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I need to fit an undertray to the front of my car as I am restyling the front end and replacing the standard bumper. It is a 911 so there is no engine heat to worry about but there are a couple of radiators I plan to leave as open and vented as possible. The picture below shows the starting point, the planned Rivnut locations to bolt the frame to, the frame design (probably steel as I can't weld aluminium) and finally the carbon Kevlar sheet.
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According to the internet a 2M Sq undertray like this can generate around 30 KG per SQ M of down force, so are 4 Rivnuts enough to hold it? The location for the Rivnuts are into a reinforced part of the floor that is used on other models for chassis struts, so hopefully strong enough, and the radiator mounting brackets which are reasonably thick steel shaped to brace against load.
Any help or advice welcome but the immediate questions are:
1) Are 4 Rivnuts enough to hold 100KG of downforce (30Kg per sq M X 2 + 40% safety)
2) Should the frame extend sideways to provide support to the edges of the Kevlar floor?
You'll need to provide some support all the way to the edge.
Whether that is more frame or a rolled/moulded edge, i dunno.
Yes, I agree. I was thinking of build a perimeter reinforcement into the Kevlar sheet that would connect to the front and back of the frame. The reason for this is that it keeps the frame simple, reduces floor flax and should be reasonably simple to bond into the tray panel when I make it. I will make a car/plastic template tray first and the use that to make the tray edge support.
In straight tensile pulling 1 M5 (grade 8.8 bolt) will take over 1100kg so your not going to break any bolts. I can't actually find the tensile pulling force for a rivnut but it is going to be way more than 24kg. The thing you have to contend with on the car is the forces coming and going (pulsating) and the flex in the undertray. I would have thought you would have to make the undertray very stiff and heavy to be strong enough to be supported in only 4 points. Most car undertrays I have come across spread the load over more like 12 fastenings. Normally the leading edge is very well supported and down the sides with less fasteneres on the trailing edge.
There is also the forces generated by hitting an object or a speed bump etc. Why would you not reuse all the original mounting points.
The outer edges will need to be supported/rivnutted in some way otherwise it may deflect downwards.
You could make a sacrificial front edge to the flat floor. TBH the best/cheapest way to do this is make it out of machined and/or routed marine plywood as you can get a nice rounded edge on it
FWIW and speaking from experience with a track car using an aero floor, Id suggest not bothering with carbon/kevlar on something that will likely look terrible after its first outing vs speedhumps, kerbs or potholes, instead you could make the whole part from marine plywood (its both pretty strong and reasonably lightweight) with a decent painted finish and assume its going to get chewed up so make it easy and cheap to repair or replace.
Will there really be a lot of downforce? I'd have though that without careful design of the whole underbody it won't do much
How much downforce? Not sure without proper CFD analysis but anecdotal evidence shows that even with some relatively crude undertrays you can generate quite a bit. Stick your flat hand out of the car window at 70mph and feel the airflow push it up or down. A simple curve that is closer to the ground at the front and then runs smoothly into the existing flat floor has a dramatic effect with a slope of anywhere between 5 - 10%. You can tweak the exact angle, even go steeper, if you want to optimize it but there are big gains with a pretty basic setup. As the 911 is normally light at the front anyway, even a small gain is significant.
The internet advice seems to be to use an aluminium and composite sandwich about 8-10mm thick for the tray material rather than composites. It is a lot cheaper to get it stiff enough and it counts as a sacrificial part. They also say build a strong wide frame to offer max support.
Happy for STW to add to my knowledge on this.
Does this help Nick? Similar sized car with composite bodywork.
https://parts.bellandcolvill.co.uk/deroure?qs=1&preview=1&make=&model=56&groupid=13281
Thanks Nixie - everything helps at the moment. A guy on another forum says not to bother with a frame and just do the whole lot in composite with a 1 inch core. Ideally with quick release brackets for fast swaps in the race pits as they can still get damaged running over debris at 150mph on the ovals.
Obviously I would be racing the ovals if I was in the US but I suspect that is less necessary on the Basingstoke ring road.
This guys voice is annoying but he does have some good information in videos. This is just a general 'flat undersides are good' video :
I've used riv-nuts extensively on Landrovers / projects, and whilst they are a good solution for adding a fastener to a sheet or flat surface I don't think the riv-nut itself has much resistance against pulling out or from its parent material. They only have a small amount of deformation/bulge behind and are highly dependent on being inserted into an accurate sized hole.
For under tray material I'd go with aluminium but with longitudinal strengthening ribs pressed in.
Fastening wise I'd go with counter sunk >M6 bolts ideally bolted through any chassis box sections with anti crush tubes/sleeves welded in.
If quick release is needed then have a search for Dzus/Southco quarter turn fasteners. In fact Southco is a good reference for sheet material fasteners.
https://southco.com/en_gb_int/resources/Quick-Access-Fasteners-Brochure
Yeah, I meant to mention that I have been warned away from Rivnuts for anything long term that is likely to get much stress loading. Thanks.
The aluminium sandwich or with ribs is a great idea but I think it is beyond my manufacturing capabilities so I might still to it in composites. If the thicknesses are 15mm - 26mm then there is loads of room to fit strengthening plates to avoid splitting or pull through.
Back onto google/YouTube this evening I guess
It's not a pure downforce problem for sure, but 30kg.f at what speed?
Downforce (equally lift/drag) don't increase linearly at speed.
If the 30kg.f is measured at 120mph, that would be 7kg.f at 60mph.
Alternatively if it was measured at 30mph (no chance), at 90mph you'd be generating 270kg.f.
I think the 30kg figure was at 100kph but I can't find the article. The point is that it is enough to need more than a self tapper and some gaffer tape.
The current plan, revised and open to more revision) is to make a composite panel with core fill so it is very still and about 15-20mm thick. It will be deeper at the leading edge and gradually slope up towards the rear so if joins smoothly with the existing flat floor. There will be fixings laminated into it so it can bolt securely to the underside of the car. I am still working out how many suitable fixing points I can find under the car. this will dictate whether I need to make a seperate frame as well. If there are not enough mounting points in the right places then I will make a frame that fixes to the points that already exists and then has lots of fixings to fit the panel to.
Follow on Youtube if you want to see how this turns out. Episode 1 here with links to later episodes in the description.
Thanks for the help and advice so far. Any more is welcome.
Can I ask why you're bothering with an undertray?