Roller diameter

You are talking about a difference from 3.14” to 4.7” before the “used” part of a roller again hits the forme.
The diameter of anything round directly correlates to it’s circumference. It is this outter area that holds the ink. A smaller roller will not carry as much ink to the image area. If the image is large or broad, the smaller roller may run out of ink before it has run over the comlete forme. A Ductor, or Idler roller on top of and in between the forme rollers may help this, but then, with smaller forme rollers this would have to be bigger diameter to sit and perform correctly.
You want the trucks to be same diameter as rollers so that, given a certain truck “speed” (rotational turning into linear) matches the “speed” of the roller. The roller is not “Spinning over” or “Skidding across” your forme.

Makes good sense - thanks!

most american presses are same size rubber and truck foreign made for a different type high needs some testing to work. Adana did use a larger truck to rubber. I alway like a fat roller but complete rotation on ink disk is important.

Deleted….duplicate post.

My long-time view on this is that the roller diameter should be slightly bigger than the truck diameter, say a point or two. This is because the rollers have to exert a slight bit of pressure on the form to contact it and transfer the ink. When they do this very slight squeeze, they will be slightly smaller, and that will make them exactly the size of the trucks when they are actually transferring the ink.

In the end though, there seem to be a number of opinions as to just what the roller vs truck diameter should be, and those differing opinions are expressed by many printers who have obtained excellent results. In addition to this, the rollers themselves can change diameter due to age, temperature, what solvents are used to clean them, etc., etc. The conclusion I draw from all of this is that there is some latitude in the roller vs truck diameter which can be tolerated while still producing good work.

What really matters is whatever your ‘truck’ (aka roller wheels) and roller diameter is, that once assembled and ready to run, that the roller surface is nine hundred and eighteen thousandths of an inch from the press bed surface.
Its absolutely critical and a pig to measure without proper roller setting gauges. As someone else remarked the Adana platens definitely don’t conform to the usual rules, and I’ve never been able to quite understand how they get away with it, but they do, images can be immaculately inked on an 8 x 5.