Wow, 50% right. That is better than I usually do. Especially when my wife is involved.
I enjoy a spirited discussion (and working from home I am pretty bored staring at a computer all day), so let’s throw some numbers in to this rather than discussing vague generalities.
The two primary concerns I read from this were:
1. Moving the seat up on spacers is unsafe.
2. The wood spacers are unsafe and should be replaced with metal.
Thinking about a worst case scenario where a lot of force is applied to seat mounting (when seat belts are not fastened to seat), that would seem to me to be a rear end collision. This would have the force of your body pushing back on the seat, potentially ripping the front bolts out of the floor. In this case, there is no loading on the front spacers, and a lot of load on the rear spacers.
On the wood spacer issue:
For worst case, let’s pick a crappy wood, pine. Pine has a compression strength of about 5000 psi. Let’s assume 2 square inches of contact area for a total of 10,000 lbs per spacer.
Seats use a 1/4-20 bolt as I recall. Tensile strength of a grade 2 bolt is 2350 lbs (grade 5 is 3800). Tensile strength and compression strength are two different things, but this gives you a rough comparison of what forces different things can withstand. From this I would suggest that the wood spacers may not be as bad as we might think.
On the raising the seat an inch issue:
Raising the seat an inch would provide more force on the mounting bolts like having a longer wrench on a bolt. On my car the top of the seat is about 27 inches from the floor. For simplicity and worst case, let’s assume I weigh 250 lbs and all of that weight is applied to the top of the seat. That would be a torque of 2.25 feet (27 inches) times 250 lbs for 562 ft lbs on the mounting system. If you add an inch, that makes the force 583 ft lbs. About a 4% change. Base in this I would conclude that the 1 inch spacer doesn’t make a lot of difference. if want to break loose a stuck bolt, you don’t get a wrench that is an inch longer. Doesn’t really matter
The reduction in the track surface area contact with the floor may be a bigger issue with round spacers. It would likely be better to have a long spacer that supported the entire track from front to back, rather than round spacers made of anything.
How about an example crash scenario? I will arbitrarily pick a 10g crash. This means the 583 ft lbs would now be about 5,830.
For two grade to bolts on the front of the seat seeing about half the the force each: about 2900 lbs on a 2300 lbs bolt. So if everything else held up the grade 2 bolts would shear. Given the sheet metal floor, this seems unlikely, but that is speculation, as the strength of the floor depends on many factors and would be hard to calculate.
This is a very crude analysis and has all kinds of unmentioned assumptions and approximations. May have slight errors or wild glaring mistakes. My point from this is that whether we are doing something useful or not depends on the actual numbers. It is easy to spend a lot of time and effort on things that don’t really matter. Is it a gnat or a camel? Hard to tell sometimes.
Changing anything from the factory design that is safety related should be considered carefully. Unintended consequences are very common.
Well, I suppose I have wasted enough time on this.....time to get back to work.
