Tire Roll-over Definition
 

Is it A) street tire tread roll-over with potential for chunking. B) Upper side-wall rubbing in a turn. C) All of the above. D) None of the above.

My understanding has always been A) based on this general line of thought...

http://www.gttrackdays.com/tires.html

...but have gotten into a healthy discussion on upper-side wall roll over which also makes sense.

Sorry, are you wanting to know the definition for the term "tire roll-over"?

The issue with tread chunking is certainly a legitimate concern. This is actually present in all types of tires with tread (i.e., street-legal tires), from all-seasons to R-compounds. For example, the Toyo RA-1's can be bought pre-shaved, in order to lessen this effect. The logic is that by removing some of the tread material, the tread will flex less you have the potential to get more "good" life than if you had used an un-shaved tire.

Personally, I just buy my tires un-shaved since I'm not looking for 100% perfect performance, lap after lap, and as long as I break them in properly I can get similar -- if not better -- ultimate life.

Was this where you were going?

This description in the link above is too vague for me. In the paragraph addressing using slicks/track tires, they should mention that they handle better, but have less margin for error at the limit than typical street tires. Also often less noisy (less warning). But I agree that "brand new" street tires would be more prone to chunking when driven at high g-loads under track driving condition. Not so much a problem under normal street driving.

I've always used "tire rollover" as a way to gauge air pressure. I've found that the optimum "rollover" is about 1/4" above the lowest edge of the tread, above the sidewall (red line in pic below):
http://farm5.static.flickr.com/4135/...8c986508_o.jpg

However, I've been noticing that its become more vague as a measure with modern low profile very wide tires. So YMMV.

As far as chunking, my understanding is that deeper tread blocks are subjected to more internal "squirm" under loads, which creates heat in the rubber. The heat + tread loading can cause portions of the tread to break off, chunk. Shaved tires reduce the height of the blocks, making them squirm less, which reduces the heat build-up in the rubber. Also, the various Tire Speed Ratings, like "S", "H", "V" etc. (as in 225/50SR16 vs 225/50VR16) relate to the ability of the rubber compound to resist damaging heat build-up. Higher speeds increase loads & squirm in the tread. Lots of info here at Tire Rack's site.

All tyre's treads roll-over and there is a potential for chunking with any treaded tyre. Proper "hot" air pressure and wheel width helps limit this. Older/harder tyres will chunk before newer softer tyres. Fully treaded DOT R tyres, properly mounted and with sufficient air should not. A brand new tyre with a wear index of say 400 might right out of the box. Since street tyres tend to have higher wear indexes, and can be years old you can expect them to eventually chuck. Especially if they still have most of their tread.

I’ve had a set of nice, but very, very old Pirellis chuck so bad during an auto-x that I thought I’d need to change them to drive home. 205/60/15s on 5.5” steelies on our ’71 Targa.

IMO it’s more a case of tyre age, air pressure and wheel fitment. If properly set up most street tyres will loose grip long before they chunk. My Pirelli’s only chunked after several runs with absolutely stupid slip angles!!

Gents,

Appreciate the feedback, but still looking for the roll-over definition. I think it means different things to different peoople. Some might consider the expended rubber left on a tire as roll-over, others use upper side-wall flex, and still others believe its inner tread movement. My problem is that those three descriptions are all very different definitions of a single term.

IMHO, "roll over" is simply defined by how much a tire rolls over the edge of the "designed" tread blocks, as per my pic in the post above. However, why its important to know & monitor this is based upon a variety of factors, including PSI, tread squirm (controlling heat), tread compound (new, old, street, R-comp, etc), speed ratings, etc. which affect a tires ability to work optimally.

Not sure how roll over means anything other than the side wall. Viewed from the front the wheel would not be parallel (offset) with the tread. At the extreme it would cause the tread to lift on the opposite side. Might want to ask who ever said the other what they meant.

Some nice graphics at http://www.toyojapan.com/tire-technology/tire-design/

Edit: clarified my post. Damn cell phones.

Can you help give some context to your request? I've personally don't use 'tire roll over' to mean 'chunking' as a term in my racing jargon, though that doesn't mean it doesn't get used. I would say it means something similar to what Tony mentioned above, but context to where you've heard it used will help.

At least, this is how I've used it.

Think of roll-over as the degree to which the edges of the tread become part of the contact patch - the bit that touches the road.

The pic that Tony posted below is a Toyo Proxes RA-1. Note the traingle marks molded into the edge of the tread [where Tony drew his red line]. Toyo put those there as a guide. If those guide marks are being rubbed, more tire pressure or less suspension flex is required. If the lowest rubed area is well above the guide marks, less tire pressure is required.

http://farm5.static.flickr.com/4135/...8c986508_o.jpg