I am going to focus here on traction bars, as they are cheap, simple, and period-correct. Caltracs do a similar (but apparently better) job, but are not period correct (they are a ‘90’s invention). TractionMasters also do a similar job, but are more complex (needing welding back to the chassis). Ladder bars and other fancy setups equally increase traction, but are rather complex. I reckon that traction bars are about right for the typical 60’s-70’s local drag strip flavour I want with the meth monster.
Traction bars (also know as traction rods, tramp bars, tramp rods and slapper bars) are used to counter the effect of leaf spring wrap. The image below shows a standard leaf spring setup, with the spring eye to the left and the spring shackle to the right. I have borrowed the image from an internet site and will modify it to explain the story as I go. For my story, image the car moving to the left, as per the red arrow.
When the vehicle is stationary and you mash the loud pedal, the axle pushes (twists) against the springs in order to turn the rear wheels. This is the force shown by the orange arrow in the image below. This force can make the rear spring deform. The spring is held at one end by the spring eye, and at the other end by the spring shackle. With nowhere for the spring to go, the twisting force tries to make the spring wrap around the axle, opening up one end of the spring pack and flattening the other end as per the image below. This is known as spring wrap or spring wind-up. As the spring deforms, it moves upwards and forwards, as per the blue arrow in the image below.
Eventually, the wheel lifts enough that traction is lost, and the wheel starts to spin. With traction lost, there is nothing for the wheel to push against, and the orange-arrowed twisting force is lost. The spring relaxes, and the wheel comes back down again, chirping the tyre.
Scrt!
If you still have your right foot mashed on the loud pedal, the whole process starts again. The rear wheel repeatedly lifts and falls, losing and regaining traction. The back of the car shakes, and acceleration is poor. This is known as axle tramp, or axle hop.
Scrt… scrt… scrt… scrt… scrt… scrt…
Not only is axle tramp bad for quarter-mile times, it is also hard on rear end components. The poor diff gets loaded and unloaded mercilessly… and with the early Holden coarse-spline axles, can lead to failure.
Scrt… scrt… scrt… scrt… scrt… scrt…bang!....................@#%$!
To stop spring wrap, we can install traction bars. There are several different configurations (more on that later), but for now I have drawn below the universal type that clamps on under the spring. I’ve shown it painted yellow, as a nod to Lakewood (one of the largest traction bar manufacturers, and now owned by Holley).
The traction bar is a piece of rectangular pipe, with some brackets and u-bolts to clamp over the spring pack. One end of the traction bar has a rubber snubber, which is positioned just below the spring eye. As the loud pedal is mashed, our spring tries to wrap as per the orange arrow. The traction bar rotates (clockwise in the image above), and the snubber rises until it contacts the spring eye as per the green arrow. This gives the spring something to bear against, spring wrap stops and traction is retained. As the traction bar moves up, it slaps the spring eye… and hence the name slapper bar.
There is of course some down sides to running traction bars. They do reduce ground clearance. They also can effect the way the rear suspension works as it goes over a bump. The image below shows our vehicle going over a bump. The diff moves upwards, and as the spring pack flattens it elongates and moves backwards, taking the axle backwards too (notice the shackle moving backwards in the image below). The axle thus moves up and back as per the purple arrows.
The traction bar also moves with the purple arrows, and can either contact the spring eye or the leading end of the spring. This can make the leading half of the spring pack tighten up, giving a harsh suspension ride. Just how much effect this has depends on the spring shape, it’s rate of response and how close the snubber is set to the spring eye – more on that later. So whilst traction bars stop axle tramp, they make the suspension ride harsher.
Another downside of traction bars is seen under braking. As the car brakes, the axle tends to roll forwards, as per the image below. The spring wraps the other direction, and can make the leading end of the traction bar drop down and hit the ground.
The traction bar leading ends are normally bevelled to reduce this risk. In some model traction bars, an additional set of u-bolts (often called the “square u-bolts”) is installed at the front of the traction bar, as shown by the pink arrow in the diagram below. The square u-bolts hold the traction bar to the spring pack and prevent the leading end of the traction bar from dropping down under braking.
Traction bars are installed flat under the spring pack. This can be either by sitting under the factory u-bolt plate (as shown in the images above for the universal type traction bars), or by replacing the factory u-bolt plate. The latter option requires a specific traction bar to suit the vehicle rather than the universal type, as the factory u-bolts and plates differ with vehicle make and model. Regardless of the type of traction bar, the traction bar should be horizontal, or angling slightly upwards. If the traction bar is angling downwards, aluminium wedge shims are installed between the traction bar and spring pack to angle the traction bar back up, as per the pink wedge shown in the diagram below.
Multiple wedges can be stacked to give more height as needs be. Under no circumstances should the traction bar be installed sloping downwards, as this greatly increases the risk of the traction bar dropping down (… and digging in to the road) when braking. Wedges are available from Lakewood as part number 20500 (2º) or 20510 (4º). Mr Gasket do the identical wedges as part numbers 1606 and 1607 and respectively. Both Lakewood and Mr Gasket are Holley brands and the parts look identical… you just pay a few dollars more for the Lakewood stickers
Note that traction bars are also of differing lengths, and that the snubber often has options for where it is bolted to the end of the rectangular bar ( a series of holes drilled in the rectangular tube). The snubber should be assembled so that as the traction bar rises the snubber comes into contact with the front spring eye, as per the green arrow in the diagram below.
The spring eye is a nice solid, reinforced location to bear the force required to stop spring wrap. The snubber should not be set so that it comes into contact with the spring itself, as per the red arrow in the image above. The ends of the spring are relatively weak, and the large spring wrap force is more than enough to permanently deform (and hence ruin) the spring leaf.
So how are traction bars tuned? A gap is normally left between the snubber on the front of the traction bar and the spring eye.
• A larger gap takes the traction bar longer to react, is more likely to allow axle tramp, but makes the rear suspension feel softer.
• A smaller gap makes the traction bar react quicker, is less likely to allow axle tramp, but makes the rear suspension feel harsher.
For dedicated race vehicles (where rear suspension harshness is not a consideration), the gap is often set to zero (referred to as preloading). For street/strip vehicles, a gap is normally used. Automatic transmission cars tend to load the rear axle more smoothly, and are less prone to axle tramp. As a starting point, use a ½” gap for automatic transmission cars. Manual transmissions apply power more harshly (as the clutch is dumped), and a smaller ¼” gap is a better starting point. The gap is then tuned by repeatedly launching the vehicle and watching for axle tramp, and keeping an eye on any quarter mile time changes. Note that the gap can be different on either side of the vehicle:
• If the car is pulling to the right (driver’s side), then the left-hand (passenger’s side) rear tyres are not gaining traction fast enough. Make the left-hand (passenger’s side) gap smaller than the right-hand (driver’s side) gap.
• If the car is pulling to the left (passenger’s side), then the right-hand (driver’s side) rear tyres are not gaining traction fast enough. Make the right-hand (driver’s side) gap smaller than the left-hand (passenger’s side) gap.