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Braking question(11 posts)

Braking questionseanf711
Oct 17, 2003 7:50 AM
Yesterday on an extremely steep hill I had to brake hard for an unexpected road obstacle. At the time I started braking I was going 47mph. When I was braking (the wheels were not locked) the bike went in to viloent left to right shaking. So much so that I almost lost it. Is this normal or is there some braking technique that I am not aware of?
skidding?DougSloan
Oct 17, 2003 7:58 AM
Were you skidding, particularly the rear tire? That would easily account for it.

Could be your fork was flexing a bit.

Hard to say. If skidding, obviously just don't brake as hard, and get your weight back.

Doug
sounds like you induced a speed wobble (nm)ColnagoFE
Oct 17, 2003 8:11 AM
sounds like you induced a speed wobble (nm)seanf711
Oct 17, 2003 8:21 AM
Do you know what the cause or the cure is?
happened to me once on my MerlinColnagoFE
Oct 17, 2003 9:19 AM
going about 40 on a steep potholed road (Lizard Head pass in CO) I smacked a pothole good and next thing I knew the bike felt like it was trying to buck me off. I grabbed the brakes and it kept shaking violently until I got to about 5MPH. How I got stopped without falling off is a mystery. It was a close call. I went to inspect the bike and found nothing wrong. I rode the rest of the week on it with no other problems. Best I could figure out that the pothole hit caused an oscillation in the frame and/or fork. If I would have been thinking more clearly I would have immediately grabbed the top tube with my legs to try and stop the spring. Might have gotten the shaking stopped sooner, but as is I feel lucky to have held on without bailing. Make sure headset is adjusted right and stiffer wheels/frame might help prevent this, but there is really nothing you can do other than that.
I think you tried to buck yourself off-filtersweep
Oct 18, 2003 6:35 AM
I've had this discussion with a riding buddy- there is a long hill where this has happened to both of us. A third friend, who has less of a sense of self-preservation does not have the same problem. He obliviously rode right through the gauntlet of terrible road surface.

The theory is, while the road surface at the huge hill is undeniably wavy, it creates anxiety in the rider (as the little dips throw the rider around), and the rider tenses up. If the pedal is fully extended (to 12 and 6 o'clock) it places more weight on the BB which adds to the problem. The rider white knuckles the bars with the so-called "death grip" and in essence, the rider is shaking the bike all over the place.

Anything can set these events in motion- but a jarring pot-hole at speed can induce panic (or a "reaction" if you prefer) as shown above. It is not a mechanical issue. If there were mech issues, you'd be affected by them at all speeds. I highly doubt a bike can resonate to the point it is uncontrollable. Most people would rather blame equipment that look at it as their own issue, but I'd bet it has happened to all of us.

I try to achieve a zen like state on these types of descents. I also find that where I look ahead of me can affect the ride- the further ahead, the better.
Was the shaking coming from the back or the front?cmgauch
Oct 17, 2003 8:21 AM
Remember that while braking, your front wheel is doing the majority of the work. Check your front end & front wheel for anything out of the ordinary, like a bad braking surface, slight bend in the rim, loose spokes, loose headset, cracked handlebars, etc.

If it came from the back of the bike, you could have been skidding like Doug asked, or maybe you have the same wheel issues I mentioned or even a cracked or broken frame – check the chain/seat stays (esp. the drive side).

In a panic stop I get my weight WAY back (pretty much behind the saddle) hands in the drops and grab my levers as hard as possible. You modulate the front lever (and the rear-ward weight shift) to keep the nose-wheelie under control. If you have to stop completely, once you scrub off most of the speed you will probably end up with your back wheel off the ground, not a bad thing if you do it right.
that shouldn't happenChazWicked
Oct 17, 2003 8:41 AM
Unless you were unknowingly skidding (which I doubt) then there's likely something not dialed in on the bike. And, if you see 47mph with any frequency, you should really make sure everything's perfect.

Suspects: Brakes-make sure there's nothing loose that might induce an oscillation. Also make sure the pads/rims are clean and adjusted. Hmmm, you *may* have a flat spot on the rim from a pothole or whatever. This would create a pulsing effect on the brakes.

Headset: I've made the bike wobble quite well with a mal adjusted headset. This was at much lower speeds however.

Hubs/wheels: Make sure there's no bearing looseness in the hubs and the wheels are true & tensioned well. Make sure the tire is mounted such that it's perfectly round (the rim could be dialed and the tire not seated properly).

That's a tricky one. I'd visit that section of road at a lower speed to see if there was anything odd about it. Grooves in the road???

I have one frightening thought. Perhaps in your aggressive braking your rear wheel left the ground. Then when it re-engaged, it was not quite straight, introducing the wobble you describe.

If this ended up being the case... it should be second nature to throw the bike forward (i.e. moving your weight back) when braking hard. This is instinctual for the mtn biking community (& Lance, given he pulled off the cyclo move on the Tour). But you probably know this given your high speed descending.

Take it easy out there.
Chaz
tankslappersSteve_0
Oct 17, 2003 9:10 AM
not necessarily indicative of faulty equipment. Precise road undulations for a given speed can result in violent oscillation (tacoma narrows anyone?).

Combine that with overcompensation (read: tension on the riders part), and yagotyourselfawobble.

Cause of the death of many a motoryclist.
High speed shimmies, how to solve.bimini
Oct 17, 2003 9:36 AM
This has happened to me at least once on each of the bikes I've had over the years.

The last was when a squirrel did a kamakasi through my spokes at 42 MPH on a steep hill. Fortunately, I was braced for the problem and the shimmies went away after only a couple of undulations.

I've taken to using my knees to brace the top tube on high speed down hill coasts. This stiffens up the bike. I also get down in the drops and have my arms and hands ready to brace in against my knees to hold the bars solid if the shimmies start.

If they start, be careful not to overcompensate, it only makes it worse. Just try to lock your arms in a stationary position (easier said than done).

At high speeds it is best to ride around obstacles and avoid the brakes. Too easy to skid out of control and you won't stop in time anyway.
Locking arms INCREASES overcompensationSteve_0
Oct 17, 2003 9:54 AM
best to be loose; the bike WANTS to stay up on its own.