need help from suspension pros

I’m writing a paper for my technical communications class, and part of the paper requires a professional opinion. The paper is about the pros and cons of independent and solid axle suspension systems. So my question to you guys is just that what are the pros and cons of each system, what are their weak points, how can those weak points be improved, and where does each type of suspension system work best?

thanks in advance

Try asking this in the General Discussions sections, you will get more answer instead of just 2wd guys answering, since no Ranger has a 2wd solid axle in the front.

Pro . . . hummmmm?

Hey, KCM

I saw your post and the suggestion that you send it to General - but I'm feeling chatty. Don't know that I'm a pro, but this is from a few years with a Trans Am car and a lifetime of loving things that go booden-booden.

To have a discussion of these two types of suspension, you have to have an understanding that there is no single best suspension for all cars/trucks.

Secondly, you need to understand that the purpose of all suspension designs it to keep the tire patch - that part of the tire touching the roadway - as flat on the ground during all phases of driving as possible.

Solid axles generally have one real big advantage over independent suspensions in that they are tough as boot leather. For instance - military vehicles, long haul trucks, trailers carrying ultra-heavy loads all are best fitted with straight axles.

This is also true of Rock-Crawling which uses uni-ball type suspension that allows for some really crazy angles.

Third, solid axles for the most part keep the tire flat on the ground except under hard cornering. The muscle cars of the 60's and 70's did a good job of holding their own with the more high tech suspensions of European and Japanese cars because the system works pretty well for the drive axle and for the rear tires. A straight front axle will suffer from a lot of steering wheel feedback as well as the effects of torque steering if it is four-wheel or front wheel drive. (An example would be the 1936 Cord)

An independent suspension, with upper and lower control arms of the same length will do exactly the same thing with torque steer. But they will not feed bouncing interference from one side of the axle to the other like the straight axle. Therefore, an independent front suspension is easier to control.

Independent suspensions begin to get interesting because of their ability to be designed to take advantage of the laws of physics that relate to the roll of the car around it's axis. If you have a model car in front of you or better - a rectangular box like a matchbox - look at it from it's smaller end. Now imagine that it is a car going into a corner. As you dive into the corner, weight shifts, causing the car to "roll" more weight onto the outside of the car: right side if turning left and visa versa. Pretend that the upper and lower control arms (A-arms) were connected at the top and bottom of each corner of the rectangle the matchbox represents. Can you see that the upper side of the box rolls over so it sticks out further from an exact 90 degrees to the roadway?

That means if the control arms of the same length are used, the inside and outside tire will tilt away from the car in a hard turn. That means the bottom of the tire is drastically effected by the angle of the tire and wheel to the pavement.

However, if the upper control arm is slightly shorter than the lower, as is the case in most cars, the upper a-arm will make a shorter arc. In doing so, it makes the tire remain closer to a 90 degree angle to the surface of the road. The best way in the world to demonstrate this is to build a two dimensional model out of poster board. That will allow you to see how much "bump" effects and "roll" effects tire deflection. If it's for school work - it's good stuff for demonstration purposes. (Use brads for the pivot points - ball joints - of your suspension.)

The changes that take place during acceleration - lifting the nose of the car, deceleration - lowering the nose of the car, acceleration and turning, braking and turning - with four wheel independent suspension become even harder to keep up with if you are trying to design the best Formula One car with a max of two inches of suspension travel and entirely different if you are working on a Baja Car that requires two feet

Poorly designed independent rear suspensions almost universally have a tendency to "tuck" under hard braking at high speed. The early Volkswagen with it's independent straight axle and the 80's 280 XZ both suffered from that problem. This is a "fault" the straight axle does not share. Those designs allowed the rear of the car to rise under braking, as the car rolled during the turn, the tire patch completely lost contact and rolled over onto the sidewall - making the car very unstable and prone to roll over. If the lip of the wheel caught the asphalt it was almost guaranteed.

IRS suspensions are a bit more complex, therefore more expensive. They are also more prone to failure, wearing of parts, and need for maintenance. They are more complex of course only if they are also the drive wheels. Independent rear suspensions on FWD (front wheel drive) are cars relatively simple - McPherson Strut for the most part. The complexity of Independent FWD cars is the reason they wear tires, clunk, need more maintenance, etc. etc.

Each and every design is some sort of compromise. The factors of the design depend upon how it is used, the weight and speed of the vehicle and to a great degree the initial cost. Although independent FWD designs are more complex, they are easy to build into one easily installed unit quickly. That saves labor cost on small cars - and labor is the bigger part of the cost on most cars.

Hope that helps.

wow thanks woddy. that helped a ton. ended up having to much information(which was a good thing).