Fortunately for slotcars, we only have the final drive ratio (FDR) to worry about. All the same, it has been an age old problem, whether you just want to change ratios for a change in circuit or whether you are trying to get one of those beautiful over geared German models to drive sensibly on your small club or home circuit.
Well, as promised some time ago, I have sat down with pencil and paper and a calculator and manually drawn up a couple of graphs to help you out. I toyed with the idea of doing a fancy preparation in Excel but not only was it over my head; it wouldn't have conveyed the simplicity and ease of reading of a line graph that I wanted.
Below are a couple of snaps of the graphs and below them are a couple of links to the full sized scan which you will be able to download (right click, save as) and print off!!
Chart one download
Chart two download
Hopefully they are self explanatory but for the benefit of some I will go through it step by step.
Chart one contains diagonal lines with the ratios they represent written at the top and down the right hand side. At the bottom is the tyre diameter reference and at the left side is the distance covered in millimetres after one full revolution of the motor armature. So let’s do a quick check. A model has a rear wheel/tyre diameter of 20 m.m. (bottom line) and has a gear ratio of 9 / 27 (3 : 1) Follow the 20 m.m. line up ‘til it dissects the 3.0 : 1 line. At the point where they cross you can move across to the left edge where you can read off the distance travelled per revolution. In this case it will be a fraction under 21.0 m.m. Likewise, if you wanted to upgrade your FDR for a faster track and you have an idea or inside information as to what to try next, simply follow the tyre diameter line upward to the nearest figure you are looking for.
i.e. Upgrading the final drive from 3 : 1 to 2.75 : 1 will give you an increase of about 1.8 m.m. per rev.
Chart two is a lot simpler and has the contrate tooth count along the bottom and the most popular pinions along the diagonal lines. Retaining a nine tooth pinion will require a crown of 24z or 25 z to give you a ratio slightly either side. A 12z pinion and a 33z crown will give you 2.75 : 1 spot on, but a 12 z pinion isn't a good choice for in line motors unless you are using 64 pitch gears! But this is just a demo.
Next, for the average collector and slotter is the over geared model reviver.
i.e. You have an earlier Carrera model with their notorious over gearing and you want to perk it up a bit. It certainly has the motor to do it but the FDR is way too tall and the result is a model which needs a ten metre straight to wind up and a three meter straight to stop! Firstly, pick a model which flies on your track. Let’s take a fly Le Mans model for instance. Rear tyre diameter 21.0 m.m. Final drive ratio of 3 : 1. You'd like your Carrera Bentley Speed 8 to behave more like this model!! (fingers crossed). I don't have the Bentley gearing details at my finger tips and they may be in the Carrera section of the model review section but it's not important anyway as we are changing them.
The Fly car travels at a speed of 22.0 m.m per motor rev. (Follow the 21 m.m. tyre size line up until it intersects the 3.0 : 1 FDR line. looking left, these two pieces of data cross at the 22 m.m. per rev line!! So, we want the Bentley to do 22.0 m.m. per rev. (in actual fact, because the Bentley is 20% heavier than the average fly car it is more prudent to drop the ratio a little more). Therefore, if we want the Bentley to travel at 22.0 m.m. per rev, which is slower than its RTR top end but it will gain heaps of acceleration and a whole lot more brakes, we then firstly need to know its rear wheel size which is, lets say, 22.5 m.m. We follow up a point from 22.5 mm on the bottom bar and where it intersects the 22 m.m. per rev line we have the ideal gear ratio. Approximately 3.2 : 1. If we lay a rule across Chart two at 3.2 we can read off what crowns and pinion combination will give us this ratio. Remembering I said we needed to compensate for the models weight I would go for a reading closer to 3.3 : 1.
Our choices are : 8 / 26 which is 3.25 or 9 / 29 which is 3.222, or 10 / 33 which is 3.3 . On inline motors it is best to use 9 or 10z pinions as 8's can be over worked and 11's may be too wide to fit in the crown gear! As it happens, the Bentley Speed 8 has a very low body section above the contrate and an eight tooth pinion is almost obligatory unless you want to hack the body about!!
So there you go, at a glance cross reference for pinions and guides. There is a very small amount of ambiguity in my graphs as they don't register tenths of millimetres and minor estimations have to be made. But it may not be possible to match the exact ratio anyway and the slight adjustment to an estimated read out will negate any accuracy anyway.
So get off now and drag out those hopeless shelf queens and dust them off, a few minutes with the 'Wixwacing' FDR calculator and a trip to the Slot.it spares counter will see some of them finally doing the business!!
