TUNING A FLY'S FRONT END

In my quest to get my Fly Classic-series slot cars to run better, I've run across a number of problems related to their front end: Some cars deslot too easily upon acceleration into curves, especially at banked curves where the car just continues forward in a straight line. The front end can add an odd intermittent sound to the mix as the wheels jangle around. The slop in the stub axles sometimes cause the tires to rub against things in the wheel well and may not be compatible with the installation of headlights. In the case of a Fly clone I recently acquired, the Spirit Ferrari 512M, the front wheels didn't even turn.

The situation would be vastly simplified if one were to remove the front tires from the equation. Some folks believe that slot cars should be run in "tripod" configuration: Two rear tires and the front guide, with the front tires not making regular contact with the track surface. Personally, despite the performance consideration, I like to see front tires turn. If not, they might as well be removed (along with the body), since from a performance perspective, they really are just dead weight. However, aethetics are an important quality for me, and I try to strike a balance between the two.

One of the salient features of the Fly Classic design is its use of independent stub axles, but there are a number of other parts that affect how the front end behaves, including the tires, the wheels, the chassis, and the guide. Naturally, the wheels and tires should be round, true, and mounted so that they spin without wobble on the axle. That's not a given since Fly rarely grinds down the sprue remnants on wheels, and these can create high spots on tires. It's easy to see that a tire that isn't round isn't going to provide a very smooth ride, and a bumpy front end can lead to deslots at high speeds. For maximum performance, some folks use ultra-low friction "zero-grip" tires, or coat them with nail varnish. The idea is sound-- front tires don't contribute to traction --but whether that level of performance tuning is appropriate for your vision of home racing is a matter of personal preference.

Opinions vary about stub axles. Some people believe that the stub axles should always be replaced with a single straight-through axle, whereas others believe that the independent rotation of stub axles confers an advantage at cornering (since the inside and outside circumference of a curve are different lengths). That's too subtle a point for me, and wouldn't be sufficient reason for me to change anything: I prefer the "don't fix it if it ain't broke" approach. However, in every Fly classic car I've worked on, the stub axles have always had far too much slop, allowing the wheels to chatter up, down and front to back... and every direction in between. If this doesn't cause performance problems from rubbing in the wheel well, it may cause a bit of noise as the wheels and axles respond to the changing directions of force and inertia. The amount of play can usually be reduced by pressing the axles tighter into the wheel. However, this doesn't totally eliminate it because the axle doesn't fit snugly within the chassis support tube (and you don't want to jam them so tightly that they don't turn freely), so there will always be some amount of play. Although this can be fixed, this may be sufficient, or even desirable.

One advantage of independent stub axles with some amount of "slop" over a fixed-position single axle is evident in tracks where the road surface is skewed along it's length, such as a section leading to a banked curve. For most of a layout, the track sections are level along their length, and all four tires touch the track in the same plane (fig. A). As you approach a banked curve, the track section will skew slightly. If all four tires are aligned in a flat plane by a fixed single front axle and rigid chassis, all four tires won't be on the road (fig. B). In this case, the rear tires may be on a level area as the front tires enter the skewed area. There, a front tire lifts the front end so that other front tire isn't touching the road; since the guide is mounted on the same axis, there's also less of the the guide in the slot. Acceleration in this state is likely to cause an "understeer" deslot, where the car continues in a straight line. Stub axles or a single axle with up/down play (fig. C) can conform to the skew of the road, and will leave more of the guide in the slot.

In the scenario described above, the performance does benefit from some up/down play in the axle, or axles. Front-to-back play doesn't seem to confer any obvious benefits though. Opinions seem to be fairly unanimous that turnable front wheels that steer with the guide don't confer any performance advantage either, but they do look neat; uncoordinated, independent front-to-back slop doesn't look neat. In small doses it may not hurt anything, but this is one area of axle behavior that I think we can do without.

A single front axle can be made to accommodate some degree of up/down play, as shown. In the case of my Spirit Ferrari, I'd replaced the noisy stub axles with a length of brass tubing: Unfortunately, this made the car into a tripod, and the front tires never turned. To fix that problem, I simply elongated the axle retaining hole in the chassis, downward, so that the tires were free to contact the road. While the front end is still supported by the guide, the axle has enough up/down play to let the tires to contact the road. When accelerating into a banked curve, the floating axle lets the guide stay planted in the slot.

Sometimes, the up/down play of an axle doesn't work out. If the headlight mounting leaves only a tiny amount of clearance in the wheel well, you may not want the axle to have any play at all. Unfortunately, this may cause the understeer deslotting problem. You can usually take care of that by working on the guide.

Fly's Classic guides are somewhat shallow, and an obvious solution is to replace them with Slot.It's deeper wood track guide-- it's an easy, practically drop-in solution. This will certainly reduce the incidence of understeer deslotting at curves. However, a deep guide does have some drawbacks. There are times when a car should deslot-- if you take a curve too fast, the car should fishtail and deslot. If the guide doesn't escape from the slot, the guide will reach its rotational limit and suddenly stop with the considerable inertial force of the car's rear-end weight. This stalls the car dead, and puts a lot of stress on the chassis' guide post retainer. I've yet to snap one, but I recognize that it's possible. To avert this potential disaster, I've experimented by trimming the guide flag to different lengths and trailing tapers, which seems to help, but every so often, I hear the awful "thwak" of a fishtailing car stopped dead in its tracks. An alternative is to extend the guide flag slightly with duct tape, which isn't as rigid. It looks tacky, but it's a cheap, quick, and reversible solution.

Another solution is to install a shim (fig. D, blue) between the guide and the chassis. Fly's Classic guides are notoriously loose and sloppy; There may be about 1 mm of vertical slop between the guide and the body, which is often compensated for by giving the brushes a slight curl. A shim will bring the guide lower to the track relative to the tires, allowing the tires to still contact the road, but reclaiming the extra guide depth that was lost to the vertical slop. I've discovered that this tiny adjustment can noticibly reduce the incidence of those understeer deslots.

Finally, adding weight at the front of the chassis can help to reduce the lifting action caused by sudden acceleration.

I prefer to try wheel, tire, weight, and axle solutions before messing with the guide, since those are general mechanical aspects that slot cars have in common with real cars. In my opinion, extending the flag to help solve all deslot problems borders on missing the point of home slot car racing. It should be about the driver and skill, not about putting the perfect machine on cruise control. Cars should deslot and fly off the track when corners are taken too fast. Otherwise, tracks would be designed with guide retaining grooves to ensure that slotcars with I-beam guides never deslotted. Otherwise, people could put huge magnets in their cars and ... uh... never mind!

--05/07/06