How do I measure for a 4-link?

[efingstein]

I really wanting to add a 4-link and bags to the rear of my 51 Merc. Can anybody give me either the basic or detailed reference points so I can get an idea as to how much work this entails? thanks

[enjenjo]

That is kind of like asking how long is a rope. There are many correct answers. There is an optimum length for racing with non symetical upper and lower bars. for the street the lengths and mounting positions may be different. If you are using bags, the bars are generall symetrical, same length top and bottom, although on the Mercurys I have done it's hard to find room to do that. I can also tell you on a Merc, you can't lay frame without major modifications to the X member, and a serious C over the rear axle. Also you have to raise the driveshaft tunnel. If you use parallel bars, you will need a panhard bar, if you angle the top bars in, you can get by without it.

So what do you want to do, and I can get more specific.

[efingstein]

Thank you enjenjo. that's the point in the right direction I was looking for. I will be using an equal length parallel system. I do not want to lay frame (personally, I've never understood that), but, I do want to get down for slow cruising. I hadn't even though of raising the drive shaft tunnel. I did figure on using the panhard bar though.

[enjenjo]

You going with a kit, or building your own?

[efingstein]

I'm looking at either the airride ARS14500 seen here,
http://www.ridetech.com/productinfo/air4link_parallel.asp
or looking into getting a friend from ark city to help me with it. I'm not to good at the measuring and guzintas. I'd like to have a watchful eye over my shoulder.

[enjenjo]

Well let us know when you decide. A kit is pretty self explainitory, but If you do it yourself, there are some guidelines.

[38racing]

I am considering building my own for a 29 A sedan and would like to know some guidelines.

[enjenjo]

I am considering building my own for a 29 A sedan and would like to know some guidelines.

On a short wheelbase car like an A, you want to mount the bags, or springs, behind the rear axle if at all possible. This makes the suspension act as if it has a longer whelbase and gives a better, less choppy ride.

For the lower bars, close to optimum for traction is 1/3 the wheelbase, and just below a line from the rear contact patch to the instant center  for that car. The instant center is the point that the car would balance, and is generally close to the back of the camshaft in both height, and front to rear.

For a parallel 4 link, the upper bars should of course be parallel with the bottom bar, at least in plan view, but can be angled in at the front to eliminate the need for a Panhard bar, or Watts linkage.

For maximum traction, the upper bar should be adjustable from just above the instant center, to just below it, and about 2/3 the length of the lower bars.

This is just a general guideline, and there have been several books written on the subject. There are other things to consider that I haven't even mentioned, but this will get you started.

For street use, don't use rod ends, incorporate urethane or rubber bushings. The rod ends will wear enough to start rattling in as little as 100 miles. At this point they can also cause handling problems.

[38racing]

Thanks, from what I've seen A's the bars are about 22 in long. That's probably because a longer bar would put the mounting brackets too far ahead and interfere with other parts e.g saddle tanks. You mention having the upper bar point in to eliminate panhard bar. Again what I have seen so far is that for this triangulated setup they mount  top bar closer to centre on the axle and point it outward to mount on the inside of the frame rail. Apparently this can be a problem on GM rears as the centre housing is cast and presents welding issues for the bracket. To point in as you say I see using a cross member to run the bar to and of course then the top bar would mount outward on the axle closer to the bottom bar. I am becoming more interested in the triangulated as my setup has a wrap behind rear cross member on a 3 in. kickup, narrowed frame and I don't see easily where I can run the panhard bar. I have read also , need to followup where in the paralllel 4 bar the role of panhard can be done by a bar going from rear bar connection of right side to forward bar connection on left side. I can see how that creates 2 triangles that should oppose side movement of the axle in both directions.

[enjenjo]

a bar going from rear bar connection of right side to forward bar connection on left side

That is commonly know as a track bar, and is used on a lot of race cars. It does work, but not as well as a Panhard bar. The problem with a track bar, it is indirectly fastened to the rear end, and typically has a rod end on each end. So figuring in the small amount of play in the rod ends, and the flex in the brackets, it still allows the rear end to move side to side somewhat. Not much, but enough to make the car a bit ill handling, particularly when decelerating. If you watch at the drags, you will notice many of the cars jumping back and forth as they slow down, this is what causes it.

Angled in, or angled out, the upper bars will work just as well to control side movement. Some GM rear ends already have bosses for mounting the upper bars on them. If not you can mount the bars on the axle tubes, angle them in, and fasten the front to a crossmember with a loop in the center for driveshaft clearance.

Stock GM 4 bar rears have lower bars about 19" long, and upper bars about 12 to 14 inches.

[Fat Cat]

If you watch at the drags, you will notice many of the cars jumping back and forth as they slow down, this is what causes it.

And that is not a fun thing to experience on the street. Especially when you are coming down from 140MPH two thirds the way down a 1 mile long road and you have to feather it out to keep from hopping sideways.

Don't ask how I know.  

[38racing]

I  do have the G body rearend with the upper arm brackets cast on the housing but my thought was that they are too high which would result in the top bars having to point down to have front ends bracketed to side frame rail. Also they would be quit short in length  because of the angle of the brackets on rear end.

[enjenjo]

Actually, for optimum traction, the G body mounts are too low. If you can get the upper bars at least 16" long, they should work fine with 22" lower bars.

[Crosley.In.AZ]

interesting stuff.

I need to build / buy a rear suspension setup for my 1962 Morris Minor that will receive the full cage & chassis under it.