When it comes to building a street\strip drag car, making concessions are part of the game. Big power coupled with a stiff chassis and stock leaf springs is a recipe for no traction. There are band-aids for leaf springs, like slapper bars and installing stiffer springs, but the inherent problem remains- there is no static connection to the chassis. The rear end is hanging off the chassis on a stack of spring steel leaves that shift, twist, and wrap up, leaving much to be desired. In addition, the leaf springs sit outside the subframe rails, seriously cutting into the wheel well area. Since we are running slicks on the Royal Scamp, we needed a better solution.
There are three main styles of rear suspension (not counting IRS, which is a whole other animal), leaf spring, ladder bars, and the 4-link. While some people swear by leaf springs, you would be hard-pressed to find a serious drag car that uses the stock leaf springs. Most drag cars use a 4-link style suspension. The 4-link offers the maximum amount of adjustability, which allows for chassis tuning at the track. The 4-link uses 4 separate bars (two on each side, upper and lower) that connect the chassis to the rear-end. These may run parallel or be triangulated, with top bars running outward from the center of the chassis to the rear. The bars connect to the chassis and rear via rod ends that bolt to plates with 8-12 potential mounting points, providing quite a few tuning options. In the middle of the three is the ladder bar. The ladder bar provides adjustability on the front side (bar to chassis) and a static mount to the rear. Ladder bars are easier to tune for the novice than the 4-link and, when set up correctly, provide the same benefits. Like the 4-link, ladder bars use a coil-over shock to support the weight of the vehicle. This also allows for better tuning, such as ride height and with adjustable shocks, the damping factor.
What makes ladder bars (and the 4-link) so effective is the controlled movement of the suspension. The travel will always occur the same way, every time, in the same arc. This static connection to the chassis controls wheel hop and promotes better weight transfer to the tires for optimum traction. Leaf springs simply cannot do this; there are too many variables, what one side does can differ from the other side. The choice for the Royal Scamp is ladder bars.
Installing ladder bars is a big task. There are no bolt-on kits for the Mopar A-body. The factory leaf-spring mounts sit to the outside of the frame, so they can’t be utilized at all. Everything had to be custom built. We chose a 36” ladder bar from Chris Alston’s ChassisWorks. The kit came with the ladder bars, coil-over shocks, track locater and tubing for mounting the system. We went with the VariShock double-adjustable shocks for maximum tuning. We could have done the 32” bars, which would have pulled the front crossmember rearward about four inches (which would have put it on the factory subframe), but the longer bars provide more stability and since we were adding subframe connectors, it was not a problem.
One key area that added difficulty to this project was the rear coil over mounts. The factory shock mounts are not suitable for supporting the entire back end of the car, the floor was pretty rusted, and the Scamp is getting a 9” housing, so much of the trunk floor was cut out. The shocks have a maximum extended length of 14-inches, and minimum travel of 13-inches. The suggested travel is right in the middle at 13.5-inches. This is where you need to set the shocks for mock up. With our rear ride height of 10-inches and the shock mounted in the middle of the shock mount, the top of the shock mount was above the stock floor. We could have dropped it down, but then we would have lost some adjustability. The shock crossmember was welded to the rear down bars on the roll cage. This might not be acceptable for some builders, so choose your shocks and springs carefully. A shorter 10-inch spring and matching shock would probably fit under the stock floor, but take your own measurements first.
This project that is not for the faint of heart. It took us about a month to get it all together, which was mostly evenings and weekends, including all the measuring, and cutting out the floors. There is probably 100 hours worth of work on this one. With our Toyo 9-inch slicks mounted to 15×10 Centerline “Fuel” drag wheels, we had about an inch of clearance on either side (stock frame rail to the outer wheel well lip). Gaining this much room alone is worth the effort of the swap, not to mention the superior traction. Another word of caution- take your time and measure 5 times, this is one area you don’t want to rush things.
01. First thing on the list was assembling the ladder bars. Each end uses threaded rod ends. Each end was threaded in 3\4 of the way, to leave room for adjustment.
02. We used a sheet of plywood on the floor to set up the bars. We squared the bars 90-degrees to the vertical center of the rear diff mounting point and measured the actual length of the bars. This is an important step in assuring the bars are located front to back in the wheel well.
03. During the mock up, we checked where the bars should sit at ride height. The lower portion of the bar (the long side) should rest parallel to the ground at ride height. We also measured and marked the wheel center to front ladder bar mounting point at ride height.
04. You must measure carefully several times to get the crossmember installed properly. Using the measurements taken earlier, we set this position on the passenger side, measured to common points (driver and passenger) both in front and behind that crossmember center point, and transposed those measurements to the driver side. This ensures the crossmember runs perpendicular to the subframe connector rails.
05. The front ladder bar brackets were used to mark the subframe. With the bracket rotated so that the pick-up points run 90-degrees to the ground (straight up and down), and the upper edge just touching the floor, the connector was marked. Note the roll cage tube coming through the floor- this will be tied to the crossmember tube for extra stiffness.
06. The rails were trimmed out with a plasma torch. Cutting half-moons with a hole saw is tough on the car. The cuts were dressed with a grinder.
07. We used a special adjustable rear-end to set the height of the car. We measured both the vertical wheel centerline and the ride height. More on the adjustable rear in a future article. This unique device allowed us to mount the wheels and set the width of the rear end so we could narrow our 9-inch housing.
08. Once the rear end had been built, the rear work could begin. The ladder bar crossmember was held in the car with jack stands and the position of the mounts were set. We measured 2 inches from the inner side of the subframe connectors to the outer mount. The ladder bar rod ends were bolted in place to keep everything lined up.
09. The mounts were tacked in place under the car. Only one side, the other side must match, and that comes later.
10. The entire crossmember was removed and the brackets were fully welded. The outside and inside edges of the brackets should be welded up.
11. Next, the mounts for the other bar were assembled in the same manner and the entire assembly placed on the floor. The fully welded side ensures the correct geometry for the unwelded side. We measured each side to ensure the opposite mounts were in the right place. Nice and simple.
12. Then the crossmember was reset into the car and welded in place.
13. The rod ends were reinstalled into the bars (with anti-sieze) and bolted in place. We set the ride height to the center position, so we had some adjustment room.
14. Then the rear end mounts were installed and placed up to the rear housing.
15. The optimum position for the rear end is 2-3 degrees down angle to the front of the car. This provides the right geometry for the driveshaft rotation. A magnetic angle protractor is really helpful for this project.
16. The ladder bar mounts were tack welded in place (only tack welds at this point), as well as the shock mounts. You can put the shock mounts anywhere, but the further apart, the better. We kept it simple and placed them between the ladder bar mounts.
17. The shocks (no springs yet) were assembled and bolted to the lower mount. The upper mount ears were positioned vertically and the placement was marked for the crossmember.
18. We measured the distance between the rear down bars and transferred that to the 1 5\8-inch diameter tubing supplied for the shock crossmember. We used a PipeMaster notching helper to get the right shape for the notches.
19. The crossmember was then installed between the rear down bars.
20. Before welding, the bar was squared to the body. Each side was measured to the inner edge of the trunk lip. Once set, the bar was welded in place.
21. We slid one of our Miller welding sleeves over the shock to protect it (they are nice looking!) and the ears were positioned to the crossmember. It is always best to set the shocks as close to vertical as possible.
22. Assembling the shocks was a little tricky. We had to use a set of spring compressors to get the upper retainer in place, even with the lower retainer threaded all the way down.
23. The shocks were bolted to the lower shock mount using the supplied hardware and spacers. The ride height can be adjusted using the threaded lower retainer on the shock. We have not gotten that far yet.
24. The finished mock-up install. All that is left is to pull the rear end and finish welding the brackets in place.
25. The last step is mounting the track locator bar. This bar centers the rear housing under the car. This can be done with a panhard bar, track locator or Watt’s Link. The track locator bolts to opposing sides (one front and one rear) for the ladder bar mounting bolts with rod ends. Turning the rod moves the rear left or right depending on your needs.
A life-long gearhead, Street Tech Magazine founder and editor Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 5 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced. You can follow Jefferson on Facebook (Jefferson Bryant), Twitter (71Buickfreak), and YouTube (RedDirtRodz).