Watts link location

[fordsbyjay]

How do you determine a good starting point to mount a watts link?

[exwestracer]

It varies with the type of link suspension holding the axle in the car. Basically, mounting the rocker pivot at axle centerline height is a good starting point.

One key thing to make sure of: The outboard link mounts need to have the same vertical separation as the length of the rocker arm.

Your build link is dead, BTW...

[fordsbyjay]

Thanks for pointing out the glitch with my link. I fixed that up. Also thanks for the starting point. That's I will start.

[Bryce]

Jason,

I started with my pivot point at axle centerline, links are parallel at ride height or center of travel, the links are the same length. Also add as many pivot point height adjustments as you can.

[jamesinark]

It varies with the type of link suspension holding the axle in the car. Basically, mounting the rocker pivot at axle centerline height is a good starting point.

One key thing to make sure of: The outboard link mounts need to have the same vertical separation as the length of the rocker arm.

Your build link is dead, BTW...

instead of the outboard links having the same vertical separation as the rocker arm couldn't you use two panhard bar mounts mounted on the axle centerline and use something like a set of short offset J-bars with the curved section at the outboard position (mounted on the P/H bar mounts) to make essentially the same geometry and work the same way.I have a drawing for an example , but I can't seem to import the file.Ray you should be able to envision what I'm saying.It might be a little easier to route exhaust ,but I may be wrong.I drew it all up at work on their computer about 6 years ago.Let me know if you think it would work if you don't mind would you Ray?

[Norm Peterson]

No. That would force the rear to NEVER rise and fall in purely vertical fashion. Think up to the right/down to the left (or vice versa depending on which side was tied to the lower end of the WL rocker/propellor/football).

The behavior of any suspension link depends only on where the pivot points are that it connects to, not on its visible shape between them. In terms of suspension kinematics, a J-bar or even something as silly as this little sketch are geometrically the same as a straight link connected to the same locations. Just heavier and not (or not nearly) as rigid as the straight link would be.




What you need is for the two lateral WL links to be at mirror-image inclinations to each other (and obviously at different heights). The usual method of achieving this involves setting the links to be parallel to the axle (and the ground) with the car at its static ride height. Better yet would be with the driver's weight (or driver's weight plus 1/2 a passenger) simulated with something like barbell plates or bags of sand. Center of axle height for the WL main pivot is as good a place as any to start from unless you have a solid reason for choosing a different height.


Norm

[jamesinark]

I get what you're saying but i have seen more than one example of a watts link with this type of configuration as on this lakewood model for mustangs (Saleen site) and it appears that the outer links are both mounted on the axle centerline.Is there a reason that this would not be acceptable?Or is there an advantage for them to have the same vertical separation that the rocker would have?

[Bryce]

The links just need to be parallel at some point during the travel.

[Norm Peterson]

I get what you're saying but i have seen more than one example of a watts link with this type of configuration as on this lakewood model for mustangs (Saleen site) and it appears that the outer links are both mounted on the axle centerline.Is there a reason that this would not be acceptable?Or is there an advantage for them to have the same vertical separation that the rocker would have?

Consider what happens to the ends of the rocker in the top figure as the car and the WL frame move downward.

The right side link will pull its end of the rocker arm toward the right (relative to the axle) as the WL frame and the WL main pivot is lowered.

The left side link will push its end of the rocker arm also to the right at the same time.

The WL main pivot and hence its frame and the rear of the car have no choice but to also get shoved to the right. That puts you right back to having the same sort of deficiency that a PHB has, except that because the WL links are shorter than most PHBs and because they have to already be at a significant inclinations the effect will be considerably more severe than the ideal PHB case (PHB horizontal at static ride height with the driver aboard). Five or six times as bad, which begs a question about why anybody would choose to spend that much more money for that much greater complexity - and that much poorer behavior.

When the vertical separation is kept the same as the rocker, one link would pull to the right and the other would pull to the left, cancelling out the lateral movement at the WL main pivot. Straight up/straight down is what you want happening as you drive down the road and encounter a smooth bump that gently heaves your car up followed by gravity reminding everything who's still the boss.

Yes, this also means that when the vertical separation is kept the same as the rocker, if you hit said bump in mid-corner with a couple or three degrees of roll present, the rear of the car won't move in precisely vertical fashion. But the sine of 3° is pretty small and the more or less steady state cornering behavior of the car is already being disturbed.



I can think of two unique situations where making the end pivot heights about the same probably doesn't matter. There might be one or two others, but they would also be for narrowly defined situations.

If your suspension were to be so rigid that it can hardly move at all, the tiny amount of WL-caused lateral migration that would then occur isn't going to matter much.

Alternatively, if your suspension has 100% antisquat and neither squats nor rises on launch and nothing else is of interest, there still isn't going to be any lateral movement (I suspect that this is where the vast majority of Lakewood's customer base lives).


Norm

[JRouche]

WOW!! A lot of great watts link talk. Ill be honest, some of it was over my head.

I have a watts link in my car, I made it. I did buy the center pivot from Jim at Fays. The mounts and links I made.

What I have come to understand is the links dont need to be parallel to anything. They can be made in any shape that will work. They dont have to be straight bars. A "U" shaped bar will work but it better be of sufficient size to reduce bending of the bar. Thats why straight bars work. They are strong and resist bending.

The point to point measurements for the ends of the bars need to be exactly the same LENGTH though, hard to do with some oddly bent bars..

Ummm the center pivot should be as close to vertical for ride height though to get all the travel you can get.

To get the most travel and rear end control the watts should have the center pivot set so its at vertical at ride height, and the links would be parallel. BUT!! Thats one of the good points for the watts link though. The center pivot can be clocked over some and the link bars can be tilted and NOT parallel. As long as the clearance is there the side to side control will be just as accurate.




The center pivot does NOT need to be centered top to bottom either, thats just an ideal point to get the most travel. The center pivot can be at any position (angle). But the more you go off up and down the more you limit the overall travel. It does NOT affect the overall control. And the links can be tilted, all you lose is travel, it will wrap the links up and bind.

You can have a center pivot clocked over 20 degrees and have the links at parallel. It will still work but the suspension travel will be lower. You dont want to pull the center pivot over to horizontal. As long as the center pivot is in motion it is good to go, but again, ideally you want the center up and down at ride height.. And the links parallel. Best control for overall suspension travel..

And knowing that, the vertical distance for the link mounts also need to be the same on both sides. Thats the KEY. The length of the pivot points.

If the vertical points are diff. Say one is three inches long and the other is four inches long you can see that they will pull or push the links at a diff rate. NOT good. The ends of the links rotate on a point. If one point is longer from center line then it will rotate slower, a perceived longer link. You just screwed up the watts link idea, both links NEED to be the same length. So the pivot points of the out board mounts need to be the same.

Now you can change the whole thing and use unequal bars, various end link mounts but you better have yer math skills with you. JR

[exwestracer]

If the vertical points are diff. Say one is three inches long and the other is four inches long you can see that they will pull or push the links at a diff rate. NOT good. The ends of the links rotate on a point. If one point is longer from center line then it will rotate slower, a perceived longer link. You just screwed up the watts link idea, both links NEED to be the same length. So the pivot points of the out board mounts need to be the same.

And that's ALL that really matters. As long as the ends are the same vertical distance apart as the rocker length, the links don't have to be level or parallel. They DO need to be the same length...unless you build an offset Watts like Mazda did. The less said about that, the better...

[jamesinark]

that is all I was saying .I could see it in my mind as well as well as on paper and it didn't seem to matter if the outboard links were the same distance apart VERTICALLY as long as they were the same length and aere at a parallel position.The only reason I used J-bars as an exampleis that it seemed to get around the having to have points the same vertical distance apart and then I realized that they would work better if they were straight.I'm glad to see I was not totally off in my thinking.Thanks to all of you for your input and taking the time to answer what may have seemed to some as inane and monotonous.Thanks again.