Arthur Bishop on 4WDs
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  1. #1
    Fellow Frogger! Paul Smith's Avatar
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    Arthur Bishop on 4WDs

    From today's Drive in the SMH, a quote from Arthur Bishop, described as one of the world's foremost authorities on automotive steering systems.

    The designers of large 4WDs, he says, are "required to meet the conflicting demands of a boudoir on wheels and an army tank".

    Perfect description!! head_ban head_ban

    He goes on to say "The knee-jerk reaction has been to install a computer .. to take over from the driver when the stability is threatened. Surely a more rigourous approach in the design of the suspension, steering gear and tyres is all that is required"

    But that might require people who actually know what they are doing, not just marketers??? mallet mallet

    Paul

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  2. #2
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    Has anyone got the article online (I couldn't find it), because I find this particular quote quite interesting:

    "The knee-jerk reaction has been to install a computer .. to take over from the driver when the stability is threatened. Surely a more rigourous approach in the design of the suspension, steering gear and tyres is all that is required"
    His comments would've made more sense to me, if we were dealing with archaic ladder chassis type machines. The latest batch have all moved on from that - everyone knows that these are primarily used on sealed roads, so they are designed accordingly. His comments are certainly fine if we're dealing some old bomb like Land Rover Defenders, but the comments about electronic aids indicate otherwise.

    Tyres - many of the premium SUVs are using tyres optimised for road performance, not off-roading. One of the XC90s I reviewed basically had the Contis on the 307 boosted up 50% in size. The Porsche Cayenne, I believe has Pirelli P-Zeros (I'll laugh at the first poor sod who takes the car offroad). You're dealing with quality tyres.

    Steering - not quite sure what he's suggesting here? The good SUVs are using quality rack & pinion systems.

    Suspension - the good SUVs are not using ladder chassis, using car like designs. That's why the best ones are car like to drive - they're using quality independent systems at the rear, not archaic live axles.

    I can't really think of what realistic improvements could be made to SUVs at a suspension/tyre/steering level (let's face it, a Porsche Cayenne system can work in a $200k car, but not at the standard price level). I suspect the Cayenne drives better than a lot of conventional cars too. Given that the better SUVs are reasonably good in this department, attention should be paid to the fundamental disadvantages of such designs - such as reducing the centre of gravity.

    When he talks about electronic aids, the main one that comes to mind is something like RSC - roll stability control. Look what they do in tall cars when they need rollover protection - at the chassis level, they engineer in more understeer, ala 307. In fact, that's what RSC does - to prevent rollover, it induces a little bit of understeer, chiefly by braking the outside front wheel of the turn, IIRC. Do you want a car that is consistently compromised by understeer, or one that uses it when needed to its advantage?

    At the other electronic level, stability control systems are welcome in my books. They don't excuse poor suspension design, and I believe the good SUVs do not have poor suspension design. Stability control systems are great for most people, especially in inclement weather. There are now plenty of cars with ESP/DSTC/STC type systems on the market - it's not just something tacked onto luxo SUVs.

    At the end of the day, there are some serious challenges posed by the mass and higher centre of gravity of SUVs. But Bishop's comments seem flawed, if he's implying that SUV manufacturers are only using electronic aids and ignoring fundamental chassis engineering in trying to make the cars more stable. If we're dealing those gargantuan trucks they drive in the USA, he probably has a point then

    Cheers,

    Justin

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    Guru davemcbean's Avatar
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    Pug307:


    Steering - not quite sure what he's suggesting here? The good SUVs are using quality rack & pinion systems.
    It's obvious. For freeway stability a vehicle with such a high cente of gravity should have a rack with a ratio of atleast 20:1 on centre and no less than 16:1 at the extremeties, assuming an average steering wheel size of around 370mm diameter. Even this would be a tad too quick for many 4WDs.

    So in other words racks with much less than 3.5 turns lock to lock are a bloody stupid idea on such a high vehicle, unless you've got the driving skills of Michael Shumacher, (or your vehicle has a computer controlled stability function wink ).

    Dave
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  4. #4
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    Just for interest, on this issue of rollover, I was reading a post from the USA.

    There was a simulation of the moost avoidance test, apparently the:

    RX300 started going on two wheels at 45mph
    X5 started going on two wheels at 75mph
    MDX started going on two wheels at 75mph, but started losing adhesion, unlike the X5.
    XC90 at over 95mph still was on four wheels.

    I know all had stability control systems, not 100% sure on the RX300 (the RX330 has it).

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    Guru davemcbean's Avatar
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    davemcbean:
    racks with much less than 3.5 turns lock to lock are a bloody stupid idea on such a high vehicle,
    I neglected to mention that variable ratio racks are not a good idea on unstable vehicles either, so I'd be inlined to design them with a linear rack of a ratio 20:1 or higher, which means the turns lock to lock will often be in the vicinity of 4 (obviously this varies on how far the wheels are allowed to turn, due to driveshaft geometry, etc).

    Dave

    <small>[ 03 August 2003, 01:20 PM: Message edited by: davemcbean ]</small>
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  6. #6
    nJm
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    Pug307
    That's why the best ones are car like to drive
    It is interesting you said that. Going on the information you have provided, I have driven one of the best ones out there - a volvo xc90. However, I did not feel quite like I was driving a car. Sure, once I was used to the huge ride height and steering ratio I got pretty confident and started to throw it around quite a bit - however from what I understand while doing that I was engaging many of its stability programs. A car would not have needed them for what I was doing.

    Granted they have come a long way (the last 4WD I drove was a mid 80s Range Rover), but I would not call them car like.
    Nick
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    "All of its cars from the 1.1 litre 205 through the ugly duckling 309 to the 2.2 litre 505 GTi had a rightness and a righteousness about them that turned every humdrum drive into a journey. Someone, I once wrote, in the bowels of Peugeot understands handling and how a chassis should feel." - Jeremy Clarkson

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