Trampolines from oztrampolines online are dangerous. Don't buy them.

Trampolines from oztrampolines online are dangerous. Don't buy them.

Here is my experience.

In March of 2014 bought a 7 x 10ft Rectangular Trampoline from Oztrampolines online.  It was delivered fairly promptly but when I got it here is a picture of what I got. 

Actually a 6ft x 9 ft trampoline with an extra rail 1 foot around the outside.  But this is just disappointing not dangerous.  The legs are complicated and not especially stable, but again not dangerous, just annoying.


From about 18 months after I got the trampoline, the springs started breaking.  They would snap and would fly off and hit the person on the trampoline causing at least bruising.  Thank goodness they haven't hit anyone in the head or eye yet.  This continued on and off since.

5 or 6 have broken this way.  I complained to oztrampolines online and they said that the trampoline had a five year warranty but the springs had only a two year warranty and they would sell me a new set of springs cheaply.  Presumably with a two year warranty.   The complaints went on for a while and they would say: did you install the springs the right way round?  Yes I had.  Did you have the guard pads installed?  Yes I did. 

Everyone seems to believe oztrampolines when they say that the spring guard pads should stop this happening but the physics is quite clear, the pads stop someone landing on the springs and hurting themselves, but they don't stop the springs flying off and hitting the person on the trampoline. 

I went to my government consumer group to dispute the fact that the trampoline I bought was not fit for purpose, but they wouldn't take it any further.  I tried the checkout but they weren't interested. 

A friend had an idea to put cable ties on the springs to stop them flying off.  We did that but they don't work as it has just happened again.

I am now throwing the trampoline out as I believe it is unsafe to use as a trampoline.

1. Who sells dodgy springs in trampolines that break?  I have owned trampolines that are decades old that no spring has ever broken.

2. These springs all break at a similar point, the frame side hook.

The physics:  When you are at the bottom of a trampoline jump, your feet or what ever part of you is contacting the mat are well below the level of the frame.  The mat applies something like 7G* of acceleration to you at the lowest point.  That is the time the springs have the most force on them, and are most likely to break.  It is also the time when the springs are below and not touching the guard pads.  If the springs break at the frame side or anywhere in the middle, the part that is on the mat side will be propelled in the direction of the force, ie towards the person on the mat.

http://iopscience.iop.org/article/10.1088/0031-9120/50/1/64/meta

You can clearly see in this picture how the guard mat does not cover the springs at the bottom of a jump.

*http://iopscience.iop.org/article/10.1088/0031-9120/50/1/64/meta

Here is a sketch of the forces at the bottom of a jump:

And the forces on a spring:

You can see that the guard pad does not cover the spring any more. If the spring breaks before the person reaches the bottom of a jump the force that caused the spring to break will launch the spring above the mat.  In the lower picture you can also see that the hook on the mat when it hits the mat side of the triangular clip will be launched upwards.

Australian speed limits rant

I read an article in the Canberra Times about using apps and GPS devices to "dodge" the law. Apps like Wikango. That particular article doesn't seem to be online. But here is a similar story

The police response:
An official statement says the Queensland Police Service ``does not support the use of any particular instrument, device or system that might assist motorists to avoid detection while committing traffic offences''.


Another similar article


I am astonished at the attitude of the writer and of Australian Police. It's like these things are helping people evade the law. How about: it's helping them abide by the law. Road law has become so complex that it's difficult often for locals and nearly impossible for non-locals to even know what the speed limit is any particular place. The speed limit on a road can change 4 or five times in the course of a couple of kilometres and often only one sign on the side of the road for each change. A sign that may be obscured by a parked truck, a moving truck, a bus. Speed limits can change depending on time of day or day of the week or school term. Anything that helps drivers know what the speed on a road is should be encouraged. I don't understand the Australian police attitude to radar detectors either. They should be encouraging them and putting out lots of radar to set them off. Similarly GPS devices with speed limit warnings.

Is their attitude that drivers should be doing all this other stuff and driving safely as well? Why not encourage anything that would help. Why not make or encourage GPS devices that would warn drivers when reaching or going over the speed limit? Even devices that could limit the cars speed? Is revenue raising more important than safety?

The attitude to GPS and other devices is ridiculous. Are they distracting? Is looking at a GPS distracting? Is looking at the dashboard rather than the road distracting? How else do you keep within the speed limit without looking constantly at the dashboard speedo and at the road signs and still keep watch on the road?

The article says: that to use the apps on a phone drivers will have to touch the phone while driving, which is illegal. You have to be joking? What about touching things on your car dashboard and instrument stalks? Is that illegal too? Is it illegal to touch a device on your dashboard that switches on your turn indicator while making a turn? Is it illegal to turn on your headlights by touching something on your car dashboard (possibly right near a bracket for a phone)?

I should add here some gripes about Canberra road speed rulage and signage.
1) My daughter used to go to a childcare centre in Canberra where every morning and afternoon toddlers were moving around outside the building on a 60km road. The ACT government would not make it a 40k zone. Most of the colleges in the ACT (senior high schools) with teenagers 16-18 years old have huge 40km zones despite the fact that the students are well old enough that they should be completely safe on roads, unlike toddlers.

2) I have noticed a number of places in the ACT where a local 50km road enters straight onto an 80km road with no 80km signs near the intersection. There are few speed signs on the 80km road so a non-local may not understand that cars on that 80km road may be travelling much faster than 50km. If you are going to have slow local roads connecting onto fast 80km roads you need to have a lot more signs. There seem to be a lot of memorials on these intersections so I assume people are dying there.

3)I have noticed in ACT and in other places like Adelaide there are roads that change from 60km to 50km with only one sign and little change in the look of the road. I notice that police often hang around the start of the 50km section to book people travelling too fast on the 50km road. This sort of thing is an almost guaranteed trap for non-locals. I think if this is not to be thought a revenue raising trap and the local councils are serious about people slowing down they need to either make the whole road 60km or 50km or sign such changes much more seriously.

I hear that people are talking about cutting the speed limit in suburbs in ACT to 40km on the general principle that speed kills. It is the case that in road accidents that speed kills. But it is not the case that going over the speed limit in particular kills. In fact most of the fatal accidents happen in the country, not in city suburbs and at much greater speeds than the speed limit and not 5km or 10km over the speed limit. What about the other options: traffic calming devices, shared road areas, pedestrian right of way areas. Why not create pedestrian right of way areas with special coloured roads or signs painted on the roads.

If schools are so precious why not put changeable speed bumps or movable traffic calming devices outside schools to use the laws of physics to enforce/encourage people to slow down and coloured roads outside schools rather than just setting speed limits and using punishment. In other words help drivers. And what about day care centres and other areas with lots of toddlers going in and out?

Edited: I was thinking about the physics of this and realised that:
Speed doesn't kill, acceleration kills.