Tire Pressure Manual Confusing

[Bernard Foy]

On 11/9/2023 at 8:29 PM, Don Mac said:

I don't think that you understand what Citroen did if you think that their system was doing something which electric power assistance can't.

The force exerted on the steering changes when a tyre deflates mainly because the wheel drops, thus altering the angle of the steering axis and the scrub radius (the distance between where the steering axis intersects the road and the contact patch of the car). In the old days cars were often set up with a positive scrub radius (intersection point inboard of contact patch) because this made the steering lighter, the downside was that they didn't deal with uneven braking from side to side or tyre deflation so well.

Modern cars, which don't rely on the driver's muscles, tend to have a negative scrub radius which has pretty much the opposite properties.

It's not the easiest thing to grasp (or explain...), but you'll find loads of material on the web (with some good diagrams). There's a reasonably simple starter on Wikipedia - https://en.wikipedia.org/wiki/Scrub_radius#:~:text=An advantage of a negative,steering control in this emergency.

My point is that having power assisted steering allows manufacturers to make cars with inherently stable steering geometry without requiring huge efforts from the driver. Compare some of the anecdotes above with what used to happen (I had a puncture once in a 1980 Vauxhall, I did not stay on the road).

There are, of course, other factors at play. Modern tyres tend to be lower profile, so the change of the steering axis' angle will be less, and more resistant to shredding when deflated. Car manufacturers have the advantage of computer modelling and decades of experience in setting up suspension sytems to behave the way that they want.

 

I'll shut up now....

Good I thought for a moment you were going after Cykers longest post record. LOL.