I've been doing research on carrying a mtn bike to the trails on my NC Miata. I see things about not using trunk racks on the aluminum trunk, and tow hitches causing cracks issues. In most cases I just need to carry a single bike, I would like the option of carrying two of my son goes with me. Here are my thoughts but all lead to more questions.
I was thinking of using the Frankenstein bolts, and fabbing up a bracket for a fork skewer on one side and a tire mount on the other. I was thinking those holes hold a hardtop they should be able to hold a mtn bike. That puts the bike ride sideways and will that cause any stress on that area?
this:
plus one of these
There is a used removable trunk rack for sale near me. It says they can hold 60lbs. Would a trunk rack with some bike tray mounts work? Too much weight or too much stress?
This
Plus one of these:
Or is the tow hitch and a regular hitch rack the best?
Thanks
Somewhere on google (can't find it now) someone mounted their bike to the roll hoop with the fork mount you found and then put the back wheel on the trunk with a Seasucker wheel strap.
As long as the bike isn't too long I would think this is the best solution if you don't want a hitch type rack. Moutning the bike sideways on the back feels like you're working against physics.
I have a plan to make a similar mount for my Boxster but I haven't actually gotten around to it yet. So I'm not actually any help. Ha
edit: not a Miata but this is the idea. Stole this from reddit.
@spandak, I saw those searsucker mounts but was worried they may simple the aluminum. And I saw one guy mount it to a roll bar and the other mount it to the Frankenstein bolts.
@boulder, I was worried about a hitch and all that extra weight all the time for the few times I'll be taking my bike with me.
I used a hitch rack on my NB for years. No issues with the rack being there all the time but I was not racing or counting milligrams. But I will recommend using a bunch of extra bungee cords and wrapping the bike well placed towels. The bike is not in the slipstream of some SUV so it will move if not tied down with extra straps.
spandak
HalfDork
4/16/20 11:16 p.m.
GTwannaB said:
I used a hitch rack on my NB for years. No issues with the rack being there all the time but I was not racing or counting milligrams. But I will recommend using a bunch of extra bungee cords and wrapping the bike well placed towels. The bike is not in the slipstream of some SUV so it will move if not tied down with extra straps.
Very important. Unless you get one of the racks that grab the wheels. The nicer racks tend to be this design now. 1up seems to be a good brand and it's aluminum.
The racks that hold wheels are nice, but relatively expensive. I've had good luck with my cheapo Swagman rack (probably 40k-50k CT miles on it).
My favorite rack setup on my MR2:
Miata with the fatty:
I have had hitch mounts now on 3 cars, including my NB and they've been great. I greatly prefer it to any solution that has a mount contacting painted body panels that can dent or scratch.
I'm also a big fan of the racks that cradle both wheels vs ones where the bike hangs or the front wheel comes off. Worth the relatively small extra cost IMO.
Unless this car is used to compete at the pointy end of some competition series, I wouldn't worry about the extra weight one bit.
If it is being used to compete at the pointy end of some series, you should definitely try adding the hitch mount as part of your chassis development program to see if the extra weight out back loosens the handling - you know because loose is fast! 
(As discussed in the Spec Racer Ford thread)
