I have seen a couple of older posts on here, but nothing very new, and wanted to bring the topic up again.
What are the latest and greatest thoughts on the rule of 105%? (Outer rim width should be 105% the width of your tire)
I just purchased some conti GP5000S TR 28mm and put them on my hunt 50’s that have an external width of 27mm. this makes my tire a bit wider than my wheel, thus breaking the rule of 105.
How much of an aero gain am I really missing out on with the latest and greatest knowledge of rolling resistance and/or comfort for long days? Would love to hear all of your opinions!
Well, Specialized has a wind tunnel and they say there’s still a benefit with wider tires.
They are using Roval CLX 50 wheels, which measure 29.4mm externally. The 26mm tires had the biggest benefit on the aero wheels over the basic wheels, but the 32mm tires(breaking the “rule of 105”) weren’t far behind them.
The tires may also stretch a little after being mounted for a while.
From a pure aerodynamics standpoint I believe 105% is still the rule. That being said, in terms of overall efficiency you would need to consider tire pressure and rolling resistance as well. For me personally I don’t mind going a little bit bigger to gain less rolling resistance and more comfort with lower pressure, but others may prefer the aero-advantage.
Personally, I’m not going to chase a tiny number of watts in any way. I’m not going to ride a smaller tire than I prefer for the sake of aero. I’m not going to spend thousands on equipment to save a few extra watts. After all, I’m not in the world tour or racing the national TT championships for a podium. But if you are then go for it!
Some would argue that the suspension gains from the larger tire counter balance the aero gains of the smaller tire. Thus ideally, if you want to run a 28mm tire you need like a 32mm wide rim. Again, most mere mortals probably don’t need ideal.
Cannondale has been pushing this concept. Having tires narrower than your rims means potentially riding on carbon if you get a flat. That scares me a little.
To conform with the Rule of 105 you would have to run a 25mm tire. The data I’ve seen for rolling resistance between similar 25mm and 28mm tires at 100psi is about 500mW to the plus for the 28mm tire. That’s half a watt.
Now, to really answer the question, we have you know your yaw distribution. Most riders have no idea what their yaw distribution looks like, but I’ll just advise you that as yaw angle goes up the Rule of 105 matters more. At lower yaw angles the difference you’ll be seeing is less than the rolling resistance benefit.
One more thing to consider! How much climbing will you be doing? The 28’s are heavier, of course. And the slower you go the more your yaw distribution is going to skew to the right (tending more towards double digits) BUT the less aerodynamics will matter. Of course, rolling resistance is linear wrt speed. So it gets complicated.
If it were me, I’d use the 28’s. I don’t have enough info to give you a cogent suggestion.
Thank you for the info. As you stated I have no freakin clue what my yaw is… i dont even know what that means. I do absolutely love climbing… but I love going very fast in the flats too. I love both about the same
Inserts. Tubeless. Its my thing for a reason. My Trek Domane had scuff marks on the aluminum box wheels from flatting with tubes. My Enve carbon wheels got replaced when they got road rash.
If you are scared at all, my first question is - are you running tubeless and inserts?
Personally I don’t mind spending money to buy some watts, work on aero gains, and have some fun while I still can. I’m chasing KOMs and some watts and marginal gains. Done nearly all of my own home maintenance for 23 years, and last 4 or 5 years we’ve only had 1 car (and wife has work car). We mostly cook food, and don’t waste any food. My wife is a pro at finding discounts and bargains. My philosophy is live a little, enjoy the things you like, don’t worry be happy.
Only if you can blame the equipment and not the engine. Last week
On wide 28c / 32c Vittoria Corsa N.EXT tires. Not the fastest tires and definitely NOT aero optimized Thankfully I don’t need to revisit that one, yet, but somebody is always targeting me, so when I do then it’s time for faster 25c/26c tires!!!
I think the counterargument is that it isn’t a loss, but a tradeoff: inserts offer you more protection against flatting. Some inserts also positively affect how tires behave, although that depends on the type of inserts. Some inserts participate in the “cushioning” and are part of the suspension. You could run lower pressures safely, for example.
That wouldn’t be optimal though, if you follow the 105% rule then optimal tyre width on a 32mm rim would be about 30mm, which likely means running a stated 28mm tyre which will inflate wider than that. Just figuring out the least bad way of writing that sentence makes me realise that manufacturers need to do a much better job of explaining tyre widths!
More broadly what hasn’t been discussed is the root cause of WHY a rim that is 105% wider than the tyre is more aero. My understanding at the time it was first discussed (I’m sure others know much more!) was that it was due to the shape of the tyre cross-section and to how smooth the tyre-rim interface was. I.e. If the tyre is wider than the rim that means it’s going to bulge out just above the rim which isn’t smooth, and then it’s also going to have quite a round shape which isn’t very aero. Vs a narrower tyre where you get a smoother tyre-rim interface and more of a U shape with straighter tyre walls.
Seems to me that both tyre shape and tyre-rim interface would be affected by tyre/rim developments which have taken place since the 105% rule was first discussed, which might at least mean that the aero penalties for going wider aren’t as significant as they were say 10 years ago. Tubeless is one change - not having a tube could potentially change tyre shape a little. Hookless is another - eliminating the hook could make for a smoother tyre-rim interface and also a less rounded tyre as you’re not pushing the bead in as much. And with both rim widths and tyre sizes increasing it’s also possible the 105% just doesn’t scale linearly. E.g. the aero penalty for fitting a 25mm tyre onto a narrow rim optimised for 23mm might be bigger than the penalty for fitting a 30mm tyre onto a wide rim optimised for 28mm.