Put some inserts in the wheels. You can get some similar foam at the local hardware store. Just make sure it's 3/4" diameter. Check the toe in or toe out of your wheels. To much ether way can cause a lot of drag on takeoff.
If you need down trim to fly you may have an engine thrust problem or an incidence problem. They can amount to the same thing. Check for to much up or down thrust. I believe it's supposed to be "0" degrees engine thrust on the .60 sized TF Thunderbolt. If the thrust line is correct the wing or tail is off. The wing can maybe shimmed a degree or two, but the tail. OOOps !!! You will need some incident meters for this. If you don't have any, see if you can borrow some. Two would be best.
Let us know what you find.
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