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Old 03-17-2008, 04:52 PM   #1 (permalink)
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Planning build, need advice.

Ok I'm in the planning stages of my (re)build and could use some advice from the go fast guys.

The plan, take my 74 bronco and make it work really good at the dunes and still be decent for trails and not be to scary on the road. My plans,

build new cage, bumpers, lighten up as much as possible

3 link w/panrod in front, coilovers and air bumps

triangulated 4 link in rear, coilovers and air bumps

stretch wheelbase as much as practical, 2"-3" front, 5"-6" rear, approx 100" overall

What amount of travel should I shoot for in the front? Is 14"-16" realistic?

I understand that rear travel should be 25% to 50% more then front, I take it getting the front setup first is the best way to go about it.

Am I better off building it with less travel and spending more time with the valving? Quality better then quantity?

What kind of numbers should I be trying to achive with the suspension? anti-squat, instent center, etc. Lots of info for crawling out there, not so much for gofast and jumping.

What kind of weight distobution should I be shooting for? 50/50? 60/40? Where does it start to get ugly?

How do you choose air bump travel? Is it a % based off of wheel travel?

Sorry for all the questions but I'm trying to build this for the last time, I built this up the first time in the early 90's and it works OK but there is a lot of room for improvement.


Brewchief

Last edited by brewchief; 03-17-2008 at 04:53 PM.
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Old 03-18-2008, 08:55 AM   #2 (permalink)
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I know where to find some of the info you are looking for... I have been looking for it myself...

From this thread: http://www.race-dezert.com/forum/sho...t=33128&page=4

Post #35
TeenyCar does a great job of summing some things up but the whole thread is good! I tried to get someone to go a little deeper into how to estimate a good height for the roll center but no one has responded yet...

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Antisquat is in part highly related to the instant center. Antisquat affects how the tires are planted but also how the chassi is affected. That old physics thing about "for every action there is an equal and opposite reaction" applies here. High antisquat numbers (say above 80%) will plant the tires hard, but will also raise the chassi, and the tires will also break traction faster as well. Forward forces are as a large part determined by what the link angle is when viewed from the side. So you are loosing force in a suspension where the links go uphill from the axle to the chassi steeply versus one where the link angle is fairly flat. Low antisquat will still plant tires but not with as much force initially and will hold traction much longer too. I tend to shoot for 50-80% at ride height by building in different points of adjustment in 1 inch increments on the upper links at the chassi. Because antisquat is in part determined off the instant center, where you put your instant center is also of importance. The lower and farther out from the rear axle you can put it the better off you are to plant tires and keep them planted. So I'd shoot for somewhere ahead of your front bumper and keep it as low as you can with sacraficing other areas.
Roll axis determines how much under or oversteer you have. Generally speaking you want the angle to be going down hill from the rear to the front axle by a degree or two. This will help to minimize the roll steer from the rear end as you corner.
Something else to consider is the roll center. The closer the roll center is to the center of the gravity the more stable your car will feel in cornering but when you run through the whoops or over bumps your car will feel stiff as the suspension is not as free to act independently of the mass of the car. Obviously the opposite is true. The lower your roll center the more freely and independent your suspension will react from the mass of the car. More on that if you wish.
You should know what you are talking about and thinking as you build your suspension. I was self taught in the beginning and later took some classes on suspension dynamics and that took me to a whole new level of thought and design. I've been at both ends of the design spectrum and I can't ever imagine going back. I feel sorry for people who claim their suspension is the best, having never tried what proves to be far superior.
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Old 03-18-2008, 09:46 AM   #3 (permalink)
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14" is about the most youll be able to get with traditional steering.

are you planning on 1:1 motion on the rear?

bumps should be for heavy bottoming out. for reference JRs bumps are destroked to 2"
check this video out ... if youll notice the suspension shot he not on the bumps hardly at all. http://youtube.com/watch?v=7WdhXlLCFgY
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Old 03-18-2008, 03:56 PM   #4 (permalink)
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Originally Posted by rcurrier44 View Post
I know where to find some of the info you are looking for... I have been looking for it myself...

From this thread: http://www.race-dezert.com/forum/sho...t=33128&page=4

Post #35
TeenyCar does a great job of summing some things up but the whole thread is good! I tried to get someone to go a little deeper into how to estimate a good height for the roll center but no one has responded yet...
Lookd like some great info there, I can see I'll have to spend some time over there.

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14" is about the most youll be able to get with traditional steering.
What limits the travel, to much angle on rod ends? Tons of bump steer past this point? Panard bar pulling the axle to far to the side?

are you planning on 1:1 motion on the rear?
I doubt it, I'm kinda planning on the shocks mounting on the lower links, this may depend on front travel(no sense in building the rear with 24" travel if all I get out of the front is 10")

This brings up another issue, how to keep the links from twisting? I understand that if the shock mount is below the center of the link it won't roll over, so how far below center should it be? Is just below center OK or should it be 2"-3"?

bumps should be for heavy bottoming out. for reference JRs bumps are destroked to 2"
Will a 2" bump be enough for a truck that will see some decent jumps? I can see it being plenty for whoops and small jumps if the shock vaving is right but will it work for a 4000-5000 pound truck landing from 6-8 feet?check this video out ... if youll notice the suspension shot he not on the bumps hardly at all. http://youtube.com/watch?v=7WdhXlLCFgY
Thanks for the help guys, I'm not starting this till fall but I'm trying to get a good plan now so I can start gathering parts over the summer.

Brewchief

Last edited by brewchief; 03-18-2008 at 06:40 PM.
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Old 03-18-2008, 11:52 PM   #5 (permalink)
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For the rear links you need to be about 1/2" below the centerline. You can do more but the links get very complicated become and rock magnets...which is one reason I'm not a big fan of the standard "bent tube sand canoe" links on most trucks. Attached is one that I designed. 1" sphericals at both ends and the shocks are both 1/2" below centerline. The giant cut-out for the shocks was so that standard rod-ends could be used. You can make the cut-out smaller if you use +2" or +3" rod-ends on the shocks.
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Old 03-23-2008, 05:36 PM   #6 (permalink)
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Originally Posted by Triaged View Post
For the rear links you need to be about 1/2" below the centerline. You can do more but the links get very complicated become and rock magnets...which is one reason I'm not a big fan of the standard "bent tube sand canoe" links on most trucks. Attached is one that I designed. 1" sphericals at both ends and the shocks are both 1/2" below centerline. The giant cut-out for the shocks was so that standard rod-ends could be used. You can make the cut-out smaller if you use +2" or +3" rod-ends on the shocks.
What thickness mild steel would you use for an arm like this 1/8"? 3/16"? maybe 1/4" on the bottom where it may hit stuff?

When mounting this type of link should it be attached under the axle or can it go in front? I realize that mounting in front will cause the axle to try to rotate and it will put more stress on the uppers but ground clearance would be much better. Since I would like this truck to still be a decent trail rig more ground clearance would help a lot.


I've seen desert trucks where the upper links are joined into a wishbone setup, any downside to this? On an early bronco the frame width is pretty narrow and getting enough angle on the upper links to locate the axle may not be possible, this looks like it may be a solution.

How do you calculate for a wishbone upper? Do you just use the four link program and set the upper link seperation at the axle at zero?
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Last edited by brewchief; 03-23-2008 at 05:38 PM.
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Old 03-23-2008, 06:04 PM   #7 (permalink)
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What thickness mild steel would you use for an arm like this 1/8"? 3/16"? maybe 1/4" on the bottom where it may hit stuff?
The ones in the picture I posted above are for a ranger prerunner, are 3/16" 4130 top and bottom, and will be heat treated after welding to bump the strength up some. You can draw any conclusion from that you want...

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When mounting this type of link should it be attached under the axle or can it go in front?
Bottom, front, top...put it wherever you want. Just make sure you have the proper separation, link/tab strength to handle the loads, and room for the upper links at full bump all while not getting your roll center too high.


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Originally Posted by brewchief View Post
I've seen desert trucks where the upper links are joined into a wishbone setup, any downside to this?...How do you calculate for a wishbone upper? Do you just use the four link program and set the upper link seperation at the axle at zero?
There are a few down sides and a few up sides. I normally prefer 4 separate links unless I need to merge 2 of them into a Y because of packaging or lateral strength/stiffness. You want to have about 45 between the 2 upper links. Yes you just model it as a 4-link with a separation of 0"


Edit: the front of the link is to the left in the pic. 3 points if anyone can figure out what the tab on the top is for I'm still tossing around 2 different names for it.
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Last edited by Triaged; 03-23-2008 at 06:27 PM.
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