grizzly59

If you have rim brakes, I would use some dish detergent like Dawn and a scrubbie pad and really clean your rims. Clean the pads as well. Rinse extremely well. Brake heavy before the turn, light as possible in the turn. Check for proper tire pressure. Tight skewers, wheels set properly in the dropouts. Proper adjustment of hub bearings, rim shouldn’t flop around.

BforumBMC

I can only comment on my experience sitting on the bike whilst it happened underneath me. not pleasant and frightening when doing 60km/hr or even 30km/hr when I crashed. reading up on shimmying it sounds similar with the frame twisting rather than handlebars vibrating. However, only happens when braking? will need to find an answer before riding bike again.

BforumBMC

thanks for reply. about 6 weeks to mend, I expect if all goes well.

BforumBMC

Thanks for interest.

I love riding this bike it is great fun apart from braking on curves at speed which I must admit is a serious drawback.

I have braked from high speed many times whilst on a basically straight road without any issues as it never deviates and stops as one would expect.

BforumBMC

Thanks for interest.

I would not wish to swap bike as it is a delight to ride apart from this serious problem when braking on a curve/corner at speed. may have to if I can't find an answer.

Will get parts checked out as you suggest. Wheel set were rather expensive Vision Metron 40carbon fibre, but I suppose could have an issue.

BforumBMC

I was not conscious of going hard on the front brake and try to brake evenly on both levers. I am aware of braking prior to a curve is best but at times you don't know quite what to expect on new routes. Will certainly have to take it more carefully in future.

The Vision Metron 40 wheels also roll really freely which probably didn't help to hold back speed.

PeteHski

Yep, totally guessing here.

PeteHski

Well he could sell it and use the money to buy another used bike. Sure if the OP can work out what the problem actually is and deal with it then great. But he has to do something. The only other alternative is to avoid those problem situations by riding super slow on descents.

unterhausen

That sounds scary. Generally, braking and turning at the same time leads to bad things. Do all the heavy braking before you get to the corner. As I have gotten older, I don't go as fast down hills. I found out I don't bounce as well as I used to.

Once you get back on the bike, I think you should get up to speed on a straightaway and see how the bike behaves under heavy braking. And maybe practice going around corners somewhere quiet.

63rickert

60kph is a normal speed on a bicycle. It is very normal on a downhill. Any bike in good operating condition should be stable at that speed. It is also fast enough that it will hurt if something goes wrong. Downhill the landing point is a lot lower than the launch point, it is a big fall.

It has been mentioned a few times already that weight goes to front wheel when braking and moreso when braking downhill. Can’t be avoided. But you can do something. Change the center of gravity of that bike. You could install a dropper post - and use it. You could stop at top of descent and lower the saddle. Or you could move the biggest weight on the bike, which is the rider’s body. Slide right off the back of the saddle. Get behind the saddle entirely and then lower your weight towards the wheel. If you aren’t agile enough to do that maybe you just have to go slower.

75% braking power on front wheel has been mentioned. Sounds right for flat ground. Downhill it could easily be 90%. Or more. Test the back brake. When applying the rear brake does nothing perceptible you are on the edge. Basically you are attempting to go down the hill doing a nose wheelie. Even Danny Mac won’t do a nose wheelie at 60kph.

Going into a downhill turn the back wheel will always want to drift out. If there is little or no weight on the back wheel the bike wants to just swing around the head bearings. A bike with any slight tendency towards shimmy is going to start to shimmy. Get some weight on the back wheel.

Lifting the back wheel has been mentioned. You want to avoid that. It does not just happen. Unfortunately on a downhill it does not take much to get it started. A little bump. A rock or a twig. This can be controlled but not for long. Even best stunt riders cannot do it for long. Change center of gravity. Get the back wheel on the ground. Sitting back further makes the bike far less sensitive to that little bump.

And then do a lot of reading about shimmy. Has many causes. If the bike is in good condition the likely cause is the rider. Until this crash is well behind you and confidence is re-established best to go slow.

cyclezen

Hi John, sorry to hear about your injury - it's a common injury for many riders. One I've had the 'un-pleasure' to also experience...
BMC has many different models of bikes - if you know which model and year you have, that would help all in some understanding of the equipment involved.
Also, knowing this is a bit personal, it would be important to know your weight ? this has impact on how and when a bike might react to forces and braking stress.
Further down I'll make some comments on possible things to look at re the bike, and some things for more rider control and stable downhill riding.

'Shimmy' would have the handlebars going into a back and forth rapid movement, increasingly as it gets worse - Does that happen?
where on the bike does, what you call, 'Flex' happen - where do you experience it? is it up front at the fork, or some other area?

When you brake going straight, is there ever an experience of 'pulsing' of braking ?

Braking... Ideally 'braking evenly' does not mean 'with even braking force'. When braking, the forces load the front of the bike (this is unavoidable), so most of your 'braking' will happen at the front. It's important to brake lighter on the rear than the front, ESPECIALLY if you must brake in a corner. Braking too hard in the rear could cause lockup of the rear wheel and rear wheel sliding.
IF you are on the limits of rear adhesion/braking, you could be causing a 'slide/grip' reaction in the rear, which might seem slight at first, but will cause what seems like wild 'flexing' of the bike.
That's the precipice of crashing...
If the Headset of the bike is not properly adjusted (is either too TIGHT or too LOOSE) ! That will also cause wild bike behaviour in cornering under braking (and even without braking. This would normally not be apparent in straight line braking. Have your tech check the Headset for proper adjustment - VERY IMPORTANT !
Tips for improving your descending (when you have healed. LOL! )
1. DON'T Stiff-arm the bike. Bend your elbows strongly, bend forward and DOD put more 'weight' on the bars. Good weight distribution is important for proper riding and best outcome 45% front & 55% rear. The nature of the bike is that even with what seems 'all your weight' on your arms, you're not at that 45/55 %....
When Braking, the weight does shift forward, BUT locking your elbows causes the front end to become rigid, which is a sure formula for disaster. GET the weight onto the outside pedal, keep the elbows bend, stay down and forward. You would only slide back if you see a 'dip' or 'ditch' in the road surface ahead - something which would drive your total mass/weight forward in a very dramatic fashion.
2. Any time you do any 'turning/cornering' descending, put the 'outside' pedal down (outside of the turn) - lowest position, put as much of your weight onto that pedal, even by ever so slightly 'raising' your weight off the saddle (you're not actually coming off the saddle, just releasing that weight to handled by the outside leg). When practicing, it's ok to actually come off the saddle a bit; but not too far, maintaining saddle contact is important for fine control. Subtly will improve as your technique improves...
3. NEVER look down at your wheel or just in front of your wheel. Your attention should always be well in front of where you're riding... Be aware of road surface and conditions well before you get to them. The faster you go, the further forward is where your attention should be.
As applied in descending/cornering. Always LOOK AS FAR FORWARD INTO THE CORNER/TURN AS POSSIBLE. In a corner this means turning your head and putting your focus as far in the corner as sight line allows, often past the main part of the corner bend ! You should already be aware of what's in front of you and can keep that area, directly ahead, in the periphery of your vision.
Much of this is counter-intuitive and not a 'natural' reaction - it must be practiced !!! Then it will become 2nd nature and your cornering, descending skills and confidence will improve dramatically.
The 'Cornering' skills are just as important 'on the flat' as on a downhill. Especially if you are riding in a group.
Practice - Practice on a short and not-steep section, keep speeds well in bounds. Extend your 'reach' for longer, steeper, faster sections as skills improve.
It's well worth the effort and practice - descending is one of the purest joys of riding the bike !!! .... when it's not in done in white knuckle terror ! LOL !
would be great if you could give us some more info, - re my questions earlier in this post.
best of healing - Ride On!
Yuri
EDIT - also, 1. Has your tech checked your wheels for 'trueness' (wobble/hops in rims) ? 2. has your tech checked for smooth brake/cable function and brake pad condition ? all V-Important...

Mr. 66

I did have a bike that handled horrible in the turns. Fine in the straits, no swerve, no speed wobble. With inspection I discovered the rear tire had defect where the tread center had some wave to it. No hump just a small switching to the side and back. I thought it was minor, anything but minor in the turns. I thought I was going to wipeout, brakes or no brakes. I put new tires on, that solved my turning issue.

_ForceD_

If the bike has cantilever brakes you may need to change the length of the hanger. See the explanation at this link:

https://bicycles.stackexchange.com/q...tilever-brakes

Dan

63rickert

Never had that one, never heard of that one. But it makes perfect sense. At speed in a turn everything has to be right. Close enough only counts in horseshoes.

And if everything is not right, go slow.

icemilkcoffee 

Start with the basics: Where did you put your weight while cornering? You should straighten your outside leg and put most of your weight there. If all your weight is on your butt, or if you are gingerly trying to put your weight on both pedals, you will feel a lot more unstable cornering like that.

cyccommute 

Although your tire doesn’t sound like it was as bad as this one, the cause of the problem is likely the same. Threads in the casing can break for various reasons and the tire ends up with an S-shape to them. These are likely 60tpi tires. In the pictures you can see multiple snake bellies.

63rickert

There’s another side to this. Bikes are remarkably stable and forgiving. If they weren’t they would not be so much fun. The bike is your friend. It wants to stay upright.

Here are a few examples of things that have happened to me on descents. In turns. All of this was at 70 to 80kph with little or no notice.

Deer bounds off the hillside onto the road in front of me. Right in apex of turn and I am already committed to my line. Which way will the deer go next?

Tree limb down in the road, blocking entire road. Explains why there is no traffic going up. That limb was not there two hours ago when I went through in other direction. Of course this is the hairpin on the descent with the least forward visibility.

Descending with a group. They are good riders. Everyone has a clean line. Everyone gives their friends lots of space. And we are fast. Another rider hits a stone - or he hits something. Bike lurches and starts to slide and drift towards me. He basically body blocks me from the side. BANG.

In every case the bike or bikes stayed up. Too much happening on a descent to stop and think about it. It happens and it passes. The guy who ran into me did take a moment at the bottom to thank me for backstopping him. But not a big deal. It’s a bike ride.

The bike is your friend. Let the bike be your friend.

Miele Man

I've read many articles that stated that a bicycle in a turn when brakes are applied wants to straighten up and that that can lead to loss of control. The recommendation in t hose articles was to not brake in a turn.

I do hope that the OP heals fully and is able to ride and enjoy riding again.

Cheers

cyccommute 

Anything in a turn wants to go in a straight line. That’s just physics. Centripetal force (not centrifugal) is the force applied by the tire friction that is used to pull the bike (or other object) around the circle. The problem comes in when you apply the brakes and transfer more of the weight to the front wheel. All of a sudden, the friction that is needed to pull you around the corner is lessened…even halved. On a corner, the bike is likely dancing on the edge of cohesion with both tires at their maximum tire friction. Lift one up and things go south fast.

Rather than not brake in corners, you need a different strategy. Use a light hand on either brake but you probably want to treat braking like you would on wet roads. In other words, use more rear brake (to a certain point) than front and push back on the saddle to load the rear wheel like you would on a downhill. It’s a delicate balance, however. If the rear wheel does start to slide (and you have enough time), get off the brakes, specifically the front, and lean more.

Hiro11

OP appears to have abandoned this thread and hasn't responded the the many questions here.

Still, I'll comment:
The main rule is do not brake while cornering. If you find yourself braking while cornering, you entered the corner too fast for your ability. If the route is unknown and you find yourself braking through corners, you need to slow down. I don't mean to "told ya so" the OP in saying this. Proper technique is to brake fully before the corner, enter the corner at the proper speed that allows you to safely navigate without changing speed and then accelerate out of corners. Braking while cornering throws off your balance and introduces all sorts of difficult to control forces on your tires and bike.

Something that worked for me: if you really want to learn this well, borrow a cross bike or mountain bike and do some flat cornering drills on grass. Navigating through flat corners offroad really teaches you how to sense the limits of grip, how to best execute bike/body separation to lean the bike and how to brake properly. Also as the speeds are much lower in this type of drill, the consequences are much lower. Get some cones, find a field and give it a try.

Pratt

This discussion reminds me of a question I have about disc brakes. Do they cause the fork to flex/deflect in hard braking?

BforumBMC

Thanks for the input

I will certainly do as you and others have suggested and develop cornering skills as I can't keep doping what I currently am experiencing.

BforumBMC

I have endeavoured to answer these queries several times but none get posted. trying again.

1. Bike - 2020 Teanmachine SLR02 D Two - 54 cm. Has disc brakes. Vision Metron 40SLwheel set.

2. Me - 76 yrs, 74kg

3. Only really riding regularly with group since retirement, hence experience limited and this bike really picks up speed downhill.

BforumBMC

Thank

This makes sense. I thought that I was applying slight pressure but probably squeezed a little too hard. Will have to enter at a lower speed and control speed gain with rear brake rather than front. Probably feathering brakes a better option.

Maelochs

Yes, that is why everyone who rides discs and braked, has died.

First off .... Every force "deflects: or "distorts" the fork, but in microscopic amounts, usually.

I am not sure what your question is---are you asking if disc brakes flex forks Dangerously or excessively? The fact that the whole pro peloton has safely been riding discs for a few years (and trust me, they brake harder than you do) and none of them have died from "fork deflection," seems to indicate that the forks are strong enough for the forces involved.

Second, if the fork deflected in large amounts under Any load, people would crash and die.

If Your fork flexes noticeably while braking then you need to take your bike to a shop.

IMO.