Need help designing a watt's link

[Motorcitydak]

I am building a car rite now using a 3 link with a billy strope design. The rear axle is a Moser circle track full floater. I need to build a way to locate it laterally so I want a watts link, just need some help with the details. I will be building the entire thing myself. The crank will be mounted on the axle itself to cut down of the overall weight of the setup.
How long should the crank be?
Where should the bolt for the crank be in relation to the center of the axle? Or is it better to orient it more towards the CG?
Should the links be equal lenght?
Should the links be horizontal at ride height or does that even matter?
Because I will mount the crank to the axle, it will have a few different height adjustments available but I still want to get it in the ball park.

[BrianP]

Click on the animated video on this wiki page:
http://en.wikipedia.org/wiki/Watt%27s_linkage
It'll show you how they're supposed to be designed.

[LowBuckX]

I would like to see pics of your 3 link. Mine is linked in my sig. Just would like to see what you came up with seeing we both used Billy as inspiration.


I am building a car rite now using a 3 link with a billy strope design. The rear axle is a Moser circle track full floater. I need to build a way to locate it laterally so I want a watts link, just need some help with the details. I will be building the entire thing myself. The crank will be mounted on the axle itself to cut down of the overall weight of the setup.
How long should the crank be?
Where should the bolt for the crank be in relation to the center of the axle? Or is it better to orient it more towards the CG?
Should the links be equal lenght?
Should the links be horizontal at ride height or does that even matter?
Because I will mount the crank to the axle, it will have a few different height adjustments available but I still want to get it in the ball park.

[silver69camaro]

I am building a car rite now using a 3 link with a billy strope design. The rear axle is a Moser circle track full floater. I need to build a way to locate it laterally so I want a watts link, just need some help with the details. I will be building the entire thing myself. The crank will be mounted on the axle itself to cut down of the overall weight of the setup.
How long should the crank be?
Where should the bolt for the crank be in relation to the center of the axle? Or is it better to orient it more towards the CG?
Should the links be equal lenght?
Should the links be horizontal at ride height or does that even matter?
Because I will mount the crank to the axle, it will have a few different height adjustments available but I still want to get it in the ball park.

I'll answer your questions in the order you asked:
1. Crank should be about 5 or 6 inches between bolt holes.
2. Crank pivot should be at the height that you believe the RC needs to be, at vehicle C/L obviously.
3. Yes, the links must be equal in length.
4. They do not need to be horizontal at ride height.

Hope that helps.

[MonzaRacer]

Also check out the Fays2 stuff very cool stuff if you can reach the guy.

[Motorcitydak]

Lowbuck, here is what I came up with

https://static1.pt-content.com/images/pt/2010/07/DSCF0711-1.jpg

Distance forward to IC 70
Tire radius 12.5
Axle ratio 3.50
Car weight 3000
Weight of axle 225
CG height 16
Wheel base 117
Anti-squat 60%

First link (Upper)
Distance forward from center 0
Vertical height from ground 18
Offset from center 5
Length 14

Left lower link (Second)
Distance forward from center 0
Vertical from ground 9
Offset -22
Length 22

Right Lower link (Third)
Distance forward from center 0
Vertical from ground 8
Offset 22
Length 22

And the results for the frame side mounts

Vertical from ground
First 15.59
Second 6.19
Third 8.45

IC Height 5.74

Link loads at 1G acc

First 2788 tension
Second 2469 Compression
Third 3073 Compression

I had to change my upper link to only 8 inches in length due to packaging issues. I know it will change pinion angles, but should not be too bad

[Motorcitydak]

Thanks for that info silver, it will for sure come in handy. How can I determine about where the roll center should be? Also, what does C/L stand for? Sorry Im still new to this stuff

I also found this site if it can help anyone else out there
http://www.spitzracing.com/index_files/Page1049.htm

I am leaning away from a fays2 because I do not want that big crossmember hanging out behind my axle. I know it weights about 25#, but to me it seems like an axle mounted setup will be lighter and take up less space.

[Norm Peterson]

For all practical purposes, the roll center will be at the main (central) pivot of the football.


Just to clarify about the links not needing to be horizontal with the car at static ride height - they do need to be inclined equally (and in opposite directions, obviously) as seen in rear view. IOW, 3° uphill from the WL football to both of your chassis side link pickups would be acceptable (if perhaps a little tougher to work with). There are limits, though, as the WL does not provide purely straight vertical motion over its entire range of motion.


If your LCAs have any plan view skew, you might want to consider that when setting the WL main pivot height. The side view slope of a line between the LCA convergence and the WL main pivot defines the roll steer.


Norm

[silver69camaro]

Just to clarify about the links not needing to be horizontal with the car at static ride height - they do need to be inclined equally (and in opposite directions, obviously) as seen in rear view.


What I always tell people is the vertical height difference between the chassis mounts must be equal to the span between the pivot bolt holes...I even see many professional builder's get that wrong.

[Norm Peterson]

Scary, Matt. Just plain scary.


Norm

[Twentyover]

Couple questions to add to the mix.

1) When the football is mounted to the axle, the distance between the roll center and the center of gravity will change as the axle moves vertically. The would change the roll angle as the axle articulates vertically, since it is a product of lateral acceleration and this distance (granted a simplification). A frame mounted football would hold this distance constant as the axle travels.

Is this noticable as a 'butt wiggle" as the car moves up and down in a corner? or is the problem in my mind?

2) Norm-

Thanks for the input on angle away from the football. I thought they needed to be parallel w/ each other, and could not figure out how to eliminate the transverse movement as the axle traveled vertically

[The WidowMaker]

What I always tell people is the vertical height difference between the chassis mounts must be equal to the span between the pivot bolt holes...I even see many professional builder's get that wrong.

i like that way of describing it. id shoot for chassis mount difference of 1/8-1/4" less than the bellcrank span if welding your mounts on solid. you can always turn (it gave me **** for the other word i used) the bellcrank to one side to make its vertical span less, but you can never get more.

id also mock the whole thing up with the body on when centering the rear with all necessary work done. my body is perfectly aligned with the frame when its also 1/8th off what the rear now measures as center. therefore my links are off 1/4 in order to get it centered. whether i was off or currie used up their +/- 1/8th when narrowing after the watts was built i dont know. im just glad i welded it all up and didnt make adjustable mounts........


I am leaning away from a fays2 because I do not want that big crossmember hanging out behind my axle. I know it weights about 25#, but to me it seems like an axle mounted setup will be lighter and take up less space.

25lbs is nothing and its in the best spot. but if your still worried, jim sells everything seperate. i would at least get the bellcrank and bushings from him.

[Roadbuster]

I am leaning away from a fays2 because I do not want that big crossmember hanging out behind my axle. I know it weights about 25#, but to me it seems like an axle mounted setup will be lighter and take up less space.

The fays2 bare crossmember weighs 10.8 pounds.

[JRouche]

I think the frame mounted center is lighter (unsprung weight) than the axle mounted center pivot. Just my opinion. I have built a watts link. But dont want to go into too much talk if you are set on an axle mounted system. JR

https://www.pro-touring.com/forum/showthread.php?t=55420&page=2