Rear Wheel Steering 3

[CKehoe17]

Hi All,

I was looking for some info on rear wheel steering. Does anyone know of some resources I can use? If there were any books that had a good section about rear wheel steering, or any papers that I could read. I found a couple of papers that were decent, but would like more information. I have Race Car Vehicle Dynamics by Milliken and this doesn't even mention much about rear wheel steering.

I know it's more of a recent trend for vehicles to use rear wheel steering, but that's not to say it's a new idea. I'm looking to be able to calculate how much of a benefit I could get out of rear wheel steering (turning radius, high speed stability, increased lateral acceleration) so I can prove out any advantages or disadvantages.

Thanks!

 

[BrianPetersen]

Presumably you are talking about "all" wheel steering - auxiliary rear steering that acts in conjunction with the normal front steering which is the main steer axis.

 

[CKehoe17]

Yes, that is correct. Having both axles steer, similar to systems like Nissans HICAS, Porsche's system, or AKC by ZF.

 

[3DDave]

Thank goodness it's simple to make those evaluations against all possible criteria.

I've worked on two vehicles, one self-propelled, that had multi-axle steering. The main effect is altering the turning center from where it is on single steering axle designs.

Without even looking at the steering geometry required you can start by making an evaluation of changing that. In coordinated steering, it moves the center so it moves towards the center of the vehicle. In anti-steering it pushes the turning center to infinity.

In these vehicles the requirements drove the design to multi-axle steering rather than deciding to find a problem that multi-axle steering to solve. I'd suggest fully understanding all those aspects of conventional steering and suspension design and then finding cases where it has a large enough flaw that vehicles using it would be impracticable for the task.

 

[gruntguru]

The effect on low speed (low slip angle cornering) is straightforward. If you were designing an autocross car it would be easire to negotiate cones. (reduced tendancy to clip cones with a rear wheel)

At high slip angles and larger radius turns it is usual to anti-steer - bringing the turning center from ahead of the front axle line to somewhere between the axles.

je suis charlie

 

[GregLocock]

The papers, and then simulations or calculations are probably all you'll get. Kinemtically it's the same as front steering, in terms of what affects what in the geometry and so on.

Cheers

Greg Locock


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[3DDave]

Our vehicles had a lot of drivers fail to understand that the rear of the vehicle went outwards, often hitting buildings and other nearby obstacles, a bit like what happens with large rear overhang vehicles such as school buses.

 

[BrianE22]

A bit off topic but related: For front wheel steering there is a nice mechanical link between tire steer and the steering wheel. Since the rear wheels can turn in either direction there must be a hydraulic or electric actuator that steers the wheels. So when there is a fault in the hydraulic or electrical, what keeps the rear wheels from turning in the wrong direction?

 

[cibachrome]

The goal of rear wheel steering is zero side-slip. The net effect is to shorten lateral acceleration response times for driver pleasability. It's effective also for vehicles that pull trailers because the ass end doesn't move the wrong way first. The safety nazis caused it to be pretty tame and almost ineffective because failure is not allowed, so all kinds of prevention measures. A simple handling simulation is all you need to deal with it. It does not increase or decrease a vehicle's maximum lateral acceleration. It does not change a vehicle's understeer gradient unless implemented with Ay dependent compliance (Fy or Mz usually). It helps on slippery surfaces because (you should know why). Too costly to sell to customers as a features for turn radius reduction. Now only available on some lawn tractors.

Next question...

 

[BrianPetersen]

The upcoming electric Hummer has it as a party trick (probably realistically because the vehicle is too big and awkward to maneuver in parking lots otherwise). Some versions of the upcoming electric Silverado will have it because they share parts with the Hummer (BT1XX) and whatever versions of the Silverado they sell with that, will help to amortise the price tag.

The rear electric drivetrain in that vehicle (and the electric F150) essentially forces the use of independent rear suspension anyhow.

 

[3DDave]

cibachrome - too costly for residential customers. I forget the precise figures, but I believe it was on the order of a 40 foot radius turn for our 50 foot long vehicle, literally we turned circles inside the competitor's. The Air Force appreciated that ability. The deck of the vehicle did the side-shifting for the alignment tweaking required.

Scheuerle makes a good living doing this. I think we got a license to use their tech.

 

[gruntguru]

Lots of current cars with 4WS. Link

je suis charlie

 

[GregLocock]

"Although there was much to like, opinion is divided over whether or not to disable the High Capacity Active Steering (HICAS) four-wheel steer hardware."
Ferzackly.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Jdogma]

I too am interested in rear steer as can be helpful to a road racing car. I notice that rear suspension designs have gone from 0 bumpsteer to geometry that gives toe-in on bump, so that when the car corners the rear wheels steer in the same direction as the fronts, but to a smaller degree. I will be experimenting with this, and am considering a design that allows incremental changes for evaluation purposes. It does not do as the ones you mentioned, steer in opposite direction at low speed. It is chassis roll dependent only.

 

[cibachrome]

You describe "Roll Understeer" in units of deg_steer per degree_roll. Just about all vehicles have it, varying from around -0.15 to +0.15 with a median value of about +0.05 deg/deg. Sometimes you'll hear this as "5% roll understeer".

Of course you have to have a certain amount of roll angle to induce it. Positive values increase the car's understeer and reduce the steering gain as a result. It doesn't help or hurt your max.lateral acceleration because tires don't hear slip/steer angle changes very well near max Fy, they only respond to Fz or camber changes. But, you will like the normal driving feel. The extra understeer from the rear will shorten the Ay response times by increasing your car's natural frequency. Keep in mind that steer doesn't really cause roll, steer causes Ay. Then the Ay causes roll in a process called convolution. Yes, caster will give you a bit of roll from steer, but not a lot.