our marmon herrington gun tractor had this same problem when we first put it on the road. the entire front axle had been overhauled and the wheels balanced. when it got around 20mph it started shaking so violently it was dangerous and uncontrolable. we fitted castor wedges to the front which made a big difference. it now only comes in mildly when you hit a bump around 50mph.
robin mawson had similar problems with his white scout car. he played around with his for quite some time. tried a number of things and still had problems. eventually he tried ajusting the toe in more than what it said to in the book. no shimmy at all from zero to flat out.
we are planning to try the same on the marmon when we get time. this obviously increases tyre ware but for the amount we drive these old trucks waring out tyres is highly unlikely.
give it a try?
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