One of the first things beginning customizers want to do is put a bigger tire on their motorcycle. This is not as difficult as other custom moves, as long as you understand some basics.

The limiting factor in most tire swaps is not tire to fender clearance, as most people think, but tire to belt (or chain) clearance. This is the closest point the belt runs to the sidewall of the tire.

So let's start with all the basic numbers on the side of your tire so we can make some sense out of all of this. There are seven different things you should be able to see on the sidewall of a quality motorcycle tire: size; direction of rotation; load rating; speed rating; lot number; wear indicators; and balance dot.

Size: There are a couple of different numbering systems that are in use today. Metric, alpha-numeric, and two different inch systems. if you have a 130/90-16 on your sidewall, your tire uses the metric numbering system. What do the numbers mean?

The 130 tells what the thread width is. One millimeter is 0.03937 inches. So a 130 mm tire is 5.11 in. wide. The next number is the aspect ratio. In the example it is 90, which means the sidewall height is 90% of the width of the tire. A tire that has a smaller number here is going to have a lower profile. The last number represents the size rim the tire gets fitted to. Tire sizes can vary from manufacturers, as the government allows up to an additional 10% in extra size to still remain at a given size. Avon tires generally seem to be the larger ones.

If you see MT90-16, that’s the alpha-numeric system.

Use Chart 1 to cross reference for the right replacements for each system.

Direction of Rotation: A front tire and a rear tire have opposite loads being placed on them. A rear tire provides thrust for the bike so its primary load is to the rear. The front’s load is braking only, so its load comes from the front. Tire manufacturers construct the carcass (inner body) to reflect this.

Load Rating: This is the amount that can safely be carried at a specific tire pressure. You must factor in the weight of the bike when considering this.

Speed Rating: As a tire rotates faster, centrifugal forces cause it to grow. Eventually the rubber will separate and chunks of rubber can leave the carcass. This tells us how fast we can turn a tire before we are in danger of this happening. The higher the speed rating, the better quality of rubber and construction. See Chart 2.

Lot Number: This allows the tire to be tracked as far as date of construction, batch, and rubber compound. This can be used for recalls if needed.

Wear Indicators: These are marks on the sidewall that show where on the tread the wear indicators are. Once you have worn the tire till the indicators are no longer visible, it’s time to change the tire.

Balance Dot: This shows the lightest point on the tire. When installing it is to be lined up with the valve stem (heaviest point of rim).

Now that we have the basics, let’s relate it to putting that big tire under that Softail®. All Harleys®, except the Deuce®, come stock with an MT90-16, or a 130/90-16. Generally, for every 10 mm in tire size we grow, we add 0.400” to the width, or 0.200” to each side of the tire. Going back to our limiting factor, the distance to the belt or chain, if we have a 0.250” space, we can put on a 140 mm tire.

When we attempt to install a 150 mm tire, it is 0.800” wider than stock, or 0.400” on each side of the tire. If we have at least 0.450” when we measure, we can put on that fat rascal. This will give 0.050” clearance, tight but adequate. If you use a wider rim that must be considered also. The wider the rim the farther out the sidewalls will be, making the same size tire appear wider. To get wider we must move the belt outward or use a skinnier belt. We’ll cover that in an upcoming Tech Talk.

Well, that covers the basics in “How Fat Will Fit.” Remember that making changes like this can affect how your motorcycle handles. Measure carefully before you buy, and please ride safe!