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Chassis 101: Suspension Design for the Normal Guy, Part 1: Basics

 

The front and rear suspension design for my chassis are heavy in the testing phase. Running roll, bounce/rebound, and turn tests through Suspension Analyzer and making incremental tweaks here and there. Up to this point in the blog it’s all been lighthearted, amusing, and for some of you at times comical. Honestly, I didnt really have much to talk about, save for where I was on the build and where I wanted to go. For this installment I decided to drop the ranting and get down to what I’ve been up to, without giving away any of my design specs.

First and foremost I’m using a software package called Suspension Analyzer by Performance Trends. Simply put, this powerful design program is indispensable. I cannot stress that enough. I would never undertake a build of this magnitude without it. The price is reasonable and it has paid for itself twenty times over.

Introductions aside, I am not an engineer, though sometimes I operate at an engineering level. I’m primarily a master mechanic, however I hold two degrees and have independently studied everything from music to mechanical engineering. I’m of the belief that experience and a logically arranged mind offer the ability and knowledge to understand the concepts in any hard copy textbook. To understand suspension design and theory you do not need to be an engineer. That is a title reserved for an individual who has a piece of paper that says so. However, to fully understand suspension theory and design you do need to be logical and mechanically minded. So, here we go.

Track Width – center of tire to green tape

I’m designing and building from scratch. Also, I am not building within a specific class of race sanctioning rules. My car will be placed in what most would consider to be an “unlimited” class. I have no constraints other than I need everything to fit and work. Reading ahead please keep in mind that in suspension design there are no absolute values. Yes, there are ranges of some geometry values that are known to be working ranges and venturing outside of these ranges usually equates to poor results. Every part and piece of geometry needs to be optimized to work towards your handling goal. There will be gains, losses, and trade-offs to every decision from here out. Furthermore, the general design layout I’ll be referencing is an unequal length, non-parallel setup. AKA: short arm long arm.

Suspension Analyzer

If you don’t have access to or don’t want to buy a suspension design program you can set up your layout on paper at 1/4 scale. Again, I highly recommend purchasing a quality suspension design program.

I like drafting paper, but you can use any sheet you’d like. Start with a vertical line. This will represent your ground plane. Next, on one side of the page add your tire. Draw a line down its center and intersect that line with the ground plane, this is your contact patch. In the center of the vertical tire line make a mark and extend the line to what you deem to be the center of your car. This distance will be known as half of the track width. Now you can locate the center pivot points of your upper and lower ball joints. Next if you draw a line intersecting the upper and lower ball joints and carry it through to the ground line, then measure the angle you will have your kingpin inclination. Now look at the ground plane where the kpi line and contact patch line intersect the ground line. The distance between these two points is known as scrub radius……think of how much easier that would all have been with a computer program.

Let’s take a deep breath and rewind. Count backwards from 25.

Good. So far, what the hell does all of this actually mean? Let’s dissect that last paragraph and break down the terminology. Engineers like to give fancy words to very, very simple theories and units. So let’s talk about this in human terms:

  1. Contact patch – the place where the tire touches the ground. Let’s not over-complicate this right now and leave it at that.
  2. Ground plane – this is a representation of the earth’s surface. For design purposes it’s perfectly level and imperfection free.
  3. Track width – the distance horizontally between the contact patch of both tires. In our case we are showing half the track width.
  4. Upper and lower ball joint – ball joints, spherical bearings, or solid bushings are the points at which the wheel upright is attached to the upper and lower control arms. All the wheel/tire forces are transferred into the upright and then transferred to the ball joints, which transfer the force into the control arms and into the chassis. They also serve as the pivot points for the steering system at the knuckle/upright.
  5. Kingpin Inclination Angle – Let’s spend some time on this. As viewed from the front of the chassis the KPI is an line drawn through the centers of both ball joints and driven into the ground plane. Where does this term come from? Well, in the old days and even today on heavy trucks, front suspensions incorporated a solid beam axle. On each end of the beam axle there is a canted bore. On the rotating spindle end there is also a bore. A large shaft known as a kingpin is driven through both of these bores attaching the spindle to the axle. The spindle is then able to rotated around this kingpin, which in turn enables the vehicle to turn. That’s where the term comes from. KPI is the axis in which the upright pivots around during steering.
  6. Scrub radius, as viewed from the front of the chassis, is the distance between the contact patch center and king pin inclination where they intersect the ground plane. This is either positive, where the kpi is to the inside of the contact patch, zero where the contact patch and kpi intersect at the exact same spot on the ground plane. Or it can be negative, where the kingpin angle sets down outside of the contact patch. Scrub radius will effect the feel and ease of steering and road shock transfer to the chassis and driver.

For my application Scrub radius will end up being a little positive. You’ll have to decide what you want if designing from scratch and keep things in mind right down to how thick the mounting flange of your brake rotor hat is, as this will effect scrub radius. Which will in turn effect the overall feel of the steering.

Keep in mind as stated before that it’s a balancing act. Going hard out of the gate and getting tunnel vision on one aspect of your design will have I’ll effects on the rest of the system. This will equate to an I’ll handling car.

I’ve briefly touched on setting up your layout and explained a few terms used to describe suspension design geometry. As complicated as it seems it really is simplistic. Hopefully non of this read like 1980’s stereo instructions.

Personally , I would not go any further into my own design without having software powerful enough to input all my data and have the software run a simulation and spit out page upon page of results. Then after interpreting those results fine tune and tweak my design until I got all my desired results. As I wont be directly going over how to use Suspension Analyzer, I will show you what it can do in our next episode. I’ll also go over a basic design setup, add a steering system, explain bump steer and how to lessen its effects, and show you what changes you can make to the suspension and the resulting outputs that will come from those changes. It is extremely powerful software and well worth its price in gold. When building your design if accuracy set in the program can be maintained on the jig you should get a chassis that handles as well in real life as it does on paper. Setting a solid design is an excellent basis for making tuning adjustments in the real world and getting that well handling chassis you’ve wanted all along.

Until next time.

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Chassis builder, engine builder, cynic

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