dwe8922

I bought a used evo 2, that didn't come with the side bolts. I had hardened bolts for attaching the bracket to the floor (no slider, seat to bracket to floor), but could not get hardened metric bolts for the side, mounting mounting the seat to the brackets. The side bolts for my drivers seat (original sparco bolts) don't seem to be hardened. Will standard bolts be safe, or should I seek hardened ones?

Thanks!
David

Alan C.

I wouldn't use anything less than grade 8.

Premier Motorsp

Grade 8 is for SAE bolts only.

Metrics generally are 8.8, 10.9, and 12.9. Even the 8.8 are ok, since the seat is going to fold up LONG before the bolt even cares about the load. Almost all Allen bolts (available at any Ace Hardware) are 12.9. The 12.9 is the only way to go for Allens since they don't strip out any where near as easy.

Chris Cervelli
Premier Motorsports

dwe8922

Thanks for the clarification on the difference between metric and SAE. The bolts I bought from Lowes have 8.8 on the head; I just didn't know what it meant, and neither did the salesman. I didn't see any metric allens there. I'll check with ACE to see if they have any 12.9's.

Thanks!
David

RedlineMan

Hey All;

Metric Grade 8.8 is equivalent to SAE Grade 5. Porsche fasteners are no less than Grade 8.8. Early Porsche seats used only four dinky 6mm bolts. Not good! Later model Porsche seat rails used six 8mm bolts to the floor. Really late model (996 at least) use 10mm. They are not "REAL" bolts, but that's another thread.

Most race seat applications use at least three bolts in each side to attach brackets/rails to seat bases. That is the minimum I like to use. In that quantity, a good quality SAE Grade 5 bolt would be sufficient, or obviously the Metric equivalent. As Chris says, the bolt is "relatively," and I reiterate, "RELATIVELY" non critical.

Something else that no one ever considers is that you do not want to create undue stress in an area by entering in a fastener that might endanger the assembly that it is fastening together. Using a grade 12.9 that was so strong it ripped out would not be as adviseable as using a grade 5/8.8 that deformed but allowed everything to remain intact.

dwe8922

John,
The 993 has 3 bolts per rail, and for the seat bracket to the floor, I ahve the 12.9 bolts. I do wonder though about the amount of give in a bolt that's designed into the application by porsche. I think a good point was made above that the fiberglass around the bolt, even using an 8.8, would give before the bolt would.

RedlineMan

Indeed;

That is a concern. All you can do is try and spread the load over as wide an area as possible. The best way to attach them is a side panel mount where the materials and fasteners are stressed in shear (see photo). This greatly increases the survivability of all materials involved.



Any mounting that has the bolts going vertically down through the seat base - loaded in tension - would not make me very comfortable unless the seat and rails were specifically designed and reinforced for such a mounting. I generally recommend alloy seats, and always mount them to the side panel with bolts in shear!

smokey

The seat is there largely for reasons of cosmetics and comfort - and to provide a place to sit when you're driving. The only case where there are significant stresses on the seat is in the case of a rear impact, in which case the roll bar or roll cage has to provide the restraint to the deflecting seat. In a 40 g impact, the load (150 lb driver) will be 6,000 lbs. It's the belts that do the restraining. The seat just helps position the belts at impact.

chrisp

Don't forget that it's not all about tensile strength. One should consider brittle-ness as well. A hard bolt is brittle and although it's yield is higher, it doesn't stretch. Whereas a lower grade bolt will stretch before it fails. It depends on the peak loads it's seeing and the duration and repetition of those loads. For example, the bolts holding my SC (and I am saying SC because I don't know how others are attached but are assuming most 911's are similar) rotors to the hub are probably better off being a medium grade bolt rather than the highest grade. With the repetetive stress it's better to have something that mixes strength and stretch. The lowest grade would stretch too easily and fail and the highest grade may take the load a few times but would snap when it meets it's fatigue limit.

RedlineMan

Smokey;

I've got to almost completely dissagree with you. The only time it is 100% up to the belts to retain the driver is in a frontal straight on hit. After that, you lost me.

At every other angle, the seat is a VITALLY important part of a modern driver retention system, and this importance increases dramatically as the direction of the impact moves away from longitudinal toward lateral.

If you look at your seat, and it does not look like it is deep enough or stiff enough to help you stay in it in a side hit, you better think about replacing it with something GOOD! Likewise, if the mounting looks too flimsy to withstand that 6000lbs you mention, this might suggest you build something stronger!

smokey

John, what is the maximum force that is applied to seats for the FIA tests? If it's over 6,000 lbs, I stand corrected.

RedlineMan

Hey Smoke;

Of course they do not put that much stress into a seat, as it would obviously fail well before that, but your premise assumes that every hit registers 40gs. What about hits that do not register that high? It doesn't take 40gs to hurt or even kill you. Wouldn't it make sense to have any protection and retention you could in the phase where a good seat would help?

smokey

I checked FIA Standard 8855-1999 for seats. It requires a rearward impact test at 20 g, and a side impact at 15 g. There were some pictures in a thread here a while ago of a car with an aluminum seat that had a 40 g side impact, and the seat was visibly distorted. The seat does play a role, and a good seat offers protection and absorbs some energy, but only up to a point. The mounts are intended as part of this system, and making them stronger than necessary is redundant. We all agreed in a previous thread that a head restraint of some sort is desirable for side impact. After 20g, some sort of restraint behind the seat also seems desirable, but the roll cage should provide that. So we're in agreement. ( I have a tendency for hyperbole.)