So, the other day, I was riding along and got a flat tire. I was at the bottom of a large hill where I often find myself travelling 30 or more miles per hour (as per my speedometer) about it's many curves. The tire came off very easily when I was trying to repair it.
Has anyone had this happen to them? How likely am I to die? Surrounding terrain is quite hostile.
It depends on which tire, but the essential thing is a flat tire has almost no grip.
Two examples:
Two days ago I was descending a little hill at 60 kph (about 35 mph) when the rear tire suddenly went flat. Moving my weight forward as much as possible, I was able to maintain control while stopping using only the front brake. Turns were perilous, because the rear tire, had almost no grip, and kept trying to slide out.
A few years ago I set out on a ride, and foolishly had not checked the tire pressures. The front tire had a slow leak and was at half it's normal pressure, which I discovered when attempting a high speed downhill off-camber turn. With barely any front grip, the bike went almost straight on. If there had been oncoming traffic then I would have hit it.
As for the front tire coming off the rim, several years ago I was in a bunch and some of the bunch were swerving around in the buildup to a sprint. To avoid crossing wheels with the guy coming across me (who happened to be my brother) I had to turn sharply. We were doing about 35 kph at the time, and the cornering force rolled the tub off the rim. If you think it through, losing grip on the road is nothing compared to what happens when the tire that is now not on the rim gets to the top of the forks. The wheel stops rotating. So I was suddenly flying through the air, sans bike.
To answer your question how likely are you to die? Yes it's possible you'll die, if you hit a rock, tree, or oncoming traffic. That possibility depends somewhat on your bike control skills, and also on luck. But it also depends on where you choose to ride and in what manner. Hairy descents are not for busy roads.
First off, the best thing to do is have your bike in good working order -- properly inflated tires in good condition, good brakes, etc.
The second thing is to ride at an appropriate speed and technique (for example, don't ride the brakes on a long descent -- you can overheat them; also, don't descend faster than you can control).
As for what will happen, there is a good chance of a crash. Injury depends a lot on the terrain, how you land, protective equipment (e.g. helmet), and how lucky you are. You could end up with some broken bones, you could end up dead (say, if an 18 wheeler runs you over after you crash), or you might luckily escape with a bunch of scrapes. Generally speaking, if anything happens to the front, you're in for a crash. Rear is more controllable, but at those speeds, I'd say you were crashing anyway.
Separate the two parts into
1) The bike failed mechanically or structurally.
2) I'm on a hill and travelling at speed.
So if your bike fails normally, there will be consequences which depend on the location, the failure, your reaction, and luck.
Taking the specific example of a flat tyre, we'll assume that its a sudden blowout or a tyre rolls off the rim completely. Both will cause your rim to run on the road surface immediately, and the rim has very little traction compared to an inflated tyre.
If the problem happens on the flat, you'll feel bump-bump-grind and you will stop with the brakes quickly. If you were turning gently, you'll probably recover after a fright. If you were turning hard, a front wheel blowout will cause you to suddenly lean even further, maybe beyond the limit and you're going down on the inside of the turn. If its your back wheel, then theres more chance of saving it, but there's also the risk of over-recovery and then you're falling off the outside of the turn.
Now, all that is bad enough, but if you're travelling fast, you have more kinetic energy (momentum)
A decent downhill speed could easily exceed 50 km/h (35 mph) and its not hard to hit 70. So the flat front while doing a hard turn will leave you with about 0 reaction time before your inside hip hits the road. Same goes for the other road users, the rider behind you who has to go left or right real quick, the car coming the other way who will intersect your path in under a second... do they brake? or cut left or right? Its all gone pear-shaped in less time than it takes to read a line of text.
In short, hills simply add speed to a situation. This multiplies the energy and enhances the risk.
Going UP a hill is no problem at all. A mechanical on a climb is generally easy to recover from.
The worst on a climb would be a chain break while standing to sprint. I've had this, and when putting foot down I couldn't reach the ground due to the profile of the road on that side. Cleats didn't help, so I gracelessly fell over, and the bike slid back down the road 10 metres or so. Again fortunately no cars were coming.
Another personal story - I had a wee fall at speed on a hill descent. It wasn't a bike failure, it was a loss of traction due to a lump in the road combined with white painted line and debris on the road.
I rolled across the wrong side of the road on my wheels, braking hard. Another metre of space and I might have held it, but instead rolled over the edge onto a steeply sloping weed area. So I chose to lean left and hit the ground nearer to me, rather than fall to the right and have further to fall.
Results - I stopped 3 metres off the road. When I stood I could not see the road and noone could see me. Noone saw me go, so I had to haul myself back up to the road. I was near enough to have a speaking conversation with someone, had there been anyone. a 10 minute sit down and damage assessment. Other than thousands of sticky weed seed pods all over me, I was merely winded. The road bike's wheels were still true, the only prob was the standard brifters pushed inwards. Nothing a quick yank couldn't fix. Then 20 minutes finding my gear which had spread itself down the slope. The only net damage was my helmet visor which broke off and vanished. This was my most major crash in 14,000 km of riding over the last 2 years.
Long term, I'm a LOT more cautious on downhills now. I will brake hard periodically to limit the top speed and alternate front and back to limit temperature increases in brakes. I don't lean into corners anywhere as much as I used to either.
