Do you know what the tyre markings actually mean?

We all have seen motorcycle tyres with lots of information written on it. Rarely did we know what that information actually meant. We rely on shopkeeper to provide us with correct tyre. Here is our effort to decode all the sidewall markings of a tyre.

This is maximum outer width of the tyre in mm. In this case, it is 100 mm -

This is tyre aspect ratio or height of sidewall in mm. In this case, it is 90 mm.

Tyre diameter is measured at the beads. 1 inch = 2.54 cm

Load Index

Load index lets you know how much load you can carry on your vehicle safely. In this case, tyre can carry 218 kg weight per tyre.

Speed Index

Every tyre is marked with maximum possible speed of the tyre. In case of the current photo, you can ride at max 150 kmph. Any speed above that is dangerous for tyres and ultimately for you.

This table will tell you the maximum possible speed for your tyre. Just check the speed rating mentioned on your motorcycle tyre.

Country Approval:

This mark lets you know that the tyre is approved for use in your country.

Direction of Rotation:

TThis shows the direction in which the tyre should rotate after fitting.

Type of Vehicle:

Type of tyre :

Know the age of a Tyre:

One should avoid buying an tyre more than 2 years old. Older tyres tend to get hard and crack. Easy way of identifying when the tyre was manufactured is to read the marking on the tyre.

The date of manufacture is engraved on sidewall of the tyre. This date consists of a series of 4 digits engraved on the sidewall : the first two digits indicate the week of manufacture and the last two, the year of manufacture. Eg. 0315 = 3rd week of 2015.

First two digits indicate the week of manufacture. In this case, third week of a year.

Tyre Wear Indicator (TWI):

Usually a logo / marking is engraved on shoulder of tyre to indicate location of TWI on tread area.

When the tyre thread is level with the indicator shown in the photograph, it is time to replace the tyre. Shape/size of TWI may vary from tyre to tyre.

Please check TWI of your tyre to see when you need to replace it. 


 

Motorcycle Tyre Upsizing/Downsizing(TWI):

One of the most commonly asked question by our readers is about upsizing/downsizing motorcycle tyres. We thought it would be a good idea to compile some information about the process and help you make an informed decision.

In simple words, using a wider tyre or a larger wheel than stock is called as upsizing. Similarly, using a narrower tyre or a smaller wheel is called downsizing.

Upsizing/Downsizing can be done is two ways:

  1. Using a wider/narrower tyre with the stock rim.
  2. Moving on to larger/smaller rim along with wider/narrower tyres.

It is always recommended to stick to manufacturer provided tyre specification. We can always go for a different brand of tyres but the size should be conserved as upsizing/downsizing has the potential to affect the dynamics of a motorcycle. If there arises a situation where in stock sizes are unavailable, then the primary goal should be to try to stick as close as possible to the stock specs.

The following parameters are affected if we Upsize/Downsize the tyres:

  1. Speedometer / Odometer reading
  2. Gearing
  3. Acceleration/Top Speed
  4. Fuel Efficiency
  5. Handling and manoeuvrability

Speedometer/Odometer Reading:

Depending upon where the speedometer sensor is mounted, upsizing/downsizing can induce an error in the reading. In most of the bikes available in India, the sensor is mounted on the front rim. So upsizing/downsizing the front tyre can induce an error. To make is easy to understand let’s consider the example of Hero Hunk. It comes equipped with 80/100-18 tyres at the front.

What this means is that the rim is 18 inch in diameter, the tyre has a width of 80 mm and the sidewall height is (100% of 80 =) 80mm.

Overall diameter of wheel(inches)= diameter of rim in inch + 2*(sidewall height in inch)

= 18 + 2 * ( 80/25.4)

= 18 + 6.3

= 24.3 inch (approx)

Circumference of wheel (in inches)= 2 *22/7 * r (r=radius)

= d * 22/7 (d=diameter)

= 24.3 * 22/7

= 76.371 inch

Note: These figures are approximate as the exact diameter and circumference would also depend on the tyre pressure and width of the rim as well.


Lets say we upgraded the tyre to 90/100-18 at the front. So now our width of tyre is 90mm and sidewall height is (100% of 90=) 90mm. The diameter of wheel and circumference will be:

Overall diameter of wheel(inches)= diameter of rim in inch + 2*(sidewall height in inch)

= 18 + 2 * (90/25.4)

= 18 + 7.09

= 25.09 inch (approx)

Circumference of wheel (in inches)= 2 *22/7 * r (r=radius)

= d * 22/7 (d=diameter)

= 25.09 * 22 / 7

= 78.85 inch

So while the stock tyre covered a distance of 76.371 inch per rotation of the wheel, the upsized tyre covers a distance of 78.85 inch. So after upsizing, when you cover a distance of 78.85 inch the speedo sensor still thinks you’ve covered 76.37 inch since it is calibrated to work with a tyre circumference of 76.371. So this induces an error of roughly 3.25%.

Gearing:

Since the rear wheel is the driving wheel in all motorcycles, upsizing/downsizing the rear wheel can affect the final gearing.. The thumb rule here states that the bigger the circumference of rear wheel the taller is the gearing.

I will share my personal experience here. I own a Yamaha R15 v1 which comes with 100/80-17 rear tyre. As the rear tyre is not easily available in the market, I had to upsize to 110/80-17 tyre. In stock profile, the circumference of the rear wheel is 23.30 inch (Similar calculation as shown in the above section) but with the new upsized tyre the circumference became 23.93 inch. So clearly there is an increase in circumference by 0.63 inch with the new tyre.

While this number might not seem like much but it does affect the gearing. With the stock tyre, at 5k rpm in 6th gear the bike does around 72 kmph but with new tyre it increased to 75kmph. Similarly if we downsize the tyre (lower sidewall height) the gearing will become shorter.

Acceleration/Top Speed:

Gearing is one of the factors on which the acceleration/top speed of a bike depends. And as discussed in the previous section, upsizing/downsizing the rear tyre affects the gearing which in turn affects the acceleration/top speed.

The bigger the circumference of tyre, the slower the bike can accelerate and vice-versa. But with top speed it’s the exact opposite. A bigger tyre will enable the bike to reach a higher top speed than stock theoretically (and practically if the bike has enough power).

With the 110/80-17 rear tyre on my bike I could feel a drop in acceleration. On the other hand, at a fixed rpm and gear I was able to do better speed than stock. However when we take into account the top speed, the changed gearing should have allowed me to do a maximum of 156 kmph theoritically but practically it was more or less the same as the engine was more stressed with the taller gearing and upsized tyre and didn’t have enough power left to push any further than stock.

For a more precise effect of upsizing/downsizing on gearing you may check Gearing Commander.

Fuel Efficiency:

An upsized tyre would mean a higher rolling resistance as well as increased weight. The wider the tyre, the higher the rolling resistance and the harder the engine has to work to rotate the wheels. This would lead to a drop in fuel efficiency (not by a huge margin). When a downsized tyre is used, the exact opposite happens.

Handling and Maneuverability:

An Upsized tyre because of its inherent nature has a larger radius of rotation/mass. This contributes to higher gyroscopic force which in turn increases the stability but on the other hand reduces the ease of changing direction. What this means for a rider is that, he/she will have to put in more effort to turn the handlebars. It’s for this very reason that adventure bikes would often come shod with 21″ front tyres.

The second important aspect is unsprung weight. It is the mass of the suspension, wheels and other components directly connected to them, rather than supported by the suspension. When in motion, surface imperfection causes the tyres to compress which induces a force on the unsprung weight. The unsprung weight responds to this force by movement of its own.

The binding principle here is that this movement is inversely proportional to the unsprung weight. Therefore a lighter wheel (rim+tyre) would readily move in response to surface imperfection and will have more grip as well as constant grip while tracking over imperfect surface. For this reason lighter wheels are sought after for high performance applications.

An upsized tyre also increases the ride height of a motorcycle affecting the entire dynamics of the bike.

Points to consider while upsizing or downsizing:

The general guiding principle to check if an upsized tyre will fit or not is that, the sum of the width and aspect ratio of the upsized tyre should be same as the stock tyre. If its the same then it will fit in all probability. For example, lets consider Bajaj Pulsar 220 which comes with 120/80-17 tyre. The sum of width and aspect ratio is 120+80=200. So a 130/70-17 tyre (MRF Zapper Revz S) will also fit easily as the sum in this case is also 200.

There are still certain cases where the sum may not be equal but still the tyre fits perfectly. So while this rule can help you identify which tyre will definitely fit, it will not help you to identify if a particular tyre will NOT fit on your bike.

To help you understand better lets consider another example of Yamaha FZ-16. It’s stock tyre size is 140/60-17 which brings the sum to 140+60=200. For a 150/60-17 tyre (MRF RevZ-C), the sum is 210 which is not not equal to the sum in case of stock tyre. But infact a 150/60-17 tyre fits perfectly on a FZ-16.

So the point here being, if the sum doesn’t match use your own discretion and get it checked before buying a set of tyre.

Another point to consider while upsizing/downsizing is the sidewall height. While upsizing or downsizing the focus should be on getting the same sidewall height or as close as possible to stock size. To help you understand better, here are a couple of examples:

  1. Pulsar 220 rear tyre: Stock tyre size=120/80-17. So sidewall height is 96 mm. If a 130/70-17 tyre is used then the side wall height becomes 91mm which is in acceptable range.
  2. Yamaha R15 V1 front tyre: Stock tyre size=80/90-17. Sidewall height in this case is 72mm. So a 90/80-17 tyre from R15 V2 will fit perfectly as the sidewall height of the later is exactly the same at 72mm.

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