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I recently bought a lightly used 2018 Supersix (non-Hi-Mod) with Dura Ace 9100 (rim brake with Dura Ace 9100 calipers), 50cm frame size. The rims, which are stock, are "Cannondale HollowGram Si Carbon Clincher, 17mm inner, 35mm deep" (I am quoting from the 2018 catalog verbatim). The spoke count is 18 front and 24 rear. The current tires are Bontrager R3 25c and I've had no problems in terms of fit. (The catalog says the original tires were Vittoria Corsa 25c)

I just ordered a pair of Continental GP5000 28c tires. Did I make a mistake? I've read on various forums that some people have said 2016-2018 Supersix rim brake versions will fit a 28c tire no problem-- but it also depends on the rim brake model (SRAM brakes not having enough clearance, but I have Dura Ace 9100). And I also read that spoke count on the rims make a difference and that lower spoke count will present issues--not sure on the reasons (perhaps the higher spoke count cause less flex and thus tire won't deflect into chainstays or brakepads??). I also see that some people have used 28c on the front and 25c on the back, but I want to be consistent.

Anyone who has been able to run the 2016-2018 SuperSix rim brake with this rim with 28c tires? I attach a photo of my bike below (excuse the excess steerer tube which is going to get cut soon!). Thanks!
 

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will 28c fit with no rubbing?
Well you have the bike. You could just measure. A 28 is 3mm wider (1.5mm per side). Plus 3mm min clearance. You should have a min of 4.5mm clearance currently.

And I also read that spoke count on the rims make a difference and that lower spoke count will present issues--not sure on the reasons (perhaps the higher spoke count cause less flex and thus tire won't deflect into chainstays or brakepads??).
That's nonsense.

I attach a photo of my bike below (excuse the excess steerer tube which is going to get cut soon!).
The picture is useless. It doesn't show the chain stay or brake clearances.

I have a 2018 Supersix. I've never used 28's but I've no doubt it'll fit them. Probably 30's.

The Catalog lists the tire size as 25-28.
Product Bicycle tire Rectangle Bicycle wheel Bicycle frame
 

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If you run 28's with rim brakes, don't plan on taking the wheels off/on a lot. You may have to let the air out of them, or hit them pretty hard to get thru the pads. It's doable, but on some bikes it is flustrating. If you have a flat and fix, just wait till you got the wheel on before airing it up.
 

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If you run 28's with rim brakes, don't plan on taking the wheels off/on a lot. You may have to let the air out of them, or hit them pretty hard to get thru the pads. It's doable, but on some bikes it is flustrating. If you have a flat and fix, just wait till you got the wheel on before airing it up.
9100 series Shimano has no issue with wide tires. They were designed for 28mm. Can easily clear wider.
 

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Oh I read his post as talking about tires flexing. But I guess he meant the wheels.
Yes wheels can flex. But that's why you have 3mm+ of clearance.

The HollowGrams are pretty stiff for for low spoke count.
My 2014 Synapse Carbon came with Shimano RS-11 alloy wheels which had a spoke count of only 16F/20R. There was definitely tire rub on climbs when I put 28mm tires on it. I built up a set of 24F/32R wheels for that bike and the tire rub went away.

Are you saying the HollowGram wheels are stiff or the rims are stiff? Remember that a stiffer rim will actually make the tire more likely to rub. Remember this:

 
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