IRS Conversion Torsion Bar Options

[lner JP]

I'm starting to gather parts to convert my swing axle bug to IRS and was woudering what length Torsion bars to use. I belive mine should be 21 3/4".

In the UK the only Torsion front beam beetles to get IRS where Semi-Autos which are rare, am I right in beliving all beetles after 69 got IRS in the USA? What lentgh Torsion Bar would a Torion front beam beetle have?

Should I stick with this or is there any advantage to going to the longer ones fitted to super beetles?

If it's of any help, I'm planning to stay stock height on the rear with a minor 1" drop at the front with 175/70 tyres.

Cheers
JP
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Stock 1973 GT Beetle in Apple Green

[Bruce]

IRS torsion bars are all 676mm long. For Type 1, they are all the same from 68 to 79. Beetles, Super Beetles and Karmann Ghias all use the same bar that's 22mm in dia.
If you want thicker bars, Type 3 bars are bigger. But not all Type 3s use the same bars. Squarebacks are thicker than fastbacks or notchbacks.
Porsche 924 and 944s also use the same length bars. Same splines, so they are interchangeable.
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[lner JP]

Mine has the Z bar so should have the 552mm Torsion bar 21mm diameter.

That would make IRS 676mm long and 22mm diameter.

Google tells me that the 552 would have 488 lb/degree twist versus 464 for the 676mm Torsion bar.

So an IRS bug had softer spring with slightly increased weight.

So should I stick to what VW did? as the common sensus on here alway is VW
really knew what they where doing, or would there be any advantage with a stiffer spring?
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Stock 1973 GT Beetle in Apple Green

[Bruce]

How did Google compute the spring rate?

My calculations say that the two bars are within 1.7% of each other. IOW, an IRS car has the same rear spring rate as a swing axle. The spring rate is proportional to the diameter to the 4th power. It is also inversely proportional to the length (with shorter being stiffer).

A suggestion is to borrow an IRS spring plate, then trace it to make a pattern. Transfer that pattern onto your existing swing axle spring plates so you can cut them down. Then use your existing torsion bars and cover plates. Swing axle torsion bars and spring plates are quite a bit lighter than IRS ones.
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[lner JP]

I found an old chart with google that had been posted on Samba and that's where I took my figure from.

If the spring rate of both bars are nearly equal what is the advantge of the longer bar?
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Stock 1973 GT Beetle in Apple Green