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Sandcar Technology - Suspensionn & Handling

Apr. 01, 2004 By Alan McCaa

I remember my first sandrail. A Chenowth frame, stock swing arm suspension with Airplane shocks in the rear. It had a beam that we later cut off and installed an aluminum beam. It was run off a Corvair motor that had a HUGE cam in it and sounded like a double A fueler at the drags. It had about 6 -7" of front travel and MAYBE 8" in the rear if we were lucky. My grandfather gave it to my brother and me. We thought we were HOT STUFF in that rail. It was so loud that everyone stared when we drove by. We use to jump that thing like it was a long travel sandcar all the time. At the time, I really didn't know what true suspension and handling was. Sure we got beat to death when we would go through whoops but we didn't know any different. After all, the off-road cars my father had always built only had 8" - 12" of suspension. 1600 desert cars were top of the line equipment in the 70's. That was just part of duning. How does suspension geometry affect the handling of the car?
This front engine sandrail (manufactured by V8 Rail) obviously handles great. By the photo you can see that a properly designed suspension and steering geometry can handle even the harshest of terrrain.
I also remember the first production sandcar that we built that was considered "long travel". It had 23" of travel with $600 King dual rate coil-over shocks. The first time I took it for a ride it was incredible. It had a Northstar Cadillac motor and tons of power. We took off and literally, it seemed to float over the dunes. 6" chop, 3' whoops, big bowls, it took on everything like it was no big deal. It was only after our first prototype that I learned the truest sense of handling, suspension & steering geometry. It was also after a ton of computer design and talking with expert race car fabricators that I realized why everything worked the way it did. Ever heard the phrases "BUMP-STEER" or "SUSPENSION GEOMETRY"? What do they mean? How does suspension geometry affect the handling of the car? Does weight affect the handling of the car and if so, how? What about wheelbase - does a 132" car handle worse than a 120" car? I thought we would take the time and discuss some of the differences that you might keep an eye out for. First let me say that we're not going to get into the scientific, mechanical engineering of things. We can spend hours talking about and try to explain scientific theory and physics and probably only confuse most people. What we're going to be talking about is the logical operation of suspension as well as the truth about handling. What is bump-steer? "Bump-steer". You hear that phrase a lot when it comes to long travel sandrails. What is it? Basically it is a change in the toe as the suspension cycles through its full motion of movement. What causes bump-steer is the improper design of suspension components. Basically the spindle configuration has to be designed to move in conjunction with the A-arms as well as the tie rods so that everything is moving in complete unison. A wrong dimension of simply 1/8" can cause bump-steer. Simply changing tires can also cause bump-steer. Tires vary in size. Some are taller & wider and some are shorter and skinnier. All can have different tire pressure. Usually a suspension must be designed around the tire that it is going to run. Running a taller or shorter tire than what the suspension was designed to run with, can change the geometry and can cause bump-steer.
Editors Note: The paragraph above seems to be a hot topic of debate. It seems that most argure that tire size does NOT change or cause bump steer but rather affects how you FEEL bump-steer.
What happens if a car has bump-steer? It can affect many things in the way of handling. Imagine going through 2' or 3' whoops and the steering wheel is shaking so badly you literally have to hold on with all your might, just to keep it from jerking out of your hand. Could that be a dangerous situation? Absolutely. Improper suspension geometry that causes bump-steer could put someone in a bad situation. Can you have zero bump-steer? It is possible. This requires many hours of computer-aided design so that every dimension is exactly where it should be. Keep this in mind though - typically in the sand a small amount of bump-steer will not even be noticed. For example: Revenge Racing builds the Sidewinder series of "high-end" sandcars that we discussed in our last article. Through 24" of travel their car has 1/32" of bump-steer. Truly a superior design to achieve this! On the average a car can have between 1/8" and 1/2" of bump-steer and operate with no problems whatsoever, especially in the sand. The sand is very forgiving. Generally when someone tells you that a car has zero bump-steer, in reality it should be considered as having very little bump-steer. Proper Geometry
Here is an example of good steering & suspension geometry. Throughout 24" of wheel travel, this Sidewinder buggy (built by Revenge Racing & Sold exclusively by Dunebuggy.com) has only 1/32" throughout the entire range of travel.
A few things to look for when it comes to proper steering geometry is the location of the tie rod mounting points in relation to the suspension arm mounting points. Compare the example in the adjacent picture:
  1. The inner tie rod connecting point is slightly to the outside of the middle of both top and bottom suspension arm connecting points.

  2. Look at the spindle configuration. Draw an imaginary line between the top bolt of the spindle to the middle of the tire. If that line goes straight through the middle of the outer connecting point, then generally you have a good idea that the suspension was designed correctly. Again this is a simple way to get an idea of what we are talking about without going into huge scientific theory. But with this imaginary line theory you can see how a different tire size you change the bump-steer in the suspension.
What about handling? In some ways, handling is a matter of opinion. Many people think that duning is simply a matter of turning. This is not true. Think about it in this way - Not everyone on the planet likes to dune the same way. Some like to turn in and out like a slalom course. Some like to simply go fast and straight. Others like to jump and carve continuously. How do you like to drive? I personally like to split the difference. I want a car that can go 130 MPH without feeling unstable. I want that same car to be able to turn well while following the lines of the dunes. At the same time, I want a car that will pound through the whoops and hit BIG jumps without coming down and bouncing out of control. That's why I like a car with a 125" - 132" wheelbase. I have found that this wheelbase gives the all-around performance that I am looking for. Does that mean that a 116" wheelbase car doesn't handle well at high speeds? No, that's not what we're saying. There are many cars on the market. There are cars that vary from 96" to over 140" wheelbase configuration. Is there a single "perfect" car? No. If there was such a thing, there would be only one builder on the market and that would be all that was manufactured. Instead, people vary in wants and needs and hence there are many cars to choose from. NOW WHAT!? Don't get confused. This is where you need to analyze your wants and needs. Do you have a family of five that you want to take? A 5-seat car may be what you are looking at, but what do you plan to do? Do you want to get out and cruise over the bumps with a smooth ride? A longer wheelbase may be what you need to look at. It's all a matter of personal preference. HENCE, GO FOR A TEST RIDE. These are words to live by. If your in the market for a car, DO NOT BUY A CAR WITHOUT RIDING IN ONE FIRST! This way you can see how the car handles in all situations. Go through the whoops, go through small choppy ground, use the steering as if you were in a slalom course, hit some jumps and pick the speed up to see how it handles. All these things need to be considered especially before you plop down $40,000, $50,000 or more dollars. What about weight? Does weight affect the handling of a car? ABSOLUTLEY! Both in good and bad ways the weight is important. Does a heavy car not handle well? Not necessarily. Does a light car handle better? Not necessarily. Remember handling is a matter of opinion. A heavier car may handle very well over the whoops and at higher speeds versus a lighter car that will tend to get bounced around over the whoops and get a little uncontrollable at higher speeds. A heavier car may not be able to hit that slalom course in the dunes as well as a lighter car can. Ultimately, the goal is to find a balance. A car with a middle of the road wheelbase, a middle of the road weight, good suspension design, good handling in all areas of the dunes and find it in a design and at a price that you like. There is no "best"
This two seat Funco is lightweight and has a short wheelbase. With a powerful lightweight motor it would be quite hard to keep up with this car through the dunes. Although it handles well in pretty much all duning aspects, it's probably not you car of choice for high speed runs of 100 MPH +.
Don't get sucked into the trap that a particular builder's car is "the best". There is no single "best". There are a lot of options and ideas that you as the consumer have to sift through. Balance & reasonableness will help you in your decision. There are other factors in the handling of a car. Chassis design, seating position as well as others. Do your homework and you'll easily find who knows there stuff and who doesn't. Keep an eye out for the next article. We'll be talking with Porter Race Cars. Porter builds top of the line off-road racecars and is considered to be one of the best in the field. We'll see what they have to say about racecars and sandcars.
References:

www.funcomotorsports.com
www.dunebuggy.com
www.v8rail.com

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