Bora WTO

[Wookski]

So maybe lightweights are incredibly bad, and Shamals are just very bad?

I have owned Shamal Milles, have ridden them in severe crosswind gusts (>50km/h) and have never experienced the issues raised on this thread. After reading some of the comments posted my 64kg body should have been blown off the road

Bora Ultra 50’s catch the wind more than the shamals ever did and are still completely acceptable even on the windiest days.

Lightweight Obermeyers on the other hand have caused me to be pushed across multiple lanes on a high speed descent. But they’re still the world’s greatest wheels.

[Attermann]

I own shamals here in Denmark, and it’s always windy, never a problem at all for me

[sawyer]

The WW consensus on wheel characteristics that make for poor crosswind handling seems to be:

- narrow V shaped profile
- deep rim
- fat spokes

obviuosly other factors, not least tyre size will contribute to some extent.

I'd rate Bora 50s (wider, current version) as about the same as Shamals on crosswind handling - that said, only really an issue in a few circumstances ... coming off a mountain in wind, riding past a gap between two tower blocks on a windy day etc

Zondas are better, no question. Thin spokes. As are Bora 35s.

[derosa2000]

How come Lightweights are probably the worst handling rim in gusty conditions out there? I don’t know why, but they are. Maybe the cross section of their spokes adding significantly to the surface area versus flat steel spokes?... the big flange at the hub? the thin flat shape? I don’t have the answer but sure do notice a big difference in gusty conditions on 47-52mm Lightweights compared to even something like a Bora which isn’t particularly toroidal shaped at all, yet the Bora is so much more stable in the same conditions, even the older pre 2015 narrower version.

I can confirm the same about Lightweights , a bit scary even few episodes were . I switched to Mavic CCU few years ago and I forgot about this problem .

[kgt]

According to TOUR (10/2017) Meilensteins do not perform better or worse in crosswinds than other similar profile wheelsets. In fact Enve and Zipp performed worse.
It's interesting because all these 'scary' experiences by LWs in crosswinds never appeared a few years ago and LWs are rather old in terms of design.
I do not doubt all these incidents but in terms of physics it does not make sense. Wind force is just a function of the surface exposed.

[TobinHatesYou]

It’s a matter of pressure differential between the leading edge of the wheel and the trailing edge caused by flow separation. This will cock or steer the front wheel. It is in fact, not all about surface area.

[kgt]

Any reference or test explaining why this phenonemon affects LWs more or in a different way than other wheelsets?
I can understand that a very light wheel is more sensitive to crosswinds than a similar depth but heavier one, but that's another story.

[TobinHatesYou]

When the leading edge of the tire/rim combo doesn’t closely match the trailing edge, such as with V-shaped rims, laminar flow is lost at shallower yaw angles. Instead there is a different amount of turbulent separation at the front of the wheel and the rear of the wheel, which acts as an external steering force.

[kgt]

I know the theory but I have not seen any actual data on rim shapes and cross-winds so far.

[TobinHatesYou]

???

Look at any of the various whitepapers? Look at any study of wing shapes? You’re basically asking for proof that the sun is hot, so just load up your favorite web search and punch in “toroidal vs v-shaped rims crosswind stability”

[Beaver]

???

Look at any of the various whitepapers? Look at any study of wing shapes? You’re basically asking for proof that the sun is hot, so just load up your favorite web search and punch in “toroidal vs v-shaped rims crosswind stability”

Torodial Zipps are more prone to crosswinds despite better aerodynamics. It's no contradiction, also BMWs with air curtain cause less drag but are affected more by crosswinds.



The most stable shape is the teardrop ("NACA") shape by Reynolds, not only according to Tour Mag. but also many users...



Hambini did a comparison of downtube shapes, but maybe one can use it here, as the shapes are quite similar:



Earlier flow seperation but no "hot spot" on the wind faced side...

Maybe Hambini could explain us why.

[Beaver]

Any reference or test explaining why this phenonemon affects LWs more or in a different way than other wheelsets?
I can understand that a very light wheel is more sensitive to crosswinds than a similar depth but heavier one, but that's another story.

LW: Very light rim and also an overlapping tire which also decreases crosswind stability.

But in the mentioned Tour Mag test 4 Nm for LW Disc, 7 Nm for the 3mm lower Zipp 303 Disc, Reynolds 46 Disc 1 Nm...

And Bontragers new round V-shape is not much better (1 Nm = 100g):



https://bikerumor.com/wp-content/upload ... aper_3.pdf

[TobinHatesYou]

Beaver, you need to show CFD analysis for the full wheel and local side forces on both the front and rear of the wheel. If the tests you are linking are only measuring the sum or the leading edge values, it's kind of useless for the purpose of figuring out "stability."

The wheel itself has a leading edge and a trailing section. Each of those sections has a leading edge and a trailing edge. Because the shapes end up being mirrored images with V-shaped rims, the parallel flow on front rim-section and the rear rim-section results in different amounts of drag...aka a steering input.

[RyanH]

I think the side wind tests are slightly misleading. I'd gladly take a deep V any day over a set of Zipp 404 for a strong crosswind day. V shape you get pushed around but 404 feels like a combination of being slightly pushed and speed wobble.

Reynolds 46 (with SLg), RZRs and Bora 50s all had nearly imperceptible influence by side winds. Two of those were V shaped but whatever trickery goes on with the SLg worked.

Sent from my SM-G950U using Tapatalk

[TobinHatesYou]

Then again, a shallower toroid rim *should* be just as aero as a deeper V at varying degrees of yaw. The closest comparison would then be 45mm 303s or even shallower 40mm toroids with 50mm Boras.

(with closely matched tire widths for each of course.)