Recirculating steering calculations

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Recirculating steering calculations

This tutorial describes setting up the steering on a vehicle with recirculating ball steering gear.

Previously, the steering gear was always calculated with the Pitman arm pointing either straight forward (or straight backward). This was OK for most applications where the Pitman and idler arm were parallel (in the straight ahead position), but was a compromise where this was not the case.

V4.501A adds the ability to specify the Pitman arm offset, so that the Pitman arm can be angled either inwards or outwards as required.

You will need a registered SusProg3D V4.501A (or later).

This tutorial uses the NASCAR demo file, but you can use your own data.

Start SusProg3D, and open the demo file, Nascar.s3d

Back to SusProg3D and change to [Front].
Go to Roll&Bump and do a Roll&Bump Calc.
For the calculations we have just done, we could have used ECalc with the "Roll and Bump" ticked. The results are the same.

When doing the basic geometry and roll&bump calculation there is no need for the steering data. The calculations assume no bump steer and hold the original toe alignment setting.

Make sure that the Geometry -> Config Wheel location & Alignment is set to any except "Top and bottom wishbones + steering linkage + turn".
I'll use the basic "Wheel location + alignment" for this exercise.
This option requires that the wheel alignment toe-in be specified, and it will calculate the length of the steering tie rod.

Do the Roll& Bump calc.

Now setup the Steering -> Configuration. Specify steering as "Recirculating ball" with a steering box ratio around 10:1, LHD and "Opposed" steering.
This is a typical Nascar front steer setup.

Step 1

We will assume that both the Pitman and idler arm pivot axes and arm lengths are specified. But we don't know the track rod dimensions, other than the overall length.

We also don't know where the tie rod ends attach to the drag link, so we will let SusProg3D calculate them.
Change the steering calculation option to [Chassis]. This will calculate the optimum position for the tie rod ends (where they attach to the track rod).

Add all the dimensions for the Pitman and idler arms.
Add the track rod dimensions (except the  tie rod pivot locations. They will all be "read only").

Leave the Pitman arm alignment at zero.

Leave the Steering Gear dialog open. Click the Apply button.

Now do a steering calc.

Looking at the graphic in plan view, you can see that the track rod has been dimensioned such that the tie rods are straight across the vehicle, and that the Pitman arm is positioned "straight ahead".
In Display, make sure that the display orientation is [Hold] (and not [NoHold]). This will prevent the display reverting to the usual front view when Calc'd.

Now, this isn't quite how the real vehicle looks, but gives us a good starting point.

Step 2

We know that on the actual vehicle the tie rods aren't "straight across" but are about 25mm (1 inch) towards the rear of the car.

So what we'll do is the change the steering calculation from [Chassis] to [Fixed].

This enables us to specify the track rod tie rod pivot locations.

In the track rod dialog, change the longitudinal tie rod pivot offsets to a dimension about 25mm (1") towards the rear.
And Apply.

Do the steering calc.

Now see how the track rod has "moved" backwards, with increased offsets.