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Sunday, July 17, 2011

What is the minimum air pressure you need to run?

Simple question but there is much confusion on the answer.

In the March 30 post we covered the fact that it is the air that carries the load not the tire.
The April 16 post showed what can happen to tread wear if you are over-inflated or under-inflated by a significant amount.

May 10, the relationship between temperature and pressure was covered.

May 23, the tire certification label or “Placard” was covered.

July 8, I covered the effect of elevation (altitude) on tire pressure.

But after reviewing all these posts it has been pointed out that there was no clear indication on the minimum inflation you should be setting your tires at when you check before a trip or travel day. There have also been questions about the minimum level for the warning from your TPMS.

I have no way of knowing every car, pick-up trailer or motorhome tire application so I can’t give you a number but I can provide step by step instructions so you can determine the values you need.

You need to create a page with the following information:

1. The tire size and Load Range.
Be sure to include the letters that are part of the size. Tires on most cars will start with a “P” but some imports have no “P”. SUV and small Pick-Up may also have a “P”. Larger Pick-ups with probably start with “LT” and many Class-B or Class-C RV will also have tire size that starts with “LT”. Many trailers will have tire size start with “ST” and most Class-A RV will have no letter before the numbers. We call these TB (truck-Bus) type.

2. The Minimum inflation and maximum load molded into the tire sidewall.
You will find that LT , TB and ST type tires will have numbers for both Single (front) and Dual (rear) position. Write them all down.

3. The Minimum inflation as provided on your Placard.
Note you may have different minimum inflations Front vs Rear. Some Pick-up may provide a light load inflation and a heavy load inflation. Anytime you are towing you need to consider that heavy load.

4. Your actual side by side load for each axle.
Here a guess may be off by 5% to as much as 20% so you need the facts.

5. Write down the minimum inflation for all tires on each axle based on your actual load for each position found in #4 above. Remember all tires on any axle should have the same inflation, so the number for the warning level from the load tables would be based on the heavier side. You can get the minimum inflation based on sacale weight from the Load & Inflation tables available from your tire manufacturer or other sources. Well over 95% of all tires of the same size have the same values as they generally follow Industry Standards, so if your tire manufacturer doesn’t have tables available on the internet find a table from another tire company and use that number.

Okay, so now you have all the information needed.

Your minimum cold inflation is the number shown on your Placard. This inflation has been established by your RV manufacturer as the inflation needed to carry the design load of the RV and to provide the best compromise of ride, wear and vehicle stability.

Your minimum for the warning level from your Tire Pressure Monitor System should be the higher of 80% of the minimum inflation on the Placard or the minimum inflation needed to carry your actual load based on the Load & Inflation tables.

This may sound complicated but here is an example from a “Super-C” application:

1. The tire size and load range.
245/70R19.5 LR-H Note this is a TBR type tire

2. The minimum inflation and maximum load molded into the tire sidewall.
120 psi 4805 Lbs single 4540 Lbs Dual

3. The minimum inflation as provided on the placard.
95 Psi

4. The actual side by side load for each axle.
LF 3242 RF 3652 LR 6914 RR 7279

5. Minimum inflation for all tires on each axle based on actual load for each position found in #4 above.


The table gives the following loads at various inflations:
Infl. 80 85 90 95
Single 3540 3740 3890 4080
Dual 3415 3515 3655 3860

So we can see that based on the heavier RF we need a minimum of 85 psi to carry the load in the front axle and 90 psi in the rear based on the heavier RR.

I would suggest that you set your warning levels at the 85 to 90 psi levels but you should be setting 95 psi as your minimum morning inflation before each trip and each day of travel as this is the manufacturer’s recommended minimum inflation for the vehicle.

Remember if you lose 20% of your inflation it is considered to have been run flat and needs to be dismounted and have the interior inspected by a knowledgeable tire tech or engineer. 80% of 95 is 76 psi and you would like to avoid the cost and time involved in having a complete inspection so having the warning go off at 85 to 90 psi provides a nice margin.

With all this information you also have learned that if you discover a tire a little below 95 psi but still over the inflation needed for the load you can plan on driving a short distance at a reduced speed to the nearest location that can provide the 95 psi needed. If you do this don’t forget to compensate for the warm tire. (See Temperature & Pressure May 10).

A final point.
Many suggest that you over-inflate your tires by 5 psi to provide a cushion for day to day temperature variations and I support this idea as long as you do not exceed the maximum rating for the tire or wheel. Many wheels are marked with their max inflation rating and if yours are not marked you need to contact the wheel manufacturer for the specifications.

2 comments:

  1. Roger, thanks for the update on this. We need to do this for our motorhome. I haven't been real good at keeping up with that part of it.

    ReplyDelete
  2. Wow! Great blog!
    I had never thought of the tire pressure listings as the "minimum"...just the pressure that the tires should be set at. I would always shoot for that pressure, and actually even thought that going over it, was probably a bad thing.

    Very informative!
    Though now, I've got to go and start doing some figuring...LOL!!

    ReplyDelete

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