Silca accommodates wider rims by you entering your measured tyre width, whereas Zipp et Al ask you for the nominal width then assume an increase.
The ground interface doesn’t understand rim width, that’s not how physics works. Rim width increases the total volume, so less pressure is needed to equate to the same tyre drop.
Therefore, for a 28c on a 25mm internal you’d enter 30mm (ish) or whatever your calipers came up with and get proportionally lower pressures.
Either way, 28c blown out to c.30mm on a 27mm wide rim (zipp 404, 303s) is a terrible aero choice.
I reckon honestly the lesson here is hooked rims end up being more aero in the end.
Josh addresses this in the above-mentioned podcast….their specs are only performance-based recommendations and they emphasize that the user needs to check with their wheel supplier for maximum tire pressure limits.
pretty sure they are clear that anything that requires over 73 psi isn’t compatible with hookless…Still don’t see how their tire pressure calculator is hiding that fact. Personal experience with the Silca calculator tells me their pressures are too high. You also continue to ignore that the wider tire, while negatively impacting aero performance, can, and often is, as a whole the faster option(unless you’re riding on pristine brand new pavement or a track surface.)
Yeah, I have found similar results. For instance, when I just swapped out to Pathfinder 42’s, their calculator recommended (IIRC) low-mid-30’s. Haha…not a chance, I ran 25psi for my 40’s.
That said, almost all the online calculators recommended higher PSI than I normally run so I am experimenting with slightly higher PSI than normal.
Because their calculator is designed to offer you pressures that are compatible with your wheel / tyre choice and are safe, not what’s fastest.
If silca says 80psi is fastest, you can’t sue them because your tyre came off the hookless rim (which is literally hookless to save money for the vendor).
If zipp says 80psi is recommended and your tyre blows off the rim, they’re paying for your jaw reconstruction.
BRR is quite clear; wider and lower can be but isn’t always lower RR, it is ALWAYS higher CdA. At same ‘comfort level’, RR is roughly equivalent.
I don’t have an issue because I worked this out and bought 303 firecrests instead for my 28c’s. You may also not have an issue because you don’t race and so care less about efficiency than comfort.
Beginner question: How wide would you go for this? I am currently on 30c (measured 31.5mm on current rims) on my road bike and contemplating going to 28c. Thought that would be faster, but the more I read about it, I‘m getting unsure. The 30c have been great so far. I don‘t race, but go on long drives (>100km) very regularly.
I just read a lot about people going to wider rims/tires and I‘m not totally sure why. I understand, the comfort/RR/aero thing - Is the difference that big?
Depends entirely on what riding you’re doing and on what kind of roads. If you’re not racing then you likely don’t care about every last watt, in which case I would have thought the extra comfort and grip of 30mm would win out. And if riding on some rough road surfaces may well be as fast or faster anyway. Or just split the difference and go 28mm on the front where aero is more important, and 30mm on the back where comfort is more important.
I agree with @cartsman 28’s are a nice compromise between aero, comfort and rolling resistance for realistic real world road surfaces. That said, outside of solo riding/time trialing the differences will be pretty minimal.
Just got myself a new bike and it came with 303S. In the box it came with the cassette and freehub off the wheel and after some fiddling I got it on and did my first ride. Just brought it in and threw the bike on my trainer in anticipation of my workout tonight and when taking the rear wheel off, the feehub slid off and I haven’t been able to fully seat it so the prawls engage the hub. Has anyone run into this?? I’m at a loss. I’ve only had cheap wheels until now and never had a freehub just slide off like nothing lol.
So, any road riders have a problem with hookless. I’ve found deals on the Zipp 303s and the Enve Foundation wheels. I’d rather buy Zipp or Enve (actually Enve sounds the best) but I’m still slightly hesitant with hookless.
Anyone run tubes in hookless anyway? Is the no tubes recommendation because they want you to run tubeless tires which may have a tighter or less stretchy bead? I guess one could still use a tube in a pinch to get home as long as the pressure was within spec.
