Tires/Nose dive and tire type
QUESTION: A certain late model SUV is considered by many owners to have excessive nose dive under straight line braking. There are numerous reports of noticeable improvement by switching to LT tires and I'm having a hard time imaging a mechanism through which this would work, given unchanged tire dimension and pressure.
I'm assuming any difference in tire deflection under the compression of hard braking would be much too small to account for this.
The only guess I can come up with is that increased stopping distance smooths out the perceived dive. If this sounds reasonable, the obvious contribution to longer braking would come from increased tire weight. Would you imagine any significant contribution from decreased dry traction due to tread pattern or even compound? And how about this for a stretch (ouch) -- differences in elastic slip?
First, I am not buying that excessive nose dive is fixed by switching to LT tires (and I assume from P type tires). Like you, I don't think the the difference in deflection between the 2 types is enough to account for this.
What I think is going on is the human failing of self-congratulations - patting ourselves on the back for being so clever, even when we aren't. And in this case, I'll bet the stiffer ride is fooling people into expecting less nose dive - and then it magically happens (since the only gauge is their butt!)
I'm assuming that the increased stopping distance due to the switch has been documented, because if it has, it would certainly explain why the nose dive is less. If not, then it is pure speculation that it is increased - although LT tires tend to be made from stiffer, less compliant rubber and are quite likely to have increased stopping distances.
Why stiffer rubber? Because these tires have to hold up more load per unit volume, and that causes a problem known as compression set, where the rubber is permanently deformed due to the load. It's a little complicated, but rubber isn't truly a solid. It is a visco-elastic material - that is, it displays properties of a viscous liquid (flow), AND properties of an elastic material (returns to its original shape when the load is released). To prevent permanent distortion, the rubber used in LT tires is stiffer.
So overall, I'm not buying into this - as I think it causes other issues.
---------- FOLLOW-UP ----------
QUESTION: Yes, P to LT.
No increase in stopping distance has been documented. I was simply assuming that the increase in tire weight (without any accompanying tire properties that could improve stopping distance) pretty much made a distance increase a foregone conclusion -- no?
The fooled perception explanation is easy for me to buy. I just wanted to explore the possible physical explanations first.
Tire weight = longer stopping distances?
Nope! While more weight does equal longer stopping distance - all other things being equal - it is the vehicle weight that is being slowed down, and increasing the tire weight doesn't change the vehicle weight enough to be noticeable.
But tread rubber has a very large effect on stopping distances and any change there would completely overwhelm any other property.