P.Po

Is the strength of the seat mount really that important?

The reason why I ask this questions is because recently I have hauled out quite a few seats from production cars. I cannot believe how most of them are mounted. The seats I saw mounted used very small bolts, with a small head area, not much of a washer going into not that much a thread, and isn't really in any really strong area of the floor. To my amazement, the whole back seat of one car was help in by four sheetmetal clips, and two small bolts.

Before you think I am nuts for even wondering about this, think about the forces that get applied during a crash. Does the seat really have to be mounted super strong? Isn't most of the force going to be between the belts and the body?

I understand that shear and tension need to be considered when mounting a seat, but I believe most mounting systems in race cars are overdone. Specially when you compare them to the way car manufactures mount thier seats.

Anyhow.... Throw in your $0.02 and am curious what others think...

regards,
Patrick Parato

Juan Lopez

Patrick,

It is really important... IMHO you cannot judge by what an OEM does for road use to the heavy stresses involved in competition driving. OEM considers many other things (mostly a balance between $$$ issues and potential liability).

Even using the heavier duty race type setups, different organizations (PCA included) have had incidents where the seats have broken off the base and left the driver tumbling inside the car.

Regretfully, some of these have had fatal consequences thus the requirement for the back brace on race seats.

In my car, the seat mount is overengineered and overdone....

Z-man

Patrick:
What about the mechanism between the seat and the bolts - IE the sliders?

I saw footage of a Speed World Challenge car (Jon Groom's 911 GT3 Cup car) get smacked up in a race. (T1 at Mosport) At the point of impact, you can see the seat literally move more than a foot laterally.

IIRC, there is a move to fixed seat (non-sliders) in such cars, since sliders are a weak point in an impact.

-Z-man.

sjanes

Well, here's how I see it.

911s typically go off backwards, so the seat helps out a fair bit This summer, I was at the body shop of the local Porsche dealer and there was a recently crashed 996 cab there. It of course went off the road backwards (proving that it is a real 911). The seat did not break free of it's mount, but the back of the seat collasped and the driver was ejected out of the car (he survived) even though he was wearing a seatbelt. If the seat itself had been strong enough to take the impact, would the mount have broken free instead?

I too was supprised when I got my Recaro SRD seats. The factory Porsche seat mount for my 87 Carrera used 6 x 8mm (I think) bolts per seat, but the Recaro slider bolted to the SRD using 6 x 6mm (12.9) bolts. I've orderd a seat back brace to help prevent the Recaro SRD (which is a reclining seat) from reclining unexpectly in a rearward shunt and throwing me into the rollcage.

As for the back seats of a car, they are typically supported by the wall of the trunk, and really can't go anywhere.

smithk993

When I was building my BMW Spec E30 (325i) track car cage and seats mounts, the builder suggested that we build the seat mounting rails as part of the overall cage construction. So my seats and harness are all basically mounted to the cage and not the frame of car. I think the idea is that in an impact, seat,harness and cage all move as one rather than have the seat move one way and the harness moving another! Seem to make sense.

Keith

chrisp

Production seats don't hold you in so well that in a crash the forces are transferred through the seat to the mounting points. Once you get into fully bolstered racing seats I think you'e demanding more of the mounting system. For me the way people mount their seats to the floor pan is a really bad idea...no matter what size hardware they use.

Jon Moeller

P. Po,
Not to make light of a serious topic, but you should talk to the 911 guys with roll cages/bars that run factory seats and have hit a wall backwards. The punchline: you probably won't find any to talk to.

OEM mountings and the seats are a compromise. If you're looking to do serious track work (multiple DE's, racing), I would strongly recommend that you avoid sliders, run fixed-back seats, and mount them securely to the floor.

I'm installing fixed back seats, 6-point belts and a rollbar in my car for this season.

-Jon

evolvedaz

i couldnt agree more, with mulitple track days and DEs a seat might wannt to be securely mounted to the floor and reinforced. I just saw an m3 race car last month at PIR(az) get the driver bucket completely ripped out of the floor upon impact and it got me thinking seriously about this topic.
-with some cars it can be really amazing how weak the factory will support a seat. 4 12mm bolts in most cases.

P.Po

Good advice all around. Thanks guys!

I was wondering because while trying to mount a Kirkey in my 951 using Sparco sliders I came up with so many different ways of mounting them. Some super string and time consuming, others simple but probably not safe.

Basically I have come to the conclusion that if you have a street car and you don't want to weld/create supports to mount the seat properly (for resale reasons), you are probably going to have to comprimise safety in some aspect.

Assume using a popular seat like a Recaro or Sparco, and using the mounting systems that they supply to have the ability to remove such a seat to put the factory seat back in. Would you say that this is probably the same or worse then stock?

The more I learn the more I realize that a street/track cars biggest comprimise is safety. Either on the street or on the track. I need to save for the truck/tralier!!

Thanks everybody,
Patrick

RedlineMan

Hey;

Let me start by saying that our one and only Club Race fatality resulted from a 911 rear hit and a broken plastic racing seat. The seat actually broke twice; once from its mounts, and then when it contacted the belt bar. Quite sadly, the next thing that broke was the driver's neck.

Everybody is on the right track, as it were. Chrisp came at it from a very sound dynamic direction. The amount of retention that even the most basic race seat offers compounds the forces involved dramatically compared to any stock seat.

It is not so much a factor of how many bolts, or what size, but of spreading the load. Any number of bolts of any size will only offer as much as what they bolt to can withstand. Four 10mm bolts (2 per side) mounted in shear will take a lot of load, but six 8mm bolts (3 per) will spread that load over a larger area, and eight 6mm bolts (4 per) even further. The weaker the mount structure is, the more you need to spread the load, so the more fasteners you should seek to use to do so.

If you have a Titanically strong mount base, four 10s would work. However, in the same or even a lesser sub structure, six 8mm bolts would spread the load further afield, which is generally always better. That's the principle you need to work under.

I generally only use alloy seats in customer car builds. I mount through the vertical side of the thigh bolster - in shear - using at least three Grade 8.8 8mm bolts per side. This is an EXCEEDINGLY strong method. I also generally seek to reinforce the floor areas as much as the scope of the project and the intent of the vehicle will allow. This can be anything from sliders (aftermarket race) to reinforcing the floor to completely custom building a new mount system.

The picture below shows a 930 install to the OE mount flanges. This was later redone by removing the OE sheetmetal longitudinal mounts on the rocker and replacing them with custom steel mounts that welded in latterally between the tunel and rocker sills. Not only was it FAR stronger, but it gained us at least 2" of head room.



Unfortunately, this car was involved in a serious track crash late this year. It T-boned a stationary 911 at approximately 50-60mph.



The front of the car was destroyed, the floor severely buckled, and the pedals ended up under the steering wheel. The seats and their moutning system performed flawlessly, and quite possibly kept the driver from more serious injury than a sore ankle.

Yes, Pat. Seat mounting IS important, but I know you know that anyway. What you see in OE apps should frighten you, eh?

Jon Moeller

John,
Can you elaborate more on the fatal crash? Not to be morbid, but can you provide more details on what the car had in it for equipment? Am I reading correctly that the seat broke from the mounts, and not the mounts separating from the body?

Thanks,
Jon

chrisp

Redlineman keep in mind that when you study the load carrying of various hardware items that your design (and all others) has the bolts in single shear. A bolt held in double shear has a much greater load carrying capability than a bolt in single shear. So if you are comparing tension to shear make sure you look at single shear values.

Z-man

Can you post a diagram of a single shear setup vs. a double shear one?

I'm assuming that the bolts aren't lined up in a straight line in a double shear setup.

Thanks,
-Z.

sjanes

This shows single vs double shear. I got it from: http://www.roymech.co.uk/Useful_Tabl...ted_Joint.html

chrisp

Single shear is when the bolt passes through two layers of material. Double shear is when there is a center layer with two layers sandwiching it and the load is being applied from the center layer.

Double shear bends uniformly whereas single shear has a bending moment as the bolt is both trying to be sheared and bent.

For example, the alignment bolts on a spring plate are in single shear. Lug bolts too.

The bolts on a front sway bar drop link to a-arm mounting tab are in double shear.

Since, on a seat application, you pass the bolt through a side mount and into the seat it's in single shear. If you could design a mount that went up and over the bolster and grabbed the bolt on the other side it would be in double shear. Obviously this can't be done because then your body would be touching the inner portion.