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Wall of Death Revisited
#1
Long term Forum addicts will recall a few years ago an interesting series of posts about Wall of Death. It was prompted by a Seven based car on wire wheels, despite the 1G side force! (Someone provided a clip of a prewar show with a lion in the sidecar!) 
I presume it has been on UK TV but was here a recent programme where Guy Martin set out to break the speed record. Some science git explained how strong centrifugal force is, but most intuitively realise you can stay up at speed. The greater puzzle is the lowest speed, not explained. (At the outset Guy did not have a grasp of!) Anyway, from memory , a 40m dia circle; he attained 70 mph, 7G on the brink of blackout. (seems to calculate at somewhat less at body height, but the undulations would have increased)
There are  some impressive recent shows on  Youtube, including one which still uses a leaf spring Indian circa 1932
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#2
There were several Austin Sevens used in Wall of Death acts; speed seems to have been under 30mph, which was still pretty impressive when only inches away!

This photo is of Frank Todd taken at a Rochdale display:

   

Anyone interested may like to investigate Ann Wright's book, available via Janus Publishing:

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#3
    .. slightly off beam but one of my favourite images.
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#4
It is interesting to think about the lowest speed that would allow a rider to stay on the wall without slipping off.  The force arising from going round presses the tyres onto the wall and this means that the tyres can resist the downwards force gravity on the bike and rider.  At some minimum speed this friction becomes smaller than the weight of the bike and rider so the bike will tend to slide off.  

The minimum speed required to stay on the wall depends on the wall diameter.  For a wall with a diameter of 10m (30ft) the minimum speed is about 20mph and for a wall of 37.5 metres, as Guy used in the record attempt, it is about 40mph.  This is calculated using a coefficient of friction between hard rubber and wood of 0.7. 

As the friction force that resists gravity acts where the tyre meets the wall, and the centre of gravity of the bike and rider is to one side of this, there is a tendency for the rider to be tipped off the bike.  This is resisted by the rider leaning the bike a bit past the horizontal, as illustrated in the attached diagram.


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#5
Bobs initial comment suggests the minimum speed holds some mystery, I haven't seen the program so not sure of the basis of this. One factor possibly overlooked is, it is the diameter (and speed) of the center of mass of the vehicle that needs to be used rather than the diameter of the track!
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#6
There is no great mystery; it is just that the more obvious question was not explained in the Guy Martin programme.
From  CM  post the vehicle must have its own weight equivalent pushing skyward on the tyres. To enable grip for 1G needs about 1 1/2 G laterally at least. (Much as GP cars with 2G aero assisted downforce can corner way beyond the normal absolute limit of about 1G )

It is surprising how little the bikes lean upward, Note that all vehicles slope up at the front and are detectably steering up to counter the strong drift down. 

Hopefully the lion never got  bored or otherwise fed up. If it had a nervous wee on the boards, reducing the friction, the result could be intereting.

The probably Indian motor bike with leaf front spring and tank under the frame would ne 1930ish.

(I am still pondering what stock Seven roll induced oversteer would do here!)
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#7
Demon Drome here in the UK use a Seven special (called Talula) in their show. I've seen it a couple of times at various events. It's a proper show. The old Indians are great too.

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#8
Hi Bob,
The skyward force needs to resist the weight.  Because the coefficient of friction between the bike wheel and the wall is less than one, the downwards force needs to be higher than the weightforce of 1G.  If the coefficient of friction is 0.7 - a reasonable figure between wood and hard rubber - then slip won't occur for a downforce of the weight divided by 0.7 - as you say, about 1.5G.  

Yes, if the track were wet and slippery, then a significantly higher downforce would be needed - insufficient speed would mean instant disaster.  This is yet another hazard from having a lion as a passenger - as if any more were needed?  That picture is priceless.

Regards,
Colin
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#9
HI All,
Back in the sixtes when NZ speedway Riders ruled the World, Ronny Moore used his JAP Speedway Bike on His fathers wall Of death on shows All around NZ this was on the small Dia wall and the wall moved as the Bike went a round.Health And Safety would have a field day 
Colin 
NZ
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#10
In Beijing, I once saw riders performing inside a sphere. Sort of an added complication. At the height of the show there were seven bikes going at the same time. If I can find a picture I will post it.
Alan Fairless
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