I think the swing arm may be more consistent than the 4-link with its fewer moving parts. Other than lack of adjustability, do you see any downside to the swing arm in this type of application?
Swing arm suspension is definitely something I would not recommend, especially in the sportsman classes.
With a swing arm car you have zero adjustability available to you in the rear suspension. If track conditions do not favor your setup, you probably won't have a very good weekend. There is no way to change pre-load or stagger in the rear suspension. You can't even make suspension changes to get faster reaction times. Due to its lack of adjustability, you will not be able to set up your car to be highly competitive, especially in the sportsman classes.
For rear suspensions, there are two other obvious choices: 4-links or ladder bars.
Most people use ladder bar suspensions because of their simplicity. Ladder bar systems are easy to install and even easier to maintain. If a ladder bar system is used, be sure to use one with adjusters. The adjusters allow you some ability to tune the chassis to fit the track, however, the solid design of ladder bars still greatly limits your ability to set the car to perform its best.
The 4-link suspensions offer you the best suspension you can have on a drag car. Four-link system can be easily adjusted to suit your needs and keep up with the constant changes in track conditions. If you need faster reaction times from the car, you raise and shorten the intersect points. When you need more bite down the track, or are having problems with a high horsepower engine (such as those in 8.90 cars), you can lengthen and lower the intersects. If your car goes right or left on the launch, pre-load can be adjusted.
In addition to the advantages you get from its adjustability, a 4-link car rides more smoothly over the bumps than a swing-arm or ladder bar car does. High quality 4-link systems are relatively easy to maintain and can last for some time. When choosing a 4-link, be sure to get brackets with adjustments that run along an arc so that you can maintain your same pinion angle even after you have made adjustments in the locations of your 4-link bars. Rod ends are just as important; 4130 rods ends are the best choice for any application due to their higher durability and strength.
With the right components and brackets, a 4-link system is sure to keep your car running consistent numbers down the track no matter what class you run.
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