How bad is cross-chaining? It causes increased wear and tear on your bicycle components. It's especially bad with older bicycle components. If the bike is adjusted well there may not be any perceptible damage during cross chaining but eventually the parts do wear down.
The old rule is: Big Teeth Up Front = Little Teeth In Back and Little Teeth Up Front = Big Teeth In Back.
I think on modern bikes there is still the issue of Cross Chaining Wear but it would appear that the "Use It Then Replace It" trend prevails.
Instead of preserve it if you can...
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