Swing vs IRS newbie Q

The VW Beetle. Everything about bugs!

Moderator: Marc

Post Reply
User avatar
Posts: 39
Joined: Wed Jul 13, 2016 11:09 am

Swing vs IRS newbie Q

Post by Gnasha » Fri May 18, 2018 3:53 am

As I'm new to ACVW so I'm thinking this is a dummy question that may raise a few smiles.

I'm converting T4 to upright to fit into a 73 Ghia. Ill be removing the current engine and IRS box for O/haul at a later date.

I'm looking for another T1 IRS box to fit to the T4 to fit as a complete unit. Current box is AN code. I'd like a IRS box to replace the existing.

What is the difference between swing axel and IRS? can you tell the difference by looking?. can I fit/convert swing axel to IRS?

I'm looking at an AM code box at the moment but need to be sure before purchase. Are there any other codes I should look for to suit this application

Many thanks for reading this

Posts: 14041
Joined: Tue Nov 14, 2006 1:17 am
Location: Just north of Seattle

Re: Swing vs IRS newbie Q

Post by Ol'fogasaurus » Fri May 18, 2018 10:05 am

Swing axle 02.jpg
Swing axle 01.jpg
Both are IRS designs getting away from a solid axle design.

A swing axle has one pivot point and it is on the transaxle. The arc of the pivot causes the tires to tip in or out during travel. Depending on what you are planning to use the car for this could be no big deal or a big deal; longer travel being one of them.

IRS has two axle pivots: one on the trans and one on the now necessary additional trailing arm. There is still some tipping at the tires but it is comparatively small compared to the swing configuration. There are also more parts involved like two CV joints per side, a stub axle, a shorter drive axle, etc..

This is a very simple definition of the differences and doesn't discuss a bunch of other things that could be relative to what you are trying to do.


Tried to make it better but it got worse
You do not have the required permissions to view the files attached to this post.
Last edited by Ol'fogasaurus on Fri May 18, 2018 11:32 pm, edited 4 times in total.

User avatar
Posts: 39
Joined: Wed Jul 13, 2016 11:09 am

Re: Swing vs IRS newbie Q

Post by Gnasha » Fri May 18, 2018 11:47 am

Lee thank you for taking the time to answer my question so succinctly.

User avatar
Posts: 23738
Joined: Thu May 23, 2002 2:01 am

Re: Swing vs IRS newbie Q

Post by Marc » Sun Jun 03, 2018 9:50 pm

Late 'Ghia "AN" transmissions have the 3.875:1 ring & pinion (same as the "AT" used on `73-up Beetles) but they retained the .88 4th of the "AH" 4.125 boxes, while the ATs got a .93 4th. This gives the AN the tallest overall ratio ever offered in any stock Type I trans, which would be especially desirable with a Type IV engine. Ideally you can find another AN to use until yours is rebuilt, but the ATs are much more common and would be my second choice. One of those will raise your engine RPM by ~200 at 70 MPH compared to the AN.

The "AM" was used on cars with 1300cc engines, never offered in the US market. Although it would function for you, it would be a poor choice IMO for your application - it has a 4.375:1 R&P making it a full 12% shorter in all gears than your AN. That's over 400 more RPM at 70 MPH.

The swingaxle configuration requires a transmission case that has two sideplates (later IRS cases from C1973 have only one).
To convert a trans from IRS to swingaxle (or vise-versa) requires fitting the appropriate differential carrier and sideplates. Older swingaxles, such as pre`67 ones sold in the US market, have ring gears and diff carriers which are mated by six bolts rather than the eight found in later boxes so their ring gears aren't compatible with any stock IRS carriers - but you won't encounter that issue if you avoid the old stuff.

Changing carriers/sideplates involves the proper setting of the ring gear backlash and the preload on the diff carrier bearings, so while it isn't as involved as transplanting a pinion shaft (which requires the setting of the pinion gear engagement with the ring gear) it's a job that should be performed by someone with the correct measurement tools and an assortment of selective-thickness shims available I would not recommend that you try to do it yourself.

Post Reply