Tire Cupping - Why

[bigjohnsd]

I spoke with my suspension Guru, Dave Moss, about tire Cupping on my front tire. It has just a tiny bit. I run 39# in my front tire, I have run as much as 42. In all cases every brand of tire has experienced Cupping no matter what pressure I ran them at though the PR4 at 36# was the worst, the Avon at 39 has been the best.

Dave provided the following opinion which I have included here as well in my Suspension Tuning thread.

The cupping is from rebound damping being too fast/ too open. As I closed down the rebound circuit by making the adjuster move closer to the closed position, it stopped the weight transfer back and forth/pogo/rocking that the bike was doing. As the fork oil ages, the cold to hot viscosity variance increases dramatically so regular servicing of the forks is a good idea between 8-10,000 miles. With a big bike like that, tire carcass is also VERY important as tire pressures have to match carcass design (soft tire carcass needs a lot more pressure). Start at 46/44 and bring it down to 38/36 to see how the bike's handling changes based on the tire flexing as you turn in and then go through the corner. Defining tire pressure is as important as getting the bike set up.

Interesting.

 

[Fred W]

Do you really mean cupping? Or do you mean scalloping? Here's my understanding of the two. (Someone please correct me if I am wrong):

Cupping is a wear spot that repeats itself around the circumference of the tire, and is the result of inadequate damping of the suspension that allows the tire to bounce (oscillate) up and down after hitting a bump. As the tire becomes somewhat cupped the bouncing and the wear is amplified and will happen without any bump. It will be mostly in the center of the tread, not off center to the sides like scalloping is.

Every front tire (and most rears) will scallop at least somewhat on an FJR. Scalloping is where the tread blocks are worn unevenly, primarily to the sides of center, while the center is worn evenly. Every tread block (of the same shape) around the circumference will be worn unevenly the same way. Scalloping is caused by the tread blocks flexing under the weight and G forces of the bike while cornering, especially while also accelerating and/or trail braking. Scalloping is accentuated by running the tire soft, as that allows the tread blocks to flex more. Scalloping generally is at its worst at the mid-worn point of a tires life. As the tire continues to wear, and the blocks become shortened in height, the twisting is lessened and often the uneven wear evens out somewhat before you have to ditch the tire.

If you feel a vibration or thumping when you are riding straight down the road, it may be due to cupping. Play with your damping.

If you only feel the tire vibration or hear the "buzz" with the bike heeled over somewhat, usually just a little is all it takes, then that is probably tire scalloping. Pump up the tires more as the carcass is flexing too much.

 

[erixun]

Waiting for minds far superior than mine to weigh in... all I know is that it seems to occur less with running my PR4's at higher than manufacturer recommendations.... subcribed.

 

[its just rain]

Great explanation Fred!

 

[Blind Squirrel]

https://www.rattlebars.com/tirewear/index.html

 

[N4HHE]

Fred is correct, cupping appears across multiple tread blocks, scalloping occurs to the tread blocks.

Cupping is due to inadequate suspension damping. Scalloping occurs because the distance from axle to tread is less the farther from center of tire tread. The rolling diameter on rounded motorcycle tires different across the contact patch yet the entire patch has to move at the same speed. So something has to scrub. Hence scalloping.

 

[bigjohnsd]

Scalloping or Cupping, I don't know, but the explanation I received from Mr. Moss is what I posted above.

As one can determine from his response, he dealt with both dampening and tire pressures.

 

[N4HHE]

Large trucks and most trailers do not have shock absorbers because they are sprung very stiffly. As are motorcycles where almost no damping is needed to ward off cupping.

 

[Fred W]

<blockquote class='ipsBlockquote'data-author="bigjohnsd" data-cid="1340898" data-time="1475462059"><p>

Scalloping or Cupping, I don't know, but the explanation I received from Mr. Moss is what I posted above.<br />

<br />

As one can determine from his response, he dealt with both dampening and tire pressures.</p></blockquote>

Yes, but only one of the two was germane to your situation (depending on which it was)

 

[08FJR4ME]

One of the reasons I love my PR2's. If I keep up with the tire pressure I never see any of these issues on my bike. The only time I had scalloping was with the BT023's. Oh yeh! most of my roads down hear don't twist anyway. Maybe that helps a bunch.

Dave

 

[RaYzerman19]

I don't get much cupping if any either. I agree keeping the pressures up (40 F/42 R) minimizes it. Some tread block patterns are more prone IMHO. Damping I think has a lesser effect, unless perhaps really light (OEM standard of 12 clicks out on an A is very little damping).

 

[Bounce]

Found this: https://www.rattlebars.com/tirewear/index.html#CUPPING

 

[N4HHE]

Found this: https://www.rattlebars.com/tirewear/index.html#CUPPING

Ah! Its On The Internet So It Must Be Correct And True! Not.

Once again, cupping is a suspension issue. Cupped sections from the tread generally only occur in a place or two around the tire and occur spread over tread blocks. Motorcycles are sprung stiff enough that just a little damping is all that is required to prevent cupping. Classically, Buicks are not.

Scalloping occurs on tread blocks, and repeats with the tread pattern.

Feathering is a form of scalloping which leaves sharp edges on tread blocks.