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Front end shimy on my bike, anyone?(16 posts)

Front end shimy on my bike, anyone?Largo
Apr 1, 2001 7:30 PM
Was on a training ride yesterday, and while on a long fast downhill in a side wind, the front end started wobbling like crazy. It was really freaky, i thought i might end up dumping. My partner behind me saw it all.
The bike is a custom Neuron steel, with a 65 c-t seat tube, and a 63 c-c top tube. It has a oversized teardrop down tube, and a carbon fork with steel steerer.
It has been a nice stiff frame thus far (it is only a year old)
Anyone have any ideas? The drive train and wheels are all fine.
This has got me puzzled.
re: Front end shimy on my bike, anyone?Starliner
Apr 1, 2001 8:16 PM
Did it occur after hitting some rough or bumpy pavement? Something like that could have caused the fork to vibrate. Other causes besides the pavement might be a headset that is either too tight or with too much play, or even a good tire that is not mounted correctly.

At any rate, assuming that the bike checks out fine, and the road was as smooth as my girlfriend's derriere, then you might just have a situation where you and your bike are not compatible when going downhill real fast. Maybe your fork is just too light and flexible for your weight. If so, the shimmy mystery could be solved with a beefier fork.
I would check...JBergland
Apr 2, 2001 4:55 AM
... the wheels again. Both wheels lined up in the drops? Have you put on new tires recently? How about the magnet pick-up for the computer... same side of the wheel as the valve stem?

Wheels are a very common source 'wobbles' and 'shakes'.
Thanks for the input, but.......Largo
Apr 2, 2001 7:17 AM
The drivetrain is perfect, the fork is the stiffest carbon fork i could get from Mizuno, with a steel steerer, Vector Comps, so the wheels are burly.
I am at a loss. Maybe its got something to do with the length of the top tube, and it setting up a harmonic wave in certain conditions.
Thanks anyhow.
I have seen this a few times w/ steel. I would get aluminum inCXer
Apr 2, 2001 7:30 AM
that big of a frame. Steel is just flexy. Especially when youre riding a 63.
hogwashjoejoe
Apr 2, 2001 2:44 PM
nm
How about asking the frame builder?Cory
Apr 2, 2001 8:31 AM
I've had the same problem with two mid-range bikes ($500 10 or 12 years ago), but I sure wouldn't tolerate it from a custom frame.
I totalled one of the bikes in a crash (unrelated to the wobble--I hit some sand), but we never did find the problem with the other one. I checked it, and then an ex-US Cycling Team mechanic who lives in town went over it. We tried swapping tires and adjusting everything. The wobble didn't come in until about 40mph, and I don't go that fast very often, so I just lived with it.
If it makes you feel any better, it's a reasonably common (or at least well-known) phenomenon. I don't know if aluminum is less prone to it than steel, as somebody else suggested, but I sort of doubt it. You shouldn't have to worry about this with an expensive frame no matter what it's made from.
I had that problem...Cartman
Apr 2, 2001 10:11 AM
only at high speeds. The last time it happened was around 42 MPH, it about scared the S*$t right out of me! It would only happen over 35 mph. I told the LBS about it when I brought it in for my yearly tune up/ check up. The mechanic checked the headset and wrote on the repair slip that "The headset needed some adjusting - I hope this solves your high speed shimy." Unfortunately, I haven't gone fast enough to see if it worked.
The lovely speed wobble or tank slapper in the motorcycle world.Jimbob
Apr 2, 2001 12:59 PM
Ive seen this happen on small-tubed steel frames. They seem to resonate or spring back and forth causing the front wheel to shimmy uncontrollably until you slow down. Ive never experienced it but Ive seen it happen to some buddies and its ugly. Ive heard grabbing the top tube with your knees can help. This is one reason you dont see many or any DH MTBs made out of steel. Too much frame flex. You can get two-wheeled steering with this frame flex and it can cause problems in trying to compensate and correct. I think this gets worse as the frame size gets bigger like the guy above with the 65cm! frame. Viva aluminium grande para la descente!!
Baloneygrz mnky
Apr 2, 2001 1:16 PM
Steel having too much frame flex? Jezus, where do you get this stuff - sniffing too much tire patch glue? I guess that puts titainium in the "fishing rod" category. Anything can be made stiffer or more flexible by using more or less material. To say that one material is bad, regardless of design, demonstrates a real lack of knowledge or understanding. DH MTB's are typically made of aluminum b/c it's easy to work with and crank out the "design of the month" as are lots of production bikes such as C'dale, Specialized, et al.

Back to the subject - one of the best ways to cure the high speed shimmies is to get your wheels trued and make sure that all critical bearing assemblies are in good order and adusted correctly. A set of wacked wheels really becomes noticable at speed - even if they are ture, but have been through hell.
ti fishing rodsclub
Apr 3, 2001 4:39 AM
actually, ti fishing rods do exist. The butt sections of some big game rods are made of it, with carbon for the tip halves. Forget whose, I'll stick with my bamboo Battenkill for trout, but I got 4 ti bikes and none of bamboo.
And, depending on the size and tube selection, some steel road frames wobble because of flexy top tubes even when they're mechanically perfect.
Let me guess, Grz mnky, you ride steel. Maybe I came across....Jimbob
Apr 3, 2001 8:01 AM
wrong. I love the look, tradition, and feel of steel (in my size frame). In fact, some of my favorite bikes have been steel. I feel bad for all these steel builders like Ritchey, Masi, etc, etc, because they have contributed so much to the sport with their knowledge and great designs. Now Team Ritchey is riding aluminum bikes. Technology moves on. Steel is great but I think aluminum is taking over. Its sad for me to say it too. Anyway, enough about that. Lets address your statements.

Yes, steel in its traditional tube thicknesses, I believe, is too flexy for a 65cm frame. Sure, you could make fatter tubes and this would increase stiffness, but then it gets heavier real quick. And as we see mentioned here, yes, fishing rods are made out of titanium.

DH MTBs are made out of aluminum because in order to get the stiffness they need, steel would be a lot heavier than alu. Aluminum is very hard to work with. I don't know exactly how it compares to the dificulty of working with steel, but my guess is that aluminum is more dificult. I have a friend that rode DH for the Mt. Dew/Specialized team back in ‘94-96. In ‘94-95 their DH bikes were steel. Why did they change to aluminum??? To get the desired stiffness. Thats straight from the rider. And to clarify your "design of the month" statement, the majority of DH bikes have remained unchanged since ‘96 when the mighty Intense design stole the show. And today is still the design of choice for most.

Yes, wobbly wheels and loose bearings could cause these speed wobbles I would guess, but my friend's bikes were in perfect mechanical condition and they got the wobbles. They were bigger frames in steel. And yes, this was years ago. It's a flex shimmy not a loose rattly wobble.
Nope...grz mnky
Apr 3, 2001 9:18 AM
I ride a ti Serotta and an aluminum C'dale MTB, but have owned steel and carbon bikes.

I understand what you're saying, I guess the the thing that bugs me is that the mechanical properties for steel (Young's modulus, tensile strength, etc.) are better than either ti or alu., except for the specific gravity. To make a blanket statement that "steel is too flexy" and cite some industry trends as proof is a bit silly. After all the bike industry isn't exactly known for making the best choices when it comes to technology. There's a whole lot of junk science and psuedo engineers that aren't much more than modern day snake oil salesmen. It's just that the public hasn't figure it out yet. Fortunately steel frame builders aren't ready to throw in the towel just yet. The steel CSI Serotta frame is still a top choice amoung larger riders. De Rosa is still going strong, etc.

I believe that the previous poster stated part of the fishing rod that's made of titainium ain't the rod, but rather the handle. BFD. Everyone is jumping on the ti bandwagon, next we'll be presented with the virtues of titainium paper clips and staples.

Aluminum is NOT "very hard to work with." Take it from someone who knows - the stuff cuts like butter, you have to use a different welding technique and then heat treat it if you want any reasonable mechanical properties. In fact the stuff is so soft that you can't use a normal grinding wheel otherwise you'll clog it. In a pinch you can cut it with a carbide tipped blade in a circular saw. Essentially you can do all sorts of exotic shapes in aluminum without too much difficulty. This isn't true for the other frame materials of choice.

When you cite the input from the riders wanting a stiffer ride without the weight, aluminum does have an advantage. Since it's SG is quite low you can use a whole lot of it and get the rigid structure you desire at a weight less than steel even though it's strength properties are significantly less. Again it all comes down to how much material (thickness, in our case) you can use before the weight piles on. So in a sense you do have a point, but a lighter and less strong material allows you to use more of it. Ever notice that companies like Cannondale use the same frame on both their low and high end bikes (now they're incorporating pressed bearings in the high end stuff, but the frame is virtually the same). Aluminum frames are also cheap.

DH guys aren't exactly known for being on the leading edge of lightweight structures - many DH bikes are pushing 40 lbs. (or more) - - - motorcycle frames without engines. If they really desired strength and light weight they'd be making frames out of carbon fiber or ti. Handling and durability are more important than pure weight. There are still plenty of racers (road and MTB) on steel frames. Gotta get a good laugh at the Intense Tracer going for $1,600 for just the frame!

Fact is there is no free lunch and various materials offer different advantages and tradeoffs. I think the reason why carbon and Ti MTB's aren't more popular is b/c of the high cost of tooling and materials and the fact that the designs aren't very stable.
i am truly puzzled.LArgo
Apr 2, 2001 2:04 PM
I've eliminated mechanical or wheel problems. As far as speed goes, i wasn't going that fast at the time, because of the high cross wind. Maybe 60kmh tops. I've had the bike up to 55mph on a 3200' descent before, and it has always been rock solid. Weird!
It does wobble if i am riding with know hands at moderate speed and up, must be do to the size, i guess.
I am in touch with the builder, and they are working on it.
Thanks folks.
Maybe a suspension road fork might helpStarliner
Apr 2, 2001 8:56 PM
Never rode one on a road bike, but I imagine it might put a damper on the problem.
EXORCISM NEEDEDnm
Apr 3, 2001 4:40 PM
nm