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kbrew8991

alignment of harness bar, harness hole n seat, and shoulder?

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kbrew8991

I've read 15.5.6: [The shoulder harness should be mounted behind the driver and above a line drawn downward from the shoulder point at an angle of no more than twenty (20) degrees with the horizontal] and understand it, just looking for more of a "what's the actual best practice" rather than looking to just meet the bare minimum requirements. I'm already in there upgrading things and would prefer to do it once close to the actual right way (I don't have an unlimited budget), but doesn't need to be perfect if there are only slight differences and what I have on hand will work fine enough.

 

The old seat that currently is in there with the cover falling apart on that I'm pulling out and throwing away puts me closer than I like to the roof and cage halo, but, did provide a nearly perfectly horizontal path from the top of harness bar, through the seat, to the tops of my shoulders.

 

I have on hand a newer aluminum seat that mounts much lower to get my head away from the roof and halo bar, increases ease and speed of egress, and eases the concerns I had with the old setup of back-brace versus fiberglass seat in a rearward impact. The downside is that the belt path alignment isn't exactly optimal I think..? If I have the harnesses come off the bottom of the harness bar they come out towards the harness holes of the seat within a degree or two of horizontal headed slightly upwards, but, once they pass through the seat they come up fairly sharply to go over the tops of my shoulders and then on to the buckle - ie - the harness holes are about 1-2" too low. How not-optimal is that configuration?

 

If I end up needing a 3rd seat to get one with harness holes even with the tops of my shoulders I think I may be choosing between coming off the bottom of the harness bar to head slightly upwards through the seat and over the shoulders OR head off the top of the harness bar and head slightly downhill to pass through the seat and over shoulders. Which of those two choices would be the better route in practice?

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Alan_Wolfe

I don't know if it makes a difference safety wise. I like the belts to pull me back in the seat (down over the shoulder), not push me down in the seat (up and over the shoulder).

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TurboShortBus

In this example, I see no point in having the shoulder harnesses touch anything or change direction between the mounting bar and your shoulders.

 

Since you have an aluminum seat, you can enlarge the shoulder harness openings fairly easily ($20 Harbor Freight air body saw) so that the shoulder harnesses touch nothing between the mounting bar and your shoulders. Weld a piece of aluminum trim around the newly cut opening to prevent chafing the harness. Take the cover to an upholstery shop and have them enlarge the harness openings to match.

 

When determining the angle of the harnesses between the mounting bar and your shoulders:

 

Harness manufacturer's instructions > CCR

 

If the harness bar isn't quite at the correct level, then you can add a pair of these to the harness bar in order to get the correct fit (flip them upside down if necessary):

 

http://www.hmsmotorsport.com/store/schroth-competition-belts/competition-hardware/double-shoulder-bracket

 

DoubleShoulderBkt.1.jpgDoubleShoulderBracket.01.jpg

 

I have confirmed with HMS that using the upper bolt wraparound attachment for the primary shoulder harnesses (and not just for the piggyback 2" HANS harness for double harness belts) is permitted.

 

If you are interested in a set of these brackets, I have a new, unused pair for 1.75" tube that I didn't end up needing with the LaJoie seat and Schroth belts. Your cage is probably 1.50", though.

 

IMO, all seat backs should be braced to the cage, FIA approval or not. Even though the FIA-approved seat and brackets might hold up in a rear or side impact, the 22 gauge floorpan to which they are attached might not (especially if you cut out and patched your OEM floorpan and tranny tunnel for seat clearance, as many Spec Miatas do). The Evo that took a header off Pike's Peak is an example of a car that could have used seat back braces, as the floor pan buckled at the seat mounts, which allowed slack in the shoulder harnesses.

 

Mark

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