Watt's Link Rear End - Do they "jack"

[Frank]

Pretty Simple question this time.

Will a watts link rear end (roll center above ground obviously), under lateral acceleration, experience "jacking"?

[Frank]

Pretty Simple question this time.

Will a watts link rear end (roll center above ground obviously), under lateral acceleration, experience "jacking"?

[js10coastr]

unless I'm overlooking something... and it's possible since you boys down under are more familiar with the watts links in the V8 Supercars...

yes...jacking is present whenever the roll center is above the ground.

[rjwoods77]

I cant speak to that so much but all i can say is if you are talking about lateral stabilizers for a solid rear I would look no where else than a mumford link.

[Dan B]

There is no jacking with a solid rear axle unless you run something like a steeply sloping Panhard (in which case you will have +/- jacking depending on the direction of the turn). But you shouldn't take my word for it. Draw the FBD!

[Frank]

my FBD says no jacking with a solid axle and watts.

Any V8 experienced people here with knowledge of this concept?

Do any of the suspension texts mention it? Claude notes?

[Mike Flitcraft]

http://totallypolished.com/watt's_link_kits.shtml

That may help out quite a bit. If the link doesn't work, go to www.totallypolished.com, (http://www.totallypolished.com,) click to enter, and click on Watt's Link on the left, they have a video of a truck on hydraulics showing the alignment range, and then a video of one installed while driving down the road.

[js10coastr]

hold on... it's all coming back to me now (that and I checked Milliken).

Frank... in your FBD, are your trailing arms parallel to each other? If they are then the instant center for each wheel reaches out to infinity and the roll center is kinematically (you have to say this in your head with the Claude Rouelle accent "keen-eh-MAT-eh-tuhk-ooly") on the ground/doesn't exist... which means that for the suspension to work in roll, something has to bend or twist (either the axle housing or the trailing arms).

If the arms are not parallel, then the ICs aren't at infinity (hopefully at the centerline of the chassis)... and then the RC is above the ground and "here we go again" (once again in the CL voice)... draw the two force members and viola, jacking!

reference p 622-624 and p 647-650 RCVD

[Frank]

js10

im talking about solid rear axle in roll (lateral acceleration).

the RC is at the pivot of the Watts link

[Wesley]

I'm not a suspension guy, but would not adding a Watts linkage decrease total jacking force by keeping the roll center at the pivot location (and closer to the contact patch,) whereas without the linkage jacking forces would remain constant?

Sorry, I'm a newb to suspension.

[Brian Evans]

what I think of as jacking forces occur anytime the roll center isn't at ground. A solid rear axle with no lateral location other than the springs has a roll center at ground. You can control lateral location with a panhard rod, or a Watt's linkage, or a Mumford Link or variations on these... Mumford link can be arranged so the roll center is just about anywhere, including below ground, from what I've read.

Brian

[Frank]

My free body diagram, and a FEA study (geometry creation methods, not auto meshing) both suggest there is no jacking.

Jacking happens in swing axle / independent suspensions because the wheels can be pushed together.

I'm almost 100% certain now that a solid axle / Watts setup does not jack in lateral accelerations.

The fact that the RC is fixed at the Watts pivot (usually quite high off the ground) means this hardware would jack like crazy (if the geometry actually caused jacking as we see with independent suspensions).

I guess the proof is data. Look at rear ride height (averaged left and right side heights) in a relatively slow (areo neglected) sweeping corner.

I'm guessing that because there is no jacking; there are only two differences between raising rear roll center height and using a stiffer rear ARB

First Difference: Roll damping ratio differences. Raising rear roll center means higher roll damping ratio. Stiffer rear ARB means lower roll damping ratio.

Second Difference: (Slightly) Differing roll rate(s) more or less depending upon the actual numbers

[js10coastr]

"Solid axles have lateral-force to vertical-force coupling, similar to the "jacking" effect that occurs on an independent suspension, but it cannot be determined as simply as is conventionally done with independents (by looking at the roll center height)."

P 622 RCVD

"As an aside, a lateral force applied at the roll center will, however, roll the unsprung mass (axle) on the tires unless the point of force application is at ground level; this "axle roll" does not occur with independent suspensions."

P 623 RCVD

For beam axle suspensions keep in mind that we really need the roll axis and not just the roll center height.

From reading the section on panhard bars and applying that to a watts linkage the roll center height of a watts linkage is at (about) the height of the center of the linkage... BUT it's longitudinal location is on the rear axle centerline... in space. And so (from my understanding) applying the forces to the center of the watts linkage is incorrect.

I don't claim to be an expert on the subject, but this is my interpretation of what I've read, applied and studied. I can say for certain that with parallel trailing arms, the watts linkage does not work kinematically in roll. I think the argument needs to go back to the definition and determination of a roll center.

[Frank]

Milikin is confused

[ad]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Frank:
Milikin is confused </div></BLOCKQUOTE>

Thats a balsy statement. Or pure idiocy http://fsae.com/groupee_common/emoticons/icon_smile.gif

[js10coastr]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by ad:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Frank:
Milikin is confused </div></BLOCKQUOTE>

Thats a balsy statement. Or pure idiocy http://fsae.com/groupee_common/emoticons/icon_smile.gif </div></BLOCKQUOTE>

I move for idiocy... partly because Terry Satchell wrote the suspension chapter.

[mtg]

I heard somewhere that if you try to model a Watt's Link suspension in Matlab, it explodes when you run the simulation as that suspension type requires compliance in the axle to move.....js10??? Ha ha ha.

[js10coastr]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by mtg:
I heard somewhere that if you try to model a Watt's Link suspension in Matlab, it explodes when you run the simulation as that suspension type requires compliance in the axle to move.....js10??? Ha ha ha. </div></BLOCKQUOTE>

...like shooting in the dark.

[Frank]

ive been doing non-linear static analysis with non-linear geometry..

the jacking forces imparted onto the trailing arms do exist, but they are likely very small forces with reasonably long trailing arms.. the forces i was seeing were actually "anti-jacking" for parallel and horizontal trailing arms

thanks to George Commins for noting the predominant source of jacking with watts links.. it happens as the tire compresses (loaded side) and de-compresses (unloaded side) during lateral G's.. the axle "rolls" and the links from the axle to the Watt's bellcrank assume almost the same angle as the axle.. this does cause a SMALL jacking force to be imparted on the sprung mass...

considering engineering measurement accuracy, (especially when determining motion ratios) I think it's safe enough to say that a well designed Watt's link rear end essentially (considering the height of the RC, and the effect this would have on an IRS) does NOT jack.

If the trailing arms are paralell, complience is not required

http://www.uq.edu.au/fsae/spreadsheet.xls