i used to have the 598 racks aswel as the 516 versions, both were good and had no problems.
i have driven to morzine from southampton 7 times in the last 10 years which is a drive of about 1200miles, with several different types of bikes on the roof, downhill bike on the 598 (giant dh team with boxxers) and 2 mtbs either side most times.
it should be said that i was very heavy right footed and i had no issues at all up to and over 100mph for long periods.
i would however recommend some extra wheel straps for the racks though especially if you run big tyres as the standard straps weren't that long although this may have changed since i owned mine (sold them because i now have a van)
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