# Deep wheels: confused about recommended inner and outer rim width

Hello,

Please excuse me for my beginner question. I have searched for this, but have only found moderately helpful answers.

I’d like to buy some deep rim wheels. Let’s say I would like to use an 28mm tire (by tire specification). If the 105% rule (for aerodynamic reasons) is to be followed, it would follow that I need a rim with an external width of around 29-30mm. But most wheels with such an external width, have an inner width of 21+mm.

For 28mm tires the recommended inner rim width seems to be 18-19mm (e.g. here). But all deep wheels I looked at with that inner width have not enough external rim width. Maybe the 105%-rule doesn’t matter that much?

Another thing that confuses me: The same tire on a bigger inner width (let’s say 21mm) will probably measure 29mm, wouldn’t it? This again means that the external width must be large enough. Is it possible to estimate the measured tire width given the tire and inner rim width? I couldn’t find a calculator.

Some wheels I looked at:

• Elite Drive 50D, inner width: 21mm, outer width: 28mm, recommended tire width: 28-38mm
• DT Swiss ARC 1400 DB 50mm, inner: 20mm, outer: 27mm, optimized for 25-28mm tire
• Lun Hyper D45 Disc Brake, inner: 21mm, external: front 27.6/ back 27.8mm, optimized for 25-28mm tire
• Specialized Roval Rapide CLX II, inner: 21mm, external front 35mm/ back 30mm, recommended tire: 24-38mm
(The only wheels where the 105% can be applied?)
• ICAN Aero 46, inner: 21mm, external 28.4mm, recommended tire: 25-30mm

I am not sure if some of those wheels were built for 25mm tires, because then the whole calculation would work out.

I think that WTB chart is a bit outdated but I think a 21mm inner width might be the max I’d go for a 25 tire.

No, since there’s no standard way to measure tire size. So that 28mm is the ‘nominal size’ or, said another way, it’s what the company measured it as when it put it on a rim. So that company could measure it on a 17mm internal rim or a 22mm internal rim. But you are right that a 28mm tire will be larger on a 21mm than a 18mm rim. IDK exactly what the best calculation for that would be though. Some tires have started to put their measured widths on different rims on their boxes though (I know ENVE does this for their road tire).

Of those wheels though, I know the Rovals are optimized for their 26mm tire. But they would work great with a 28 also.

In the end, I think if you are ‘close enough’ to that 105% rule then you’re fine. The differences will be marginal. It’s not like the first crosswind with just throw you into the bushes if you are at 98% vs 102%.

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I am not sure of the answer, interested to learn, I dont own any aero wheels.

From the Zipp info (was looking at getting some 404s…

404 Firecrest A1
Int 19mm
Ext 27mm
Tire Minimum 23mm (recommend 25mm)

858 NSW A1
Int 18mm
Ext 25mm
Tire Min 23mm

858 NSW C1
Int 23mm
Ext 27mm
Tire min 25mm

808 A1
Int 19mm
Ext 27mm
Tire Min 23mm

Is it the external width that matters?

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It would be the inner width that matters for measured tire width, cornering stability, and maybe safety of it staying on the rim.

External width matters for aerodynamics. If the tire is a bunch wider than the rim then it can wreck some of the crosswind stability of the airfoil.

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Firstly I’d say not to pay too much attention to that link, they seem to be a predominantly off road site.

Secondly I’d say the 105% rule needs to be weighed up against other factors including grip, handling, comfort. For most wheels you list 25mm tyre is going to be the most aero. But if it’s a wet road race with some sketchy tarmac where you’ll be spending most of your time in the pack, the extra grip and comfort of a 28mm probably trumps the marginal aero gains of going narrower. A TT on good tarmac? Go with the 25mm. Other option is split the difference and go with 25 on the front and 28 on the back where aero is less important. Also worth noting that with wider inner rims I think generally the aero penalty of going wider is less than it used to be.

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If you want to use a 28mm tire, I’d definitely recommend a wider external rim. I’m running Bontrager Aeolus Pro 51 wheels…23mm internal/31mm external. 28mm tires end up around 30mm.

Specialized has a wind tunnel video where they showed aero benefits still exist with deep wheels and wider tires.

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