Rear Suspension 101 FAQ including Torsion Bar and IRS items

[enesset]

Guys,
This forum continues to be a wealth of information and I'm extremely grateful. I have done some searching and it doesn't seem like we have a "sticky" for Rear Suspension and Torsion bar knowledge.

Any advice on what length and diameter to choose for a given project style would be greatly appreciated. Also any tips/tools/etc when installing and adjusting would be great additions to this thread.

We have been talking about my project in another area regarding brakes and since I'm an Early Ford Bronco and Spec Miata guy and am learning about VW and Manx's as my project continues I thought this would be a great reference thread to start.

My specific project is a 1969 Manx body and pan and I am starting from the bare canvas pan. My use is going to be Baja desert terrain and the intention is for NORRA 1000 racing (at a gentleman's pace "not a Walker Evans/Robby Gordon/Troy Herbst pace).

When choosing a torsion bar for the rear suspension, I generally (and hopefully correctly?) understand that the shorter the torsion bar chosen, the more harsh/stiff the ride.

If I were to go much longer than stock (29-9/16) then I am generally understanding that a common (?) wheel size (31") may not be possible or advisable due to limited clearance (if using stock IRS components -- more on that below).

Torsion bars also come in a variety of thickness from 24mm to 30mm (any shared knowledge here would also be wonderful).

SECOND PART:
The torsion bar is ultimately attached to either an OEM trailing arm or many use custom built micro-stub rear trailing arms.

This would be a good place to discuss the plus and minuses of both options. To start the discussion, I have considered using OEM trailing arms as they would be an easy "plug and play" with standard shocks and the remove/replace/repair might be easy in the middle of baja/nowhere.

Micro-stub allow more clearance if you are using a long torsion bar AND your vertical "range of travel" could be very long if you are building something to jump the dunes in Pismo?

I am learning in the Brakes Thread that you must understand the "trailing arm" question before you make the "which type of brakes" am I going to buy because if you go OEM trailing arm, perhaps the brakes situation is not interchangeable if you decide to to "more hardcore" and upgrade to mini-stubs.

I think this is a good start to the thread. This is started by a noob and I'm learning so thanks for the patience and the collective member knowledge being added to this thread. -- Eric

[enesset]

As a follow up. I should ask.....if you use an OEM trailing arm setup is there a specific length torsion bar that is optimal so you can use a 31" tire without issue.
Thanks again -- Eric

[GotManx]

A 31" tire will normally fit with a stock IRS trailing arm and the longer torsion bar. It is close but it usually fits.

The torsion bar diameter determines how stiff it is. A shorter bar of the same diameter will be slightly stiffer than a longer bar.

The size of the bar needed depends on the weight of the car and the intended use. A smaller bar or even stock works well for a cruiser Manx. If you are planning on running hard through whoops and bumps you may want a 27mm bar. The smaller diameter bars are better for letting your suspension move and work better for climbing and keeping traction on varied terrain.

Anything other than a stock trailing arm is going to require new upper shock mounts. Even boxed stock arms remove the original lower shock mount. Stock un-reinforced IRS arms over time and especially with off road use will sag creating negative camber. They are fine for mild off road travel, but over time they will bend.

Hope this helps you out some.

[enesset]

You and this type of info/insight is awesome, I am SO appreciative. Thanks so much -- Eric

[jsturtlebuggy]

If you are going to use stock length trailing arms and a 31in tire, you can end up with it rubbing on the longer torsion bar spring plate. The stock length torsion bar for IRS is 26-9/16in long. Tires grow when they get hot. I speak from experience from seeing the tires wear a groove in the tread from the spring plate in off road racing when I started back in the 1970s.
In the early days of racing converting back to the short torsion bar 21-3/4in with modified swingaxle spring plates was the normal thing. SwayAway came out with heavy duty spring plates that did not bend. You can use a 31in and larger diameter tire without clearance issues.
It also depends on how much the off set the wheel has. Most wheels sold for off road use in the wide 5 pattern have less inset then a stock VW wheel. This sets the tire out further and may our may not clear the IRS spring plate that using a 26-9/16in torsion bar.
There is also a 24-11/16in torsion bar length used in 1950s Bugs and some type 3 vehicles. Being shorter in has better clearance then the longer one.

A longer torsion bar is able to be twisted further. Ideal for long travel suspension. It also can give a softer ride.
Since you are planning on using stock trailing arms. They do need to be reinforced. The stock shock mounts is not in the best location for controlling the suspension through its' movement. Moving the shock mount to the top and further back help. This does require fabrication work.

If you are planning on just using stock suspension with stock shock mounts, I would stay with less than a 24mm diameter 26=9/16in long torsion bar. And use a 30x9.50x15in tire. BF Goodrich has them. The shock mount on the trailing arms will need to be reinforced to keep from breaking off.

Are you going to have a full roll cage, which is required. You will also want to tie in the front axle and rear torsion housing and shock towers to it.

[enesset]

Again, this is fantastic info.

You asked about caging the Manx and the short answer is yes.

I have talked in the past with Dave about building my cage and he was a decent distance from me (Santa Cruz-ish area) when he was in Fresno but now that he has moved all the way to AZ it will be a much longer tow.

I actually just called him to talk about a few things and this has expanded my knowledge which I'll share here and then I'm still interested in feedback/info.

Being new to VW suspension and having a bare pan, I'm a knucklehead and did not fully appreciate that I could go the +1 fabricated trailing arm route and then just transport the vehicle with spacers and no shocks (having the torsion bar carry the weight of the vehicle). Then when the cage is built, at that time have the custom shock mounts made on the custom trailing arm.

[enesset]

More research....more questions....

It sounds like one of the "trailing arm guru's" is a gentleman named "Todd Francis"..... http://precisionalloy.com/pages/contactus.html

I'll share what I learn.....if anyone has experience history with his stuff, this would be a great thread to share it.

[enesset]

Talked with Todd Francis. Good guy, he is building some very robust engine cases/blocks.

His friend is a guy largely responsible for these very robust trailing arms (Dave Brown).

The "hot ticket" that I'm continuing to learn perhaps is some of what we've talked about in this and other threads which is bus stub axels and 930 CV joints, ultimately terminating at a mercedes disc set up which incorporates a parking brake.

More here as I learn more.

[jsturtlebuggy]

Since now you are talking about using longer and wider trailing arms, you are going to want to match the width with the front beam.
How much suspension travel do you plan on having in the rear?
If you are trying to get more than 10in of travel, you may want to rethink the use of a type 181 front end and use a King and link setup.
Also you may want to get the chassis modified. Talk with Dave he knows what is needed to be done with building a chassis and cage combo.
How much have you budgeted for this build, because it sound like you may have to double or triple what you are thinking.

In the early days of off road racing people found out very quickly the limitation of the stock VW pan and how they would break apart.

One book you need to get is Baja Bugs and Buggies: Prepping for off road use by Jeff Hibbard. It is dated some, but still has much useful information for what is needed to go off roading and to keep from breaking things.

[enesset]

I am grateful for your 10" travel statement as that reflects the discussion I had with the guys in the PacNW that will make the trailing arms.

You mentioned longer and wider. Going to have the arms made just 1" longer. Dave Brown and I talked about 2.5x2.5 but I really don't want anything goofy looking so we settled on +1 for torsion clearance and general strength per our discussion here how OEM isn't going to cut anything more than light trail riding and if you box OEM you are going to need fabricated shock mounts anyway.

You also asked about budget and you are right on it and I am cringing when I think about how I will have to "disclose to the boss" the fact that there is zero chance this is going to be a 4-digit build like I promised. She insisted that unlike my Early Bronco build and my turbo Miata track car build before that, there is a budget and that really isn't going to be abided to. 8-(

You mentioned those reference books....
my reference books have been "HP Books - Baja Bugs and Buggies" and then "Building a Dune Buggy the Essential Manual" by Paul Shakespeare.

Back to the torsion bar discussion, with a '69 IRS pan, I now understand it's a 26-9/16 bar and need to think about which diameter thickness? I was looking at Kartek for the range and from memory I think it is 24mm-30mm?