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Rear Steer Thoughts


simonr
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Although I'm still a couple of months away from having to build any of the rear steer control system for my truck, I thought I'd share my thoughts so far.

I've looked at a few different rear-steer setups and most use a separate control lever to move the wheels left or right with an indicator of some sort to show when they are central.

At the Belgium National one year there was a MaxiDrive Disco which had a lever built in to the centre cubby box which the driver or passenger could operate. This seemed a good idea as it's one less thing for the driver to worry about a lot of the time. The down side of course is when the passenger decides to make it a bit more exciting for you or accidentally leans on the lever!

I wonder if there are many/any circumstances where you want something other than either:

Rear Wheels steer straight

Rear Wheels mirror front wheels

Rear Wheels follow front wheels (crab steer)?

If not, then it would make sense to build a simple servo control system to move the back with the front.

It occurred to me to use Range Rover axle position sensors as used with the air suspension to measure the axle positions. The sensors are pretty robust and water resistant so are probably a good choice. Inside, they just contain a variable resistor - so easy to interface to.

Although I considered using a computer for the control - my experience with the air suspension makes me think that an analog, hardware solution might be more bulletproof.

So, here it is - for those of you who speak circuits...

post-74-1185797857_thumb.jpg

Can any of you think of scenarios where you would realistically want the rear steering to do something other than the above three modes?

Si

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Si from my experience with plany i have found rear steering axles 'steer' the truck more for the same degrees as the front if that makes any sence, therefore i would say a rear steer only aswell as he options above would be helpful. Also the jcb fastrac has a headland setting wich turn the front and rear axles at a ratio of 3:1 ie 15degrees on the front to 5 on the back this means the back follows the front and makes for accurate turning with implements ect, i would think this could be very handy in a trials environment as you could steer the front end more towards the inside of the gate as opposed to having to 'swing in' alowing for the back end to clear.

No idea whether any of that makes sence :lol:

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Rear steeer is definitely more effective for a given angle, hence it's use on the commercial ride on lawn mowers - these will pivot on one wheel if required to line up for the next run .

Therefore this is possibly use 4 - rear steer only to pivot on inside front wheel

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From videos I've seen of rear-steers in action most of them seem to use a combination of mirror/follow to get round things so an automatic linked system may not provide the flexibility required. I'd suggest a good trawl of YouTube / BitTorrent to see how people use them. I don't know about rock-crawlin' but I've got some video from European events with 4WS in action is terrain that's more familiar.

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I assume that its hydro steering?

If so then why not pinch the valveblock complete from a small telehandler or other 4ws plant? Then you have a hardware solution that will take a lot of abuse, and a long time to fail (the excavator at our workshop is over 30yrs old and the valveblock is still quite functional)

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As lewis said Si just get a manual steering mode valve out of a telehandeler, then you have the choice of front, crab or follow steer.

No electrics, nothing to go wrong.

Most of the newer stuff i've used ('94 onwards), wil not let you change the steering mode untill you have the wheels straight ahead, thus eliminating the need for rear position sensors.

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Agreed. The 'co-driver leaning on the lever' problem is, of course, easily solved by lever shape/placement etc.

The only reasons I can think of for over-complicating it beyond the simple valve block approach are:

1. To imitate systems such as the modern electric rear-steer axles from the States which alter the contribution of the rear axle depending on road speed, so at low speeds they mirror the front to augment manoeuvrability, and at high speeds they match the front to 'crab' across motorway lanes etc - more stable, apparently. At speeds in between, the rear axle dynamics are interpolated between these two extremes, resulting in a neutral speed where the contribution is zero, if that makes sense. Unless you plan a lot of high speed work (and even if you do), its hard to see the point of this complexity in our context.

2. You could use sensors / control loops to automatically re-centre the rear at the touch of a button - this would save you straightening up by eye, which seems a slight issue in use for some people - although others claim to be able to do it no problems by the 'seat of their pants'.

I have to vote for the simple option too!

Al. :)

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1. To imitate systems such as the modern electric rear-steer axles from the States which alter the contribution of the rear axle depending on road speed, so at low speeds they mirror the front to augment manoeuvrability, and at high speeds they match the front to 'crab' across motorway lanes etc - more stable, apparently. At speeds in between, the rear axle dynamics are interpolated between these two extremes, resulting in a neutral speed where the contribution is zero, if that makes sense. Unless you plan a lot of high speed work (and even if you do), its hard to see the point of this complexity in our context.

Al. :)

Not a modern system by any means - Honda did this 20years ago

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Steering on the front is just ram-assist. Steering on the back is hydraulic via a 4 port valve and an electric compressor - both of which I already have!

post-74-1185816318_thumb.jpg

I'm not convinced about full hydro's legality on the road, at least at higher than agricultural speeds, so the front at least needed to be mostly mechanical. Full hydro would have been so much easier - but that bit of it is largely done now.

My reason for wanting to automate it is just that with 4 wheel steer, 4 wheel fiddle brakes and 4 wheel height adjust - there is too much to think about! Too many buttons!

Even if there is an override for more or less manual use, I think to get the max benefit from them there needs to be some degree of automation. The fiddle brakes could operate in sympathy with the steering, but be disables when using lockers for example.

The height adjust isn't intended as an active system like my EAS setup, just something which allows me to change the ride height by moving the coil spring hangers up & down. I'll have a 'low rider' for use on the road and 19" under the diffs off road (if I want). That at least won't need thinking about until it's bellied out.

Si

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I wonder if there are many/any circumstances where you want something other than either:

Rear Wheels steer straight

Rear Wheels mirror front wheels

Rear Wheels follow front wheels (crab steer)?

Si

Not sure how useful it would be (or if you have enough lock to achieve it) but a concept Jeep had a neat fourth option of applying toe in at the front and toe out at the rear enabling the truck to effectively spin about its own centre. You can't get a tighter turning circle than that and look fun in the videos :D

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I had a 4WS Prelude for many years, the 4WS was totally automatic and most of te time you forgot it was there. The only time it caused problems was if you pulled up next to a wall with very little lock then some one parked next to you and you had to use full lock to pull out, if you weren't carefull the back end went into the wall, in those cases it would have been nice to be able to turn it off. Not sure if this would be a problem at times off road or not but a simple switch to turn it off would solve that.

The Honda system was totally manuel in operation (no electrics) and ran with a second steering rack on the rear axle and a steering rod slaved from the front axle. It was quite complicated though as in operation if you turned the steering slightly the rears turned the same way and let you crab steer a bit then as you applied more lock they straightened and started turning in the opposite direction to give a better turning circle, if that makes sense. The improved turning was usefull at time as the Prelude was a long car (longer than most 4WD's !), it wasn't as low as it looked just longer to give the same look, I drove mine across several off road sites (often just to access road was bad enough) and generally made it, just.

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Thanks for the input.

Trevor - even if one could achieve toe in at the front and toe out at the back, there is a bit of a problem persuading the left hand wheels to go in the opposite direction to the right! That concept truck had two separate gear trains to achieve it.

Bill - I'm just using Jag diffs with the standard UJ based drive shafts. By most accounts they are up to the job - probably 50% bigger than LR.

Not having CV's is generally a problem when you add steer into the equation. Off road, it probably doesn't matter too much and on the road the 4wd is selectable so it will be running 2WD with the rear steer disabled such that the pair of UJ's in each shaft will behave as a CV as the offset angles are the same.

Si

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.

Bill - I'm just using Jag diffs with the standard UJ based drive shafts. By most accounts they are up to the job - probably 50% bigger than LR.

Not having CV's is generally a problem when you add steer into the equation. Off road, it probably doesn't matter too much and on the road the 4wd is selectable so it will be running 2WD with the rear steer disabled such that the pair of UJ's in each shaft will behave as a CV as the offset angles are the same.

Si

Oh yes I forgot about the Jag driveshafts and UJ's, They are quite substantial.

Bill.

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Oh yes I forgot about the Jag driveshafts and UJ's, They are quite substantial.

Bill.

The next step after the Jag parts, for a bit more travel & steer is these:

http://www.4x4review.com/feature/HalFrost/

(Down the bottom of the page). They can run at double the offset of the Jag UJ's at the same strength.

Si

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In my experience the biggest handicap with rear steer is not being able to get it straight again quickly enough after coming out of a turn.

Trialing in particular you need to be able to keep your eyes on the course and prepare for the next obstacle rather than be concentrating on some sort of indicator to see what the rear wheels are doing.

I built a circuit so that when the joystick was pushed forwards the rear wheels immediately returned to centre. It made a vast difference to the usability of the system.

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I still have a copy of this circuit diagram if you would like to compare, of course if Neil allows.

I'd be interested to know what you used for position sensors? Were they just limit / position switches or more analogue?

My circuit is likely to be quite different as it is a servo control system giving accurate positional control - hence it's easy to make the back follow the front, or a proportion of it. A 'steering wheel' for the back could be as simple as a sprung to center potentiometer - or a joystick.

Si

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I'd be interested to know what you used for position sensors? Were they just limit / position switches or more analogue?

I used Proximity sensors, it is only a simple circuit that knows if the wheels are left, right or centred.

You say that yours will re-centre when disabled or turned off, but I would strongly recommend that you include the ability to centre on demand.

Having driven vehicles with and without there is a massive difference and it frees your mind to concentrate on other things.

And Roger - No I don't mind the circuit diagram being circulated - its all for the greater good ;)

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A couple of things on the rear steer idea (i've been thinking about this idea myself for quite a while)

@ present they are not road legal unless vehicle was designed this way - Any significant modifications to steering etc is theoretically an MOT Failure ! - However My idea to get over this was to have som for of mechanical "Lock" so it could be put in on-road mode and be impossible for the rear steer to move whilst on the road.

I saw the system on the Eagle / RR hybrid @ Billing and his system was totally manual and hence it looked a complete nightmare to use as the driver was constantly trying to get the wheeel in the right position and having to hang out of the window to do this.

Another Mode I was thinking about for my system was if using twin RAMs on the rear to get independant control was "Ground Anchor Mode" so when winching you could set both rear wheels out to the maximum to give the maximum amount of drag etc for when winching.

Just som little ideas !

Lyndon

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I think you'd run into tracking issues very quickly with two rams for the rear axle, for theoretically no gain - the resistance of the rear wheels is the same if they are toed-out or just locked with the brakes.

The Eagle jeep belongs to Bathtub Pete, who has been known to post on here when taunted - he might be better able to explain his steering.

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Shaun Stapley at POS makes a rear steer system with an auto park function, he also makes a kit with a mechanical "locked in straight ahead" mode for the US army if its of interest?

how about.... closed circuit rams mounted where the chasis end trailing arm bushes sit, (mid stroke being straight ahead) effectively increasing and decreasing the length of the trailing arms.. its an easier idea to build IMO, prox sensors can be mounted well out of harms way.

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