Most MTB cranks are wider than their road counter parts, at least for 24 mm spindles. In fact BB are a different sub standard. BB 86, 9i, 92. Not sure about bb30.

I would Google SRAM technical manual frame compatibility to find a good SRAM technical doc that would clarify the doubts.

Do you have an alloy hollowgram crank that you could put a dm ring on?

https://absoluteblack.cc/xx1-can

Praxis has a road crank that will run a 32 chainring. Couple that with a long cage derailleur and an 11x34 cassette. Ping them - they have great customer service.

The issue you’re going to run into is BB shell width and spindle width. The swap won’t work without an absurd number of spacers and an unusable chain line.

The bigger problem here is that if you are indeed running 35 rpm in a 39/28 at a sustainable effort, you'll need a different cassette *and* crank to get anywhere near a regular cadence.

39x28@35 rpm ~= 24x34@70 rpm ~= 22x40@90rpm

There are plenty of 46/30T road crank options these days. For a short-term use situation, this is one of the cheaper options:

https://shop.fullspeedahead.com/.../omega-crankset-3822

What rear derailleur are you running? Many of the newer short cages are rated to run cassettes up to 30 or 32T. With a Wolf Tooth Road Link, you may be able to run a 34T, and possibly even a 36T, as long as you're careful about cross-chaining big-big.

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"They're made of latex, not nitroglycerin"

Just borrow a mountain bike.
Put some 28mm conti tyres on it and go for it.
24 x 42 should get you up the hill.

Looking at this pic looks like things would line up just about perfectly to me. Haven't actually tried the swap yet but measurements seem to line up fine.

https://photos.app.goo.gl/XoELYH5duns0y4S73

Appreciate the run down of other options. I am familiar with them and bought a roadlink have a long cage MTB Rd etc. Just seemed like way more work for a similar max gear. I don't need access to the full range of gears just one or two maybe three gears.

Is the route important to you? Personal significance?

Otherwise I’d pick a hill without a wall to Everest. Probably a boring but steady 4 to 6 percent climb. You don’t want to have to mentally or physically handle shifting all the time in that challenge. I wouldn’t add a mechanical difficulty.

Just me.

Hill isn't an option. Doing it with upward of 20 people for charity.

http://www.adventuresforthecure.com/campaigns

jbank wrote:

Appreciate the run down of other options. I am familiar with them and bought a roadlink have a long cage MTB Rd etc. Just seemed like way more work for a similar max gear. I don't need access to the full range of gears just one or two maybe three gears.

I don't get why a 1 x mountain bike crank is any easier to swap than a 2x road bike crank ¯\_(ツ)_/¯

What's your plan for the FD with a 1x setup? If you were only going to use the small ring of a 2x setup, you wouldn't even have to faf with the front derailleur other than to maybe fiddle with the low limit. And if you're going to be spending the majority of your time in the biggest cog, and all your time in the three biggest cogs, the chain line will be better anyway.

Now, if you decide a 30T front wouldn't work without changing the rear cassette, but a 28T would, then sure. But I suspect you're eventually going come to the conclusion that you want a bigger cog in the back either way.

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"They're made of latex, not nitroglycerin"

The 1x MTB crank would mean changing one thing. Just the crank (well, and chain). If I need more gear, I can swap from a 28T to as low as 24T (I think based on what RaceFace direct mount shows online). I think 28x28 will be plenty for the wall, will get me to a reasonable standing cadence and then can seated spin the rest, but if test rides show it doesn't could go to that 24x28. The other suggestion involves changing the crank, the RD, the cassette (and chain) and maybe the roadlink, for the same effective gear range during the event.

Hadn't really thought about the FD with the 1x, figured it wouldn't be a problem to leave it on, but maybe I'm missing something there.

 " I think 28x28 will be plenty for the wall, will get me to a reasonable standing cadence and then can seated spin the rest"

Do you really want to climb this section 100 times where you have to stand plus all the other times you will need to stand during what I am guessing will be a 12+ hour ride? I think you should really consider taking the advice of the poster above and just run a MTB with slick tires.

rich

A BB30 mt bike crank should work in this instance if you have one available. If the spindle is a little wider, you would put some flat 30mm washers/spacers on each side until it's centered and snug. You will have to figure out what to do with your FD since it will be in the way. It wasn't designed to work with a chainring as small as a 28 or 30, so even routing the chain through the cage is probably not an option. I'm fairly sure your bike has a tab to hold the FD, so it couldn't be raised up the seat tube enough to get it out of the way (so you don't have to remove cables, housing, etc.) If the FD is held on by a band going around the seat tube, you may be able to raise it up enough to get out of the way after loosening the cable.

The chainline will move over from the center a few mm, but probably not enough to matter a ton.

Cyclenutnz was correct in that if you have a SRAM road crank with a removable spider, you can add a direct mount mountain bike chainring like the one Wolf Tooth sells.

You will also likely have a chain that's too long if your only front ring is a 30 (or 28). Knocking out a few links out to do it, but you couldn't use that same chain when it was time to get back to your 53/39.

Which rear derailleur is it? Most short cage Shimano and SRAM RD's of recent years can handle a 32 tooth rear cassette in a 1x configuration without any mods (except adjusting the b screw for clearance). It depends on the actual bike (specifically the derailleur hanger), but it's likely.

Thanks for the constructive and detailed response. My RD is a few year old SRAM Red short cage. I got a WolfTooth RoadLink and confirmed that I could mount up a 36T rear (pulled from my old F29er that I would get the MTB crank from). I did some reps on the target hill today with a 39/36 max gear combo. I think it would be doable that way, although perhaps not ideal, would be nice to have a bit more margin to go even easier. Of course at this point, now that I've got a 36 on the back, a regular road compact on the front would provide at least as much gearing as the MTB crank + 28 in back. I am still curious if I can make it work though because I have a number of other friends who are looking for ways to setup their bikes for the same event (and have lower power #s, meaning a bit more gear would be very helpful).

The main thing you'll start to have issues with is since you will need a bit more chain to make such a wide range system work, you will need a longer cage rear derailleur to manage all that chain. If you commit to not using the big chainring, then it doesn't matter. A zip tie on the front shifter could help with that commitment.

I think the mt bike crank would be fun though if it is parts you already have. A 30 or 28 up front with a 28, 32, 36 in the back would give you a few options.

Heck. If you go from a 39T down to something like a 28T, you MIGHT get lucky and be able to leave the FD right where it is and route the chain under it.

What about finding a cheap sram crankset with removable spider and putting a new (tiny) direct mount ring on that?

I kinda think the MTB crank would be fun too! Seeing a 36 on the back of my Evo is pretty funny. Makes the rear wheel look like a 650. It is sitting next to a MTB right now with a 50T Eagle on back, so next to that it still looks puny. I'd kinda get a kick out of seeing a 28T single chain ring on the front, and as you say a 28 or 30 would give a whole lotta bail out options.