredths of a second.

As in the bind on the rear of these T/as, this comes from pinion adjustments. On any T/a you have a set measurement from the lower bolt hole on the rear to the upper blot on the rear. When ever you adjust pinion angle by making the lower rod end longer or shorter then you change to measurement from top to bottom. This will cause a bind. This is what tears up rod ends and mounting brackets.


NO NINTENDO!!!!
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Old 01-27-2004, 08:41 AM   #6
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Quote:
Originally Posted by RAMnGO
so what about the T/As that no longer mount on the transmission, aren't they usually a bit shorter??
i was just wondering the same thing in reading this since i don't have a trans mounted tq arm.
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Old 01-27-2004, 08:53 AM   #7
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Tire size will also have alot in determing what the "best" IC is, due to better holding ability of the bigger tire.

There is always a trade off...you can have the car hook REAL good on the line, but you risk the chance of loosing traction on the top. The reverse of this can happen to. What you want is the happy medium between the two...that is why a GOOD chassis man is important on your small tired, 8 sec cars. I talked to Skinny Kid about this the other day. He was telling me of Gliddens old black GT. It has a 4-link. He said by the time they were finished getting it dialed in, it looked like a ladder bar car, the bars looked wrong, but that is what worked. I doubt any of you went to the WFC when Bob Glidden drove this car....it was an awesome site to see that car leave and the second stage hit right off the line. They had the car SET UP! I think it ran a 7.7@177 in the small tires

First time out with a car, I'd set it up to spin more off the line and make damn sure it hooks up top. That is my plan anyways
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Old 01-27-2004, 09:00 AM   #8
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Picture your car from the side. Draw a straight line from the axle mount of the LCA to the body mount of the LCA. Now, draw a line from the front mount of your torque arm to the center of the mount on the rear end.

I threw together a quick drawing in CAD to help a little.
http://www.quarter-mile.net/images/instantcenter.gif

What puzzles me is that by moving the front torque arm mount back, it raises and moves the I/C back slightly. I'm sure this works though, since any chassis-mount torque arm (Spohn, Madman, Billingsley, etc) is shorter than a stock-style. The only realistic way I could see of moving it forward, was to drop the axle mount of the lower control arm (relocation bracket).

Good info Brian. I knew how 4-links were determined, but I wasn't completely sure about torque arm cars.
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Old 01-27-2004, 11:23 AM   #9
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i can visualize it, but i wish the cad thing you made up would show up, its not working for me
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Old 01-27-2004, 11:35 AM   #10
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Dunno why not. It's a regular gif image. It's a little large (1024x768) but nothing nuts.
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Old 01-27-2004, 12:38 PM   #11
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I may be reading this wrong, but If I draw a line front back to front of my LCA and from back of the TA to the front of the car, these lines will be parallel. How would they cross?
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Old 01-27-2004, 12:41 PM   #12
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OK, maybe I have it figured out now. You must view where the lines cross from the SIDE of the car, not from the BOTTOM, correct?
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Old 01-27-2004, 12:48 PM   #13
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so what happens to a lowered car? how does that change the LCA angle? looks to me if you lowered a car and didn't have relocation brackets it would move the IC forward more..also is there any way to modify or change the front LCA mounting position??
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Old 01-27-2004, 12:54 PM   #14
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Good Drawing, good info
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Old 01-27-2004, 12:57 PM   #15
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Something to think about...

The shorter the arm the faster the tires plant and then unload but with the longer arm it is generally easier to wheelstand the car and that can present its own problems if front suspension limiters do not work...

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Old 01-27-2004, 06:38 PM   #16
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Quote:
Originally Posted by Chris ARE 360
Something to think about...

The shorter the arm the faster the tires plant and then unload but with the longer arm it is generally easier to wheelstand the car and that can present its own problems if front suspension limiters do not work...

Chris, I think you have that backwards... A shorter arm would promote a more violent initial launch and wheelstand, at least that's the way I've always understood things.

A longer torque arm (IC more towards front of car) doesn't put as much "mechanical hit" on the tires like a shorter bar would.

Of course, I could be wrong...
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Old 01-27-2004, 10:33 PM   #17
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madman, what would you reqamend for a 9 second, 3500 lb race weight car to hook good. take in consideration that a heavy iron block will be used so alot of the weight will be on the nose.
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Old 01-28-2004, 07:31 PM   #18
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I would obviuosly use my T/A. We normally put weight in the front of these cars anyway so you will be ahead of the game.
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Old 10-12-2005, 05:56 PM   #19
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bring it back!

couple of questions,
in the line diagram where should the TA rear portion of the line be pointed? in "the middle" but is it the center of the rear axle or what? I would think it would not be the middle between the two attachment points on the rear bracket because those do not move so it shouldnt really matter... is it actually pointed through the axle center?

What effect would lowering the rear bracket where the TA connects to the housing have? it would seem it would not affect the suspension geometery as it is still fixed to the housing, so does it just reposition the forces acting on the TA?

If you moved the front TA point back but still on the origional "diagram line" (ie back and down a little), this should keep the same IC right? but how does this affect the settings because it should put a bigger force on the body as compared to a longer TA (torque force pushing up on body I guess)

Is the length and IC 2 different variables then for the "shock" vs "steady" settings

When you shift hard with a manual car, does it act like you are launching, ie the tires will plant and unload like in the launch? but to a lesser degree maybe?
is it more the long-arm forward-IC you want to keep traction when you shift or what?
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Old 10-13-2005, 07:39 PM   #20
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Quote:
Originally Posted by MADMAN
As in the bind on the rear of these T/as, this comes from pinion adjustments. On any T/a you have a set measurement from the lower bolt hole on the rear to the upper blot on the rear. When ever you adjust pinion angle by making the lower rod end longer or shorter then you change to measurement from top to bottom. This will cause a bind. This is what tears up rod ends and mounting brackets.

Please explain further, I have a Spohn arm in my car with Chrome moly rod-end in the front, the rear does not have a "rod-end" per say, it has a fixed"eyelit", but should pivot around each one when setting pinion angle, should you loosen the bolts when setting pinion angle to prevent bind? Then re-tighten? More info please.

On another note, IMO instant center should be somewhere under the drivers thigh area or close to that point in space..do you agree?



David
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