Community Under Pressure - Air Pressure For Motorcycles, That Is

Under Pressure – Air Pressure For Motorcycles, That Is

Years ago, when I was a young punk just getting into the idea of personal mobility, an old mechanic succinctly explained to me how pneumatic tires work: “It’s the air, stupid!”

Turns out that blunt lesson delivered with a little spray of his Copenhagen chew from between his teeth was absolutely correct. The tires don’t really carry the load—it’s the air.

Take the air out of the tires, and you don’t have a functional vehicle. Tires become misshapen lumps that make it tough to even push, let alone ride, your bike. That superbike of yours moves from road rocket to “disabled vehicle.” Inflate the tires to the correct psi, kPa, or bar, and you have your freedom back.

So, what, exactly, is the correct tire pressure for motorcycle tires? The succinct, but less than satisfying, answer to that one is, “It depends.”

The best answer is whatever the motorcycle manufacturer says it is. However, the type of tire can lead to big differences in what that number may be. For example, those big, fat tires on the ATVs and UTVs are often low-inflation types that may be correctly inflated with only 2 to 10 psi in them. On the other end of the spectrum, some small diameter trailer tires may call for up to 60 psi. Typically, street motorcycle tires are in the 28 to 40 psi range.

Air Pressure - motorcycle tires
A label like this may be on the bike that tells you what tire pressures to use for different load conditions.

Another factor is the anticipated load conditions and road conditions. The manufacturer may recommend slightly increased tire pressure for heavy loads (see the image). So, if you’ve got the saddlebags, tail bag, tank bag loaded to the max, and are planning to ride two-up, check your manual or the motorcycle for a label that provides information for that riding situation.

Under-inflated tires can run hot and, if the ambient temps are high, temperatures in the hull of the tire can soar. The combination of sidewall flexure, high heat, heavy load, and high speed can lead to a blowout.

Michelin Commander 3 review
Heavy loads require properly set tire pressure.

There are more than 50 standard units of measure for pressure. For the purposes of measuring air pressure in your tires, we really only need to think about three: psi, bar, and kPa. Most know psi is pounds per square inch, while bar means atmospheric pressure at sea level (100 kPa), and kPa being kilopascals (1000 pascals). Type the conversion you want into your Google search box, and the answer appears. Sites such as UnitConverters have a more comprehensive range of unit choices.

Which unit will be most common for the specifications on your tire’s inflation depends on where you live, where the tires were manufactured, and under which standards. In North America, psi is the most common. That said, when picking out a tire pressure gauge, it may be handy to think about getting a digital pressure gauge that offers all three scales, such as the Digital Sport Tire Gauge from Slime.

Most of the pencil-style mechanical pocket gauges provide psi and kPa, while most of the dial-indicator gauge types are either psi only or psi and kPa.

Air Pressure Gauges
There are a lot of pressure gauges out there. Which one will work best for you depends on what pressures you need to measure.

Just a word here about gauge accuracy. In my experience, which dates back to just after the third global extinction event, pressure gauges that are all plastic and cost very little tend to have the widest variations (5 to 8 psi high or low) in readings when compared to more than one metal-body unit such as those shown on the left in the image.

Accuracy and precision are not the same. Accuracy is how close the reading the gauge gives you is to being the actual pressure. Stated another way, it is how far off the reading is from the true pressure, plus or minus, on any given reading. Precision is the degree to which the gauge can reliably and repeatably measure in units—that is, can it read down to fractions of psi, or a single pound per square inch, or is the smallest degree of precision that can be read off the gauge only to the nearest five pounds per square inch. How accurate are the gauges in the hose at the gas station? Well, that’s anybody’s guess. I always double-check with my own gauge.

Tires may not be the only thing you need to monitor pressure in air-adjustable rear suspension, for example.

Also, how low can the instrument reliably measure pressure? Some simple pencil-style pocket-size gauges have a scale that starts at 10 psi. This matters if you have a bike that air-adjustable suspension elements that must not be inflated to a pressure greater than 6 psi, as is the case in the compression damping adjustment in the forks of my 1984 Honda VF1100S. Over-pressurizing the forks can cause damage that is a pain to fix. So, if you have suspension or tires that require such precision, you will want to consider spending the extra money on a high-precision gauge.

Precision and range also matter. An air-adjustable fork, such as on the Honda VF1100S, must not get more than 6 psi.

You may have a motorcycle with tire pressure monitoring systems (TPMS), either factory-installed such as a Honda Gold Wing Tour or retrofitted (Blu Technology Products and others), those are great. But, they are not infallible.

Carrying a pressure gauge along with you is still a good idea because, if your bike is starting to handle like you’re riding in a banana cream pie, but the TPMS still says all is well—you might want to stop and get a second opinion.

Air Pressure - Harley-Davidson Sportster
An angled valve stem, such as this one on a Sportster, can make tire inflation a little easier.

 

Another consideration is where the valve stem is on your bike and what contortions you and your gauge may have to go through to get at it. Somebody was thinking when they designed the rims for my Harley-Davidson Sportsters with a valve stem that is angled out to make getting a gauge and air hose fitting on it a little easier. On other motorcycles, such as the Road King, for example, the rear rim may have the valve stem vertical and with the brake disc, exhaust pipe, saddlebag, and low clearance of the rear end, that makes it a pain to even see, let alone get a gauge and filler nozzle on the valve stem.

Air Pressure - Harley-Davidson Road King
Difficult access to the valve stems, as in this example, should be considered when picking out your gauge and compressor equipment.

If you have your own compressor, having an extension with an angled head can help with getting at some valve stems, but in some, it’s just plain going to be a pain. No matter, though. The pain of that kind of inconvenience is very minor compared to that which results from an under-inflation blow out miles from home or poor traction caused by over-inflation.

 

For more info on motorcycle tires on UltimateMotorcycling.com, see:

 Sorting Motorcycle Street Tires

Motorcycle Tire Safety Standards Explained

Top 10 Things to Know About Motorcycle Tires

MIC Releases Booklet and Video on Motorcycle Tires

 

 

 

 

 

 

 

 

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