After the four long-haul grands prix that started the season, the Formula 1 circus has had time to return to base and regroup and every team has new parts for this weekend.
McLaren was not unique in having a major upgrade package, but the first significant update of the MP4-28 after struggling since pre-season testing was the big story of practice.
The changes centre on two key areas - the front wing and the sidepod/exhaust set-up, although the rear wing also featured a new endplate.
As these changes focus on the front end and the exhaust, it's fair to conclude these are the areas where the car was failing in its original guise.
If these parts show less aerodynamic sensitivity to a lot of suspension movement, McLaren can run the car's suspension softer for more mechanical grip and increased driver confidence.
McLaren's updates have drawn the most attention © XPB
The front wing remains a relatively simple three-element design, gaining its downforce from using the full area allowed for the wing rather than its rivals' method of creating the most downforce near the tips and having six or more elements in this area.
To prevent its larger wing upsetting airflow over the centre of the car, McLaren has feathered the inner edges of the flap to reduce the disruption.
The main addition to the wing is the cascade mounted to the endplate. This is now a larger winglet with an l-shaped winglet mounted to its inner edge. This replaces the previous smaller winglet and separate l-shaped vane.
Further down the car, the sidepods have been completely re-sculpted. They are now very similar in shape to the Mercedes design.
The sidepod tops form a ridge that directs airflow over the exhaust outlets. This effect is further bolstered by a new vane arrangement around the front of the car.
Like Ferrari, McLaren has gone down the Sauber route of arched pod vanes, although it has added vortex-generating fins to the top of the sidepod to aid the effect or direct airflow over the exhaust and deflect it towards the diffuser.
The other noticeable change is to the rear wing endplate. It has gained a slot in its leading edge similar to the one Force India has run for some time.
This slot allows more airflow to pass through the duct formed by the wing and its endplates. By having more airflow to pass through the wing, more downforce can be produced.
Additionally, McLaren has added vanes to the outer face of the wing's endplate similar to the ones used by Ferrari for several years.
Having tested blown wheelnuts in Bahrain, it was a surprise Red Bull didn't run them again in Spain.
Instead, the two cars were different with one running new front brake ducts. These feature a small recess in their leading edge, which appears to be closed off with a covering panel when the car is not running.
The recess has a small hydraulic actuator, which is nowhere near the brakes or any other normal part of the car powered by the hydraulic system. Judging by its position, the actuator may be linked to a flap forming part of the brake duct.
This appears to be similar to McLaren's 2012 idea of adjustable brake ducts, which altered the exposure of the hot brake discs to the wheel to alter tyre temperatures.
These ducts would be adjusted (legally) when the car was at rest at a pitstop by a mechanic with a tool. Therefore, the tyre temperature could be tuned to a small degree to react to race conditions.
Using a hydraulic system is a surprise. If the system is powered by the main hydraulic system it might be considered illegal, so it's likely it uses a manual hydraulic system rather than simple cables to move the brake duct flap.
Red Bull has been experimenting with sidepods and exhaust detail © XPB
The car also sported revised sidepods and front wing details, both these areas being tested with flow visualisation paint and a rake of aero sensors.
The sidepods featured even more curved exhaust bulges and a flatter section of the tunnel formed by the sidepod.
The front wing doesn't appear to be very different, as the flow viz was applied under the wing the changes might be to the fences under the wing, which redirects the flow around the inner face of the tyre.
Having completed a straightline test at Spain's Idiada facility, Ferrari ran its cars in two slightly different configurations during Friday practice.
Both cars featured changes to the aero detailing around the front of the sidepods, with Felipe Massa's car having a lot more sidepod vanes and a longer exhaust exit panel.
All of the changes appear to be aimed at getting more performance from the Coanda exhaust set-up. The more complex sidepod on Massa's F138 features Sauber-like vanes that arch over the top of the sidepod.
Ferrari's work focused on sidepods © XPB
Ferrari already had a two-part sidepod vane. The forward part of the vane now reaches over to the cockpit's edge and aligns with the other fins around that area.
The trailing part of the pod vane now meets the shoulder of the sidepod front, with the vane pointing downwards as if to create lift rather than downforce.
The team ran flow-visualisation tests with this set-up and the flow patterns showed the vanes were turning the airflow down over the sidepods.
This suggests Ferrari is aiming both to deflect the exhaust gasses down to the diffuser and also feed more flow through the coke bottle tail of the sidepod for yet more airflow over the diffuser.
This was removed from the Brazilian's car for second practice and only the additional fin ahead of the wing mirrors remained.
Compared to other teams, the Lotus upgrades were far simpler but no less innovative with new features on both its front and rear wings.
Ahead of the Spanish GP, these were tested in a straightline test at Duxford Airfield.
While the majority of the front wing and its endplate appear to be similar to the version introduced in Bahrain, the endplate has gained a small l-shaped vane hanging from its outer edge.
Lotus's changes look outwardly simple © XPB
This piece presumably creates a vortex that spirals around the front wheel to aid the airflow to the rear of the car. No other car has run such a device in recent years, but as long as this part does not extend past the maximum width of the wing, it's legal.
Another grey area in the bodywork rules exploited by Lotus is around the rear wing. There is a small area ahead of the beam wing that is allowed to have bodywork, but is rarely exploited. Lotus has fitted a thin aerofoil section below and ahead of the beam wing.
In this position it will act as a slat to help feed air to the beam wing to make it more efficient.
Mercedes features only small changes © XPB
Aside from a switch to a new colour scheme, there only appeared to be one change made to the W04.
Since its launch the car sported three fins around the cockpit. These have been simplified into a single vane.
The wet weather upset Sauber's plan to run new aero parts to tackle the car's rear-end instability.
Sauber is hunting for rear-end stability © XPB
One key part in resolving these problems is a new rear wing, the third design Sauber has run this year.
This new version features a dip in the centre of its leading edge and a new DRS actuator.
The cover for the DRS mechanism was so new it had to be made in metal with a rapid prototyped method, the polished silver finish being similar to that run by Lotus around its exhaust outlets.
A completely unexpected departure for Toro Rosso was a new Coanda exhaust design unlike the Red Bull tunnel concept or McLaren's semi-Coanda solution.
Practice suggests a big improvement from Toro Rosso © XPB
The sidepod is undercut below the exhaust outlet, but unlike Red Bull's idea it never closes in to form a tunnel. Instead, the exhaust plume makes a short 5cm jump down to the floor, to blow along the edge of the diffuser.
In some respects, this takes the concept of the undercut sidepods run by Toro Rosso for the past two years and updates it into an effective Coanda design.
The sidepods are now effectively raised and flat bottomed in the coke bottle area; this cleverly allows the exhaust to find the diffuser and the general airflow to reach the diffuser, without upsetting each other too much.
After a successful update to the CT03 in Bahrain, Caterham completed straightline testing in France in preparation for the Spanish GP.
The car features further updates, forming the definitive 2013 specification compared to the interim version run up until Bahrain.
Like most teams, the updates focus on the front wing and sidepod package, although the car also gains a more graceful vanity panel over the nose.
Caterham is now in full 2013 spec © XPB
The front wing is largely a three-element set-up, with an added slot near the wing tip. The cascade set-up is all new and mounts to a longer once piece vane on the endplate.
The cascades are now a made up of a much wider winglet with a vane splitting its main surface, and then a smaller winglet added to its inner edge. This is very reminiscent of the current Lotus design.
Aiding the flow over the sidepods, as well as the Bahrain spec pod vanes, are several fins to set up vortices to tease the airflow into the right places. A pair of vanes sits under the raised section of monocoque and another pair under the wing mirrors.
Caterham has found its updates both from Bahrain and these new parts matched its expectation, unlike the less successful upgrade at Silverstone last year.