First rain on a mountain road, then a tunnel
It is the last week of September and the first rain for three weeks is falling steadily. The temperature is 4 °C. You are riding a 471 cc twin west on riksveg 7 across Hardangervidda with a top box and panniers, about 25 kg of luggage. The road has been dry and warm for weeks. There is a sheen of water on a surface that still holds old diesel at the filling station exit and polished tar snakes in the wheel tracks. Halfway along you enter a 1.2 km unlit tunnel. The mouth is wet from rain running off the rock, and 200 m inside the road is dry but dusted with fine grit. Your rear tyre is a sport-touring model with about 2.5 mm of tread; you checked the pressure two weeks ago at 2.5 bar, against 2.5 solo and 2.9 fully loaded. Your visor is beaded with rain, and the sun comes out low and directly ahead as you exit.
1. How does the first rain after a long dry spell change the grip, and how do you ride the first 20 minutes?
Read the worked answer
Three weeks of dry weather leave a film of rubber, oil and diesel dust on the asphalt. The first rain lifts it into a slippery emulsion, concentrated in the wheel tracks and wherever vehicles stop and turn - the filling station exit is exactly such a place. Grip is worst in the first 15–30 minutes; after a long heavy shower the film washes away and conditions improve.
So: one gear higher than normal, both brakes applied progressively and squeezed rather than grabbed, the bike kept more upright through bends, and no hard acceleration while leaned. Increase the gap beyond the normal three seconds, because stopping distances on a wet contaminated surface can approach double the dry figure. Look for the rainbow sheen of diesel and ride around it, not through it. At 4 °C also expect shaded sections and bridge decks to be colder than the open road, so treat dark patches as possible ice rather than water.
2. Describe your line and speed through the tunnel entrance, the grit inside, and the exit into low sun.
Read the worked answer
Entering an unlit tunnel in daylight, the eye needs several seconds to adapt, so the entry must be slow enough that you could stop within the distance you can actually see. The wet rock at the mouth carries run-off and grit onto the tarmac - a thin film of mud on the curve. Ride in the wheel track rather than the centre where grit collects, and do not brake while leaned on it.
Inside, the dry dust reduces grip slightly but visibility is the bigger issue: use the road markings and the tunnel lights for your line, not for your speed. On exit, low sun plus a beaded visor combine into near-blindness. Slow before the mouth, clear the visor with the back of a glove, keep it down and use the peak. If the sun is straight ahead, ride the near-side wheel track and use the edge line as a guide - but never cross it.
3. Your tyre pressures are 2.5 bar front and rear with 25 kg of luggage. What is wrong and what is the consequence?
Read the worked answer
The recommendation is 2.5 solo and 2.9 fully loaded. With panniers and a top box you are in the loaded condition, so the rear is roughly 0.4 bar short. Under-inflation makes the carcass flex more, heats the tyre, slows the steering and makes the contact patch less stable in standing water - exactly the conditions here. Combined with 2.5 mm of tread, which is legal (the minimum is 1.6 mm) but marginal for dispersing water at speed, aquaplaning risk rises.
Set the pressures cold before loading, ideally 2.9 rear and 2.5 front, and check the sidewall and tread for cracking. Carry your own small gauge, because forecourt gauges are often 0.2–0.3 bar out. If the tyre is old or perished, replace it before the trip rather than adjusting pressure around a worn tyre.