four link help

kid rok · Mar 10, 2008

Ok, need a little help. My lower links are already there so I'm going to work around that.
40" tares.
Wheel base 104", I was 101-102 but some how I am now at 104.
Separation at the axle is 9.5". Top link to bottom link
Lower link on axle tube 37.5" C.C
Lower link at frame 12" C.C
Lower link lenghth 39.5 Center heim bolt to center heim bolt
Lower link separation from axle to frame 7" Maby more my truck isn't sitting dead level. You could add 1-2 inchs easy.
With that as my lower where would my uppers be.
How long
How wide at the frame
How far forward of center line of my axle.

The current upper was almost dead level and at the height as my lowers at the frame. It was a three link and I want a four link. Tell me if I need to post more info.

I really don't understand a whole lot about four links separation and all that, and don't care too much. I just need to know how to set up my upper links using the existing lower link setup.

Speeding · Mar 10, 2008

Since you have all the numbers might want to plug 'em into "Trianged's" fourlink calculator.

http://mysite.verizon.net/triaged/files/4BarLinkV3.0.zip

I think that's the right link. You put in your link info and it tells you how it'll perform basically. Gotta have Excel to run it though.

kid rok · Mar 11, 2008

That's my problom I don't know what #'s to put where.

vanguard · Mar 11, 2008

I wrote this a little while ago. I think if you read it you'll have a good grip on a 4 link in about 5 minutes.

http://www.ncttora.com/forum/viewtopic.php?t=6050

kid rok · Mar 12, 2008

Geometry Summary:
Anti-Squat 91 %
Roll Center Height 26 in
Roll Axis Angle 1 degrees (Roll Oversteer)
Instant Center X-Axis 320 in
Instant Center Z-Axis 76 in

Are these #s ok?
Thanks for all the help on this :

. If any of you need to know how to frame a compond roof, and you need to know what your plumb cut needs to be to make your soffit drop work out on a hip, just let me know. :flipoff1:

Hardline4 · Mar 12, 2008

#'s don't look to bad. Depends on if your a true crawler or if you just point and shoot, are the numbers for the front or rear?

Speeding · Mar 12, 2008

are there any "ideal" fourlunk numbers to shoot for? I have no idea what that stuff like "roll axis" and "instant center", ect means anyway.

kid rok · Mar 13, 2008

#'s don't look to bad. Depends on if your a true crawler or if you just point and shoot, are the numbers for the front or rear?

I'm not a crawler, I like to hit it and get it. I need to get in between a crawler and point and shoot. More towards point and shoot.

vanguard · Mar 13, 2008

Speeding said:
are there any "ideal" fourlunk numbers to shoot for? I have no idea what that stuff like "roll axis" and "instant center", ect means anyway.

I'll take a shot at it.
Roll axis = If I push on the side of a buggy it will sort of rotate around an imaginary point in space. That is the roll center and sometimes called the roll axis.
Roll axis angle (steer) = Ok, think of the half circle below as the arch that your wheels/axle articulates along. If your links are more or less parallel and your in the center of your shock travel you'll have very little rear steer because as the axle moves up and down it moves forward and back very little. Now imagine your links are already points down toward the axle (up toward the chassis). As the axle drops further it will move forward (toward the center of the rig) as it moves through its arch. When one side does this and the other side it up you get an axle that's not perpendicular to the buggy and it steers. (Personally, I think this isn't really work worrying about for rock racing/crawling.)
Instant center = Take your upper and lower links and find the imaginary point in space they would touch if they kept going forward. That's your instant center. It's used, along with COG and wheelbase, to determine your anti-squat. (my post above has a link to a writeup that I thought was easy to understand)

Disclaimer: I'm not a race car engineer, this is just based on my research and a little hands on work too.

The numbers look ok to me. The AZ guys shoot for lower anti-squat numbers. If you wanted lower AS numbers you would need to move your instant center down/forward. The easiest way is probably to raise the lower link on the axle side.

kid rok · Mar 13, 2008

I was actually thinking about going with a bigger anti squat, but am worried about excessive wheel hop. The way the #s are now it will be easy to weld and work with what I have. Thanks for the input Vanguard.

vanguard · Mar 13, 2008

kid rok said:
I was actually thinking about going with a bigger anti squat, but am worried about excessive wheel hop.

Right, that's the whole focus of anti-squat for rock crawling. With numbers over 100% (neutral) the rear tends to rise under power, then when traction breaks it drops and initiates the hop. When AS is under 100% in theory the rear lowers under power and it doesn't drop when traction breaks.

As for what the perfect number would be, I'm not sure. Lots of guys on pirate seem to aim for around 75%, mine is lower than that, maybe 60% but I haven't measured it at the current ride height. Good luck.

Hardline4 · Mar 13, 2008

Excellent job Vanguard!! I think my AS is ~68. You are right about the west coast guys wantin to keep it in the mid 70s. They are more "true crawlers" as they have sandpaper for rocks while we have greased pig rocks. I like to be technical with my car so I went with a little higher AS than most on the east coast perfer.