# How hot can rims get?



## karen.488walker (4 Jun 2009)

My rims were really hot coming down today (radiator temp). I have v brakes. I'm trying to feather brake. But ended up stopping to let them cool down. I am very nervous of having a blow out. Is this a real risk?


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## mickle (4 Jun 2009)

I've got rims hot enough to burn my finger-prints off at the bottom of the downhill course at Big Bear, during a California summer, at altitude. I weigh 16 stone. You've got nothing to worry about.


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## mickle (4 Jun 2009)

User3143 said:


> Certainly is




No it isn't lee. Unless you can tell me by what mechanism such a thing could happen.


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## karen.488walker (4 Jun 2009)

Rim brakes also heat the rim because the brake functions by converting kinetic energy into thermal energy. In normal use and with lightweight bicycles this is not a problem, as the brakes are only applied with a limited force and for a short time, so the heat quickly dissipates to the surrounding air. However, on heavily-laden touring bikes and tandems in mountainous regions, the heat build-up can increase tyre pressure so much that the tyre blows off the rim. If this happens on the front wheel, a serious accident is almost inevitable. The problem is worse when descending cautiously at slow speeds because the brakes are "always on" and the cooling airflow over the rim is insufficient. The risk can be reduced by not over-inflating tyres and adopting an aggressive riding style, only braking for the corners, but the real solution is a drum brake or a disc brake which avoids rim heating.


wikipedia. how hot is hot though. need a new bike.


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## RedBike (4 Jun 2009)

A trapped quantity of gas at constant pressure has Volume proportional to its 
Temperature.

So if you start off at 0 degrees, ride unitl you rims hit 100deg you're probably only looking at a 20% increase in pressure. If I get time I will do the sums later. 

Now although this sounds a lot if your riding a MTB your tyres are probably only at 40psi to start with so they'll only inflate to about 48psi. Your tyres are still well bellow the recommended maximum pressures on the side wall and miles below the pressure that they will fail at.


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## dodgy (4 Jun 2009)

I think this is in the same category as 'your tyres will explode in the hold unless you deflate them first' isn't it?
I've ridden down Snowdon with V brakes several times with the brakes on a lot at the height of summer and not suffered heat induced blow outs!


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## Amanda P (4 Jun 2009)

I've had a tube split when the rim was very hot during a loaded rapid descent (Cheddar Gorge, I think it was). 

It could have been a coincidence. I didn't realise the rims were so hot until I grasped one to fix the puncture and burnt my hand.

More recently, descending mountain passes in South Africa in about 38 degree heat on a small-rimmed Moulton with 120psi tyres, my rims have got very hot - but now I'm aware of the possibility of tubes popping and stopped to let them cool off. (I weigh the same on any bike, but the potential energy is being dissipated into a smaller mass of aluminium).


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## dan_bo (4 Jun 2009)

I've had a blow-out at the bottom of wessenden head-luckily after i'd heaved myself to a halt.


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## PaulB (4 Jun 2009)

I had two front tyres burst on me last month when I was cycling in very hot temps in Cyprus and both times were coming down steep hairpin decents. In each case, the rims were far too hot to touch and as I couldn't afford to waste water, I had to wait for them to cool down. Also, Paul Kimmage makes the point that several riders in the TdF had inners burst due to them melting in the heat in all of his rides there.


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## montage (4 Jun 2009)

PaulB said:


> I had two front tyres burst on me last month when I was cycling in very hot temps in Cyprus and both times were coming down steep hairpin decents. In each case, the rims were far too hot to touch and as I couldn't afford to waste water, *I had to wait for them to cool down.* Also, Paul Kimmage makes the point that several riders in the TdF had inners burst due to them melting in the heat in all of his rides there.



Be honest...you wee'd on them didn't you..


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## PaulB (4 Jun 2009)

montage said:


> Be honest...you wee'd on them didn't you..



 yes


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## 02GF74 (5 Jun 2009)

they can get so hot you cannot put your finger on them without going *ouch!*
not had it happen but the reason I was told is that the inner tube can melt or become soft enough to blow.

a good reason to convert to disc perhaps?

my theory on braking on long down hills is to apply the brakes hard then release as opposed to lightly hanging on the brakes all the time.

what this does it to heat the rim to a higher temp than when applied all the time and due to temp difference, more heat is transferred. (newton's law of cooling).


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## fossyant (6 Jun 2009)

Had a mate melt his tub glue coming down Alpe D-Huez - he needed to stop as the tyre was seriously in risk of rolling off....

Normally, I'd say you were OK.


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