Downhill + vbrakes = crazy rim temp
Discussion
Did a bit of single track and some downhill as part of a tour down a mountain. The bikes were hardtails with vbrakes, half way down I felt rim where the spokes come out and it was baking hot, can innertubes/tyres withstand this abuse, has anyone suffered blowouts with vbrakes downhill? The heat was mental. Just another advantage of disks?
On a side not a couple of Austrian lads were banging on about the bikes they had back home (Santa cruz full suzzies with xtr etc etc) they were both crap - the better of the 2 coming off twice. Made me feel a bit better about having an absolute dog of a Claud Butler back home and being able to keep up with these lads riding at 7/10ths. Happy days!
On a side not a couple of Austrian lads were banging on about the bikes they had back home (Santa cruz full suzzies with xtr etc etc) they were both crap - the better of the 2 coming off twice. Made me feel a bit better about having an absolute dog of a Claud Butler back home and being able to keep up with these lads riding at 7/10ths. Happy days!
Yes discs get hot enough to brand your fingers, I know, but that's all they do, brake, they don't have to hold the tyre on or take repeated impacts like rims do.
I've never heard of anyone blowing a rim because it got too hot, but I know Roadies can wear through expensive road rims after a while!
I've never heard of anyone blowing a rim because it got too hot, but I know Roadies can wear through expensive road rims after a while!
To give you an idea of the heat generated, I had a pad fail on a set of front Hope 6 pots on my MTB Tandem. It was running as a road bike at the time and had panniers fitted - so carrying a lot of weight. We were in a group of about 6 other MTB tandems, all of whom were 'playing' on a mile-ish long steep road downhill doing around 40-55 MPH as it was fast smooth bends. I thought the squealing was coming from the pads getting a work out.
When I stopped, the leading Ti piston had completely melted its way through the copper pad backing and was acting directly on the disc. Number 2 piston was halfway through and number three had simply indented it.
On the same descent my brother-in-law got the fluid so hot (old fluid too which is why I suspect it happened) on their bike that it eventually vaporised in the hose and popped off the calliper in a cloud of white smoke. Scary as hell to ride through at 40mph.
All good fun though.
I think I kept the pad somewhere so will post up a pic if I find it.
When I stopped, the leading Ti piston had completely melted its way through the copper pad backing and was acting directly on the disc. Number 2 piston was halfway through and number three had simply indented it.
On the same descent my brother-in-law got the fluid so hot (old fluid too which is why I suspect it happened) on their bike that it eventually vaporised in the hose and popped off the calliper in a cloud of white smoke. Scary as hell to ride through at 40mph.
All good fun though.
I think I kept the pad somewhere so will post up a pic if I find it.
I've heard of blowouts on road bike running 100+ PSI but an MTB probably running 35-45 PSI has probably got a shed load of headroom for the heat causing a PSI rise, most tubes for MTB's are rated to 60-65 and will probably go a fair bit higher, don't know the physics but i imagine a 50% volume increase in air is a serious amount of temperature...
Aluminium should dissipate the heat fairly quickly and there is a large quantity of material that has to be heated before the tubes get serious warmth directly - the braking surface is isolated from the tube by the rubber tyre bead/sidewall, which is a good insulator, so direct transfer from the hottest areas is reduced.
With a big enough hill, maybe, but I don't think most would be able to put enough heat into the material of an average mountain bike rim to make it happen before something else gave out.
With a big enough hill, maybe, but I don't think most would be able to put enough heat into the material of an average mountain bike rim to make it happen before something else gave out.
I was once on a cycle tour in Norway using a touring bike with rim brakes.
The bike was well loaded with panniers and on some of the descents (2500m-sea level) the wheel rims would get so hot that the inner tubes melted and burst.
After multiple punctures, we had to stop several times on the way down, to let the rims cool off!
A great holiday but only if you were fit, as the climb back up on the other side of the fiord, was hard going!
The bike was well loaded with panniers and on some of the descents (2500m-sea level) the wheel rims would get so hot that the inner tubes melted and burst.
After multiple punctures, we had to stop several times on the way down, to let the rims cool off!
A great holiday but only if you were fit, as the climb back up on the other side of the fiord, was hard going!
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