ColorChange

Nice job John.

Z-man

Thanks for the explanations. Now it makes more sense.

-Z.

Bill L Seifert

First, I am not an engineer. But I remember from Army Helicopters that our armored seats were on sliders, and held in the helicopter with 4 half inch bolts. (about 3/8 inch shafts) The seats weighed close to 100 pounds, the pilot averages 200. The lap belts were anchored to the floor, but the shoulder belts were mounted to the seat. I was told by Bell (The company that made the helicopters) Reps that a human could only take 18 G's and survive, and even then you were going to get hurt real bad. Anchoring anything down beyond that only secures your dead body so that you can be easily found.

Also, you do want a certain amount of give in everything. On impact, the belts stretch, and you want them to. It is the sudden stop that kills, so anything that can slow that stop helps, whether it is belt stretch, mounting bolts bending, car crushing, etc. Don't get me wrong, you don't want you seats held in with wire ties, but the mounts you get with good quality seats, Momo, Sparco, Corbeau, etc, are fine.

Bill Seifert

1987 944S Race Car

RedlineMan

Jon -

I honestly do not know if it was the seat or the mounting that failed. Somehow I thought I'd heard it was the mounting, but that is nothing more than a hunch. Suffice it to say that it was probably done like most installs, and it did pass tech. No one setup will save you in every conceivable eventuality, but there is good and there is better. Always strive for better. If you're not sure, don't assume, ask.

Crisp -

I'm not an engineer, I don't play one on TV, and I have no current plans to stay at a Holiday Inn Express anytime soon. As you know, there is no doubt as to the increased efficacy of a double shear mounting. Whether in all cases it significantly enhances safety or reliability is an arguable point. I feel that seat mounting is such a case where it does not. Now, if we are talking suspensions...


Thanks Color!

Bill -

This goes off topic, but here goes anyway. With much due respect, I must dissagree with a lot of what you have said, technically. It is indeed the sudden stop that kills you, but the current wisdom is that it is the old model of trying to dissipate energy slowly that CREATED the sudden stop. This was dissccussed at length earlier in the summer.

Kinetic energy is equal to one half the velocity squared. KE=0.5V^2. The amount of kinetic energy that is created is a function of time. The longer a body stays in motion within the time frame of the incident, the more KE builds. The old model had belts that stretched dramatically to dissipate energy. However, when the stretching stopped, the whiplash effect took over. THIS is the truly deadly aspect of the incident.

Fatal kneck tension loads start to appear around 30g. The modern model calls for MINIMAL belt stretch and rigidly mounted seats. You DO NOT want to dissipate energy over time because motion relative to time is the enemy. Retain the body as completely as possible. Stop the motion quickly, and you stop kinetic energy from running wild.

Indeed, it is a bit of a lesser-of-two-evils situation. With modern belts, super rigid containment seats, and Head & Kneck equipment, you've done the best you can do.

mitch236

I'm just thinking aloud here but wouldn't the belts do much of the retention in a crash? It would seem that as long as the seats are reasonably well mounted and the belts are either mounted to the cage or secure floor mounts, that one would be reasonably secure. I am also assuming there is a seat back brace, properly mounted.

BTW, John will you be at 48 hours? If so, how can I find you?

smokey

When you're connecting two pieces of sheet metal, the moment in single shear is probably negligible. Also, if the bolt is threaded into a solid mount on one side and pretensioned, the moment will not be a major issue. Finally, the moment really only becomes an issue once the bolt has passed its yield strength in shear, by which point it's academic. Re seat mounts: in a frontal crash, seat mounts are a bystander, the belts do the work; for a rear crash, the seat and roll cage stop you, but the standard test for composite seats is only 17 g's, and it doesn't take much of a mount to withstand that. In any serious crash, the seat will break. There is one composite seat tested out to 44 g's, and a good mount would be worthwhile with that one.

macnewma

Smokey, what seat has been tested to 44g? Is it the RaceTech seat used in the Viper GTS-R?

Also, could people post pics of beefed up seat mounts that they have built?

Thanks,
Max

Greg A

I'm not Smokey, and I'm not sure which seat he is talking about, but according to a Cobra distributor's website, the Suzuka Technology "[e]xceeds 3 times the FIA 8855-1999 standards @ 63g".

Sube Sports

Greg A

smokey

I was thinking of the RaceTech, but it looks like the Cobra also does well. Survivability is somewhere north of 50 g's, so the Cobra is worth mounting properly. With an FIA seat, you want to look at what you will hit behind you when it breaks, and what effect that impact will have on your physiology. There is no point in making the mount stronger than the seat. In a rear impact, the front bolts on the seat will fail in tension if the seat doesn't break. Take the maximum tensile strength of the bolts, calculate the maximum moment around the back of the seat, and figure out the max accelaration of the seat/occupant at yield of those front bolts. If that acceleration is more than the seat or occupant can survive, the bolts are strong enough. Got to get back to work.......

smokey

Ok, I couldn't resist. Here goes. An 8.8 metric bolt has a tensile strength of 120,000 psi and and an 8mm bolt (Porsche's standard seat mount bolt) has an area of 0.078 sq. in. Thus the Porsche bolt has a tensile strength of 9,300 lbs. Two bolts give you 18,600 of capacity, and with a moment arm of 1.5 ft for the track length, can take a torque of 27,900 ft lbs. Say the seat and occupant weigh 250 lbs, and the centre of pressure of the seat/occupant is 2 ft above the seat track. The maximum force at this centre of pressure is thus 27,900/2 = 13,950 lbs. Thus the max acceleration of the seat/occupant that those two 8mm 8.8 bolts can sustain is 13,950/250 = 55.6 g's. Maybe Porsche chose the right bolt.

A930Rocket

That's what I was thinking....

P.Po

Scroll down this link and check out the seat mounting kit -> http://www.cscracing.com/se2.htm

It really simplifies life for the at home welder guys...

ymmv,
Patrick

mdbickell

Ok, I think I get the issues here but since I'm not an engineer, mechanic or anything close I'm going to ask a dumbed-down question and would like to hear some thoughts....

I have a sparco seat w/ side mounts. I don't use sliders and have bolted the seat to the existing mounting points in my '81SC. Is there an issue of mounting to the existing points as supplied by the factory or is bolting to the floor preferable? If the floor is preferable, why? This stock mounting points appear to be a solid mounting point of reasonably thick steel welded to the body.

I don't have a seatback brace right now, but I'm thinking I'll get one for all the reasons stated in this thread.

thanks in advance for your thoughts.

Glen

Hence my repeated concern regarding the mandatory seat back brace...no give at all..... while a FIA approved seat alone has quite a bit of give built into its equation for the reasons Bill brings out here.

fatbillybob

Redlineman,

FG and Carbon seats limit us to 4 mounting points on the seat. I understand how you are doing 3 per side with the alloy seat. Can you show us some pictures of some ideas on how you mount to the sheetmetal floor?