I agree with Brad..people have a natural reluctance to cross a physical barrier..it doesn't even need to be particularly high to do the job. You could go a little further and put a 'DO Not Cross' tag on the cordon.
My fear is that 'little Jimmy' would get crushed by my M20 should it go over and then his irresponsible parents would hold me liable.
Kids are best kept on a short leash in public places and their parents should be held responsible for their actions..
...the little darlings...Adults should have more respect for things that don't belong to them
...Ian
email (option): julie@wright52.plus.com
Rope ? Wot, No Dannert wire ? 
How about a wee sign like this.....................
Or a big gun like the one in Henks forum picture!!!!!!!!!!!
A friend of mine used to put a fake plastic dog turd on the seat of his Welbike to keep the kids off at shows.
email (option): robmiller11@yahoo.co.uk
Nice Cent, but I wasn't expecting a happy ending there.
A horrible child tightened the steering damper right down on my M20 at a show once, it was almost unrideable but I managed 8 miles with lots of stops before I worked out what the problem was.
email (option): robmiller11@yahoo.co.uk
what worries me is going to a bike jumble on my m20 having a look round the stalls and then finding that thing have been stolen off it an nobody has seen a thing less one steering damper knob and rear number plate plus light which i didnt notice till i went to use it a few days later
Maybe it's simpler to just take a bike cover with you and hook it down all around (something in khaki, of course). Out of sight, out of mind, etc. I know people like to show off their bikes, but you can do that later when you've finished looking around and have decided to stop for a bite (or something) where you can see your wheels.
If a tarpaulin or cover just doesn't do it for you, maybe you need a fairly fine (but strong) net that allows people to see the bike, but makes it hard to actually remove components. A net does provide a minor physical/psychological barrier.
Ultimately, of course, you can't stop these things unless you sit on the bike. At shows, I try to park my bike where it has multiple lines of sight. Doesn't always work, of course. But if I'm not happy leaving it, I just don't leave it.
Maybe it's time to boycott shows that fail to take security seriously. Modern CCTV is cheap. There's little excuse for not having it. I've got a £15 key fob camera that will record for one and a half hours. It's easy to hide on the bike (or a friend's neighbouring bike) and might at least show you who nicked your damper knob or whatever.
email (option): dannydefazio@sumpmagazine.com
Google motion detector or motion sensor alarms.
Here's a link.
http://www.amazon.co.uk/BML-Motion-sensor-alarm/dp/B000TR9OT2
It wouldn't be hard to rig one of these on your bike (in a dummy Vokes filter maybe, or on a bracket/box attached to the seat or something). Leave it pointing upwards. A little experimentation with height and angle will probably sort something out that will keep most people away. You can make it fairly discreet perhaps. They cost less than a tenner these days.
Also, a little note warning that the bike is alarmed and being watched by CCTV will perhaps keep some of the more serious thieves away. Might be worth considering.
Footnote: If you're handy with electronics, you could probably rig up a motion sensor using your bike's battery and horn. This would have the advantage of no-delay. The factory-produced motion alarms usually give a ten second delay for (remote key fob) deactivation. A home-built system would allow you to set the sensor pointing up (110 degree monitoring angle), and still allow you space to approach is fairly low/cautiously and flip your hidden off switch. I'm assuming that these sensors will still work in a "tunnel" whereby you can control the monitoring angle (so that they monitor an area directly over the bike or jeep or whatever). Needs thinking through. But you don't really want a ten second delay, or any delay. Remember to wire in a timed "alarm cut" switch. Talk to Maplin.
email (option): dannydefazio@sumpmagazine.com
I bought a new Harris Matchless back in 89 and wanted something simple as an alarm (not integrated into the bikes electrics) I bought an alarm, about the size of a matchbox or slightly bigger which just attached to the bike with an adhesive strip. It worked without a delay and only had an on/off switch and nothing else externally. A couple of small batteries, a (sensitive) movement sensor and an alarm (loud) were the only working parts. Effective, cheap and easy to tuck away on the bike (I fixed it under the tank). If my memory serves me correctly I got it from RS Components...the wholesale electrical supplier.
No doubt there are numerous versions....Ian
email (option): julie@wright52.plus.com
Hi Ian, that can work too, I suppose. My worry with a movement sensor is the issue of sensitivity. Too sensitive, and the wind sets it off. Too little sensitivity, and people are unscrewing your float chamber top or damper knob. I think movement alarms work best to stop theft of the vehicle (where more substantial movement is involved). Maybe a combination of the two is needed. Trouble is, we all pay too little attention to security - until it's too late. It's human nature. Did you regret buying the Harris Matchless?
email (option): dannydefazio@sumpmagazine.com
In Normandy, if I want to visit a museum or patisserie, I just leave Ian on guard. He's happy to sit there as long as he's equiped with his Golden Virginia roll ups. Ron
email (option): ronpier@talk21.com
Hi Danny..No I didn't regret buying the Matchless.There were a few issues which you might expect from a small volume producer but mainly detail stuff. I didn't approach the bike expecting an old style single as I was quite familiar with the Rotax 504 engine fitted as standard.An excellent engine and superior to Japanese singles in a number of areas. Forks were Paoli as fitted to the Harris Bonneville, front disc Grimeca with Brembo caliper. Base model parts but quite serviceable. Switch gear was also trouble free.
The speedo drive was the cheap and nasty Veglia type (also fitted to the Bonneville) and subject to a short life span.
There were problems with cracking of the airbox..a common fault. That was easily cured by binning it and fitting a more compact K&N filter.
A Dellorto carb was also standard and some work was required here to achieve easy starting and a reliable tickover. The latter could be a problem with the light flywheels.
I personally thought Harris got his marketing wrong. People who wanted a modern bike bought one and few of the 'rose tinted spectacle' brigade were up for buying any new bike, whatever was on the tank. 'It's not a proper Matchless'..blah...blah..a refrain currently heard (substitute Triumph here)in some quarters when they talk about Hinkley Triumphs.
I fitted wider alloy rims to mine with track/road compound Avons, a larger carb, flat track cam and flat track piston. Also some better spec shock absorbers. That bumped power from the standard 37BHP to just over 50BHP and in such a light bike with good tyres and suspension that made for a 'fun package'...too much fun as it happened, as I picked up two speeding fines in nine months. So I concluded it was either the bike or my license that was going.
I think if Harris had marketed the bike with more power, the optional electric start as standard, a higher level of equipment spec., more sports oriented styling and called it the G50 instead of the G80 he would have attracted 'modern' riders...the people who actually buy new bikes. I know some of the people riding Japanese singles I blew away were highly impressed with the bike. I bought the last one he sold of approx. 250 examples and ran it for about 2 years in total...Ian
email (option): julie@wright52.plus.com