Formula 1 teams are still awaiting fundamental information from the FIA on how 'Halo' load tests shall be conducted before chassis designs are finalised.
Teams and engineers have been supplied with the full details of the loads that the cockpit protection device and its mountings on the chassis will endure.
Force India had previously suggested the 'rushed' Halo introduction was going to cause delays with the 2018 car.
What is not currently known however is how the loads will be applied, and how forces will be distributed through the chassis.
Practically, a relevant load test, which would involve much higher loads than a halo can withstand. should see the cockpit device fail before a chassis' mountings fail.
But without a specific method which replicates how the loads are distributed, several chassis design parameters cannot be defined.
"The actual halo that we're going to run has been defined," Force India tech boss Andy Green told Autosport.
"But the actual chassis that it bolts to hasn't.
"To try to get a chassis to cope with the sign-off loads that the FIA have imposed is a challenge.
"The halo that we're going to race can't withstand those loads, so there's no point in using it to do the load tests on the chassis, because the halo will fail first.
"So, you have to do it with something else, and that's what we're trying to define at the moment.
"Depending on what that something else is will change how you design the chassis, and how strong you need to make the chassis, because it delivers the load in a different way, depending on the geometry.
"At the moment we're missing the details on that device, and it's quite fundamental to the design of the chassis. If you fail the test, you can't run."
In order to solve the pressing testing problem, the FIA has suggested the interim use, for this year only, of a virtual alternative destined to model loads on each team's chassis for 2018.
This solution which would use Finite Element Analysis - a computerized method for predicting how a product reacts to real-world forces - and would serve as a back-up to a physical test.
The issue is set to be discussed at a Technical Regulations meeting scheduled for next week.