I had never noticed a lorry with this feature before, apart from those enormously long low-loaders used for moving ships and railway locomotives, which have a man at the back with another steering wheel.
I followed one yesterday, and it was fascinating to watch. It looked just like an ordinary artic, except that the rear of the 3 trailing axles was visibly steering.
It appeared to have a different action depending on the kind of corner. When it moved across the road to pass a parked vehicle or edge over to avoid a bollard, the back steered in the SAME direction as the front. But when it went round a tight corner the back swung out in the OPPOSITE direction.
How does this work? Does the driver have a separate control, perhaps an additional inner steering wheel? Or is it entirely automatic? If so, how does it know whether he wants positive or negative assistance? The steering that would assist getting round a tight corner would be lethal on a motorway.
|
I'm almost sure that there have been cars with a degree of 'rear steer' built in deliberately to aid handling.
All the characteristics outlined above are present in the Prelude 4WS I have been used to for years. Up to something like 12mph the back wheels turn in the opposite direction to the fronts, to aid manoeuvring, above 12mph they turn the same way to assist cornering and lane changing. All done by an ECU and servos. It's brilliant. People watching are always astonished how it can manoeuvre in tight spaces (and U-turn in the width of an ordinary road).
Takes a bit of re-acclimatisation though after driving my friend's canal boat for a weekend.
|
I'm sure some Beamers have that.
I was going to add my tuppence-worth and talk about the physics because it's simple yet clever, but as I have access to journals, this paper says it quite well: pid.sagepub.com/content/224/12/1501.full.pdf especially page 7.
|
Thanks Bobbin - that's quite an interesting paper.
About 15 years ago, I was told that there was nothing left to say about vehicle dynamics using linear modelling. That paper shows that that point of view wasn't true!
|
>> Thanks Bobbin - that's quite an interesting paper.
Indeed, I have never seen a journal written in native klingon dialect before!
|
The 'new' Mercedes London cabs have a rear steering axle to get them within the 25 foot turning circle requirement.
Watch one of those turn round in the road, quite strange!
|
...to get them within the 25 foot turning circle requirement...
Can I be the first poster in this thread to mention the Triumph Herald?
Full-lock turns had to be exercised with care, some owners snapped one of the stub axles by going too quickly.
|
>>
>> Full-lock turns had to be exercised with care, some owners snapped one of the stub
>> axles by going too quickly.
>>
Splitting the steering front/back presumably reduces the force on the front half yet achieves the same turning circle?
What happens if the car or lorry accelerates past the 12 mph point while turning? Does the back suddenly lurch as the steering switches from negative to positive?
(Like my tractor, whose normally self-centring steering suddenly swings to full lock beyond a certain critical point!)
|
...Takes a bit of re-acclimatisation though after driving my friend's canal boat for a weekend...
I had a brief play with an AWS Prelude and couldn't tell the difference. :)
Although I read somewhere the rear wheels lock-out straight if it's kerbed, so the AWS may not have been working on the one I tried.
|
>> How does this work? Does the driver have a separate control, perhaps an additional inner
>> steering wheel? Or is it entirely automatic? If so, how does it know whether he
>> wants positive or negative assistance? The steering that would assist getting round a tight corner
>> would be lethal on a motorway.
>>
I can't speak for the STGO ( low-loader) trailers, but our new curtainsider 8-wheeler Renault has a rear-steer axle. Despite appearing to be a dirty great lump of a thing it's incredibly manoeuverable, and very easy to drive; all automatic or course. I can get into farms with that thing, even with nearly three tons of demountable fork-lift hanging off the back, which would defeat many four wheelers. The only thing we're not too sure about yet is how it will handle more extreme winter conditions.
|
Rear steering axle - Mike Hannon
