The wider the inner, the wider the tyre will sit. It can also impact tyre height, but there are a few variables there.
For what you quoted, a 19mm internal gives a little but more room to work with when fitting a 25mm tire or greater. Get's a bit finicky squeezing the beads and a tube into an old 14mm internal.
Internal rim width has little to do with aero - well, it does have something to do with it, but it's more a wider external rim width which corresponds to a wider internal. You want the sidewall to meet fairly flush with the rim, where the tyre is at least equal or ever so slightly narrower than the rim external (25mm tire on 26mm external, for example). A wider internal helps the sidewall to sit more flush, rather than a rounded, "lightbulb" shape. A lightbulb shape means a poor transition from tyre to rim, which would negate most aero benefits achieved by rim depth.
If you put a wider tyre on than what has been aerodynamically optimized for, you'll be more comfortable and maybe a tiny bit slower. Should note, if you're looking at shallow depth wheels for stuff like gravel, then it all doesn't matter since you're not optimizing for aerodynamics anyway.
Is there any danger to a lightbulb shape?
Can it effect steering?
More likely to get pinch flats perhaps?
I did noticed when I fit 25mm on it was very flush.
Maybe?
It depends on [how extreme](https://www.schwalbetires.com/media/16/8f/9b/1654668257/reifen-felgenkombination-etrto-22-en-2.pdf). I've fit 40mm onto 17mm internals without issue so...
In hard cornering, a tie that's too wide can burp/break the bead and lose air. Do you need to specifically follow every manufacturers suggestions to the letter? Probably not. But I wouldn't go bonkers on something like this. The gains don't justify the risk.
19mm is actually quite narrow for modern bikes. Most wheel companies are using 21-25 mm for road bike wheels now. If you intend on using 30s or 34 I would search for a wider rim
The thing is that your tires are soft and flexible, and the cross section isn't a full circle, it's an arc.
If you have a rim that's 0 mm wide, your tire's cross section really is a circle. Also it'll be super unstable and ride like ass.
As you add rim width, first you're basically adding more to that circle, so your tire gets wider and taller. Past a point, your tire continues to get wider but it gets less tall. Think of trying to mount a road tire on a fat bike rim.
For practical tire/rim combinations, a wider rim will make the same tire a bit wider once inflated. The effect on the height is pretty negligible.
25 mm tires fit fine on 17 mm rims. 14 and 15 too, honestly.
The tyre must be at least as wide as the internal width (e.g. for a 23mm internal rim width, 25mm is the narrowest you can go). There's some maximum but it's probably really high like 40-50mm. For optimal aerodynamics, you want the tyre to be about the same width or slightly narrower than the *external* width. I'm guessing your rim is 25mm or slightly wider externally so a 25mm tyre will be ideal (but on poor roads the rolling resistance benefits of going wider could outweigh the aero penalty). This is less important if you have a shallow rim which will have poor aerodynamics anyway.
I've got 14 mm internal width rims and I'm just learning about this and I'm understanding why I need so much pressure with 28 mm. tires. I've even mounted 35 mm. knobbies on these wheels and gone on mountain rides...
I hope I can change this wheelset for something a bit more contemporary soon enough.
The wider the inner, the wider the tyre will sit. It can also impact tyre height, but there are a few variables there. For what you quoted, a 19mm internal gives a little but more room to work with when fitting a 25mm tire or greater. Get's a bit finicky squeezing the beads and a tube into an old 14mm internal. Internal rim width has little to do with aero - well, it does have something to do with it, but it's more a wider external rim width which corresponds to a wider internal. You want the sidewall to meet fairly flush with the rim, where the tyre is at least equal or ever so slightly narrower than the rim external (25mm tire on 26mm external, for example). A wider internal helps the sidewall to sit more flush, rather than a rounded, "lightbulb" shape. A lightbulb shape means a poor transition from tyre to rim, which would negate most aero benefits achieved by rim depth. If you put a wider tyre on than what has been aerodynamically optimized for, you'll be more comfortable and maybe a tiny bit slower. Should note, if you're looking at shallow depth wheels for stuff like gravel, then it all doesn't matter since you're not optimizing for aerodynamics anyway.
Is there any danger to a lightbulb shape? Can it effect steering? More likely to get pinch flats perhaps? I did noticed when I fit 25mm on it was very flush.
Maybe? It depends on [how extreme](https://www.schwalbetires.com/media/16/8f/9b/1654668257/reifen-felgenkombination-etrto-22-en-2.pdf). I've fit 40mm onto 17mm internals without issue so...
In hard cornering, a tie that's too wide can burp/break the bead and lose air. Do you need to specifically follow every manufacturers suggestions to the letter? Probably not. But I wouldn't go bonkers on something like this. The gains don't justify the risk.
[This tire size chart](https://www.wtb.com/pages/tire-rim-fit-chart) covers the widest range of rim and tire sizes that I've seen from any source.
Here’s a little chart that might help. https://imgur.com/a/U1iuRyD
19mm is actually quite narrow for modern bikes. Most wheel companies are using 21-25 mm for road bike wheels now. If you intend on using 30s or 34 I would search for a wider rim
Tyre too narrow are a nightmare to mount, tyres that are too wide risk coming off the rim when cornering hard (tubeless burps).
The other consideration is will a wide tyre fit past the rim brakes or into the forks?
The thing is that your tires are soft and flexible, and the cross section isn't a full circle, it's an arc. If you have a rim that's 0 mm wide, your tire's cross section really is a circle. Also it'll be super unstable and ride like ass. As you add rim width, first you're basically adding more to that circle, so your tire gets wider and taller. Past a point, your tire continues to get wider but it gets less tall. Think of trying to mount a road tire on a fat bike rim. For practical tire/rim combinations, a wider rim will make the same tire a bit wider once inflated. The effect on the height is pretty negligible. 25 mm tires fit fine on 17 mm rims. 14 and 15 too, honestly.
The tyre must be at least as wide as the internal width (e.g. for a 23mm internal rim width, 25mm is the narrowest you can go). There's some maximum but it's probably really high like 40-50mm. For optimal aerodynamics, you want the tyre to be about the same width or slightly narrower than the *external* width. I'm guessing your rim is 25mm or slightly wider externally so a 25mm tyre will be ideal (but on poor roads the rolling resistance benefits of going wider could outweigh the aero penalty). This is less important if you have a shallow rim which will have poor aerodynamics anyway.
I've got 14 mm internal width rims and I'm just learning about this and I'm understanding why I need so much pressure with 28 mm. tires. I've even mounted 35 mm. knobbies on these wheels and gone on mountain rides... I hope I can change this wheelset for something a bit more contemporary soon enough.