Fernando Alonso: McLaren driver spins off in Formula 1 testing in Barcelona
McLaren had a difficult start to Formula 1 pre-season testing when Fernando Alonso's car lost a wheel after just six laps in Spain.
The two-time champion lost control at the last corner of Barcelona's Circuit de Catalunya when his right rear wheel came off.
Red Bull's Daniel Ricciardo set the fastest time ahead of Mercedes' Valtteri Bottas.
Alonso and McLaren recovered to set the seventh fastest time of the day.
McLaren blamed a "wheel nut issue" and Alonso lost valuable running in the best conditions.
Alonso was initially classified fifth but was demoted to seventh when it was confirmed he had cut the chicane on his fastest lap to ensure he was not delayed by Toro Rosso's Brendon Hartley.
World champion Lewis Hamilton was sixth fastest after taking over the Mercedes from Bottas for the colder afternoon session, when most teams limited running because of the poor conditions before rain hit with half an hour of the day remaining.
Ricciardo's pace-setting time of one minute 20.179 seconds was 0.17secs quicker than Bottas, both using the medium tyre.
The Australian not only set the best time, but also completed the most laps, making Red Bull the only team to do more than 100 laps on the first of eight days of pre-season testing before the first race in Melbourne, Australia, on 25 March.
Red Bull team boss Christian Horner said: "It certainly was a positive first morning.
"It is always difficult to read too much into these things, but over 60 laps and sensible lap times was a good start to the pre-season testing.
"And it was exactly what we have been aiming to do over the winter to get ourselves onto the front foot coming into the testing.
"So the whole team has worked incredibly hard to get into the position where we are today.
"But, of course, it is just the start of a very long journey, and it is all about how we develop RB14 from this point all the way through to Abu Dhabi."
Ferrari's Kimi Raikkonen was third fastest, 0.157secs behind Bottas, on the faster soft tyre, and 0.041secs quicker than Renault's Nico Hulkenberg, who was on the medium, on what was an impressive first day for the factory Renault team.
Hulkenberg was consistently among the fastest runners when out on track on the first morning of testing and the German said his first impressions of the car were "positive".
Renault, who supply Red Bull and McLaren with engines as well as their own team, have said they are taking a conservative approach to engine performance at the start of the season and will only start to bring in extra power after the first few races.
Horner told Autosport he was "concerned" by Renault's strategy and feared that the gap between them and Mercedes on engine performance could widen as a result.
Hulkenberg's team-mate Carlos Sainz used the soft tyre in the afternoon session to go fifth quickest
McLaren's problems restricted Alonso's running to 51 laps, most of that in the less-ideal and colder conditions of the afternoon.
Ironically, Alonso did fewer than half the number of laps achieved by the Toro Rosso team, who are this year using the Honda engine that McLaren abandoned at the end of last season after three years of poor performance and reliability.
Hartley was eighth fastest. The New Zealander did 93 laps, comfortably surpassing the highest number McLaren-Honda has ever achieved on a single day of pre-season testing.
Alonso's off-track incident happened about an hour into the day's running and on a lap on which he was on course to set the fastest time of the day so far at that point.
The 36-year-old Spaniard was running the super-soft tyres on his first real run of the morning when the right rear wheel worked loose and then detached from the car as he went through the flat-out right-hander that leads on to the pit straight.
He closely inspected the right rear corner of the car while it was sitting in the gravel trap and returned to the pits holding what appeared to be the wheel nut.
McLaren executive director Zak Brown said: "There is no drama. It is quite an easy fix. It is what testing is for. All the teams will have a variety of issues. we will lose very little time. No big deal at all."
Fastest times from day one
1 Daniel Ricciardo (Aus) Red Bull-Renault - One minute 20.179 seconds
2 Valtteri Bottas (Fin) Mercedes - 1:20.349
3 Kimi Raikkonen (Fin) Ferrari - 1:20.506*
4 Nico Hulkenberg (Ger) Renault - 1:20.547
5 Carlos Sainz (Spa) Renault - 1:22.168*
6 Lewis Hamilton (GB) Mercedes - 1:22.327
7 Fernando Alonso (Spa) McLaren-Renault - 1:22.354**
8 Brendon Hartley (NZ) Toro Rosso-Honda - 1:22.371*
9 Lance Stroll (Can) Williams-Mercedes - 1:22.452*
10 Romain Grosjean (Fra) Haas-Ferrari - 1:22.578*
11 Marcus Ericsson (Swe) Sauber-Ferrari - 1:23.408*
12 Nikita Mazepin (Rus) Force India-Mercedes - 1:25.628
13 Sergey Sirotkin (Rus) Williams-Mercedes - 1:44.418*
* = time set on soft tyre; ** = time set on super-soft tyre; all other times on medium