drilling the floor

Moderator: The Midas Forum Staff

drilling the floor

Postby billybob » Mon Aug 03, 2015 12:09 pm

Hi all,

I picked up some smart car seats last sunday and they are a pretty good fit.

the front holes lined up with my existing brakets which was a stroke of luck to say the least. I have no idea what the original seats were from though so I can't really advise on that front.

Anyhow, the back holes were way off- for the drivers seat I made up some extension plates and bolted through (no welder unfortunately).
However, I would rather re-drill the floor than use bolted brakets, so my question really is if there is any harm in doing this for the strength of the floor? If I am to drill new holes then of course I will fibreglass in the old holes. The new holes will be a good 15 cm away from the old as well and I have 2-3 mm thick stainless plates to use either side of the floor where the bolt goes through.

Also, I have 10 mm bolts to use but have been advised that only 8 mm are required- the 8 mm would be much easier to use as the bolt head would fit in the runners with no modification.

Thanks,
billybob
 
Posts: 73
Joined: Sat Jun 06, 2015 2:24 pm

Re: drilling the floor

Postby ACourtney » Mon Aug 03, 2015 1:41 pm

I've posted several times on this before, so regular forum followers will have to forgive me for repeating myself.

The floor of the Gold convertible is a sandwich structure with a soft polyurethane foam in between two walls of glass reinforced composite. I recommend reinforcing the sandwich area around each hole by scraping out some of the foam, filling with a glass loaded bridging filler (such as Isopon P40) and then redrilling the hole. This has two benefits: Firstly it strengthens the area around the hole and ensures that the foam doesn't get crushed, so you don't get the underside of the floor distorting as the bolt gets tightened. Secondly it keeps moisture out, which can get in between the polyurethane foam and the GRP, weakening the joint. This is just good practice when working with sandwich structures - with aluminium honeycomb panels it is common to bond in aluminium bobbins and with carbonfibre based panels it is common to use an epoxy filler.

My technique when drilling holes in the floor of a convertible is to mark up and drill the hole as per normal. Do a trial fit to make sure they are exactly where I want them and then I will use a hole cutter, typically of 20mm diameter, to cut through the lower skin of the floor (from the underside). The upper skin is the stronger of the two so I prefer to leave that intact. I then use the tip of a screwdriver to scrape out some of the foam around the hole so that the area to fill is more like 30-35mm in diameter. Once the hole is filled and the filler has hardened I re-drill the hole from the top, through the filler.

This seems to have been done on some of the earliest Midas cars, but was clearly forgotten during the GTM days and no mention was made of it in the manual. However, as I've already said it is good practice that is found elsewhere in the composite industry.

I would also suggest filling any of the redundant holes with the same filler, to put back the strength and prevent moisture from getting in.

On the subject of bolts, M10 bolts would obviously be stronger than M8, but M8 should be adequate for seat bolts. My preference is for zinc-plated 8.8 (look at the numbers on the bolt head) high tensile bolts as these have very good impact properties. I would steer clear of Stainless Steel and higher strength (12.9 or higher) steel bolts as they tend to be more brittle and therefore more likely to snap in an impact. If you think about it, the seat bolts only see a high load in a rear impact, where your weight is pressed against the seat. In most other cases the seatbelts take the load. It is better to have bolts that stretch and absorb the impact energy than ones that don't stretch, but snap suddenly under a high load. The stainless steel spreader plates will be okay as they are compressed by the bolts, but won't see the same loads that the bolts do.

Regards,
Alistair
User avatar
ACourtney
 
Posts: 488
Joined: Thu Dec 04, 2008 12:22 pm
Location: Oxford

Re: drilling the floor

Postby billybob » Tue Aug 04, 2015 9:22 am

ah yes I remember the fibre glass filler deal now..
I picked up some P40 last night so will give this a go. What I noticed is that the original holes had not had this treatment- I also managed to get the seats in with the braket setup so I will remove everything, fibreglass the original holes, re-drill and stick with the brakets.

I will see if I can get some decent bolts from work, is there a real issue with using stainless? Perhaps they are too weak but this is what I currently have.

Thanks!
billybob
 
Posts: 73
Joined: Sat Jun 06, 2015 2:24 pm

Re: drilling the floor

Postby ACourtney » Tue Aug 04, 2015 7:04 pm

The stainless bolts are fine for 99.9% of the time, but unfortunately that 0.1% is when you get a severe rear impact. It is the brittleness of stainless fasteners that is the issue. An 8.8 high tensile bolt will absorb a much higher impact energy than the equivalent stainless steel one, which is why they tend to be used in most lifting equipment, pressure systems etc.
At the end of the day it is up to you what bolts you use as it will be you sat in the seat.
User avatar
ACourtney
 
Posts: 488
Joined: Thu Dec 04, 2008 12:22 pm
Location: Oxford

Re: drilling the floor

Postby billybob » Wed Aug 05, 2015 10:18 am

I picked up some hardened bolts....

I do have to wonder though, if the impact was that severe in a midas then is the liklihood of survival pretty small anyway? When I was stripping the doors down for example, there is next to no protection in there at all. I think if any midas owner (or probably any kit car owner) were that concerned about safety then they wouldn't drive their cars. I'm not overly concerned about it and also have a motorbike.

Obviously though it is best to be as safe as possible with what you have got, hence the hardened bolts!
billybob
 
Posts: 73
Joined: Sat Jun 06, 2015 2:24 pm

Re: drilling the floor

Postby Alan D. » Wed Aug 05, 2015 12:38 pm

Just a thought, when I used to fly Micro lights, they used bolts which were designated A & N. these bolts were soft, the idea being that they stretched, rather than sheared. Just another variation on "Hooks" law I suppose.

Alan
User avatar
Alan D.
 
Posts: 945
Joined: Fri Dec 05, 2008 10:34 pm
Location: Newcastle upon Tyne

Re: drilling the floor

Postby billybob » Thu Aug 06, 2015 10:50 am

I don't know why but this last post really tickled me!

I'd be packing a parachute rather than worrying about bolts!
billybob
 
Posts: 73
Joined: Sat Jun 06, 2015 2:24 pm

Re: drilling the floor

Postby Jin » Thu Aug 06, 2015 2:35 pm

you could always go with titanium like i did on my subframes

viewtopic.php?f=14&t=2178&start=50

corosion protection and strength
https://easywider.co.uk/ universal flexible wheel arch extensions
Jin
 
Posts: 454
Joined: Fri Aug 01, 2014 10:33 pm

Re: drilling the floor

Postby Hans Efde » Fri Aug 07, 2015 8:22 am

I am really not bothered by the bolts. I recall the tensile breaking strength for a 8.8 bolt is 80 kg for 1 square mm. For shearing I use 1/3 of the tensile strength. For accidents I tend to take a max g of 40g and calculate backwards. There was so much redundancy in M8 bolts I never looked at it again. A few years ago I make my own seat belts from safety strap (cut up old red sabelts) and also kept to the 40g rule. Breaking strength was 9000N of the webbing. 4straps will break at 36000N. At 40g with a weight of 100kg (heavy man plus seat) the load is 16000N so a safety margin of 36/16= 2,25 is good enough for me. And it looks cool too:
Image
Image
The only thing I still need to do is make large spreader plates for the mountings on the rear wall.
User avatar
Hans Efde
 
Posts: 1731
Joined: Sat Dec 06, 2008 10:11 pm
Location: IJlst, Netherlands

Re: drilling the floor

Postby ACourtney » Wed Aug 12, 2015 5:00 pm

The strength of the bolt isn't that important in an impact. It is the ability of the bolt to absorb energy and to stretch that will make the difference. If you look at Hans' seat belts you can see that the straps are deliberately long as that allows for more stretch, which absorbs more energy. The standard inertia seatbelts use the same technique, which is why the strap runs freely through the top mounting and down to the inertia mechanism housing on the floor. Short lengths of seat belt, such as fixed belts attaching to a roll hoop directly behind the drivers seat are not a good idea.

Going back to the bolts, if you look up some mechanical property tables for bolts you will see that as well as Tensile strength (strength at failure) and Yield strength (strength at elastic limit) there are usually figures given for Elongation after failure and impact energy. These are the figures that are of interest when selecting bolts for mounting impact critical components such as seats, seatbelts, roll-hoop, subframes and suspension. As Alan posted, you want bolts that will stretch without breaking. You also want bolts that will absorb energy as the kinetic energy that the seat belts and seat anchor bolts absorb will be energy that would otherwise go into you!

So comparing the 8.8 HTS bolts that I suggested to the much stronger 12.9 bolts:
8.8 bolts: Tensile strength 800MPa, elongation after failure 12%, impact energy 30Joules.
12.9 bolts: Tensile strength 1200MPa, elongation after failure 8%, impact energy 15Joules.
These figures can be found on several websites for example: http://www.fullermetric.com/technical/i ... rties.aspx
In other words, the 8.8 bolt will absorb twice the energy and stretch by 50% more than a 12.9 bolt of the same size.

The Titanium bolts that Jin mentioned are stronger still. The most common grade of Titanium, i.e. that used for fasteners in the aircraft industry, is Grade 5, aka Ti64. This has a notched tensile strength (across the threaded part) of 1450MPa and an ultimate bearing strength (across the shank) of 1860MPa, the elongation after failure is 14% but the impact energy is just 17Joules (source http://www.aerospacemetals.com/titanium ... -4911.html ) So they are good from the stretching point of view, but not so good from the energy absorption point of view.

The comparable figures for Stainless Steel fasteners are harder to find. The commonly found A2-70 grade (sold in Screwfix, Toolstation etc) has a Tensile strength of around 700MPa and the less common A4-80 grade has a tensile strength of around 800MPa, but the elongation and impact figures are harder to find. In fact the international standard for stainless fasteners, ISO 3506 doesn’t refer to impact energies and gives elongation not as a percentage, but as a factor of diameter 0.4d for A2-70 and 0.3d for A4-80.
“Stretch like toffee, snap like carrots” was how a senior engineer described stainless fasteners to me once. However, when strength and impact resistance are not issues, but corrosion resistance is then stainless fasteners are the best solution. Use a stainless nyloc nut with a little copper-slip on the thread and you should have no problems unscrewing the nut ten, or twenty, years later.

Alistair
User avatar
ACourtney
 
Posts: 488
Joined: Thu Dec 04, 2008 12:22 pm
Location: Oxford

Next

Return to Mk 3 Convertible

Who is online

Users browsing this forum: No registered users and 2 guests