Sorry Ronan I added the previous comment before reading your post.
I believe I'll do something similar to what I saw elsewhere ie two large square tubes with a synthetic material beween and screws to adjust the play. I already have a 3 mm square thread screw/nut with a maneuvering 8" wheel from the same large belt saw that I could use.
The width of the contacts/blows on the anvil (lower) wheels determines how quickly the panel stretches for a given pressure setting. The radius at the shoulders is for clearance/support.
The upper/rolling wheel is mostly a smoothness versus tiredness thing in that a small dia struggles with a bumpy panel (roughed into a bag) while a large/heavy wheel has more inertia to start/stop at the end of each pass. The inertia thing doesn't like a big deal but wheeling a large panel can be knackering so every little helps...
I'd say 7 - 9" is probably about average dia for the upper wheel and even at 8" the upper wheel of an Edwards is scalloped out at the sides to reduce weight.
While pretty much anything that looks like a wheeling machine will physically stretch sheet metal they are, or should be, precision machines... if there's any runout in either of the wheels then pressure settings change as they turn and the result is ripples. Torsional stiffness of the frame can have the same effect as can play in anything and/or a flexy anvil post. Alignment of the wheels is easier in that the anvil yoke can be shimmed or made/bought with levelling adjusters to ensure that the contacts/blows on the anvils don't sit p***ed relative to the upper wheel
Thanks hotrodder for taking the time restating some obvious facts to the beginners.
Your answer confirms what I already learned after having looked a lot of machines, ie precision & stiffness are the key.
You also answer indirectly to what was my initial question, my large wheel with a relatively thin rim should do the job as long as I make sure it is true.As I initially believed I can't see any drawback to a 13" wheel.
Thanks to everybody here.
Renaud
The diameter of the top wheel doesnt really matter, I woulnt want more than 80,90mm wide.
The real enemy when spending days wheeling small parts is when the top wheels and anvils are to HEAVY giving to much momentum when your starting and stopping.
I have a big hd vickery wheel and that thing will suck you in!