Chevy Chevelle Rear Axle Swap - The Great Rear Axle Comparo

This Strange Engineering drawing of a 9-inch rear gear assembly clearly illustrates the relationship of the pinion to the ring gear and hypoid offset. The closer the pinion is to the ring gear centerline, the less power is required to turn the gearset.

This Strange Engineering drawing of a 9-inch rear gear assembly clearly illustrates the re

Hypoid Offset
What the hell is that? This dimension is defined as the distance between the pinion gear and ring gear centerlines (see illustration). If the pinion gear were placed directly in line with the ring gear centerline, the hypoid offset would be zero, and this would really be a bevel gear rather than a true hypoid. All automotive rearend gears are a hypoid design because the pinion gear is positioned below the centerline of the ring gear to increase tooth contact and strength. Since free lunches rarely occur in engineering, the cost of this increased tooth contact is greater sliding friction between the gear teeth.

The Ford 9-inch has the greatest hypoid offset, which means it should require more power to drive this ring-and-pinion set compared with a typical Dana 60 rearend that employs the least offset. When you look at the accompanying chart, you can see that the Dana 60 rearend appears to have an advantage based on both ring-gear diameter at a massive 9.75 inches and the smallest hypoid offset.

This comparison of a Dana/S60 9 3/4-inch ring gear to the 12-bolt's 8.875-inch overall diameter redefines the concept of big.

This comparison of a Dana/S60 9 3/4-inch ring gear to the 12-bolt's 8.875-inch overall dia

The Ratio Game
If you are looking for the broadest spread in gear ratios, the 9-inch has the field covered, but only slightly. Between factory and aftermarket ratios, the Ford spans from a Bonneville-busting 2.47:1 all way up to a tach-twisting 6.14:1. But even that isn't as deep as the S60's 7.17:1 rear cog. As a gear ratio becomes numerically larger (lower in terms of a ratio), the pinion gear diameter becomes smaller with fewer teeth. This reduces durability, which is when the 9-inch's pinion support becomes really important.

By Jeff Smith

Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!