The VDO folks tried to make one basic speedometer fit a whole batch of applications. For Saab alone, this basic speedo worked for 10 or so different cars!
Different transaxle final drive ratios meant that they had to have different ratios INSIDE the speedometer so any one car had the correct speed and miles recorded. This photo shows the left side of a Saab VDO speedo with the little side transfer gear set removed.
At the bottom of the photo you can see the transfer gear set.
I did the exploded view drawing below to show the side transfer gear set at the extreme right. The lateral transfer gear set is just to the right of the main speedometer body. The ball is used to retain the lateral gear set. You need a special, very small, puller and press to push out the ball so the lateral shaft can be removed without damaging it.
The exploded view also shows the trip register reset mechanism [extreme left in the drawing]. The tiny cog wheels at the top and bottom advance the number wheels at the appropriate time.
This is a close-up drawing I did of the main speedo body, with the side transfer gear set still in place. This gear set must be adjusted for the correct mesh with the gear on the end of the top [primary] mileage register.
The four arrows show the worm drive gears on the two shafts and the two gears--that are selected for a given transaxle ratio. The gears are different diameter and have a different number of teeth. So a single speedometer "main frame" can work for a 2-stroke with 5.23:1 final drive as well as Sonett V4 with a 4.67: final drive, by changing the gear sets. Clever, eh?
My next blog will be about some special tools and why I needed them.
City Car Design
7 years ago