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Beveling Slot Car Gears

You have probably heard the term 'Beveled Gears'. Neat! Now, what are they?
Well, by changing the shape of your gears a little bit, you can actually control certain aspects of performance, movement and overall speed. Read on...

A great, but under used way to reduce friction and control your gear assemblies interactions is by Beveling the edges of the gears. Depending on how it's done, beveling will either slightly increase or greatly reduce the amount of surface area where one gear meshes with another - thus controlling power robbing friction.

We found the best way to bevel is by mounting the gear in a Dremel Mandrel using a tapered head screw (hard to find this correct size) and applying it to some 300 or 400 grit sandpaper to remove some of the sharp edge. If you use a Cardboard Nail File (or standard metal file) you can also begin to change the actual 'shape' of the gear.

The HO Thunderjet 500 Chassis Now some say the idler is the only gear you'll need to bevel, but we believe you can bevel all 4 gears and realize some performance gains. The choice is yours depending on your application. For this installment, we're just going to cover the Armature Pinion, Idler and Driven gear.

Depending on the 'shape' you give the gear , different things are going to happen. If you dramatically change the shapes, you'll dramatically change their behavior. Typically, the Brass Idler can be be modified fairly heavily, while the the other 2 brass gears really only need a VERY slight 'rounding' or 'shaping' of the tooth edges you are trying to affect. Did you know that some later magna-traction cars actually came 'stock' with an Idler that was beveled only on the top edge, in order to better interact with the Nylon Driven Gears found on those cars. Yup - even the 'scientists' (who always worked overtime) at Aurora believed that there was value in doing this. If you can find one of these 'Small Hole' MT Idler Gears, you can 'tighten' the hole with a nail-set tool and use it as a T-jet Driven Gear! ; We digress...

After beveling, you should take your Dremel Tool, fitted with a Soft Wire Wheel, and remove all tiny burrs between the teeth (you ARE wearing safety glasses, right?). Tedious? You bet. But it's what you'll need to do before you go 'all the way' and Lap In the gears. Look at the following examples. In each of the following side-views, the Armature Pinion is on the left, Idler in Center and Driven on Right. Angles may be exaggerated for illustrative purposes.

This is by far what most people think about when they think of Beveling the gears. The thinking is that because the Pinion and Driven gears are relatively stationary in their vertical travel, that by simply reducing the surface contact area of the Idler Teeth will provide an increase in performance. Well, they're right!   If the gears are set up like this and Lapped In, you will actually create a 'resting place' for the Idler in the Vertical 'Center' of the Pinion and Driven 'teeth'. Notice the Tight Gear Clamp over the Idler. Remember, you ALWAYS have a bit of control by increasing or decreasing the Gear Clamp tension.   Good Road Racing setup & General usage. Oiling intervals usually long. Benefit Smoother overall operation. Idler should 'float' between Pinion and Driven once Lapped In. Drawback None - though operational predictability is not as controlled as in other examples. Things to Watch for Correct cluster operation. More predictable operation than non-beveled. Keep in mind that the Driven gears up-and-down travel can always be adjusted but will have direct impact on the crown pinion. To some extent the Armature Pinion's vertical travel can also be adjusted, either by shimming the upper arm shaft under the Gearplate or by pressing the gear further down on shaft to reduce travel.

(For specialized situations) This is an example of a Tight Gear Clamp with an inverted Idler Bevel. What it going to happen here? Yup, the Idler is going to want to raise up as RPM increases. The by-effect is that the Driven and Pinion will be 'forced' downward. Of course, since the Pinion IS directly attached to the motor, the Idler will have a tough time 'forcing' it down at moderate-to-high RPM.   If the top of the Idler Gear is smooth and the Clamp is Tight, this design may help assist in the Armature riding 'lower' on its Vertical Travel Plane until higher RPM is achieved.   Should be used on a case-by-case basis as will quickly produce heat.   Benefit Predictable operation. Drawback Idler and gear clamp have a large amount of surface interference. Arm Pinion will generate heat (friction) faster. Driven is now a known friction factor. Things to Watch for Correct cluster and arm shaft travel. Brush Tension.   Keep in mind that the Driven gears up-and-down travel can always be adjusted but will have direct impact on the crown pinion. To some extent the Armature Pinion's vertical travel can also be adjusted, either by shimming the upper arm shaft under the Gearplate or by pressing the gear further down on shaft to reduce travel.

(For specialized situations) Now, what will happen here? The Idler will remain pushed down against its Boss throughout the RPM range. The Idler Boss that the Gear sits on is smooth, oils well and is a stable place for the Idler to operate - not so bad. This design will almost immediately bring the Armature to the top of its Vertical Travel and keep it there throughout the entire RPM range. Will you need to adjust your brushes? The Gear Clamp is loose here, because the idler will not have a natural tendency to raise up at speed. May be applicable to Drag Cars and cars which only do shorter, controlled 'sprints'. When properly polished, this will also work well for longer Road Racing - though attention may need to be paid to Idler oiling intervals.

Benefit Predictable operation. Allows for exacting brush tolerances. Low surface interference due to both Pinion and Driven gears raising to top of travel almost immediately, allowing smaller Armature top shaft and cluster Pinion Gearplate bosses to be fully utilized. Typical lower heat build up on Arm pinion due. Idler shouldn't 'transfer' too much heat and is not a heat producing gear. Drawback Idler friction is now a known factor. Things to Watch for Correct cluster and arm shaft travel. Brush Tension.

Once you are satisfied with your work, rubbing both faces of each gear on some 600 or 700 grit sandpaper until smooth to begin your 'polish' of the gears. If you look closely at the face of each factory finish gear, you see the they are anything but "smooth as glass". The smoother you can get them, the better they will spin, oil and interact. Grab a few unmodified idlers from your box and feel the edges - sharp aren't they? All 4 brass gears are like that stock. That 'sharpness' usually creates friction and slows you down..   Another beneficial by-product is that by removing material from the gears, you also make them a little lighter - therefore lowering their 'rotational weight'. This means you make it easier for the motor to turn the gears. It's kind of like a fan-clutch on your real cars water-pump - or Aluminum Rims on a race car. If it takes less power to turn, the 'saved' power is realized at the rear wheels. Polishing achieves the same end result - the weight isn't necessarily reduced but the gear becomes easier to turn..   Of course when you are done lightly sanding, you can polish each gear to a mirror-like finish with some Brasso, Noxon or chrome polish and a soft cotton cloth (like an old T-Shirt). Now when you go to Lap In the gears, you will make all 3 gears work together as a 'unit'. A lot of work for $1.05's worth of brass - how bad do you want to win?

If nothing else, this now gives you one more thing to think about and tinker with. Just to keep you thinking, what would happen if you turned just the driven gear over in example number 3?

The next time you are behind a car that you 'just can't figure out why' is always faster (and usually quieter), remember what you've just read - cuz maybe the other guy read this too! And of course, see if he's repainted his magnets!

Many times rules state 'no sanding' or even 'no beveling' but say nothing of polishing. Only the micrometer knows for sure...