Is there a limit to safe downhill speed on a bike

Question

I have a very steep hill on my route which levels out after about 2 km. The first quarter is extremely steep and shortly after I enter it the bike skips around a little bit and by the time I'm at the bottom I'm gripping the handle bars and working pretty hard to keep it under control.

Is this a normal issue or is it indicative of problems with the bike? It happens both with my mountain bike and the touring bike. Less on the 29'er mountain bike perhaps due to the wider tyres and less twitchy handling.

There is a lot of acceleration right from the start going down.

Clarification:- I'm not concerned with other people's safety or traffic, my question is more about at what speed does my bike become unhandleable and/or fall to pieces.

Answer 1

I don't think there's anything like a specific safe speed for going downhill. From my experience there are many reasons for a bike becoming unhandlable when going downhill:

your bike: i'd say the main factors that enable you to go downhill fast & safe are:

• frame & fork: a super stiff carbon frame or an aluminium frame tend to be less shakey on a fast downhill ride. the same is valid for the fork. however, the stiffer your frame, the more sensitive it is to bumps etc.
• wheels: aero designed wheels (i.e. "high" rims and flat spokes) allow higher levels of control at high speeds.
• tyres: the wider the more friction and more absorption of bumps.
• suspension: generally hinder control at high speeds, however if the road is very uneven it might still be of benefit nontheless
• handle bar: wider handle bars and a longer stem make your bike less twitchy at high speeds
• stance: the wider your bottom bracket / the pedals, the smaller becomes the issue of balance at high speeds

fear / the size of your balls: right after the hardware, i'd say the software (as in your brains) determine a safe downhill speed.

the road: not gonna go to details here, that has been discussed in may ways in the comments.

summing up: if your bike is properly set up and well maintained, i don't assume it would just fall apart when going fast.

Answer 2

The maximum safe speed is the speed where you still have control you think is needed.

There are levels of control

• Can you stop if a car suddenly reverses out of a driveway?
  This is the highest level of control; it means we'd never exceed about 25kph (16mph) on the flat. Obviously we frequently exceed that, and so we're trusting that such intrusions into our "road space" will not occur on this ride.

• Can you swerve to avoid a small child or animal that suddenly runs onto the road?
  Since such intruders are notoriously unpredictable, we also need to brake and swerve. If I know there are kids / dogs on the loose, I slow down to that 25kph level, or slower. But if it's reasonably open road I figure it's reasonably safe to clock 40kph (25mph). I've encountered oncoming cars in my lane as I round a bend on a descent at this speed, and there's plenty of time to evaluate my options, brake, and take evasive action (time flows really slowly in such cases).

• If the road is really clear, i.e. you can see well to the sides, and a long way in front.
  In this case we can easily do 60kph (35mph). Around this point (and faster) some rider / bike combinations can experience shaking, which can be quite frightening. You need to experiment a bit to learn what's going on. In many cases resting your knee against the frame seems to dampen the shakes. In other cases I've had success relaxing my grip on the handlebars - this seems to suggest that it was my fear of death grip on the bars causing the wobbles :-)

• Ok the road is clear, and you're feeling lucky. How fast can you go?
  If we keep going faster, it seems we pass the zone where the speed wobbles occur. If you are not a pro (being paid to do it), and are not confident, or are just a sensible person don't do it. However, I can confess to cracking 120kph (70mph) on a descent on a road bike. I found that as I went faster the bike became more stable, I'm guessing from the gyroscopic effect of the wheels. The same effect seemed to make turns more difficult. The first time I also found it was extremely hard to see properly, since I wasn't wearing any eye protection. And it took over 400 meters to stop on the flat. We can be sure that if any of the above mentioned road intrusions occurred then I would have been toast.

After all that, given the pictures you've posted, I would recommend at top speed of 30 to 40kph (16 to 25mph), because it looks like a residential area. Even that may be too fast, especially because of the incline, which means your center of gravity is higher and so you have less braking capacity.

Answer 3

High speed stability can be dramatically affected by the geometry of the bike in question. There's a chance that the head angle on your 29'er is slacker which should give greater stability.

I know that on a car at certain rpm's you can get vibration through the car if a wheel is not correctly balanced. You might want to check over your wheels and make sure spoke tension is correct for safety's sake.

I don't know what bike you're riding and I'm not a road rider really but perhaps there's something in your posture that can aid stability. On an MTB you'd hang further out the back to keep your weight centred over the bottom bracket.

Answer 4

There is no limit to safe downhill speed on your bike. The limit is when you come off your bike. In which case even a low speed crash of 20 to 30 mph can be pretty nasty on chip and seal type surfaces or where there are obstacles (oncoming traffic anyone?).

You can ride a modern road bike up to 60 mph, even a little more but I would say that it is not safe to ride at that speed with nothing but spandex and a lightweight crash helmet.