Article was originally published on MXStore.
The majority of content written on the side of a Dirt Bike Tyre is to help identify what size, model and terrain the tyre is suited to. The first series of numbers you will need to learn are the ones which relate to sizing followed closely by the tread pattern the tyre is suited to.
HOW TO CHOOSE THE RIGHT SIZE MOTOCROSS TYRE:
The sequence of numbers which give you the tyre sizing are set to give you first the tyre width, then the tyre wall height and finally the circumference of the rim the tyre will fit onto. (See example below)
110/90-19
The “110” refers to the width of the tyre in millimetres as does the “90” which refers to the wall height as a percentage of the width of the tyre. The “19” refers to the circumference of the rim which the tyre suits in an inch measurement. (Some variations of this are listed below)
- 100/90-19
- 130/90-18
- 80/90-21
- 120/90-19
Standard Dirt Bike Tyre Sizes
The majority of full sized dirt bikes in the world run a 21” front tyre and most motocross bikes run a 19” rear. Trail bikes vary with many running an 18” on the rear to suit the different types of terrain covered in enduro.
An 18” wheel typically has a taller wall height, the taller the tyre wall means the more the tyre can flex and roll which lowers performance through corners. Contrary to this the added flex gives a smoother, more controlled ride over tree roots and rocks which you encounter more on enduro.
Wider tyres make the bike harder to turn, the wider the foot print the more it will want to stay upright which can cause a “front end push” feeling through turns. Also for soft terrain or rutted tracks a wider tyre will struggle to fit into ruts easily which will affect your performance.
A wide tyre comes into its own on hardpack dry conditions, a wider footprint means more grip providing the tread pattern and tyre rubber compound are suited.
No matter what bike you have it should have the wheel sizing written on its tyres to identify what you have. Then its a matter of narrow down the tyre you need by thinking about what type of terrain you ride on most.
It is important to choose the right sized tyre recommended for your particular bike. If you decide to run a wider than normal tyre on a narrow rim the performance of the tyre will be compromised as the shape of the tyre carcass will have been changed as it is manipulated to fit onto the odd size of the rim. The same applies if you put a narrow tyre on a wide rim it will push the tyre wall out and change the tyres’ footprint which will effect performance and grip!
How to choose the right tread pattern of motocross tyre:
Different tyres suit different conditions, hard terrain tyres require more knobs with a larger, shorter block close together to increase the footprint which gives you grip. Soft terrain requires fewer knobs with a smaller, taller block which are spaced further apart to allow the tyre to dig into the soft terrain giving you more grip.
HARDER RUBBER COMPOUNDS SUIT SOFT TERRAIN SURFACES WHILE SOFTER RUBBER COMPOUNDS SUIT HARDER SURFACES.
Similar to muddy conditions the added advantage of larger spacing between the knobs allows the tyre to self clean easier. As the tyre spins the mud collected will fling off so the clean knobs bite into more terrain every revolution.
But with the varying nature of dirt bike riding means you will usually encounter an array of different terrain, so unless you have a factory race teams’ budget for tyres you will more than likely need to settle for an intermediate which will handle most things you can throw at it.
Have a good think about the terrain you are riding on most as with an intermediate tyre there will always be a compromise in performance in one terrain compared to another so choose wisely!
HOW OFTEN SHOULD YOU REPLACE YOUR MOTOCROSS TYRES:
Like anything tyres have a finite lifespan, the shelf life of a tyre is around 4 years from manufacture but if you are riding regularly this will be much quicker!
You can check your tyres by simply looking at them, if the knobs are rounded down or short they aren’t going to perform very well. Knobs can actually crack and become ripped off completely so if you are missing any knobs you are up for a new set of tyres.
If you ride frequently this will occur quickly but if you only ride on the odd occasion (poor you) you will need to keep an eye on the health of the rubber. Over time the rubber will become dry, cracked or discoloured. New tyres have a very dark colour which fades out to a grey with age, the older the tyre the harder and more brittle the rubber becomes which will effect performance.
A hard terrain tire is made of a softer compound to give you more grip on hard and slick surfaces, whereas a soft terrain tire is made of harder compounds so the tire can dig in more for traction. If you don’t always know what terrain greets you on riding day or you typically end up riding both hard and soft terrain, grab a set of intermediate tyres.
THE MOST IMPORTANT POINT OF TYRE MAINTENANCE IS TYRE PRESSURE:
The correct pressure to inflate your dirt bike tubes to will vary from track to track but the majority of dirt bike tyres run between 10-20psi. If you aren’t sure you can check your owner’s manual, the harder you inflate your tyre the less grip you will have. If your tyre is too soft it will give a very spongey feel and roll more on the rim through corners affecting your performance.
Inflating your tyres to around 16psi tends to be a popular choice but again, check your owner’s manual and if you are at a track don’t be scared to ask other riders. You should check your tyre pressures before every time you throw your leg over your bike as they can vary a lot every hour and this will affect your performance a great deal!
OTHER ISSUES YOU CAN ENCOUNTER WITH INCORRECT TYRE PRESSURES ARE:
- Tube or tyre failure, punctures
- Cause uneven tyre wear
- Under inflation can cause the tyre to come off the bead
DO NEW MOTOCROSS TYRES HAVE A BREAK IN PERIOD?
There is no break in period for new motocross tyres, however we do suggest to ease into riding on a fresh set of rubber particularly if you have changed size, tread, brand or rubber compound. Get a feel for your new shoes to work out how well they hang on before you go full throttle into a corner or you may end up on the ground before you have even worked up a sweat!
