That is my point, it will undermine stability to some degree
At least if a yarn test was done it could show the effects to a certain degree and it does make sense that it would stall the air and cause lift.
It would be nice if the manufacturer actually did some basic testing before offering such parts for sale.
The car was designed with a rather loose rear end to make it more fun, at very high speeds taking away more grip could end up if not causing at least contributing to lose of control and a very serious crash.
It is not to likely we will ever see a crash attributed to the risers due to the fact it will not be looked at that way, only in hard core racing does such analysis of crashes take place. Public road crashes at higher speeds are nearly always just noted as being due to excessive speed, lose of control, etc, accident investigators are not even trained to look much beyond that and maybe a blown tire.
Most drivers are not going to push their car to that level and most should not be doing so unless on a track and have some real experience but we have in our midst some that have not shown enough self restraint and tragic results followed, it could happen at lower velocities due to less rear grip using such a mod.
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My posts are about a specific part, not the overall company product line as they build many excellent parts, this one they got a bit off course on unfortunately and they are not the only ones making parts that destabilize the rear of the car aerodynamically.
I have discussed these maters with somebody that knows more about this subject than all of us combined on this forum, it is a real issue that might only come into play on certain and rare occasions but those are the occasions where life can be on the line.