True center-point steering geometry?

[mark512]

I'm looking at the concept of "center-point steering" which seems to mean that there is zero mechanical trail and zero scrub radius (steering axis passes through the geometric center of the tire contact patch) or by some definitions, just zero scrub radius (steering axis passes through the geometric center of the tire contact patch in the front/rear view only.)

Practically, does this mean that the scrub radius (and mechanical trail) are *literally* zero, or just "relatively small"?



...In particular regarding scrub radius, the designers of the 1st-gen Mazda MX-5/Miata talk a lot about designing zero scrub radius into the car, and how the effects of having a scrub radius are bad, how zero scrub is good for the feel of the car, and how that was high priority when designing the front suspension.

Other sources say that zero scrub radius is bad, and results in "squirm" - the effective scrub radius goes positive and negative as a result of pneumatic scrub (and/or the loaded radius of the tire changing in combination with having steering axis inclination). The consensus here is that the scrub radius should be slightly positive or slightly negative but not zero, so that the aforementioned factors don't cause the effective scrub radius to change signs. Right now, this latter concept makes sense to me...

 

[GregLocock]

Centrepoint steering literally means the steering axis passing through the centre of the contact patch. Zero scrub and zero mechanical trail.

However that ignores pneumatic trail, and what I suppose we could call pneumatic scrub, or overturning moment. You will not easily evade pneumatic trail, but I suppose if you fiddle with camber you could probably eliminate overturning moment at straight ahead. Similarly for trail if you went to negative castor, and a lot of hub lead, you could eliminate trail, at straight ahead.

Given that back in the day plenty of cars had 100 mm of scrub, I find it hard to believe that the current fad for restricting it to 25 mm has much to recommend it, and I certainly agree with the idea that having it weave from side to side across the neutral location is unlikely to be helpful.

Trail is a bit more complex. It's a fairly important part of being able to detect where the tire is on its curve of lateral force vs slip angle. Too much trail and the car feels too linear (as you drive off through the sandtrap), too little and you lose all force feedback as you approach max latacc.




Cheers

Greg Locock


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[cibachrome]

Pretty keen idea for a lawn tractor. What Greg said.

All I would add is that it requires specific knowledge of the tire construction you plan to use. The aftermarket tire situation would also be involved. Even the same brand, size, model, etc, of a replacement set of tires does not guarantee that the pneumatic scrub (Mx factor mentioned) would be the same. And if it should happen to wind up 25 mm POSITIVE, no telling where you would land.

Plus, how will you account for dynamic scrub under full power and heavy braking ? I would dare you to to keep your car in a straight line for a wot start followed by a panic brake stop. And I would win the bet with ANY tire even the OEM Mazda skins.

What they say they do and what they do is do-do.