I am making a large tractor cake, I've already made 1 back tyre out of fondant which is massive, I've used about 500g of black fondant but it still isn't as thick as I need it to be.

I've left it out over night but it's not very hard yet, when I turn it on it's side (how it will go on the side of the tractor) it has a lot of weight on top so it slowly sinks down so looks like it's a flat tyre!

I have got time to leave it out for a week or so but I can't see the inside getting much harder and strong enough to stay on it's side.

I didn't want to make them out of cake as I figured that would also droop down and not be strong enough.

If anyone has any ideas on what I could use or anything please let me know!

Thanks so much in advanced!

Have you thought about RKT. They are much lighter.

I just made one of those, (in my gallery) and I did use rice crispy treats covered in fondant. Then I used royal icing to pipe the treads on hth.
Diane

Jessicakes has a great tutorial on how to make tyres:

http://jessicakesblog.blogspot.co.uk/2011/05/monster-truck-how-to-make-tires-part-1.html

Her's a for a monster truck, but the principle would be the same for a tractor, I'm sure.

I used RKT covered in fondant for my jeep. Worked very well! I attached the tires to the cake using melted chocolate for glue. If you dont want to go that route, why not use Tylose powder or gumpaste/fondant mixture just to make sure your tire gets hard. A week should be more than enough to let them harden enough to support their own weight.

Or use a styrofoam base.

Each tyre being 600+ gram will need extra dowels for support.