vibration at 75 to 80 mph
check that the wheel-nuts are tightened to specified torque
Wheel Tech - Wheel Lug Torquing
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not sayin' that IS the cause .. but it could be
i.e. assuming tyres are in good shape to begin with
( i had steering shimmy at the same speed range .. a few weeks after fitting new tyres
( checking the wheel nuts was the solution
best to avoid those speeds .. if you can .. 'till you find the cause
because the vibration will just advance wear on the bearings
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am guessing that IF a wheel balance weight had fallen off ..
then a vibration would set in at a lower speed ..
but .. check that out as well
there be other things on the hippo that would cause bad vibes ..
but going by problems posted on this site over time ..
they be usualy felt at lower speeds than 75/80 mph
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if it ain't wheel vibe yer on about .. then discount the above