Cracking and Chunking Pirelli MT21 Tires

I just returned from a 1300 mile trip on my KTM 690 Enduro with a set of Pirelli MT21 tires. They were brand new before the trip. One of my riding buddies also had a 690 with MT21's that had 200 miles on them prior to the trip.

We obviously did a fair amount of street riding at 80+ mph but also some pretty aggressive dirt that was rocky and steep. After an inspection half way through we both found that there were chunks missing from many of the center rear knobs and some were missing completely. Also many had cracks on the base of the knob on the leading edge.

From what I've read the MT21 is a very well respected tire and perhaps we're not using it was designed to be used. Does that seem right to y'all? And if it isn't right, what would you recommend as a replacement?

the 690 is hard on tires (and chains) I am not sure if there is a knobby tire out there that holds up to it without compromises in traction or will last very long.

I haven’t had experience with the MT21’s but currently have the stock TKC80 on my 701... Just did a 1200 mile trip up through the OAT and some TAT into Arkansas and they seemed to perform ok.. Lots of pavement as well... Last year I had the Shinko 804/805 on my KLR for a 3000 mile trip up to and along the CDT and really liked those tires. Another guy had those tires on his Tiger 800 (lots of power) in the same trip and he loved the way they performed... I will be replacing my TKCs with the Shinkos soon!


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Doesn't sound right unless they were ran under inflated.

I've seen that when riding in really rocky terrain, sharp edged rocks rather than the smooth rounded rocks often seen in creek beds. The simple fact is that rocks are hard on tires. I have had that happen with the D606s and the MT21s. I ran the D606s on my KLR 650. Most of the time there were just cuts and pieces of individual knobs missing. Occasionally, a few knobs had cracks near the base or were just gone. The MT21s are used on my KTM 530 EXC, same thing. I saw a Terra-Flex get chewed up pretty good on a KTM 625 in one day of riding in the Arizona desert near the Gila River. It was brand new at the start of the ride and had maybe 100 miles on it at the end of the ride...

Terror Flex--Quite possibly the worst knobby currently made for a dirt bike. Wait, I have had time to think about it and it IS by far the worst.

TKC--Great 50/50 tire if you are only doing light dirt. Not a serious dirt tire because of the very flimsy sidewalls but the fronts last a long time. Rears are useless for me and what/how i ride (actually so are the fronts).

MT21: You got 1300 miles out of an MT 21? My best for a front is maybe 800 miles on my 950 Adventure. I have run several and they are my favorite true dirt DOT knobby by a long shot. There are many variables on why yours chunked knobs. Tire pressure, speed, temperature etc. If you were riding 80 mph on road in warm temps at less than 32 psi then of course it will chunk knobs. At the end of the day it is really a 90% dirt tire that is DOT legal. You have to pay to play on those types of tires. Dirt performance vs. longevity. I will take the dirt performance every time.

I heard something once about breaking in your tires so they don't chunk. Who's got time for that.

Ok, did all those things. Rode in very rocky areas with sharp edges and rode 80+ in high temperatures at 30psi.

Heard a recommendation to switch to a Metzeler MC 360 in the rear and Dunlop D606 in the front. Any thoughts on this combination?

copb8 said:

Ok, did all those things. Rode in very rocky areas with sharp edges and rode 80+ in high temperatures at 30psi.

Heard a recommendation to switch to a Metzeler MC 360 in the rear and Dunlop D606 in the front. Any thoughts on this combination?

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You don't need 30 psi in any tire on a 690 unless you like having your fillings rattled out...
I've had 3 different 690's, all of which came with MT 21's and never had any chunking or cracking.

15 -18 psi in rear

20-22 in front


usually worn out in about 1500 miles of mixed pavement and off road

skinny said:

You don't need 30 psi in any tire on a 690 unless you like having your fillings rattled out...
I've had 3 different 690's, all of which came with MT 21's and never had any chunking or cracking.

15 -18 psi in rear

20-22 in front


usually worn out in about 1500 miles of mixed pavement and off road

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I did 30 based on the KTM sticker but you're saying that's not necessary if you're doing a lot of higher speed street miles?

copb8 said:

I did 30 based on the KTM sticker but you're saying that's not necessary if you're doing a lot of higher speed street miles?

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Only way I would run 30 PSI is if you had it loaded pretty heavy
I run 25psi on XChallenge and I am a big boy

My 32 PSI comment was from the perspective of a KTM 950, so not correct for you 690. If I was running 80 mph for extended distance on the 690 I would run a MT21 at the max tire pressure KTM recommends.

Big fan of the MT21s on my XChallenge. I've worn out several sets including some hard off road in Big Bend and California. I've never had a set chunk. My guess is that they were either old when you bought them or the inflation was wrong. I run 20lbs in mine unless I forget to check, then I may run them as low as single digits. There is no telling how long they sat in the warehouse before selling.

copb8 said:

I did 30 based on the KTM sticker but you're saying that's not necessary if you're doing a lot of higher speed street miles?

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30 psi for nothing but pavement would probably help extend the mileage,
but for off-road, especially in rocks, the carcass has no flex so the knobs get ripped off or chunked.


For the weight of a 690, at 30 psi it must ride like the tires are full of concrete...

As a follow, I mounted another MT21 tire and will run it at 22 psi and see what it does. My friend with an identical bike and who had the same issues as I did on the trip is mounting a Metzeler (can't recall which one) on the back and we'll see how they compare.