Equal Length Driveshafts
If you find that your car turns for you with heavy throttle input, this page is for you. This is most commonly called torque steer. A major contributor to torque steer is a force resultant from unequal driveshaft angles. Let me explain what I mean:
The diagram above is a simplified representation of the driveshaft layout used in the stock 2GNT. The driver's side driveshaft is significantly longer than the passenger's side driveshaft. Measuring from horizontal, the acute angle formed between the rotational axis of the driveshaft and the rotational axis of the outer CV joint is greatest with the shortest driveshaft. Why should you care about that angle? You should care because that angle diverts a portion of the torque applied to the driveshaft to the suspension instead. As the angle increases, the percentage of torque applied as a force on the suspension also increases. That force applied to the suspension will try to steer its wheel in one direction or another, depending on suspension geometry. Since the driver's side and passenger's side wheels are tied together by the steering rack, you must sum the forces applied to each side and see which "wins". Back to the diagram, angle D is clearly larger than angle P. This means that force D is larger than force P. If you sum those forces, you end up with a smaller resultant force R in the same direction as force D. Force R represents the bias to the driver's side suspension. This doesn't necessarily mean that the car will steer to the left. Your exact suspension geometry will determine which direction the car steers when force R is applied. Since the force applied to the suspension is a percentage of the torque applied to the driveshaft, the steering effect becomes increasingly noticeable as engine output increases. The average consumer might not notice any torque steer on a stock 2GNT. Throw a turbo on and the car wants to jump into the other lane as soon as boost comes up.
What Can You Do About It?
Clearly, it would be best if your angles D and P were both equal to zero. That would make force D and P equal to zero as well. You can adjust your ride height with shorter springs and engine height with lower mounts to bring your driveshafts in line with the CV joints. Keep in mind that weight transfer due to acceleration and deceleration, as well as humps and dips in the road will affect the angle between the driveshaft and CV joint. In the real world, this angle will often not be zero and should be taken into account. So, the next step is to ensure that angle D is equal to angle P. This will make force D equal to force P, canceling out any bias to one side and the steering effect. The easiest way to do this is to add an intermediate shaft such that the driver's side and passenger's side driveshafts are equal in length. This will make angle D equal to angle P.
MS Paint diagrams are all well and good, but how can we actually install equal length driveshafts on a 2GNT? In case you didn't know, the driveshafts on a 2GNT with T-350 manual transmission are the same as a 2GNT with A604 (41TE) automatic transmission. Fortunately, the Dodge Avenger has an option for a version of the A604 (41TE) mated to their 2.5l V6. This option included an intermediate shaft, shown above. If you hit the junkyard, look for 1995-2000 Dodge Avengers or Chrysler Sebrings with a V6. The four cylinder cars do not have an intermediate shaft. Since these cars are built on the same platform as us, the driveshaft that mates to the intermediate shaft fits right into the 2GNT.
If you prefer to buy a new driveshaft rather than reuse a junkyard one, they are cheap and available. The EMPI 80-1964 shown above was $70 when I wrote this. That would be a stock replacement strength part. If you are looking for something stronger, The Driveshaft Shop could probably build one, although the inner joint might have to be custom.
Holding Your Shaft
So now that you have an intermediate shaft and driveshaft, you need something to hold all of this together. The Avenger intermediate shaft comes with a sealed bearing and mounting flange. You need to fabricate a bracket to hold the mounting flange rigidly to the engine block. There are many ways to do this, but I will only discuss my final design, shown above. The two holes at the corners accommodate bolts that attach to the engine block and the two holes in the center accommodate bolts that attach to the intermediate shaft flange. I used a spare engine block and transmission to mock this up and drew my design in SolidWorks.
I sent the drawing file off to a machine shop and had them laser out the flat parts from 0.25 inch 304 stainless steel plate. The tubular spacer was cut and drilled from 1 inch 304 stainless steel bar stock. It would have been cheaper to use painted mild steel, but I didn't want to worry about rust. You can download my DXF files needed to cut the plate here and here, as well as the PDF for the tubular spacer here. I TIG welded up the parts to make the final bracket shown above. This thing is a little over-designed, but I don't like to lay awake at night worrying about whether my parts will break.
Speaking of breaking, I recommend that you use strong fasteners to keep all parts firmly in place. I used Class 10.9 with a black oxide coating all around. A parts list and exploded diagram with part numbers is shown above. McMaster Carr has all of these parts on the shelf at decent prices.
Wrench sizes and torque values for the bolts and nuts are shown above. If you substitute alternate bolts or materials, you will have to adjust the torque.
Here is the equal length driveshaft (ELDS) bracket bolted to the block, with a 1997-1999 crankshaft position sensor. Note that I have rotated the sensor away from the bracket and cut a notch to clear the mounting bolt. The 1995-1996 sensor is smaller, but you will probably still have to notch it.
The inboard joint is fairly close to a part of the block right above the oil filter. The crimp on the metal band that secures the rubber boot to the joint may have to be smashed flat to clear. If you still need more clearance, you can add a thin M10 washer to space the intermediate shaft flange away from the ELDS bracket. You will need to trim some plastic from the splash guard that the driveshaft runs under. If you need some help removing or installing the original or replacement driveshaft, see Changing_an_axle. If you haven't had a four wheel alignment in a while, you might consider doing that after the installation. You'll get the best results from this modification if your car is properly aligned. To test your installation, find a flat road with very little crown to it. Run as much torque as you can through the driveshafts without breaking traction. The car should tend to drive much straighter. For me, it went from jumping into the opposing lane to only needing a finger to hold the wheel. If your car still wants to jump into the opposing lane, you may have another issue (worn suspension bushing, soft motor mount, or bent suspension arm).
1. Torque steer can come from a variety of sources. Only one of them is affected by this modification, but I believe that we attacked a major one.
2. There are a variety of factors that can magnify the torque steering effect. That is why some owners experience it more profoundly than others.
3. For my simplified explanation at the beginning of the article, I have made several assumptions. Most important are the assumptions of constant angular velocity of the driveshaft and constant load. Because of that, we can ignore the rotational moment of inertia and torsional stiffness of the components. This allows us to view the part of the problem which we can most easily solve.
4. For people that don't believe in equal length driveshafts, I ask you to consider the high output FWD vehicles supplied from the factory. The majority will incorporate an intermediate shaft. Especially interesting are low output models with unequal length driveshafts and high output models of the same vehicle with equal length driveshafts. They don't use equal length driveshafts to save space, weight, or cost and they certainly don't brag about them in commercials. They do it because it improves driveability enough to outweigh the penalties in space, weight, and cost.
Contributed by Corbin
Cars Modifications Control Transmission Equal Length Driveshafts Alt
Last modified on 2015-03-03 21:30:59