4-link design principles

[Foxtrot]

I'm starting to work out some basic fundamentals for designing a good 4-link rear suspension for wheeling, and want to hash it out with the rest of you.

I've done a tonne of reading on Pirate4x4 and found some great info, but it takes hours to weed out all the bullshit from the genuine info.

The best threads I read were these, if anyone else is interested:

http://www.pirate4x4.com/forum/showthread.php?t=283954
http://www.pirate4x4.com/forum/showthread.php?t=113044
http://www.pirate4x4.com/forum/showthread.php?t=94327
http://www.pirate4x4.com/forum/showthread.php?t=93576

I haven't played with this 4-link calculator yet, but it sounds pretty slick:

http://www.pirate4x4.com/forum/showthread.php?t=204893


Example of some design fundamentals

-Build adjustability into your links
-Keep the links the same length, upper and lower
-Keep the mounting points the same distance apart on the axle and on the frame.
-Keep the angle on the arms down, level is best.
-Wishbone uppers work just fine
-double wishbone arms eliminate a lot of rear steer.
-really really long arms tend to get in the way.



Post what you know.

[Foxtrot]

Here's a good link and explanation of Roll Center. Something often overlooked.

http://www.pirate4x4.com/forum/showthread.php?t=205125

[ClintEastwood]

I've been looking around on pirate aswell. I am going to 4 link the front and rear of my truck and was looking around for what type of coils. seems they're using alot of different coils. the aerostar coils look like a good choice aswell the xj coils and tj coils. Any ideas? I am thinking either EB or XJ coils for the front and aerostars for the rear. If for no other reason then aerostars will be cheap an i can always switch to EB or XJ in the rear after.

[ClintEastwood]

Rusty's offroad sells 9 inch lift coils for an xj. with a spring rate of 180 pounds. would this be a stiff spring? aerostar coils are somewhere around 110-130 i believe. would the 180pd. coils be stiff? how much is too stiff? What weight rating should i be looking for? sorry if these questions are in the wrong section.

[ClintEastwood]

http://www.pirate4x4.com/forum/showt...fourlink+front

[Ryand]

Link Suspension For Dummies...
http://www.pirate4x4.com/forum/showt...ension+dummies

Lots of good Link-Info in here...
http://www.pirate4x4.com/forum/showthread.php?t=255609

[Captain Air Time]

Originally posted by Foxtrot
Example of some design fundamentals

-Build adjustability into your links
-Keep the links the same length, upper and lower
-Keep the mounting points the same distance apart on the axle and on the frame.
-Keep the angle on the arms down, level is best.
-Wishbone uppers work just fine
-double wishbone arms eliminate a lot of rear steer.
-really really long arms tend to get in the way.



Post what you know.

For starters, what you've posted might work OK for a dedicated rock crawler, but for going fast (which generally includes street/hwy) and rough stuff, you might want to make some changes there. Typical desert truck fare(works pretty well everywhere, except a pure rock buggy, maybe) would be more like this:

-Adjustability is always good, but research planning and mock-ups is better
- uppers about 70% of lower length
- vertical frame spacing about 50-75% of axle spacing
- axle vertical spacing about 25%+ of tire diameter
- close to level, 5-15* drop at most. (that leaves the upper usually close to level)
-wishbone uppers work fine, put more stress on one point, limit where you can put your roll center/axis (not enough to be a major issue in most 4x4 situations)
-arm length should be a function of overall travel, and angle at rest, but generally they end up being whatever fits the chassis and gives you the desired geometry. Its all one huge compromise, but generally longer works better if you have some wheel travel.
-double wishbone makes for some really freaky geometry...
-one set of links or wishbone (typically the uppers, or shorter links) should diverge at least 45 degrees, and the other set should diverge in the opposite direction as much as reasonably possible. Typically I've found it difficult to get more than 5-10 degrees without mucking up either the rest of the geometry, or having spacial issues.

For anyone REALLY interested, 4Wheel and Off-Road magazine actually did a very good article in July 2003. Most of the home builts I've seen could have stood to read that before starting to work, for sure! Another priceless tool is here. That's version 3.0. I find v2.1 to be a lot simpler to use, with no glitches, but I see that he's removed it from the download area. This program doesn't give you any indication of what geometry values are suitable for your application, but it does show you how each parameter affects those values. THe 4WOR article is probably the best one I've seen in print on what to shoot for and how to achieve it. I've got a few books on race car chassis dynamics that help too, but they do get a little over technical, and sometimes backwards for what we are trying to achieve with a long travel off road vehicle.

Just remember that while virtually ANY combination of links will function (within reasonable levels of stupidity), for it to work WELL accross a broad range of terrains (as most of us require), takes a LOT of careful planning and re-designing.

[ClintEastwood]

here's a link to a pretty good site with quite a bit of info...maybe a lil outdated now but still good technical stuff....

http://www.akfabshop.com/alaskaoffroad/suspension.htm

[ClintEastwood]

I've used a few this one is the best i've found yet....pretty much everything you could need to calculate....

http://www.gregblanchette.com/images/ExcelCAD_v2.1a.xls

[Captain Air Time]

Originally posted by ClintEastwood
I've used a few this one is the best i've found yet....pretty much everything you could need to calculate....

http://www.gregblanchette.com/images/ExcelCAD_v2.1a.xls

Ahh, Version 2.1. good to see its still up somewhere. I don't care for v3 (can't make it work properly)

any idea off the top of your head what revisions wend into 2.1a from the v2.1 that I've been using? I haven't had any issues with it, and I've used it a lot.

[ClintEastwood]

haven't used the 2.1
just stumbled across this one today, so i haven't gotten much time to play with it. but it looks like it's got everything you could possibly need and is quite user friendly......gonna take some measurements tommorow so i can get some real use outta this thing

[Captain Air Time]

I downloaded it, its listed as v2.1'a', but its actually the same old v2.1 that I've been using. Glad to see that I wasn't missing out on some update LOL! Check out my previous post for version 3, there's even more parameters involved, although I can't make it work properly. Might work for you, tho v2.1 is a superb little calc, and I don't have need of much more than what it offers.
The materials selection part is priceless, as is the load calc. Shows you just how much longer your shit will last by changing this and that to reduce stresses, which in turn allows you to reduce component weight. Very, very cool toy. All simple calculations, and I have various calculators for each function, but to have them all integrated has saved me countless hours of pencil scratchings and drawings.

[ClintEastwood]

yeah i really like the program....didn't get outside today as it was rainin a lil more then i was willing to endure

defintaley helps turn all the numbers into something you can picture....i like that it plots it all out too so you can see just what adjustments will do as it relates to you actual wheelbase an tire size. i haven't used the materials calc but i also think it's a great tool.

one question i've got is...how much vert seperation between the links at the frame should you have? the best answer i could come up with on pirate was 75% of the seperation at the axle..but from the many pics i've looked at on the site most aren't that far apart at the frame....even with the buggies.. does anyone know?

[Captain Air Time]

Originally posted by ClintEastwood
yeah i really like the program....didn't get outside today as it was rainin a lil more then i was willing to endure

defintaley helps turn all the numbers into something you can picture....i like that it plots it all out too so you can see just what adjustments will do as it relates to you actual wheelbase an tire size. i haven't used the materials calc but i also think it's a great tool.

one question i've got is...how much vert seperation between the links at the frame should you have? the best answer i could come up with on pirate was 75% of the seperation at the axle..but from the many pics i've looked at on the site most aren't that far apart at the frame....even with the buggies.. does anyone know?

*edit* keep in mind that what works perfectly, or even acceptably in a dedicated rock buggy can be a deadly horror show on a high speed bump (or lane change). IMO if you are building an all around 'wheeler (driver) you should start with a desert style (great at speed, good all around, but squats too much for crawling, most say), and then fine tune toward what works best for your chosen favorite (rocks, etc). You'll be happier in the end.

As I stated earlier, about 75% seperation at frame vs. axle is a good goal, but really anything from 50-90% should be fine. Everything is one huge compromise, and its pretty rare that you can ever use your 'ideal' geometry once you start on the chassis. There's always something in the way, or you don't like the clearance, or you can't have a transfercase and linkage too... Be careful of making one parameter perfect, (pinion angle change, for instance) because usually it throws something more important right out the window.

Juggle juggle until its an ACCEPTABE compromise, its all you can do. Find a copy of that 4WOR article, its really very well done (for once). Typically the magazine guys have trouble spelling their own names, let alone getting the facts straight, but this one is pretty good.