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A couple of days ago I fell off my bike. I was on a wet, steep downhill, asphalted road with a lot of curves. I didn't expect neither the weather, nor the 15+ km downhill route. I was always reducing my speed to 9-10 kph before taking a turn. However, just after a taking a turn and before another curve, when I pressed the rear brake lever, the wheel stopped turning immediately, despite the slight pressure, and started to slide. Worse, the bike didn't stop, it started to accelerate. I pressed the front brake lever, slowly initially, but since the bike was still accelerating, I pressed more was thrown off. Never mind the damage; I got off with minor injuries, the bike as well. This could have been due to my being careful to begin with, but not enough, apparently, so my question is how can I be better prepared in the future? And what might have caused the tyre to slide? The road was wet and with minor holes and bumps, sure, but I had gone through similar terrain seconds ago and I could brake just fine. And I'm sure I didn't press the lever way too hard than perceived. Also, there wasn't any wet sand. None that I could see anyway.

In hindsight, there are a few things that came to my mind. I'm not entirely sure about any of them, however, since I have just recently started to cycle since my childhood/high school years and I just discovered that there was stuff I was missing out on back then (such as bike fitting and geometry).

  1. The saddle could've been lowered. I normally ride so that my knees bend at a 25-30 degrees at the pedals' lowest position. I assume lowering the saddle would've necessitated much more effort from the front brake to throw me.

  2. My bike is suited for XC racing. Would picking a downhill bike, with the appropriate frame geometry, have made a difference, provided we're talking about an asphalted road?

  3. Should I have kept the rear or both brakes engaged constantly? At the time, I didn't want to do this out of fear that they, being disc brakes, would overheat and the rotor would be destroyed. The route I was on was at least 28 km long, mostly steep downhill, save for the last 10 km or so, which were a rather pleasant, slight descent.

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    The main thing is being careful. Wet roads are tricky, and some things people don't recognize are things like wet leaves being like sheet ice. – Batman Oct 10 '16 at 16:23
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    I have fallen a few times on ice or leaves or an oily surface. If the surface is at all "questionable" you must carefully control your speed, and try to do all your braking only when going straight. – Daniel R Hicks Oct 10 '16 at 23:01
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    What kind of tires are you riding on? Are they smooth or are they knobby? Sounds like you might have hit an oil slick and slid. Did you go over the handlebars when you were using the front brake, or did the front wheel lock up as well? If the latter, I'd bet heavily on an oily surface. The former seems improbable — if you had enough traction to grip with the front tire, your back shouldn't have slid out, and if you put pressure on it slowly as it sounds, you shouldn't have gone over the handlebars. – Stephen Touset Oct 10 '16 at 23:06
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    Unfortunately, there's not much you can do about an unexpected oily surface except exercise due care. Particularly if it's rained for the first time in a long while, roads can be extremely slick. – Stephen Touset Oct 10 '16 at 23:08
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    It doesn't sound like he grabbed the back brake very hard. "Slight pressure" on the back brakes and "flip over handlebars" from the front brakes are typically contrary to one-another. My guess is both wheels locked up under light braking, as a result of low traction conditions (wet + oil, in all likelihood), possibly exacerbated by knobby (and possibly muddy, due to CX) tires. – Stephen Touset Oct 10 '16 at 23:33
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when I pressed the rear brake lever, the wheel stopped turning immediately, despite the slight pressure

if it locked up with only slight pressure, either your brakes lack modulation, or it had really poor traction.

The first (modulation) is generally adjustable.

The second depends on your tyres, the road surface, possibly oil/petrol/diesel spills, loose material etc. etc.

Since you don't think there was loose material, and say the traction was fine previously, an oil spill is certainly possible - look out for those rainbows on the road surface.

Worse, the bike didn't stop, it started to accelerate

If the back wheel is locked, skidding it should still have some braking effect, so I'm inclined to view this as evidence for poor traction.

I assume lowering the saddle would've necessitated much more effort from the front brake to throw me

Did you go over the handlebars, or just lose grip with the front wheel and crash?

If the former, which I think is what you're suggesting - lower saddle might help on steep downhills, but generally you need to brace yourself down & backwards with your arms. The goal is to stop yourself sliding forward relative to the bike as it slows down, and also to reduce the angular moment about the front axle by keeping your weight low.

Interestingly, this does suggest there was no traction problem with your front wheel, so something happened to impair traction at the back only.

My bike is suited for XC racing

Does that mean it has knobbly tyres? More contact area is generally better on asphalt, so slick or textured tyres will have better traction (and, as a bonus, generally lower rolling resistance).

Should I have kept the rear or both brakes engaged constantly

I'd usually drag the back brake to control speed on descents - to avoid overheating, don't do this continuously, but pump it and ease off to let it cool. Alternating with the front will also give the back brake a chance to cool - but note the front brake affects steering more, so definitely keep braking to the straights as much as possible.

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    Painted lines and tram/rail tracks are particularly slippery in the damp. I've seen a FWD sedan fail to move off when a traffic light changes, because both drive wheels were resting on steel tram tracks, and the driver used too much power. White Paint has caught me out personally once. – Criggie Oct 10 '16 at 19:26
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    Yep, road paint is generally slippy, wet metal, wet leaves ... even otherwise good asphalt can get slippery if spilt oil was hiding in the nooks and crannies, because it floats to the top. – Useless Oct 11 '16 at 9:34
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    In wet weather (I suppose even a rainy day) and with such slow speed as the OP describes, I don't think overheating of disk brakes could be an issue here. – Pere Oct 11 '16 at 14:26
  • @Pere Does this mean that if I keep the brakes engaged so that I maintain a low speed, they won't be as negatively affected as engaging them to keep a medium-high speed? – unintelligible Oct 12 '16 at 13:18
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    @unintelligible Physics major here. The total amount of energy (heat) you need generate in your brakes is fixed (per m of height). You want to go slowly enough that you only generate as much heat as your brakes can dissipate. Disc brakes have holes in them to speed up the dissipation of heat... Ultimately though, is very hard to judge the heating up of the brakes, so it's safer to a) go slower b) keep your front brakes as cool as possible. – Aron Oct 17 '16 at 10:54
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A lower saddle will compromise your riding comfort at all other times. Instead you should have unweighted the saddle and put your backside further backwards to weight the rear wheel.

Also, were you braking on the straight only? Or were you braking into the turn? In the dry we can get away with a lot of bad behaviours like braking while turning, that cause problems in the wet.

Technique - I'd alternate between a good squeeze on the front and then release, and then the back, then release. If its steep you can alternate so one brake is always on, and if its getting away from you then use both together, but only on the straight

If you're cornering and only realised your speed is too high, then you have failed to read the road right. At best its going to be a scary turn, at worst you're trying to mitigate the upcoming slide/fall.

Tyres may help - Slick ones are generally better at contact with a smooth road, but if there was gravel or ice or leaves or even a painted line then smooth tyres lose their advantages. Big knobbly tyres are generally poor on the road, both on cornering and general power transfer.

28km is quite a long descent too - its easy to get blasé about each corner and get a bit faster on each one. An unexpected camber or kick on a corner could be all it takes to throw you.


Obligatory "I've been there" - this particular corner has a kick, and is known for catching out both cyclists and motorcycles.

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    I'd be interested in your diagnosis of what exactly happened in that video, could you elaborate some? – BSO rider Oct 10 '16 at 23:38
  • @BSOrider Sure bit.ly/2dLtoI9 shows kick above road sign strava.com/activities/492996444/analysis/4249/4367 shows speed profile. I slowed to about 20-25 km/h for the corner but that was a bit fast, so I was still braking while turning as hands show. Leaning left, front wheel hit damp white-painted line and slipped to the right. I held that by instinct, turning right and moving body weight, but then I was on the wrong side of road, aiming sideways, and still moving too fast to stop. Thankfully no oncoming traffic! Chose to fall left because ground was closer. – Criggie Oct 11 '16 at 0:34
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    I see... you slipped on the white line but got your traction back on the pavement. At first it looked to me like one of your pedals hit. I'm not sure what you're talking about with the 'kick' though. – BSO rider Oct 11 '16 at 11:53
  • @BSOrider its a short sudden drop in the height of the road surface as it goes around the inside of the bend. Made worse because you're already leaning for the turn. We ride/drive on the left side of the road here. – Criggie Oct 11 '16 at 19:17
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And what might have caused the tyre to slide?

When braking, push your weight as far back as possible.

When emergency-braking, standing on the pedals (easier with bike shoes attached to the pedals), you might even stick your bum behind (off the back of) the saddle. That maximizes your weight over the back wheel. It also puts your weight low and away from the front (not high and near the front), and your arms pointing forward instead of pointing down, so you're less likely to flip over the front.

Slow down. After falling you may be more careful in future. I slipped (and fell) recently on a little mud in an otherwise dry path, which reminded me that mud is much more slippery than a dry surface.

Brake with both brakes (front and back) at the same time. Weight is pushed onto the front wheel (by deceleration), and friction is proportional to weight (no weight implies no friction): so if you brake equally on both wheels, your first warning that you're braking too hard is that your unweighted back wheel starts to skid (at which point you start to fish-tail, but might not fall off ... I think the point of no return is if the front wheel starts to skid). This (the back wheel skidding) is early warning that you're braking too much and are near to skidding the front, at which point you ease off slightly on both brakes.

disc brakes, would overheat and the rotor would be destroyed

I don't know whether that's possible? I've heard of people bursting their tires by overheating with rim brakes.

I don't know about the rotor but maybe it is possible to glaze the brake pads by overheating: and maybe that depends on what the pads are made of (e.g. whether they're "organic" or "sintered").

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    When braking on the straight, weight back is an excellent idea. If braking while turning, it will decrease the front wheel traction, and that will lead to a front-wheel washout, which is generally very hard to recover. – Criggie Oct 10 '16 at 21:37
  • Disk brakes can certainly overheat. Rotors will warp, hydraulic fluid will boil leading to no brakes at all. Rim brakes can head the rim and make the tyre pressure increase, and can thin/soften tubular glue. All this requires some pretty fast descents and braking, and for a fair duration. The technique is to brake hard alternately, then off the brakes for a few seconds, as opposed to dragging them for minutes at a time to bleed off speed. – Criggie Oct 10 '16 at 21:41
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    @Criggie I think I've been warned that I shouldn't brake at all while turning: that the wheels' friction is already being tested in a turn, and any braking (requiring even more friction) on top of that might be too much: so brake before the turn. – ChrisW Oct 10 '16 at 21:52
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    @ChrisW That's correct, in theory at least. You should brake to the required speed before entering a turn, so you have maximal traction for the turn itself. Of course, in practice, you can misjudge how much speed you need to bleed off in a turn, so some braking inside of a corner is at times unavoidable — one important reason to not take corners so fast that you're at the limit of your traction. – Stephen Touset Oct 10 '16 at 23:11
  • @chrisw Yes that is good safe advice. Racers want to push though, to shave seconds off the turn. Like most things on two wheels, its all about balance. Weighting and lowering the "outside" foot helps press the tyres onto the roadway, but that's beyond getting-started level. – Criggie Oct 10 '16 at 23:12
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It sounds like you have your weight far too far forwards when braking.

My reasoning comes from 3 things:

  1. Your rear wheel skidded as soon as you applied a small amount of rear braking
  2. You flipped over the front as soon as you applied a small amount of front braking
  3. You were going downhill.

You should shift your weight further back on your bike and brace against your handlebars when braking (especially when downhill).

Now here is the science bit.

Friction is limited by

  • Normal Contact Force (Weight)
  • Coefficient of Friction (Grip)

Normally when going downhill, your weight is further forward than it is normally.

  • More weight on your front wheel
  • Less weight on your back wheel

Also, it was raining. Water, reduces grip.

When you tried to brake with your rear brake, you had no weight on your back wheel, AND you had very little grip. THEREFORE, there was no friction (slippage).

When you tried to brake with you front brake, you had ALL your weight on your front wheel, and very little grip. THEREFORE, MUCH MUCH MORE friction. Your BIKE (not you) slowed down.

However, since your weight was already far forward on your bike. Your bike slowing down, causes you to be FURTHER forward on your bike. At some point, your bike stops, and you ... don't...

By shifting your weight further back, you give yourself more time to react to your bike slowing down. You weight down your rear brake (giving it more friction/braking power). Put yourself in a better position to brace against your bike.

  • Hi Aron. I don't think the OP has said he went over the bars; it's a fairly unclear question. But I think your conclusion that there was too little grip is at odds with the idea that he went over the bars. I don't fully agree with the accepted answer, but I do think it makes a better argument. – andy256 Oct 21 '16 at 22:50
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I have a very steep portion of my ride which has a sharp corner at the end which I need to slow down for. During the rainy season the only way I can do this is to use both brakes carefully AND put a foot down. I put my foot down with my calf against the pedal and apply pressure downwards.

My old bike had very dodgy brakes so I started doing this. But even my new one which has excellent brakes needs this technique during the rainy season when the road is very slick.

1

However, just after a taking a turn and before another curve, when I pressed the rear brake lever, the wheel stopped turning immediately, despite the slight pressure, and started to slide. Worse, the bike didn't stop, it started to accelerate. I pressed the front brake lever, slowly initially, but since the bike was still accelerating, I pressed more was thrown off.

It seems that you were braking only with the rear brake and used front as an emergency. This is the least effective way how to slow down. I often brake with rear wheel sliding, sometimes locked.

When braking, the most of the energy is absorbed by front brakes. The deceleration puts mass to the front wheel and remove it from the rear wheel.

The best way for braking is brake more with the front brakes, brace againts the handlebars and move the centre of mass as far as possible to the rear and low. This will reduce the risk of throwing off. For long descents it is better to brake hard and then release, then brake hard again.

But, sometimes the best way is to find a soft place to land.

  • I generally use both brakes when I have to stop quickly. I did that time as well, it's just that despite my engaging the rear brake and back wheel skidding, the front brake flipped me over anyway. – unintelligible Oct 12 '16 at 12:24
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    Incidentally, had I not fallen on that particular curve, but the next one, I could've fallen over a cliff, so you're right about that last part :) – unintelligible Oct 12 '16 at 12:26
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Under suboptimal condition, tyre will lost traction and slide even under slow walking speed (4km/h) when you apply brakes. Ironically, you need to release the brake in order to regain traction, quickly re-apply it to slow down. And this is how Anti-lock Braking System(ABS) works.

Few people are trained to deal with rear wheel slide : most of us went panic and apply the front brake, cause both wheel locked up and the weight shift just throw us out.

Unfortunately, "ABS simulation" is not an easy skill to acquired. What we can do is slow down to walking speed to reduce the impact damage.

  • If you wan to learn to deal with wheel slide, ride a bike on frozen lake or pond. – Crowley Oct 13 '16 at 11:26
  • One thing I've done, on "iffy" surfaces (but not pure grease) is alternate rapidly between front and rear brakes. But this only works when you need to burn off a little speed -- if you're going way to fast you're still in trouble. – Daniel R Hicks Oct 13 '16 at 11:50
  • @DanielRHicks : Agree. ABS doesn't shortened the stopping distance, it is just mean to regain control. – mootmoot Oct 13 '16 at 12:05
  • ABS doesn't serve to "regain control" so much as to not lose it in the first place. Once you're started a sideways skid it's hard to recover -- you just hope to not make it any worse. – Daniel R Hicks Oct 13 '16 at 12:12
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I'm not 100% confident in this as an answer, but I do have a bit of experience. I don't know anything about over-heating breaks.

I commute year-round, and part of this ride is on a small hill on a quiet road. Snow provides the perfect conditions to ask "What can my bike do?"

So, I'm going straight down an icy hill and lock the back wheel... The back of the bike instantly whips towards the front. The distance between the front and rear obviously has to stay the same, so the bike becomes sideways in the road (sometimes, with me still on it). The best course of action seems to be exactly what you would do in a car, turn the handlebars to keep the front wheel in the direction of your movement and release the breaks. The difference with a bike is that you also have to keep your weight over it, so don't resist your bike going sideways or it will slide out from under you.

As a side note: I don't know why the rear of the bike goes do much faster than the front, but it's significant. It could be that the lack of traction causes the back to slide out from your weight, but then the front still has enough to transfer that sideways force to forwards. I'm no physicist, just a guy locking his wheel. :)

I've always had better luck slowing the bike down by using only the front break.

Turns are not easy. When I'm on ice, leaning into a turn will cause instant crashing. I take a turn slow enough that I can do it by turning the handlebar alone.

Winter is coming up here, I'll be thinking about this.

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