While it may be common knowledge to the veteran gearheads among us, the existence of pinion offset and all that it entails is potentially foreign to those new to the automotive and racing hobby, and with the help of Curt Perry at Chassis Engineering in West Palm Beach, Florida, we’re going to provide a little 101 education on that very topic so you don’t get in a bind…quite literally.
Before we go any further, it should be noted the difference between pinion offset and pinion angle. The latter, pinion angle, refers to the vertical relationship between the crankshaft/transmission and the driveshaft and differential. While pinion angle is a Pandora’s Box topic of its own that we could easily devote a few thousand words to and emit as many opinions as there are readers, the end goal is to have a perfect line drawn through the driveline, from the centerline of the crankshaft right through the pinion while under load. However, this topic renders a range of thoughts, depending on who you ask, on the proper angle at rest to achieve this perfect angle once the suspension and other factors come into play under load. Too much angle places the universal joints in a bind, robbing performance, causing unwanted vibrations, and ultimately stressing parts to the breaking point if left uncorrected.
Pinion offset, meanwhile, is a bit more simplified of a topic. Referring to the lateral position (the side-to-side positioning of the driveshaft in the car), pinion offset is the pinion’s location relative to the chassis centerline, and is a result of both the inherent design of many OEM differentials and the need to offset the engine and transmission for a variety of reasons.
The vast majority of domestic OEM high-performance applications, be they GM, Ford, or Chrysler, feature rearends with the pinion offset to the passenger side of the vehicle to match the position of the engine and transmission. Each manufacturer has its reasons for doing this, from fitment of the steering column to providing the driver more room in the cabin. The tried-and-true Ford 9-inch is offset 15/16-inch, Ford’s 8.8-inch housing is offset 9/16-inch from the factory, and GM 10- and 12-bolts rears are offset 1/2-inch, for example.
Regardless of the horsepower range you’re in, the goal is to achieve a straight line through the crankshaft centerline to the pinion, if you’re looking down from above. Like pinion angle, a driveshaft that’s crooked in the car puts unneeded strain on the driveshaft and driveline components and could result in vibration and breakage.
If they don’t make it so that the pinion is offset that same amount, then the driveshaft is crooked in the car. – Curt Perry, Chassis Engineering
This, Perry shares, is where the confusion comes along, as racers cut apart factory vehicles for back-half chassis jobs and choose to center the engine — once you do so, the pinion, and therefore the entire housing, have to be moved. With that comes cutting of drivers’s side axle tube and measuring for the proper axle length to accommodate the adjustment (back-halving the car and fitting larger wheels and tires, which many of you reading likely are, will necessitate cutting both axle tubes down in order to narrow the housing).
“Where the confusion comes in is when guys are going to back-half a car — they don’t know or realize that the factory moves the engine and transmission off to the passenger side. If they don’t make it so that the pinion is offset that same amount, then the driveshaft is crooked in the car.
“Most GM cars are shifted over half an inch, for example, so if you aren’t moving the motor back to the middle of the car, but you’re building the rear end as center pinion, then it’s going to be off that half an inch,” Perry continues. “They need to have it built so the pinion is half an inch offset to the passenger side to match.”
Most GM cars are shifted over half an inch, for example, so if you aren’t moving the motor back to the middle of the car, but you’re building the rear end as center pinion, then it’s going to be off that half an inch. – Curt Perry
Perry uses the ever-popular Fox-body Mustang to illustrate a typical scenario for a racer modifying the chassis and/or rearend.
“Let’s say you have a Fox-body you’re going to back-half and you want to narrow the rear and it’s on the stock engine mounts. That’s pretty straightforward — everyone knows they’re offset to the passenger side by half an inch, and everybody can go on about their business. The problem is, if you want to back-half it and you want to put it on motor plates, the question is, are you moving the engine back to the center of the car, or are you going to leave it with the half-inch pinion offset? That’s why we go through the whole scenario with the customer and have them measure everything. Because we don’t know if the previous owner did it correctly or not.”
Of course, accuracy of course is key here, but performing this measurement isn’t difficult. Chassis Engineering, as does any good chassis builder, advises using a framerail, a suspension pickup point such as leaf spring mounting locations, or similar to perform your centering (or offset) measurements. This can be done with a tape measure from point-to-point, or with a plumb-bob on the jig or floor with markings to measure to and from. This applies to both the rearend/pinion positioning and the engine location itself, although the engine and transmission location is generally more simplified since you can use the front framerails as your measurement point.
“Any good pickup point that’s the same on both sides will work. You could go from inside a framerail to inside a framerail a lot of times at the front and do the same out by the tailshaft of the transmission. That offset should be the same front to back and if it’s not then you need to figure out where your pickup points are,” Perry shares.
In the rear, Perry explains, “you’re better off using the frame more than anything else — you’ll have to drop a plumb-bob off the inside or outside of the rail, drop it down and then measure point to point.”
A mismatched pinion offset, as alluded to earlier, simply puts too many stresses from too many different angles on the u-joints and bearings, and in due time, that error will rear its ugly head.
“Your u-joint is going to be doing too many things — if you have some pinion angle in it, and the driveshaft is off to the side, now you’re making the u-joint do what it’s designed to do, but it isn’t going to be happy about it, because everything is going to be in at an angle. With pinion angle, you’re shooting to have the driveshaft, under load, in line with the pinion. That way the u-joint is neutralized and all it’s doing is driving the car — it’s not having to move or flex forward, back, left, or right. If you have the driveshaft off to one side, now you’ve exacerbated the problem.”
A key thing to factor into any change in pinion offset is axle tube length and axle length. If the engine is centered, the pinion also has to be centered in the chassis. This is where it becomes decision time.
If you opt to leave the offset in the driveline and plan to keep the existing housing, then you can order your axles and you’re all set. If, however, you want to center things up but use the existing housing, it becomes a bit more complicated. In order to shift the pinion to the center, the housing itself has to be moved to the driver’s side of the car, leaving the axle tubes uncentered. To compensate, you’ll need to measure the housing from the pinion center out and cut down the driver’s side axle tube. This also leaves the passenger side housing end narrower than it was previously. Although there are certainly examples out there of housings with extra tube welded on, it generally isn’t advised, and at this stage, Perry recommends either narrowing the reared overall, or having a custom rearend housing built with the third member centered to the desired length of the axle tubes.
Of course, either approach will require a custom set of axles — you’ll want to measure from the outside of the housing end to the center of the pinion on each side and order a new set of accordingly.
In truth, there’s no science to pinion offset nor is there a right way or a wrong way — center or offset pinion — but like anything else on a racecar, you can save yourself a lot of headache down the road by doing your homework up front, asking the right questions of the right individuals, and being deadly accurate with your measurements and calculations to get it all lined up the first time. While it is a simple concept, it’s most definitely not an easy fix if you get it wrong.