Fitting 3 or 4 point harnesses ?

[interloper]

As I'm now using my 944 for the odd track day I have decided to fit race style harnesses. Has anyone done this with my sort of car what would you recomend ? I've heard that the belts should not extend at an acute angle from the back of the seats, so that would rule out using the mounting points on the rear seats (for the rear seat belts) then I would assume ?

Your thoughts would be apreciated.

Cheers
Steve

[davyboy]

I once saw a bar that fits across the back of the car - dont ask me for anymore details as it was on ebay!

What seats are you using, not the standard ones?

Dave

[dontlift]

I have seen the rear seat belt mounting points used for harnesses on some 944 race cars.

[Melv]

davyboy said:

I once saw a bar that fits across the back of the car

Brey-Krause -marketed by somebody in UK.

Melv

[interloper]

The seats I've got are Cobra bucket seats, fixed back with holes for the straps. Just like this.



Did you happen to notice if the belts were attached at the wheel arche mounting points or the points that are on the rear seat area itself ?

>> Edited by interloper on Tuesday 25th November 14:15

[johnny senna]

My E30 M3 Sport Evo has a half cage and harnesses. The harnesses come from the rear seatbelt mounting points. I have been told that this is NOT the best place to mount them from because in a crash, your collar bones can be damaged. I don't know how true this is.
You need at least a half cage if you are going to fit harnesses, period. That's because if you flip the car, you will be held bolt-upright and the roof will crush your head. You effectively become the B-pillar.
I would talk to Safety Devices. I am told they bought Matter cages, famous for Porsche RS's. I don't know for sure. Ask them to quote you for a half cage with a harness bar. This will get rid of the problem of the seat belt mounting points and the danger of being crushed in one go. It will cost about £300-350.

[james]

A lot of people at track days just find a convenient mounting point for their harnesses, and then go with that. Then you come along, see what they've done and copy it, assuming that they have some sort of mystical expert knowledge. The trouble is, they might just have picked a point randomly based on what they thought would be ok, and you don't know that you've done it wrong until it's too late and your arms are lying in a different field to the rest of you.

If I were to fit harnesses to a road car for track day use, I would be inclined to go by the MSA rule book. Basically, if it's ok to get through scrutineering for a race, it should be ok for you on a track day. The rules for the mounting points are as follows:

Lap belt. You will generally be ok using the floor mounting points of the regular seat belt, assuming that you haven't moved the seat from its original position.

Shoulder straps.
Horizontally these should ideally go no higher or lower than 10 degrees up or down from the shoulders (although it is acceptable for them to go as far as 45 degrees BELOW horizontal if no other mounting point is available).

Laterally they should lie within 20 degrees of the center line of the seat.

If you can find mounting points that fit within these cones, then you're ok. If not, you will need to get something made up. A rollcage will generally have suitable mounting points built into it if the car does not present any suitable mountings.

Cheers

James

[GR4]

Think the Bray Krause bar is a guide rail rather than a mounting, so you can mount the belts low down and then feed them up through the slots in the guide bar which should be about level with the shoulder slots in your seats.

[interloper]

Right so what your saying is that the shoulder straps should in an ideal world extend straight back in both a vertical and lateral plane. Looks like a half roll cage or similar would be very much in order then.

I think I will make some proper research into the matter. Thanks people.

[DontLift]

interloper said:

Did you happen to notice if the belts were attached at the wheel arche mounting points or the points that are on the rear seat area itself ?

They were on the mount points for the rear lapbelts as I also questioned this with the builder of the car, and they are apparently within the angle.

[DontLift]

interloper said:

Right so what your saying is that the shoulder straps should in an ideal world extend straight back in both a vertical and lateral plane. Looks like a half roll cage or similar would be very much in order then. I think I will make some proper research into the matter. Thanks people.

I am collecting my roll cage next weekend, it is a bolt in one, and therefore will have a rear welded section and then forward bolt on bars, I will make enquiries if the rear section can be purchased on it's own for you

[heyou]

johnny senna said:

My E30 M3 Sport Evo has a half cage and harnesses. The harnesses come from the rear seatbelt mounting points. I have been told that this is NOT the best place to mount them from because in a crash, your collar bones can be damaged. I don't know how true this is....

I've also read this before but i don't understand why collar bones would be broken if the shoulder mounting points are much out of horizontal? Is it something to do with the way the belt tightens differently in an impact compared to when the belts are horizontal?

I was thinking of doing the same thing on my 968.

Del.

[ninemeister]

You will find that most off the shelf 4 points do not fit the 944 particularly well since the existing seat belt mounts are too close to the seat and the lap straps will not be able to be shortened enough. Secondly, as mentioned, if you use the rear seat belt "bum" mounting points the angle of the shoulder straps is too acute.

I suggest that you buy eye fittings and plates and simply drill through the floor in the rear footwell for the lap mounts, then either fit a rear cage with a harness bar across behind the seats & use the lower rear mounts, or take the shoulder straps back and put another pair of eye mounts in the boot floor. HOWEVER be warned that the petrol tank is under the boot floor, so you must not just drill it willy-nilly. If the car is pre-86ish and has a metal tank you can drill the floor in front of the gearbox crossmember. If it has a plastic tank you must drop the tank to drill it, then fit the anchor blocks underneath and cut down the eyes so that the thread is flush underneath. It would do no harm to tack weld the anchor blocks in place since it makes it easier to remove and refit the eyes if required.

[james]

The thing about your collar bones being broken is a bit of a red herring actually. Ok, you might be pulled down in your seat if the seat back fails, but that isn't the reason for the horizontal angle rule. It's actually to prevent the seatbelt being overloaded.

You must all remember about vector forces from your O Level physics. If you mount the shoulder straps at a steep angle, the load will be much higher in the same impact.

Taking an example.

Say you had a 100N load under decelleration. If the straps are horizontal, there will be a 100N load on the straps.

Now you have the same load, but the shoulder straps are inclined at 60 degrees below the horizontal. The load exerted on the straps will be

100N
---- = 200N
cos60

So you've doubled the force acting on your shoulder straps simply by angling them down at 60deg. If you go to 80 deg, it becomes

100N
---- = 576N
cos80

As you can see, it doesn't take long before a fairly minor impact is going to make your shoulder straps fail.

The reasoning for the lateral angle limitation is much more simple. It's to stop them from falling off your shoulders

Cheers

James

[interloper]

Interesting reading guys, thanks ! Dont lift if you remember to get a price on a half cage please post the info up here.

Cheers
Steve

[heyou]

james said:

You must all remember about vector forces from your O Level physics.... James

Physics!! I do recall our science teacher telling us about the time he did some car maintenance and he cleaned his brake drums with parafin, he then took it for a test drive and went straight through a cross roads the first time he used his brakes! I sort of lost faith in his ability to teach me anything useful after that.

Thanks for the info

Del.

[DontLift]

interloper said:

Interesting reading guys, thanks ! Dont lift if you remember to get a price on a half cage please post the info up here. Cheers Steve

Will do Steve, My complete cage is only costing £250 so i wouldnt imagine it would be that much