Bike handles poorly over about 23mph

[tincaman]

I have no problems until I get up to speed and especially downhill. It feels just like you are riding with a gusty crosswind, the bike will not track straight and wobbles around.
Its a new frame, PlanetX RT-58 v2, new headset, new bottom bracket.
Nothing has any slack in it, wheels bearings are all in good condition, no slack in the headset, and not too tight.
An ideas?

 

[Sharky]

Are the tyres seated OK?

 

[numbnuts]

Too shorter stem ?

 

[Deleted member 1258]

Headset too tight?, rear tyre not straight in the frame?

 

[Deleted member 1258]

Or handlebars not straight, pointing slightly to one side.

 

[Heltor Chasca]

Shift your weight back?

Consider that maybe it’s time to lose your Mohican hair style?

 

[Dayvo]

You're not braking too hard, are you?

 

[Tin Pot]

Nothing has any slack in it

All indications are that this is not true.

I’d first reseat the wheels, placing body weight on the frame.

Examine the headset and fork. It can be hard to see gaps, so again add weight possibly from a friend to see movement.

Check torques on all bolts, note anything overtorqued as well.

 

[Tin Pot]

Or handlebars not straight, pointing slightly to one side.

In the absence of obvious flaws I’d be tempted to take the whole headset/bars apart and put it back together again.

 

[Deleted member 1258]

In the absence of obvious flaws I’d be tempted to take the whole headset/bars apart and put it back together again.

That would be a good place to start.

 

[Globalti]

Nobody has asked about the wheels. How deep are the rims? 45 mm or above can make you feel like an invisible hand is pushing you around.

Headsets can be too loose even though you can't feel any slack. Release the pinch bolts and tighten the top cap down by half a turn.

 

[I am Spartacus]

I would also let us know the stem length.
I run RT58 as the hack bike, it is fairly innocuous in the handling stakes. I am able to throw up 2 hands waving the Vs when necessary after being hooted at for being on the road - no issue with the bike still going straight on.
Are the wheels, especially the rear - dead centre in the chain stays?

 

[freiston]

I had a bike like that once (didn't notice it starting at any particular speed though) but it wasn't worth enough to spend the money sorting it out (there were other issues with the bike too). I suspected that my problem was either a badly seated headset cup or the head tube not being faced correctly - I couldn't tell from a visual though (I suspected a bend to the forks but when removed and placed on a flat surface, they appeared good)

 

[cyberknight]

Dont ride fast simples

 

[DRM]

rear wheel slightly out of true, had this on my Defy, was terrifying above 28 mph, replaced the wheels for better quality/lighter ones had 41 mph downhill out of it this morning, not a whiff of instability.