Hill climb chainring/crank setup

So admittedly, I’m in over my head on this one, so please take it easy on me.

I’ve signed up for a ridiculous hill climb that will require to me put together a one-time bike setup in order to make it more manageable for my fitness. The hill climb in question is the Mt Washington Auto Road. From the site…

The Mt. Washington Auto Road Bicycle Hillclimb is known as the toughest hillclimb in the world at 7.6 miles in length, has an average grade of 12% with extended sections of 18% and the last 50 yards is an amazing 22%!

The bike I’ve got has:

  • Ultegra di2 11 speed
  • 52-36 chainrings
  • 11-30 cassette
  • Medium cage derailleur (shimano says it’s cable of a 34, shop says they have put a 36 on this type of setup without issue.

So, if I put the 11-36 cassette on that will give me 1:1, but ideally I’d like to go lower as I’m not a small man.

My understanding is that my setup is compatible with a 34 tooth chainring that I could slap on, but that is the limit of the changes I can make and have a working di2 setup. It does get me a <1 gear ratio though with that 36 tooth cog.

All that said, I don’t believe I need a 100% working di2 setup. I just need the rear derailleur to work. The front will stay in the small ring for the duration. So my idea is this…

Is it possible to slap a mtn bike crankset on my bike just for this ride? That would give me access to a wider variety of chainring sizes, and depending on that choice I wouldn’t have to modify the rear at all. My thought is that the rear shifting would still work like a champ, and that’s all I need.

Does this idea have merit, or am I way off base here?

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Sounds like a fun challenge!

Typical mountain bike cranksets are too wide for your bottom bracket, so that won’t work. However, there are plenty of 1x cranksets for gravel bikes that will work. If you can find one that takes direct mount chain rings, you should be good. There should be quite a few inexpensive options on Aliexpress that will work well enough for a singly event.

Do you have a Shimano crankset currently? If so, you will need one with a 24 mm spindle diameter.

Non-electronic Shimano road derailleurs can handle much larger cassettes with a WolfTooth Roadlink or similar, but I don’t know if there are any similar options for the Di2 derailleurs.

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Now that I think more about it, a Shimano GRX 2x or similar crank would work if you remove the front derailleur. That will also give you the option of manually moving the chain to the big ring if needed.

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Shimano MTN cranksets are supplied with three 2.5mm spacers which allows them to work with both 73mm (one spacer on DS) and 68mm (two spacers on DS, one spacer on NDS). Most bike shops have many spares.

The chainline will be offset outboard and this will cause shifting problems, a chain guide / keeper could help.

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As luck would have it, a buddy of mine is willing to lend me is grx 2x crankset with a 31t small ring, so I think that’ll do it for me. I’ll put a 11-34 in the back, and I should be all set.

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I just talked to a shop guy about this setup (mtn crankset), but he didn’t mention anything about shifting problems. Good to know.

I may be wrong but I think the spacing on a GRX crankset different to a regular road Shimano crankset which necessitates the use of a GRX front mech.

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I’m not planning on using any front mech. Ideally the front mech that I have can just sit there and not interfere. If I have to remove it, that’s fine too.

The course is consistently steep enough that I’m happy to just let it sit in the little ring for the duration. There is no part of the course that even comes close to flat.

Do you see any issues with that?

I rode Great Dun Fell (in the Lake District) last year; that’s somewhere between 8-9km (depending on exactly where you start) with an average gradient of somewhere between 8-9% (ditto). It touches 20% in a few places.

I rode up that with a 46/33 and 10-33 AXS setup. It was very tough but manageable. I’m just over 70kg (155lb) and sit at c.4w/kg FTP.

You’re right I think to want to go below 1:1. 31-34 is probably the minimum I’d consider for this climb, personally. It’s always better to have a gear you don’t use than need a gear you don’t have.

As far as possible, you want gearing that won’t force you too far into the red on the steeper sections, or force you to adopt a very low cadence (<60rpm). In an ideal world, you’d want ratios that would let you hold near steady power all the way up, but that’s not realistic for most of us when the gradient gets well north of 10% for prolonged periods.

A final, unsolicited piece of advice: don’t forget the descent. I don’t know Mount Washington, but coming down GDF, especially the hairpins at the top (narrow road, howling wind, no safety barriers, sheer drop 6ft off the road side) was terrifying. Make absolutely sure your brakes are in order and practice your technique.

Good luck!

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Oh in that case it should be fine. I’d keep the front mech on to act as a chain catcher just in case.

If you wanted bigger than 11-34 at the back, I’ve made a 11-36 and 9-39 work absolutely fine with a long cage Ultegra rear mech and with no range extender.

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I’ve got a medium cage rd so I’ll probably just stick with a 34 in back (largest it’ll take). I’ve heard rumors that a 36 will work, but so far, nothing reliable.

You can absolutely use a 36t in the rear….I have an Ultegra med cage rear on my gravel bike and run a. 36 w/ no issues.

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My mistake, I should have said med cage, not long cage.

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I see Joe_Totale put an 11-36 on without a range extender. Do you use one on your gravel bike, or is it standard setup?

No extender when I use a 36 rear cog. I did put on an extender last year for The Rift in Iceland and I ran a 40t rear cog.

But if you have a med cage derailleur, it can accommodate a 36t w/o an adapter. it is technically outside of Shimano’s spec, but it works.

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I’ve run a SRAM 11-36 on a 105 long cage (max rating 34) before without problems. Do check chain length, though; it won’t work if you’re already running a chain that’s a link or 2 short.

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Be careful if you swap in a GRX chainset. As I understand it, the position of the rings is offset to the drive side by a few mm to accommodate wider tyres.

Your original front mech may have issues with this even if you don’t plan to shift with it.

Also, there could be issues with chain line in the largest sprockets. It might be an idea to try taking off some of the smaller sprockets and using spacers to push your larger sprockets outboard. You’ll have to leave the 11 as the lock ring has to oppose that and also to adjust the RD limit screws. It would however minimise friction loss to have the sprockets you’ll spend the most time in aligned with the GRX 31 tooth ring.

Thoughts anyone?

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An adjustement of the FD low limit screw could counter this issue?

The sprockets would be in the same position as they would be on a standard GRX gravel bike so it shouldn’t be an issue?

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I’ll likely let the local shop do the work, so I’ll ask them about it.

Thanks for the feedback. The good news is that we aren’t allowed to ride down. Frankly, it would be terrifying on the best of days. It’s not a friendly road.