Scary loss of stability - wheels the culprit?

[biwa]

Wheels: Farsports Kaze 35, 20/24 on Carbon Ti. Rim brake.

Are these tubular or clincher? Do they come significantly underweight than spec'ed, e.g. >5% lighter? If so, you may want to consider underweight rims (non-uniform and/or insufficient layup) a possible cause.

[kode54]

Maybe overthinking this. I ride with a few that ride TT bikes (not when I'm riding with them) that have experienced a similar occillation. What they tell me they do to stop it is to pinch the top tube between their legs to stabilize the bike. WHat I was told...its like when you run your finger to make an occillating noise on a glass (like at a wedding). I have not experience this vibration even with my O2 VAM, but I don't descend on curvy mountain roads at 65kph.

I have a 120mm stem which I prefer for stability...not sure a longer stem would help you.

[tleo]

Sounds a lot like what happened to Chloe Dygert last years UCI TT Championship race.

https://youtu.be/jI8zkvgTVGQ?t=72

[Mr.Gib]

Wheels: Farsports Kaze 35, 20/24 on Carbon Ti. Rim brake.

Are these tubular or clincher? Do they come significantly underweight than spec'ed, e.g. >5% lighter? If so, you may want to consider underweight rims (non-uniform and/or insufficient layup) a possible cause.

10 grams over spec - clincher.

Actually, do not apply motorbike technique to road-bikes. On a road bike you want to lean the bike more and keep the body further upright, the opposite of what motorbike riders do. The reason for this is that you want to keep as much mass pointing down over the tire, on a motorbike this is achieved by keeping the heavy bike straight, on a road bike it's the rider who has the most significant weight and so you want to keep your weight over the tire.
Sent from my LYA-L29 using Tapatalk

You are preaching to the Pope. People see a knee protruding a bit and think its moto gp. It's not. In fact all good descenders have their torso shifted the other way - over the toptube to the outside of the turn which allows for more lean and thus a tighter radius at extreme speeds. Even some counter rotation of the torso (turning the back slightly toward the inside of the turn) is helpful in maximizing control of the outside leg which is the key to traction. The opposite knee might protrude the other way to widen the base of support at touch and make room for lean angle adjustments.
This guy's technique is as good as I have seen and is IMO the only way to descend aggressively. https://www.youtube.com/watch?v=Zkpnj8WQGrA

[gurk700]

This would be a bad bet and there is no braking involved (reaching for the brakes with the bike leaned over at 65 km/h is not an option.)

Braking is always an option. Even at full lean. How and to what extent you apply is dependent on how close to the limit you are.

But if there was no braking, my next guess would be tire losing traction.
But this is where it's impossible for us to tell you what it might be. I have downhills here that have sweeping superfast turns up to 50mph where you don't even feel you're at the limit and ones that are sketchy at 20mph. How close you were to the limit is an unknown.

As far as lean goes, you can always take a look at what motogp riders do and apply it to some extent. Lean your body over a little more (obviously not as much as them) and keep the bike straight up more. See if you're still having the same issue. If no and you're just as fast, chances are tires were losing traction.

I try to apply this myself. I had friends who'd brag about dragging elbows in moto racing. We would take the same turns at same speed but I would keep the bike more upright and myself off the bike more. Result is less crashes, more traction.
It works on the bicycle too.

Anyway. Just a thought.

Actually, do not apply motorbike technique to road-bikes. On a road bike you want to lean the bike more and keep the body further upright, the opposite of what motorbike riders do. The reason for this is that you want to keep as much mass pointing down over the tire, on a motorbike this is achieved by keeping the heavy bike straight, on a road bike it's the rider who has the most significant weight and so you want to keep your weight over the tire.

You can see that with the pros descending but even more in crits.

Some of this (although with focus on MTB) is explained here: https://www.leelikesbikes.com/lean-angl ... ycles.html

Sent from my LYA-L29 using Tapatalk

MTB and Roadbike technique is VERY different IMO.

Just like Street bike vs Dirt bike.
I don't think above article applies to what I said at all. Road bike riding is closer to riding road motorcycle riding. Except you don't have the same limits (better traction cause wider tires more overall grip etc)

Keeping a bike more upright is always safer. The images above show crazy cambers that MTB riders are on. Where do you see that on the road? Lowering your center of gravity and putting more weight "inside" of the turn is not opinion or magic it's just physics. If you want to go fast, safer, that's what you do. Especially in not ideal grip conditions.

[Mr.Gib]

Isn't this a textbook example of a speed wobble? The Damon Rinard video/article covers it in depth. Changing the system via knee-against-toptube or similar should bring a rapid recovery.

Sort of, but the "shimmy" only occured at the traction limit with the bike leaned to the max. As soon as I backed off the bike was rock solid again.

Sounds a lot like what happened to Chloe Dygert last years UCI TT Championship race.
https://youtu.be/jI8zkvgTVGQ?t=72

The thought did cross my mind.

Ultimately I think what I am experiencing is the limit of this bike in a a very specific situation. I suspect it is a combo of the wheels, frame/steering geo, and tire pressures. Maybe 65 psi is just too little for these tires (28mm front, 30mm rear) on these rims in such an extreme situation. I was out again today and although the terrain I was on limited my cornering to about 50 km/h, the bike feels really good. It is silly light but also very solid.

[ghostinthemachine]

Do these rims have a slightly larger inner width than the last ones you used these tires on?
If you've widened the base of the tires you can change the limit performance of the tire. Also if there is any variation of the inner width (or the wall height) it can do weird things.
Someone up there mentioned the tread coming off the carcass, i've had this on a handful of tyres over the years. That's, errr, interesting.
Also, some tread compound/carcass/pressure combinations do exhibit a stick/slip behaviours at the limit of adhesion, had that as well. (At well over 65kph.)

[Cubist]

Balance your wheels with golf lead.

[Mr.Gib]

Balance your wheels with golf lead.

An excellent point. There are stories of people solving issues by balancing wheels. However the wheels appear very close to balanced on the stand - aluminum valve stems on my latex inner tubes probably help. They also feel well balanced at high speed in a straight line, but still something I will check. Tks.