Information regarding the width of seat the DW rail can accommodate is pretty clearly mentioned in the shop advert "
They are adjustable so allow varying width bucket seats to be mounted up to about 41.5cm's between bolt hole to bolt hole (The width of the base of the seat). It may be possible to fit even larger seats, but some 'massaging' of the frames may be required."
How low the seat can go depends on the seat, and most importantly where the mounting lugs are set into the seat. Every brand is different.
Regarding quality I'm extremely surprised to hear anything negative about this. Compared to the other options out there the quality of the DW rails is extremely high. If you have a problem with it rattling then it's either fitment error or there is a fault with the product, in which case you should return it to us so we can do something about it instead of just moaning on the internet
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