Speeding downhill

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Speeding downhill

Postby Sauce » Wed Feb 13, 2013 6:23 pm

The topic of this thread was sparked by a recent Courier Mail article regarding cyclists speeding down Mt Coot-tha.

My opinion is that it is physically impossible to descend the mountain under the speed limit without dismounting and walking. Having heard all the horror stories of tyres puncturing due to overheating rim brakes, I'm led to believe that if you were to coast the brakes the whole way down, you would more than likely end up in a ditch on the side of the road before you even made it half way. I do my best to manage my speed during the descent so I am in less of a hurry to brake for the intersection at the bottom of the hill, but I still end up sitting above 60km/h for most of the descent.

I feel that Queensland Police have failed to recognise the fact that the road is a one-way single-lane configuration, with significant room to maneuver around the corners, and a very wide separation from oncoming traffic.
Regardless of this latest campaign, I would like to continue to ride Mt Coot-tha but suspect doing so would result in a hefty fine, which I would very much like to avoid.

As road cyclists, what are our options? Disc brakes? Won't these warp under continuous braking and then jam the caliper, resulting in skid / somersault over the handlebars?
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by BNA » Wed Feb 13, 2013 6:46 pm

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Re: Speeding downhill

Postby human909 » Wed Feb 13, 2013 6:46 pm

If you control your speed to sub 20kph then your brakes will be fine. But yes you are correct, with low braking capacity sometimes there will be an intermediate speed where brake overheating is likely.
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Re: Speeding downhill

Postby RonK » Wed Feb 13, 2013 6:54 pm

Rubbish. I have no difficulty controlling the speed of a heavily laden touring bike on longer and steeper descents than Mt Coot-Tha, using only cantilever brakes.
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Re: Speeding downhill

Postby London Boy » Wed Feb 13, 2013 7:02 pm

RonK wrote:Rubbish. I have no difficulty controlling the speed of a heavily laden touring bike on longer and steeper descents than Mt Coot-Tha, using only cantilever brakes.

I'd have to say that the last time I rode a laden touring bike was around Ireland 20 years ago, with simple cantilever brakes. Whatever the gradient, I do remember that braking was no problem at all. And Ireland is far from the flattest country on the planet.

This was with tent, cooking kit, water, clothing and everything else I needed. No credit card touring for me back then...
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Re: Speeding downhill

Postby jonbays » Wed Feb 13, 2013 7:42 pm

It goes against the grain to brake on downhill runs but I could if I wanted to keep below the speed limit with my old centre pulls. Of course it is damn hard to read the cateye with my chin on the bars!
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Re: Speeding downhill

Postby Lukeyboy » Wed Feb 13, 2013 8:52 pm

jonbays wrote:Of course it is damn hard to read the cateye with my chin on the bars!


I solved a similar problem a long time ago.

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Re: Speeding downhill

Postby Chillibones » Wed Feb 13, 2013 8:58 pm

Sauce, it's not the QPS that set the speed limits on the road, so it's not the coppers that have got it wrong.
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Re: Speeding downhill

Postby il padrone » Wed Feb 13, 2013 10:13 pm

What is the speed limit for the road? What is the gradient of the descent? Is it not feasible to control your speed with simple pulse braking?
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Re: Speeding downhill

Postby Lukeyboy » Wed Feb 13, 2013 10:53 pm

50kph. 5-13%. ~110m elevation difference.
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Re: Speeding downhill

Postby il padrone » Wed Feb 13, 2013 11:03 pm

Hmm..... OK. 50 kmh is ridiculous on that sort of gradient :shock: :roll:

But if I recall, 45-50kmh should be enough speed to ventilate your rims enough to avoid a blow-out, even if you're on the brakes the whole time.
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Re: Speeding downhill

Postby Lukeyboy » Wed Feb 13, 2013 11:39 pm

What about 60kph speed boards on a 15-25% gradient for 2km :P
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Re: Speeding downhill

Postby il padrone » Wed Feb 13, 2013 11:57 pm

Death wish :shock:

Don't they use hob-nailed gloves for breaking anyway ??
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Re: Speeding downhill

Postby human909 » Thu Feb 14, 2013 12:08 am

il padrone wrote:But if I recall, 45-50kmh should be enough speed to ventilate your rims enough to avoid a blow-out, even if you're on the brakes the whole time.


I believe it is less about simple ventilation and more about the power (heat) going into the rims. At high speeds a greater percentage of the potential energy is lost in wind resistance and so brake overheating does not occur. At very low speeds (say 15kph or less) the power (heat) input into the rims remains low. At moderate speeds (30km-45km) the heat input into the rims is high enough to cause run away temperature increases.

The exact speeds where it is dangerous would be very dependent on gradient however.
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Re: Speeding downhill

Postby GH » Thu Feb 14, 2013 12:31 am

This line of thinking that it is not feasible to maintain the speed limit in/on a particular vehicle is heading in the direction of having the roadway signposted such that those vehicles unable to comply with the speed limit would be banned, be careful when you protest that it is not possible to maintain the speed limit. That simply means your vehicle is not suited to the conditions encountered on that roadway and therefore is not "roadworthy". If carbon rims are not capable of remaining at safe temperatures on those roads then they are not fit for the purpose to which you put them. There will be rims (steel) which are safe, want to be forced to use steel rims on Mt Coot-tha? Just joking but you see where this is leading.

There are no points associated with bicycle speeding, and I think the fine is not too bad either, just as an aside.
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Re: Speeding downhill

Postby VRE » Thu Feb 14, 2013 6:28 am

il padrone wrote:What is the speed limit for the road? What is the gradient of the descent? Is it not feasible to control your speed with simple pulse braking?

That was my thought, although I'm unfamiliar with Mt Coot-Tha. I've descended Mt Baw Baw east of Melbourne, and just used pulse braking, keeping my average descending speed probably at about 40km/h. Some might think that's too slow, but you have to ride to the conditions, because we don't have a God-given right to descend hills at the speed limit.
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Re: Speeding downhill

Postby zero » Thu Feb 14, 2013 7:54 am

RonK wrote:Rubbish. I have no difficulty controlling the speed of a heavily laden touring bike on longer and steeper descents than Mt Coot-Tha, using only cantilever brakes.


Cantilevers don't impact heat generation over long periods of braking.

Loaded tourer typically won't blow a tire off because it has larger tires and heavier rims which take considerably more energy input to raise the temperature (and thus raise the pressure), and are lower pressure so they start further away from the pressure required to blow a clincher off (not convinced thats linear but it is favourable to larger tires), and the loads are usually relatively unaerodynamic compared to the stored potential in their mass.
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Re: Speeding downhill

Postby RonK » Thu Feb 14, 2013 8:17 am

zero wrote:
RonK wrote:Rubbish. I have no difficulty controlling the speed of a heavily laden touring bike on longer and steeper descents than Mt Coot-Tha, using only cantilever brakes.


Cantilevers don't impact heat generation over long periods of braking.

Loaded tourer typically won't blow a tire off because it has larger tires and heavier rims which take considerably more energy input to raise the temperature (and thus raise the pressure), and are lower pressure so they start further away from the pressure required to blow a clincher off (not convinced thats linear but it is favourable to larger tires), and the loads are usually relatively unaerodynamic compared to the stored potential in their mass.

Rubbish! It doesn't matter what type of rim brake, the heat generated by braking forces will be the same.

My 15kg touring bike with 20kgs of luggage and me sitting on it will generate far more braking heat than my 7.5 kg CF road bike. And with tyre pressures at 80psi they are not much different either.

And to rubbish again, I regular ride my road bike on Mt Coot-tha. It's not unusual to see a speed trap near the Legacy Way tunnel workers car park, and it's not at all difficult to stick to the speed limit.
Last edited by RonK on Thu Feb 14, 2013 8:25 am, edited 1 time in total.
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Re: Speeding downhill

Postby william » Thu Feb 14, 2013 8:24 am

Over heating brakes and subsequent failures of any mechanical design would imply an unroadworthy vehicle.
How do truckies manage it?
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Re: Speeding downhill

Postby human909 » Thu Feb 14, 2013 8:55 am

william wrote:How do truckies manage it?

Engine breaking and a radiator to dissipate the heat. On long steep hills cars will have the same problem. Though in Australia we don't have too many of them. :wink:


Personally I haven't had any problems with overheating bike brakes but then the only big hills I have ridden is the 450m down into Lorne. I didn't really brake much until the bottom. 8)
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Re: Speeding downhill

Postby Sauce » Thu Feb 14, 2013 9:17 am

Dont mistake this thread not looking to have the speed limit changed, simply wondering what we can actually do to make our bikes "roadworthy" in this aspect. I simply dont trust rim brakes to do the job safely.
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Re: Speeding downhill

Postby il padrone » Thu Feb 14, 2013 9:21 am

Truckies avoid long steep hills. The risks are too great.

Riding up the lower reaches of Dead Horse Gap, the smell from burning brake pads from the descending cars was overpowering. Lots of people driving automatic transmission cars know nothing about use of gears to slow the car - they just hang on the brakes and overheat them :roll:

Descending into Lorne is just a gentle roll. On the bike Mt Baw Baw was one of the most extreme descents I've done under brakes. In 6kms you descend about 800m. On a light road bike I was forced to sit on the brakes continuously to hold the speed. After about 1 km I blew out the front tyre. I walked a good bit after that. Gradient is about 12-15%.

If you want another road to ride fully-loaded for some very challenging descents (and climbs) do the Western Explorer through the Tarkine in NW Tasmania. Sheer hard graft - but some beautiful country.

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Re: Speeding downhill

Postby high_tea » Thu Feb 14, 2013 9:40 am

Sauce wrote:Dont mistake this thread not looking to have the speed limit changed, simply wondering what we can actually do to make our bikes "roadworthy" in this aspect. I simply dont trust rim brakes to do the job safely.

Shrug. Coot-tha isn't that long or steep. Pulse brake, as il padrone said, or alternate front and rear if you're concerned. Plus sit up as high as possible to maximise frontal area. This assumes metal rims - carbon might be another matter, I don't know. But still, 2.5k on, what, 8% average or so is something that any decent bike should handle.
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Re: Speeding downhill

Postby zero » Thu Feb 14, 2013 10:25 am

RonK wrote:
zero wrote:
Cantilevers don't impact heat generation over long periods of braking.


Rubbish! It doesn't matter what type of rim brake, the heat generated by braking forces will be the same.



Can't see how that differs materially from what I said. Not sure where the Rubbish! is coming from.


My 15kg touring bike with 20kgs of luggage and me sitting on it will generate far more braking heat than my 7.5 kg CF road bike. And with tyre pressures at 80psi they are not much different either.



its undoubtedly more upright as well, and with load it has considerably more frontal area and thus drag. For me, adding 27kgs of weight would increase the stored energy in the bike at the top of the hill by 25%, and increase the drag considerably as well. ie (with my weight) the brakes probably have to dissipate at most 20% more energy to maintain the same descending speeds. Making the bike 3x as heavy doesn't make the system 3x as heavy.


And to rubbish again, I regular ride my road bike on Mt Coot-tha. It's not unusual to see a speed trap near the Legacy Way tunnel workers car park, and it's not at all difficult to stick to the speed limit.


I agree, but not starting at 120psi is a big factor in how far you can drag the brakes before blowing the tire off - ie you pretty much have to burn the tube (180 deg c might not even do it).
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Re: Speeding downhill

Postby InTheWoods » Fri Feb 15, 2013 1:22 pm

I don't really buy your argument and I suspect you are just seeking excuses. I have no braking problems staying within the speed limit on the front of cootha. I do *choose* to briefly let it go to 60kmh on that dip before the only uphill part of the descent, but not because I'm worried about my gear. Pulse brake, alternate between rear and front, and sit upright. The bit down past the botanical gardens is 60kmh, and with no braking and a non-aero position I can get to about 63kmh max at the very bottom.

If anything I think the back of cootha is worse. But again pulse braking, front and rear, and its no issue. My rims are about lukewarm by the time I get to the bottom.

At the end of the day, if your gear can't safely keep you under the speed limit, then you shouldn't be riding there - the problem is yours, not the road's. What would happen, if, gasp, you had to slow right down or even stop to avoid an obstruction! Maybe even your frame would melt from the heat radiating from your red hot rims :)
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Re: Speeding downhill

Postby elantra » Fri Feb 15, 2013 2:18 pm

I have been around Coottha perhaps 100 times or more over the last 30 years.

There is the back way and the front way, it is a circuit several kilometres in length.

Most cyclists ascend the back and descend the front, for no better reason than that the back descent is more dangerous.

Descending the front is a lot of fun, and i generally reach a speed of 70 km/hr, but not for very long.

I have been doing this for over 30 years, and i will probably continue to do this.

The current publicity about bikes speed is appropriate as a general reminder to exercise care, but i don't use a speedo.
So i am not going to get too neurotic about controlling my speed if it seems appropriate (depending on conditions) to go a bit over 50 km/hr.

BTW, in the last few years there have been 2 car accidents up there resulting in driver fatalities.
As far as i know, there has been one cyclist fatality.
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