Coil over rear suspension options

[vujade]

Good point Shad

[Alex]

A good way to save money is to get the coilover and take off the pressure of the torsion bar but leave it in for support. This way you will not have the problem Shad mentioned.

Alex

[Sandeep]

Could you not just adjust the torsion bar so the car was really 'slammed', and then install the coilover to take up the ride height ?

After re-reading Wally's post, I beleive this has been addressed... but could you not lower it more than 1 notch ?

Sandeep

[Alex]

What you do is to take the torsion bar out and put it back without pressure....which means the car will be slammed without coilovers. This way there are just used to support the spring plates.

Alex

[vwlownslo]

I kinda doubt there would be any change in the spring plates behavior (other than the obvious) when the torsion bar is removed, providing the bushings are in place and the bolts are properly tightened... I mean really there isn't really any room for play (other then up or down) and besides there would have to be less play (side to side) then a Heim joint!

But, if the eagle "kit" seems worth $200 bucks go for it... (although the same could be easily fabricated for about $60 CDN)

[ricola]

There is also the suggestion that putting the torsion bars in at a relaxed position will make a difference.
They are springs which work with angular movement, their spring rate is near enough constant in the range that they are used so all you would be doing is transferring more of the load through the coil-over rather than the torsion bar. The effective suspension rate will be the sum of both components. The only benefit of this is easier adjustment for setting corner weights by adjusting the coil-overs...

I have used aftermarket 944 'bushings', actually quite a hard aftermarket replacement I got from Pelican Parts. Mine is a road car so I still want an amount of compliance and this has worked perfectly for me...

Rich

[RonR]

I am currently in the process of converting my IRS to coil-over. Even though I’m not completely convinced that just taking out the torsion bars and installing new neoprene bushings won’t work just fine, I purchased the Eagle Products’ conversion kit. This seems to be a well designed kit and the quality is excellent. As for as the price, I think it’s reasonable considering how expensive a pair of heim joints are. Unfortunately, I was unable to use the kit because of wheel clearance problems. Therefore, I had to build my own setup where the heim joint bolts on the inside of the mounting plate, rather on the outside. I’m pretty happy with the way they turned out.

Shad, I’m curious about your comment that the one of the spring plate's function is to maintain toe. I’m not sure exactly what you meant by that. It appears to me that the adjustment for toe is through the placement of the washers on the inner pivot bolt. Also, unless you have a lot of slop in the inner bushings, you can’t substantially change toe by adjustment of the spring plate. It seems to me that you adjust the spring plate to accommodate the position of the suspension arm based on the placement of the aforementioned washers. The spring plate would then help support the arm and prevent a change in toe due to wear in the inner bushings. Is that what you meant? I’d be concerned that if you try to adjust toe with the spring plates, you could put the arm in a bind that could cause premature wear of the bushings or worse. Does this make sense? Bottom line, I think the Eagle Products’ conversion will work just fine if adjusted properly.

Also, I think Ricola explained very well why just relaxing the torsion bars does not work. That is unless of course, you’re looking for a very very stiff rear suspension.

[Rob]

I'm getting seriously interested in this Eagle Performance setup, mostly because of it's simplicity and cost.

So what are you using for Coil-Over shocks and how much are these ?

Thanks,

Rob.

[Shad Laws]

Hello-

Shad, I’m curious about your comment that the one of the spring plate's function is to maintain toe. I’m not sure exactly what you meant by that. It appears to me that the adjustment for toe is through the placement of the washers on the inner pivot bolt.

Changing the washers is a _major_ adjustment. Often times, it never needs to be done. Minor adjustments, like +/- 1/8" of toe, are done by moving the trailing arm bolts in the slotted holes of the springplates.


Also, unless you have a lot of slop in the inner bushings, you can’t substantially change toe by adjustment of the spring plate. It seems to me that you adjust the spring plate to accommodate the position of the suspension arm based on the placement of the aforementioned washers.

No. Basically, the correct procedure is to measure the toe you have, then decide whether a major (and coarse) adjustment is necessary by washer moving, and then do a minor (and fine) adjustment by moving the bolts in the slotted holes.

Even 1/16" of toe difference can make a big change in the way the car handles.


The spring plate would then help support the arm and prevent a change in toe due to wear in the inner bushings. Is that what you meant?

No matter how you cut it, the rear suspension is held in place by two pivot points of rubber. It is therefore flexible (which is why you can move it in the slotted bolt holes).

Toe is not a constant thing. We like to think of it that way - we measure it with the car on the ground and it stays, right? I wish :-). The truth is that during use, the rear suspension undergoes many different forces in different directs under different conditions, and the rubber bushings flex. That's why it's a benefit to use polyurethane bushings... a little more rigidity for better toe control.

The torsion bar helps keep the springplate in place, too. It takes some force to "bend" the outer ends out of place and change the toe. This is good - it increases the rigidity of this pivot point compared to the rubber alone.

By contrast, the inner trailing arm bushings are much smaller - there's a lot less flexible rubber there. This is because there's no torsion-bar-ish-thing there to contribute to rigidity, so they made the rubber pieces' geometry more rigid to start with.

Long story short: I wouldn't simply remove the torsion bars and call the conversion "done." If you don't put something back there, handling will suffer.

Germanlook - form follows function, right? I know the form of no torsion bars seems cool, but the function doesn't follow. Unless, of course, you do something else to compensate.

I’d be concerned that if you try to adjust toe with the spring plates, you could put the arm in a bind that could cause premature wear of the bushings or worse. Does this make sense?

I understand what you are saying, but it's wrong. The washers alone don't allow a fine-enough toe adjustment.

Bottom line, I think the Eagle Products’ conversion will work just fine if adjusted properly.

One concern I have about their system is that there doesn't appear to be any isolation mounting. While this makes for a ton of rigidity, I do wonder if this is ultimately problematic... no "shock absorbers" and perhaps a much harsher ride. <shrug>

Also, I think Ricola explained very well why just relaxing the torsion bars does not work. That is unless of course, you’re looking for a very very stiff rear suspension.

Well, hang on... we have to back up and ask why the heck we want coilovers in the first place.

If the answer is to look cool, then nothing I said applies. Form doesn't follow function, so the book is open on what you want to do :-).

But, I'm assuming that it isn't. Okay, so before we go changing what we have, we should ask this: what's wrong with the stock torsion bar system? The stock torsion bar system provides a constant linear spring rate. Now, we may desire to change this for three reasons:
- make a higher linear spring rate. This can be accomplished with stiffer torsion bars - very simple, staightforward, lightweight, and cheap (<$200 for everything). IMHO, adding coilovers for this purpose is a waste of money, time, space, and weight. Not to mention that you are imposing new stresses where the system was designed to take them... while these problems are curable, why cure them if they don't provide a net benefit?
- make a progressive spring rate. This is a good thing to do! There are two ways to introduce a progressive spring rate. The first is pneumatically - gas-filled shocks. We can do this with torsion bars, too. Many of the cheaper coilover setups use only this method, and therefore, changing our system to use this is pretty much a waste if this is our only goal. The second way is with special progressive springs. These kick ***. Here, the spring itself looks different from top to bottom - part of it is wound more tightly than the rest. These are a good reason to add coilovers! Or, simply reduce the torsion bar's springing by a bunch and make up for it with these springs (i.e. relax the torsion bars and add the coilovers). Unfortunately, these aren't cheap... <sigh>
- add adjustability. While one could argue that the stock torsion bar system is adjustable, it's a pain in the ***. If this is your only goal, consider 944 springplates. They are lightweight, relatively cheap (junkyard items), and work well. Of course, adding coilovers accomplishes this, too. But, they also add weight. But, they also "look cool." Ah, well.

Take care,

[Wally]

Originally posted by Shad Laws
Hello-

Now, we may desire to change this for three reasons:

Take care,

Hi Shad,
Acually you forgot reason number four, which would be my only reason for the change although I'am still not sure if this reason would apply:

- making room for the nose cone section of a 915 gearbox'in my 1303. Since the 915 install might imply that I need to remove (some of) the center section of the inner mountings of the torsion bars, I would than need an alternative suspension system.

I strive to have the bell housing of the 915 gearbox in the same spot as the original beetle gearbox was. Don't know what can be shaved of the torsion housing untill critical strength is reached...Also don't know what can safely be shaved of the 915 nose section without it becoming critical either.

I know you did the 923 install in your Fastback recently, but the front clearence is a lot different on a type 3 than on the 1303.
Well, we shall see in a few months when I go install the box. No reason to have the gearbox fitted if the engine still misses its (porky) heads :-) (just teasing)

Greetings,
Walter

[Alex]

Shad,

I need some more explanations now.

Why do most modern cars not use torsion bar setups then if it is as good and cheaper?
Why do the 911 and 944 guys upgrade their cars to coilovers?
Why did Porsche change the suspension with the 964?

If you consider buying shocks as well you can add some money to the entire rear setup.....not only the 200$ for other torsion bars.
If you are serious about your cars performance you will have to add bracing to the rear to eliminate wheel hop. This ties in the top shock mount so there is nothing to worry there.

Alex

[Shad Laws]

Hello-

I need some more explanations now.

Okay :-).


Why do most modern cars not use torsion bar setups then if it is as good and cheaper?

Most modern cars don't have torsion bar tubes and torsion bar tube supports, either. If you eliminate the entire torsion bar system (not just the torsion bar itself) and replace it with a coilover, you save weight.

What we were talking about was modifying an existing semi-trailing arm system with torsion bars to coilovers. If the page is blank, and you can do what you want, then starting over completely from something else lends itself to different solutions.

Why do the 911 and 944 guys upgrade their cars to coilovers?

Ah! Now this is applicable.

They do it for the reasons I said above:
- add easy adjustability
- add a highly-progressive spring rate
- add more simple linear spring rate

Also, the labor cost of adding stiff torsion bars vs. coilovers is much higher. This is important when you figure that many Porsche people have other people work on their cars for them :-).


Why did Porsche change the suspension with the 964?

The 964 saw a removal of the torsion bars, but the MacPherson strut front and semi-trailing arm rear basic designs stayed in place.

The rears has to go to make room for a long transmission (hey Wally - here's your application :-) and the fronts for a pair of new axles for 4WD. Plus, it allowed a less labor-intensive flexibility of adjustment and modification for different applications (i.e. racing).


If you consider buying shocks as well you can add some money to the entire rear setup.....not only the 200$ for other torsion bars.
If you are serious about your cars performance you will have to add bracing to the rear to eliminate wheel hop. This ties in the top shock mount so there is nothing to worry there.

Yes, but what about the lower shock mount? After 30 years of "normal" use, many VW trailing arms are bent, leading to excessive negative camber in the rear that really can't be adjusted out. Putting the entire suspension load on a stock trailing arm at that point will just make it worse. Now, if you already have more rigid trailing arms, then it's a different story :-).

Alex - I'm not saying that coilover systems suck. I'm not saying they don't work. All I was trying to do was give a very specific analysis of _why_ you would want to use them.

It seemed like many people just wanted them "because they're better," but they couldn't pinpoint why the torsion bars were inferior in this regard, except for that it is "common knowledge" that they suck. They want to bolt them on with little other consideration for the big picture (like the trailing arm problem, for example). I was trying to give some perspective - unfounded opinions of "sucky" and "cool" are a bad idea to design a car by :-).

BTW, the prevailing thought of "torsion bars suck and coilovers are better," without any further analysis, is also another major reason why 911 and 944 people do it :-).

For example, let's compare the following configurations:
CONFIGURATION A:
- aftermarket torsion bars
- adjustable gas shocks
- 944 adjustable spring plates
CONFIGURATION B:
- stock torsion bars, relaxed
- coilover shocks (constant spring rate springs, but gas-filled shocks)
CONFIGURATION C:
- the Eagle Products' setup
- coilover shocks (constant spring rate springs, but gas-filled shocks)

All three can be configured to give basically the same springing characteristics.

Configuration C may have a problem with isolation mounting. Configuration B and C require stronger trailing arms. Configuration A is slightly lighter in weight (not by a whole heck of a lot, but it's there). Configuration B and C consume more space (Oil coolers can get close to rear shocks...). But these are all minor points - there's no huge benefit to A, B, or C.

However, it is the common opinion that A is the worst, B is next, and C is the best, and difference between these three is substantial. While this may look better on a spec card for bragging rights, from a performance standpoint, this is false.

Take care,

[Alex]

Hey Shad,

I did not write my reply because I do not believe you but more so that someone spending some serious coin in the suspension setup does not get the feeling that he wasted money on it.

Maybe I just read your reply wrong......you know.....me foreigner. (It is just such an easy excuse ).

I just wanted it to be clear that if you do it right you will have the added benefit as you mentioned in your last reply and I also agree that if you want to save money on coilovers and springplates that it would be just as good to stick to the torsion bar setup.


You forgot to add D....the springplates that are used on 944s and 911 witht he left/right thread (more $s) for added adjustability with a gas filed coilover.

Thanks Shad,

Alex

[RonR]

Shad, thanks for your response to my questions. Also, thanks for not bashing me for referring to “neoprene” bushings instead of “polyurethane”, hope I didn’t loose all creditability. I’m pretty sure I now understand what you're saying and don’t really disagree. But even though I think they're going to be a real pain to adjust, still don’t understand why the heim joint will not work as well as the spring plate in maintaining toe as long as they are adjusted properly. I’ve spent a lot of time trying to understand how the various IRS components work in relationship to each other. I’ve assembled my IRS setup and made sure that everything worked smoothly (i.e., there is no binding). One of things that I noticed was that there is very little movement in the inner bushings, even without the outer bushings installed. But maybe enough to make the fine adjustment required to obtain the correct toe. At any rate, that situation has nothing to do with the fact that I’m not using the spring plate. I don’t think the toe will change any more with the up and down movement of the arm than it would with the spring plate, but I guess I'll find out. I have not been able to detect any toe change at this point. It appears that the spring plate must twist as well as bend inward (two seperate motions) in order to abe able to follow the arc of the IRS arm which is pivoting about a different axis. The heim joint setup is able to accommodate that same arc because of it allows movment about the pivot point.

I don’t disagree at all with your statements about the merits of the torsion bar versus that of a coil-over. One of the reasons that I have opted to go with coil-over was to get additional wheel clearance. Also, I thought the adjustability and availability of different spring rates would be a plus. And, less importantly, I just don’t like torsion bars, no good reason just don’t like them.

Thanks again for your comments, but unfortunately I’m past the point of no return and can’t go back to torsion bars. Maybe I’ll go to A frames if this doesn’t work out.

[Shad Laws]

Hello-

Shad, thanks for your response to my questions. Also, thanks for not bashing me for referring to “neoprene” bushings instead of “polyurethane”, hope I didn’t loose all creditability.

Nah. If we all lost credibiliity when we made typos, we'd all be screwed. Me especially :-).

But even though I think they're going to be a real pain to adjust, still don’t understand why the heim joint will not work as well as the spring plate in maintaining toe as long as they are adjusted properly.

It will do fine. I was referring to the suggestion of simply removing the torsion bar and leaving the springplate and rubber bushings in there alone.

The only concern I see with the heim joint setup is the lack of isolation mounting. I do stand to be corrected (there are isolation heim joints), but I don't see any on the setup. This may cause other problems down the road.


I’ve spent a lot of time trying to understand how the various IRS components work in relationship to each other. I’ve assembled my IRS setup and made sure that everything worked smoothly (i.e., there is no binding). One of things that I noticed was that there is very little movement in the inner bushings, even without the outer bushings installed. But maybe enough to make the fine adjustment required to obtain the correct toe.

You are correct: it usually takes some effort to pull/push the trailing arm along the slotted bolt holes, but it is necessary and does allow the fine adjustment of toe. BTDT.


At any rate, that situation has nothing to do with the fact that I’m not using the spring plate. I don’t think the toe will change any more with the up and down movement of the arm than it would with the spring plate, but I guess I'll find out.

That is called bump steer. Bump steer is the change of toe with the change in suspension position. You are correct that the bump steer characteristics won't change much at all with the Eagle setup vs. stock.

With a isolation-mounted, rubber-bushed system (a la all OEM semi-trailing arm rear suspensions), the problem isn't bump steer. The problem is that the mounts are somewhat flexible. So, under high loads, it is possible to change the toe of the suspension due to lateral forces imposed on the suspension system, not so much vertical ones that just change the suspension's position. This effect creates the "classic oversteer" problem of an early 911 on a hard turn when you start decelerating. My point was that removing the torsion bar and leaving the springplate with the bushings left in there as the pivot, this effect would get far worse.

For more info on this, do a google for Porsche 928 Weissach Axle. It should give some diagrams of the semi-trailing arm rear suspension, how they lead to weaknesses, and how the weissach axle suspension system corrects these problems.

The Eagle setup won't have the problem of a grotesquely flexible pivot like removing the torsion bar and leaving the springplate in there would create. Rather, I fear that it *may* (again, I stand to be corrected on this one) have the opposite problem - too rigid, leading to a problem with absorbing shocks (may fatigue parts faster) and a harsh ride. Solid mounts don't do well on street-driven cars...


I have not been able to detect any toe change at this point. It appears that the spring plate must twist as well as bend inward (two seperate motions) in order to abe able to follow the arc of the IRS arm which is pivoting about a different axis.

Yes. This is why the springplate setup has almost no effect on camber... camber is controlled almost exclusively controlled by the trailing arm itself.

The heim joint setup is able to accommodate that same arc because of it allows movment about the pivot point.

Correct.

I don’t disagree at all with your statements about the merits of the torsion bar versus that of a coil-over. One of the reasons that I have opted to go with coil-over was to get additional wheel clearance.

I'm confused - how do the coilovers give more wheel clearance? Do you mean that you are just stiffening the suspension to prevent bottoming out?

Also, I thought the adjustability and availability of different spring rates would be a plus.

Sway-a-way makes a billion different torsion bars (springrates)and 944 arms are easily adjustable. :-). Again, I'm not saying that coilovers don't have other benefits, but if these are the benefits you are looking for, you don't need them.


And, less importantly, I just don’t like torsion bars, no good reason just don’t like them.

Well, okay :-).

I like F^3 - Form Follows Function. When I spend money and perform a modification, I like to know that a performance gain will result. If you're going to do the coilover modification, then take advantage of what else it can give you! Make it not only look cooler than when you started, but perform better as well!

Take care,