'crossing' 4 point harness straps

I think the crossed is more important if you don't have harness slots.
In a crash if the belts are straight they may slip off the side but the cross tends to pull the belts together reducing the tendancy to slip off your shoulders.

I suppose it prevents sideways movement in a crash. The thing that worries me in your pic though is the twisted strap. That means that in an accident the load may not go in the designed direction. The best bet is to read the instructions and follow what they say. The company making the belts will have tested them in mutiple fitting methods and the instructions they give will have the best overall results.

That's not his pic!!
Besides which, it's only because it's been rolled off the shoulder and is unclasped.




between horizontal and a 45 degree angle, is what I understand to be acceptable, which generally means rear seatbelt mounting points are ok.

I thought it was between horizontal and 30 degrees? They look too steep in the pics.

Matt

Full guide on fitting here.

www.schrothracing.com/sdocs/2009_Competition_Instructions.pdf

This guide is for competition harnesses.

SHOULDER BELT ROUTING (APPLIES TO ALL RESTRAINT TYPES)

Shoulder belts must run from the shoulders horizontally or down, at no more than a 20° angle.

In cases where the shoulder belts must be routed down to the chassis floor, support by a roll cage bar or harness guide at the appropriate height is essential to establish the horizontal shoulder strap routing off the shoulder/HANS®. Most racing seats are not designed and tested to carry shoulder belt crash loads from downward installation. Severe injury or death could result. A 45° downward shoulder belt installation is possible with seats that SCHROTH has positively tested to take a load measured during a 50 kph [31 mph] and 28 G impact with a 75 kg (175 lb) dummy. Refer to the list of SCHROTH approved racing seats in section “About Seats”. WARNING: 45° downward shoulder belt installation is not recommended with HANS®.

For the best restraint of the occupant’s upper torso, ideal anchor points should not be further back than 200 mm [8”] from back of user’s seat.

In the event that the anchor points are further towards the rear of the vehicle [e.g. using a roll cage bar for wrap around attachment] the distance between the strap anchor points will narrow or even cross over as described in following graphs and tables.

FIA Blue Book states rear straps are to be fitted between 0 and 45 degrees, which would make those shown in the original post absolutely fine - as shown here:

www.securon.co.uk/fitting/extras/fiaharnessfitting.htm