by Richard Atwell
Have you ever wondered how the VW 4-speed transaxle works?
Bentley as always has some nice cutaway diagrams that show how a transaxle combines the features of a transmission and differential into a single case. However there is only the briefest of overviews about how the transmission actually works.
The VW transaxle is fascinating study for the compactness of its design. Of more general interest, is an explanation of how it works its magic as your change gears. Before I can explain how gear shifts are accomplished, let's get familiar with basic components using the same identification numbers in Bentley's cutaway diagram.
You can find the diagram below on Page 4 of section Transmission and Rear Axle in your Bentley manual (click on the diagram and open it in another browser window for a better view if you wish). I have colored the b&w components I'm going to talk about.
Let's follow the transmission of power from the engine to the wheels. The parts of interest (in order) are:
25. Rear driveshaft - (red) engine torque is delivered to the transmission via the clutch/flywheel that live in the clutch bell housing (21). The clutch has a splined hub with mates up with the splined end of the driveshaft seen in the diagram.
10. Mainshaft - (orange) connects to the rear driveshaft (red) by axial stud which is a little difficult to see in the diagram (where red and lime green meet). The red and orange shafts are coupled and spin at the same speed.
There are 8 forward gears paired into sets of two. The gear ratio (r) you hear about is the ratio of the number of teeth between two gears (bottom:top) for any set and expressed as ratio:1. The mainshaft (red) has 4 gears (1/2 of the pairs) and the other 4 are part of the pinion shaft (brown) which is located below the mainshaft.
Power is transferred from the mainshaft to the pinion shaft via the gearing and this collection of gears and other components is called the gear stack.
12. First gears - (yellow) ratio 3.80 (38/10)
9. Second gears - (green) ratio 2.06 (35/17)
8. Third gears - (blue) ratio 1.26 (29/23)
4. Fourth gears - (violet) ratio 0.82 (23/28) or 0.89 (27/24)
17. Reverse gears - (lime green) located in the differential half of the case, are a set of 3 gears (the extra gear is required to reverse the motion of the mainshaft). Similar ratio to 1st gear.
Why do these numbers go down as the gear position increases? The gears exist to multiply the torque from the engine to do useful work. The transmission is essentially a torque multiplier: it takes the torque output from the engine and multiplies it at the drive wheels so the engine doesn't have to (nor can it) spin at all possible wheel rotational speeds.
Initially the engine is spinning faster than the wheels (1st gear). As we upshift to maintain the rotational speed of the wheels, we need to use successively lower gear ratios because we've lowered the engine speed with our upshift.
By the time we shift into 4th gear, the engine is rotating slower than the wheels. This is because 4th gear has a ratio that is lower than 1 and for this reason the 4th gear in all VW transmissions is called an overdrive gear. See the Gear Ratios section below for more info.
16. Drive pinion - (brown) welded to the pinion shaft and connects to the ring gear (not shown) of the differential which drives the wheels.
Those are the basics of power transmission. How the gears mate to the shafts and to each other is the really cool part.
The gearstack is confusing for owners to understand because it's a constant mesh design (a very typical setup). All Bentley says is,
The transmission gears are of the constant-mesh type with balk ring synchronizers. The 3rd and 4th gear synchronizers are on the mainshaft and the 1st and 2nd gear synchronizers are on the pinion shaft.
Constant mesh means that all the gear sets (except reverse) are in contact with each other at all times. How can this be when you need to shift from gear to gear? The design depends on each shaft having two fixed gears and two gears that run on needle bearings (white arrows) opposite to each other:
That's the basic design but it takes a lot of smaller components to make it all work together smoothly.
It's hard to imagine how the gear changes occur unless you've got a mechanical engineering degree already.
I had access to a cutaway VW transmission and motor one day and shot this low-res video with my digital camera. The crankshaft pulley was driven by an electric motor below the engine and this 1.5MB H.264 (MPEG-4) video shows how the gear changes occur from 1st through 4th then reverse.
Notice how the gears on the pinion shaft (lower) reverse direction as the transmission is put into neutral and then reverse.
When you shift gears, the transmission has to switch from one set of gears to another. During this time, the engine has dropped in speed because you've put your foot on the clutch and disengaged the clutch from the flywheel.
The mainshaft which was spinning at engine speed has dropped in speed. The pinion shaft which is still spinning at the same speed as the wheels is also slowing down but they are slowing down at different speeds. To allow the shafts and gears to re-engage smoothly during this time, the rotational speed of each gear must be the same.
The earliest transmissions required you to change gears only at certains engine rpms. This was done to speed up the mainshaft to allow the gears to mesh at the same speed. These old transmissions were referred to as crash boxes because the when the gears would try to mate out of sync there were be a loud grinding noise as the teeth chattered against each other.
In a synchronized constant mesh transmission, each gear has a synchronizer attached to it. The synchronizers are made from brass instead of the hardened metal the gears are made from because two metals of the same hardness will wear out much faster than two metals of differing hardness.
In the video you can see a disc moving back and forth. This disc is the clutch gear assembly (sometimes called the synchronizer hub). There is one per shaft and it's placed on the shaft in between the gears it switches. The pinion shaft hub switches gears 1 and 2. The mainshaft shift fork switches gears 3 and 4.
Each hub is loosely connected to a shift fork which is directly connected to the shift lever. This is the shifting motion you can see in the video.
I've explained how the 68-79 bus transmissions works. The beetle transmission in the video has both hubs on the pinion shaft but the operating principle is exactly the same.
The differential transfers the geared power from the engine to the wheels but have you ever wondered how you can turn corners while the transaxle allows each wheel to rotate at a different speed?
This is a scan from Page 11 of John C. Muir's ubiquitous How to Keep Your Volkswagen Alive manual. Peter Aschwanden's diagrams are works of art and I really like his explanation of the inner workings of the differential.
Here's another 1MB H.264 (MPEG-4) video that shows how the gearstack and differential work together. Again, sorry about the poor video quality but this video was also shot with a digital camera.
The spider gears inside the hypoid rotate against the direction of either wheel at any one time and keep the wheels from sliding on the pavement. Ingenious!
To compute your road speed based on the engine speed and gear selection use this formula:
MPH = RPM * TIRE HEIGHT / GEAR RATIO / R&P / 336
For example: 3400rpm * 26" / 0.89 (4th) / 4.571 / 336 = 65 MPH
The 336 is a constant that simplifies the full formula which is a product of the ratios of all rotational components from engine to wheels. For KM/H replace the 336 with 210. The full formula is:
= engine * gears * differential * wheels * unit conversion factor
= rev/min * gear ratio * R&P ratio * (pi * d")/rev * 60min/hr / (63360"/mile * 1hr/60min)
where: rev/min = RPM,
gear ratio = selected gear,
R&P = ring & pinion ratio,
pi = 3.141526535,
d = diameter of tire in inches
Download these spreadsheets:
They show all of the gear sets that VW transmissions can be built from and the road speeds they produce at various RPMs. The stock 091 gearing is highlighted by row.
All 68-79 bus transmissions have the same 1st, 2nd and 3rd gear ratios.
These are the variations in gear ratios for US models:
|VIN||Reverse gear ratio||4th gear ratio||R&P ratio||R&P * 4th|
|218 000000 - 211 2 276560||3.61||0.821||5.375||4.413|
|211 2 276561 - 213 2 068547||3.80||0.821||5.375||4.413|
|213 2 068548 - 213 2 300001||3.79||0.889||5.375||4.778|
|214 2 000001 - 215 2 300001||3.79||0.889||4.857||4.318|
|216 2 000001 - 219 2 300001||3.79||0.889||4.571||4.064|
There is some debate whether or not all 73 models had the 0.821 4th gear or whether the cutoff VIN is higher. The table above is what the factory published in the US. See the references section for more info.
091 Transmission (6 ribs on top)
091 Bell housing
04/2/07 - Created
09/08/11 - Fixed broken photos, added translate button, updated footer
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