I was inspired by this thread and bought some 700x35 Rene Herse Bon Jon Pass tires for my Roubaix. The frame can handle a 33mm tire, so I though a few extra mm wouldn’t hurt…but the tires are just about 38mm. That leaves 2.5mm on each side at the chainstays. There looks to be plenty of room at the seat stays and fork.
My wheels are 51mm deep Bontragers and 23mm internal. I’ll only be on the road (though surfaces around me can be quite rough).
Am I tempting fate with 2.5mm clearance?
Hard to capture but here are a few. I can just stick a 2.5mm Allen wrench in between the tire and chainstay. I only did the back wheel so far…knew that would be the tricky part if size was a concern.
Be careful. Sometimes rims flex and then you’ll get rub. I’ve seen situations where people have worn down the carbon on their frame in that spot. I’d keep an eye on it.
That’s what I’m afraid of, but I’m also wonder how much these rims could flex. I’d think deep-ish carbon would be pretty solid.
Just a quick comment here since you mentioned the Firecrest 404 wheelset explicitly: I think many roadies still use tire pressures that are too high. The recommended pressures for modern, wide rims are much lower that what you quote. The tire pressure recommendations by e. g. Zipp and Enve are consistent for rims with the same internal width. For example, Enve’s 4.5 ARs, Zipp’s Firecrest 303S and my 3T Discus C45|32 LTD all have an internal rim width of 25 mm. For a 28 mm tire, Enve recommends 53 psi at my weight (about 73 kg), Zipp’s recommendation in the dry is a little higher with 58 psi/62 psi. Zipp’s wet weather pressure recommendation is closer to Enve’s. I run my 28 mm tires at about 60 psi (I add 2-3 psi extra in the rear). When you look at the recommendation for 25 mm tires, Zipp recommends 67 and 71 psi front and rear, respectively.
Of course, these recommendations are just starting points. You might want to lower pressures if you are riding in the rain or across roads with bad road surface. I’ve lowered my tire pressures in a race on my old road bike with traditional rims to 65 psi, and it felt great. Was I minimizing rolling resistance? Nope. I was maximizing grip in the corners. Felt pretty good on a disc brake bike with 28 mm wide tires, many competitors with narrower tires and fancy carbon rim brake wheels really got spooked.
@AJS914
Not just flex, but also debris like small stones may get stuck and scratch your paint. On the other hand, I don’t have that much more clearance (from the top of the tire) on my 3T Strada. Probably you will be fine, just be ok with occasional scratches and keep your bike clean.
My advice would be to respect the 4mm clearance European spec. I’ve had road debris kick up between tire and chainstay, pretty sketchy when that happens.
You’re all right…I’ll keep these for the gravel bike and try something else. Thanks!!!
Put me in the “tire is too wide” group. Just because we can doesn’t mean we should.
Joe
On the plus side, it looks awesome
This is my primary motivation…
Rene Herse tires are the perfect cream color. Might have to try the 32mm but then I’d need tubes. 
I really wanted to try these with all the talk of them being supple and fast.
I’d love to try them, but it would have to be the extralights, and like you I’m very reluctant to try them in a tubed version: I have visions of multiple punctures every ride. If that wasn’t the case, I could maybe bring myself to stomach the cost… 
In response to myself (lol) I didn’t have great luck with Vitoria Corsa 2.0s on local lanes here in the UK. Lovely ride feel and very fast (and looked gorgeous in tan wall) but absolute puncture magnets.
I might try Pro Ones. The tan wall GP5000’s require tubes I believe.
I have the Extralight RH Bon Jon’s on Zipp 404 Firecrests as my road wheels on my Diverge. Great looking set up on your Roubaix. Enjoy the supple life!
Personally I am sure this will happen. Flow says their wider wheel (allsport maybe?) is faster with a wider tire than their rim brake wheel with a narrower tire so it might not be any slower to get to the 35-40mm range with a wide aero rim.
Joe
You might already know but Challenge makes some nice 36mm.
I’m curious, what are your 40mm wide gravel aero wheels? 3T discus, Nextie? Thanks!
3T already makes a 40 mm wide aero rim for gravel bikes, their Discus 45|40. An I reckon for some circumstances, people want wider slicks. 
If Rene Herse 32s were tubeless, I’d give them a go, but I’m reluctant to go back to light/thin tyres with tubes, especially as we move towards winter here in the UK.
Thinking logically about it, I should probably save the speed focus for the TT bike, and just fit out the groad for long-distance comfort, which would lead me to the 35s. Mentally, though, that seems a step too far. The commuter would be an ideal place to experiment, but a) the max clearance is 28 and b) I’d rather not deal with a puncture on the way to work!
I will ponder until the current rubber has seen better days…
Thanks for the reply!
1500g is a nice weight too.
Do you feel tangible aero gains at high gravel speed (30 to 45km/h) with the 105% tire rule ?
Or it is quite marginal compared to let say a standard-wide/30mm external and 40-45mm deep carbon wheels. Thanks!
This is a question in which I’m interested and to which I’ve never found a really definitive answer. How much do you lose from breaking the rule of 105, and at what speed? It would be really helpful if anyone knew, for example, if you were running 32mm width tyres on 27mm external width rims, that would cost you x watts at y kph. My own sense is that the loss is probably pretty marginal at typical long ride speed (27-29 kph average) but it would be useful to know.
