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4 Wheel Steering Discussion by MichiganReefer
Started on: 11-16-2021 03:18 PM
Replies: 13 (226 views)
Last post by: pmbrunelle on 11-22-2021 07:30 PM
MichiganReefer
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Report this Post11-16-2021 03:18 PM Click Here to See the Profile for MichiganReeferClick Here to Email MichiganReeferSend a Private Message to MichiganReeferEdit/Delete MessageReply w/QuoteDirect Link to This Post
Hey all,

This is my first post to the community as I'm a fairly new Fiero owner- what I'd like to discuss is the history of 4 Wheel Steering in a Fiero.

So for work, I'm an engineer and happen to have a background in automotive steering and vehicle dynamics. I was digging through the internet to see if 4 wheel steering had been done to a Fiero because I knew the rear suspension for 84-87 was a repurposed front end from the Phoenix. Turns out that it has been brought up a couple of times over the years in the forum here and they linked to an article from the 80s showing what Saginaw Steering was doing to make it happen. For those who haven't heard or seen:

Popular Science Nov 1987-Fiero Article

Simply put, they were trying to put a Steer-by-Wire system into the rear end to increase handling performance. I have a buddy who works at Nexteer Automotive (previously Saginaw Steering) and knows a Senior engineer who happened to work on it all those years ago. There is a lot of history there and I'd love to ask technical questions to him but I doubt he can legally answer them or fully remember.

Has there been anyone out there that has attempted to home engineer a rear steering system yet? Now that we have universal EPS kits I see it being a lot more possible than 10 years ago when EPS wasn't really even in production cars. As if I don't already have a ton of resto-mod ideas for my 88 GT (3800SC Series 2 swap), part of me wants to engineer a rack that we can put into the 84-87 cars to make 4 Wheel Steer possible.

Let me know if there are any engineers in the community that would like to talk shop/feasibility.
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Report this Post11-16-2021 05:05 PM Click Here to See the Profile for 87_specialClick Here to Email 87_specialSend a Private Message to 87_specialEdit/Delete MessageReply w/QuoteDirect Link to This Post
I don't know of anyone that has done 4WS on a Fiero. The EPS has been done in the front.

Pre-88 cars would be a much easier starting point than the 88. The pre-88 cars had a front suspension in the rear. The rear knuckles have toe-links that would be replaced with a rack and pinion (for "variable toe" aka steering).
Physical space constraints and suspension geometry also come it play. I don't know if such a system would even fit without extensive modification to the car.

Syncing the front and rear steering would require a steering angle sensing system and the electronics to support it. You would need a steering control module of some sort. I'm just spit-balling this as a thought exercise.
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MichiganReefer
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Report this Post11-16-2021 06:22 PM Click Here to See the Profile for MichiganReeferClick Here to Email MichiganReeferSend a Private Message to MichiganReeferEdit/Delete MessageReply w/QuoteDirect Link to This Post
Right- completely agree. It's not feasible on the 88 cars. I'm thinking of where on in the normal system to install a steering angle position onto which will be the input into the rear steer unit including the transmission gear, and the speedometer. The rear gear seems as though it'll fit because Saginaw Steering fit their system onto unmodified cars in the 80s. The major project will be the control systems.
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pmbrunelle
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Report this Post11-16-2021 06:40 PM Click Here to See the Profile for pmbrunelleClick Here to Email pmbrunelleSend a Private Message to pmbrunelleEdit/Delete MessageReply w/QuoteDirect Link to This Post
My everyday job is in the design of electric power steering.

This project doesn't come across as overly difficult.

In the rear of the 84-87 Fiero, if you trace a straight line between the inner tie-rod ends, the only thing that's in the way is the muffler. Mufflers/exhaust can be relocated. I don't think there are major obstacles to putting a steering rack in the back.

Adding a steering angle sensor (SAS) to the front should be pretty trivial. It should probably be redundant (along with some other items in this project) to avoid any epic fails. SAS does not take much room. IIRC Paccar trucks have a multi-turn Bendix SAS (steering angle sensor) that can be gutted and replaced with homebrew electronics.

In the back, the easiest route will likely be to take a rack EPS, gut the electronics, delete the torque sensor, and convert it for steer-by-wire use. To limit project scope, I would try to use a maximum of production junkyard hardware.

Any further questions?
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Report this Post11-16-2021 07:05 PM Click Here to See the Profile for 87_specialClick Here to Email 87_specialSend a Private Message to 87_specialEdit/Delete MessageReply w/QuoteDirect Link to This Post
Very good points. Like you said not overly difficult, but programming the control systems are above my knowledge-based. I'd like to clarify that this is steer by wire as mention by pmbrunelle not hydroelectric powersteering.

So making it work in the back isn't a stretch too far. Would you want the wheels to steer opposite eachother or the same way (crab walk)?

[This message has been edited by 87_special (edited 11-16-2021).]

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MichiganReefer
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Report this Post11-16-2021 10:46 PM Click Here to See the Profile for MichiganReeferClick Here to Email MichiganReeferSend a Private Message to MichiganReeferEdit/Delete MessageReply w/QuoteDirect Link to This Post
You'd want opposed steering for low speeds and transition to the parallel steering at a certain point when acceleration. Still to be determined.... If you did it the opposite way you'd have some instability.
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Report this Post11-16-2021 11:42 PM Click Here to See the Profile for fierosoundClick Here to visit fierosound's HomePageClick Here to Email fierosoundSend a Private Message to fierosoundEdit/Delete MessageReply w/QuoteDirect Link to This Post
There's quite a few manufacturers that spent million$ developing this on some of their products.

Current vehicles that have this is at end of article.
(quite a few manufacturers have dropped it)
https://www.thedrive.com/ca.../41613/rear-steering


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[This message has been edited by fierosound (edited 11-17-2021).]

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pmbrunelle
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Report this Post11-17-2021 08:15 AM Click Here to See the Profile for pmbrunelleClick Here to Email pmbrunelleSend a Private Message to pmbrunelleEdit/Delete MessageReply w/QuoteDirect Link to This Post
For low (parking) speeds you might want some driver-operated lever to vary between opposite or crab walk. For tight turns you might want it one way, for parallel parking to a curb, another way.

Either way, as speed increases, you probably only want one behaviour, unaffected by the driver's lever.
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Neils88
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Report this Post11-17-2021 10:22 AM Click Here to See the Profile for Neils88Click Here to Email Neils88Send a Private Message to Neils88Edit/Delete MessageReply w/QuoteDirect Link to This Post
Having control over the type of response might end up with an unpredicted response in a sudden emergency maneuver. The brain resorts to muscle memory in an emergency. If the wheels are doing the opposite of what you are used to when you don't have time to think about it, it will likely not end well.
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Report this Post11-17-2021 12:42 PM Click Here to See the Profile for pmbrunelleClick Here to Email pmbrunelleSend a Private Message to pmbrunelleEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Neils88:
Having control over the type of response might end up with an unpredicted response in a sudden emergency maneuver.


That's why the effect of the driver's lever should taper off as vehicle speed increases.

Emergencies aren't likely to occur at parking speeds.
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MichiganReefer
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Report this Post11-18-2021 04:36 PM Click Here to See the Profile for MichiganReeferClick Here to Email MichiganReeferSend a Private Message to MichiganReeferEdit/Delete MessageReply w/QuoteDirect Link to This Post
I think something I will also consider with a switchable steering button is to have an override/mode that defaults to 0 rear steer angle. Does anyone with a 84-87 have a decently clear picture of the gap between the engine and the cradle that the steering rack could fit? I might need to buy an older cradle to use as a bench system.
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Report this Post11-18-2021 06:51 PM Click Here to See the Profile for pmbrunelleClick Here to Email pmbrunelleSend a Private Message to pmbrunelleEdit/Delete MessageReply w/QuoteDirect Link to This Post
The engine wouldn't come close to the rack.

Assuming the muffler is removed, the trunk would be the most likely source of interference for a rack, followed by the automatic transaxle. The standard transaxle is much more compact (i.e. non-existent) towards the rear.
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Frenchrafe
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Report this Post11-19-2021 03:07 AM Click Here to See the Profile for FrenchrafeClick Here to Email FrenchrafeSend a Private Message to FrenchrafeEdit/Delete MessageReply w/QuoteDirect Link to This Post
This sounds like a good idea. A bit out of my capabilities for the electronic control/piloting, but hey!
I would like to point out that the Fiero has quite a short wheelbase and that handling is quite good already. Ask my track friends who drive GT86/BRZ which have longer wheelbases!
The only ones who out perform me (on the bends) are the Lotus Exige drivers!

Regards,
Rafe

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Report this Post11-22-2021 07:30 PM Click Here to See the Profile for pmbrunelleClick Here to Email pmbrunelleSend a Private Message to pmbrunelleEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Frenchrafe:
I would like to point out that the Fiero has quite a short wheelbase and that handling is quite good already.


I think that OP simply wants to explore, not that there is a specific problem needing fixing with rear wheel steering.

I happened to be looking at Texense products today, and I had a flash that a sliding potentiometer like this would be easy to add to the 84-87 Fiero rack:
https://www.sens4pro.com/en...ers-4/112-rsl-1.html

As Rodney Dickman sells special hardware to add a second steering damper to the rack, I think that special hardware could be used to mount a potentiometer on top of the steering damper.

A small magnet mounted to the end of the pinion (can it be made accessible easily?) could be used with an end-of-shaft magnetic sensor for some limited (modulo 1 steering wheel turn) redundancy, but I think it would be enough to catch the likely failures, by comparison with the potentiometer's reading.

It is ideal for redundant systems to use completely different technologies. In this case, a magnetic field could make the magnetic sensor read incorrectly, but not the potentiometer. A broken mounting stud would make the potentiometer read wrong, but not the end-of-pinion magnetic sensor.

Two of the same sensor are likely to share the same design flaws, or to be influenced the same by some external environmental factor... so even though the measurement is wrong, both measurements will be equally wrong, no problem will be detected, and the redundancy will have been for nothing.
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