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Geometry advice
Last Post 07/22/2020 02:19 PM by Orange Crush. 17 Replies.
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6ix

Posts:375

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07/20/2020 09:04 AM
Considering how long I've been riding, I really should have a better grasp on this but here goes. 

When descending on my Chapter2 Rere (combined with new-ish Mavic Cosmics mind you,) if I get any sort of crosswind, I feel like I'm quickly going out of control.  I drag the brakes and am scared to death as it just seems like it's extremely unstable.  Could it be the trail on the fork?  Is some sort of weird aerodynamic thing happening between the frame and front wheel?  I used to descent like a madman but now I'm going about as fast downhill as up.  I start going downhill and will have to grab the brakes thinking that I'm going to completely lose control.

Obviously, I'm still shook up from my crash this past fall when I got blasted from a huge crosswind that caused my bike to violently shake, sending me off road and crashing at pretty high speed.  I'm still experiencing the same shaking/pulsating as I did in that episode. 

Perhaps I've lost my nerve, that's very possible, but something else is going on. 
Cosmic Kid

Posts:3325

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07/20/2020 11:34 AM
Do you have another set of wheels you can try on the bike? That will start to eliminate where the problem is....throw some low profile alloy wheels on it and see how it feels. If it feels OK, then try some wheels in between those and the Mavics....maybe 35mm tall. See how those feel..repeat as necessary.

Heck, even some same-depth wheels of a different brand would help you zero in on the cause.

Overall, crosswinds should not impact general control as it relates to the frame.....instability in a crosswind is usually a factor of the profile in front of the steering axis.
Just say "NO!" to WCP!!!!
zootracer

Posts:721

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07/20/2020 11:51 AM

Agree, try another set of wheels to eliminate that problem. Most LBS will rent you a set of wheels. I had a set of Rolf Vector Comps on my old '99 Klein Quantum Race. Crosswinds made my bike unstable. Not as bad as yours though. Did you crash on on the bike you are riding now? Maybe you damaged the headset? I crashed years ago descending. Last thing I remember is my front end violently shaking. Knocked unconscious, no recollection of going down (fractured 4 ribs). I had my LBS go over the bike. Could find nothing wrong. Never happened again. I'm on my 3rd bike since then. Never a problem with crosswinds with any of the bikes.

Orange Crush

Posts:3133

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07/20/2020 12:26 PM
Agreed that eliminating factors is the way to go.

That may include having someone else take the bike for a downhill rip.
6ix

Posts:375

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07/20/2020 12:33 PM
In the spring and fall, I typically leave a pair of Ksyriums on there (have to swap out brake pads.) Can't say I've experienced the exact same situation so more than likely it is the wheels. If it's windy at all, the Cosmics are dangerous to the point I should really just get rid of them.

I did have my frame inspected and repaired following my crash by Broken Carbon here in Boulder. The only casualty was a small crack in the chainstay, likely from a rock or the chain whipping it really hard. Headset seems good. No play in it whatsoever.

Guess I've answered my own question. The wheels probably need to go. I'm just shocked if that's really the culprit here as these are their latest wide-profile, U-shaped versions, not the V-shaped earlier design. And they aren't even that awful deep.

I also have a set of beater Syncros wheels that are slightly deeper than the Ksyriums I could try out too.
Cosmic Kid

Posts:3325

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07/20/2020 01:22 PM
try the reverse for grins and giggles, too....throw the Mavics on another bike and see how they feel. if they are still a handful in the crosswinds, you have likely found your culprit.
Just say "NO!" to WCP!!!!
Orange Crush

Posts:3133

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07/20/2020 02:21 PM
Just get rid of the crosswinds and you should be all good and dandy.
6ix

Posts:375

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07/20/2020 02:27 PM
Man, I wish I could get rid of them at times. For instance, Saturday morning was pretty calm until I was descending back into the canyon. The wind builds up and comes at you from all over the place.

It's a shame because the Cosmics are otherwise awesome wheels with the Exalith braking but safety first. Think I'll shelve them for now. Not going to buy another set of wheels to replace as saving my pennies for all-new bike. Thinking the new BMC team machine that was released last week.
longslowdistance

Posts:2192

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07/20/2020 10:30 PM
"I feel like I'm losing control" is very legitimate . . . but also vague. It may be helpful if you can clarify your experience for us. E.g., is it or does it include front end shimmy? Visual or balance disturbance? Delayed response time? Just a sense of disassociation of your mind from your body's situation? Something else? Does the position of your hands on the bars, weight on the bars, or position of your head affect this? Caffeine? Any history of tremor in the family? (Google benign familial tremor)
79pmooney

Posts:2511

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07/21/2020 12:57 AM
6, have you tried different tire sizes? I went to the Mavic website and looked at the CXP Pro Carbon UST rim. I wonder if the wind flow is staying attached as it wraps around the rim, producing lift, the suddenly detaching to a very different flow. I'd a-thunk Mavic would have caught that early but - say the tire you are using is not what Mavic tested and that dynamic is happening. Well, if so, you just did some good research for Mavic (if you care to tell them) but maybe it's time to try a tire they envisioned.

This is all speculation. But I did study all the aero and incompressible fluid flow I could get my hands on in college as I was an obsessed sailboat racer and on track to be a designer. (Air at the speeds of bicycles is basically an incompressible fluid, behaving just like water.) The leading edge of an airfoil is critical to its performance - drag, lift and especially the resistance to the separation of flow. If the flow stays attached, that foil can produce high lifts - good for an airplane wing, less so for your wheel. Some of the second generation rims were very good at detaching flow from cross-winds making them far more predictable.

First thought - is your tire bigger than what Mavic envisioned? A bigger radius leading edge might encourage better flow retention. Good TT'ing in no wind, not with a crosswind. Also the discontinuity between the large tire and the rim might be making the flow attachment unstable. I'd see what Mavic recommends and try that. (I'm speculating that is a smaller less forgiving bump, traction and comfort-wise tire, hence you went larger.)

Feel free to tell me how out far to lunch I am. Being humbled is always good.

Ben
79pmooney

Posts:2511

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07/21/2020 12:59 AM
To follow up on lsd - if it is just this bike but in your head, you might have to take it with you to counseling.

Ben
Cosmic Kid

Posts:3325

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07/21/2020 11:24 AM
Zinn addresses high speed wobbles in this week's Technical Q&A....

https://www.velonews.com/gear/technical-faq-needing-a-better-seat-clamp-system-eliminating-speed-wobbles/
Just say "NO!" to WCP!!!!
6ix

Posts:375

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07/21/2020 01:23 PM
All good suggestions. No doubt some of it may be stemming from my mind as I'm still really shook up from that crash. BUT, I've had this bike for a couple years now and have descended at 60mph at times without issue. This was, of course, on the Ksyriums and not Cosmics which leads me to believe it's the wheels. Running 25mm tires so nothing out of the ordinary there.

I'm throwing the Ksyriums back on for now. My guess is that will solve the problems, leaving an expensive set of wheels hanging in the basement.
longslowdistance

Posts:2192

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07/21/2020 08:06 PM
Whoa, not "in his head" implying a confidence thing or similar, but rather in his organinic body was my gist. Heck, some people simply cannot ride a wheelie or a unicycle but otherwise can do many wonderful motor skills including brain surgery, I've seen it. I think there is some genetics to certain things, overlayed with wear and tear or preprogrammed later life things. Blame your parents. Or more fairly blame your parents' parents, etc. as your genes come from many generations.
Orange Crush

Posts:3133

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07/22/2020 10:52 AM
LSD - not an expert but wouldn't the fact that 6ix used to descent like a rocket rule out the generic explanation?

I'd think the "between the ears" explanation to be the more logic one (aside from fact that it could simply be the wheels). Minor nervous trembles amplifying while on bike. I have a bit of nervous trembles sometimes, although it is always before diving in, never during activity (this used to be literal as a youngster before diving into pool for a swim race). Also the worst bike wobbles I've encountered were from cold shake body tremors (OK aside from getting out of slip stream from a panel van and associated turbulence).
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