Theoretical watts lost due to too wide a tire

[spartacus]

In this blog Josh from Silca is sharing some data from an old Zipp 404 with a 23mm GP4000 and a 25mm GP4000 at different pressure levels (widths).
https://silca.cc/blogs/silca/part-5-tir ... rodynamics

Thanks this is interesting.

Kinda mind blowing that they're assuming a nominal tire width instead of measuring them though, both silca and hambini, I thought it was common knowledge those tires ran wide.

[xav]

Thanks, this makes sense, I was just hoping there were tests done where someone intentionally tested tires that are wider than recommended (3-4mm wider than the wheel) on various rims

I've got a bit of data I can share, this is a full tunnel set on a few different time trial specific wheels using 23mm Vittoria Corsa Speeds, 25mm Corsa Speeds and also 25mm GP 5000 TLs. Tyre choice really does change the yaw curves and performance of wheels, even if you compare a "25mm" tyre from two different manufacturers - I find on WW people are really helpful at posting the measured tyre width on different rims as it can be a bit of a minefield really. From our perspective (ie. TT focused) as manufacturers are moving more towards 25mm and away from 23mm it will change things somewhat, pretty much what happened when 20mm tyres got phased out in favour of 23mm. Not sure where it will end!

[spartacus]

Thanks, this makes sense, I was just hoping there were tests done where someone intentionally tested tires that are wider than recommended (3-4mm wider than the wheel) on various rims

I've got a bit of data I can share, this is a full tunnel set on a few different time trial specific wheels using 23mm Vittoria Corsa Speeds, 25mm Corsa Speeds and also 25mm GP 5000 TLs. Tyre choice really does change the yaw curves and performance of wheels, even if you compare a "25mm" tyre from two different manufacturers - I find on WW people are really helpful at posting the measured tyre width on different rims as it can be a bit of a minefield really. From our perspective (ie. TT focused) as manufacturers are moving more towards 25mm and away from 23mm it will change things somewhat, pretty much what happened when 20mm tyres got phased out in favour of 23mm. Not sure where it will end!

23mm Corsa Speed.png

25mm Corsa Speed.png

GP5000 TL.png

Thanks for sharing your graphs. It seems like the people want 28c tires now, but there must be a limit to how wide the rim can ultimately go in order to maintain the ratio of the airfoil?

[Maddie]

I think it's worth noting that Hambini doesn't seem to have access to a windtunnel. What he does though is computer simulation which can also be interesting of course.

[xav]

Thanks for sharing your graphs. It seems like the people want 28c tires now, but there must be a limit to how wide the rim can ultimately go in order to maintain the ratio of the airfoil?

Yes absolutely - just from a fork clearance perspective and downtube width interaction point of view there is a maximum. Right now 23-25mm, with the current available tyres and the best wheels, is still the right choice for high performance/high speed.

[Lakal]

In this blog Josh from Silca is sharing some data from an old Zipp 404 with a 23mm GP4000 and a 25mm GP4000 at different pressure levels (widths).
https://silca.cc/blogs/silca/part-5-tir ... rodynamics

Thanks this is interesting.

Kinda mind blowing that they're assuming a nominal tire width instead of measuring them though, both silca and hambini, I thought it was common knowledge those tires ran wide.

Hmm.. My understanding is that Silca measures the tires?

[greycat]

I have always wanted to know these questions?

If I run 38mm tires is there significant benefits of a deeper dish? Does using a 45mm rims with 38/40mm tire result in a 55mm rim and 28mm tire?

Is it more important to go shallower/lighter/comfort on the rim and use the tire sidewall for aero benefits with bigger tires?

I know asking about 38mm and aero is weird. Yet I see manufactures offering deeper wheels for gravel.

[choochoo46]

In this blog Josh from Silca is sharing some data from an old Zipp 404 with a 23mm GP4000 and a 25mm GP4000 at different pressure levels (widths).
https://silca.cc/blogs/silca/part-5-tir ... rodynamics

Thanks this is interesting.

Kinda mind blowing that they're assuming a nominal tire width instead of measuring them though, both silca and hambini, I thought it was common knowledge those tires ran wide.

The Silca article actually does show WAM (width as measured). See the chart titled 25c GP4000sII on ZIPP 404 Firecrest Clincher. The values in parentheses are the measured width. And the previous chart also plots the actual measured height & width.

And the table titled "105% Rule" while not stated should be read as measured width. For example, if your mounted tire measures 20mm at its widest point, than to stay within the 105% Rule you should have at least a 21mm external width rim.

It doesn't matter whether the tire runs wide or not. The point of the article is that for best aero performance the rim should be 105% the width of the mounted tire. In some cases just a few mm increase in tire width will push you past the 105% rule and add drag (1-2 watts at low yaw angles to 9 watts at higher yaw angles in this article).

One interesting effect is that drag goes down as yaw angles increase but then rebound at some point. I wonder if the decrease in drag is similar to the sail of a boat...

[xav]

FWIW I don't agree with the blanket 105% rule. These days with varying rim shapes, tyre profiles and rim depths it's not a nailed on performance target/outcome.

[Aryeh]

I like running 28mm tires and I specifically bought 32mm wide wheels (Light Bicycle WR50) because my 28mm tires measure around 30.5mm.
But it seems like there aren't too many wheels out there that comply with the 105 rule for 28mm tires.

[choochoo46]

FWIW I don't agree with the blanket 105% rule. These days with varying rim shapes, tyre profiles and rim depths it's not a nailed on performance target/outcome.

Agreed. Blanket rules by definition won't work for everyone. And many don't care about marginal gains.

For those without access to a wind tunnel any suggestions how to approach the rim width, tire width, tire pressure decision?

FYI here's a podcast featuring the inventor of the 105% rule, Josh Poetner. Around min 30 he talks about tire width and tire pressure. In an extreme example he found at Paris Roubaix a difference of over 24 watts from the worst to best setup.

https://play.acast.com/s/8ed8adfa-6342- ... 00121b9a5e

[JayDee81]

For those without access to a wind tunnel any suggestions how to approach the rim width, tire width, tire pressure decision?

Manufacturer or rider?

Anyway it always depends on what is your target, whether comfort or speed or combination.

[Elmersen]

I like running 28mm tires and I specifically bought 32mm wide wheels (Light Bicycle WR50) because my 28mm tires measure around 30.5mm.
But it seems like there aren't too many wheels out there that comply with the 105 rule for 28mm tires.

For a wheel like this that is wider a little further down from the edge (the WRs are something like 33,5 mm at its widest point), wouldn't it be OK to have a tyre that is almost flush with the edge, just shy of 32 mm, since the rim itself widens out a litte further down?

[xav]

For those without access to a wind tunnel any suggestions how to approach the rim width, tire width, tire pressure decision?

I would definitely ask the manufacturer first, as they should have done some testing on it (or at least have recommendations or feedback from customers). As narrow a tyre as the manufactuer will allow to be used on their rim is more likely than not to be an acceptable aerodynamic option. If the manufacturer has no information then that's a bit of a red flag.

For RR I would make a judgement call based on the road surface and go wider/low pressure without worrying about aero if it's going to make your riding experience better.

Most of the time, at the speeds recreational cyclists go, you will have a better overall experience focusing on the tyre and tyre pressure than the aero. For a competition cyclist it's a bit more involved but I would much rather go with manufacturer recommendations or whatever independent testing you can find rather than a 105% blanket calculation.

[Hexsense]

Note the origin of 105% rule.
It was observed from old rims which are widest near the middle of the rim. The rim hook end is a few percent (5%?) narrower than the widest point of the rim.
So... what if your modern rim is widest at the rim hook. Maybe we don't need the rim that much wider than the tire for smooth transition.