07-13-2009, 03:04 AM #1
- Join Date
- Sep 2008
- Clarksburg, WV
How To: Find the right trailer air pressure....Chalk Test
Does anyone do a chalk test with their trailer tires?
I searched and searched on this but couldn't find any posts about it. I'm actually quite surprised no one has mentioned it before.
Most people say to inflate trailer tires to the max recommended tire pressure. Usually 50-60 psi. I've been reading of some tires that say 90 psi. Trailer shops or salesman will usually tell you to inflate to the max amount.
We don't inflate our cars to the max amount do we? Some may It is pretty easy on our vehicles, we just look in the owners manual or on the door sticker and it says what to inflate to. What about when you start adding weight to the car or truck? What about when you install bigger tires on the same wheel. It changes the needed tire pressure. My Jeep, stock, is supposed to have 31 psi. I run 33x12.50 on the stock rim with 24 psi and no wear issues.
Same with store bought trailers, they will tell you what to inflate the tires to (most do anyway). Or, at least the salesman will tell you what to inflate them to.
How do they know what you are going to be hauling on your trailer?
To properly determine the air pressure necessary for your trailer (or vehicle with modified tires) you need to perform a chalk test. It is pretty simple. This is how i do it.
-First you will need an empty street or an empty parking lot. If you have a large driveway where you can pull forward and back that will work just fine.
-Air pressure gauge with valve stem depressor.
-Sidewalk chalk. It's cheap.
-An extra hand. Someone can drive while you check the tires and deflate air.
1. Put normal operating load on your trailer. 1, 2, or 3 skis, 1 boat...whichever.
2. Inflate tires on both sides to max psi. I say the max psi because this is just a starting ground...you shouldnt need any more than that.
3. Park where you have an open area ahead of you.
4. Mark a thick line across both tires. Only one side is really necessary.
5. Pull forward enough for line on tire to mark the ground.
6. Observe chalk mark on ground, Mark should be of the full tread width. If too narrow, deflate. If it only shows the outside edges, you need to inflate more, BUT you shouldn't need to inflate if you are at the max pressure. You may need to go up in tire size or rating.
7. Deflate accordingly. If you think you need to take a lot out, start at 5 psi increments. Once you start to get close to the full tread, move down to 1 or 2 psi increments. Make sure you deflate both sides the same so the trailer is even!
8. Repeat Steps 4-7. (Remark, Drive, Observe, Deflate)
9. Once even tread contact is observed, Record that air pressure for future use. That pressure will provide the best traction, best wear and be less likely for blowouts.
07-13-2009, 01:46 PM #2
- Join Date
- Jul 2007
- near Toronto, Canada
Trailer tires work differently than vehicle tires. They don't have steering loads or traction loads the same way car/truck tires do.
ST trailer tires are often made with a rounded tread shape, so the center rib intentionally has the highest contact pressure. This helps the trailer track straight behind the tow vehicle, and not wander or tail wag.
The main thing a trailer tire needs to do is survive. Survive curb impacts, sidewall impacts, pothole impacts. The higher the air pressure, the better the tire can withstand these stresses.
Most trailer tires don't wear out the tread - they die from old age with plenty of tread remaining. At 5-6 years old most trailer tires have dried out and begun cracking between the treads. They are due for replacement. No longer flexible, continued use is likely to cause an unscheduled tire failure.
Trailer tires also carry the weight of the trailer and the load, which causes the sidewalls and tread to flex.
Sidewall flex causes heat and rolling resistance, as does tread flex. The higher the air pressure, the less sidewall and tread flex, which reduces tire heat, reduces rolling resistance, and extends tire life.
ST trailer tires are typically made with multiple belt layers, which increases load capacity, and makes the tire stiffer and tougher.
When this thicker tread carcass flexes, it creates more fiction and heat than a normal car tire. At lower air pressures, the tire stiffness works to maintain the tire shape, but at the cost of additional heat from the additional flexing.
The main benefit to lower air pressure in trailer tires is a slightly softer ride. This comes with a reduction in tire toughness, reduced fuel economy, and shorter tire life.
07-13-2009, 01:59 PM #3
- Join Date
- Sep 2008
- Clarksburg, WV
That is informative.
Seems like there could be method to the madness. The guy i got my ski/trailer from ran 20 psi in his trailer tires and the outside edges of the tire tread are worn more than the inside.
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