Preheat the oven to 180 degrees C. Grease and flour a 20x30cm baking dish.
In a large bowl, stir together 1 1/4 cups sugar, bicarb soda, salt and 2 cups flour. Stir in the eggs and sour cream until smooth, then fold in the rhubarb. Pour into the prepared dish and spread evenly. In a smaller bowl, stir together the remaining 1 cup sugar and butter until smooth. Stir in 1/4 cup flour until the mixture is crumbly. Sprinkle the mixture on top of the cake then dust lightly with cinnamon.
Bake in the preheated oven until a toothpick inserted in the centre comes out clean, about 45 minutes.