Tyre size and cross-wind instability

I recently bought a new set of 55mm wheels, 23 internal / 27mm external. I have used the same depth wheels for the past 7 years, and never have I experienced instability on cross-winds as i do now. The only real change (other then the wheelset) is Conti 25mm GP5000 to Conti 30mm GP5000.

Im not sure if this is something that the aero smart people lean into, but has anybody else experienced this before? Can you introduce instability by using larger (30mm tyres) for the rims? Does the tyre stand up taller creating an even deeper profile, or the rim/tyre lightbulbing causing more wind to catch.

In wheelset reviews I often see mentioned stability in crosswinds. Some wheels are just more stable than others

Most likely by installing tyres wider than the rim you have moved the stall angle into the yaw range you are experiencing on your rides. Fitting a narrower tyre may move the stall angle. Silca have a write up on it here

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What was the exact wheel and tire combo on the old setup?

What new wheels?

Keep in mind that depth as a number is only a small part of the story with actual rim shaping being the key here.

I highly doubt the issue is tires, but the functional difference in rim shaping.

30mm tire on a 23mm IW wheel is not lightbulb shape. It’s well within the proper size range and not likely your issue. [I incorrectly wrote 25mm initially, but the likely result is that 30mm is still good. Exact wheel info will be useful to confirm this.]

I read at as 30mm tyre on a 23mm internal, 27mm external rim width. I’m guessing that the 30mm tyre is inflating to 32mm+ on 23mm internal/27mm external rim width so the tyre is much wider than the rim.

In the chart on the Silca website it seems to show that the wider the tyre is than the rim the closer the stall angle moves towards zero degrees yaw. Obviously it will never reach zero but it may push the stall angle into a range of yaws that is experienced more frequently.

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What were the old wheels’ manufacturer vs the new wheels? I just moved from first generation ENVE 6.7s with 26mm tubular wheels to Light Bicycle wheels of slightly less depth (56mm, I think) but a 28mm tubeless.

While the ENVEs were not perfect (a horrible braking surface and internal spoke nipples, meaning ripping out the tubular in order to tension the spokes) they were very solid in crosswinds. I never had a time where I felt more than steady tension on the handlebars, even in pretty gnarly crosswinds.

By comparison, the LB wheels are very sketchy in crosswinds. I was bombing down a hill at about 30mph and as cars passed by, the turbulence was inducing a pronounced wobble that alarmed me enough for me to slow down. I’ve never had that experience before, and it definitely made me appreciate the fact that, yeah, maybe all of that expensive wind tunnel testing actually does make a difference!

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I think they are, but on the website, I can’t find the exact specs. This is a small independent shop. I’ve had them for a few months now, thinking it must be me, but like you, I’ve had some wicked snaps that caused me to question if I was doing something wrong.

Is a 23internal/27 external rim profile too narrow for this width of internal?

I’d say it’s a slightly odd combo for a deep aero wheel. 27mm external width on an aero wheel would typically mean it’s optimised for a tyre of 25mm or narrower (to follow the rule of 105 I.e. External rim width should be ~105% of tyre width). But 23mm internal width would typically mean it’s designed for a tyre of 28mm or wider (and I think 28mm is also the narrowest “approved” tyre width for a 23mm rim per the ISO and ETRTO standards)

So it’s a bit unclear as to what tyre width that rim is designed for.

What is the shape of that rim crossection? Does it have a round or sharp back? I was once an aerodynamicist. Knowledge half-life expired by now but I have a theory based on my own experiences that the stability of aero rims in CW is down to the Kutta Condition on the back of the rim profile. No real data to justify it, just a hunch and two sets of aero wheels with vastly different CW performances a 30mm AL set with a sharp backface and a 60mm C set with a radiused backface. The 60mm are the most benign wheels I ever rode in a CW.

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The 105% rule was introduced some 20 years ago, when the rims were narrow and V-shaped and the tyres were also much narrower than today.