Independent single trailing arm suspension w/lateral links. Further Discussion 1

[Isdtbower]

Reference Thread: thread800-419247

The referenced thread is fairly new and I have been working on an off-road IRS design that might bring some light to moving the Lotus lateral links.

It really helps to have a fresh paper design as you usually see more in the development of the geometry. To make a long story short, I was having problems making the lateral links do to the wheel camber what I wanted ...without putting the inboard links into the center section. My main problem was at the hub where the hub CV center was 5" outside and where I could mount the closest link mount points.

Offroad racers will understand this better, but here goes. I wanted 22" of wheel travel, with the bottom of the tire running along the same driving line through out travel. I also wanted the camber to not go positive up to 5* of body roll to the ground. I also wanted to minimize CV axle/half shaft plunge (desert guys are used to 4"), and maximize ground clearance while still preserving a tub in the rear for an ice chest, etc. I am an old draftsman and not a CAD guy and had to play a lot with different suspension systems and was getting no where until I read about "virtual points." This was the key to moving me forward. Some recent OEM and racer offroad suspensions with trailing arms and lateral links started to make sense. Long trailing arms have less plunge and more consistent "drive". I set that at 56" which mounts under the seats. Then I worked on the lateral camber geometry from the rear view...inline vertically, with the rear half shaft/hub/CV.

By placing the bottom link at the height of the half shaft (ground clearance), plunge went to almost zero. After working the camber curves and trailing arm radius, The bottom link was almost to the center of the car/differential and plunge was near 1/2". The top link, which is 12" above the bottom link/half shaft was just missing the IRS quickchange diff.

So those were my perfect "virtual points." Since everything rotates from the front trailing arm mount, I figured the inboard link mounts would be on a line from the "virtual points" to the arm mount. This allowed me to move the inboard link mounts forward toward the pinion and preserve the geometry. Right now, the geometry works great in moch-up and workiing toward figuring out the toe out problem. Usually that is "compromised/solved" by moving the arm mount inboard under the seats. Since the lateral links are so long, I don't expect the change to be much, and should change the inboard link mounts only slightly.

So life is good, but I have not found any internet reference or real life setting of the toe for this type of system. I'll either get it or understand why it doesn't work!

All of this brings back the questions on the referenced Lotus thread. By moving those links, you are changing the original designers "virtual points." at the half shaft. That can be OK or not, depending what you are looking for.

Always listening for helpful information. Thanks.

(The bottom link mount at the hub is inboard 5" from the CV, 6" in front of the half shaft, and on the CV to arm mount center-line. 85" outside of tire. 39" tires. Compressed, camber moves from -4 to -1 with 5* body roll. At ride height camber ranges from -2 to 0. At droop camber ranges from -4 to -1. This will not be a track racer so a big sway bar cannot be used to minimize body roll. From tons of video, rear roll was usually below 5* before a tire was lifted during turn-braking at speed. For IFS we check to 10* of body roll as turn-braking exaggerates everything quickly and no front sway bars are preferred. A 10" rock is a nuisance )

 

[GregLocock]

A picture is worth 653 words!

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Tmoose]

Lincoln's Gettysburg Address was only 272 words long.

Hi Greg, you // almost // got me to count the words in the OP's OP. Did you ?

regards,

Dan T

 

[Dave K]

I got 663, per

http://www.countofwords.com/

I have nothing on the original topic

-Dave

NX 9, Teamcenter 10

 

[Isdtbower]

Attached is the articulation drawing to find the virtual points inline with the half shaft and a very recent build by JIMCO Racing Inc. You can see where the lateral links are canted forward. Some UTV's have the links behind the rear end located very near the car centerline. Maybe missed, but to maximize axle length and minimize axle plunge...and the associated wear and heat.

The discussion is about how you get there and probably only of interest to someone searching for the specific info. It should be of interest to the UTV racers as their bottom links are below the hub and bent regularly. Not many have played with the UTV OEM suspensions as, per the rules, can't modify the inside mounting points. Many of those seem to lack camber gain during articulation which is helpful to counteract the camber loss to the ground with body roll. It is this camber loss to the ground, and short links why the rock crawlers and fast trophy trucks run live/straight rear axles. (From what I saw, once you get to about 23* CV angle, the wheel scuff, or loss in tread width really takes a toll. 24" articulation in a IRS is good but not compared to 30-36" in a live axle. The double triangle linked live axle suspensions have been developed to minimize rear steer on uneven ground which was the "problem" with leaf sprung offroad rigs. The evolution of offroad racing in the West went from Bugs to leaf sprung trucks, to independent rear engine'd buggies, to truck bodied, front engine, rear double triangle linked straight rear axles with IFS fronts. There is more coming with 4wd IFS. ) (Yeh... I am long winded sometimes, as I want others to share ideas and thoughts)

 

[sierra4000]

Only quick observation, what rear toe lateral compliance?

Radek

 

[GregLocock]

I'd aim at zero bump steer, as a first guess. You'll probably find you want toe in in jounce, but not very much, say 1 degree per m. Rear axle bump steer is good for linear range understeer, but is bad for off road antics, it'll make the car darty on single wheel bumps.

Generally for a rear suspension (I have never come across an exception) you want everything behind the wheel to be as stiff as possible laterally, and then you can tune the stuff in front of the wheel to give you whatever lateral steer compliance you want.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[BrianPetersen]

Isn't this type of rear suspension prone to toe-out with motion either above or below whatever ride height has the lateral links horizontal? Seems to me that the links would pull the back of the big trailing arm in, and the front of it is at a fixed location, and that sounds like toe-out to me.

 

[GregLocock]

Brian I think you are right, but haven't checked it.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Isdtbower]

Yes. I am pretty sure all you guys are generally right about toe out and darting. Thanks for the comments.

If you go by the links, they move inboard about 1" at full range. This would relate to a little over 1* toe out. But if you look at the tire contact patch, it intentionally stays along the same track. The rear camber is pretty high also which tends to understeer and combat the rear toe out. So I am not sure. I made a full size moch-up to see what happens statically. The trailing arm is 51" long cl-to-cl to minimize any angles and radii. Most trailing or semi swing arms are mounted inboard, like the buggy above, at the front to combat the toe out. I can do that or just machine in the 1* toe in at the hub. Some have said that is not too much.??

Generally longer links have been beneficial in offroad as the drive is generally forward and not up and down into the frame. The anti's are affected by the centers rigid mounting. We have generally minimized the anti's, thinking we should have active shocks soon. Holding that back has been the extreme amount of fluid continually exchanged thru the "shocks." The thinking is to keep freedom of motion for the suspension over rough ground and not mechanically binding it when the tire goes from loose to tight.

It would be nice to get it right the first time. Milling two of these is going to be a lot of work. Not the outside, the lightening reliefs which are not in the moch-up. 7075, 3 and 4" slabs, 1.25" heims, 1.5" front wide spherical bearings, 12" lateral link separation, 40" - 150lb tire/wheel. 50% shock/spring and 60% bypass shock motion ratios.

 

[Isdtbower]

Toeing the arm in definitely "fixes" the toe out issue through travel. The exact answer will probably have to happen in live testing. In a similar build in the 70's I did make the front adjustable by moving spacers but basically used that for initial alignment and have never moved them since. That was 8" suspension. This one is 22", and bad things happen quickly at the far ends.

As GregLocock alluded, there is a ton of bendiing force on the arm just in front of where the hub mount meets the arm. I was actually hoping by moving the links forward it would help that...but still have the arm stiff enough to not bend the arm and hub on acceleration or braking behind that. When I clearance for the links, I'll have to take the post bending into consideration. Thanks for that tip.

"Generally for a rear suspension (I have never come across an exception) you want everything behind the wheel to be as stiff as possible laterally, and then you can tune the stuff in front of the wheel to give you whatever lateral steer compliance you want." I got it, and by moving that lower link forward things happen fast, but because I am using a QuickChange center diff. in the rear there just isn't real estate to do what I needed....And it does help to have a working racing example in the second post.

My real world experience started with a stock Corvette swingarm and unit bearing. It lived with 1966 hp of maybe 250. When I bumped it to 350hp, it didn't make it one trip and bent. The new fortified one again got bent, when the hp went to 450. Always in wet dunes with paddles and all the hp to one rear axle. A real fun Jeep that can sidehill anything. I use this example as knowledge what we are doing, among other things with that 22" suspension.

I was a good draftsman and easily created that way, but with CAD, I just couldn't visually get the hang of designing and it was just better to sketch and give the details to experts. I thought this would be easy...but now I want to minimize chaois and fit this part into the free Suspension Program 3D. I understand it has some limitations. Does anyone know if I can move links and toe?? Page 56 4.16 looks the closest. If not, it is into full simulation programming, and I learn nothing. Only "this works."

Again, Thanks for the comments.

http://www.susprog.com/images/Susp13a.jpg

 

[GregLocock]

For kinematics Susprog3d is fine, and I'm sure a relatively common architecture like your is included.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Isdtbower]

The Can Am X3 has a solution to the Toe issue.

 

[Tmoose]

"The Can Am X3 has a solution to the Toe issue."

And what looks to be a fabricated trailing arm.
Not as cool as one milled from billet, but maybe easier to love for development work.

 

[Isdtbower]

Here is a picture of one front IFS upright. Our first pass was a 1/4 scale printed part. That allowed me to re-design a little and to figure out how to hold the part for machining. Then I made a full size pressed wood part. We again made changes as I could break some areas. Then I got the 150lb 7075 chunk (15" x 12" x 7") onto the mill and about 20 set-ups later got to polish it. The upright takes a series 30 GKN CV (6"dia.), with a relatively small 35 spline axle. This allows us to get 20" of suspension and turn 39* and still be within 85" outside-of-tire. That took about 5 race cars built from CroMo to figure out the geometry...with no positive camber..except above 10* roll. Finished weight of the upright is 37lbs and represents about 200 hrs for two. Hand milled. Weight with brakes, unit bearings, etc is 90lbs.

I have just about completed the change drawings on the rear arm above and should be on the mill soon. "It's the adventure."

 

[Isdtbower]

I didn't realize that the toe plate and toe link on the CanAm X3 above took much of the wheel torque. They are running tires up to 33" with the extra weight of tire balls. They can instantly stop and recede when hitting any rock, etc. As a result, the aftermarket is making sturdier hubs and toe links. I was wondering how they got away with such a bent trailing arm.

 

[cibachrome]

" ... such a bent trailing arm..." They have to make it through a car wash rack.

 

[Isdtbower]

LOL ... Well they are pretty tough even for the above average driver on fairly tough terrain. Not your Sunday Golf Cart. The guys I key off of are the top 5 guys winning the brutal races. Not track. They are usually problem solvers first with help from small shops and then Racers, then "engineers." A knowledgeable engineer would probably also fix the Camber issues that you-all have worked on for years. (Or, I am missing something.) Photo credit unknown, but in a recent Offroad Expo handout magazine.

In defense to the photo and UTV. They seem to be stepping down from an off-camber rock ledge so the rear weight is not totally from a weight shift....like the front.