I’ve been watching my digital temperature gauge (attached to the rear sparkplug) as I’ve been riding around. In my observation, there are a few (obvious) things that impact engine heat.
<ol>
<li>ambient temperature – the bike runs hotter on hot days. Of course, but the effect is less than I first thought</li>
<li>engine revs – the faster the engine is running, the hotter it gets</li>
<li>engine load – going up a hill or accelerating generate heat, going downhill or decelerating tend to cool the engine</li>
<li>air flow – the less the airflow, the hotter the engine, all other things being equal.</li>
</ol>
Now, when I ride around town at an average of 50km/h I’m usually in 3rd gear, so the engine is revving at 2017rpm. At 100km/h on the highway it revs at 2349rpm in 6th gear. So the revs aren’t too different, but the airflow in the city is half that on the highway. Add to that frequent high load as I accelerate between traffic lights.
No wonder it runs hot in town!
At any rate last week, a federal judge ruled that a class action overheating burn lawsuit against Harley-Davidson could go forward, siding with four bikers who claimed their Harley-Davidson motorcycles were defectively designed because their engines ran so hot as to pose a constant danger to riders of being burned and were therefore not fit for their intended use.