Safe Speed Forums

Strangely enough - all of my ancient mentors had one bit of advice - "keep your thumbs outside the wheel" - but this was n the days before power steering etc, when you could end up with a wheel in the rough ( unmade road on the nearside/ grass verge) and have the wheel torn out of your hands, or alternatively have a blowout/sharp deflation and have the wheel decide to follow the front wheels.
The idea was that with your thumbs in one piece you could control the car.

Still not convinced. When a power-assisted rack encounters a disturbance from the road wheels, it "fights" the disturbance. It does this in two ways. Firstly, the power steering rack (the hydraulic types, not these new-fangled column-mounted electric types), is one big steering damper. With the engine running, you can hit the wheel at the rim with a dirty great sledgehammer hard enough to damage the rim and not feel much at the steering wheel. Secondly, it is "active". When the driver starts to turn the wheel, the valve block in the rack senses that the steering wheel is starting to turn relative to the pinion and opens the appropriate port to allow high pressure fluid into that half of the rack which will assist the turn. When the opposite happens (i.e. the road wheels start to turn without being asked to) this happens in reverse. The wheels turn, the rack bar moves, but the steering wheel hasn't. This opens the valve (effectively) the opposite way and allows fluid into the half of the rack that the valve "thinks" needs assistance. This is the opposite half of the rack to the way the whels are moving so it pushes them back towards where they came from - i.e. it tries to make them go the way the steering wheel is pointing. Added to that, there's the moment of inertia of the steering wheel itself, resisting sudden inputs.

I'm not saying that Paul's scenario couldn't overcome these forces and maybe in the circumstances Paul describes, it COULD do you some damage, but this is a crash situation rather than driving along and hitting a kerb or pothole hard or having a front wheel blow-out.

Have you got a vehicle that meets that spec? Try it in reverse at 25mph in a big safe non-public space. Put on a quarter turn of lock and let go. You'll get scary forces at the steering wheel from castor (working backwards) alone.

_________________
Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Servoed steering systems reduce but don't eliminate feedback and kickback from the road. They make the steering lighter, but they don't make it weightless. They damp kick-back, but they don't eliminate it. The forces that occur during a collision can be dramatically greater than in normal driving. You may be able to hold 50 lb-ft but 500 lb-ft is just going to rip the wheel from your hands.

_________________
Peter Humphries (and a green V8S)

But what makes you feel that a torque of (say) 50 lbft at the wheel will go up to 500 in a crash situation? The fluid in the steering rack (assuming it's PAS) will act as a damper so the faster the rack bar tries to move, the more that movement will be resisted.

I've seen a fair few crashed cars with steering tie-rods buckled like fudge. and I know that this couldn't have happened as a result of the wheel being ripped out of someone's hand and going to full lock (and beyond) because there isn't enough clearance between the inner wheel arch and the tyre to allow the wheel to do that. I assume, therefore (and I freely admit that this IS ONLY an assumption!) that the reactive force at the steering wheel rim could only have been a combination of the driver's grip, the gearing in the system, the inertia of the rotating parts and the damping in the rack.

Just twigged your username by the way - I guess you'll have plenty of experience of steering feedback!

not that sure how they compare but electric column drive systems may be as much if not more of a damper as the hydraulic systems as the motor & gearbox adds a fair bit of inertia to the system.

Funny you should mention that!

I haven't quite got a vehicle I'd try that in! It's a company Peugeot 807 and I'd be a bit worried that it might fall over if I tried that! (and I'd get told off!) That said, I think it's probably not a very representative test - using the castor the wrong way round! I DID, however, try gently clipping the kerb with the nearside tyre at low speed a few times. Hardly representative, I know, but the steering wheel didn't do anything that I couldn't prevent with a pretty light grip and only one hand! What did strike me, however, was that once the wheel WAS moving quickly of its own accord, there was little I could do to stop it! This, I think was the inertia of the wheel acting the opposite way. I still stand by what I said regarding the inertia of the steering wheel (and other rotating parts) acting to help in a sudden impact situation but once the system is moving, it's quite hard to grab the stering wheel and stop it before it goes to full lock! Also, if I jack the car up (no engine running) with the front wheels clear of the ground and turn the road wheel (as in "steer "it, rather than "roll" it), the steering wheel moves pretty fast and whacks the lock stop quite hard.

What I HAVE just remembered is that about 18 months ago, I was in an 807 on a single track NSL road and some muppet came flying round a blind bend and hit me. Being sat high up, I could see him coming and pulled right over into the hedge. I even had time to come to a complete stop but couldn't reverse because by then I had two wheels on the mud and was pointing down hill. HE came round the bend, jumped on his brakes and slid right into my front corner. My wheels would have been pointing straight ahead (ish) and he hit the driver's front tyre hard enough to rip the bottom balljoint out of its housing, snap the steering track rod and push the wheel back into the trailing edge of the footwell. The interesting thing is that I don't remember feeling ANYTHING SIGNIFICANT through the steering wheel or having sore fingers / thumbs afterwards. The wheel didn't move appreciably because the road wheel on the other side was still roughly straight ahead when the tow truck arrived....