How do you judge when a rim is getting too hot?

[swansonj]

This summer, for the first time in a decade or more, I got the chance to cycle up and then down some really long steep hills in France, on a touring bike with rim brakes. I know that the best way to descend is to let the bike roll so that your energy goes into the wind not the rim, but I'm sorry, I'm too much of a wuss to let my speed build up that much. So I was braking quite a lot and the rims were predictably getting hot. I was quite prepared to stop at intervals and let them cool down, but my question is, how hot is safe (before you risk exploding a tube) and how do you judge it?

 

[Davidc]

Don't know how you can gauge it while riding.

I've been there 3 times over the years and been caught by surprise each time.

I wish you luck in your quest. The answer is probably disc brakes but presumably it's possible to overheat those and do some damage.

 

[Born2die]

Discs are ok but you don't want to boil hydraulic fluid (trust me on this one )

 

[fossyant]

You need to break hard into bends, then not brake for the straight bits. Bit hard on a loaded tourer.

My mate once had to stop coming down the Alpe, his tub glue started melting.

 

[Tim Hall]

Discs are ok but you don't want to boil hydraulic fluid (trust me on this one )

But cable discs will fare better.

 

[Matthew_T]

When braking going downhill (very rare) I always grab a big handfull of brake, then release. It slows me down and prevents the rims from getting too hot.
I once had to stop going downhill so held the brakes for a long time. The rims did that 'wet rim braking sound' like sandpaper was rubbing. I stopped and inspected the rims. The front had warped a little and the rear was a little bit out of balance.
The problem has rectified itself now though.

 

[swansonj]

I do understand the theory here. If you're prepared to go fast enough, you can let the wind to all the work of limiting your speed, and not have to do any braking at all. But it does rather depend how fast you're prepared to go. I was lucky enough to be descending several continuous kilometres of greater than 10% gradient. Even with my not very aerodynamic riding position, I was still accelerating past 35 mph when freewheeling and terminal velocity would have been well over 40 I reckon. Plenty of cyclists are happy to descend much faster than that; good for them; I'm not on those sorts of roads, hence my use of the brakes, and hence heat in the rims. I'm quite content to stop at intervals to let them cool, after all, if you're in that sort of terrain in the first place the scenery is almost bound to be worth savouring, and what's an extra 5 minutes cooling time when you've spent 5 hours climbing the thing in the first place. I'm just interested in how hot it is actually safe to let the rims get.

 

[biggs682]

when you can fry an egg on it