I'm doing Dirty Kanza 200 for the first time this year. To date, my gravel experiences have tended toward drier events where crashing was the thing to watch out for rather than mechanicals. 

(I've torn off a rear derailleur once in cyclocross when I crashed, bent my hanger, and then shifted into my spokes. Not worried about that situation at DK.)

Now I'm looking at how many people DNF DK by tearing off their RD and I want to be sure I'm clear on what happens. My understanding:  1) Mud packs up on the tire 2) The mud gets scraped off by the chainstays and plops onto the chain 3) Chain picks up the mud 4) Chain drop at front or 5) Gums up jockey wheels which stop turning and SNAP.

Assuming that's correct, it seems that tire clearance in sticky mud conditions is paramount. I currently run G-Ones at 40.5mm which give me about 5mm clearance at chainstays, but that seems inadequate if DK is really muddy. So I'm thinking I'm going to want something more like 35mm if it's a mud day. 

Am I right about RD failure causes? Any thoughts on good tire choice for a muddy DK?

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Truth in advertising - I have no personal experience riding in extreme mud, but I'm an avid reader about bike tech. You may want to consider tires designed specifically for riding in very muddy conditions. These typically use special rubber compounds and somewhat wide lug spacing to improve shedding of mud. Here is a review of an example - the Panaracer Mud:

You may also want to consider the Compass Steila Comp designed for muddy CX conditions. The Steila Comp is manufactured by Panaracer using a supple tubular carcass. Although it is rated tubeless compatible, I've found that it takes a lot of TLC to get Compass tires to seal up. If you go with this tire I would recommend the "standard" casing both for durability and for tubeless mounting: 

Our brethren in the CX world frequently have to deal with extreme mud. Last week the UCI CX World Championships were held in very muddy conditions in the Netherlands. Wout Van Aert was an upset winner of the race. One of the tricks he used in preparing his bike was to spray it down with non-stick baking spray. It apparently worked. I kid you not:

You may also note that Van Aert switched to somewhat narrower tires to help deal with the mud -- addressing your concern about tire clearance in very muddy conditions. For DK, you may want to have 2 wheel sets on hand, one for mud , the other for dry dusty conditions. Apparently, both are possible.
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You’ll want at least 5mm on each side of your tire. You can get away with that if it’s dry, no problem. Tire choice at DK is always a big question - keep an eye on the weather, consider a cyclocross mud tire. And never ever try to ride the peanut butter mud.

I had a mud tire on my rear in 2015, and it was just fine (I also ran a clip on rear fender). Last year I ran the G-One and it rained for four hours, which meant riding in the (non-peanut butter) mud. And that was just fine too.
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There might be 3-5ish miles out of 207ish where it could be a mud bog, and no tire will keep you moving in it.

A skinnier tire could help for that hike-a-bike stretch, so if you have two wheelsets, follow bobknh's advice and put a skinnier tire ~30-35 on your backup wheelset. As of 2017, you had to start and finish on the same frame. I interpreted that to mean you could switch out wheelsets at the checkpoints.

If you don't have a second wheelset, assuming approximately 98% of the course probably won't be a mud bog, I'd run whatever tire you prefer. If you aren't riding to place, I wouldn't run a tire skinnier than 35. Even if you are riding to place, 32-35-38 is what the winners/placers roll mud years and dry years alike.

In 2015, I did the 3-mile hike-a-bike, and the people who moved the fastest through it:
(1) picked their bikes up and carried them before the mud packed in,
(2) were wearing Camelbaks (no bottles to weigh down their bikes and fill up the triangle space),
(3) had small pads on their top tubes (so they weren't having to constantly stop and readjust) to carry the bike (cyclocross style) and protect their shoulders ,
(4) had a stick to clean out the mud,
(5) and were running 32-35 tires so they had lots of clearance.

If it's looking in the weeks leading up to DK like there will be a hike-a-bike mud bog, train to carry your bike on your shoulders, and figure out what makes it a more bearable experience for you. If 15 lbs. of mud accumulates on each of your feet and it's slippery as snot to walk, and you've got a 25-35 lbs. bike with all of your nutrition and fluids and tools and tubes, what will make the experience go faster and smoother?

If your bike gets packed with mud, and your pedals/crank catch for any reason while pedaling, DO NOT force it. Get off and free it up without pedaling. If you do force it, you're probably going to lose your chain and/or rear derailleur.

You'll be waiting a LONG TIME if you call in a bailout ride because even the Jeepers didn't want to drive into some of the mud in 2015. You'll probably have to walk some distance to be picked up anyways, since the muddy sections had everyone walking pretty much the entire width of the ~50-foot road right-of-way, and Jeepers couldn't safely drive through all of the people.

I would estimate 99.9999% of people who snap off their derailleur have obvious mechanical warning signs before it happens.
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This is super helpful, thanks! I keep thinking that I've thought of every eventuality, but I hadn't given any thought to how fun it'd be to carry my bike with a frame pack on there. I prefer to store extra water in a frame pack rather than a backpack if it's really hot, but if it's cool and wet the backpack could be the better way to go. 

I've got a few sets of cx tires in the 33mm +/- range, but I'm not sure any of them would be great for flint hills--they're optimized more for low pressure and supple sidewalls instead of puncture/slice resistance. I'll travel to KS with two sets of tubeless wheels & tires, so for the narrower mud tires I'm thinking maybe the Vittoria Terreno Wet Gravel Tire 33 or the Panaracer Gravel King Mud 33. I'm not wedded to a mud-focused tire though. Durability and clearance are more important to me. I'll be mounting my "wet-day" tires on HED Belgium+ rims, which measure 25mm external--so my tires will likely net out ~2 mm wider than their nominal width.

So, any recommendations for a narrow 33-ish tubeless tire for a wet day?

Thanks again, y'all.
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This is just me, but since 96+% (at the worst) won't be the peanut butter mud, I'd run whatever is the most comfortable/durable/fast-rolling tire that you prefer. Since you have to carry the bike through the peanut butter mud no matter what, trying to get a mud tire isn't worth my while. At most, just get a slightly skinnier gravel tire (i.e. 35-38 instead of 40).

Just remember to pick up your bike and carry it immediately as soon as you see big herds of people walking. If you do that, you'll be fast/comfortable/more reliable on the 96+% of the Flint Hills gravel that isn't peanut butter mud, and you'll have several dollars to spend on other bike components/beer/celebrating.
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Land Run is a different story. If you're doing it, the investment in mud tires is worth it, but if they're just for DK, I wouldn't.
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Also remember that if you destroy your derailleur you can always set up your bike single speed and at least finish the race. Easiest done if you have a chain breaker and spare link, but that’s not the only way to do it. Easiest for me just to say “google it”. It’s useful knowledge, and I’m surprised by how few people don’t know how to do this (I am in no as suggesting you don’t).
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RE SS conversion--I'm actually going to carry not only a spare derailleur hanger but also a cheap chain tensioner I could slap on there if need be. And I'll have a spare rear derailleur in my SAG car. The tensioners only weigh a couple ounces and cost about $25. Seems like cheap insurance to me, and a lot easier than trying to find a SS gear combination that doesn't keep dropping. I did see a pic from a past DK of somebody fabricating a "tensioner" by tying a rubber spatula (former mud scraper I assume) to the seat stay with an innertube... desperation can breed amazing ingenuity.
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SeaPaul wrote:
so for the narrower mud tires I'm thinking maybe the Vittoria Terreno Wet Gravel Tire 33 or the Panaracer Gravel King Mud 33.

This is my exact combination (albeit in the wider widths) that I purchased for LandRun.  Unfortunately I have not been able to test either of them out yet (winter in the northeast) but if I get some miles on them I'll report back.  I'm thinking the knob spacing and depth will provide better mud traction than other patterns, while not sacrificing too much speed on the hardpack or pavement.  
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Well, FWIW I settled on Gravel King 35's. Because gum walls are sexy. Now I just have to wait a month or two for Toronto's winter to abate... thanks again for all the help, all!
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I was riding in the 2015 DK200 and witnessed the carnage first hand. The OP is correct in how it happens Secondly, this situation was caused by people trying to ride through the 3 mile dirt road section which was pretty much clay mud. Third- Even the scrapings people were leaving on the road after cleaning their bikes once they reached gravel was still sticky enough to latch on to tires and rip off derailleurs. No tire you choose will be an effective defense against this mud, if it happens. Finally- WALK, don't ride- your bike if you see a muddy dirt road at the DK200! [smile]
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