Rule 344(f)(2), R.C.P. Only in exceptional cases may questions of negligence, contributory negligence or proximate cause be decided as matters of law. Rule 344(f)(10), R.C.P. See Kuhn v. Tank, 261 Iowa 800, 156 N.W.2d 127, 128. We have reviewed the record and conclude it establishes trial court was justified in finding substantial evidence supported plaintiff's claim that defendants were negligent in the operation of the truck-mounted cornsheller.

I. The trial court's findings of fact are binding if they are supported by substantial evidence, and it is only when the findings are undisputed or no conflicting inferences may be drawn that the Supreme Court may interfere. Kuhn v. Tank, Iowa, 156 N.W.2d 127, decided February 6, 1968, and citations; Rule 344(f) (1), R.C.P.; Werthman v. Catholic Order of Foresters, 257 Iowa 483, 491, 133 N.W.2d 104; France v. Benter, 256 Iowa 534, 536, 128 N.W.2d 268. Moreover, the finding of a trial judge in a jury-waived case has the effect of a special verdict, and the function of this court is not to weigh the evidence but to decide whether the trial court's finding is supported by substantial evidence.

II. Plaintiff-appellant asserts the trial court erred in finding him guilty of negligence which was a proximate cause of the accident. Defendant as an affirmative defense so alleged. That he was required to so plead, see Schultz v. Gosselink, 260 Iowa 115, 148 N.W.2d 434; Kuhn v. Tank, 261 Iowa 800, 156 N.W.2d 127, 129; section 619.17, Code 1966. III. The trial court's findings of fact in this law action are binding upon us if supported by substantial evidence.