Castor angles on multi-wheel wheel steer vehicles

[mloew]

For multi-wheel wheel steer vehicles is there a relationship between the castor angles at the front axle(s) and those at the rear? Specifically, when both modes (in phase and contra-steer) are considered, should the rear axles have castor angles that are in the same direction as the fronts or are they generally opposite? Any fundamentals that can be traced to stability?

Best regards,

Matthew Ian Loew
"Luck is the residue of design."
Branch Rickey

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

I haven't worked on active 4ws, so treat this with a large grain of salt.

My first thought was that the trail always needs to be consistent to get good steering feel.

Then I got confused.

I'd start from the desired SWT vs SWA vs yaw velocity vs vx characteristic, this might suggest that for low speed manouevres, if that is where you are countersteering, you could get away with zero trail.

Howver, if you are steering the rear wheels at high speed I bet you end up with needing (roughly) the same trail as the front wheels.

For stability you will always want to have a net self aligning torque in the direction of travel, I guess.

For in phase high speed steering I'd have a look at reverse engineering the 4ws Honda.



Cheers

Greg Locock

 

[GraviMan]

Sorry Matthew, this forum is not a regular haunt of mine. To some extent it depends on whether you are trying to correct for tyre slip angle.

The best on-highway multisteer system puts back wheels in anti-phase to front. The angles are arranged so that the steer centre is behind the mass centroid, rel to vehicleline. In this way the centroid always moves inwards during cornering, so there is a natural self correcting mechanism. I.E. the mass moves outwards on straightening up.

The absolute best appraoch is to then also have tyre slip angle compensation all round. You need both understeer compliant correction for the front, and oversteer compliant correction for the rear. Designing compliant systems before ADAMS was a tricky task to get right. For this reason the Honda 4-wheel steer system aims to get the hub angle right for a given cornering accel. Slower than this you get understeer, faster than this you get oversteer - but the Prelude is FWD anyway, so no problem.

The system I am suggesting will outperform the Honda mech. There are only two problems:
1. Defining tyre slip angle in an off-highway application can be difficult - normally speeds are not deemed high enough to warrant slip angle measurement.
2. Should the vehicle oversteer, steering into the skid may provoke further oversteer (since rear hub reacts the wrong way). This is where full electric steering comes in.

In truth if the rear tyre is skidding, it has already lost traction, so better to get the fronts "unstalled". Also by altering the rear steer bias, from say rear 80% of front to 70% of front, this should never happen. Ideally the system is also used with all wheel drive, to avoid excessive power oversteer. Not sure why Prodrive don't try this, but a 4x4 rally friend of mine recons gravel/mud throw-back help to produce cornering thrust.

Regarding castor angle (or better trail/lead distance - in good geometry). You need to arrange for the resultant centrigugal forces to generate a self aligning torque. The front should obviously be trailing. The rear is ideally 0 to avoid fighting the front, in the mech I mention above. If you go for a seperate rear steer mech, then you need a lead distance. This is all why you need to keep steer centre behind centroid. Interestingly SST didn't do this - probably why Andy Green swore so much! ;-)

Apologies for not seeing this sooner. I gather my knowledge of active suspension worried you. Active suspension was nearly my final year project, but hybrids won out - I prefer Helicopters now anyhow.

Mart

 

[GraviMan]

Forgot to mention:

For the rear steer lead mechanism, you need to make sure there is adequate damping in the system. With oversteer correction compliance, a natural frequency exists. Without the correct damping trolley wheel "flutter" will exist above a certain speed.

And you thought us ADT boys were jus' a buch of pivot steer luddites! ;-)

Mart