Rear IRS Suspension Question

[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