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 From: aero.spike@mail.com 
  
 Simon Parker  wrote: 
 > On 03/11/2025 07:59, Spike wrote: 
 >> Simon Parker  wrote: 
 >>> On 01/11/2025 10:13, Spike wrote: 
 >>>> Jeff Layman  wrote: 
 >>>>> On 31/10/2025 10:50, Simon Parker wrote: 
  
 >>>>>> I agree with your thoughts on gravel (run off areas on tracks are 
 larger 
 >>>>>> than one might think and are in addition to a safety barrier, not 
 >>>>>> instead of it). 
 >>>>>> 
 >>>>>> From memory you mentioned "a largish pond".  I do not know your 
 >>>>>> definition of "largish" but I do not consider 7.5metres to be 
 "largish" 
 >>>>>> as ponds go - certainly not around here. :-) 
 >>>> 
 >>>>> Nor do I, but we had no idea of how big the front garden the OP 
 referred 
 >>>>> to was, other than it was a lawn and pub beer garden. My thought was 
 >>>>> that if it was too small it wouldn't be effective as it could be missed 
 >>>>> by the car. I think you're right with around 7.5m, but I would have 
 >>>>> thought that 2m is too deep. Unless you had an overflow drain at 1m 
 >>>>> depth, rain would eventually fill it to 2m, and there's a possibility 
 >>>>> that, as you infer, the car occupants might drown if they were 
 >>>>> unconscious at that depth. Might that result in some sort of 
 >>>>> contributory negligence claim if the depth wasn't limited to 1m? 
 >>>>>> I am more than happy to agree with you on your proposed, seemingly 
 >>>>>> legal, solution. 
 >>>> 
 >>>>> Yes, but this would be in a pub garden. The pond would have to be 
 >>>>> properly fenced to avoid the hazard of children or inebriated customers 
 >>>>> falling in, or I can see your litigation colleagues having quite a few 
 >>>>> lunches there in case something "interesting" happened while they were 
 >>>>> eating. 
 >>>> 
 >>>> It’s interesting to crunch some numbers on the suggested size and 
 depth of 
 >>>> the lake that’s intended to restrain a speeding vehicle, namely 7m 
 diameter 
 >>>> and 2m deep. 
 >>>> 
 >>>> 100mph = ~44m/s. At this speed it will take ~0.16s to travel across the 
 >>>> lake, and due to gravity will lose ~0.12m in height as it does so. 
 >>>> 
 >>>> On this basis the wheels will likely hit the far bank of the lake, the 
 >>>> water it contains having almost no function in retarding the vehicle’s 
 >>>> speed, and in these circumstances the lake as suggested is merely a 
 >>>> landscaping feature. 
 >>>> 
 >>>> However, use can be made of the fact that acceleration under gravity is 
 a 
 >>>> squared function of the time the vehicle is ’in the air’. 
 >>>> 
 >>>> If the lake was 15m across, things are a little better in that the 
 vehicle 
 >>>> will drop about 0.6m, possibly not enough to get a significant 
 proportion 
 >>>> of the front area of the vehicle into the water to perform the desired 
 >>>> retardation. 
 >>>> 
 >>>> At 22m diameter, the drop would be ~1.25m, very likely to put a 
 significant 
 >>>> proportion of the front height of the vehicle into the body of the water 
 >>>> and so bringing it to a halt. The lake would need to be slightly deeper 
 >>>> than the drop, to reduce the risk of the wheels supporting the vehicle 
 and 
 >>>> so ensuring maximum contact with the water. 
 >>>> 
 >>>> Disclaimer: the above calculations assume that g = 10m/s^2 and that 
 there 
 >>>> are no other factors operating to slow the vehicle’s progress. 
 Accurate 
 >>>> calculations would need to be made for any real-life situation which 
 would 
 >>>> need to take into account relevant factors other than the physics in a 
 >>>> multi-disciplinary approach to finding a suitable solution. 
 >>>> 
 >>>> IANAP 
 >>> 
 >>> As I've just said to Jeff Layman, my dimensions are for a wedge shaped 
 >>> structure, 7.5 metres in length, 2 metres deep at the far end, filled to 
 >>> a depth of 1 metre.  (Please note my use of the words "sloped ditch" in 
 >>> my previous post, which many people seem to have missed.) 
 >>> 
 >>> Regards 
 >>> 
 >>> S.P. 
 >> 
 >> The problem with your wedge design is that its slope is greater (steeper) 
 >> than the ballistic trajectory of the vehicle once it leaves the 
 horizontal. 
 >> It’s a variation of the brow-of-a-bridge phenomenon where a vehicle 
 arrives 
 >> fast enough to leave the ground - it’s not that the vehicle is 
 ‘launched’, 
 >> it is merely the case that the surface, on the other side of the brow, 
 >> falls away more steeply than gravity can accelerate the vehicle in the 
 >> vertical direction. 
 >> 
 >> The calculations made above show that at 100mph (44m/s) the car will fall 
 >> only 12cm by the time it crashes into the 1m bank at the far side, some 
  
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