preventing trailing whitespace when using vim abbreviations

pb2q answer is exactly what you want in your current scenario, but does not fully answer the question presented in the title. This exact problem is addressed in the vim help file. See :helpgrep Eatchar. The example it gives is this:

You can even do more complicated things.  For example, to consume the space
typed after an abbreviation: >
   func Eatchar(pat)
      let c = nr2char(getchar(0))
      return (c =~ a:pat) ? '' : c
   endfunc
   iabbr <silent> if if ()<Left><C-R>=Eatchar('\s')<CR>

You would put the Eatchar function in your ~/.vimrc file and then use like so in your abbreviations:

iabbr <silent> diR1 C:/dirA/dira/dir1/<c-r>=Eatchar('\m\s\<bar>/')<cr>

This would "eat" any trailing white space character or a slash. Note that I used iabbr instead of just abbr, because it is rare to actually want abbreviations to expand in command line mode. You must be careful with abbreviations in command line mode as they will expand in unexpected places such as searches and input() commands.

For more information see:

:h abbreviations
:helpgrep Eatchar
:h :helpgrep

Instead of abbreviations, you could use mappings. They're expanded as soon as you have typed the last character of the mapping, so there won't be a trailing space:

:inoremap diR1 c:/dirA/dira/dir1

The downside for this approach is that the letters you type while a mapping could be expanded are not displayed until the mapping is finished. This takes some using used to.

This is possible, without more customization than just abbrev.

The abbreviation is being triggered by the space character, as you know. The space is a non-keyword character, and remains after the abbreviation is expanded.

But there are other ways to trigger the expansion, such as other non-keyword characters, including /. So if you instead define your abbreviations like this:

:ab diR1 C:/dirA/dira/dir1

That is, without the trailing path separator, then you can type diR1/, have the abbreviation expand for you because of the slash /, and continue typing, appending to your path with a file name.

Alternately, you can force abbreviation expansion using Ctrl-]. That is, type the abbreviation: diR1, with no following space or other non-keyword character, and then type Ctrl-]. The abbreviation will be expanded and you'll remain in insert mode, and can append your file name to the expanded path.

Check out :help abbreviations, there may be something else useful for you there, including more complicated constructions for always consuming e.g. the space character that triggered the abbreviation.