Wheel base

[tegfan railway]

Is there a way of working out max fixed wheel base for a given radius of curve?

[The Denying Dutchman]

Yes there is, but it involves a lot of maths. You have to calculate the maximum distance of the wheel flange (the point in which the flange is in contact with the rails) and then you have to calculate if the given radius is wide enough to allow the maximum flange distance.

There has to be a tool somewhere on the net which calculates this, you won't be the first nor the last with this question.

Edit: You have to calculate the contact points of each wheel and calculate the max distance of the contact points of the outer wheel flanges. Else you get a rectangle which might fit the radius, but the inner wheels don't and while writing this I'm not sure if that is true.

It's complicated!

[The Denying Dutchman]

This is what Wikipedia (the most reliable source in the world.  ;) ) has to say about it:

In rail vehicles, the wheelbase follows a similar concept. However, since the wheels may be of different sizes (for example, on a steam locomotive), the measurement is taken between the points where the wheels contact the rail, and not between the centers of the wheels.
On vehicles where the wheelsets (axles) are mounted inside the vehicle frame (mostly in steam locomotives), the wheelbase is the distance between the front-most and rear-most wheelsets.

So my first assumption is true, you can see the max. flange distance and the width between the flanges (outside not inside of the flanges) as a rectangle.

If you want to know what the largest flange-contact-point-distance is for your track you can just draw and cut out the rectangle as described above and see when the rectangle doesn't fit the track anymore. Mark the point on the paper which doesn't fit the track and you have your maximum distance.

If you want to know what the minimum radius is for a loco, which of course has a fixed lenght between the outer flange contact points, then I don't have a practical solution.