Flat spotting or something else?

Took my pickup truck for a balance/align/rotate recently, and was told that the two rear tires have “slipped belts” and need to be replaced. Having done some reading and thinking, I am skeptical and would appreciate some feedback:

  • The tires are modern steel-belted radials that have ~15k miles on them. There are no surface irregularities on the tread or sidewalls. They appear to be in very good shape.
  • The truck was, however, parked for almost 2 years, during which time I drove it ~1/2 mile every couple of months. The shop said the two rear tires were the ones with the problem, but that may have been after they rotated them from the front, where one would expect flat-spotting to develop.
  • There is a tiny amount of vibration >65-70 mph, but everything feels great at lower speeds. Watching the truck drive, I can also see a barely noticeable amount of wheel wobble up and down at highway speeds.

I’ve read the threads where “slipped belts” are said not to exist any more, except when the term is used to refer to tread separation. So, how does one tell the difference between flat spotting, tread separation, and (if they do exist) slipped belts by looking at the tires?

This site tirerack.com has the answers to your question.


How old are the tires?

2.5 years…

Thanks. I read that article. But unless I’m missing something, it doesn’t actually say how to tell the difference between flat spotting and other things that can happen to tires…

What kind of shop is it? How much do you trust this shop? Is there another that you might try that specializes in tires and alignments?

This is a specialized tire/alignment shop, a regional chain. No basis to trust or not trust this particular shop. I do plan to take it somewhere else for a second opinion, though.

Still interested in how the symptoms of flat spotting differ from tread separation and slipped belts (if that’s a real thing).


Thanks. I saw that video.

Still trying to find a guide re how to tell the difference between that and other things…

Then why don’t you listen to the tire experts?


I have only experienced flat spotting with nylon cord tires. Used to just drive it out. Get a second opinion my thought.

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Tester: Have had mixed luck at tire shops before, and never used this one before. Plus, generally, I like to understand things and prove them myself before spending $500 or $1k…

I’d get a second opinion. The tires didn’t sit in the same spot for all that time. Every time you moved the truck it is highly likely the tires came to rest on a different part of the circumference.


True. And I will.

Still looking for a guide to how to tell the difference myself, though…

Agree with the nylon cord tire’s do they still make them ? I have not seen them for many year’s.

No idea. These are definitely steel-belted radials, though.

Flat spots are rather uniform, looks like the pavement they sat on.

Tread separation is usually an irregular bulge across the tread. In some cases there may not be a bulge but give the tread a zig-zag appearance while rotating.

The remedy for either problem is the same, replace the tires.


I sure hope not !!

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Retired tire engineer here. Here’s my website: Barry’s Tire Tech

Among the things I did was analyze failed tires - so, Yes!, I can tell the difference between a tire with a separation and one with a flatspot, but it requires specialized equipment - something only a tire manufacturer would have - to answer this exact question.

BTW, a tire can have both! - but it would be rare.

You will NOT find a guide - too complex and too many exceptions. But you sound determined, so let’s play the percentages.

Step 1) With a gloved hand (work glove, not surgeon’s glove!), rub your hand over the entire tread surface. You are looking for a bulge = separation. HOWEVER, no bulge does NOT mean no separation (1st exception!)

If you have a separation, replace the tire IMMEDIATELY!. Not tomorrow, TODAY!

Step 2) Assuming no bulge, look at the sidewall of the tire. On it will be a listing of what fabric was used in the tire, divided by sidewall and tread. Typically it will say “Tread: 2 steel plies + 2 polyester plies”. In this case it is very unlikely that there is a separation. (more exceptions!)

But if nylon is mentioned in addition to the steel and polyester, then a separation can be disguised. - again, no bulge does NOT mean no separation! Plus the use of nylon makes it more likely that there is a flatspot!

Another indicator: If the vibration gets worse (over a couple of hundred miles), that’s what separations do. Flatspots do the opposite.

That is as far as it can be taken without specialized equipment.

But your narrative sounds more like these are flatspots. Flatspots can be generated in as little as 2 months given the right conditions. And the longer the tire sits, the stronger the flatspot. The small vibration you are feeling is unlikely a separation and likely a flatspot.


Nylon is currently used in cap plies - an overlay on top of the steel belts to restrict growth due to centrifugal forces.

Why nylon, if it flatspots? Because nylon has this remarkable property of shrinking when it is heated, making it very advantageous for high speed tires. So tire manufacturers take steps in the manufacturing process to minimize the flatspotting tendencies. That means that flatspotting only becomes a problem under certain conditions - and tires sitting for a long time is among them.