Hi all. Given my spate of flats lately (always at the bottom of a descent...) I was interested in how much I was heating up my rims through braking. So I pulled together a quick power calculator. I know there's a number of engineers among us, so thought folks might find this interesting (sorry if this is a "been there, done that" thing - I couldn't find anything online that worked well)

The cells in orange are inputs. The coefficients of wind resistance and rolling resistance came from internet sources but seem to work when compared to other online calculators. The calculation of time/acceleration seems a bit slow, I need to work on that one a bit more.

File should be attached, hope that works.Bike Energy

Brad

Views: 175

Comment by Christopher Wu on June 18, 2016 at 6:44pm

Interesting stuff.

I don't know what to do with this information, but anyways...

The temperature calculation is just the worst case scenario, which probably won't be reached in the real world. This is instantaneous energy. The dissipation of heat through 1. the time elapsed in the stopping process and 2. the rotation of the rim so the brake pad is not contacting the same section means the T won't get nearly that high. There's also heat at the brake pad, which probably has a higher heat capacity and loss of energy through vibration.

Ultimately, I'm a little surprised how little heat is actually generated. Have your flats been predominantly just the front or rear? Do you use one brake more than the other when descending?

Comment by Brad Reddick on June 18, 2016 at 7:00pm

Nothing for anyone to do with this, just thought if anyone was interested I'd post it.

You're absolutely right, I didn't factor any cooling into the model.  For heating the rim I assumed I was heating half the mass of the rims (that's my fudge factor in there). Was really just interested in undertanding the forces involved and how they vary with wind, grade, mass, etc.

Comment by Jonathan McCrea on June 20, 2016 at 11:20am

Interesting spreadsheet Brad.  I've heard about some issues with carbon rims heating up and delaminating when pro-riders do some hard braking when riding in the mountains (which would mean heating up >120°C)... so the heat-up theory seems very plausible. 

Comment by Jeff N on June 30, 2016 at 1:29pm

Fun model!  Sensible result, it makes sense from an energy balance that the more time going faster, the more energy will be dissipated by air resistance and less needed from braking.

Time for an experiment?  Just need a temperature sensor to get the deltaT.

On the specifics of the flats, I wonder if it is another problem.  The tires survive some very high road temperatures on hot days. 

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