Yes, lower pressure leading to higher friction coefficient is generally
true in a practical sense, but I believe the mechanism causing this is
still tread distortion. IOW if you take an unstressed chunk of ***
and put an even pressure across it, the friction coefficient will be the
same no matter what the pressure is (unless it gets into sliding
friction, more about that below). Put that same chunk of *** on a
tire that changes shape depending on the pressure, and now the traction
coefficient changes (I'm old school, we used to refer to traction
coefficients for tires to emphasize the fact that it does not work
exactly like classical friction).
Apology accepted, and I'm sorry I jumped on you so hard. The classy
thing to do would have been to ignore you. ;o) What can I say, I was
tired.
Okay, you probably already know some of this, but bear with me so I can
explain it fully. Because tires are made of a flexible material, they
stretch when you pull on them. So when a piece of tire rotates through
the contact patch, and there is a force applied to the tire, the
material stretches as it passes through the contact patch, and then
snaps back as it comes out of the contact patch. This makes it appear
like the tire is always slipping, so under acceleration or braking we
have a slip ratio, and in cornering we have a slip angle. This
stretching occurs not just in the tire carcass itself, but also in the
tread elements. The first thing that greater tread stability
accomplishes is to reduce this slippage. So slicks will not go to as
high a slip angle as a fully treaded tire, or as high a slip ratio in
acceleration or braking. But here is the first part of my premise - I
don't think a longitudinal groove in the tire will have much of an
affect on the distortion of the tire under acceleration or braking, so I
don't think the slip ratio will change in those situations. Also, since
there is so little material that has been removed, I don't even think it
will have a big effect on the slip angle behavior in cornering. It is
hard for me to see how those small grooves and some slight additional
thickness in the *** between them will change the amount that the
tread stretches when a force is applied to it.
But slip behavior and traction are not the same thing, admittedly. I am
not a tire expert, but I have looked at a fair amount of data and worked
with some simulations, and I have formed an opinion on what is going on.
And BTW my opinion is fairly similar to a number of SAE papers I have
read, so there is at least a slim possibility that I might be right.
;o) If you look at a tire, with or without tread, you can mentally or
computationally cut it up into small elements, calculate the forces on
those elements, and add them up to get the full force. Each element
that is towards the center of a tread block is going to be fairly
stable. As you get towards the edges the elements will start to bend
significantly when force is applied. Also each element will have a
slightly different normal force applied to it. Now under classic
friction theory, as long as we are talking about static friction, the
coefficient is constant. That being the case, I could distribute the
load any way I wanted to over these elements and the total lateral force
they generate should come out the same. BUT, if the normal force gets
too low, and the element is free to move, it will behave more like
sliding friction than static friction. That is exactly what I would
expect to happen at the edge of a tread block, especially if the force
is applied transversely to that edge. Since sliding friction
coefficients are generally lower than static friction coefficients,
these elements are no longer doing a proportionate amount of the work,
and the overall traction is decreased. That could also happen if the
tire were simply so distorted at some point that a part of the tire had
insufficient pressure on it to maintain static friction. This is what
happens at the rear of the contact patch as you approach loss of
traction. It is also what happens with improper inflation pressure, or
incorrect camber angle. Basically the more even the pressure across the
contact patch, the more likely that the entire contact patch is doing
its job.
If I understand your question correctly then yes, and that is exactly
why I think it has little effect. The grooves are spaced widely apart,
and the tread between them is not very tall, therefore it should be very
stable still.
I know that was kind of a long winded explanation and I didn't really
take the time to proof read it very well. I hope it made some sense.
Please feel free to ask more questions if it does not make sense. I may
get grouchy occasionally, but I honestly do not mind a well intended
question. ;o)
