engine angle

[Tom Broome]

Originally Posted by Mike Savelle

No big tires for me , its a stock eliminator small block Camaro, and I finally got it to 3 degrees. what does the broken back mean? Thanks for all the replies

Due to the increased diameter of big tires, the axle centerline sits lower in the car than original. Essentially the car is lowered in the rear. The drive shaft has to travel "uphill" from the transmission to the pinion. We tend to want the pinion to driveshaft angle smaller on the upper plane for drag racing, think "positive pinion angle". You end up with the shallow driveline angle on the upper plane at both the transmission and pinion, forming a very shallow "U" or broken straight line, the driveshaft breaks upward at the transmission and again at the pinion. I have a friend that refers to that condition as the "skip rope angle". Not good. Doesn't "broken back" sound easier?

Ideally, you should be able to draw a straight line from the front of the crankshaft, through the transmission, and straight on through BOTH pinion bearings. That way, when you adjust the driveline/pinion angle at the axle housing, the included angle above the pinion is equal to the included angle BELOW the transmission. That way, just like the referenced guide, the "uncancelled angle" is zero.

[FireSale]

Here is a graphic that shows what Tom is talking about in slightly different words. The results of the output and input angles should be as close to 0 as possible in theory. In practice you need to add some down angle to cancel out the tendency of the differential to try to wrap up the springs under launch load. I've read 4 degrees down for the pinion but I don't know how bars like Calverts cancel that out.

[CMcAllister]

A common configuration on a big tire car with the engine in the stock location is having the engine 2-3 degrees down from level towards the back, the pinion down 2-3 degrees from level and the driveshaft very close to level - the broken back or skip rope effect. It's a compromise to avoid a positive operating angle on the rear u-joint.

An ideal situation is having the crank/trans CL pointing directly at or just below the rear u-joint CL with the pinion angle (operating angle) being negative (pinion down) to some amount which is determined by suspension/traction device used. i.e. the crank/trans shaft and pinion at or near parallel with the appropriate pinion/driveshaft operating angle. Not easily done in many applications.