Aero rig - steering geometry question

[markocosic]

Hello!

I'm lashing up an aero testing rig (for kitesurfing kites if its of interest)

20 foot long affair, weighing in at ~1500kg (500kg + ballast as the kites can generate a terrifying amount of lift) that is hinged (hinge runs across the bumper) to the front of a pickup truck and pushed up and down the nearest strip of tarmac at speed (<60mph) on a still day.

I need a pair of wheels on the front that are self-centering and will steer themselves if I steer the truck (just to correct for slight deviations from straight-ahead)

I've pulled a Volvo 240 frontend to use - a macphearson strut indepentently sprung frontend. Nice beam with lower a-arms/wishbones & rack a mounted to it, and two strut upper mountings and you're done.

I can tweak the caster and kingpin on this by moving the strut upper mountings. Was thinking for this application of throwing in a bunch more caster and kpi by pulling the top mounts inboard and backward slightly relative to the lower beam as it is in the car.

How far would you go for self-centering, self-steering? Caster = kpi = 15 degrees sound reasonable?

[markocosic]

Hello!

I'm lashing up an aero testing rig (for kitesurfing kites if its of interest)

20 foot long affair, weighing in at ~1500kg (500kg + ballast as the kites can generate a terrifying amount of lift) that is hinged (hinge runs across the bumper) to the front of a pickup truck and pushed up and down the nearest strip of tarmac at speed (&lt;60mph) on a still day.

I need a pair of wheels on the front that are self-centering and will steer themselves if I steer the truck (just to correct for slight deviations from straight-ahead)

I've pulled a Volvo 240 frontend to use - a macphearson strut indepentently sprung frontend. Nice beam with lower a-arms/wishbones & rack a mounted to it, and two strut upper mountings and you're done.

I can tweak the caster and kingpin on this by moving the strut upper mountings. Was thinking for this application of throwing in a bunch more caster and kpi by pulling the top mounts inboard and backward slightly relative to the lower beam as it is in the car.

How far would you go for self-centering, self-steering? Caster = kpi = 15 degrees sound reasonable?

[Chris Lane]

I think a large mechanical trail creates the 'self-centering' effect while steering. This is (generally) the longitudinal distance between where the kingpin axis meets the ground and the center of the tyre contact patch.

Large trails and scrub radii make it really hard to steer the wheels.

[Nik Garrett]

Interesting one, I guess if you're not steering the wheels yourself then you can get away with more self-centreing torque to give more straight ahead stability, though presumably with too much caster/kpi you'd start to see some strange effects.

15 degrees sounds sort of reasonable, difficult to tell how 'driveable' it'll make the rig as a whole though. Can you do some test runs at different settings? Also I wonder if there's anything else with caster wheels (certain forklifts? not sure...) that you could have a look at to get ballpark figures.

Good luck!

[HenningO]

If you run caster wheels (or something similar) make sure to have an appropriate amount of damping (in rotation), since a small lateral disturbance will cause the caster wheels to oscillate (you know, running with shopping carts!).

With the kites high up in the air (?), I don't see any reasons why you can't pull the rig?

As for the KPI, make sure the KPI offset at ground (scrub) is zero, otherwise you'll end up having longitudinal forces generating a turning moment of the wheels. (Which once again can result in oscillations)