Rear IRS Suspension Question

[fastbacker]

Im looking to build my 69 bug into a street car that doubles as an autoX car. I currently have a rebuilt 901 Porsche tranny with a lsd and a 200hp type IV 2.4 liter (yet to be installed). I'm going to run stock sized front fenders and 3" flared rear fenders (custom offset wheels with 275's).

I'm currently weighing rear suspension options. Having never built up a bug, I need some advice. From my research, I'm thinking of running a kafer 5 bar type brace custom made by welding in tubes - would tie into a roll cage. Also thinking of using 944 (or 944 turbo) arms and brakes.

Couple of questions:

1. What are the pros and cons of 944 vs 944 turbo arms and brakes on this set up with 3" flared fenders and 275's?

2. Any reason to stay with the stock irs arms?

3. Any better suggestions for the rear suspension?


Thanks a bunch-
Craig Backer

[evilC]

Originally Posted by fastbacker

Im looking to build my 69 bug into a street car that doubles as an autoX car. I currently have a rebuilt 901 Porsche tranny with a lsd and a 200hp type IV 2.4 liter (yet to be installed). I'm going to run stock sized front fenders and 3" flared rear fenders (custom offset wheels with 275's).

I'm currently weighing rear suspension options. Having never built up a bug, I need some advice. From my research, I'm thinking of running a kafer 5 bar type brace custom made by welding in tubes - would tie into a roll cage. Also thinking of using 944 (or 944 turbo) arms and brakes.

Couple of questions:

1. What are the pros and cons of 944 vs 944 turbo arms and brakes on this set up with 3" flared fenders and 275's?

2. Any reason to stay with the stock irs arms?

3. Any better suggestions for the rear suspension?


Thanks a bunch-
Craig Backer

Hi Craig,

As I see it Auto-X is like a giant slalom? If thats the case then your rear tyre proposals are probably too large - you will be wasting power. A good set of tarmac racing tyres in say 180 - 200 section width should be sufficient and allow the ability to slide the rear end to compensate for a tight turn.

The kafer cup brace should brace the end of the frame horns to the damper top arm and then down to the front of the frame horn. I would use bolted connections as the purpose of the brace is to triangulate the top of the dampers and stiffen up the frame horns. By using pivot joints rather than welded joints the brace members are put in (mainly) compression and tension. If welded, the members may be put in bending that means increasing the section sizes.

Re your queries:

1) The 944 came with steel and alloy arms. The alloy ones in two widths the turbo one being the wider track. I would use the turbo ones as that will give a wider track. The turbo brakes should use 16" wheels and come with 4 pot calipers. The N/A 944 rear calipers are very small single pots that don't provide enough braking IMO. Also use the alloy arm spring plates as they are more adjustable.

2) Junk the stock arms - the alloy ones come with bigger bearings.

3) Add the turbo torsion bars (25mm) and include for coil overs as well with lightweight coil springs to tune the wheel rate. Add an AR bar 16mm, a Whiteline is the doodahs.

4) Junk the ball joint front suspension in favour of double A arm or coilover McPherson struts suitably cross braced with adjustable AR bar and rack and pinion steering. Spend some time getting the Ackermann right as it will definitely help with the (relatively) slow speed steering response.

evilC

[fastbacker]

So you're saying you'd use the turbo arms and brakes, run 3" flared rear fenders and use wheels that run about 200mm wide tires. With this setup, I'd have the same sized wheel that could fit under a stock rear fender but the track would be increased by 6".

Thanks for the detailed reply-
CB

[Steve C]

Hi

An over wide rear track will make the car handle like a tricycle, it will want to fall over towards the front of the car, think of how a tricycle acted if you went around a corner in your driveway to fast, you would get tipped out of the seat.

So you need to the track wider on the front than the back or at the worst the same.

Stick with what you have, but heavier bars for now.

Steve

[Clatter]

Ooohhh, those wide-track alloy arms just LOOK so tasty, don't they..

Couldn't bigger, stiffer torsions in the rear upset the handling if the fronts are not stiffened to match?
My experience with motorcycles is that stiffening up one end really makes the opposite end go much more soft.

[fastbacker]

Originally Posted by Steve C

Hi

An over wide rear track will make the car handle like a tricycle, it will want to fall over towards the front of the car, think of how a tricycle acted if you went around a corner in your driveway to fast, you would get tipped out of the seat.

So you need to the track wider on the front than the back or at the worst the same.

Stick with what you have, but heavier bars for now.

Steve

My 911 track car's rear tires are 2.5" wider than the fronts making the rear track about 5" wider. The bug starts out with a 1" wider front track so with the proposed 6" added to the rear both cars would have the same stance. This is a great set up for the 911 so I can't see why it wouldn't be for the bug. But hey, I don't know.

[wrenchnride247]

The rear on a 911 weighs a bit more than a T1.

[Humble]

Fastbacker, here's a little inside knowledge from someone who's been there...

There's a saying about D-modified cars: that when d-mod cars are built the builder didn't read the rule book. So D-mod is usually a catch-all class for cars that don't fit in any other class. I know this because I made a d-mod super beetle. If you build it the way you are saying, the car will end up in d-mod just because of a type 4 swap and while you'll have a lot of fun at the autox you won't be competitive in the class unless you've got about 400hp/ton (just basing this off of the local d-mod cars)

That being said you will have a kickass street car, and if you take it to an autox you will get a lot of attention just because it's a beetle. But if you want to race competitively you'll be better off stripping the bug, going through the stock suspension, hopping up a type 1 motor, beefing up a bug tranny and racing it in FSP.

What I tell people who are building or planning to build cars is to start at the end of the project and work backwards. Figure out if you want a race car or a street car and then work your way backwards to the start from there.

[fastbacker]

Thanks Humble. Good reading.

What I'm actually looking for is more of a street car that I can autoX non competitively - maybe a couple of times a year.

I actually took the route you recommended with my 911. Built it (w alot of help) for a specific class before I even knew how to drive on the track.

[evilC]

Originally Posted by fastbacker

My 911 track car's rear tires are 2.5" wider than the fronts making the rear track about 5" wider. The bug starts out with a 1" wider front track so with the proposed 6" added to the rear both cars would have the same stance. This is a great set up for the 911 so I can't see why it wouldn't be for the bug. But hey, I don't know.

Agreed. The 911 is in very simplistic terms a more powerful bug. The racing 911s all had IIRC wider rear than front tracks that stablised the straight line running.
If Steve C read my reply fully he will have noticed that I commented on the front suspension that was in addition to the subject of the thread. The WHOLE suspension design is important not just one end and I would have expected the front track to be increased in additon to the rear.

evilC

[fastbacker]

OK. So I think I'm going to look for some wide track aluminum 944 rear arms and brakes (along with t-bars, spring plates and axles). The information out there is a little confusing as it seems there are different versions of the same width arms (?). I did come up with this info on aluminum arms via some searching:

U.S. Models:
==========
86-89 944 S
==========
86-89 944 Turbo
89-91 944 S2
==========
89 944 Turbo S
90-91 944 Turbo
==========

Will all these options work to widen the track by 3" per side. Are all the brakes used the same? All things being equal (if so), are some more easily obtainable?
Any pros and cons in comparisons?

Thanks-
Craig

[wrenchnride247]

ALL 87 and later arms are the same. Four pot brembo's fit.

[evilC]

And besides the rear bearings are so beefy that Porsche racers can regularly use up to 50mm spacers to further increase the track. don't worry get some alloy arms and then fine tune the track with the available spacers.

evilC

[fastbacker]

I just sourced a complete 86 narrow track, aluminum arm, 4 piston caliper rear suspension for $400 usd. It comes with brand new bearings and, as luck has it, the pivot bushings removed. Everything's included - even the torsion bar housing and bare axles.

This seams to be a good deal. Right?

[evilC]

Sounds like it contains everything you are going to need including the inner pivot brackets that you will need to weld onto the original torsion bar housing. Make sure you have the handbrake cables as well as the outer cables fit the alloy arms and the VW ones don't. With the inner cables you use the VW ones but have two options - a) use the VW end and buy new converter swan necks or b) Cut the VW end off and braze the Porsche end on (I did as it was for me a no cost conversion). BTW if you need new handbrake shoes buy the BMW ones - they are 1/2 price of Porsche ones and cut a notch in the end to match the Porsche unit. Use the Porsche dust shields for the discs as they also keep the dust away from the handbrake side of things. The dust shield tabs are usually in poor condition so just pop rivet some new tabs on.
If you've got 4 pot calipers then you wil normally need to run 16" wheels but 15" ones can be used with some 40-50mm Porsche spacers.
Use the Porsche torsion bars they will be either 23.5mm or 25.5mm (if you're lucky) they add some welcome stiffness to the rear. Use also the Porsche spring plates that will give full adjustability together with the alloy torsion bar end cap suitably cut down. The alloy end cap will require the Porsche bush that is wider than the VW one but this will cause problems with the heads of the eccentric adjusters on the spring plate fouling the forged damper arm. I over came this by adding a 5mm PVC thick 'washer' to either side of a standard VW poly bush. The diameters of the washer and the bush should be the same. This spaces the spring plate away from the torsion bar tube to allow the eccentric adjuster to clear (by ~1mm!). The eccentric adjusters are worth having as they control camber and toe-in. You can use the VW spring plate but it will need redrilling to connect to the alloy arm - that IMO weakens the spring plate so on balance the Porsche one with mods work best.
Does the axle come with AR bar if so, it will be a very welcome addition. It should be 16mm and you will have to fabricate new brackets for the VW torsion bar tube or cut off the old brackets and weld them on.
Did the rear axle come off an auto gearbox car? If so, then the driveshafts will be different lengths. Get equal length driveshafts. Keep the CVs as Porsche supplied and change the IRS gearbox flange for a 100mm one from a Trekker/181 - they are available new = £80.00/pr. Hopefully, the CVs will still have the bolts intact with them. If not you will require the retainer plates and new 12.9 grade socket head bolts.

All of that and more is the learning curve that I went through. My axle cost me complete £67.00 then added 4 pot calipers £110, equal length driveshafts (original was a 924S autobox) £60/pr and new bearings/seals £54/pr so all up it was £291 or $436 at todays rate so your $400 is about right. On top of that cost was new discs/pads (never use the old ones), handbrake shoes, driveshaft output flanges and all new bushes.

evilC