Wheel stability affected by disc rotors/ballooning of the tires

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R2power
Posts: 17
Joined: Thu Apr 04, 2019 2:54 pm

by R2power

I’ve just bought a new disc bike with 40mm deep wheels that are highly unstable when it comes to speeds above 50kph and side/cross winds. My previous bike was with rim brakes and 38mm deep wheels and never experienced any aerodynamically stability issues. I was suggested that this behavior might be caused by added surface of disc rotors. One other guy at work said that it might be related to the width of my tires combined with narrow rim width. Current set up is 23mm wide rims and 29mm wide tires, while on the last bike I ran 23mm tires combined with 25mm wide rims.
Is there any merit in this statements? Can steering be affected so much by disc rotor and/or ballooning of the tires?
Unfortunately I do not have access to any other disc wheels to test have they behave.

tjvirden
Posts: 540
Joined: Sun Nov 22, 2020 9:21 pm

by tjvirden

My immediate thought is that no, it's absolutely not the rotors or tire/rim interface; it's the rims themselves [the front rim]. Some rims do develop high (lateral) aerodynamic forces and that force is at the edge of the wheel, meaning the lever is as big as it can be. The rotor is insignificant by comparison.

Rim/wheel model? Can you borrow a shallow rim front wheel to compare?

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TwiggyForest
Posts: 191
Joined: Wed Jun 03, 2020 2:06 am

by TwiggyForest

I would say it's the rim/tyre combination. Instability gives when the air detaches from the tyre / rim trailing side of the wheel, creating a low pressure area. The big difference between the 29mm tyre and 23mm rim means that it detaches far earlier than on your old setup.

CampagYOLO
Posts: 698
Joined: Thu May 06, 2021 3:58 pm

by CampagYOLO

23mm rims strikes me as being very narrow, are they quite old wheels? If so it may also be the case that they're a shape such as a V shape which is more affected by crosswinds.

tjvirden
Posts: 540
Joined: Sun Nov 22, 2020 9:21 pm

by tjvirden

It's worth noting that an aerofoil produces maximum force when the air flows "smoothly" over it (so-called laminar flow, not turbulent); so a smaller tire (within the confines of normal road tire sizing) will almost certainly enable a higher lateral force..........

This for a "deep" rim.

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