Dirt Bike Tire Pressure, Pinch Flats And Rim Locks

Dirt Bike Tire Pressure Specs From The Manuals

Every current KTM and Honda motocross manual we checked lists the same number: 1.0 bar, which they print as 15 psi, front and rear, measured cold. KTM calls it the off-road tire pressure. Honda prints it as 15 psi, 100 kPa or 1.0 kgf/cm2. That is far lower than a street bike, because a knobby on dirt needs to squash over rocks and roots and spread its footprint for grip.

One bar is 14.5 psi if you convert it exactly. KTM rounds it to 15 in its manuals, which is why you will see both numbers online.

Tire makers publish their own numbers, and they are often lower than the bike maker's. Michelin recommends 0.9 bar (13.1 psi) front and rear for its Starcross 6 motocross tire, with 0.8 bar (11.6 psi) as the minimum. For its Enduro Medium it lists 1.0 bar (14.5 psi) with a rear minimum of 0.6 bar (8.7 psi). Racers go lower too. Motocross Action's test riders settled on 12.5 psi for a Dunlop MX33 front, running 12 to 13 psi in wet dirt and more on hard pack.

So the sources disagree by two or three psi. Our read is simple. Start at the bike manual's 15 psi. If you want more bite, follow the tire maker's range for that exact tire and stop at its minimum. Michelin says rocky, hard ground needs a little more pressure and soft sand or loam can take a little less, but never below the published range.

How To Check Dirt Bike Tire Pressure

1. Check cold. Honda's manual says to check pressure when the tires are cold, before each ride. KTM says the same.
2. Use a low-range gauge. A 0 to 30 psi or 0 to 2 bar gauge reads small changes far better than a 100 psi car gauge.
3. Change it a little at a time. Honda's manual says to make changes a little at a time when you tune for conditions.
4. Never bleed a hot tire. A tire read right after a moto shows more pressure than it will have once it cools. Michelin warns against letting air out of a hot tire to hit the cold number, because it will end up low.
5. Look while you are down there. Check the valve stem angle, the rim for dents, the spokes and the rim lock nut. Honda puts all of them on its pre-ride list.

How A Pinch Flat Happens

With a tube inside, the air is the spring that holds the tread away from the rim. When you hit a square edge, like a rock ledge or the lip of a pothole, the tread gets pushed up toward the rim. The air pressure is what stops it. Lower the pressure and you lower the spring rate, so the same hit pushes the tread further. If the tread reaches the rim, the tube gets trapped between the tire and the two rim flanges and gets cut twice. That is a pinch flat, and the two slits side by side are why riders call it a snake bite. TUbliss calls pinch flats the most common risk of running low pressure.

Drag the pressure slider down in the figure below and watch the tread reach the rim edges. Then raise the hit size and see how much pressure it takes to stay safe.

The same model shows the upside of low pressure. The footprint on flat ground grows as pressure drops, because the tire has to spread the same load over more area. At 12 psi the footprint in the model is about 25 percent longer than at 15 psi. TUbliss describes the same trade: lower pressure means a bigger contact patch and less "ping pong" off small bumps. KTM's manual names the cost of going too low on any tire, which is abnormal wear and overheating.

Fixing A Flat The Right Way

• Replace the tube. Honda says a punctured tube should be replaced as soon as possible, because a repaired tube may not be as reliable as a new one and can fail while you ride. A patch gets you home. A new tube goes in for the next ride.
• New tire, new tube. Honda says to fit a new tube with every new tire, since the old one is probably stretched and can fail inside the new tire.
• Inspect the tire. Look for cuts, cracks that show cords, embedded objects and sidewall bulges. Both manuals list these.
• Check the age. KTM recommends replacing tires after 5 years at the latest, whatever the tread looks like. The last four digits of the DOT code are the week and year it was made.
• Find the cause. Two slits side by side point to low pressure or a hard square edge. A single puncture means something went through the tire. A tear at the valve stem points to the tire slipping on the rim, which is the next section.

If you are not confident mounting a tire, have a shop do it. A pinched tube from a tire lever or a bead that is not seated will put you right back on the trailer. Honda's manual tells owners to have tires replaced by a dealer.

What A Rim Lock Does

At 15 psi there is not much air pressure holding the tire bead against the rim. Under hard drive or braking the tire can creep around the rim and drag the tube with it. A rim lock is a clamp that sits inside the tire. Tightening its nut pulls a pad up against both beads and pinches them to the rim so the tire can't spin. Rear wheels on motocross bikes have one, and many fronts do too.

The tell that something is slipping is the valve stem. Honda's manual says a tilted valve stem means the tube is slipping inside the tire or the tire is slipping on the rim. If it keeps creeping, the stem eventually tears out of the tube. Run the figure below to see how a few degrees of creep tilts the stem.

Honda's owner's manual does not print a rim lock torque. It tells you to check rim locks for looseness before every ride and again between motos. Use your service manual's number for other brands.

Spoke Tension And The Tap Test

Spokes keep the rim round and centered on the hub. KTM explains what goes wrong when they are off. Too tight and they break from overload. Too loose and the wheel develops side to side and up and down runout, and then the other spokes loosen in turn. KTM calls incorrect spoke tension a safety issue because it affects handling.

1. Tap every spoke. KTM's check is to strike each spoke with a screwdriver blade. Spokes of the same length and gauge should ring at the same pitch. A different note means different tension.
2. Torque the odd ones. Bring loose spokes to spec with a spoke torque wrench. On KTMs that is 6 Nm. Don't assume your bike is the same; DIY Moto Fix rebuilt a wheel whose rim called for 2.2 Nm.
3. Go slowly. DIY Moto Fix suggests turning nipples an eighth to a quarter turn at a time and working around the wheel in a pattern rather than cranking one spoke.
4. Spin it. Turn the wheel against a fixed pointer to see wobble and hop. DIY Moto Fix says most service manuals allow about 2 mm (0.079 in) of runout. Check your own manual.

A wheel that is badly out of true, has a cracked rim or needs several new spokes is a job for a wheel builder. A wheel is a safety part.

TUbliss And Mousse Inserts

Two common ways to get rid of pinch flats replace the standard tube.

TUbliss swaps the tube for a small inner bladder you fill to 100 psi. It seals the beads to the rim and acts as a rim lock around the whole wheel, so the tire itself runs tubeless at low pressure. TUbliss says to start at least 2 psi lower than you ran with a tube, then drop 1 psi at a time until it feels worse and go back up 1 or 2 psi. The bladder loses about 0.5 to 1 psi a day and several psi each time you put a gauge on it, so check both pressures before every ride. Fit it to a new tire, because the old rim lock leaves marks in the bead that can leak.

Mousse is a foam insert with no air at all. Michelin says its Bib Mousse feels like 0.8 to 1.0 bar (11.6 to 14.5 psi), is limited to 130 km/h (80 mph), and should be used for 6 months at most after fitting, less for motocross than enduro. It needs Michelin's gel every time it goes in or out, and it does not like heat. Michelin says not to store it above 30 C for long and to avoid 40 C.

How Often To Check Tires, Rim Locks And Spokes

The hour-based items are the ones that slip. A new bike's first spoke check comes up after one hour, which can be the first afternoon. Our KTM 250 SX-F maintenance schedule and Honda CRF250R maintenance schedule list the rest of each service by hours.

Common Tire Pressure Mistakes

• Running the sidewall number. The pressure molded on the sidewall is a maximum, not a riding pressure. Use the manual or the tire maker's chart.
• Going as low as possible. Michelin says to stay inside its range, not to go as low as you can. Below that, pinch flats and tire creep come fast.
• Cranking spokes tight. Overtight spokes snap. Torque to spec and check them often instead.
• Ignoring a tilted valve stem. It means the tire or tube is moving. Fix the pressure and the rim lock before the stem tears.

Pressure is one part of how the bike handles. Sag and clickers are covered in how to set dirt bike suspension, and the order to set everything up is in our dirt bike setup guide. Wheel bearings, which live inside the same hub, get their own guide on dirt bike bearing maintenance.

Track Tire And Spoke Checks

MotoMind keeps a maintenance log for each bike, with tasks like tire pressure, spoke tension and wash and inspect. Hour-based tasks show as upcoming, due or overdue as you add engine hours by hand or with the GPS ride tracker, and date-based tasks can send you a reminder. Log the parts and cost of each tire change and export the whole history as a PDF.

FAQ

What psi should dirt bike tires be?

KTM and Honda both list 15 psi (1.0 bar) front and rear, cold, for their current motocross bikes. Tire makers sometimes go lower: Michelin recommends 13.1 psi for its Starcross 6 with an 11.6 psi minimum. Start at the manual number and stay inside your tire maker’s range.

Why do dirt bike tires run such low pressure?

Low pressure lets the knobby tire squash over rocks and roots and spread a bigger footprint on loose dirt, which gives more grip and a smoother ride. The cost is less protection for the tube and rim, so the window is small.

What causes a pinch flat on a dirt bike?

A hard hit squashes the tread so far that the tube gets trapped between the tire and the rim flanges. It leaves two small cuts side by side, often called a snake bite. Low pressure is the usual cause, because the air is the spring that keeps the tread off the rim.

Should I check dirt bike tire pressure hot or cold?

Cold. Honda and KTM both say to check when the tires are cold, and Michelin warns not to let air out of a hot tire to match the cold spec, because it will be low once it cools.

How tight should a dirt bike rim lock be?

KTM specifies 12 Nm (8.9 lbf ft) for its M8 rim lock nut. Honda’s owner’s manual does not list a torque but says to check rim locks before every ride and between motos. Use your service manual for other brands.

How often should I check spoke tension?

KTM’s 250 SX-F motocross schedule calls for a spoke tension and rim runout check after the first operating hour on a new bike, then every 15 hours. Honda says to check spokes before every ride and between motos.

Can you patch a dirt bike inner tube?

You can patch one to get home, but Honda says a repaired tube may not be as reliable as a new one and should be replaced as soon as possible. Always fit a new tube with a new tire.

Why is my valve stem crooked?

Honda’s manual says a tilted valve stem means the tube is slipping inside the tire or the tire is slipping on the rim. Check the pressure and the rim lock, and straighten it before the stem tears.