I have not seen a front disc hub that has a lower spoke count than 24.

I second that. Everything I can think of is 24 front and rear, with 2-cross lacing all around. Typically on j-bend hubs, too.

I managed to get some dt swiss 350's in 20 for a center lock front hub.

Good luck finding much if anything under 24.

All of Roval's current generation Carbon Road Disc Brake rims (32,50,64mm) have 21 spoke Fronts and 24 spoke rears with Straight Pull DT Aerolites

21 spoke fronts?
How does that work?

More spokes on one side of the wheel?
If yes, that seems like a first for a front wheel !

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Yes...2:1 Lacing...All Roval rims utilize this technique

I thought the same thing, but it looks like he's right: https://rovalcomponents.com/...heels/products/cl-50

Gotcha, now I follow. Kind of a wild pattern for a front wheel.

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I haven't seen anything less than 24h. IMO a 24h disc front wheel is pushing the envelope of how low you can go. That's the same as rear wheels. Rear wheels generally support a lot more weight on road bikes, but not so much on TT and Tri bikes. Rear wheels also need to transfer torque (whether they are disc or not), but this torque is never as high as what can be experienced in extreme braking.

Maybe if I'm bored I'll do the calculations later (or look them up, I know I've done them in the past), but I know the effect of braking torque on spoke tension and rim/hub stress can be quite high. I think that most designers are assuming that the average person will rarely if ever experience an extreme braking event, so they don't really need to design a margin for that.

The Roval approach isn't that novel. 2:1 lacing has been used for decades on some rear wheels to help compensate for highly uneven spoke tension due to dish. Disc front hubs have lots of dish now as well, so it makes some sense. On a disc hub this also allows you to make one of the hub flanges very small and take it out of the torque picture altogether. And if you make the other flange very large you can mitigate the amount that spoke tension needs to change to deal with torque.

The bad thing about 2:1 lacing is that if you break a spoke on the "1" side the wheel will usually warp so badly it becomes jammed in the frame or fork. That's why I quit building wheels that way.

Rolf still makes (nice) wheels and they're (generally) 16 spokes

My mountain bike runs 24 spoke front and rear with zero problems.
With carbon rims there is far less deviation when a spoke breaks, but I don't see why I should be worrying about spoke breakage, just spec and build the wheel well.

Close. Rolf rim wheels are 16spoke, but Disc wheels are still 20 spokes (which is still lower than anything else I've seen).

https://www.corima.com/carbon-wheels/road/disc-brake.html?isorc=1

technically 2 Spokes would be the least amount of spokes you could get on a disc brake front wheel

http://www.2-spoke.com/product/cfd-clincher-front-disc/

Yup, good point.

And such a failure on a rear wheel would not be fun.
But such a failure on a front wheel?
Yikes.
That would probably be life changing, and not in a good way.

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I've never actually experienced it, but there were some who did. I performed some experiments where I built a wheel and loosened a spoke, and it was bad. As was mentioned earlier, a stiff rim will help though.

I have been involved with rim development for Roval for a few years now, and this is not something I have seen...Sure it can happen but EVERY single Specialized SWorks and Pro level Tarmac, Venge, Roubaix, etc...have been spec'd with these 21H Front Disc wheels (CL and CLX levels) This is not a brag, but that is 10's of thousands of bikes.

Spokes can break (or pull thru) but that does not result in catastrophic failure generally speaking. The 2:1 lacing is a very robust way to build a wheel and we tested it heavily to validate our decision.

Good to know that the wheels are reliable.

But I was not saying that the wheel would just buckle if a spoke broke.

I was more talking about if losing a spoke on the radially-laced 7-spoke side would cause the rim/tire unit to deform to such a degree as to hit the fork blade. Which I imagine would probably lock up the front wheel. Does that happen? Or, would that be possible?

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There are a lot of variables to consider in this hypothetical scenario but generally speaking this is not an issue to be concerned about...of course designing a rim for low spoke count is key...I would not say that you can take the same exact rim and drill it any way possible. You need to consider spoke count upfront.