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I recently bought a 2007 victory vegas. I discovered that the rear tire is a radial 180/55 ZR 18 while the front is a 90/90 21 bias. I have been looking all over the internet for advise. Some say it is OK, some say NO. Do Victory's come this way ??
 

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I don't think they come mixed that is not advised to do but A raidal on the rear will work. but never the other way around. The stock rear tire Is a Dunlop D417 bias ply Victory dealer only tire. Mixing tries may make the bike handel strange.
 

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Radial Ply Tire vs Bias Ply Tire

Mixing is no bueno. Grip levels change at different rates between the two at various speeds, which changes the balance of your bike as it changes speed. But that's not to say you cannot ride it. Cowboys have been riding wild bulls and survive. ;)

What's a radial tire, anyway?
Tires have plies inside, which are reinforcements that hold up the tire's body in its manufactured shape and resist the tensile forces applied to it. These plies (think of them as trusses on a bridge or roof, or as steel reinforcement inside a concrete wall) can be designed to run at 60° to the path of travel (bias ply tires) or at 90° to the path of travel (radial ply tire).

The tire ends up having different elasticity characteristics if it's a bias ply or a radial ply tire. The amount of grip also changes at different speeds from one tire type to the other. For example, at 15mph the radial might have less grip than the ply tire, but you won't feel that difference since you're going too slow. At around 40mph, the two tires might end up being a perfect match when it comes to grip. Then you take it to 70mph, and all of the sudden the radial has a lot more grip than the ply tire. This creates the imbalance necessary for a speed wobble to show its ugly head.

P.S.: If you were looking for a longer explanation, there are already a few threads on the forum hammering this very topic. Here are a few to get you started:
http://www.victoryforums.com/showthread.php?t=1121
http://www.victoryforums.com/archive/index.php/t-6913.html
 
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