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Old 12-13-2010, 01:12 PM   #1 (permalink)
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Rear Triangulation

I know that this topic has been discussed in great detail in other threads but not here in the FJ Cruiser section. So with a little more knowledge, some measurements and the version 3 (4 link calculator) I bring it here so that those of you with expierience can help disect the numbers and give some insight into this complex subject both for my benifit and others who will be linking thier FJ 's in the future. There is a great article on the 4 wheel offroad site that I found very helpful in understanding the basics. Here are the #'s

Anti Squat 116

Roll Center 28

Roll Axis 15 Oversteer

IC X 79

IC Z 38

If I am correct, Anti squat is not very important in crawling but more than less is better fast in the desrt. Still seems like too much

Roll center at 28 seems acceptable

15 degrees of oversteer is not that bad but could be responsible for my rear wheel steer.

The IC X and IC Z are #'s that I have no grasp on what so ever except for where thay are on the graph, I'm not sure what thier importance is.









Last edited by Mad Dog 1; 12-16-2010 at 09:36 AM.
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Old 12-13-2010, 01:58 PM   #2 (permalink)
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Do you have any numbers from the the calculator?

Mine also has rear steer but moves the opposite direction, upper wheel moves forward of the lower one
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Old 12-16-2010, 10:12 AM   #3 (permalink)
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I originally thought and was told that If I lengthened my uppers and thier horizontal measurement at the frame it would help with the rear wheel steer but plugging in several different options into the calculator it didnt help at all. So I started messing with the lowers and came up with something I think is a little better by moving them up onto the side of the axle and out horizontally by 2" this made them about 4" shorter.

Anti Squat 93

Roll Center 27

Roll Axis 10

IC X 114

IC Z 43
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Old 12-16-2010, 11:53 AM   #4 (permalink)
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Ok

So we should start by achieving some common ground.

Read this:

http://hstrial-4wheelundergr.intuitw...nsion-411.html

It's a pretty good total summary of everything I've heard about linked suspension design in plain english. There's some other sources that hit about the same level, but this is as good as I've seen in one spot so far.

The reason I bring this up is that you're putting some numbers out there that are out of context, and I want you to see what the context is that would make me ask other questions.

Antisquat, for example, isn't thought to be meaningless. It's thought to be important in being able to get a linked suspension to drive up a steep climb without inducing wheel hop.

The instant center is used as a theoretical point where the force is being applied to the system. The question becomes where it is relative to the center of gravity.

The BIGGEST question in making these numbers meaningful is where the center of gravity ACTUALLY IS. There are people who put forth guesses about where you can expect it to be, but I have no independent experience to verify that these are correct. More than a few people have suggested that the estimated point can be the top center bellhousing bolt, but I'm pretty sure that mine is nowhere near that since I have almost a perfect 50/50 forward back weight distribution.

You can deduce the location of the center of gravity using 4 wheel race car scales and some trigonometric trickery.

You first weigh all 4 corners with the vehicle flat. Add up front end and rear end. Add up total. Measure the wheelbase and track width.

Front over total is front end%. Front end% times wheelbase is the distance from THE REAR to the CoG (rear over total is the distance from THE FRONT, in case you want it).

Do the same thing, side to side and you have an X,Y type position on the ground right under the CoG when the vehicle is flat.

Write down the number as inches forward of the rear, inches in from driver's side, to be consistent.

Next... jack the front end up about 2 feet and put the racing scales on blocks. Put the wheels back down on the scales. RE-measure the new wheel base (shorter by an inch or so) as the points ON THE GROUND directly below the hub centers (use plumb bobs to the ground and measure from tip to tip) Accuracy is key.

Now do the same 4 way measurement.

Calculate the X,Y (distance from rear, distance from driver's) of the CoG when the rig is front end up.

Now, calculate the angle that you've raised it to =arcsin(height of block/wheelbase)

X1-X2 is the distance between the two CoG measurements

(X1-X2) / TAN(angle) should give you the height of the CoG.

... but you're not done. That's the height while the front is up on blocks.

Now you're going to have to calculate how much you need to subtract to figure out where it is when you're flat on the ground.

TAN(angle) x X2 should be the extra height. Subtract it for your TRUE CoG height.

Plug *THAT* into the 4 link calculator for meaningful numbers.
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Old 12-16-2010, 02:58 PM   #5 (permalink)
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You always have a way of making me feel very small, which is good because generally my ego is too big.

It is true about the COG I used the top bolt of my bell housing and I only estimated the wieghts, so if the #'s are meaningless with out these exact measurements and wieghts then it will be a while until I have them . I do not have a truck scale to wiegh each wheel. I think I have seen something on line that you can buy that will give you that wieght. I will check and go back to the drawing board and read the above article.

Thank you, Good stuff.
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Old 12-16-2010, 06:14 PM   #6 (permalink)
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No way... not worth buying car scales for this.

This, coming from the guy who uses projects as an EXCUSE to buy stuff.

I just got home, having spent 86 bucks on a strap wrench strong enough to crack my shock bodies off the top caps!!

You want to find a shop that has scales and just borrow the opportunity to use them.

A lot of hot rod, custom car, race, and offroad fab shops would have them because they're super useful for getting spring rates correct.

You might look for a place that makes sprint cars or midgets, it kinda depends on where you live.
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Old 12-17-2010, 08:47 AM   #7 (permalink)
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[QUOTE=BellyDoc;12261797]No way... not worth buying car scales for this.

The cheapest I could find on line was about $1900.00 normally I wouldnt have hesatated to buy this but seeing as we are still in the great recession I may do as you say. Besides, Mad Dog is currently not drivable but am finnally having some great luck with the wireing, my new gauges,and my steel braided fuel lines( needs another thread)

Hey, I bet if you bought one of those you could take it out to events like KOH and others and make some money. When I was at the summitt last year there was all kinds of talk about how much every ones rigs wieghed with all of the afteremarket crap on them.

Hmmm.$$$

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Old 12-17-2010, 10:06 AM   #8 (permalink)
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You can find scales for under a thou on Summit, but even so, at 900+, that's a hell of an investment for something that's going to mostly take up shelf space and collect dust. Even if you have to drive a full tank of gas to, and another full tank of gas back from where ever you can find scales, you'll save 800 bucks.
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Old 12-19-2010, 10:54 AM   #9 (permalink)
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i use the scale at the recycling place down the street from me. they don't charge me to use it and i can get 3 weights, front, rear, and total.
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Old 12-19-2010, 11:12 AM   #10 (permalink)
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There may be accuracy issues if you don't have your mass centered on the plate, but other than that, this is a really good option. If you could do front half, rear half, driver's side half and passenger's side half, you could deduce corner weights.

If you could use the scale long enough to do all that and then park your front end on, and Hi-Lift your rear axle 24", you could deduce center of gravity.

I guess it depends on how patient the operator is... or whether or not they wheel!
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Old 05-28-2011, 07:15 AM   #11 (permalink)
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i found a some what quick way to find the center of gravity assuming you have a somewhat flat frame. Have to have level ground and use 4 pin jacks, 2 on each side. use the early t4r to example. place one stand up front on the frame to point were it starts to bend upwards, right before the frame mount for the ifs. do the same for the back, right before the leaf spring mount, should be something like 65 inches apart. do the same for the other side. now jack up the rear of the truck and move the rear pin jacks forward say 10 inches, then let the jack down. repeat, what your looking for is the truck to balance on the rear jack stands, that is the line of center of gravity. the actual center of gravity would depend on how tall the truck is. i have used this tech for setting up road race cars with great results. hope this will help anybody setting up any truck

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Old 05-28-2011, 05:40 PM   #12 (permalink)
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Hmmm

Wow, I am like Donnie in The Big Lebowski...completely out of my element. I like learning from you guys though.
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Old 05-30-2011, 07:43 PM   #13 (permalink)
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so how does set up feel? from what ive gathered from doing my research is your anti squat and roll center looks pretty good, but that seems like a lot of oversteer. seems like you would want alittle bit of understeer if anything... is this a street driven rig? thats where you will really notice it.
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Old 05-31-2011, 06:35 PM   #14 (permalink)
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Quote:
Originally Posted by locost7 View Post
i found a some what quick way to find the center of gravity assuming you have a somewhat flat frame. Have to have level ground and use 4 pin jacks, 2 on each side. use the early t4r to example. place one stand up front on the frame to point were it starts to bend upwards, right before the frame mount for the ifs. do the same for the back, right before the leaf spring mount, should be something like 65 inches apart. do the same for the other side. now jack up the rear of the truck and move the rear pin jacks forward say 10 inches, then let the jack down. repeat, what your looking for is the truck to balance on the rear jack stands, that is the line of center of gravity. the actual center of gravity would depend on how tall the truck is. i have used this tech for setting up road race cars with great results. hope this will help anybody setting up any truck
This logic would help you find the point on the ground that the center of gravity is over, but the height of the center of gravity is the real question. This is the value that gets "guestimated" and used in the link calculator. The resulting values that come out of the calculations are therefore just as much guesswork as the numbers that go in. If I'm remembering correctly, the link calculator only needs the height of the center of gravity (z), and not it's (x,y) position.
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Old 06-01-2011, 02:52 PM   #15 (permalink)
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i was giving a quick and dirty way to physically find it. using simple math is best way to. check out this link. but figuring it with out unsprung weight or taking into account that the center of gravity will shift when the suspension travels.
http://www.longacreracing.com/articles/art.asp?ARTID=22
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Old 06-01-2011, 04:41 PM   #16 (permalink)
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Originally Posted by BellyDoc View Post
This logic would help you find the point on the ground that the center of gravity is over, but the height of the center of gravity is the real question. This is the value that gets "guestimated" and used in the link calculator. The resulting values that come out of the calculations are therefore just as much guesswork as the numbers that go in. If I'm remembering correctly, the link calculator only needs the height of the center of gravity (z), and not it's (x,y) position.

This is exactly correct, it is the hieght of the COG that is important for acurate calculations. We are going to do it on MAd Dog when she gets back from Tucson. It invlolves wieghing all 4 wheels independantly then front and rear independantly and then doing a certain amount of lifting from the front and rear that relates to the amount of travel that you have and wieghing in these conditions, then these wieghts fit into some formulas which I am not familiar with but will be later when we do it. I will try to post pics and explaination when the time comes.
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Old 06-01-2011, 04:46 PM   #17 (permalink)
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Originally Posted by locost7 View Post
i was giving a quick and dirty way to physically find it. using simple math is best way to. check out this link. but figuring it with out unsprung weight or taking into account that the center of gravity will shift when the suspension travels.
http://www.longacreracing.com/articles/art.asp?ARTID=22
That is pretty cool, I think it is the basic thinkinig behind what we are doing with a few more variables.
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Old 06-02-2011, 12:52 AM   #18 (permalink)
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Quote:
Originally Posted by locost7 View Post
i was giving a quick and dirty way to physically find it. using simple math is best way to. check out this link. but figuring it with out unsprung weight or taking into account that the center of gravity will shift when the suspension travels.
http://www.longacreracing.com/articles/art.asp?ARTID=22
I'm familiar with this method. It's basically how I did it, but I'm not in love with the result. There's a lot of error built in when you do this for a massive buggy compared to a little low riding race car.

At some point, I'm going to re-do my measurement just for kicks, but the next time, I'm basically just going to tip it over with a fork lift till it balances/flops to the right and left, and mark lines with masking tape that go vertically from the contact points on the ground. When it's back flat on the ground, the point where these lines cross will be at the height of the CoG.
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Old 06-02-2011, 02:15 PM   #19 (permalink)
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another thing about doing things "physically" on top of the mathmatics, its a check to make sure you in the ball park. also using strings or laser along side your rig that are parallel to make sure its straight on the center line of the unit
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Old 06-02-2011, 04:26 PM   #20 (permalink)
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Physically checking it is deffinatly real life acuracy. It would be interesting to check it that way. Until it flops or rolls while in the drivers seat this would also give you a feel for where that point is as you drive. Ive been teetering before and wished that I could have had a way to measure that point.
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