Rear 4 link Help

rcmbronc · Sep 18, 2018

I need some help with some advice to get rid of the rear roll oversteer. Here is my 4 link calc. I cant seem to figure out what I need to change to get rid of the over steer. I am assuming flatter lower links but dont want to drop the front mount and just moving the rear mount up and in front of the axle does not really help the numbers. The rear lower mount is where it is because that is as far back as I can move the rear axle with the 40" long lower arms. I may need to shorten or build new lowers. Front lower mounts are already a bit below the frame. It does help to lower the entire truck but not alot. Any ideas?

73azbronco · Sep 18, 2018

You might want to look at, "gasp", how a jeep rear axle uses a 5th link like the front axle does to keep it from moving.
https://www.google.com/search?q=jee...UIDygC&biw=1640&bih=835#imgrc=vHHm_LjmishpoM:

Yeller · Sep 18, 2018

Ugg yeah that's kinda ugly. For the ride height that you have entered there are only a few options, to lower the ride height it would have to be a lot to be much help. At that ride height, I would move the lowers above the axle and the uppers a corresponding equal amount. Then you might need to raise the frame side of the upper some, or lower the frame side of the lower some to get antisquat under control. And yes the control arms above the axle works very well. Me, I'd lower it a lot but this is your build not mine.

And for what you are building a 5th link is not going to help in anyway and will actually be a bigger problem. IMHO Heeps drive like ass unless they have been heavily and properly modified.

73azbronco · Sep 18, 2018

My 2013 jeep drives like a scalded ape at 85 with no over/understeer, plenty of 5 links are out there functioning just fine.

You want it to "look" good or drive good? Just a suggestion, his build he can do what he wants but it's not working right now.

Yeller · Sep 18, 2018

73azbronco said:
My 2013 jeep drives like a scalded ape at 85 with no over/understeer, plenty of 5 links are out there functioning just fine.

You want it to "look" good or drive good? Just a suggestion, his build he can do what he wants but it's not working right now.

My apologies if I touched a nerve, that was not my intention to make an offense. I looked into what the OP is working on and a 5 link is not the best option for his desired performance and wanted to not have a debate and remove it from the table. It can look good and drive good, just takes some some sound, creative engineering. Again my apologies to both you and the OP

rcmbronc · Sep 19, 2018

I did not think my ride height is crazy high. Yes a bit high but I am going crazy buying coil over springs. I do want it lower so I will check some new numbers.

rcmbronc · Sep 19, 2018

Right now it looks like I need to drop the truck like 4 inches and move the trailing arm to the front of the axle. New springs again. Love it. No problem. I will try lowering it first and go from there.

I am not debating that the 5 link set up works good. I am sure it works great. Not an easy set up to use on a set up like this with 18-20 inches of travel in the rear. I assume there would be a ton of side travel with that.

JSmall · Sep 19, 2018

Any pics of your lower axle side mount? How are you able to mount it behind the axle? Listening to Yeller got me an awesome handling Bronco

73azbronco · Sep 19, 2018

No stress taken my friends, just talking.

As an engineer, I'd say the links are to close together and you have no triangulation offering side to side rigidity. Thats why I mentioned a 5th link.

Look at this setup using double triangulated setup:

https://genright.com/products/double-triangulated-rear-4-link-suspension-kit-links-optional.html

rcmbronc · Sep 19, 2018

Here is what it looked like before when I had it right below the housing. Basically I rotated it some to help with my longer links. Dont have a pic of it after rotating.

Seems to me to have quite a bit of triangulation. More is always better but hard to get with this much travel.

73azbronco · Sep 19, 2018

sup4201-2-installed-1200__21146.1470689533.1280.1280.jpg

the double triangle setups I'm seeing would keep everything you have the same except either place the front upper mounts further apart on the frame, or the lower front mounts closer together on the frame, or both. That gives a double triangle which would eliminate side to side play.

What you have now is a 4 link with each side of your system running basically parallel, offering little prevention of side to side movement of the axle, in other words top bars need to cross bottom bars.

rcmbronc · Sep 19, 2018

My upper mounts are fully triangulated. They are as far out on the front top links that I can have. They are attached to the inside of the frame and on top of the rear end housing. The lowers are traingulated 5 degrees which is not alot but not parallel. I dont think side to side movement is the issue but I could be wrong. The lowers I could put more angle into with front mounts closer to the inside. Does not leave room fro the coil overs. Got me thinking. Note I am also an engineer so I have this on the spread sheet and cad and also in my head alot thinking of answers. My set up is like most pre runner set ups with high travel. Got to keep thinking

JSmall · Sep 19, 2018

The Bronco frame is really narrow so relying on the uppers for the triangulation could cause some issues. I got around that by mounting my frame side lowers inboard of my frame and the axle lowers as wide as possible on my full width axle. Your setup currently has 31.5" horizontal separation between your lower links at the frame and mine has 19.5". With my full width axle I have 47.5" separation at my axle and you have 43.5". That is a few more degrees of overall triangulation between our lowers. I have 0* Roll Axis Angle in the calculator.

*Disclaimer* It is highly possible that I have no idea what I am talking about as I have only setup one Bronco 4 link ;D Yeller and others will chime in and get you headed in the right direction.

73azbronco · Sep 19, 2018

See even those dont cross they do create a vivid double triangle. The OP's are closer to two parallel sets.

rcmbronc · Sep 19, 2018

I’m not sure yet how to get more triangulation. I can maybe go wider at axle lower. Can’t really change lower front frame because shocks are mounted to arms running outside frame rail. Would need to remove frame rail to change. Need to look at it. I am still convinced the roll understeer is the issue and think I have some good ideas to change that

73azbronco · Sep 19, 2018

I would say se what can be done to get your red lines closer at the frame side. As close as the axle set on the top.

Yeller · Sep 19, 2018

Nice looking work! I'm not an engineer but a long time player in learning the hard way and playing with several hundred different set ups to see what works, what doesn't and why they do or don't. Also been chasing this knowledge for years before I ever saw a suspension calculator, I still don't use it while I'm building, I use it when I'm done because the owner usually wants to see it. The triangulation is sufficient, you have more than the average monster truck that is heavier and abused harder than anything you are going to do. Triangulation is not helping but not the biggest cause of the handling issues you are struggling with. The geometry of the links in the relationship to the ground are the biggest culprits. I have some basic rules that always work without fail. At ride height the upper control arm needs to be as close to parallel to the ground as possible to pointing down toward the front. The lowers need to be mounted with the frame end 35-50% of the vertical separation at the axle, depending on how much anti squat you want. For a 4 link I try to get 20% of the tire size for vertical separation at the axle, more if available, packaging always creates compromises to everything but usually can be worked to a compromise that functions well. Packaging permitting I make all of the control arms the same length, but will shorten the uppers up to 33% shorter than the lowers if necessary but still follow the same basic rules.

here's a sample of what the calculator looks like on my personal bronco. Please keep in mind I do not have a bronco frame, I built a rectangular tube frame that helped to accommodate this geometry, but have managed to get close on bronco's.

bamabaja · Sep 19, 2018

Hey Yeller, how about some pics or drawing for us non engineer, no experience students in the back of the class ?

rcmbronc · Sep 20, 2018

Here is what I get if I lower the truck 4 inches. Looks better. I will try this first and go from there. I do appreciate all the comments and help.

rcmbronc · Sep 20, 2018

Yeller said:
At ride height the upper control arm needs to be as close to parallel to the ground as possible to pointing down toward the front. The lowers need to be mounted with the frame end 35-50% of the vertical separation at the axle, depending on how much anti squat you want. For a 4 link I try to get 20% of the tire size for vertical separation at the axle, more if available, packaging always creates compromises to everything but usually can be worked to a compromise that functions well. Packaging permitting I make all of the control arms the same length, but will shorten the uppers up to 33% shorter than the lowers if necessary but still follow the same basic rules.

Tires are 35". Vertical separation is 10" now and would be 6.5 if I remounted my lowers higher on the axle.

I am confused about the 35-50%. So with the vertical separation of 10" I would want vertical separation of the front mount on the upper and lower links to be 3.5-5"? I think that is what you are saying. I have 3.75" right now and can adjust two more holes up for a total of 6.25".

What number for oversteer is OK? I assume none is best and understeer would be better.

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