I was talking with Glenn yesterday, discussing tire wear on my Kuhmos and how I had lowered pressures because of my increased negative camber.

As some of you might recall, I used the Nitto DR for a while and I know others do as well. It performs rather well, once aired up to road course pressures.

I recalled following Chuck, shod with 315 Nitto DRs, and watching his left front trie as we went through a mid 3rd gear sweeping left.

A while later, a C5 comes up behind me, I point him around and now he's between Chuck and I.

Through the same corner, I am watching the C5's left front tire, shod with Hoosiers.

In my minds eye, the snapshot I have of both of those cars from my vantage point, the Hoosier maintianed it's general shape. A squared off bottom, where the tread is.

However, my recolection of the Nitto was much more rounded, like it was balloning. And maybe the other side was squishing?

I have always thought the only differences between the Nitto DR and the Nito RR tire would be the sidewall. However, I am beginning to think the cord might be stiffer as well.

My point? Hell, I don't know ... just random thoughts. At my age, when you have a moment of clarity, you document it.

 

I can see that. Does the cord go across the face of the tread?

Between the Nitto and the Kuhmo, which do you prefer and why?

When I pulled the rear tires off my car after that weekend, there was rubber spattered all over the LCA's. There is about 1/2-3/4" between the tire and inner fender in the driveway, but either the tire was flexing that much or car was swaying (which it feels pretty stiff).

 

I'm not quite sure what you are saying about the Nittos. If you push down really hard on a tire, I would picture the result as the sidewalls ballooning out as the tire is squished, so I'm not sure what you mean by one side ballooning and one side squishing.

In side/cornering loads, the tread will move a shocking amount. I've seen some footage from suspension-mounted cameras that leaves me amazed we can turn a corner without wrecking. I guess the sidewall toward the inside of the turn could be described as ballooning and the outside sidewall could be described as squished (though I picture it more as a parallelogram). Is that what you mean?

Are you suggesting the cords are stiffer in the sidewall or tread? I'm guessing you mean the DR sidewalls may have a softer sidewall cord package than the R2, and not JUST a sidewall insert.

Sorry, not trying to pick at your post. I've recently come to the conclusion that I'm not paying enough attention to the tires (even with a pyrometer) and need to seek a few moments of clarity myself.

The current issue of Sports Car has excerpts from a new book on race tires that looks "interesting", though the idea of reading a whole book about tires is, uh, well, less than thrilling.

 

In drag racing, a slick will balloon or grow taller because of centrifugal forces overcoming the tire's carcass. It does that to help a car's overall final drive ratio.

I believe the the Nitto is doing just that. As weight transfers off off the tire it ballons up. I can only specualte that a similar "squish" is happening on the other side.

Further ... it appears that would make a linear spring become a progressive one.

 

Interesting obervation. I wouldn't have thought it would be that significant at road course pressures.

Z-rated tires usually (or always, as far as I know) have an outer tread belt of nylon. Although nylon tends to flat-spot in cold weather, it has the usefull property of shrinking when it gets hot, or so I've read. This is useful because tires get hot at high speed, which causes the outer tread belt to shrink, which counteracts the tendency to grow.

I guess it might make sense for a DR to have less, looser, or no nylon in the cap ply.

It probably does change the effective spring rate as the speed changes.

It also reduces rolling resistance by reducing the size of the contact patch as the speed increases. That hardly seems desireable on a road course

Fascinating...

 

Here is a photo of 315 drag radials under load at an autox:
http://24.219.180.66/prosolo/lg/tire_flex.jpg

 

Chuck, I think your picture of Nitto DRs at an autox isn't that unusual for any DOT type tire under heavy load. The Hoosiers are probably the only ones that might be an exception to that rule. Their sidewalls are much firmer as anyone would tell you after trying to drive to an autox on them on Detroit roads.

 

I have never driven on Hoosiers, but I imagine they have a stiff sidewall much like the old BFG R1's. Those tires would beat you to death on the street.

 

Hoosiers have a VERY stiff sidewall, much more so the a Kumho.

 

The tire isn't growing at all. What you are seeing is the simply the sidewalls flexing under a load. Turning left, and looking at the left front tire, the sidewall will bulge outward. The Hoosier wasn't doing that because the tire's sidewall is much, much, MUCH stiffer than the Nitto's.

You are right in assuming the same thing is happening on the loaded side, right in a left turn, but in the opposite direction. The outer sidewall gets pulled under, the inner sidewall pulled up off the surface and a loss of contact patch results. This is why those are drag radials and not road race tires.

FWIW, radial tires really do not grow much at all. One of the reasons NASCAR switched. Diameters are much more consistant from tire to tire and with much less change than bias-ply tires when driven.

The more a tire deforms under load, the more static camber and/or camber control you need to compensate for the greater deflection.