Dopefish905

Been a problem with a couple different types of wheels. Riding a Cervelo s2 headset is tight, brake pads toe in is correct, both wheels are true to .5mm I've had front wheel shimmy with Mavic cosmic sls, and bontrager aura 5. Mostly only descents and braking when over 35kmhr. Both these wheels are alloy wheels with Carbon farrings. I put on my Mavic cosmic elites 30mm solid aluminum rim And no shimmy. It seems like the shimmy is more vertical then horizontal. Both Mavic and bontrager rims have different Tyres conti's and pro4's but both are round with no bulging spots. I talked to a guy at the lbs and he said most alloy rims with carbon farrings experience this do to being more flexible because the carbon farring is mostly for aero benefits? Anyone else riding aluminum rims with carbon farrings the experience some vibration on the front rim when braking at high speeds due to flexibility of the rim. The back wheel is fine it's just the front, the wheel runs perfect true and sound even speeds 70km+ it's just when braking it gets a up and down vibration!?

VNA

Check either for a flat spot on the rim or grease spot, or the head set is loose.

FBinNY 

Since it's a speed related event, I suggest it may be a harmonics issue. Uneven braking action due to variations in rim width, stiffness or surface cause pulses when braking, which flexes the fork or brake, and when the speed is right the effect is amplified through harmonics.

I wish I could do better, but with the limited info, all I can give is a possibility to consider.