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I have Ksyrium Elite wheels from around 2012 or 2013, 15mm internal rim width.
I use 25mm tyres on them, and despite the wheels being lovely and stiff, there is sometimes enough flex for the tyre to contact the inside of the (carbon) chainstay.
Would wider rims definitely make this more of an issue? My grasp is that narrow rims make tyres more bulb-like but narrower and lower - whereas wider ones make them more dome-like, wider and taller.
Have you found that to be the case IRL?
I'd have thought the tyres would be taller on a narrower rim. The bead to bead measurement around the carcass will remain constant. If you're squeezing the beads closer together, the only direction the carcass can be displaced is up and away from the rim bed.
Regarding your flex/rub issue, if it's the sides of the tyres that are rubbing, I would guess that wider rims would exacerbate matters (assuming the new and old rims flex equally).
Ksyrium wheels are usually pretty stiff so it may be that your frame is flexing rather than the wheels?
It must partly depend on the angle of the stays. A wider rim might give a wider tyre but if that width is further from the bottom bracket it could increase clearance
The tyre will be less tall on a wider rim so less 'flexy'. However the place where a tyre would flex would be where the force is applied to it - i.e on the ground so not sure what effect it would have on it hitting your stay.
The only relevant visual illustration I could find is this...
[img] 
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It doesn't quite look "right" to me, but does seem to suggest I shouldn't get wider rims!
I don't think that diagram is accurate. I think on narrower rims, the tyres are a bit more "bulb" like, whereas on a wider rim, they match the width of the rim.
I have some Pacentis with GP4000s 23c tyres and the widest part of the tyre is only slightly wider than the rim, with narrower rims like an Open Pro, the widest part of the tyre was much wider than the rim.
Go down a tyre size; most 25c tyres are massive anyhow. My 23cs are a touch over 25mm wide. They're quicker than the wider tyres they replaced.
Narrower tyres on wider rims is an option if they provide more cushion.
I went from 23c to 25c Michelins and stopped getting punctures overnight, so I don't think I could go back on the same rims.
Could just helitape the inside of the stay and keep on top of changing it.
Are they newer Michelins or old ones? The old 23c were very narrow, but I had some of the Pro 4 25c tyres and they were MASSIVE on my rims; swapped the for some 23 Contis which just feel loads better.
Try tighten your QR up alot? Are your spokes still tight?
Pro 3 in 23c to Pro 4 25c - assumed they'd be consistent in the difference.
Actually using Conti Grand Prix GT now, which may be a touch smaller than the Pro 4s for the same stated size.
I think it just very occasionally rubs when I'm giving it loads of welly. It a top-end carbon frame so must be really thin in places, but the stays down near the BB will probably have a decent amount of material?
Yeh, the pro3 were pretty flimsy and not very wide.
Certainly see plenty of carbon frames with rub marks on the chainstay. I'd try a decent (shimano) QR and do it up tight; might be all it needs.
Is it a qr? Try a dt Swiss rws skewer. Made a massive difference to my bike. They are supposed to be 50% stronger. Might help reduce flex.
OK will try an alternative QR when I get the chance.
Are Mavics considered sloppy?