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2-link rear suspension

thread800-89736

2-link rear suspension

2-link rear suspension

(OP)
I've been involved in a "discussion" about using a 2-link suspension on a street driven truck.  Simply put, they are remving the leaf springs, and using 2 paralell rectangular bars, fixed to the axle on one end, and with bushings at the forward (chassis) end.  It's even been claimed that if the bushing ends are tight enough, a panhard bar isn't needed.

I find flaws in the design, which I will list below, but I'm being told that I am totally off base.  This by employees of, and people that have had this work done by various "custom" shops.

1)  A lack of side to side articulation, resulting in stressed link members, and stresses being transferred to the chassis.  This is increased by not using a panhard bar is due to stresses when cornering.

2)  Since this 2-link doesn't allow any rear suspension compliance, I think the rearend would tend to be "loose", and cause oversteer.

3)  A 2-link like this...wouldn't have any real roll center, since it can't pivot about anything, except what results from link and chassis twisting.

Am I off base on any of this?  I can't find any reference to a setup like this in any of my books, and I think there is a reason for that.

Thanks,
 Dave

-Dave
http://www.moslerauto.com
"Everything should be designed as simple as possible, but not simplier"

RE: 2-link rear suspension

What you are describing is essentially a Ladder Bar setup, less the Panhard and Heims at the rear end? If so, I believe your points are on target. I sure wouldnt want to drive it very far.

My best advice? Find another custom shop.

RE: 2-link rear suspension

(OP)
"What you are describing is essentially a Ladder Bar setup, less the Panhard and Heims at the rear end?"

Exactly.

-Dave
http://www.moslerauto.com
"Everything should be designed as simple as possible, but not simplier"

RE: 2-link rear suspension

Now, if they were to replace the solid rear connection to the axle housing with a bushing, ON THE LEFT SIDE ONLY, and add a Panhard, they'd have the equivalent of an asymmetric 3link, similar to that which was used on the Jaguar C-Type. If properly done, this could cancel driveshaft torque, providing equal rear tire loading during acceleration.

RE: 2-link rear suspension

This suspension should fit well with the other work that I sure was done to the truck first. Most of the bump travel wased used to lower the truck to where the body kit scraps on the ground. Then the springs rates were chossen so that the suspension wouldn't move.

As Carroll Smith said. It doesn't matter how bad the suspension is if it doesn't move.

I have seen this same setup before. It was used on a bagage tractor used at airports. A solid rear axle was bolted at both ends onto a huge triangular 1.5" thick steel plate. At the point of the triangle was a 8" long pivot with rubber busings. The front end had a kind of semi trailing arm setup. Things were so bad that the last thing they atempeted to make this work was to make the front a duplicate of the rear.

ProEpro
www.whitelightdesign.com

Pro/E FAQ www.whitelightdesign.com/servicestips.htm

RE: 2-link rear suspension

(OP)
"This suspension should fit well with the other work that I sure was done to the truck first. Most of the bump travel wased used to lower the truck to where the body kit scraps on the ground. Then the springs rates were chossen so that the suspension wouldn't move."

These trucks in particular are on airbags(aka air springs), so the frame, etc. has been notched/reworked to allow the frame to lay on the ground when the bags are deflated, but can provide 6+ inchs of lift when the bags are inflated.

-Dave
http://www.moslerauto.com
"Everything should be designed as simple as possible, but not simplier"

RE: 2-link rear suspension

Quote:

1)  A lack of side to side articulation, resulting in stressed link members, and stresses being transferred to the chassis.  This is increased by not using a panhard bar is due to stresses when cornering.

     Supporting lateral loads by bending & twisting the rectangular tubes, essentially cantilevered @ the chassis... couldn't think if a much worse situation.

Quote:

2)  Since this 2-link doesn't allow any rear suspension compliance, I think the rearend would tend to be "loose", and cause oversteer.

3)  A 2-link like this...wouldn't have any real roll center, since it can't pivot about anything, except what results from link and chassis twisting.

     Pt. #2 emphasizes that the rear roll stiffness is essentially infinite, I would say you're probably right about the oversteer.
     Pt. #3 --> agreed, how can something that cannot rotate relative to something else have a roll center... if you think in terms of forces, I think you could technically say that the lateral forces are fed into the chassis @ the "bush" mounts, but there will always be an accompanying moment @ chassis mounts as well,due to the zero roll compliance condition, that exactly cancels the moment created on the chassis by the lateral forces... so that all means that the roll center is basically always @ the ground (b/c of zero rear roll compliance) and all of the load transfer is taken @ the rear... sure does simplify load transfer analysis... of course everything will bend and twist such that there will be some finite rear roll stiffness, but I think you are right about why you haven't seen this anywhere else.  

RE: 2-link rear suspension

I don't see much hope of making a great suspension out of ths, but you might want to check out twist beam rear suspensions as used in many front wheel drive cars.

These use two drag links connected by a cross tube that provides the roll stiffness and some semblance of structure between the two arms. There is no inherent reason why this could not be used with a live rear axle - I have seen it proposed for a RWD car, albeit with a DeDion diff.

Designing the cross tube is rather tricky, I know of one car whose launch was delayed for more than a year because of durability problems with the welds, but if you inspect it regularly then you can just weld it up again.

The longitudinal position of the cross tube is very important, further forward makes life simpler, I think.



Cheers

Greg Locock

RE: 2-link rear suspension

are you referring to the "truck-arm" suspension that Winston Cup cars use?

if so, i can't add anything except that a lot of people really like them for cornering on high performance street cars.  the most common remark is that they hook sooner coming out of the corner.

RE: 2-link rear suspension

Yes, that is pretty much a twist beam suspension. Using the axle as a torsion/lateral stiffenening member is a bit ugly, but by tuning the compliance f its mounting system it can be made to work.

Designing that joint is going to be a bit more complicated than just drilling some holes in a plate and chucking a U bolt through it. Note that your lateral compliance is probably going to be controlled by the integrity of that joint, EVERY compliance in that suspension will give you compliance oversteer - a big no-no.

Cheers

Greg Locock

RE: 2-link rear suspension

http://www.hotrodstohell.net/cars_from_hell/P9270022.jpg

that's a kit sold by hotrodstohell, but the geometry is way off from what you show there.  i've read that angling the front links together brings the binding/torsion down to an acceptable level.

i haven't found any good information about this design, though, so it's all just guesses that i've seen.

RE: 2-link rear suspension

Dave - Linkee no workee.

crashbox - Converging the lower arms like that makes the axle steer less dependent on the angle of the arms as seen in side view, meaning that it will vary less over your suspension travel.  In addition, the "anti's" also vary less with suspension travel, and the SVSA length remains essentially fixed.

Bushing stiffness(es!) is/are still important tuning elements (I think that some racing bodies have some requirements here), and you'll notice that the arms are torsionally flexible open cross-section (normally an "I" - shape) as an additional means of reducing the overall torsional rigidity between the axle and the chassis while maintaining the necessary bending rigidity to resist axle torque reactions.

Norm

RE: 2-link rear suspension

That one worked.  Thanks, Dave.

Leaving it in its 2-bar configuration, I'm trying to imagine how to get sufficient lateral restraint out of the bushings without introducing a large amount of torsion in the two arms in roll (oval and voided?).  Without a PHB or Watts link, the arms now need to be rigid about both bending axes, the result being either torsionally rigid closed sections or fussy cruciform conglomerations of "T" shapes.   Never mind the structural considerations of strength, wear, and fatigue for the chassis side brackets and dealing with the concentrated loads against the axle tubes (for which read: thicker wall, heavier, and all unsprung).  Seems to me that using a PHB would be easier and more reliable assuming that it could actually be tuned to drive acceptably well without one.  That brings it back to being a unique truckarm configuration by virtue of the parallel arms and a plan view arm axes intersection point that's tending towards infinity ahead of the vehicle.

Norm

RE: 2-link rear suspension

Just finished playing, on a spreadsheet, with the asymmetric version of this 2link (see my earlier post in this thread) and the results were quite encouraging. I used reasonable location dimensions for a "typical" car and then diddled with the numbers until I got no rise or squat and equal rear tire loading on acceleration. Yes, I took unsprung mass into consideration.

Would have liked to solve explicitly for, say, the left link angle, but I got bogged down in the algebra. Yeah, it's just high school algebra manipulation, but there's a point where I just throw up my hands and say it's not worth the effort. I'm sure I spent less time with the spreadsheet.

RE: 2-link rear suspension

supercustoms are my biggest area of interest.
I have seen a lot of creative ideas for reworking suspension. some good some bad. If you throw enough math at it I think you could get this system to work but I don't think it is going to leave you much room for error or change. if you were building a truck specificly for drag racing and shows (or a trailer queen) then it would probibly be fine but for cornering and daily driving I would be uneasy with it.

also the picture makes it look like the whole setup is behind the rear wheel. so when you mash on the brake pedal the back end would lift like crazy.

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