Different results for support vector machine(SVM) using R

I came up with following issue when I try to extract the predicted probabilities using support vector machine (SVM). Usually the probability cutoff for a classification algorithm is 0.5. But I need to analysis how the accuracy changes with the probability cutoff for SVM machine learning algorithm. So it will only store the predicted class labels. To extract the predicted probabilities, I need to specify classProbs T inside the trainControl.