What causes "cupping"?

[Audi Junkie]

I'm not even sure I'm using the right term because I have basicly no experience with any uneven wear, besides light feathering on the front tires. This is where the treadblocks on the edge get faceted. Not "scalloped" where there is a bit scooped out like from a spoon, this is where those blocks get worn at an angle, in some cases every 3rd block has a high spot.

I don't think it's my rotating or lack of rotating them, since there is another new set that came on the CRV, still in use, that wore perfectly flat (Nexen). This is the TRZs, and all 4 tires have it. All have about the same total wear too.

I hate to whine about it, but they are loud, and it bites. I plan on installing them, so the "sharp" part with the oblique angle hits the ground first. Sort of a cross rotation. I suppose that wears them flat eventually. that's why I also shaved them square too. yep, I shaved them.

Anyway, what causes it and what can be done to avoid it?

 

[Shannow]

I've oft wondered the same thing.

I've seen "cupping" on steel rollers in a coal grinding table mill in the power stations. In fact it's the norm. The rolls go in perfectly round, and roll on a bed of coal, with no mechanical gearing between them.

Place metal balls in a big cylinder with some coal, and rotate it for 8,000 hours, and the balls will wear to cubes or pentagons.

Best I can ascertain is that a tiny imperfection creates a unique loading at a point. This creates a slightly larger imperfection, which then reacts more noticeable, creating a bigger imperfection or something.

I think it's sort of like potholes growing.

 

[glennc]

Motorcyclists get that a lot and usually do call it cupping, though technically it still isn't. It happens because the tread blocks flex under traction and the side of each block forced into the pavement wears more quickly than the other.

The reason it's so common on bikes is that the traction forces are always braking-only on the front tire and often predominantly acceleration on the rear tire, and obviously the tires can't be rotated.

If you install them so the sharp part hits the ground first, then as the tread block comes down to the pavement the most-worn part will be in front. You can visualize the braking force by imagining the pavement being forced rearward against a single stationary downward-pointed tread block. That would have the block flexing rearward and I would expect the front to be forced harder against the pavement, causing the front to wear faster than the rear, promoting the uneven wear.

Since braking is usually more powerful, I THINK that would be the cause of the wear you are seeing. However, if you have an awd Audi and do not usually brake aggressively, it could be the opposite, caused by power application.

You would need to know which way the tires were mounted when the wear was caused, and then reverse them, to correct the problem.

 

[CapriRacer]

First, let's discuss terms:

"Cupping Wear" is generally an isolated "scooping" out of the tread and is generally caused by bad shocks or an out of balance tire. On bias ply tires, the wear pattern looked very much like the name implied. On belted tires (and radials being 100% belted), this wear is spread out over a larger area and it washes out the appearance, making it difficult to distinguish from:

"Diagonal Wear" - this is caused by misalignment and aggravated by insufficient inflation pressure and insufficient rotation practices. This wear is pretty much what AJ described - it goes across the face of the tread in a diagonal. Both camber and toe can affect this. Not to mention the "akerman" - which is basically "toe during cornering" .

As a result, I tend to use the term "Irregular wear" which covers both.

Prevention:

1) Make sure the alignment is within the inner half of the alignment tolerance. Vehicle manufacturers specs are pretty wide (with a few exceptions)

Side issue: Some vehicle manufacturers have alignment specs that promotoe this problem. A good example is BMW. Their camber spec results in an excellent handling vehicle that doesn't have good tire wear. Try to fix the alignment to get good tire wear and you lose the handling.

2) Drive gently: Partially this is keep the car within the "akerman". "Spirited" driving causes slip angles during cornering - which is functionally equivalent to "toe in" (or out)

3) Frequent rotation: No matter how you slice it, the front of the car does different things that the back of the car - and that affects the wear pattern that is being generated. Rotating tire evens out the differences.

Hope this helps.

 

[Audi Junkie]

Good discussion. I did shave them flatter and still can distinguish which ones went where. What is hard to figure out is that I think they came off with the sharp edge down (hitting first) My instinct tells me to set-up with that again, and hope for the best. I will monitor it more closely. It is just the blocks on the edge, mostly the innners.

 

[glennc]

I get that sometimes too. BMW doesn't recommend rotating tires, as Capri sort of alluded to, and they sometimes get funny wear. The fronts have braking-only force fed through them (and cornering of course). I have never had it get noisy though. My front Nokians on their third season are almost worn out on the inside but still have maybe 4/32 on the outside.

Sounds like yours is also from braking force. If you reverse them I think they will wear the other way but I don't think it hurts anything regardless (except the noise). If they are directional then of course you can't reverse, only do front-to-back.

 

[Audi Junkie]

Well, I hand carved the high edge of the treads and re-installed with the low edge striking first. They run much smoother. That's all I can say now. I hope they wear flat.

 

[BuickGN]

Is this on the Accord? I ask because my TL is bad about cupping the softer summer tires. I drive easy 99% of the time, have had the tires balanced a few times, aligned a couple times, and rotate very often and still it has this problem. It's very sensitive to road noise as most Honda products are.

 

[Audi Junkie]

Nope, the CRV. I'm gonna double check the direction I placed them. iirc, I crossed them, but don't remember how they are oriented now.

 

[NutBuster]

http://www.toyo.com.au/tech_info9p2.html

I think what you're describing is shown on the 3rd pic from the top.

 

[Audi Junkie]

Good link. It's a combo of 3 and 4. Although my rear tierod end links are worn a lil, I don't think it's a suspension problem since I have a set 2nd set of Nexens that wore completely flat. It says reversing direction will help and as the tire wears the problem goes away, both things I suspected. Unhappy with the TRZ in this respect, never had a wear problem before.

 

[BuickGN]

For what it's worth, I had mine aligned today because the car has been acting like it has too much toe out. It's been following ruts in the road, getting blown around by the wind, and feeling unstable. Even though I had it aligned when I got the tires not that long ago, my toe was way out on the front and rear on the right side. I've been having cupping problems and the only thing I can think of is the shop that's been doing my alignments (went to a different one today) has been off the whole time.