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

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post #1 of 119 Old 12-18-2011, 04:42 PM Thread Starter
RaggedOleMan
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Keeping the front end "down"/hill climbs/Instant Center...

There's this one particular obstacle in my area that I often fail at. It's an uphill, over some large rock outcroppings. The line to take is obvious, but I only make it up this thing approx 40%-50% of the time. As I discussed this with a well known JeepForum Guru some time ago, he advised me NOT to disconnect my disco's the next time I tried the obstacle. I took his advice and it worked. However, subsequent attempts yielded the same 40%-50% success rate.

Time passed, and as my understanding of suspensions grew a little bit, (I need to emphasize "little bit") I began to hear the more experienced guys & gals speak of the "front unloading", "instant center", etc. That same guru I spoke to earlier attempted to explain "instant center" to me, too, but I just don't get it, yet. Can someone patient explain this?

Also, it seems to me that with mid to long control arms in the rear, the tendency for the front to stay put during an ascent would be better, as opposed to the short arms I currently run. That's just physics, eh? Is my thinking in the right direction?

To summarize, what are the equipment configurations/conditions and/or theory that contribute to keeping the front end down during hill climbs?

Thanks,
~John

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post #2 of 119 Old 12-18-2011, 05:39 PM
rubiconrich
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On a steep vertical climb, a longer wheel base on a vehicle will keep the front end down longer before it lifts. Torque in the drive train keeps the front end down. having lower gearing in the diffs helps with maintaining torque plus choosing the right gear. If the gear is too high, you need power to keep the axle spinning.
Try going on a really steep hill and start to back down...you should feel the front end lift. When that happens, shift into first gear and the front will come down. long arm suspensions have a tendency to raise the front end easier when backing down a steep hill.
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post #3 of 119 Old 12-18-2011, 05:47 PM
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I have very limited knowledge in this area. But isn't that what suck down winches are for?

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post #4 of 119 Old 12-18-2011, 06:41 PM Thread Starter
RaggedOleMan
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Originally Posted by rubiconrich View Post
On a steep vertical climb, a longer wheel base on a vehicle will keep the front end down longer before it lifts. Torque in the drive train keeps the front end down. having lower gearing in the diffs helps with maintaining torque plus choosing the right gear. If the gear is too high, you need power to keep the axle spinning.
Try going on a really steep hill and start to back down...you should feel the front end lift. When that happens, shift into first gear and the front will come down. long arm suspensions have a tendency to raise the front end easier when backing down a steep hill.
I can see how a longer wheel base is part of the solution, as is torque/numerically high gearing. Nonetheless, stretching my Jeep isn't in my upcoming plans, though gearing/axles are. That said, doesn't suspension play a supporting role in keeping the nose down on an ascent, too? And I'm still not understanding the theory behind "instant center".
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post #5 of 119 Old 12-18-2011, 07:21 PM
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Originally Posted by RaggedOleMan View Post
I can see how a longer wheel base is part of the solution, as is torque/numerically high gearing. Nonetheless, stretching my Jeep isn't in my upcoming plans, though gearing/axles are. That said, doesn't suspension play a supporting role in keeping the nose down on an ascent, too? And I'm still not understanding the theory behind "instant center".
The instant center is simply the center of the arc created by the wheels as they travel up and down. It is found by following the upper and lower links, the instant center is where they intersect.

Here is a picture from the 4-link calc, I changed the values so you can actually see the IC in the graph. You can see where the red and blue lines intersect, that is the instant center. In this case static anti-squat is 142%, which means the rear end is going to lift up under acceleration. In this case if I increase separation between the links at the frame end by 1.8" I get 100% anti-squat. That means the under acceleration the rear end neither rise or squat. Obviousy this plays a huge role in climbing because if you have a lot of squat your front end is going to lift up a lot more. 100% is what most people go for, but is very hard to achieve because you have to know precisely where your center of gravity is because that dictates where your anti squat line is.

screen-shot-2011-12-18-7.04.08-pm.jpg
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post #6 of 119 Old 12-18-2011, 07:23 PM
rubiconrich
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When you are going up a trail that there is a definite flip over capacity, sure you use a winch to hold your nose or you get a friend or two with straps to do the same.
Center of gravity changes with the angle of your tj. It can not go lower but it can rise. While I am at it, most old jeepers don't go over a 3.5 lift or a 1 inch body lift. Also they stay with 33's on tire size. They do this so they can climb steeper ascents. 15x8 inch rims are a given because they allow a 12.5 more flex, strength and more sidewall.
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post #7 of 119 Old 12-18-2011, 08:16 PM
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Originally Posted by freeskier93 View Post

100% is what most people go for, but is very hard to achieve because you have to know precisely where your center of gravity is because that dictates where your anti squat line is.
100% anti-squat when sitting on flat ground or on some percent incline?

What is so hard about knowing precisely where your center of gravity is?
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post #8 of 119 Old 12-18-2011, 08:21 PM Thread Starter
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Originally Posted by freeskier93 View Post
The instant center is simply the center of the arc created by the wheels as they travel up and down. It is found by following the upper and lower links, the instant center is where they intersect.

Here is a picture from the 4-link calc, I changed the values so you can actually see the IC in the graph. You can see where the red and blue lines intersect, that is the instant center. In this case static anti-squat is 142%, which means the rear end is going to lift up under acceleration. In this case if I increase separation between the links at the frame end by 1.8" I get 100% anti-squat. That means the under acceleration the rear end neither rise or squat. Obviousy this plays a huge role in climbing because if you have a lot of squat your front end is going to lift up a lot more. 100% is what most people go for, but is very hard to achieve because you have to know precisely where your center of gravity is because that dictates where your anti squat line is.

Attachment 318147


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?
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post #9 of 119 Old 12-18-2011, 08:49 PM
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Originally Posted by RaggedOleMan View Post

That's not making any sense to me. Am I hopelessly confused here or what?
Click Here for a link to information written by a mechanical engineer for non-engineers. Scroll down to the middle of the page and read the "Getting Started" first. Then read page 12 in the index. Then poke in on the other pages. Spend a few days reading it. Then think about it and then go back and read it again.

Most importantly, try to forget most of what you've read on forums about suspensions because most of it is WRONG or very incomplete, including who ever told you that your anti-roll bar would make a difference in hill climbs with regard to your suspension unloading.
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post #10 of 119 Old 12-18-2011, 09:14 PM
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[quote=ftgiles;12703418]Click Here for a link to information written by a mechanical engineer for non-engineers. Scroll down to the middle of the page and read the "Getting Started" first. Then read page 12 in the index. Then poke in on the other pages. Spend a few days reading it. Then think about it and then go back and read it again.

Quote:
Most importantly, try to forget most of what you've read on forums about suspensions because most of it is WRONG or very incomplete, including who ever told you that your anti-roll bar would make a difference in hill climbs with regard to your suspension unloading.
When I told him to leave the sway bar connected, it was because he related that one of the front tires tended to drop into a hole it couldn't climb out of on that one obstacle.

Since he was running discos instead of an offroad swaybar like the Currie A/R, I suggested that he try it with them connected and see what happened and only in that one spot.

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post #11 of 119 Old 12-18-2011, 10:12 PM
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Originally Posted by ftgiles View Post
100% anti-squat when sitting on flat ground or on some percent incline?

What is so hard about knowing precisely where your center of gravity is?
Yes, calculations are based on level ground. Not many people have access to a scale to accurately determine COG. It's obviously not impossible to calculate it.

Quote:
Originally Posted by RaggedOleMan View Post


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?
There is no rule or law that says the IC has to be in front of the vehicle. Like you said, follow the imaginary line (extension) of the control arms, the IC is where they intersect. Look at the Rough Country long arms, there is zero vertical separation in the arms at the frame end, and that's where the IC is, at that point vertical separation at the axle doesn't matter.

It's hard to understand and hard to explain, obviously took me quite a while to figure it out. If you haven't found it already, I would download the Pirate 4x4 4-link calculator; http://www.pirate4x4.com/forum/showt...ink+calculator. There is a text document with it that tells you how to figure out all the x,y, and z values. I would then go out and measure your current set up. Measuring from the ground to around the top of the bell housing is a pretty good measurement for COG.

I also have this cool little program called "Phun". It's a 2-demensional sandbox physics simulator, I was able to build simple little vehicles and watch how different designs reacted to acceleration.
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post #12 of 119 Old 12-19-2011, 12:30 AM Thread Starter
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Originally Posted by freeskier93 View Post
...Measuring from the ground to around the top of the bell housing is a pretty good measurement for COG.
Well, I've spent a few hours reading on that link you provided. Thank you for that...and no, haven't digested much of it, yet.

Here's what I'm understanding to this point, freeskier93; 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? Assuming that it is a fair statement, is there a point of diminishing returns with respect to the IC in this Jeep/Tj scenario, given that the objective is to achieve maximum front end grip/bite/load?

For example, in your reference to the Rough Country long arm essentially having the IC at the frame end of the rear control arms, that would seem to me to be terribly in-effective with regards to keeping the front end of the rig down. Or have I got that backwards?
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post #13 of 119 Old 12-19-2011, 12:12 PM
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Originally Posted by mrblaine View Post
When I told him to leave the sway bar connected, it was because he related that one of the front tires tended to drop into a hole it couldn't climb out of on that one obstacle.

Since he was running discos instead of an offroad swaybar like the Currie A/R, I suggested that he try it with them connected and see what happened and only in that one spot.
Mis-quoted and/or misinterpreted? Lol

That's my point about trying to learn about suspensions from forums. Forums are a great place to ask specific questions but not such a great place to learn about stuff that takes many pages of information and where some well written text should be sought out.
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post #14 of 119 Old 12-19-2011, 12:21 PM
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This is something I posted in my build thread a while back. It's just my view on it so maybe it'll help clear up the muddy water a bit. I couldn't be happier with how my suspension performs so I think my view was pretty accurate.



Here's the quick and dirty on my view of squat and anti-squat, just to help explain what the adjustment in the upper mounts actually does:

Antisquat tells you if a car is going to squat or rise when accelerating. Now go back to physics class and apply what you learned--for any action, there is an equal and opposite reaction. When the rear end of a car squats for that first split second when launching (action), the rear tires lose traction (reaction). Conversely, when the rear end of a car raises during acceleration (action), the tires push down onto the pavement....or rocks, or whatever surface you want to talk about(reaction). So, it may appear that high AS is a good thing, but here's why it's not--and it's the exact reason I want to reduce my AS--after that instant downward force on the tire, the weight transfer is over and the tire loses traction. This continues to happen if you're on a loose surface and results in hopping (or rock humping as we call it). The higher the AS, the more severe the hopping.

The percentage of anti-squat is determined by the location of the instant center. This point is determined by the imaginary lines drawn through the middle of the suspension links--where they intersect (from a side view) is the instant center. The percentage of anti-squat refers to the amount of weight taken through the suspension links. For example, 80% AS refers to 80% of the weight transfer being taken through the links and the remaining 20% is taken through the springs, resulting in downward squat. 100% antisquat is a 'perfect' situation in which the links take all of the weight transfer and the springs take none, resulting in no squat OR rise of the rear end. In this situation, the instant center falls on a line that passes through the rear tire contact patch (100% AS line) + the intersection of two other lines--a flat line through the CoG and a vertical line through the front tire contact patch. Lastly, +100 AS (110% for example) induces the rear end to actually raise when accelerating. It sounds crazy technical but if you draw it out or download the pirate 4bar calc, it makes perfect sense. That is what I'm trying to achieve but it's difficult since the exact location of the CoG isn't perfectly known. Instead, you can only make a close prediction and I used the throwout bearing, which is approx. 25" above the ground on my rig. That's why there's adjustment built into the upper link mounts--as you should now be able to see, raising or lowering the frame end of the upper link changes the angle, resulting in a higher or lower instant center.

A couple other things--the CoG is a very important location. The weight transfer occurs/pivots around that point. So, if the IC is behind the "100% AS line", the rear end will raise. If the IC is in front of the 100% AS line, the rear will squat. Again, if it falls on the line, the rear will be neutral, the ideal situation. I'm going to do my best to keep the AS just a few points under 100% since my shocks counter squat (Bilsteins). Hopefully this helps clarify the muddy subject of suspension geometry--after reading a lot and messing with the suspension calculator, things became very clear. There's literally a ton on the subject out there and really, once you apply basic fundamentals to the case, it all just makes sense.

FYI, that should explain why I never recommend RC and BDS's long arm kits. The rear upper and lower mounts @ the frame meet at a single point--ie, no vertical separation. So, the IC is far back and low, giving you a ton of AS.



Help at all? You need to download the calculator to get a visual. Plot your current suspension on there, study up on link suspension some more, and you should start to get it. Once you do, it'll all click and you'll say, "ooooohhhh, now I get it." Don't shoot for 100%. Shoot for 60-90% AS. You want that small bit of squat. On one very tough wall, I compared the previous setup with a ton of AS vs. my current setup, which should be around 8-90%. I feel much more comfortable hitting the gas hard on this super sketch, steep wall. The front end will always feel light unless you have a suck down winch but the more you do it, the better you'll get to know your rig and you'll "feel it." You'll go from being scared any time you feel the front end lighten up to knowing that you're OK. It takes a while and it takes some ***-clinching but eventually, you'll start to feel more comfortable.

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post #15 of 119 Old 12-19-2011, 12:49 PM
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Originally Posted by ABQ-TJ View Post
why is it called "instant" center? I've never heard that term before.
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Originally Posted by ftgiles View Post
Click Here for a link to information written by a mechanical engineer for non-engineers. Scroll down to the middle of the page and read the "Getting Started" first. Then read page 12 in the index. Then poke in on the other pages. Spend a few days reading it. Then think about it and then go back and read it again.

Most importantly, try to forget most of what you've read on forums about suspensions because most of it is WRONG or very incomplete, including who ever told you that your anti-roll bar would make a difference in hill climbs with regard to your suspension unloading.
Did you read this /\
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