Here are some different Losi hex adapters that can be used on the TC3/TC4, or Losi XXX-S/JRSX cars that come in different widths (
http://www.horizonhobby.com/webapp/w...pe=productgrid)
Not sure how well they would work on cars with smaller axle diameters like the TC5 or Xrays.
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The best way to change wheel base is by using different offset wheels. Using spacers or larger hex adapters to make the car wider can reduce the amount of threads available on the axle to secure the wheel nut to the axle. This can result in loose wheel nuts, which are more prone to falling off during racing.
One thing to be aware of with offset wheels is that it can be harder to get a wrench into the recessed area of the wheel to secure the nut. This isn't usually an issue with cars that use smaller wheel nuts (Tamiya, TC5/6, Xray). However with cars like the TC3/4 and some Losi 1/10 TC you may have a problem.
If using a TC3/4 with offset wheels, you may have to modify the outside diameter of your wheel wrench.