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Think I might swap the rear rim on my wheels, which are DT XM 1501 35mm inner width, Im guessing they are XM 521 rims (?)
What is the next strongest 30-35 internal DT rim that will swap straight over- EX 511 ?
Correct on both fronts.
Yes, XM1501 wheels use the XM series rims, in your case the 35mm wide run the XM521 rims.
Replacing with an EX511 isn't a bad idea, as long as you don't need the extra rim width. It'll be quite a bit stronger, and more use for "normal" width tyres (that is to say 2.3-2.5 rather than 2.6-2.8 that a 35mm rim is better for).
Yes I use 2.5 tyres generally, thanks will have a looky for some 511s, think they are in stock at CRC.
If you do want the extra width and to go up a notch strengthwise the HX 581 E-MTB rim is 35 mm wide and a very sturdy beast indeed.