I think I've finally found the answer. Have I got it right?

[fossyant]

That is super low. My Full suspension MTB is 21.5" (42/28 and a 12/36 cassette) and even a 30 x 50 MTB is 16.5 inches. And that's off road climbing ! I'd be more worried about falling off.

My vintage road bikes can only go as low as 41" - I can't fit anything lower unless I change out chain rings/mechs etc but they are 'all in keeping' with their age.

Make sure the mech can handle the varying chain lengths.

 

[Deleted member 121159]

That is super low. My Full suspension MTB is 21.5" (42/28 and a 12/36 cassette) and even a 30 x 50 MTB is 16.5 inches. And that's off road climbing ! I'd be more worried about falling off.

My vintage road bikes can only go as low as 41" - I can't fit anything lower unless I change out chain rings/mechs etc but they are 'all in keeping' with their age.

Make sure the mech can handle the varying chain lengths.

It is super low and I'm excited to have found this hack. But some touring bikes have a 22-24t granny ring and 11-36 at the back, so it's not something unheard of.

Now that I've ordered the components, I realised that I haven't accounted for the chainline. My current setup is GRX 2X so that's 47mm chainline. Senicx PR3 with normal road chainrings gives you 43.5mm chainline. But as I'm fitting an MTB chainring to it, I'm not even sure what the chainline will be.

 

[fossyant]

It is super low and I'm excited to have found this hack. But some touring bikes have a 22-24t granny ring and 11-36 at the back, so it's not something unheard of.

Now that I've ordered the components, I realised that I haven't accounted for the chainline. My current setup is GRX 2X so that's 47mm chainline. Senicx PR3 with normal road chainrings gives you 43.5mm chainline. But as I'm fitting an MTB chainring to it, I'm not even sure what the chainline will be.

We want photos of you riding up a vertical wall !

 

[neil_merseyside]

We want photos of you riding up a vertical wall !

I am really sick to death of all you (not just Fossyant - but seems likely from wall comment? - but hey assumptions are like that...) with the "I can't understand why you don't have the ability to spin the BIG (AKA roadie) gears like I/we/us/(some internet clique) do - so then "you" ask are you climbing trees/walls/sinkholes (delete as appropriate - for your personal (un)funny dig at the less able) " - strangely (to you?) it is a fact people differ in what they can do (or even want to do), we are all different and what is good for you really doesn't really translate to anyone else.

 

[Ajax Bay]

I can't understand why the OP doesn't have the ability to climb anything with a 25" gear unless they are on a fully laden tourer.
Is the OP actually climbing trees/walls/sinkholes (delete as appropriate - for extra laughs)?
What is good for you is a reasonable range of gears of which the rider has need (power/hilly terrain/load).
The OP has said they hanker after an "astonishing sub 17" gear" - I guess so as to get up very steep hills, without stepping off.
Which is fine: we are all different, with a spectrum of views and chat preferences.
May you vomit less and live long, @neil_merseyside - what's your shortest gear length?
(Mine's "an astonishing" 25" but I've got a triple, rim brakes (currently, the dark side beckons), a BSA BB, square taper cranks, spd-sl cleats and don't like wearing them down. A friend has a 12" gear: but then his shoes are Size 11.)

 

[Deleted member 121159]

I can manage most hills with a 25" gear. But it's just damn hard for me. Most of you here are probably older than me so this sounds a bit ridiculous. I seem to get overtaken all the time on the road and I used to blame Schwalbe Marathon (37mm), but the hard reality is that I'm not as fit as most 'serious' cyclists.

From a psychological point of view, my pain threshold must be lower than most athletic cyclists for whatever reason. Things get physically hard, and rather than thinking, 'I'm going to do this', I instead think, 'Huh what's the point of suffering like this', and get off the bike. But it's partly to do with my approach to cycling. I don't do it as a sport but as a way of travelling and seeing the world. So, I try to reduce pain as much as possible. That it keeps me somewhat fit is a bonus.

I had a rather obese colleague to whom I recommended cycling (not for weight loss, but I was just talking to him about how nice it is). Only later did I realise that first, he must easily exceed if not double the weight limit on most aluminium and carbon bikes, and given his power to weight ratio, it must be almost impossible to climb any hill. So think of me as being somewhere between him and a very fit amateur. Does that help to explain why I feel I need a very low gear in order to climb very steep hills relatively comfortably?

 

[fossyant]

I am really sick to death of all you (not just Fossyant - but seems likely from wall comment? - but hey assumptions are like that...) with the "I can't understand why you don't have the ability to spin the BIG (AKA roadie) gears like I/we/us/(some internet clique) do - so then "you" ask are you climbing trees/walls/sinkholes (delete as appropriate - for your personal (un)funny dig at the less able) " - strangely (to you?) it is a fact people differ in what they can do (or even want to do), we are all different and what is good for you really doesn't really translate to anyone else.

It was a joke - I can point you in the direction of a wall - there is a monsterous hill between Prestatyn and Gwaenysgor that's not too far from 'The Wirral' - you might just lake it up with that gear - I avoid it these days, most unpleasant.

 

[Ajax Bay]

Yes, locally, we have two hills out of Sidmouth which I've not managed on a road bike and now wimpishly avoid. One was used as the National Hill Climb Champs in ?'16. I have got up that, but on a 'mountain bike' with (I think) a 22t inner ring and a 28 on the back with 26" wheels so a 20" gear length - not much longer than your 'astonishing' 17" @tripletail52 , the requirement for which you've justified.
For all those significantly heavier than me, I marvel at how much power they must have to generate to get up such hills without going so slow they unbalance. Once you go real slow, the tiniest mistake will result in a fall (clipped in and determined not to put a foot down). Rosedale Chimney zig-zag was touch and go. The last time over Wrynose I had an extra challenge: unpredictable chain suck. Managed a workaround by the time I had to assault Hardknott.

 

[DCBassman]

@tripletail52 you can't go too low on gears, ignore the nay-sayers.

 

[Deleted member 121159]

@tripletail52 you can't go too low on gears, ignore the nay-sayers.

Chainring and chainset finally left China. Still a little worried about front shifting & chainline as my GRX800 derailleur is optimised for 47.5mm chainline + 16-17t gap in the front. There are loads of cheap FDs going everywhere, though, so not a big deal if it doesn't work. Whoever started the 1x trend, I'm very grateful. I love FDs and they are reduced everywhere.

 

[T4tomo]

I will say the crank set up is quite neat if it works, as you get over the "small chainring wont fit my BCD issue" by the centre fit. The OP is also being quite sensible by only having a 10T gap on the front, so they are going to be pushing the boundaries of FD capacity. 36 -11 is that high as a top gear, but its not too bad. My gravel bike runs 38-11-36 and I can spin that up into mid 20's mph on the flats and freewheel down hill if I run out of rpms. OP doesnt sound like they are bothered about top end speed anyway.

chainline is the only thing to watch i guess if its different to GRX.

good luck.

 

[Ajax Bay]

Take it the FD is band on, not braze on. The FD will need to be about 15mm lower and the cage profile will be for a 46t not your Chinese 36t.

 

[Deleted member 121159]

I will say the crank set up is quite neat if it works, as you get over the "small chainring wont fit my BCD issue" by the centre fit. The OP is also being quite sensible by only having a 10T gap on the front, so they are going to be pushing the boundaries of FD capacity. 36 -11 is that high as a top gear, but its not too bad. My gravel bike runs 38-11-36 and I can spin that up into mid 20's mph on the flats and freewheel down hill if I run out of rpms. OP doesnt sound like they are bothered about top end speed anyway.

chainline is the only thing to watch i guess if its different to GRX.

good luck.

This is exactly what I'm on about.

If your bike has a 68mm BSA shell, then you can use square taper BB + one of Spacycles' touring double/triple chainsets to get super low gears. Or install 73mm MTB BB using spacers then use MTB cranks (but then chainline might be an issue).

With a BB86 pressfit frame, you have to use hollowtech style road cranks (or Sram DUB style road cranks). So the choice of chainrings is going to be very limited to what Shimano and other big brands sell. This is why I was excited to discover this option.

Aliexpress is the only place I can see that sells 36-26 direct mount chainrings which give you a reasonable range of gears. My experience with Aliexpress has been pretty good. You can't expect the level of fit and finish of upper end Shimano parts. But if you know what you are looking for, you can bag yourself a bargain. (Having said that, prices have been going up on Aliexpress as well)

 

[Deleted member 121159]

Take it the FD is band on, not braze on. The FD will need to be about 15mm lower and the cage profile will be for a 46t not your Chinese 36t.

Yes, it's band on so easy to lower it. I don't get the second part about the cage profile? I'm hoping that it'll work like what this person has done with a regular FD. 13t difference there, so a bit more than 10t.

 

[Ajax Bay]

Two observations on the image you share, which you can read across to your cunning plan.
This guy has their braze-on FD at its fastening position limit. And the cage is too high (because they can't get down far enough, I assume, not ignorance). There should be a maximum of 1mm between the bottom of the cage and the tip of the large ring teeth. With your band-on, this should not be an issue.
This is a 39t large ring. Your plan is for a 36t. So the curvature of your FD cage (designed for 46t+) will mean it's a long way away (along the chain) from your new small ring (?26t) engagement tooth and therefore changing will not be as 'secure' (it will work).
You may wish to consider a chain catcher.