As a semi-permanent track, the Circuit Gilles Villenueve circuit poses a unique challenge to the teams. It combines both fast and slow, with long high-speed straights interspersed with a variety of relatively slow corners.
As there are no fast turns, downforce isn't so important. Good top speed and low-speed mechanical grip are, however, critical.
Getting the car from these high top-speeds into the slow corners means brakes are tested hard six times every lap. So brake cooling is crucial in order to prevent them overheating and wearing out prematurely.
But too much cooling costs braking performance and adds drag, slowing the car on the straight.
The cool weather will make it harder for the teams to predict wear and brake cooling levels as the weekend progresses.
In the Friday morning rain, Red Bull set to work on a series of aerodynamic tests, with the car trailing flow viz paint from the rear.
Red Bull is working on preventing tyre squirt © XPB
As well as the Montreal-specific low downforce/drag wings, there were a lot of detail changes around the back of the car to the floor and brake ducts.
In Monaco, a pair of slots were added ahead of the rear wheels and these were joined in Canada by two extra pairs. They are to help prevent 'tyre squirt', which is the bow wave of air spilling off the rear tyres upsetting the airflow into the diffuser.
Above the diffuser itself, a new vertical vane has been added. Red Bull ran several of these vanes sat between the diffuser and rear suspension last year, but has removed them for most races so far this year.
The weather interfered with Ferrari's running of a new aero package during the Spanish Grand Prix and the parts appeared again during Friday practice in Canada.
Joining the previously-tested revised front wing mounts, Coanda sidepods and sidepod vanes, was a new front wing design along with a chin being fitted under the nose.
Ferrari introduced a new aero package © XPB
Throughout practice, the team ran back-to-back tests with differing set-ups. It's not clear what will be used for qualifying and the race here.
The front wing is the most interesting part. The cascade winglet mounted to the wing's endplate has completely changed. Rather than being a horizontal wing, the element is now curved such that it meets the wing in an 'r' shape.
The revised cascade wing will probably create downforce, but the change is more likely to be about modifying the way the winglet bends the airflow around the outside of the front tyre. The new outer vertical section of the winglet will be quite effective at turning the airflow.
Further aiding front end airflow is the small chin below the nose. Most teams have these 'pelican' or 'pregnant' bulges under the nosecone. They work with the neutral wing shape in the middle of the wing and the wings mounts to alter the airflow back along the edges of the car.
I'm told these chins also add downforce to the front end even though their design is counterintuitive at first sight, as it appears they would add lift not downforce.
Mercedes focused on the engine cover area © XPB
Mercedes ran a new shark fin engine cover to suit the demands of the track.
Rather than the bulged Red Bull-style cooling exit, the spine of the engine cover is slimmed to a thin fin. This removes the cooling outlet area, but will cut drag at the same time.
The fin is simply there to meet the regulations that demand a minimum size of triangular bodywork at the back of the car. This area is known by the design teams as the 'toblerone' section because of its shape.
Immediately on the pace in the tricky Friday practice conditions, Force India was running a relatively simple Montreal low downforce package.
The biggest visual change was the absence of the under nose chin, which had been a feature of the Force India since its launch.
It's not unusual for Force India to alter the detail of its nose chin and front wing mounts for different circuits' downforce levels.
Still searching for the elusive balance that has eluded it all year, Sauber brought more aero parts to Montreal. These were at each end of the car, with a revised nose and a new rear wing.
The new nose appears to be a rework of the normal C32 nose, shorn of the usual modesty fairing and fitted with a wider tip, shallower scallops leading up the chassis and a deep 'pelican' chin underneath.
Sauber is still trying to unlock the speed in its 2013 car © XPB
These changes may be Canada-specific and part of a low downforce set-up, so we should see these again at Spa and Monza.
At the back, the three rear wing specs have been seen; the usual delta V-shaped wing, a wing with a slight 'W' profile and another with lift in the middle of the leading edge.
Allied to the flatter wing that started testing and the Barcelona wing, that's five rear wing iterations seen on the car so far this year.
Back-to-back tests and a split set-up direction was the order of the day for Williams on Friday. The cars switched between nose and front wing designs and sported different floor too.
For the nosecone, it was a mix of the launch spec 2012 nose with a modesty panel and the FW35 nose with both large and small wing mounts were tried. These were matched to variations of the 2012 front wing and pre-Monaco wing design.
Williams trialled alternative packages back-to-back © XPB
Both cars had different floors, the external difference being the flick-ups along the sides of the floor.
Pastor Maldonado had a single long flick-up, while Valtteri Bottas had smaller flick-ups, the forward one having an extra flap mounted above it to help direct airflow from under the floor.
These flicks are there to manage the airflow travelling outwards under the floor, redirecting it back under the floor and into the coke bottle area above the floor.
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