The pinch bolt was incorrectly tightened and/or not installed properly - I've had exactly the same thing on two cars, and on mine I had total failure like you.

If a shouldered bolt is used and the shoulder is too long, it can be tightened as tight as you want but the clamp will not close the splines together as the nut will bottom out.

You also need to ensure that there is still a gap in the clamp, even when done up tight.  If the two sides meet and aren't clamping, you have no way of checking that it is working properly.

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Thank you guys for the helpful suggestions.  Here's the triage.  I don't understand how steering column splines can vanish like this since these pieces aren't supposed to move against each other.  The pinch bolt was on solidly.  

It appears there was movement between the two - though I never had a hint of steering issues or wheel movement before.  Interesting that Miguel was parking, I was going really slow too.

Mike Kimball just told me now that he had two cars with this, and it was due to the outer spline slot not being big enough.  So cranking on the pinch bolt was no longer tightening it.  Exactly as Dr. Mini says, same fix too with the angle grinder.

Again, day before yesterday I was blasting through the trees for a good few hours, getting sideways etc.  Well at least I would've died happy...

(Edit) more from Mike:

what's shown in yer pics is that the slit pretty much ends about at the end of the shiny wear area which means that the tip of the rack spud prevents the rest of the column from (effectively) clamping down.
- the "tube" can't crush down around the rack stub at all evenly - it's prevented by the small area where there is effectively no slot at the end of the stub. It kinda makes the whole assembly like one of those rubber cone-shaped jar-opener aid things on late-night as-seen-onTV channels. Only not as effective.

you only need to extend the slit about 3/16-1/8" beyond where the spud will end when it's all mated up. I have a couple of racks and columns in the garage, I'll measure how deep the hole needs to be and how long the slot needs to be a touch later and get back to everyone...

My steering rack retired last year after 49 years of faithfuol service, as I was parallel parking to go to a pub.

However it did gave me some warning it was going to fail just before it did. Altough it's possible for the steering rack to fail at high speeds, most times these components fail under low speed manouvers due to the incresed stress. (ie steering feeling lighter at high speeds).  

Changing the rack wasn't a fun job.

Miguel

www.MinipassionMini.com
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A little OT but not much. I installed a new rack this week. While having the power unit and subframe out I checked the rack and found free play in the off side link. Ordered a new rack the OE rack had a groovre machined around the pinion. The new rack had only a notch for the pinch bolt to locate. While lining up and centering I noticed the notch was in the wrong place to center the wheel. I took it apart and rotated the pinion one tooth and assembled. I like the idea, if the splines strip or wear the bolt will hold position.

Had our mini out this week. Ours is 15 year old son's car which we drove every day when I dropped and picked him up from school each day. In spite of the light rain followed buy a down pour people were smiling and waving and that can't help but make me feel better. Steve (CTR)

Be careful with that thing, glad no real harm done.  

or did the pinion break off?

Glade you're ok. And the car is for the most part. Man, that could have gotten ugly quick ! 

Check there is still a slot where the pinch bolt goes through. If not, run an angle grinder disc into it.
If the spline in the end of the column is totally gone, you can buy a new splined end for it from minisport.com.au in Australia to weld on. Not cheap, but they are available.

[edit] p/no 21A3099.

Fantastic that you and your family are OK!

Very scary - ideally automotive components shouldn't fail without giving some warning, but behavior after +-50 years or wear probably wasn't taken into consideration.

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Edit:  pull your steering column shaft and check the splines inside where the shaft meets the steering rack.  Several people have crashed when these splines go away due to previous misalignment and age.  No matter how well you aligned your rack, somebody else may have screwed it up before and your splines are toast.

My car provides entertainment variety in its disasters.  Not one week after I put in Jemal's new engine, the steering rack croaked quite suddenly today.  This is after staring at an empty engine bay and exposed, accessible rack for 3 months.

With uncanny fortune I was going maybe 25 U-turning around a roundabout and the hard required pulling did it.   This was to pick up my 15 year-old, and perhaps missing a turn and having to u-turn saved us both.  And to think I spent half the day yesterday wringing out the new motor on single-lane mountain roads whizzing through the trees at 70.

The thing I don't like is it went away instantly.  No warning at all, just clicking from the rack and you could practically spin the wheel around with one finger.  If I was going fast it would've been bad.  Add two or three seconds for your brain to process what is going on, let alone pushing the brake pedal.

This is a similar comic immobility experience to the one last year in a busy downtown left-turn lane with my wife.  The PO's chinese knockoff KAD quick shifter broke at the base and at the green light I just held the shifter in the air and thought what the hell do I do now, and why is my wife face-planting.

Helpful people like Hunter have asked if I did a column drop without adjusting the u-bolts, and the answer is no, but I adjusted the u-bolts last year by pushing the column carefully up and down and twisting the rack to ensure they were both in the sweet spot.  I think this is just wear.  The pinch bolt was tight, I haven't been out to the garage yet until this &^%@# car sends me flowers and chocolate.