Do 36" rims flex sideways a lot?

[monocyclism]

I have set up my Nimbus 36er with Magura HS33 brakes. I have experimented with setting the distance of the brake block against the rim. I thought it seemed OK with a close setting as long as the brake block doesn’t touch the rim as it rotates.

However, I notice when riding if I twist or turn suddenly I can hear a brake block momentarily touch the rim without the brake being applied by me. From this I assume the rim or the frame is flexing slightly and resulting in the rim coming into contact with the brake block.

I think this is normal but I'm not sure. With the unicycle standing I can grasp the top of the frame where the wheel rim runs past it and by gently pulling sideways on the wheel I can slightly flex it. Clearly, I’m not attempting to bend anything but I suppose this slight sideways flex is inevitable in a 36er being such a large wheel? Would I be right in thinking this?

[tholub]

Yes, 36" wheels tend to flex. Having an ultra-wide hub is partial compensation for this tendency. You can improve it by raising the spoke tension. But there will always be some flex.

[Rowan]

Originally Posted by monocyclism

However, I notice when riding if I twist or turn suddenly I can hear a brake block momentarily touch the rim without the brake being applied by me. From this I assume the rim or the frame is flexing slightly and resulting in the rim coming into contact with the brake block.

I think this is normal but I'm not sure.

I've heard that the Nimbus frame is much more flexy than the KH frame. I don't use brakes and I've never noticed the flex but I am now wondering about the usefulness of the super-wide hub. I want to try a KH frame maybe (or wait a while for a V-frame) but that would require a wheel build since it uses a standard width hub.

Originally Posted by tholub

Yes, 36" wheels tend to flex. Having an ultra-wide hub is partial compensation for this tendency. You can improve it by raising the spoke tension. But there will always be some flex.

I was looking at my super-wide hub and thinking it is probably wider than my cranks are long (114mm) and surely that is not ideal. Some side to side wobble must be caused by having cranks so far apart. I don't ride my Nimbus 36 offroad so maybe the wide hub is overkill.

[saskatchewanian]

If the wheel is tensioned properly a 36er wheel with a super wide hub is pretty solid but the Nimbus frame is fairly flexy side to side.

[corbin]

Originally Posted by Rowan

I've heard that the Nimbus frame is much more flexy than the KH frame.

I have verified that is true by riding both. The KH frame is much stiffer. My nimbus frame has wear marks on the side of the frame where the tire rubbed. One of the reason I moved to a cable brake (from maguras) was due to this problem. I may try maguras again on my KH36 when I start riding it a lot and get the hub rebuilt.

corbin

[johnfoss]

All the large wheel frames I've ridden are noticeably more flexy than smaller frames. I think it's the frame more than the wheel, and of course the degree of flex goes with the specific frame. Even my Coker V2 (squid) frame made the occasional brake squeak under steep pedaling. My original Coker (old) frame does this a bit more, and my old 45" unicycle (with Schwinn-style frame) does it like crazy. The Nimbus is probably the most flexy of the commercially available 36" frames due to its small-diameter tubing but they all flex some.