This is one of those subjects where you can ask 10 people and get 10 different opinions, and there can be more than one thing contributing to the problem. Here are some things I think can cause it:
1) I think suicide front ends are more prone to doing it than a spring over axle configuration. I never really had an issue with it until I started running suicide front ends. Maybe something to do with the axle being way out in front with less dampening action by the spring. (I know springs don't dampen, but they do control bounce to some degree. If you doubt that statement, run a weak spring vs a stiff spring and see how much more control the strong one has over each wheel.)
2) Shocks are a biggie in my opinion. If the shock isn't keeping bouncing up and down to a minimum, the wheels will start hopping, progressively getting more out of control as speed increases, or if you encounter washboard road surfaces.
3) Toe in and toe out are also key. Some cars actually handle better with a little toe out. All the books tell us 1/16 to 1/8 toe in, but that isn't always the case. You have to play with your particular settings to find one that your car likes the best. I ran my 27 with 1/8 inch toe out to cure a slight death wobble that developed after driving it for a few years.
4) Tires are a major contributor. We think we buy new tires and they are perfectly round. In the case of bias plies, that might not be as true as for radials. I don't know if bias plies are worse today than in the past, but we are seeing thread after thread here and on other forums about death wobble. Some people have said they had to have their tires shaved (ground perfectly round) to true them up. One guy said one of his tires was 5/8 of an inch out of round!
5) Rims. Wheels can be bent or out of round too. We sometimes use 60 and 70 year old wheels on our cars, and some of them can be very much out of round or bent. Even new rims can be bad. After getting my Son Don's T running I was following him one day and the rear of his car was hopping up and down a little. We did some checking and found one brand new steel rim was bent badly. Coker replaced it at no charge, but we had to paint it to match the others and have the tire remounted. New wheel cleared up the problem.
6) Wheel balance. It may not be enough to balance the wheel and tire only. Sometimes the brake drum can be out of balance. I had a Toyota that even brand new tires would not fix for a wheel hop problem. Finally the dealer put the car up on a spin balancer that spun the entire rotating assembly, rebalanced it, and the hop went away. Some truck shops still carry those balancers that roll under the tire and spin the whole assembly.......you have to look around to find them nowadays though.
7) Caster. Here is another area where you get all kinds of opinions. Too much caster can cause problems as can too little. We originally screwed up and had my Sons at 11 degrees, and when we reset it to 6 degrees it made a huge difference.
8) Having a square chassis. To track right a car has to have both axles square to the frame. If you have ever seen a car that was wrecked and going down the road slightly sideways, you know what I mean. There are lots of ways to do this, but I like to square up the rear axle first by measuring from the exact center of the front crossmember to the same spot on each side of the rear axle. Then I make the wheelbase exactly the same on both sides. That is just one way, but it works.
9) Steering dampener. Some people call the dampeners that go between the tie rod and axle, or tie rod and wishbone, a bandaid. However, they do work great. No car leaves SoCal Speed Shop without one installed, they believe in them that much.
10) Tire pressure. Too little or too much can cause problems. I recently bought a $ 12 digital pressure gauge and found my tires were inflated all over the place. Since I redid them with the accuracy of the digital gauge the car handles much better.
11) Worn parts. On a straight axle car these can be kingpins, tie rod ends, wheel bearings, and steering box. Even wheel bearings that are too loose can cause it.
So, there can be tons of reasons for handling problems in a hot rod. You simply have to do process of elimination, make sure all parts are in balance and tight, and then make one change at a time until you get it to stop bouncing. Death wobble got it's name because it can kill you if it happens at the wrong time. Any of us who have had it happen at speed know how scary it is, and how it makes us not want to drive our cars, especially on the interstate at high speeds.
Hope some of these areas help anyone experiencing the problem.
Don
Oh, one more that I just thought about.............slop in the steering box. The steering box has to control the front wheels, and if there is play in the box it can't do that effectively. We found even the brand new Flaming River boxes in our two T's were too loose. We tightened up the adjusting nut and it helped a lot. Steering felt better too.
