Keeping the front end "down"/hill climbs/Instant Center... - Page 2 - JeepForum.com

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post #16 of 119 Old 12-19-2011, 01:05 PM
ABQ-TJ
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Quote:
Originally Posted by ftgiles View Post
Did you read this /\
Yes I could Google it, I was hoping someone could explain it, though.

Nevermind.

and I didn't ask WHAT it is, I asked WHY it was called "instant" center...I get that it's the point where those two link would intersect, if extended. But what's the reason for the term?

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post #17 of 119 Old 12-19-2011, 02:03 PM
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It's called an instant center because at any instantaneous moment in time, it has a velocity of zero due to the rotation of the rest of the system taking place around it.

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post #18 of 119 Old 12-19-2011, 02:16 PM
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It's called an instant center because at any instantaneous moment in time, it has a velocity of zero due to the rotation of the rest of the system taking place around it.
At a particular instant in time, not every instant in time, since it is dynamic.
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post #19 of 119 Old 12-19-2011, 02:54 PM
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As opposed to "actual centers" that are not dynamic that exist when the frame pivot point is the same for both the upper and lower arms (no separation) or only has one pivot point at the frame like a radius arm.

Actual centers don't move when the suspension moves. Instant centers do move which ads a level of complexity when setting up your suspension. Do you set it up for hill climbing or flat ground? Or do you point the instant center out further to reduce the rate of change in anti-squat (undesirable percent decrease) because the instant center moves when your suspension sucks down in the rear when climbing a hill?

But then maybe your arms are too long to be able to get the instant center to fall on the percent anti-squat that you desire...
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post #20 of 119 Old 12-19-2011, 04:47 PM Thread Starter
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Imped, thanks for taking the time to share your wisdom. I'm still reading & re-reading the links provided, as well as the posts you guys made. To help myself understand this better, I'm going to attempt to plot out my suspension on AutoCAD, and try to make practical sense of all the input you guys have provided. For me, the learning is often in the doing...thanks again.

~jL/OP
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post #21 of 119 Old 12-19-2011, 06:47 PM
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To keep the front end "loaded" to maximize front end traction on an ascent, the IC should be on a plane below the actual COG "point" of the rig, in this instance & per your assertion, that the top of the bell housing (approximately) is the actual COG of a Jeep Tj, then some distance below that would be desirable to achieve my objective. Fair statement?

I added some more info to your post on PNWJeep that'll explain it better, but the gyst of it is the antisquat is based on the IC's relation to the antisquat line, NOT the CG point. The antisquat line is the uplsoping line going from the rear tire contact to the IC. The antisquat is calculated in comparison to the height of that line.
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post #22 of 119 Old 12-19-2011, 07:08 PM
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I'm going to do my best to keep the AS just a few points under 100% since my shocks counter squat (Bilsteins).
If you want to build that close to the AS line then the location of CoG seems critical. I've found that the the exact CoG location is reasonably forgiving when plugging in the numbers. But it would seem you have a pretty presice game plan.

I don't know if you've seen this. I plan to visit a local Hwy truck scale where a friend works. If you're going to adjust your links inorder to get that close to the AS line then fuel level, passenger weight even tool boxes and recovery gear will all play a factor.

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post #23 of 119 Old 12-19-2011, 07:19 PM Thread Starter
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I added some more info to your post on PNWJeep that'll explain it better, but the gyst of it is the antisquat is based on the IC's relation to the antisquat line, NOT the CG point. The antisquat line is the uplsoping line going from the rear tire contact to the IC. The antisquat is calculated in comparison to the height of that line.
You must be "Paul", eh? You've been extremely helpful. Thanks a million, Paul. Hope we get to meet face to face to face some time...

~John
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post #24 of 119 Old 12-19-2011, 07:21 PM
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Thanks, but...I can't make heads or tails out of the image you posted. I was under the impression, perhaps mistaken impression, that the "instant center" was an imaginary point somewhere in front of the rig. The extension of the control arm lines, similar to what your image illustrates, converge at a point in front of the rig, and that is the "instant center. But, I must be getting it wrong, based on the image you posted, eh?

Vertical separation between upper & lower CA's at the axle end as well as at the frame end play a role. While the image you provided seems to consider that, the IC, according to you is where the lines intersect. That's not making any sense to me. Am I hopelessly confused here or what?
Because he is a novice? He's young and I doubt very experienced other than what he has read!driving for maybe 2 years?
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post #25 of 119 Old 12-19-2011, 08:29 PM
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If you want to build that close to the AS line then the location of CoG seems critical. I've found that the the exact CoG location is reasonably forgiving when plugging in the numbers. But it would seem you have a pretty presice game plan.
That was a thought early on in my suspension design days. I learned exactly what you're saying pretty quickly, hence why I suggested after my quote to shoot for 60-90 on the calc and leave room for adjustment so that you can dial it in by feel instead of just going by the estimations in the calculator. So, I can't tell you exactly where I'm at but it doesn't really matter. The suspension works flawlessly in all situations so far, including some very steep walls. No bouncing, no drama at all. It just grabs and goes and if there's not enough traction the tires just spin (as opposed to bouncing). If I were to guess, it'd be around 80% since there is a small amount of squat.

http://pirate4x4.com/forum/showthread.php?t=168577

http://www.pirate4x4.com/forum/showthread.php?t=204893

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post #26 of 119 Old 12-19-2011, 09:06 PM Thread Starter
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It was dark by the time I got a chance to get started on this, as a result I rushed it a bit.That said the dimensions are a reasonable approximation. Is there enough information on this drawing to develop the AS percentage?
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post #27 of 119 Old 12-19-2011, 09:28 PM
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Look at my link above. Input the same info into the Pirate link calculator. It's an Excel program that will give you all of the information you could ever need to know.

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post #28 of 119 Old 12-19-2011, 10:01 PM
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Divide your IC height of 21.8 by the height of the AS line directly below it. Pretty sure you'll need the calculator to get that.


Edit: Because your IC is above that line your AS is above 100%.
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post #29 of 119 Old 12-19-2011, 10:30 PM
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Originally Posted by RaggedOleMan View Post
It was dark by the time I got a chance to get started on this, as a result I rushed it a bit.That said the dimensions are a reasonable approximation. Is there enough information on this drawing to develop the AS percentage?
Your cog is not that far forward. The "cog height" looks reasonable, but the cog is closer to the middle of the car.

The 100% anti-squat line goes from the rear tire contact patch and intersects the "cog height" at a point on a line that is vertical from the center of the front wheel. So the anti-squat line you're showing is too steep.


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post #30 of 119 Old 12-19-2011, 10:34 PM Thread Starter
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Your cog is not that far forward. The "cog height" looks reasonable, but the cog is closer to the middle of the car.

The 100% anti-squat line goes from the rear tire contact patch and intersects the "cog height" at a point on a line that is vertical from the center of the front wheel. So the anti-squat line you're showing is too steep.
Yes, as I've been looking at my drawing, I suspected that the COG I pinpointed was to far forward. I'll re-measure tomorrow in the day light, and revise the drawing. Thanks FTGiles & Paul...
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