Scary loss of stability - wheels the culprit?

[Mr.Gib]

An interesting one for the wheel people.

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

Bike: Factor O2 VAM size 56

Rider: 77kg

I experienced a loss of stability in a high-speed left-hand curve, at 65 km/h, and with the bike at maximum lean. At the point of maximum g-force, the bike suddenly struggled to keep its line and I struggled to maintain a consistent lean angle. And it happened a second time on a different but equally challenging and slightly faster high-speed curve. It felt like the level of grip was oscillating, sort of a repeating bite and release (shimmy?) sensation which forced me to stand the bike up and straighten out a bit. It seems to begin with the bike turning in quicker than expected and than giving up the line and feeling like it is going to seriously wash out and put me down. After that it goes to hell if I try to force it to stay on line. In the second instance I needed every inch of road to finish the curve on the tarmac. It is noteworthy that the bike is perfectly stable in a straight line and corners nicely but for these extreme situations. Also noteworthy is that I have ridden these two curves hundreds of times on different bikes with different wheels at the absolute limit without problems.

Although it seems logical that the front wheel would be the problem, I suspect that the culprit is the rear wheel. Could it be possible that some of the non-drive side spokes are going slack as pressure and uneven forces build up? The front has seemed relatively solid in a range of cornering situations including some bumps and compressions whereas I have heard pad contact at the rear brakes in some slower but tighter cornering situations (when not applying the brakes). The front also feels laterally firm tugging on the rim, while the rear is noticeably less so. I also shift my weight back a touch during hard cornering so more load on the rear. And there is only modest shimmy sensation coming through the bars - its more of a "whole bike" sensation. Spoke tensions are within spec.

Another factor that has me suspecting the rear wheel is that the problem has only appeared turning to the left. Unfortunately, the fastest right handers in this area are about 55km/h so I can’t do an exact comparison, but it feels very solid turning right a 55km/h.

Of course, the quick test is to stick my Shamal Ultras in and hit the same curve again. But I wanted to check with the WW wheel gurus first before going through the hassle of adding spacers to the EE brakes.

Naturally the frame is a suspect. It is relatively new to me but OTOH it is also Pro Tour tested, and I imagine if there was an issue no one would ride it. I think even a few ISN riders selected it for certain stages of Romandie even though the Ostro is also available.

Thoughts?

[gurk700]

Front wheel issue
Headset issue
Road Surface
Bad technique (too much weight on hands, bad braking technique etc)

I'd bet money on one of those before the rear wheel or frame / fork but it's impossible to know reading about it on an internet forum

[alcatraz]

65km/h at maximum lean. You are brave sir. Hehe...

Maybe you should get a camera and film the wheel.

What about your tires and grip? Isn't it possible you went over an oil patch or something on the road?

Is your bearing preload set properly to take the play out?

[Mr.Gib]

Front wheel issue
Headset issue
Road Surface
I'd bet money on one of those before the rear wheel or frame / fork but it's impossible to know reading about it on an internet forum

I'd make the same bet but no. Obsessive over every detail of my hardware and the road is the same as always.

Bad technique (too much weight on hands, bad braking technique etc)

I'd bet money on one of those before the rear wheel or frame / fork but it's impossible to know reading about it on an internet forum

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.)

65km/h at maximum lean. You are brave sir. Hehe...

Maybe you should get a camera and film the wheel.

What about your tires and grip? Isn't it possible you went over an oil patch or something on the road?

Is your bearing preload set properly to take the play out?

I have a background in downhill skiing. Been over 100 km/h on skiis. If the technique and equipment is good, brave is not required. Same on the bike though during yesterday's "incident" bravery did enter into it - I did not soil my bibs

Hubs pre-loads are set perfectly. No oil on the road. Tires are Corsa 2.0, fabulous grip.

[rollinslow]

Check all the spoke tensioning, check hubs for play, check headset, check that the skewers are tight, check tires. Maybe take off your tires and carefully inspect you rims?

[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.

[alcatraz]

Chinese are practical people. 50km/h is already considered fast without cornering. I wonder if the wheels simply haven't been tested or weren't intended to handle well at speeds over that.

My guess is it's spoke/rim related. Too lightweight.

Measure the tensions just to be sure. Maybe it's an easy fix.

[gurk700]

Chinese are practical people. 50km/h is already considered fast without cornering. I wonder if the wheels simply haven't been tested or weren't intended to handle well at speeds over that.

doubt this. those chinese wheels aren't any different in design than most big brands. difference comes in quality control and marketing only not design.

hell, some big brands let manufacturers design the molds and BS us with aero / stiffness / etc marketing.

[alcatraz]

Chinese wheels often have the lightest spokes. The rims are also quite often lighter.

That's what I mean. Built for casual riding before extreme riding. But there are exceptions of course...

[Mr.Gib]

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.

It's possible, but I have had the same tires on these turns at these speeds many times without problems (just on different bikes and wheels).
As for how close to the limit... the way I look at it is that I was taking the curve at the maximum speed that it can be taken on a bicycle without sliding out. The bike lean is in a sense both the maximum and the minumum by necessity, a factor of the turn radius and bike speed. Perhaps some very light feathering of the brakes would be possible without losing it, but it would never occur to me knowing the curve(s) as well as I do.

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.
...It works on the bicycle too.

This is a super interesting point which begs a question - does a bicycle tire have more traction when the contact patch is closer to the center of the tread all other things such as speed, radius of turn, rider mass, surface, etc., being equal? If so this would be a good justification to go with moto gp style of cornering. BUT the rubber is the same (assuming uniform rubber compound), the contact patch is the same, and the total force vector on that contact patch is the same, so why would there be more traction in the center of the tread?

My intinct tells me that the traction is the same, and that by standing the bike up and leaning the body more, all that is accomplished is putting the body in a very unathletic position ill equiped to control the lean angle of the bike or any deal with road imperfections. Moto gp riders are also physically engaged with their machines with the inner thigh, crotch, back of the knee, calf, even the torso, etc. A far cry from hanging a torso sideways off the top of a tall, tippy bicycle. Furthermore the angle of forces against the frame and wheels become more lateral as opposed to being stacked in line when the body and bike are in line. The stacked-in-line arrangement of the bike-rider system should be a more stable overall structure. Also hanging off the bike also limits the ability of the body (particularely the legs) to act as a shock absorber, a critical element absent in bicycles but of course present in motorcycles.

My understanding of moto gp cornering technique was that it was a product of the mass and traction of the motorcycle, and that the motorcycle was already leant to its maximum given the corner radius and speed, and to corner faster still, additional mass away from the contact patch was required to keep it from standing up and going straight.

I am not a knee hanger in high speed turns on a bicycle - I thinks it's bs. Slower stuff, sketchy surfaces sure, but that is really just to facilitate steering, but never in high speed sweepers.

[Mr.Gib]

Chinese are practical people. 50km/h is already considered fast without cornering. I wonder if the wheels simply haven't been tested or weren't intended to handle well at speeds over that.

My guess is it's spoke/rim related. Too lightweight.

Measure the tensions just to be sure. Maybe it's an easy fix.

Chinese wheels often have the lightest spokes. The rims are also quite often lighter.

That's what I mean. Built for casual riding before extreme riding. But there are exceptions of course...

I'd like to start with the assumption that my sketchy moment is not a product of some distinctive cultural characteristic. And besides, practical would imply that all best practices were followed to provide a safe and satisfying product for all customers - which is I am sure what Farsports have done. These are businesspeople afterall, and good one's based on my experience.

The spokes are Sapim CxRay and perhaps you are right that the build is too light - just not robust enough for the forces I am generating. I take my descending seriously, although I have significantly mellowed with age. Used to be insane. Lucky to be here when I think about.

Good news is I have a deeper set of wheels coming with Sprints on the drive side rear.

[gurk700]

It's possible, but I have had the same tires on these turns at these speeds many times without problems (just on different bikes and wheels).
As for how close to the limit... the way I look at it is that I was taking the curve at the maximum speed that it can be taken on a bicycle without sliding out. The bike lean is in a sense both the maximum and the minumum by necessity, a factor of the turn radius and bike speed. Perhaps some very light feathering of the brakes would be possible without losing it, but it would never occur to me knowing the curve(s) as well as I do.

That's why I meant that comment to be "to an extent"
There's no way we could do nearly as much as we could on a moto but it's all about changing center of gravity. We're already doing it on the bicycle with our knees. We lift knees that are on the side we're turning towards. You can also drop your shoulders a bit and use your elbows without sacrificing much. On a 50km/h curve I feel like you're not pedaling so it's all about center of gravity and aero.

As far as the contact patch, while motogp tire compounds are very complicated (i.e. different hardness on the edge [even different from left to right side depending on track they ride sometimes] and in the center) I think regular non-prototype racing tires (dots or slicks in superbikes and more amateur racing that you can just go out and buy) are pretty much same compound everywhere and you still utilize the same techniques. I think it's more to do with the rounded off profile of a tire that makes the center squish more and let you have more contact patch rather than different softness / compound.

In any case, even if contact patch size and material is the same, you're still changing the center of gravity in favor of grip. I crashed going around the speeds you mentioned in 2017. It was warm and dry but the shaded parts of the mountainous areas in descends were still a little damp. I lost traction while not even braking. The first half of the corner was all dry while the second part was in the shade. Temperature, moisture etc has a lot to do with traction. I think it's never same day to day or even at different times of the same day.

But who knows. I'm throwing ideas out there as possibilities. Might very well be some mechanical issue.

PS: Even on moto, Ken Hill, one of the top instructors around here for moto mentioned once that made me go "huh!" Our heads is a crazy proportion of weight of our whole body. Even on moto where the system weight is anywhere between 450-550 lbs dropping your head a little lower helps turn the bike easier. Imagine what that does on a bicycle! I personally test this every time and see the difference. Find a long sweeping corner and start turning with your regular body position. Then bend your neck just a little and make your head go lower. You'll start turning tighter.

[naylor343]

Interesting you say you are on vittoria corsa tyres. Have you made a detailed inspection of the tyres? I had problems with my corsa controls, the bonding between the tread and the tyre carcass failed in multiple places. In some areas it wasn't obvious as at rest the tread would sit flat and look to be normal, but when moving it with my thumb it lifted quite easily.

It sounds like the awareness of your equipment is high, but worth another look possibly.

[numberSix]

"At the point of maximum g-force, the bike suddenly struggled to keep its line and I struggled to maintain a consistent lean angle. And it happened a second time on a different but equally challenging and slightly faster high-speed curve. It felt like the level of grip was oscillating, sort of a repeating bite and release (shimmy?) "

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.

As to hanging off/lowering CG, the purpose is that for a given speed less lean angle is required. In low traction situations the MX body position is better. Less lean angle generally permits recovery from a small slide.

YMMV
6

[Cycomanic]

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