If Your Car has the Shakes, Tremec’s Simple Driveline Angle Finder App Can Help Identify the Problem

By Jeff Smith

Photos: Jeff Smith and Tremec

It’s one of those annoying things that often happen on modified street cars – especially street rods. You’ve just finished installing a whole new drivetrain – a new engine and transmission – you had a custom driveshaft built by a reputable company and it bolted up nicely. But on the initial freeway test drive, you notice a vibration – it’s subtle but it’s there. The car cruises nicely at 55 to 60 mph, but once the speed climbs to 70 or above, there’s a vibration. Or perhaps it’s during deceleration, and it feels like the driveshaft is going to fall out because it is vibrating so badly.

These situations crop up quite often, usually in conjunction with a major transmission swap like to a six-speed manual. This scenario prompted a discussion with Tremec’s Nate Tovey. He told us that many builders who convert a muscle car to a late model transmission experience vibrations that are often only remotely related to the transmission itself. The issues, as Nate showed us, are more closely related to the way the transmission is installed.

In an attempt to educate Tremec’s customers and gearheads everywhere, Nate showed us Tremec’s new digital Driveline Angle Finder program that is a free app for smart phones that looks like an excellent tool for deciphering those confusing angles. We decided to try this tool out on our pal Scott Gillman’s nice ’65 Chevelle with a Tremec T-56 gearbox. Scott had to raise the floor on his Chevelle and make his own crossmember, so this afforded us a great opportunity to evaluate his effort.

The Tremec Driveline Angle Finder is a free application that loads into your smart phone. The program takes advantage of the built-in digital angle finder that automatically orients the display vertically or horizontally. This built-in level will display angles out to a tenth of a degree. Before playing with the app, we’d read stories on measuring driveline angles but frankly, the material was confusing. The Tremec app approach brings the whole situation into perspective and does all the calculations for you. But in case you don’t own a smart phone, we’ll also show you how to measure these angles with an inexpensive angle finder and do the math to determine your driveline angle. To put this in perspective, we measured four cars with the Tremec Driveline App – and only one of them passed. You might be surprised at what you learn – we certainly learned how much we didn’t know.

The Tremec App

First thing we had to do was find the app on Tremec’s website. We’re digitally challenged so it took us awhile. Click on the “Aftermarket” icon in the lower right corner of Tremec.com. In the light gray box entitled Motor Sports & Aftermarket there are a series of different paths – find the one marked Driveline Angle Finder App – click on that and you’re in. Or, since you are on the internet already, hit the link right here.

Once the app is loaded, all measurements must be made with the car at ride height. Scott’s Chevelle sits too low and it’s too challenging shooting photos while crawling under cars so we put the Chevelle on a two-post lift. The best lift for this test is a drive-on style. Unfortunately, ours lifts the frame. The trick is to have the rear axle at ride height, so we removed the rear coil springs to allow us to place the rearend at ride height. We also used small shims under the rear lift points to ensure the body was level. But the reality is that all we’re really doing is comparing angles, so the actual numbers relative to true horizontal are really not important.

With the car situated properly, the Tremec app begins by measuring the engine/transmission angle. Locate a suitable machined surface to test the engine and transmission angle. Don’t use the engine oil pan or the transmission pan as these pieces are notoriously not level. The machined oil pan rail on either the engine or automatic transmission will work. If the trans is a manual, sometimes you can access the bellhousing’s trans mounting surface as it is machined parallel to the bellhousing. Other options include the starter mounting pad, or even the vertical machined portion of the transmission output shaft with the driveshaft removed. We removed the driveshaft and used the output shaft and then compared that to a machined pad on the driver side of the T-56 that was identical. This way we could leave the driveshaft in place for the test.

The second measurement is as easy as laying the phone on the driveshaft. The third measurement determines the pinion angle. Again, it’s important to ensure accuracy by using a machined surface. Scott’s Chevelle uses a 9-inch Ford so we used the machined end of the pinion flange. There is an illustration used in the beginning of the app that helps explain how the engine/trans and pinion angles affect the driveshaft operating u-joint angles. In nearly all production cars the engine and transmission module is installed with a slightly down-at-the rear angle. This is one of the most important points to remember.

The result screen may seem a little confusing because what is indicated as Angle 1 is not the engine/trans angle but instead the front u-joint operating angle. This is the difference between the engine/trans angle and the driveshaft angle. If the engine is tail-down at 2.0 degrees and the driveshaft is 1 degree, then the Angle 1 result is 1.0 degree. Angle 2 on the result screen is another computed angle. If the pinion angle is roughly parallel to the engine/trans angle (meaning it is in a nose-up angle), then the 1-degree driveshaft angle is subtracted from the 1.5-degree nose-up pinion angle to get a green rear u-joint angle of 0.5 degrees.

As long as the two u-joint angles are in spec, our overall operating angle will also be good – indicated in green. Using our above example the overall driveline operating angle would be engine/trans of 2.0 – 1.5 degrees of pinion up-angle = 0.5 total operating angle. Where many applications run into trouble is when the engine/trans tail-down angle is an opposing angle to a nose-u pinion angle. Opposing angles create vibrations and will always produce a failed result.

The goal for a proper driveline angle is to create parallel angles between the engine/trans and the pinion. Now let’s look at a combination that falls outside the acceptable range. Let’s make up some numbers to see how this works. Let’s say the engine/trans (Reading 1) is 1.8 degrees down at the rear. The driveshaft angle (Reading 2) is at 2.0 degrees and the pinion angle (Reading 3) is 2 degrees nose down. This creates two intersecting lines that will create a vibration and a driveline operating angle that is out of spec. Again, the Tremec app results do not display the measured angles, but instead displays the u-joint operating angles. Angle 1 on the results screen would be 1.8 degrees minus 2.0 which is a negative number but is less than one degree so it is in spec.

The second u-joint operating angle (Angle 2 on the result screen) is the pinion compared to the same driveshaft angle. If the angles are parallel, then we subtract but if they are opposing (as in this case), then we add them. With a driveshaft measurement of 2 degrees and an opposing pinion angle of 2 degrees, this gives us a total of 4 degrees which is past the maximum spec of 3 degrees. It’s easy to know the system is out of spec because the engine/trans and the pinion are at opposing angles. The fix requires changing the pinion angle to raise it to a near parallel nose-up attitude of roughly 1 degree. This is a change significant of over 3 degrees. The modified pinion angle will also affect the driveshaft angle (Reading 2) and produce an overall operating driveline angle that is within spec.

Tovey emphasized that what the app is really measuring and displaying is the overall operating angle of the driveline. Tremec’s maximum spec is no more than a 3-degree overall operating angle for use in applications where the driveshaft may spin over 5,000 rpm. Having said this, if you are measuring a system where the driveshaft will never see the high side of 4,000 RPM, then perhaps measurements slightly beyond spec may still be acceptable. With current overdrive transmissions, 4,000 RPM is not an unusual spec. As an example, take a Pro Touring car with aTR-6060 six-speed running at the Silver State Classic in the 150 mph class with a 3.31:1 rear gear and a 0.63:1 overdrive. With a 26-inch tall tire, at 150 mph the engine will only be spinning 4,042 RPM, but the driveshaft will be whipping around at 6,416 RPM! That’s when you discover if your driveline is in spec.

If you find when measuring your car that the angles are in the red, there are several options available to change the angles. Lowering or raising the engine/trans (Reading 1) isn’t overly difficult but may require spacers or perhaps a redesigned crossmember. Raising the back of the trans may cause floorpan interference. Changing the pinion angle depends upon the style of the rear suspension. Leaf spring cars can be adjusted by using angled shims available from Lakewood by placing them between the leaf spring and the spring mount. Altering the pinion angle for a coil spring car like a Chevelle or a Fox body Mustang is a bit more involved. This will require a pair of adjustable upper control arms. Altering the length of the upper arms changes the pinion angle, making the adjustment easy. A key step is to ensure that both arms are exactly the same length so no preload is inadvertently added into the chassis.

It’s important to note that this Tremec tool only measures these angles in the side view plane. A true driveshaft angle discussion must also include the top view where the pinion may be offset slightly from the engine centerline. Combining the top and side angles creates what is called a compound angle. The top angle offset problems are less common with production-based components, but they can occur when dealing with custom-built machines where the lateral pinion offset can create an inadvertent compound angle and the resultant driveline vibration. We won’t get into compound angles in this story, but it is worth mentioning as a possible vibration source that may need to be measured.

Right about now you may be thinking that this description can’t be correct because it is counter to many recommendations you’ve read for drag race cars with leaf springs where the pinion angle is usually placed nose-down by as much as 3 to 4 degrees. This severe nose-down pinion angle is in anticipation of the leaf springs wrapping up under load, which shortens the leading portion of the leaf spring causing the pinion angle to move up. Tremec acknowledges that setting pinion angle and the resultant driveline angle should be in anticipation of suspension movement in drag, circle track or road race vehicles relative to how the driveline will be used.

Obviously, this means that if you think the pinion angle will change under load and that is the way the car is to be used, then setting the static location of the components will change depending upon the amount of anticipated pinion angle movement. It’s also worth noting that minimizing driveline angles within acceptable limits can pay rear-wheel horsepower dividends. There are situations where a change in pinion angle can pay off with gains in rear-wheel horsepower on the chassis dyno.

A big variable that directly affects these angles is ride height. For cars with adjustable ride heights, this may create problems for driveline angle. The best way to optimize this is to optimize the driveline angle at the driving ride height. This way, minor changes to ride height may not affect the driveline angle enough to vibrate. On the other hand, a system that is barely in spec will probably exhibit problems with major changes in ride height. For example, we’ve had problems with driveline vibration when the car is loaded down with gear in the trunk. The car may run fine at normal ride height, but extra weight in the trunk changes the ride height and could cause a significant vibration at highway speeds.

Hopefully, this short driveline drive-by has helped you discover a nice, vibration-free freeway experience or at least educated you for what to look for when trying to solve a sticky problem. Once you understand how a system truly operates, it’s much easier to come up with a solution.