Jul 27, 2012 02:45 PM
69511 Views
(Updated Dec 02, 2012 09:20 PM)
When the time comes to change your worn out OEM tyre for your motorbike we get confused as to what to buy. Often the grip in wet conditions reduces as much as 2/3rd of the dry grip on tarmac.
While selecting tyres we need to consider the following points:
1) What type of grip you want - Dry/Wet tarmac, Slush, Gravel, loose soil?/Type and depth of tread pattern.
2) Tubeless or tubetype
3) Load rating.
4) Rim size.
5) Clearance between Swing arm and the tyre. Extent of Modifications required to fit in your favourite fatter tyre.
6) Compound of tyre(Soft/Hard) used in manufacturing the tyre.
7) Profile of tyre - Round/Flat.
8) Width of tyre/Wider contact patch.
Point No. 6 is the most important point while purchasing any tyre.
1)Type of grip you want - [Dry/Wet tarmac], [Slush, Gravel, loose soil]
If you want dry grip on tarmac you need tyres which would give maximum contact area with the road that means a tyre with no tread grooves at all! also called Slick tyres as used in racing.Slick tyres also require a lot of warm up before they start gripping so they are nearly banned for on-road usage.These tyres will be quite bad in wet on-road conditions also because they don't have any grooves which will draining out water during wet operation. Slush, Gravel, Loose soiled will require a higher depth tread pattern and like a button(like motocross bike tyres). The more looser the soil the more the distance between two studs/buttons. A Dual purpose tyre will have closely packed buttons because they have to perform good on tar road as well as little off-roading and so contact patch needs to be good for more grip. A motocross type tyre will have very less contact patch and so will perform poorly on tarmac but the studs will grip the loose soil(sort of a positive contact) better and so good grip in off-road conditions.
2) Tubeless(TL) or tubetype(TT):
If you are having alloys in your bike then one should go for tubeless tyres. Tubeless tyres have one advantage that in case of punctures they dont leak air abruptly(also called as "burp" -lingo) and will deflate slowly. For repairing a TL one need not remove the wheel from the motorbike.It can be repaired on the bike itself. So easy to repair a big boon for long distance tourers. IMPORTANT: DO NOT FIT TUBETYPE TYRE AS TUBELESS ON WHEEL RIM. IT CAN CAUSE ACCIDENT.
3) Load rating:
It is mentioned on the sidewall of the tyre and it represents the capacity of the tyre to take the weight of the bike. Do not go for a lower Load rating than that is mentioned on your OEM tyre.
4) Rim Size and Tyre width:
Tyres are built for a particular RIM size. Its dangerous to fit any random tyre on any random rim. Eg. A tyre specification is 120/80 x 17 means a Tyre width of 120 mm, a Tyre height(radial distance from rim to the outer surface of the tyre.) of 80% of 120 mm and 17 is the Rim Diameter in inches. A tyre specification does not mention rim width. Eg: Rim width: 2.15j means 2.15 inches. For eg. the maximum tyre width a 2.15j rim can support is 120 mm.There are various charts available on internet to suggest the fattest tyre your existing rim can support. Confusing! So the thumb rule is you can go to a next bigger width of tyre on your motorcycle than what is provided by your OEM. If my motorcycle came with a 110 mm width tyre fitted on 2.15j rims the max I can go for is 120 mm.
5) Clearance between Swing arm and the tyre. Extent of Modifications required to fit in your favourite fatter tyre.
This clearance is very important as the tyre can rub your swingarm or your bikes body parts especially during the bike going through a pothole. And what if the tyre punctures and there is no space for the flattened tyre. I am still to find out the acceptable gap between the swing arm and the tyre but I feel a minimum of 7-8 mm will be required for all the conditions the tyre has to go through.(The tyre expands width wise while going through a pothole). You can adjust a fatter tyre in your existing bike by pulling the rear tyre more rearwards by adjusting the chain. You might be required to fit an additional link in the chain to do so. You will also might be required to cut the chain guard to make clearance. I would never recommend cutting the swing arm to make space for fatter tyre as it will lead to serious problems with the rigidity-read stability of the bike and also the reliability of the bike.
6) Compound of tyre(Soft/Hard) used in manufacturing the tyre.
Compound of the tyre is one of the most important even more important than contact patch. A softer tyre compound has better grip than a similar hard compound tyres. But a softer compound tyre will wear faster leading to less life of the tyre. A harder compound tyre has less on-road grip but will last longer. On pure loose soil as in off-roading the compound of the tyre will not matter much but the tread depth and tread pattern will matter more.(Read button type). A Dual purpose tyre will have a right blend of the Soft and Hard Compound, Hard enough so that it lasts long during the off-roading and at the same type soft enough to get a good grip on tarmac which is quite difficult to get.
How to check: Press your fingers nail into a hard and soft compound tyre and you will notice that the depression formed will be more and will also take more time to recover in a soft compound tyre. Also I observed that a soft compound tyre when rubbed hard with your thumb will give out flakes(similar to your pencil eraser) and the surface will be rough to touch. A hard compound tyres surface will be smooth with a nice finish which we don't want it to be!
7) Cross-section Profile of tyre - Round/Flat.
Tyre which are found on race bikes will have a more rounder profile which is good for cornering at high-speeds. Tyres which are found on touring bikes will have more of a flat profile to get better contact patch while straight-ahead driving position. One more point to be noted here is that a thinner rim width will make a tyre more rounder and a higher rim width will make the tyre more flat so one might need to select the correct rim width size to get the correct tyre profile he desires. The tyre profile depends on the tyre make the tyre manufacturer can make a tyre which is mounted on thinner rim to have a flat profile where as a tyre which is mounted on wider rim to have a rounder profile so it all depends on the type of tyre. One should always check whether a certain tyre width can be mounted on a particular rim width from the manufacturer itself.
8) Width of tyre/Wider contact patch.
We always want a wider contact patch at the contact between the tyre and road contact point. Now one would question that Mu Friction coefficient is not dependent on area but then that is only true of ideal bodies which don't deform.For those bodies which deform like the tyre rubber the above law does not hold true and the law of micro asperities rules the situation. So the more wider the tyre the more grip it will give. But remember a wider/heavier tyre and more so with a soft compound due to more grip is bound to reduce your bikes acceleration and also to some extent fuel efficiency.
I have put down the tyre comparisons in the below comments section.
