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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.



Enough Theory!





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 PT Cruiser turbo has an option for a version of the A604 (41TE). This option included an intermediate shaft, shown above. This is part number 5274912AC. If you hit the junkyard, look for 2003-2006 PT Cruiser turbos with automatic transmissions. Non-turbo PT Cruisers do not have an intermediate shaft and manual transmission PT Cruiser turbos use a different style intermediate shaft. Neon SRT4s use a different style intermediate shaft as well. Your best bet is probably buying direct from the dealer. When I bought mine, it was under $65 from the dealer. As you may have guessed, this intermediate shaft is of appropriate length to accommodate a custom passenger's side driveshaft of nearly identical length to the driver's side driveshaft. The transmission side of the intermediate shaft fits right into our transmission. The driveshaft side of the intermediate shaft has a unique spline design. Your custom driveshaft will need an inboard joint that mates to that spline design. The outboard joint of the custom driveshaft just needs to mate to your wheel hub. That custom driveshaft is shown above.



The Custom Driveshaft





The basic components of the custom driveshaft are shown above. The overall compressed length will be about 20.8 inches. I have given you EMPI brand part numbers because they have good literature and were helpful over the phone. The EMPI parts would have a capacity similar to stock and will hold most of what you can throw at them with street tires. I had Constant Velocity of Ocala make my driveshaft in a strength similar to stock. At the time, it was $280. The Driveshaft Shop offered to build a Level 2 axle for $450. That would hold about 500 hp (this is approximate because rotational inertia and tire grip have a lot to do with breaking driveshafts). A third alternative is to have a machine shop cut and respline your stock 2GNT passenger shaft to mate up to the inboard joint of a PT Cruiser turbo driveshaft. I attempted this route only to find that the axle was hardened too much for the machines my shop had. Depending on how deeply the shaft is hardened, your newly cut splines may be softer than the originals. I suppose that you could have the shaft annealed and hardened after cutting, but that will require even more experience to do right. A driveshaft that has been overhardened might fracture and cause massive damage. The point is that you have at least three options for making this driveshaft. You can spend as little or as much as you want.



Holding Your Shaft





So now that you have an intermediate shaft and custom driveshaft, you need something to hold all of this together. The PT Cruiser 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 larger holes accommodate bolts that attach to the engine block and the two smaller holes 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 spacers were cut and drilled from 0.875 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.






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. The bolts that attach to the block are M10 and 50 ft-lbs is an appropriate torque for that. The bolts that attach to the intermediate shaft flange are M8 and 25 ft-lbs is an appropriate torque for that. I used a couple heavy M8 flat washers to adjust the depth of the intermediate shaft splines into the differential. You want as much of the splines engaged as possible, but you do not want the shoulder of the shaft to apply an axial load to the differential (don't let it touch).






Here is a picture of the equal length driveshaft system (ELDS) installed on the car. Maybe you noticed how close the bracket is to the crank sensor. It doesn't touch the 1995-1996 sensor, but does touch the clip on the electrical connector. If you have a 1997-1999 sensor, you can either hit the junkyard for an earlier sensor/connector or try Equal_Length_Driveshafts_Alt. The inboard joint is fairly close to the oil filter, but did not interfere. 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 custom 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).



Note:
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
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Cars Modifications Control Transmission Equal Length Driveshafts


Document statistics: Last modified on 2012-12-08 20:15:19 by Corbin


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