Establish the basic suspension geometry

Herb Adams tutorial

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Basic suspension geometry

This is a basic "get you going" tutorial, using the registered SusProg3D software.

It is based on Chapter 7 of the book "Chassis Engineering" by Herb Adams ISBN 1-55788-055-7
Available from Society of Automotive Engineers ( product code B-753) and BevenYoung

It is not necessary to have this book to use the tutorial, but Herb does discuss various design options and decisions, which may be useful.

You will need a registered copy of SusProg3D, version 4.77C (or later).
The evaluation version cannot be used for this tutorial because the evaluation version does not allow for changing the upright and wheel dimensions.

Start SusProg3D.

This tutorial will use measurement units and axis system the same as Herb's book.

Go to Settings -> Settings -> Units. Select Imperial, then OK.

Hint for Metric users.
After completing the tutorial and saving the data file, reset the units to Metric. Close and reopen SusProg3D, then reopen the data file. It will display a message regarding the units. Click on 'Yes' to automatically convert the data from imperial to metric.

Go to Settings -> Settings -> Axis System.
Change the lateral axis identifier to "Z" and "+ve LH"; the vertical axis identifier to "Y" and "+ve up"; and the longitudinal axis identifier to "X" and "+ve rear". Click OK.

Hint for users with alternative axis nomenclature.
After completing the tutorial and saving the data file, reset the axis system match your preferred system. Close and reopen SusProg3D, then re open the data file. It will display an input dialog showing the axes. Click 'Cancel' to keep your preferred system.

Check that the wheel mounting flange dimension is correctly specified.
Go to Settings -> Settings -> Wheel mounting flange.
Select Offset, then OK.

You should also setup the display. Some of the items will not be calculated, so can be 'turned off'
Go to Display -> Control and in the 'Show' section make sure that all items are turned off (they are not ticked).
Then 'OK' to close the Control panel.
Make sure that the Display tabs are [Front] [Grid] [NoHold] [Both] [Wheel]. If any are different, click on them to flip to the correct one.

Your screen should now look something like this.

Setup the Vehicle data.

Select the Vehicle tab, then each of the following items in turn.

Config

For the front geometry, choose "Double A-arm". The remainder of the items can be left as-is. Click OK.

We are designing the front suspension, and will input data for the LH side.
After Config, the next menu item should be [Front]. If it is [Rear] then click it, and change it to [Front].
The next menu item should be [LH]. If it is [RH] then click it, and change it to [LH].

Datum

To keep the numbers the same as the book, we will use the ground as our vertical datum, and the front axle centreline as the longitudinal datum.

Change the longitudinal datum to axle centreline (nominal) rear dimension to 101.000.
Leave all other dimensions as zero. Click OK.

Wheelbase.

This is a calculated figure, derived from the actual positions of the front and rear wheels. It should show 101.000. Click OK.

Ride Height.

This is used to specify and adjust the vehicle ride height. It allows for the chassis ride height (and consequently all the chassis pivot points) to be adjusted in one single step.
For this example, assume that we are measuring ride heights from the chassis top tube.
The front ride height location will be 0.000, 20.000, -6.000 and the static ride height 20.000.
The rear ride height location will be 0.000, 20.000, 105.000 and the static ride height 20.000.

It is important that both front and rear ride height locations and ride heights are specified, as these two points control the calculated position of the chassis mounting points, relative to the ground. In this case, where the ground is also the chassis vertical datum, then the critical dimensions are that the "Y" dimension and the Static ride height must be identical, and that the two location points are "longitudinally separated".

Click OK.

Mass.

This isn't used for the basic geometry calculations, but some approximate values can be entered.

Assume that the unsprung corner weights are each 100lb and the corner weights are each 750lb.
Set the centre of gravity height to 18.000.

Click Apply. Notice that the total weights and distribution is updated. Click OK.

Front Wheels.

Hint.
You can also use the "All Wheels" item and set all four wheels in one go.

The following dimensions are the minimum required for geometry calculations.
Rim mounting offset 1.500; tyre rolling radius 12.500

The following dimensions are used to draw a nice looking wheel and tyre assembly.
Rim diameter 15.000; rim width 10.000; tyre tread width 10.000; tyre section width 13.000; toe reference length 20.000.

Make sure that "RH identical" is ticked. Click OK.

Rear wheels.

There is no need to enter the rear wheel as we are only doing the front suspension.

At this stage there is no graphic display, other than a small cross in the centre. You might like to setup the background colour.
Go to Settings > Settings > Display to bring up the Display Settings. (With older versions, go to Display - the top line menu item between Settings and Help).
Click on Background then Background colour. Choose a light rather than a dark colour.
You can also setup the grid. Specify the grid spacing (major) as 10 and the divisions (minor) as 10. This will draw a grid with the minor lines at 1 inch and the major lines at 10 inches. Click OK.

This has now completed the basic vehicle data. At this point it is probably wise to save the data.

Go to File -> Save As.
Specify an appropriate directory and file name. I have saved it as "HerbAdams.s3d"

Now we can enter the geometry dimensions.

Select the Geometry tab, then each of the items in turn.

Config.

This is where we specify the controlling dimensions.

Specify the instant centre location as "Swing axle lengths and roll/pitch centre heights".
This allows us to specify the swing axle and roll centre dimensions, which will then calculate the wishbone chassis mounting heights.

Specify the wheel location and alignment as "Wheel location and alignment".
This allows us to specify the wheel location (both laterally and longitudinally), which will then calculate the track, wheelbase and wishbone link lengths.

Specify the "wheel location point" as "On ground".

Click OK.

Wheel.

This has already been done, as part of the Vehicle setup.

Track.

There is no input for track. It is a calculated dimension which depends on the actual wheel location.
In this tutorial, the wheel location is specified in the Alignment input.

Alignment.

Specify the caster as 1.000 degree. Leave the camber and toein at zero.

Specify the "half track" as 30.000 (this will then result in a track of 60.000")
Leave all other values at zero.

Make sure that "RH identical" is ticked, then click OK.

Upright (knuckle design).

Enter the top wishbone mounting (upper ball joint) dimensions: Z 5.500; Y 5.750; X 0.000

Enter the bottom wishbone mounting (lower ball joint) dimensions: Z 3.750; Y -5.750; X 0.000.
Note that the Y dimension is negative, as it is below the axle centerline.

The steering ball joint is not needed for the basic geometry calculations, but they can be entered now.
Enter the steering arm ball joint dimensions: Z 3.500; Y -5.250; X -4.000

Leave the spindle reference point at zero.

Make sure that "RH identical" is ticked, then click OK.

Herb's analysis package requires the actual vehicle coordinates of all the mounting points, and these are calculated by Herb, but the calculations are not shown in the book.

We will use SusProg do all our calculations.

As discussed by Herb, we will use a swing axle length of 124" and a roll centre height 1/4" below the road surface. We will use these to determine the height of the wishbone chassis mounting points.

ICs.

This is where we specify the swing axle lengths and roll centre location.

Specify the front view swing axle length 124.000, the roll centre height -0.250 and the roll centre offset 0.0.
Specify the side view swing axle length as 370.000 and the instant centre height 6.000.
Don't worry if the RH values are wrong. Because we will specify that "RH identical" the RH values will automatically be updated on "OK" (or on "Apply").
Make sure that "Calculate using" is set to "Height", and "RH identical" is ticked, then click OK.

Why the side view swing axle instant centre height of 6”? Basically, it’s a guess. But we know that we want the bottom wishbone pivots parallel to the ground, so that means that the instant center height will be around 6”. We will do the initial calculations, and then vary it later.

Chassis.

Because we are specifying the swing axle length and the roll centre height, we only need to specify the lateral and longitudinal dimensions of the top and bottom wishbone mounting points.
The vertical dimensions will be calculated from the swing axle and roll centre dimensions.

The lateral location will be estimated from the track, upright and wishbone length.

The top wishbone lateral dimension will be 30.000 (half front track) minus 1.500 (the wheel mounting offset) minus 5.500 (the upright top ball joint) minus 7.000 (the nominal wishbone length) giving 16.000.

Specify the top wishbone mounting Z dimensions as 16.000 for both front and rear.
Specify the top wishbone mounting X dimensions as 0.100 and 6.100 for front and rear respectively.

The bottom wishbone lateral dimension will be 30.000 (half front track) minus 1.500 (the wheel mounting offset) minus 3.750 (the upright bottom ball joint) minus 13.000 (the nominal wishbone length) giving 11.750.

Specify the bottom wishbone mounting Z dimensions as 11.750 for both front and rear.
Specify the bottom wishbone mounting X dimensions as -1.500 and 10.000 for front and rear respectively.

The steering rack ball joint is not needed for the basic geometry calculations, but they can be entered now.
Enter the steering rack ball joint dimensions: Z 12.150; Y 6.400; X -4.000

Note that you don't enter the chassis mounting Y values, as they will be calculated from the swing axle instant centers.

Make sure that "RH identical" is ticked, then click OK.

Calc.

Just click the Calc.
This will do all the static geometry calculations, and update the graphic. It should look like this.

Hint:
If the graphic doesn't fill the screen, press "+" key to enlarge the graphic. The "-" key will shrink the graphic.
To keep this enlargement, go to Display and then change [NoHold] to [Hold]

Save. Just use Ctrl-S.

Have a look at the Chassis dimensions again. Open the Chassis input, and slide it to one side. Notice that the bottom wishbone rear Y value is less than the front Y value. We want them both the same. So this means that we must raise the side view swing axle instant center height. Open the IC input, and slide it to one side. You should be able to see both the Chassis and the IC inputs at the same time.

Change the side view instant center height to 7.000 and then click “Apply”, and then Calc. Notice that the Chassis input has refreshed with the new wishbone mounting Y values, and that the bottom wishbone mounting Y values are almost identical.

Change the side view instant center height to 7.100 and then click “Apply”, and then Calc. Now the bottom wishbone mounting Y values are identical.

If you are following this tutorial and comparing the results to the book, you will notice a discrepancy. The top wishbone mounting points are at 16.150 and not 16.000. As a consequence, the heights are a little low. Change the top wishbone mounting Z values to 16.150 for both front and rear. Click “Apply” and then Calc. Now the top wishbone mounting Y values are the same as Herb's: 17.120 and 16.920

Also, because we guessed the initial value of the roll centre height, the bottom wishbone mounting heights are a little out.

But this illustrates how we can start from the swing axle and roll centre dimensions to establish the wishbone mounting points. Now we will swap around.

Open Config and change from “Swing axle lengths…” to “Suspension link chassis mounting points”. Click OK.

Notice that the Chassis input has updated, and now the Y values can be input. The Instant Centre input has also updated, and now all values are “read only”.

Change the Chassis bottom wishbone mounting Y values to 5.900 (front and rear). Click “Apply” and Calc.

Notice that the instant centre values have also updated. The swing axle length is now 123.881 and the roll centre height is –0.246. Also the side view swing axle length is now 370.165 and the height 7.094.
These agree very closely (although not exactly) with Herb’s figures.

I think that this is caused by Herb not allowing for the fact that when caster is applied to the upright, the top ball joint will drop (in height) slightly and the bottom ball joint rises slightly.

So now you see how to use the chassis mounting points to calculate the swing axle and roll centre values.

Save.

Now lets see what happens when the suspension bumps and droops.

Now switch to the Roll Bump tab.

Config.

There are three sections in the Roll and Bump Config.

In Bump and droop increments, set the maximum and minimum travel to 4.000 and both bump and droop increments to 0.500

In Roll increments, set the maximum roll to 4.00 degrees, the roll increments to 0.50 (for display) and 0.25 (for calculate) and the initial bump position at “Static”.
In Roll centre and axis, leave the roll centre at “semi-dynamic” and the roll axis at “horizontal”.

Click OK.

Calc.

This will do all the roll and bump geometry calculations.

Results.

Now you can see how the wheel alignment values change as the wheel is bumped and drooped.

Save.

Now lets see about the bump steer.

We had previously entered the steering rack and steering arm ball joint dimensions, so we will use them do the calculations.

Now switch to the Steering tab.

Config.

Set the steering type to “Rack and pinion”; the steering gearbox location to either LHD or RHD; and the rack and pinion config type to “1”.
We have previously specified the steering arm ball joint as X = -4.000" so this will be “Front steer”.
Click OK.

Make sure that the item between Wheelbase and Toe turn is [Fixed]. If it is [Chassis] or [Upright} then click it until it is [Fixed].
What this means, is that we will use the specified rack and steering arm dimensions to calculate the bump steer.

Toe turn.

Set the toe-out in turn increment to 5.000 degrees.
Click OK.

Calc.

This will do all the bump steer calculations, and update the graphic. It should look like this.

The steering rack and tie rods are now shown, along with the wheel and tyres.

If the steering rack and tie rods are not shown, then go to Display -> Control and "tick" the "toe link", "rack" and "wheel" in the Show section. The steering rack and tie rods are now shown, along with the wheel and tyres.

Results.

Open the results, and look at the bump steer values. Note the values at the 4.000 bump and droop positions.

Maybe we can do better.

Close the results.
Change the [Fixed] to [Chassis]. This will calculate a rack position to minimize the bump steer.
Calc, then reopen the Results. Much better.

Notice the new steering rack ball joint position. Slightly more outboard and a little higher than originally. This highlights the need to ensure that there is some adjustability built into either the rack or the steering arm ball joint mounting.

Well, that's enough for this tutorial.

We have gone from essentially a blank piece of paper, to a basic suspension geometry.
The bump steer is pretty good, but the roll centre control may need a bit of work.