I don't, really.

Anyway, girlfriend went to a festival last weekend, they have 18 hours of rain the first day, about 12 hours of rain the second day, she ended up driving out over the equivalent of a large, saturated, ploughed field.

Car has now got a vibration in the steering wheel that is noticable above 70mph. I wondered if anyone could work out what is causing it:

So yeah, steering wheel rapidly judders left and right, and then doesn't, and then does, and then doesn't. It's governed by a low-frequency oscillation, a sine wave-ish pattern - the juddery movement fades in and fades out, over a wave that takes approx. 8 seconds to complete at 75mph. 4 seconds of shaking, 4 seconds of nothing.

So I suppose my question is: What car part, that has some effect on the feedback to the steering wheel, cycles at wavelengths that long (around 8 seconds) at 75mph? I can't imagine anything taking that long to cycle at that speed.

Thanks, nerdy anorak types.

Car type would help

You might need to get at least your front wheels balanced. The offroading into the field might have caused an already installed balancing weight to drop off a rim.
There could be something worse… eg a bent rim or faulty suspension bushings. But better start from the easiest scenario.

Something is busted. Duh.

edit: Fine, to answer your question: "none"
But you do have something(s) stuck somewhere(s) that hasn't come loose probably on both sides with masses that just happen to cause your 8 second issue.

Listen, she drove through mud - and mud (and/or it's associated components) has had opportunity to invade various locations, causing an oscillating imbalance. Same principle as having a large stone stuck in one's rim, but probably two different masses in different places on opposite sides.

Or, maybe something is busted. Duh.

Edit 2: WASH THINGS

Yup, wash it. It's pretty amazing what a little mud will do underneath a car. Take it from an American ex-4wheeler, you need to give the underneath and behind the wheels a good hard soaking with a hose. After that, you can try to figure out what is wrong. Do the cheapest first, wash it.

It could just be mud/dirt stuck in the wheels. Blast them with a hose. Or you have lost a balancing weight.

Similar thing happened when I spun off the race track at 100km/h into the muddy grass. Front right wheel filled up with mud and caused sever vibrations at speed, so I had to abort the run and return to the pits. Clean the wheel out with water and all was good after that.

Your car has Michael J. Foxitis.

Yeah I suppose we could clean the car but we never do that, it didn't occur to me.

Quote from thisnameistaken amidst other nonsense about some car something something broken car something :

a festival last weekend, they have 18 hours of rain the first day, about 12 hours of rain the second day

Ah, the world-famous British Festival. Did they get to appreciate varying levels of downpour as the poster says or was that just hype to get people from the League of Drizzle Enthusiasts to attend?

If you think that's bad, try doing what I do. Spend a few hours messing around in the snow, then go for a drive the next day only to realize that all the snow packed into the wheels/axles/etc... has become frozen blocks of weight, lol, couldn't even go above 50 mph once for a few days the vibration was so bad.

Check wheels and tyres (already mentioned) and anything else that spins. Driveshafts joints and bearings can create vibration easily, but should make some kind of noise. Power steering system could cause vibration, but it would be possible to feel at any speed. Anyway, you take your cars to repair shop to get light bulbs changed over there, so why are you even bothering?