A protective sleeve can be inserted through tubing and latched in a manner that resists rotation. The process for producing 3-methylthiopropanal, includes reacting acrolein with methylmercaptan in the presence of a triallylamine compound represented by the formula :wherein each of R1 to R3 represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. The vehicle may include a rear radius arm suspension.