BforumBMC

I have been riding a fantastic bike for several years now and have really enjoyed my time on this bike. I live in Perth, Western Australian and many roads here are reasonably flat. Hence steep hills are not always experienced. However, there are still a few around to test out skills.

I have enjoyed climbing many and also descending as well although probably not quite as confidently as speed makes for extra problems. Since I have had the BMC (previous bike, a Trek, was stolen), I have experienced some scary moments whilst descending. Coming down a reasonably long and quite steep hill, I braked whilst negotiating a slight bend. The bike reacted by flexing uncontrollably and would have tossed me off if I had not released the brakes. It quickly righted once the brakes were released.

On Wednesday last I was again descending a 5 - 8 % slope. I braked prior to the corner and braked into the corner when the bike again reacted by flexing vigorously and uncontrollably. I released the brakes, the flexing stopped but I ran on into a curb, crashed and broke my collarbone.

I have read many articles since and none describe this extreme response when braking during cornering. I figure that there must be something else happening here, such as some bike fault. It has been regularly serviced, by the cycle shop where I picked it up, so not any maintenance issue.

Any thoughts as this experience has been painful.

John

jgwilliams

First of all my condolences on breaking your clavicle. It happened to me last year - my first ever broken bone at the age of 63 - and I now have a piece of titanium in my shoulder. Not a pleasant experience.

Can you explain what you mean by 'flexing uncontrollably'. I suspect you are referring to a very unpleasant shimmying motion, which was also the cause of my crash. This does seem to be an issue with some bikes; it appears to be down to hitting a resonant frequency which is down to a combination of all the components involved, but I assume that the wheels and the frame are the major contributors. I read an article about it that suggested the way to stop it is clamp your knees to the cross bar.

rumrunn6

dunno, get well soon. how long will you be off the bike?

PeteHski

That was my first thought too. Never experienced it on any bike and if I ever did I would be selling it immediately!
Anecdotally I've heard that BMCs are stiff and aggressive handling bikes (massive generalisation I know, as we don't even know what BMC model this is). But I've read enough comments along these lines not to consider buying one.

jgwilliams

Ah, well, sadly we can't all afford to go out and swap a bike at the drop of a hat. However, it is a combination of factors which causes it so swapping any part may affect the behaviour, even something as small and insignificant as the stem. Certainly different wheels may produce very different results and that's a much more affordable change.

Troul 

uneven braking can be a result from wheel wobbling from an out of spec'd hub, poor spoke tension, PSI set inaccurately, brakes misadjusted, warped braking surface, bent fork, or a rare instance of one side of the braking surface being contaminated.

seypat

Seems like there was another thread about this within the last year. That thread had some videos documenting the shimmy/wobble.

PeteHski

Even when the alternative is having multiple scary moments descending, finally leading to a major crash requiring surgery? Sure you could start "experimenting" with parts, but this would be a red flag for me if the bike was well maintained (as the OP indicated it was).

jpescatore

It sounds like this only happens when braking on a curve downhill, so only when going faster around a curve than you would go when turning on the flats and not braking?

Using the front brake on downhill curves can cause the bike to move laterally in the wrong direction, I've always been told to only use the rear brake if you have to slow on a curve and ideally do all the braking before the curve. That was also what they taught us when I had to take a course to get my motorcycle license.

Do you think you were going heavy on the front brake on the curve?

nomadmax 

I'd start the ball rolling with going down descents a little slower and gain more handling experience. Speed is Feet Per Second, FPS. Roughly translated speed x 1.5 is FPS (actual is 1.466). The faster you go, the less time you have to correct a mistake. In my OPINION, when someone doesn't know why a bike started shaking it's head on a descent, it's a lack of experience. What I mean to say is, good descenders can "feel" what the bike is doing every second and know from experience what will induce untoward chassis movement.

OP, you're never going to get a clear and truly correct answer here (that includes mine) because no one here observed you descending. No one can say what your body position on the bike was, how much braking pressure you used, which brake was biased (if at all), the road surface, road camber, it really goes on and on. The slower you go to learn the importance of these things, the better off you'll be. As speed increases, ALL of these things become important, sometimes you can only get one of them wrong and you're on the deck. I hope you heal fast.

shelbyfv

I'd suggest you first read up on "shimmy" and determine if that is what you are experiencing. You'll learn that it's a bit of a mystery but there are a number of things you can check, fingers crossed. If your issue seems more pronounced and random, you might check the frame for cracks, especially in the rear triangle. Tap test and flexing the bike while standing on a pedal would be a good start. Good luck

WhyFi

How old is the BMC? They had a fork recall a few years ago.

PeteHski

I could be wrong, but the way I read the OP's post was that this was not an issue with his previous bike. So more of a bike specific problem rather than a general lack of experience.

jgwilliams

Well, if you don't have the money for a replacement there really isn't much you can do. As it happens, I broke my frame when I had my crash. I managed to get a used good frame at a reasonable price which only involved getting a new bottom bracket. There's no way I could have afforded to replace the bike, or even get a new frame. If you've not been in that position then you are fortunate.

Flip Flop Rider

gotta be a brake and wheel issue right? The shimmy only happens when braking. Sounds like the brakes are grabbing the wheel but I'm no expert

njkayaker

The OP is complaining about the bike "flexing uncontrollably".

It seems that he might have had the BMC "for several years". He's not really clear about anything. The OP's words are not really enough to impugn the bike.

njkayaker

There isn't enough info.

We don't know the model of BMC. We don't know the model of Trek it's being compared to.

BMC's shouldn't be doing any "uncontrollable flexing".

It's certainly possible that it's some sort of "death wobble".

He takes it to a shop for service. What do they have to say about it?

WhyFi

Tire flop could also be a culprit - pressure too low, loading the front tire while braking and cornering.

Maelochs

Unless the frame is actually damaged or faulty ....

I cannot see a BMC frame "flexing uncontrollable" or whatever .... wheels, maybe. I'd check the headset, the wheels (hubs and spokes.) I'd check the QRs if it has them. I'd go over the brakes carefully.

Disc or rim brakes, by the way?

Litespud

you’d be nuts to not use the brake that provides maybe 75% of the braking (ie the front brake). If you can’t use the front brake without the bike “moving laterally in the wrong direction”, time to go back to school.

noimagination

I doubt that the bike was flexing. Most likely, what you experienced was the brakes "pulsing", which can have various causes (brake pads old/worn; contaminated rims or rotors; brake arms or fork flexing {unlikely}). You could also get this feeling if the road surface is uneven.

As you've discovered, you should do your braking before the turn. Braking in a turn shifts the weight of the rider/bike system to the front wheel, which is trying to turn. This will exacerbate any road bumpiness, imperfections in brakes/braking surfaces, etc. It requires stronger steering inputs. It puts your weight towards the front wheel and towards the outside of the turn, increasing the risks of high-siding (going over the bars on the outside of the turn) and it interferes with the lean of the bike (causing it to stay more upright) which makes your turn wider and may cause you to run off the road towards the outside of the turn. You should brake in the turn only in an emergency, and you'll need to be ready to counteract the forces pushing you upright and towards the outside of the turn. Usually you're better off not braking but rather increasing the bike lean, increasing pressure on the pedal on the outside of the turn, and increasing downward pressure on the handlebar on the inside of the turn. And looking at the exit of the turn (i.e. where you want to go) rather than where your bike is trying to go.

fujidon

My touring bike (which I no longer ride) came with cantilever brakes which were marginal so I replaced the brake pads with grippier pads. On a downhill, but not turning, the front fork started oscillating uncontrollably causing me to be unable to stop. Fortunately, I had a place to turn and roll safely to a stop. In order to turn, I did have to release the front brake. The rear brake alone was not sufficient by itself to stop.

Basically, the new pads caused harmonic resonance in the fork. I still don't know how or why. I re-installed the original pads since I didn't have that particular problem with them and planned my stops a little better.

My new touring bike has hydraulic discs. Stopping is a non-issue.

Iride01 

Descending turns are something you admit you aren't use to doing. Don't do them so fast. At least not till you get some experience with braking while in a turn.

Sometimes I have to force myself not to brake and when I do brake, it's ever so slight a braking force I use. A tiny road bump while braking can momentarily reduce the tires grip on the road and it's all over with at that point if you are braking in a descending turn and the wheel suddenly locks up.

cyccommute 

You are describing braking in a corner wrong. You aren’t likely to “go over the high side” on a corner unless your wheel slides and hits something like a curb. Slight imperfections in the road aren’t going to catch the wheel and cantilever the rider up and over the bike. Braking in a corner does transfer the weight to the front wheel but it also lifts the rear wheel. In a straight line, this translates into a lifted rear wheel that locks easily and goes into sliding friction rather than rolling friction, i.e. a skid.

In a corner, the same thing happens but the connection of the wheel to the ground is slightly lower so the wheel slides more easily. When it does slide, it slides in a straight line that is tangential to the corner. In other words, the tire slides towards the outside of the curve. Being in a lean with less contact with the tires front and rear causes the tires to slide more easily and to “wash out” and the rider goes down. As the rider you aren’t likely to be pushed up but you will be pushed out. It happens very quickly and is hard to correct for. It’s not as fast as going down on ice but pretty close.

You are right that you should be delicate with brakes in a corner and to put more pressure on the outside pedal as it increases force and friction on the little edge of tire you are using. But stuff happens and if you happen to find yourself braking (and sliding) in a corner, the best course of action is to get off the brakes. You should get off the front brake specifically but getting off both is usually the response. This stops the transfer of weight to the front wheel and settles the bike back down to improve the friction on the rear wheel. The rear wheel transitions to rolling friction instead of sliding friction.

Ironfish653

Hard to say what's exactly at play here. Heavy braking while turning a two-wheeled vehicle makes it want to stand up and go straight. One of those things a 500 lb motorcycle won't let you get away with, but a cyclist can 'muscle through' (until you can't)
Rider position / weighting can change the way the bike handles as well; too much weight forward can cause headshake like that, especially with too-aggressive braking, and/or riding with your arms too stiff can also lead to the rider fighting against the bike