I agree with getting a custom. Maybe do one at a time, wife's bike first. Doing this conversion is a pretty radical modification on these bikes and a bit of an experiment. You may find that you don't like the result. At least with the new bikes, it's a known quantity

The stays on my bike aren't 853. Apart from the colour, the thing that worries me about the Norco is the press-fit bottom bracket.

From what I can see as the cost of a custom frame locally (at the more affordable end) it would be getting toward double the cost to get a custom frame from scratch.

We've pretty much abandoned the idea of modifying my wife's bike as it's not worth it. Might look at other frames such as the Curve GXR steel or maybe a Shand Stoater if she wants a particular colour! Cost wise would still be less than a custom frame. Mine I'll think about for a while. I still enjoy the ride as is, and there's always the risk the modification would stuff it up. Although I do find the lack of braking power for emergency situation a bit of a worry in traffic, particularly now I'm used to disc brakes on the mtb. Perhaps for now I might look at trialling v-brakes with a travel agent and see how that goes.

I just realized how I'd left this thread and thought I'd update it. We found a great frame builder in the end and have gone ahead and had my wife's bike modified. Well, it's more a case of a new bike with a modified/re-purposed frame. The wife is very happy with the result. Except now the bike lives in the house, can't possibly go in the shed!!!! The builder managed to keep the existing geometry. There was one minor tweak made in that the new fork was made with a custom crown which enabled better tyre clearance without altering the geometry. Enables a 35mm tyre where previously could only fit a 28 or perhaps 30mm but only just. Both the fork and the rear drop-outs are now thru-axle and the build included GRX-800 2 X componentry and hand-build wheels.
My bike is in getting worked on now. The frame modifications and new fork are done and it will soon go off for painting. Mine was a little more complicated apparently as the seat stays needed to be removed and modified to make room for the disc caliper...

Thanks for the follow up. Andy

I'm certainly not a professional framebuilder, but this sounds like simple BS (or maybe fear, or just up-selling) to me.

What difference does the steerer size make in how the braking forces are applied to the wheel? Unless the disk allows for much higher braking forces (only if your rim brakes are so weak they can't lockup the wheel), then the forces at the steerer are the same regardless of brake type.

Steerers flex quite a bit. Disc brakes are going to make them flex more. That's why the mountain bike industry has gone to tapered steerers. If it's you that's potentially getting sued, you can sell whatever ill-advised designs you like. Good luck finding anyone involved in the industry to testify that it was a good idea, nobody thinks that. I noticed that Crust, the last bastion of 1" disc brake forks, is now selling a bike with a tapered steerer.

1” is fine and better than 1 1/8 since mundane and hard impacts with a 1” steer tube will not crumple the down and top tubes like a 1 1/8 can with a steel steer tube. 1 1/8 steel steer tubes focus too much impact energy into the front end of a bike. There is history on this that happened with the .883 steer tubes Schwinn went to back in the day and that was a beefed up 1” steer tube. Second if you want to convert to through axle a bit of creative manufacturing can convert existing drop outs to be through axle without drop out replacement.

pwyg- If you think using a 1" steerer will prevent main frame damage in a frontal impact I think you will find yourself wrong. The Schwinns used a .833" stem, not steerer. As much of what we know of the US made Schwinn product design was based on left over WWII tooling (some paid for by the government) and by the grades of steels commonly available back then. Schwinn did do a good thing and choose a steerer with a thicker wall to help off set the low strength grade of steels they usually used.

As to converting horizontally slotted drop outs to a through axle without replacing the drop outs- Sure one could. The adaptor/conversion part would likely be specific to each brand and model of drop out and also need to accommodate the various stay attachment shapes and such. Fine to one off if your time is not expensive. Please keep us in the loop of your efforts on this. Most here enjoy seeing others' cool creations, even if some will also shake their heads at the results:) Andy

If we are going to talk about history of steerers, people used to be so afraid their steerers would break that they put broom handles inside. I assume there was some reason for that. I'd rather this not turn into another bad advice thread, particularly since the OP already got what he wanted.

Sorry, but how exactly does a disk brake make the steerer flex more? Unless your saying that disk brakes have more stopping power than rim brakes?

Flex the fork blades more? Definitely.
More flex at the crown? Sure, since the forces have to go thru the crown with a disk setup, and are applied directly to the crown in the case of a caliper brake. But, the steer tube and headset are above that.

A tapered steerer is definitely stronger/stiffer. And, the bearing should last longer being larger. If you're building a new bike, for sure use the latest/best. But, I just don't see a larger steerer being a necessity brought on by disk brakes.

Glad the OP got what he wanted.

Where do you think the forces go? They definitely apply a bending moment to the steerer. That's why there is an upper headset bearing. The lower headset bearing will not support a moment, it takes both upper and lower headset bearings to do that. A rim brake is generating a force right there at the lower headset bearing, and the moment arm is much shorter. Draw a free body diagram.

When the wheel is locked up (skidding) we can look at the fork and wheel as one rigid body applying a moment to the steerer. The force is applied at the ground and the lever is the length from the ground to the lower headset race. This is the same for rim and disk brakes. The force on the blades is much higher with disk as we know, but the distance from the axle to the disk attach point is so much shorter that we get the same moment up at the steerer.

I'm not advocating for smaller steerers, just saying the brake type doesn't matter. A larger steerer is equally sensible for any type of brake, if stiffer is what you want.

Oh and of course this analysis assumes the rim brake can lock up the front wheel, not always true. But at any given level of braking, steerer flex will be the same. So a stiffer steerer makes sense to whatever degree the disk brake allows stronger braking. If your rim can lock up the front wheel (or eject you over the bars), then the steerer doesn't care which brake. I locked up my front wheel countless times on my old MTB with 1" steerer, but dirt gives lower grip, so the force was not so high. It's difficult but still do-able on pavement, with some fairly extreme body english to not endo. Or on a tandem, which can never endo as far as I know.

Seems to me, tapered steerers for road bikes were only to allow low-density materials (e.g. aluminum) and/or straighter paths for the fibers in CFRP, from steerer to blades at the crown race. I can't imagine a use case for a 1-1/2" steerer in steel, other than triplet tandem maybe. The triplet we made at Santana in the '70s rode and braked OK with a 1" steerer, but I shudder at the thought (no pun intended) — what were they thinking? "OS" steerers definitely existed back then (and back to the 1800s) but Santana was too lazy to tool up for it, until a few years later when 1-1/4" headsets came out.

Mark B

I don't think that tapered steerers are particularly useful on road bikes. But they have taken over on everything but steel. And it seems like the 1 1/4 taper is less common than 1 1/2. With a steel fork, I see no reason to use a tapered head tube. People like to have options though, and there are a lot more carbon options with tapered head tubes.