XL 29er. 480mm reach. 40mm stem. 780mm bar, 40mm rise.
65.5 HTA
2.3x Hillbilly grid front 23psi, 30mm internal rim
2.3x Purgatory grid rear 28psi, 30mm internal rim
160mm Lyrik ~95psi no tokens, 2019 spring. ~20% sag, 51mm offset
150 travel out back on a Super Deluxe Debonair. ~190psi, 28% sag
All components free of play.
I'm 6'2", 85kg.
Generally it all feels really good. Hooks up in corners. Pedals. Has liveliness. Feels planted.
Up to speed on a road downhill out of the saddle the other day it all felt solid and then, best I can describe, I twitched the handlebars and the front went into a very squirrelly little flick. It felt that unchecked it would have developed into a speed wobble. From initiation, the wheel was driving where my hands went rather than my hands determining where the wheel went. Because I was alert I got it under control and then had a few more goes at inciting it; truly weird sensation.
So what is this? Too square tyres? Too little trail? Too soft fork? Too slow rebound? Mud tyre on tarmac?
I'm lost. Any suggestions?
When you have slack head angles, then there's more flop so an instantaneous flick of the bars can result in the wheel slightly lifting off the ground unless you have sufficient weight on it. You normally wouldn't notice it because your weight and gravity would push the fork down.
As your weight was presumably further back, that might explain it - a feature, not a flaw.
Nothing wrong, probably. Often just resonance between steering geo and frame front end twist/flex. Only takes a small amount of flex to set up a speed wobble motion to some extent and you want some of that flex anyway.
I suspect it's the short stem.
When I fitted my 747mm Knuckleball bars, I also got a 35mm Corto stem. For general riding, it felt great. But then I started doing tarmac hill reps, had one too many split second super twitchy moments on the sweeping descent, swapped stem back to an old 110mm (the one that was on my Tricross when I tried to make a jigsaw out of my upper jaw just over 5 years ago)... So much more stable.
bit dry but a good explanation of a tank slapper here:
https://en.wikipedia.org/wiki/Speed_wobble
I had this exact same problem once. Turned out I'd snapped the rear triangle on one side where it joined the bottom bracket. Took me a while to spot it under all the mud. I hope it's not the same
You're riding a 150mm suspension bike at speed on a road downhill. It's not designed to behave well there.
Mud tyre at low pressure at speed on a road descent is never going to work nicely.
I'd say it is all to do with the tyres. The back one has quite a bit more pressure than the front.It may just be the tyre tread.
Unless you're planning on riding the bike exclusively on the road downhill, I wouldn't be concerned at all. In those conditions it is well below sub optimal.
My bike is almost identical and going down a quick tarmac descent commuting earlier this week I did the same, purely out of curiosity. It did NOT feel nice, even though on rough terrain at similar speeds it feels fantastic.
I presume the root of the issue is the huge gyroscopic forces from a wheel that big and heavy, combined with very high levels of grip to the tarmac, and shifting steering trail as the effective contact patch centre point moves.
Never had exactly what you describe, but with some (more aggressively treaded) tyres I've noticed there's a bit of a transition where more knobs come into contact with the ground that can tug the steering a bit on tarmac. I assume that's your issue.
With that head angle, it's certainly not too little trail!