Transmission

This page is... UNDER CONSTRUCTION

WARNING: I am not an expert on cars, automotive engineering, VW Beetles, beach buggies or anything else discussed in these pages. What I have written is my current understanding of the issues involved in building Tina's buggy. These opinions are based only on my own research in books and on the web. They, therefore,have no basis in fact, may well be wrong and potentially downright dangerous if taken to be gospel truth. If you intend to use any of this information for any purpose other than pure entertainment, then please get its validity confirmed by someone who knows what they're talking about! You have been warned...

This page contains the following sections:

• Transaxle Overview
• Transaxle Code Location
• Understanding Transaxle Codes
• Transaxle Type Codes & Ratios
• Transaxle Ratio Calculator

Transaxle Overview

As the name implies, the transaxle combines the gearbox ('transmission') and the differential (usually part of the rear 'axle') in a single unit. The lightweight Magnesium alloy transaxle case contains the four speed gearbox at the front, the differential in the middle and the clutch bell-housing at the rear.

There have been 4 basic types of transaxle used in the VW Type 1; Split case swing-axle; Tunnel case swing-axle; Dual differential cover IRS; Single differential cover IRS.

As I have already decided to use IRS, rather than swing-axle rear suspension, on Tina's buggy, I will have to use an IRS transaxle. The dual differential cover is the weaker of the two IRS transaxle types, whereas the single differential cover is stronger, as one of the differential side covers is cast into the casing, thereby increasing the rigidity of the whole unit.

Transaxle Code Location

It took me a while to find where the transaxle code is located, as there are a lot of different numbers cast into different parts of the transaxle, none of which contain the useful type codes referred to in the table below. The transaxle code is stamped (not cast) into the gearbox casting and has only been visible after cleaning the area with a wire brush on all the transaxles I've looked at. So where is it? It's on the RHS of the gearbox when viewed from the clutch bell-housing end (the same side as the starter motor), stamped into a flat area of the gearbox casting, just below the horizontal plane, on an area that's angled slightly downwards, as shown in the picture above.

Understanding Transaxle Codes

Transaxle codes are of the form: 'AM 24 02 3', where 'AM' is the transaxle type code, '24' is the day, '02' is the month and '3' the last digit of the year of manufacture. A transaxle with this code therefore has the gear ratios shown in the table below for the 'AM' type code and was manufactured on 24th February 1973.

Transaxle Type Codes & Ratios

Throughout the development of the Type 1 transaxle, VW made many changes to the design internally to suit it to cars of different weights, with different powered engines in order to achieve different driving characteristics. One of the significant changes was to the ratios used for the ring and pinion gear used in the differential and for 4th gear. It can be seen from the table below that the ratios for 1st, 2nd and 3rd gears did not change significantly throughout the development.

The ring and pinion affects the overall gearing, with all gears being affected equally. A higher numeric value for the ratio means lower gearing, which roughly equates to higher acceleration but lower top speed. The ratio for 4th gear affects cruising and top speeds. Again, a higher numeric value equates to lower gearing, giving a lower top speed.

The table below gives a summary of the various transaxle types which can potentially be used in a beach buggy. The ones shown in bold italics are the ones which I have. The AH box came with the 1302S donor Beetle, the AM was bought for its lower gearing and stronger single differential cover case and the DC was bought because I thought the Type 3 box may be stronger internally (I don't know whether this is true or not).

If you have any further details to help me fill in the blanks, or if you think any of the above information is wrong, please let me know and I'll update the table.

Transaxle Ratio Calculator

I used the VW Transaxle Codes & Ratios table above, in conjunction with the Transaxle Ratio Calculator below, to work out the most appropriate combination of rear tyre size and transaxle ratios for Tina's buggy.

A 'reasonable' target (IMHO, of course) for a 1600 beach buggy, given its light weight, but poor aerodynamic performance at high speed, is to aim for a comfortable cruising speed of somewhere between 60 and 70mph @ 3000 rpm in 4th gear.

The default values in the Transaxle Ratio Calculator below are for an 'AM' code transaxle from an IRS 1302/3 Type 1 (Note: these were 1300cc, rather than 1600cc), fitted with 235/75R15 rear tyres but you can change them to anything you like.

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