Van steering shimmy

[Screwdriver]

So I am borrowing a van and it had a serious shimmy. The steering wheel got the shakes at around 55-60mph and you really had to hang onto it. Had the two front wheels balanced and pressures set, went for a test drive.

It is still there, a lot less noticeable but at a set speed (55) it wobbles 5-10mm in each direction and you can hold it loosely but it threatens to go out of control. The weird thing is, it does that for 3.5-4 seconds, stops shimmying, then does it again. The shimmy is periodic at 7 seconds/55mph.

It is possible that the period reduces at high revs, I ran out of motorway before I could fully test the theory but it seemed to go down to 3 seconds in 4th while running at 55mph. I guess that must be power steering related if it changes with rpm. How could a faulty pump (?) come and go though?

It is possible the steering is worn, the van is unloved. I have yet to check the track rod ends or bearings but could a mechanical fault like that cause a periodic shimmy that comes and goes??

Got a big drive tomorrow (hence the van) and if anyone can think of something I could try, let me know!

S.

[Skub]

Tyres themselves at fault? Is it worth swopping the front and rear if possible to check?

[Davie lad]

As above have a looky at the tyres to see if one or both are out of shape, swapping front and rear can sometimes sort this out..

[Screwdriver]

How could the wear pattern on a tyre cause an intermittent/periodic shimmy?

Thumbnail calculation suggests tyre is spinning 12 times per second at 55mph so I don't see how a perfectly timed 3.5 second shimmy could develop with such regularity. Maybe I am overthinking it but my guess would be something in the power steering rather than a random but periodic vibration.

<edit> by the way, when the first tyre was taken off it had a fair amount of water in it (from a poorly drained compressor?) which I really thought was going to be the culprit. I could see how a liquid weight might become periodic or set up a resonance. Wasn't that though sadly.

[Mussels]

Try changing the tyre pressure, softer tyres may dampen the effect.

[Screwdriver]

Tyre pressures were way low. 35 psi on a large Vivaro. They are now 55.

Ignoring the periodic nature which could be some sort of diversionary tactic, I am going to assume it must be poor tracking and worn suspension (track rods, ball joints or wheelbearings). None of which I can fix before a big trip tomorrow.

It is fairly mild and I think I can overspeed beyond the worst of it, it's just that I like to hypermile to save fuel as much as possible. Might just have to bite the bullet and drive at whatever speed it's comfortable at.

Thanks for all the replies. I'll fix it when I get back.

[Taff]

Liquid in a tyre helps to improve the balance.

[Mr. Dazzle]

My car used to do this to. It did it once while I was giving a McLaren suspension engineer a lift - his answer was simple and unequivocal. "Somethings worn and loose, change the rubber bits".

Which rubber bits is the question though innit? Sadly it's generally just a case of changing 'em one by one until the problem goes away

[porter_jamie]

Probably shit tyres. What make are they and what's on the back?
Any the person you borrowed it off? They were just living with it?