Quote:
Originally Posted by Chequered Flag Racing
The Diff 
Each time you change the rear foams you need to reset the diff each time
King Pins
e-clips are easily lost and then you lose the spring also
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It depends on what you do the most.
Running the F109 kit diff you can adjust the diff settings easily, but you have to adjust the diff settings each time you swap tyres. The better option if you aren't replacing tyres all the time, fit the Tamiya one piece thrust bearing (part number 53136) and you'll struggle to get the F104 diff as smooth. It's also a lot easier and quicker to adjust, to tighten a slipping diff takes a couple of seconds pulling over at the side of the track, with the F104 style it's back to the pits.
The F104 or F109 externally adjustable diffs mean you can change tyres without having to reset your diff settings, but you do have to remove the wheel and end cap if you want to adjust the diff settings so you can't tweak the diff settings in practice by just pulling in and turning the nut.
And considering the most often reason most people go near the diff is to change the spur gear the F103/F109 style is much quicker to replace and much simpler to reset the diff again.
The F104 style diff is only necessary if you change your tyres every race or two, or you run at several clubs and have to run different tyres at each club.
I've run the kit diff, the external adjustable diff and the Tamiya F104 diff. I've gone back to the kit diff with the Tamiya thrust race as I find it more user friendly, the only downside is if I have to take the wheel off to set gear mesh I have to reset the diff, but as it can be checked and adjusted while warming up for the next race I've never found it a problem.
E clips are also a problem, they fly off the kingpin at the slightest knock. If you use Tamiya steering knuckles grub screw them to the kingpin so you don't lose anything when the top E clip comes off, which seems to happen much more often than the lower one. When the top e clip comes off if the kingpin is held by the steering block you can usually complete the race as there isn't enough clearance for the upper arm to come off the kingpin. Replacing the E clips with Tamiya ones also seems to keep them on for longer.
Nothing wrong with the kit FRP chassis, in fact it's tougher than the carbon upgrade while generating more grip by being a little bit flexible. The only reason I can think of to fit the carbon chassis is to save weight, although you will usually want to add weight rather than take it off.
Build an F109 as in the kit and use the kit tyres is a good starting setup for carpet, stiffer side springs and front springs sharpen up the handling. The
front castor mount really wants to be replaced with the alloy upgrade although there are plenty still running fine with the kit one so it's not absolutely necessary and the alloy version seems virtually impossible to find these days.