Carisma F14 Blog - Part 7

The gear diff is plenty good enough for general fun and to get a taste of what the F14 is all about, but to allow fine tuning of the way the F14 drives you will want the F14 Optional Ball Diff from Carisma. Optional Ball Diff Set Up The biggest benefit of a ball diff over a gear diff, is having the choice of how tight your diff action is. A free running diff will give you lots of steering for tight circuits, even allowing you to dial in a little slip to make it easier to accelerate on low traction surfaces. A tighter diff that is almost a solid axle will promote more under steer and less "diffing out" when accelerating hard out of tight hairpin turns. The choice is yours; all you have to do is tweak the right hand wheel nut. It?s worth keeping your eye on it for the first few races as the diff balls bed into their drive rings but once they settle down the drive is very consistent. F14 Ball Diff Kit includes everything but the spur gear (left). F14 Ball Diff Built with option spur fitted (right). The spur carrier has a ball race inside against the axle, so drive is always smooth and equally shared left to right. The six main 3mm steel balls need to be greased for a smooth action; typical silicone diff grease is perfect. The plates key to the outdrives with a D Ring design so they will not slip round at all and the whole diff builds very easily. There is no thrust race, and no tension spring, but instead a coned plastic spacer is tensioned against the inner race of the wheel bearing and it works very well, though adjustment between slip and grip is a very fine margin indeed. So go carefully with that socket wrench! The Spur gear bolts to the outside of the carrier so you can swap gears for repairs of ratio changes very easily without disturbing the diff balls or plates. What a great idea! All you need do is remove the wheel nut and wheel before removing the three small screws that retain the spur gear. If you want to remove the whole axle to work on the diff away from the car, simply undo the 3mm grub screw with a 1.5mm hex wrench to release the left rear wheel mount, and slide the whole axle through its bearings.

TOP TIP - The spur will clash with the tips of the rear wing and venturi below, but by slightly trimming away the most forward points of these you won?t have to remove or twist the wing and venture in order to slide the diff assembly out.

The wing and venture forward tips stop you sliding the axle and diff out as an assembly. But that can easily be remedied with a pair of side cutters. The Ball Diff kit doesn?t include a spur gear, and while the gear diff 76/80 spurs can be modified to fit the ball diff if you have a steady hand and a Dremel, it would be best to also order the option spur gears for the ball diff at the same time. These 72T and 64T spurs (as a set) will raise the overall gearing for the F14 quite significantly, so a set of optional 10T-16T pinions (along with the kit 12T pinion) allow you to dial the F14 into the size of tracks without having to limit the throttle end points, allowing you to get the desired top speed with proper acceleration. The larger 5mm rear axle is silver, but Steel not alloy, so sadly the additional weight of the alloy wheel carriers and diff halves isn?t offset by a lighter axle, though I have no doubt an alloy or even carbon rear axle will appear on the option list shortly as the F14 gains a larger following, the racing starts in earnest. The F14 took to Ardent Raceway with glee, as we could finally unleash their full potential. The new axle is larger than the kit 4mm gear diff axle, so you also get new bearings in the diff kit, and these run in the same eccentrics to allow ride height adjustments.

Top Tip - I found these plastic eccentrics dragged very slightly on the inner races of the larger ball diff bearings so I opened the hole out a bit until it held only the outer diameter of the bearing and the axle ran a lot smoother.

To clear the bigger bearings that come with the bigger axle, the eccentric carriers need to have their hole slightly opened. The alloy diff assembly is predictably a little heavier than the plastic gear diff assembly, including axles the gear diff is 22g while the ball diff is 30g, which is not all bad news as it might add a little more useful weight over the rear axle. Since I?ve been running the ball diff my F14 has been pretty much invincible, but it won?t be long until everyone else catches onto just how nice they make the F14 handle. Then I?ll have a fight on my hands for sure, as I found out when I gave John Dawson at Ardent Raceway my spare car so he could join in the fun, and he beat me with it! Dez Chand John Dawson took to the F14 with relish, showing Dez and Gordon the way home!