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The other thread in chat on this made me think of a local road here in Spain that I sometimes use which is pretty much a short 1 in 8 hill which has escape lanes.
Athough I have ridden down it the last time I chickened out and walked a bit of it as I would be unable to take the exit I wanted as cooking the brakes would be baaaad and I’m not wanting to hit the exit at 30-40mph.
Going up no problem but down, meh I hate not riding it but did make me think there’s just some sections of road route combination which don’t work on a bike depending on direction.
(I also then wondered about the legality in Spain as your supposed to walk against the traffic- and it’s fineable)
Wots yer collective musings/ancedotes on this ?
1 in 8 is 12.5%. I've descended hills steeper than that without even thinking about my brakes. I guess it depends on how long the descent is though.
I can't say I have ever felt a lack of brakes on any hill.Discs help of course as does not dragging them but using them in bursts
I don’t really understand. I’ve never even contemplated not riding down a steep hill.
What do you mean by cooking the brakes? Has this ever happened to you before?
I can’t say I have ever felt a lack of brakes on any hill
I certainly have in the wet. In the dry, I tend to brake very conservatively when I know I don't have very good brakes.
Re unrideable roads - there are a few places where I've very nearly fallen over sideways on a road bike. There was one near Cwmbran that I have not been back to, but a memorable one is @51.5480058,-3.2780727,3a,75y,303.43h,74.61t/data=!3m6!1e1!3m4!1s7HoK-P1u15AABClrm6_IkQ!2e0!7i16384!8i8192">this tiny hairpin on the back road up the Garth near Cardiff. If there's anyone coming down you need to keep left and the gradient on the inside is a bit bonkers to say the least, especially with road bike gearing. I have done it, but I was quite afraid of breaking something.
I've definitely managed to get the brake fluid to start boiling before, but that was on a looong descent. Really disconcerting when the brakes start to not work!
Never had an issue in the UK and that's with rim brakes. Happily go down 1 in 4 or 1 in 3 in UK. Mate of mine cooked his tub glue on a long French descent many many years ago. He had to stop for a while for the rims to cool, and the glue to set.
Only time I've had brakes 'fail' on me coming down from the coate de Midhopestones, brakes were doing nothing, really hot day think I must have been dragging them on an earlier decent coming down to the A616. There was no chance I was making this hair pin, but managed to scrub off some speed by uncliiping and shooting off up the little farm track dragging my feet on the floor. Came to a stop just before the gate. Heart beat was going crazy! So I sat on the wall for a few minutes befroe setting off again.....then pulled into the Strines Inn for a drink when I got there!
Its quite easy to overheat brakes if you try - drag them down a hill at low speed - takes maybe 100m to boil them ( hydro discs)
takes maybe 100m to boil them ( hydro discs)
I don't recognise that phenomenon. What are you experiencing in terms of feel?
It is 110m from the corner at the top of my street to the bottom. I always drag my brakes down there as it's a cul-de-sac with lots of parked cars and kids playing etc. I am maybe doing 10-15mph at the top at zero at the bottom. I estimate it's about 20m of descent. If my brakes boiled down there I would be unable to ride them out in the hills!
I often sprint down the long hill into Caerphilly to try and beat my top speed, currently about 75km/h. I use it as a brake test because I have to turn right half way down, the braking distance is about 100m looking at the map. I have never boiled disc brakes on there either. So I don't think 100m of slow brake dragging is enough to boil brakes.
100m does sound incredibly pessimistic.
100m, at say 1in3 (i.e. the steepest roads in the UK) is ~30kJ of energy for an average bike + rider of say 100kg (30m x 980N).
So even if you just used one brake, say 200g of material in the rotor and caliper, heat capacity of Steel @ 200C is 500 kj/kg.K, so ~100J per C, so if you could drag one brake, in a vacuum, and eliminate radiant losses, then you could just about hit 300C which would boil DOT fluid.
Then in the real world just about every component is designed to either conduct heat away from the pads and dissipate it or insulate the fluid from the hot parts.
It was on a loaded tandem! with only one brake working so I dragging down a steep hill at walking pace. the slower you go the less cooling you get
I don’t recognise that phenomenon. What are you experiencing in terms of feel?
the fluid boiling! first the lever pushes out into your hand due to expansion with some high pressure vapour in the calliper then when you release the brake it boils so when you pull it again its back to the bar. Recovers in 5 mins and feels normal afterwards
molgrips - try rolling down that hill at walking pace dragging one brake - you will be surprised at how hot it gets At 15 mph you get a lot more cooling
my cargo bike has roller brakes on it. Loaded up with a couple of kids and me, the total weight is around 190kg. Going down ditchling beacon (~130m drop over 3/4 mile, so around 1 in 10), the brakes started smoking hard (the grease in them was burning off).
On a road bike its fine though 🙂
On separate occasions I've boiled fluid, and had pad fade to the point of not being able to stop using the brake. In both cases it was because I was deliberately only using one brake due to maintenance fail on the other, so obviously not normal use, but I found it interesting that the safety margin wasn't huge - I'm not a particularly heavy rider. One was a long steep Welsh hill, with no scope for letting speed build to give the brakes a rest, the other was a series of hairpins on an alpine descent. And yes, in both cases, the other brake was still working in a grindy metal-on-metal kind of way when I really needed it.
The boiled fluid was exactly as TJ describes. One moment it's working, you release it, and the next pull goes straight to the bars. I was able to get some braking back immediately by pumping the lever.
The pad fade was pulling harder and harder on the lever with it doing less and less braking.
More issues with tyres rolling off rims due to heat from rim brakes
Obviously I don’t know the road in question, but recommend either:
Alternating brakes, maintaining a constant speed. Maybe five to ten seconds on each. If you feel one is fading, switch immediately.
Go as fast and as long as you dare, then do a hard but controlled brake down to near stationary, then let gravity get you up to speed again.
Is the right answer!
I blued the front disc on our tandem dragging the brakes down Lydgate lane, Hope Valley.(reverse of this segment https://www.strava.com/segments/4564536) Luckily it was a cable disc so the fluid didn't boil but the pad material started burning off and I could feel the brake fading. Had my wife and 1yr old on the back, they started screaming whem the speed suddenly picked up as I released the brakes halfway down to cool them off enough to be able to stop at the bottom.
It was on a loaded tandem! with only one brake working
That's a bit different....
A bit of physics for you: if I start at 15mph then the brakes have to dissipate or absorb my kinetic energy plus my gravitational potential energy in whatever it is, 10 seconds. If I start at walking pace then they have to dissipate just my gravitational potential energy, and they have far longer in which to do it so I would expect them to be a lot hotter having ridden down it than if I walked. I can take my thermometer out and try it later.
My preferred tactic for steep tricky hills is to scrub off as much speed as I can at the top or before entering. That way, if my brakes fail for whatever reason or it goes horribly wrong I can bail at 10mph instead of 40mph.
Ah It’s actually a 8% in metric.
@37.3992971,-1.700305,3a,75y,121.92h,88.51t/data=!3m7!1e1!3m5!1sH2BK_FqinojfKS8fh__fxQ!2e0!6shttps:%2F%2Fstreetviewpixels-pa.googleapis.com%2Fv1%2Fthumbnail%3Fpanoid%3DH2BK_FqinojfKS8fh__fxQ%26cb_client%3Dsearch.revgeo_and_fetch.gps%26w%3D96%26h%3D64%26yaw%3D6.26378%26pitch%3D0%26thumbfov%3D100!7i13312!8i6656">Google Earth street view
It’s probably a lovely road to actually go down fast if that’s your thing but it seems to mess with my head, not sure if it’s the optics of the rock side or the the winds around it but I do enjoy going up it.
It was on a loaded tandem!
All bets are off then!
Had a customer years ago who'd rigged up a ~200mm rear mechanical disc as a drag brake, using a friction shifter to activate it. Using magura drop bar brakes as their "normal" set up.
Came back from their alpine tour with a disc that had overheated so badly that it'd turned itself concave, completely blued the disc and melted the seal on the idle side bearing... They'd had to reroute the second half of their tour to avoid the really high passes, until they got a replacement disc. (Not easily available in the middle of the alps in the mid 90's)
Athough I have ridden down it the last time I chickened out and walked a bit of it as I would be unable to take the exit I wanted as cooking the brakes would be baaaad and I’m not wanting to hit the exit at 30-40mph.
Unless the bike has never been maintained, you're overthinking this. A lot.
I've done some really really stupid descending (repeated decelerations from 100+kph to ~25kph on a pass in the Pyrenees for instance) and never had the brakes cook, rim or disc. I'm now nearer 100 kilos than 70 and still haven't cooked brakes on road (and only a couple of times off road, with brakes that'd never been maintained...)
That’s a bit different….
Just a bit.I was literally holding it on the rearbrake all the way down with 200+ kilo and 1 in 3 and slippery so I couldn't let any speed build
At speed the heat is removed by the air rushing past. Its when you drag the brake the whole way down it cooks
Glen Quaich, descending into Kenmore on Loch Tay.
On tours I used to recommend that riders pulled over at half way to let their brakes/rims cool.
I had exploded a rear wheel on the descent and waiting for an ambulance would take forever.
Distance: 5.2 km
Elevation gain: 423 m
Average gradient: 8.2%
Maximum gradient: 18.0%
It's a steep, long one !
I had a front tyre blow due to overheating on rim brakes coming off the Col du Glandon a few years ago. That was exciting.
I descended my hill at walking pace on one brake yesterday. When I got to the bottom the disc was not at boiling point (I tested it with spit) but too hot to hold; the caliper was barely more than stone cold.