You might drive like a saint, but can you guarantee that the person you are buying your part worn tyres from, have not hit every kerb and been speed testing on the local car park gravel? How do you know if you will be safe? How do you know what you are getting, will the part worn tyre be safe at high speed or in the wet?Here are some tried and tested guidelines to help you make a decision. Your safety and that of your passengers and other road users is the most important thing for you to consider.Firstly, you must consider why you are thinking of buying used tyres. Have you heard of aquaplaning? Well it’s all physics in the end, tyres are your only contact with the road, and you are not Fred Flintstone! Without good grip, your brakes and your steering will be seriously compromised and at a particular speed in the wet you could lose control.The more tread you have the better. The patterns are designed to clear water from the road surface so that the tyre can make contact with the road. The deeper the tread, the more water can be cleared from the road, meaning the less likely you are to find yourself aquaplaning.Aquaplaning happens when a level of water builds up between the road surface and the tyre and causes your brakes and steering to stop working. If this happens, ABS (anti brake system) and electronics will be of no help to you. Gravity and cause and effect, i.e. force and friction will have taken over. It really is down to science! Have you ever seen a skipping stone? Well, the effect is very similar.You will find with a part worn tyre, that the tread will not be as deep as on a new tyre, meaning much less grip in nasty wet conditions. The legal limit is in fact very low, a few millimetres. However this should be seen as an absolute minimum rather than the point at which a tyre should be changed. All tyres have wear indicators that are little raised areas between the tread. If you can see them, then the tyre manufacturer is trying to tell you they are past their safety level, steer clear of them.Sometimes, tyres that have been damaged will actually literally come apart. What happens is the tread parts company with the carcass of the tyre. The results can be devastating. Ask BBC Top Gear star Richard Hammond what happened when the tyre laminates blew up! It caused him to crash that jet-powered car. The accident almost killed him. Have you ever seen a truck tyre come apart? Not very nice!A tyre should be changed every 20k to 25k miles and definitely every couple of years. Remember every time you push the mileage beyond the limits you are endangering the public and yourself. Do not be so tight with your budget!A major component of a tyre is the carcass. This is made up of a entwined weave of steel wires and fabric bonded to rubber casing. The rubber compound that then goes to make the tread is wound around the carcass before being placed into a mould where pressure and heat imprint the tread, all very technical but designed to provide a very strong tyre.Always check the carcass as it is surprisingly easy to damage (kerbs, punctures etc.) Look for obvious signs such as areas where the tread is uneven or lifted bubbles (especially), unusual wear all around the tyre.You can often see damage better by removing the tyre and looking inside. This is only possible if it is not mounted on the rim of the wheel. Take the sides of the tyre and pull them apart you can then see the inside, quite easily. There should be no granules; rubber or tacks inside, you should find the pattern inside uniform. Still holding the sidewalls apart, press the tread of the tyre down onto a step or other (not sharp!) corner. Rotate the tyre, it may make you dizzy but its important to look for any damage.Do you know what a bead is? It is the part which joins the inner rim of the wheel; make sure you check it for wear and tear. Finally check the general condition of the compound, sometimes the rubber appears to be broken up or the sidewalls badly marked, please steer clear of these part worn tyres.Part worn tyres in my opinion are dangerous, my question to you is would you let your loved ones buy one?