I have a configurable product with a few associated simple products and have to set a weight for configurable so i go to associated products tab and manual set weight, save and ... after reload its still empty. Simple products have weight but cant set to configurable. Any idea why ?
You don't need the
weight field for a configurable product as it technically doesn't exist (Only it's child products). The weight field also isn't available to configurable products by default so it should not be available to be filled in.
As far as the Associated Products tab goes, it sounds like you're inserting the weight into the Quick Product Creation and then saving the configurable product which then reloads the page and removes this value, as this is used for quickly create a new associated product instead of editing the configurable product.
Matt Antley's answer is technically correct in that weight is redundant for configurable products in a Magento website, but it doesn't really answer the question.
There are scenarios where a weight may be useful for a configurable product, for example, when linking to third party marketplaces like Amazon, so to answer more fully, yes it is possible to add the weight field to a configurable product.
Simply change the settings of the weight attribute via Catalog > Attributes > Manage Attributes > choose 'Weight' attribute > Attribute Properties > Apply To then add Configurable Product to the product types or change from 'Selected Product Types' to 'All Product Types'.