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From the Workshop of the Accidental Mechanic

Wheels and tyres, the most essential of components but often neglected. Whether they cost £40 per wheel or over a £1000 they can transform how a bike performs and of course the expensive ones tend to look good. This article isn't about the merit of one wheel over the other but simply a guide as to how to look after your wheels and tyres.


Let's start with some basics. Your road bike uses 700c wheels. 700c used to refer to the diameter of the wheel in millimetres, but it doesn't anymore! Just about every country at some point used a different method for measuring wheels and tyres and as a consequence some wheels with the same size markings were in fact not the same size, which really did not help when trying to buy  a new tyre or inner tube. Thankfully things have improved and while your wheels are still called 700c they are in fact 622 millimetres in diameter. You will find that number written on the side of your tyre somewhere. Along with the supposed 700 millimetre diameter of your wheel you will also find the width which is simply in millimetres. Road bikes most commonly use 23mm or 25mm tyres so the size of your tyre and inner tube will be marked 700c x 23mm or 700c x 25mm or a very similar combination.


Rim Wear

Guess what? No matter how cheap or expensive your wheels, they can only be considered as consumables, at least those that use rim brakes. If you have spent a small fortune on wheels, ride high mileages in all weathers and are either an aggressive or heavy rider  do not be surprised if they only last a year or less. Of course all sorts of factors accelerate rim wear but how can you tell when your rims need replacing?

Many rims are fitted with wear indicators. They typically take the form of one or two small holes on the braking surface or they will have a thin groove that runs around the entire circumference of the rim. Both perform the same function in that when they are no longer visible or as in the case of the example in the picture (clik on the thumbnail for an enlarged view) they are flush with the braking surface the rim is worn. A rim can be replaced but it is often cheaper simpy to replace the wheel, particularly for low to mid priced wheels.


Don't want to replace your wheel or are keeping your fingers crossed that it will last another season? It is a false economy as a thin rim wall will fail when least expected. One of two things typically happens when a rim fails. The very high pressures present within a rim can cause it to split catastrophically or a pot hole will splay the walls of the rim outwards. In either case the wheel if it hasn't collapsed will be unuseable and you will need to call your friendly broom wagon. If in any doubt replace the wheel or seek expert help.

If your rim does not have wear indicators a simple visual check will tell you how much the rim has worn. The pictures on the right show a rim that failed due to excessive wear (It had a hard life communting in London). The braking surface when new is flat. Look carefuly at the second picture and the thickness of the metal. At the point where the tyre meets the rim the metal is less than half of its original thickness. In this case it is under half a millimetre thick.

The third picture shows the wear more clearly. Look at the straight edge held against the right hand side of the rim. The gap between the rim and the striaght edge shows the amount of metal that has been worn away by braking. If you lay a straight edge against your rim (with the tyre removed) and you can see a signiifcant amount of 'daylight' between the rim and the straight edge it is time for replacement. Clearly a rider that weighs 40kg is not going to wear a rim at the same rate as a rider twice that weight regardless of the conditions.

Brake Pad Abrasion

Have a close look at this picture of a fairly new brake pad, with less than 1000 miles of use. Apart from the horizontal markings which are normal the silver specs dotted around are in fact metal shards from the rim. As brakes are applied the pads literally abrade the surface of the rim and wear it down. Unfortunately, that abrasion causes flakes of metal to become embedded in the pad which then accelerates wear as metal to metal contact occurs. This is what casues the 'stripes' on both the rims and the pads. When changing a tube, particualrly in the wet, your hands will usually be covered in a black substance which is a mix of alloy, rubber and dirt. This combination acts like sandpaper on a rim. The best way to extend the life of you rim and brake pads is to keep the rim clean and remove those metal particles. Clean and shiny is best and quality products usually last much longer.

Tyre Wear

A wheel of course is of little use without a tyre. There are hundreds of different tyres with each manufacturer offering a variety of widths, compounds, colours and tread patterns all intended for different uses and each boasting about their performance credentials. You will have your favourite but regrdless of the tyre that you choose they all wear out. Some survive to travel thousands of miles but most reach an early retirement as a consequence of damage. As the key contact point between you and the road it pays to look after them. Make certain they are at the right pressure, check them before every ride. Both under and over inflated tyres are dangerous for differing reasons. So you have the right tyre inflated to the correct pressure for your weight and riding style. How long do you keep them for?

If they haven't been damaged in anyway Continental make that decision very easy. Most of their current tyres are fitted with wear indicators much like those used on some rims. The two 'holes' in the picture are not manufacturing anomalies but are there to indicate wear. However, on the roads around Buckinghamshire your tyre is unlikely to last that long. Every puncture damages your tyre to a lesser or greater extent. Have a quick look at the cross sectional picture of  the tyre above and note the thickeness of the tread area and the sidewall. Your tyre will survive very well with hundreds of small holes and cuts. Tyres are rendered unsafe by large cuts or slashes but it does very much depend on where they are and how large. When a tyre is at normal pressure an inner tube should never be visible. If it is there is a real risk that it will suffer a rapid deflation which will not be fun going downhill at 40mph. Cuts on a side wall

through to the inside should be considered terminal as this is the area of the tyre that is both the thinest and flexes the most. The same applies to car tyres when they are checked at an MOT. Cuts in the tread do not necessarily mean that the tyre needs replacing. Many are just surface cuts and do not penetrate through to the inside. Many can be repaired simply by applying a patch to the inside of the tyre. The rule of thumb is that on inflation if the cut begins to distort (outwards) the tyre should be replaced as there is a risk that as the air in the tyre heats it may expand and push the tube through the cut which will cause the innner tube to burst. Through normal use a tyre will picjk up small flints and thorns. These must be removed before every ride or they could eventually fall out, good news, but more likely work their way through the tyre and into the inner tube. Mentioning inner tubes just for one moment there is no reason at all why these cannot be patched and reused. A patch properly applied with glue will be stronger than the tube itself.


And finally...... Do not assume that just becuase you have a 20 year old tyre on your 'shed' bike and it looks ok that your road tyre will last just as long. For one you probably ride much more now than you did then but more importantly if you look a that old tyre it will have perished and have numerous splits on the sidewall. Fine for crawling back from the pub but not going downhill at speed.