Thursday, September 21, 2023


Is that a tire sidewall bulge? All about sidewall undulations

In an RV Forum post on Facebook recently I read… “Sailun S637 14 ply 110 psi made in Vietnam less than 2 months old suffered bulging after a 50 mile maiden trip. I made a mistake/forgot to click warranty during the purchase but I did click warranty on the truck tires but not on the trailer tires. Speed was 55-60 mph. I don’t remember hitting a curb or a freeway pothole. Per SimpleTire customer service, I’m out of luck.
Update: 8/9/23. I reached around on the opposite side and it had the same undulation.

My reply:

In the photo, note the area near the arrow below the 3. The depression makes the sidewall at the 7 look like it is sticking out a bit. Almost all tires have some level of undulation. “Indents” are just locations of a “lap-splice” made when the body ply is assembled. An outward bulge would indicate either an “open splice” or an area of broken body cord.

If you watch this video you will see the “radial” body cords at 1:58 to 2:12 time in the video. The “splice” is where the “ply” starts and stops.

There were over 110 posts by other RV owners. Comments included:

“It’s because the tires were made by Company ‘X'” and “They were made in CHYNNA!” Someone added they were made in Vietnam, in addition to the original poster mentioning that.

Believe it or not, a few correctly identified the “sidewall undulation” as normal and not a problem. Others started to ignore the correct answers and advised the tire should be inflated to 100 psi so it increases pressure as it warms up. The original poster, after reading my reply, said “I’ve never had a tire show like this in my entire life.

I added another reply hoping that some would read and learn that you can’t just tell others what inflation to run based on what you run in your RV.

My final post follows:

I saw your posts about the tire sidewall depression (undulation). I am a retired Tire Design Engineer with 45 years of experience. In addition, I am a court-recognized “Expert” on the forensics of tires in RV applications.

Tires are made to support different levels of load. Since it is the inflation that supports the load, not the tire sidewall, we see different levels of inflation for tires on different RVs. Small pop-ups and many single-axle trailers may come with Load Range “C” tires. That would mean 50 psi might be on the Certification Label. My Class C has LR-E tires that can run 80 psi. Large Class A RVs run truck/bus-type tires with 19.5″ and 22.5″ wheels. Their Load Range may start with F and go up through G, H, J, and even L, which have inflations in the range of 95 psi to 130 psi.

In ALL cases, the inflation found in the load and inflation tables is the “cold” inflation. This means the inflation needed to support the load stated in the tables is measured when the tire has not been driven on or in direct sunlight for the previous 2 to 3 hours. Each of your RVs came with a federally mandated “Certification Label” that identified the GAWR, Tire Size including the Load Range, and the inflation required to support the stated GAWR.

This thread started with a question on sidewall undulation that can be found on almost every tire. The video I provided showed how tires are “built.” The slight overlap of the body ply can, in some cases, result in a slight depression, as there are a few extra body cords in that area that resist the slight expansion that can be seen in tires.

Bottom line:

Be careful when you see some posts as replies to questions on Facebook or other RV forums are not always accurate.

Roger Marble

Check out my Blog www.RVTireSafety.Net

Read more from Roger Marble on


Roger Marble
Roger Marble
Retired Tire Design and Forensic Engineer w/50+ years of experience. Currently has Class-C RV. Previous Truck Camper, Winny Brave, Class-C & 23'TT. Also towed race car w/ 23' open trailer and in 26' Closed trailer. While racing he set lap records at 6 different tracks racing from Lime Rock CT to Riverside CA and Daytona to Mosport Canada. Gives RV and Genealogy Seminars for FMCA across the USA. Taught vehicle handling to local Police Depts


  1. These undulating bulges have been on every tire I’ve had since radial tire took over the market. I never noticed these bulges on bias ply tires.

  2. I have lots of experience with tire failures on my 5th wheel, which after 4 years of frustration I solved by moving up from “E” load rated tires to heavier “G” rated tires. So I learned to pay careful attention to inflation pressures and load weights. I also learned to do a close examination of each tire before a camping trip.

    One year in Arizona, in the bright sun, I noticed a very faint sidewall indentation in a straight line corresponding to a radius from the center of the wheel. I was shocked and prepared to replace the tire. I looked harder at the other 3 tires and found the same thing! At a local tire shop, the tech looked and in about 5 seconds proclaimed these (very slight) indentations to be a factory mark from the tire mold. Whew! Just to be sure, I had my tire shop back home look and they said the same thing, just about as fast. Maybe they could look at the tire and tell, but an important clue for me was that the marks occurred on the exact same place in the printing on the tire for all 4 tires.

    On an unrelated subject, when I had a slow leak on a car tire, I could find nothing wrong with the tire. The tire tech looked and both times took seconds to find a screw in the tread! This happened to me twice over the years. So don’t underestimate the skill of these tire techs!

  3. Thank you Tony for setting the record straight. As an engineer, I can sympathize with you having to put up with the misinformation that spreads so rapidly on the internet. Now, off to the Glen for the Vintage races.

  4. My Goodyear Endurance tires have a slight bulge around the tire from below the tread and rim. Goodyear said it’s normal and somewhere I have a sheet the Jayco Dealer sent me. I noticed them when tires were new.


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