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I have a 2015 Cannondale Bad Boy 3. I'm planning on making a trip tomorrow that goes across a very long, fairly steep road (It's almost completely straight).

I've had the bike for about six months, and I absolutely love it. I trust the construction and control of the bike, but everything has it's limits.

Hopefully someone would be able to tell me what a "safe" top speed is for the bike. I've personally hit just under 40Km/h on this bike on a different downhill, much shorter than the one I'll be riding tomorrow. I just don't want the tire to fly off while I'm cruising down this hill.

I'm going to have the bike serviced tonight. The tires are the standard Schwalbe Kojack. They're still in good condition, and everything else is stock from the factory.

Thanks in advance for any help!

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    People routinely pedal 40Km/h on flat roads -- doing that downhill on a smooth road that isn't too twisty is relative child's play. But if you get much above that speed, or you have to deal with potholes or whatever, it can get dicey. – Daniel R Hicks Sep 8 '17 at 3:08
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    The only way you really know what your (y) is to keep going faster until you crash, then it's (y-1). I prefer to go as fast as possible until I scare the poop out of myself and then go (y-1) but that's just an approximation. – RoboKaren Sep 8 '17 at 4:26
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    I very much doubt that manufacturers would give a recommended maximum speed. It depends too much on the rider and too many people will interpret "Cannondale said I should never do more than 60km/h" as meaning "Cannondale said that under 60km/h is safe so how come I'm in hospital with three broken legs?" – David Richerby Sep 8 '17 at 11:22
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    @RoboKaren - I prefer to stop before the poop thing. – Daniel R Hicks Sep 8 '17 at 11:48
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    With hydraulic discs and 35mm tires, as long as they're pumped up within the recommended pressure I very much doubt your tire will come off. Tires problems on long descents usually happen because rim brakes can heat up the rim and cause the tube to burst, or if it's a tubular (tire and inner tube are one unit) you can melt the glue attaching it to the rim and it could roll off in a turn. – Jamie A Sep 8 '17 at 16:49
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If you are expecting a number, sorry can't give one.

However these are the variables you will have to consider

  • Bike - some bikes shimmy at high speed. This can be caused by minor unbalancing like a yellow spoke reflector, or a small variation in rim weight and balance.
  • Wind speed and direction - side gusts have a greater impact at speed.
  • Traffic - Cars can get in your way, plus they act a bit random when cyclists start doing unusual things
  • Road - a descent often goes down a hill, and tends to wind in and out of valleys and bluffs. Your line of sight can be quite limited by the hillside or vegetation. So a corner may cry out for shortcutting, but you run the risk of going head-first into an uphilling vehicle or other rider.
  • Weather - road temp and dampness will affect tyre grip. Heat will cap your ability to cool down by sweating
  • Reaction time - as you move faster you cover more ground. Means you need to anticipate better and to predict further ahead.
  • Wind - if you go fast enough you get cold ears and eyes, nose, fingers and toes. Also, the passing wind can set up a howl or whistle as it passes your helmet straps. I find this may be one cause of my gradual hearing loss.

Ultimately the limiting factor is likely to be your Confidence, and the point at which you reach for the brake levers.


To calculate your likely downhill speed, you need to know your CdA, your input power, and so on. A website like http://bikecalculator.com/ helps give a guesstimate.

One example, assuming you're putting in 50 watts of power (and that you're not spinning out the gears) you weigh 90 kilos, your bike is 10 kilos, the grade is 5% down, there is no wind relative to the ground, its a 10 km long segment with an air temperature of 25 degrees C and its 1000 metres above sea level (not sure if thats start or finish elevation sorry) and that you're in the drops the whole way, you'll be doing 62.7 km/h and do the whole thing in ~9.6 minutes.

If you don't pedal at 50W you'll do 60.9 km/h and do the whole distance in ~9.86 minutes, so 17 seconds slower.

Of course these numbers are completely calculated, so you play about with the inputs. EG a 250 kilo rider on a 50 kilo MTB bike would do 81 km/h on coasting ?


On most downhills you spin out your gears quickly. http://www.sheldonbrown.com/gear-calc.html is another good calulator for this.

It shows that a 52 tooth chainring with an 11 tooth rear would give you 71km/h at 120 RPM, assuming normal road wheels. Thats really fast pedalling and feels crazy. At 90 RPM that same gear combo would be 54km/h. Hence why I used 0 Watts in the previous calculator, meaning coasting.


Lastly - do ride with something like strava or some other recording tool to measure how well you did. This also gives you fairly accurate proof for next time you do the ride to compare.

Doubtless there will be strava segments defined so you can compare your output with other riders too.

And ultimately, your time to ride UP the hill is just as interesting :)

  • I've tried to point out the variables in a factual way, without resorting to opinions in this answer. – Criggie Sep 8 '17 at 3:08
  • Thank you so much for all the information. This is really cool stuff to know! – Puma Sep 8 '17 at 4:06
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    +1; my own answer could almost be comments on this one as I've tried to put the same variables into practice – Chris H Sep 8 '17 at 5:56
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    @ojs Could you elaborate on what you mean when you say shimmy doesn't work that way? I'm not familiar with how it works. – Puma Sep 11 '17 at 0:44
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    @Pomerinke sheldonbrown.com/brandt/shimmy.html: "Shimmy results from dynamics of front wheel rotation, mass of the handlebars, elasticity of the frame, and where the rider contacts the bicycle.". This is the first google hit, and the description there matches exactly my experience. – ojs Sep 11 '17 at 6:21
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It's been said that you're the limit, rather than the bike, so this is about how to safely approach that limit.

Purely as a point of comparison for what the manufacturer might exist a typical cyclist to do, on my not dissimilar hybrid (but v brakes) I've done over 60km/h down a hill I know well. 50+ is more common. Road bikes will be faster but fundamentally aren't all that different. Although I've got 70km/h out of my tourer, I wouldn't at that spot even if traffic and the speed limit weren't an issue.

I have had to stop suddenly from that sort of speed down hill (e.g. a minor bump causing my lock to fall out of its bracket) and the stopping distance is huge.

So build up to it gradually to:

  • get practice dodging debris and otherwise manoeuvring at that speed
  • get the hang of what cars do round there
    • if they don't expect bikes to be going fast, will they pull out in front of you?
    • if a car catches up with another, slower, bike and brakes hard, will you be ok?
  • get used to the stopping behaviour - while braking hard going downhill fast the bike will handle very differently

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