請問”where the rubber hits the road” 點解同點用？

其實這是一句美式常用idiom,有時亦叫where the rubber meets the road.Meaning:Where the rubber hits/meets the road is the most important point for something,the moment of truth.例句:An athlete can train all day,but thr race is where the rubber hits the road and they'll know how good they really are.一個運動員可以整天不停地進行訓練,但只有比賽才是最重要和最真實的,參加比賽才可體會到當中的樂處.Where the rubber hits the road.解作:最重要,最實在的事情.

In this sentence, rubber = tyre, because a tyre is made by rubber
So this sentence is actually asking you :
Where the tyre hits the road
I believe this is not a single sentence by itself. May be it is coming out from a paragraph saying that : The car broke down because the surface where the rubber hits the road is very thin due to long time driving.
Hope this can answer your question.