I’ll take a stab at this. 2 points you mention or ask about in this question...
longer steer... assuming asking why not more than 4” based on your wording?
Keeping DFW off of the rail... meaning keeping the wheel from contacting the rail as much so as to not scrub speed based on your wording?
These 2 contradict each other. More steer makes the cars steer harder into the rail. The only way to have DFW not connect the rail as hard or as much is less steer.
Point of steer... keep car from bouncing/wiggling back and forth which scrubs a ton of speed. Each subsequent pull away then back to impact rail is a speed killer.
What we are doing with these little beasties is trying to do controlled chaos. Making a fast moving car at this small size down such a long (in relation) track it is physically impossible to make a car go STRAIGHT. Just can’t be done. That’s why we use the rail. It’s a necessary evil. Better to ride smoothly on that rail than bounce and wiggle all over the place.
The amount of steer creates the amount of force your car’s tendency to drift to one side is creating up against that rail. Not enough steer and the cars tendency to get all squiggly will overcome the DFW’s steer. This results in the dreaded wiggle. The cars wiggle will make it drive away from the rail. You know what happens next including loosing the race.
Too much steer and the car will be rock steady with no wiggles but it will have more force than is needed to hold the car steady. This then slows the car down as the DFW is forcing the car harder than is needed into that rail. Now you have your brake affect after the transition.
Main goal... find that sweet spot between too much force holding car too tight to the rail, and not enough force allowing the cars inherent tendency to wiggle off of it.
It does seem after a lot of folks before us have gone through a ton of trial and error that a typical car on an average track does well enough at about 4” of steer.
This is a safe starting point and what a lot of guys who do not have a track to tune on will go with. This is also why “easy steer” drill jigs like Micro’s 5 bushing clear jig have that 5th built in steer bushing set to give a car 4” over 4’ of travel. Quite a nice tool for typical Scout/Awana level racing.
Now you can then of course dig deeper as you get better, do more classes and race on different tracks AND have a test track to tune on. There are a lot of variables which at the league level (.0001 make or break level) that can cause a car to do better at a different steer amount but it is really best not to cross that bridge until you get there.
Case in point.... some fat wheel classes guys who really know what they are doing actually go less, even as little as 2” or less steer. Then the mini class... those little freak shows need more than 10”!!!
Again... best advice on that, don’t cross that bridge until you get there. Me, I don’t even want to go near that bridge right now. lol
Hope that jibber jabber helped some and was what you were asking and didn’t add confusion.
Race day is here!!! Time to see whatcha got!!!