Back from the dead.
I was recently looking for the answer to this same question except no one could tell me why.. Then I found someone..who I should've asked in the first place.
"There is a Life and Death Reason! The Tires are made from 1 strip of Rubber wrapped around a Mould. At the Joining section, they GLUE the ends together at a 45 Degree angle.
Think about getting on the Gas with the rear tire and that seam edge AGAINST the direction of Rotation. Eventually you can Split the Seam and tear the tire apart.
The Front tire does Braking which applies force 180 degrees Different from the rear. And under Braking you can force the Seam to come apart and destroy the tire and maybe your Behind. Understand now?"
Thought this might be of interest to someone else as well.
This is half true. The part that's not true is where "the rubber is wrapped around a mold." It's not done that way at all, the rubber is fully 100% injection molded.
The internal plies (in many tires) however are wrapped inside the first injection molded inner rubber portion. The way they are wrapped, they overlap in one direction. Because these plies overlap, serious acceleration in one direction makes the plies tighter, while running it backwards makes the plies loose, which can start the outer rubber delaminating from tension stresses.
I know this because I talked to a Dunlop engineer and asked why everyone's race tires are delaminating. It's because racers are flipping their tires after a couple races. One side wears out depending on track direction and the tire is flipped to get more life out of it. Many tires can't handle this, especially high hp supersports. You end up with something like this:
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If it gets bad enough, chunks start coming out and then you have catastrophic deflation at high speed. This is only a problem with some tires, mostly Dunlop Q3's and older Ntec race tires. Pirellis are designed with interlocking plies that don't come apart if run backwards. New Dunlop GPA Pro race tires are bi-directional with a new internal ply design. It's a design change Dunlop had to make because racers need to flip their tires. People were running more expensive Pirellis because they could be flipped and therefore lasted longer. At one point my race tire budget was $3K a year, and if I can't flip a tire to save money, well F that tire.
Now on a bike like the Z125, this type of delamination will never happen. There's not even close to enough power. The tire rotation direction is determined by tread siping for water ejection. And it differs depending on the primary force a tire sees - on the front it's braking and on the rear it's acceleration.
Racing rain tires - run those backwards on a wet track and you are going down HARD the moment you try to brake for turn 1. The siping HAS to go a specific direction, and the front is opposite from the rear because of the primary forces each tire sees. (unless it's a bi-directional thread, there are some of those - but normally fronts only.)
You better run these the right direction in the rain....
Sorry, I have some race history behind me and a lot of what I talk about comes from that perspective.
