Are Reversing 4 link bars good? 2

[nmorel]

I have a few questions that in another forum

http://streetsourcemag.com

is an extremely heated arguement between this is ok, and not ok to install and daily drive with.

In the custom truck world using air ride (air bags) and laying the truck on the ground and lifting up for driving. The fad in the past 5-10 years have been using 4 links with the top 2 bars triangulated "V". Now the issue is due to the gas tank being in the way with the top bars, so the easy way out has been simply facing the top bars backwards on the frame. Meaning they mount where the spare tire used to be. The air bags are mounted on each side of the solid axle. Which more and more are doing this due to simplicity and not having to move the factory gas tank. What are the pros and cons to this setup and what saftey measures are gained or loss from this type of setup?

The other question is the same as above, but reversing all 4 bars. What are the pros and cons and saftey measures gained or loss from this for a normal daily driving setup?

There are alot of people that swear to these reversing bars setups for years with around 50K+ miles driving from coast to coast and no problems. Stating that by the book the design has alot of flaws and should not work, but they do work, and more and more people are doing them... By designers and engineers what are your ideas, points, and explinations to these types of setups?

 

[Dave K]

Half-reversed 4-links will induce pinon angle change, because any suspension travel will cause the diff to rotate.

Reversed 4-links put the link bars in more compressive loads, when hitting bumps, curbs, etc. I would think under acceleration, the torque would tend to lift the rear of the truck too. Instant center could be really odd.

I think of a reversed 4-link, like trying to push a rope.

-Dave

http://www.moslerauto.com

 

[nmorel]

In addition to my previous questions what is opions on having around 20" of suspension movement with the above 4 link designs? I'm not talking potential movement, I'm talking actual movement on daily driven trucks by incorporating the air bags on the bars (after being reinforsed ofcourse) to provide a leverage enhanced air bag setup. Most people like to call these "cantilever" setups.
This is to provide enough lift with channeled/body dropped trucks to lay flat on the ground with large wheels, and have enough lift to clear the wheels for daily driving. So what would this gain and loose with the half reversed bars and full reversed bars?

 

[crisc]

im one of the guys from streetsourcemag.com. there is no need to ask about 20" of lift. we are only asking the advice of people who are not in the custom truck world their opinion on fully reversed triangulated 4 link suspension setups. we have multiple examples of fully reversed setups working day in and day out with no apparent braking, accelerating, handling, traction or driveline problems. take note these setups are all on small and midsize trucks with relatively low power that lay frame and body on the ground. apparently to some at this website having a truck perform well for 60k miles+ is still not good enough proof. rather than pointing out real life situations, they just spout out "a fully reversed 4 link is not in a suspension book."

 

[nmorel]

You're absolutely right, I agree with you 100% on everything you mentioned crisc. Yes there is no publication on reversing 4 links and fully reversing all 4 bars. Maybe due to there has not been enough research for someone to make sometype of suspension book on these crazy setups that custom truck builders make? These 4 link designs have really only been out for less then 10 years, and within 5 years with the bags being placed on the 4 link bars. Yes majority of the trucks are mini and midsize, but more and more there are full size to dullies doing this also AND TOWING trailers with these setups!!! Since there is no such literature on these setups, I'm curious as to what this site/forum thread would have to offer as far as advise physically, theoredically, and what forces and physics are acting on what parts of the suspension being reversed and is it really as safe as "everyone's doing it" says? So far only gotten one response. Never seen any manufactures or any types of other vehicles besides custom trucks run these types of reversing setups... Is it because taking short cuts, and by luck of the draw it works, or is it really wise desision to reverse the bars other than to keep the stock gas tank location?

 

[patprimmer]

If everybody doing it is evidence that it's safe, I guess smokeing is safe.

Do the drawings, study the arcs and the effects of suspension movement on wheel, axle and pinion motion and alignment.

Consider the direction and magnitude of the forces involved and run the numbers.

eng-tips, by professional engineers for professional engineers

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Dave K]

Half-reversed is obviously bad, the pinion angle change defeats one of the benefits of a 4-link, esp. when you try to subject it to 20" of lift. I don't think there is much to debate on this one.


A reversed 4-link...look at the instant center, its someplace behind the truck.

Try this once...take a push broom, the handle represents the 4-link bars, and the broom head is the axle. Now pull it around the room, and over some bumps, even over your couch. See how it follows you? Now put the handle against your stomach, and push it around the room, and over that couch. See how when it hit the couch, it transferred the load back into your stomach, aka frame? it also put the handle/bars into compression. On a car/truck, these compression loads are going to want to push that axle to the side.

A reversed 4-link also pulls the axle away from the transmission as the suspension is lifted, which slides the drive shaft farther out of the tranny, and if it goes too far...

I suspect the reason you don't see much literature on these setups, is because they were found to be inferior, not because its new and revolutionary. In my opinion, even though it may work, its far from the best way to do it.

-Dave

http://www.moslerauto.com

 

[NormPeterson]

I suspect that some of the specific reasons are structural in nature. It's one thing to place the lower bars in compression under acceleration, where the maximum loads are essentially limited by available traction. Quite a different matter to put the lowers in compression from crashing into a pothole at speed. Perhaps this is less of a problem for aftermarket bars, which are generally closed section giving improved buckling resistance over the normal OE open-section (channel, or modified channel). Of course, this mostly means that larger loads can be passed on to the frame brackets and frame rails . . .

Another issue is the rigidity of the chassis as measured between the front and rear suspension linkage attachment points. Having the rear bar chassis side attachment points aft of the axle means some extra length of frame rail and added flexibility due to the bends necessary to clear the axle.

To get the same geometry would likely involve longer (read: either less rigid or heavier) brackets for the lowers, which in turn increases the load to the frame rails themselves.

It's not that you can't get it to work; obviously it can physically be done. It's just that there are more efficient solutions available. And isn't that one of the hallmarks of good engineering?

Norm

 

[SusTestEng]

1/2 reversed is not that bad of an idea. Basically, you end up with a Watt's linkage. I have dealt with this suspension setup in a Dirt Late Model race car. Basically, to help out the pinnion rotation you need to adjust the lengths of the links. The lower link faces forward, while the top link goes to the rear of the car, but the rear facing link is about 2 times the length of the lower forward facing link. This helps alot with pinnion rotation.

I think most have covered the problems of all 4 links facing rearward. Although I think I've seen this concept used in the front axle on old race cars(like old Indy 500 cars).

 

[patprimmer]

Also, if you are only running it at the same height, the pinion angle should be straight at the running height. If it is bad while dropped to the ground, but the truck is never driven in this state, I see no problem, proveded it does not actually bind while being lowered

Regards
pat

eng-tips, by professional engineers for professional engineers

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[patprimmer]

Gramatical error.

I meant if you always run it at full height (20&quot.

Otherwise it should run straight about halfway between the min and max run heights considering travel while running.

You should also consider that it should be as straight as possible in the most common drive position.

On the above two points, you will need to set a compromise

Regards
pat

eng-tips, by professional engineers for professional engineers

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[NormPeterson]

Actually, I seem to recall that either the Dodge Challenger or Plymouth 'Cuda Trans-Am teams of the late 60's used a 1/2 reversed arrangement. It was described in some contemporary magazine article as being essentially a Watts link on both sides of the axle, though I don't remember whether it was triangulated or if it required a separate lateral constraint. The rest of my recollection was that either it never got fully sorted or it just didn't work well enough in wheel to wheel competition before being discarded in favor of a more conventional arrangement.

Norm

 

[WickedSlammy]

Well the pinion angle issue isnt really the concern we have.Most people realize that aslong as the pinion angle is set at their ride height and every time it is driven, it is returned to that ride height it will be fine. We are questioning what happens under braking and handling at highway speeds and how it can be dangerous. Look at the bar angles and taken into consideration the rotation of the axle, it seems that a fully reversed setup will try and actually lift the rear axle off the ground which will increase braking distance. Whereas a all forward setup actually pushes the wheels into the ground to aid in braking.So looking at it this way, what are you guys thoughts?

 

[carnage1]

a lot of these thing depend a lot on the lengths and angles of the various bars
The thing I keep in mind is just because you can get away with it does not always make it the best soultion.
how many low riders have you seen with terrible suspension geometry and bad bump steer?
all suspension designs are a compromise but I wouldn't mount my bars based on the location of the fuel tank, decide what will work best and truely customise it.