Accurate method for measuring gearing....

[Cosmic Bilby]

Measuring gearing accurately.... the good oil on gearing!

By Rob Wessling






The Formula:



Number of pinion teeth X 3.141 X Tyre Diameter (mm) DIVIDED by Number of Spur teeth = MILLIMETRES PER REVOLUTION



I first came across this method of measuring overall gearing whilst competing in Remote Control Electric on-road racing back in the late 1980's. A fellow club member had been skilled and fortunate enough to attend the 1/12th scale World Championships in Singapore. He did well in the event and ended up being sponsored by one of the major American manufacturers of On-Road racing equipment. One of the first things he learnt from his new sponsor was this method of measuring gearing, something he freely shared (thankfully) with the rest of us racing at that time.



Fast forward a number of years, I find myself in a new (but similar) scale racing hobby, I realize this method of measuring overall gearing has relevance in the world of 1/32 scale slot cars.



Many of us use the traditional way of determining overall gearing, that being dividing the number of spur teeth by the number of pinion teeth to give a final drive ratio (eg. 27 spur divided by 9 tooth pinion equals a 3:1 final drive ratio). Whilst very easy to calculate, this method of calculating gearing does not take into account the size of tyre being used on a particular model (circumference).



Differences in overall tyre diameter can and do affect overall gearing. I will demonstrate this by doing the MMPR calculations for a number of 1/32 models that share 9:27 (3:1) final drive gearing.









Firstly the SCX Arrows Formula 1 model, its tyre diameter is 19.5mm:



9 X 3.141 X 19.5mm divided by 27 = 20.42 MMPR











Secondly the early Scalextric Ford Taurus Nascar, its tyre diameter is 20.9mm:



9 X 3.141 X 20.9 divided by 27 = 21.88 MMPR











Lastly (and perhaps most graphically) the Carrera Dodge Charger, its tyre diameter is 23.6mm:



9 X 3.141 X 23.6 divided by 27 = 24.71 MMPR



As seen above, cars with identical mechanical gearing can and do have often substantial variance when overall gearing is calculated by this method.



I have found this equation a useful tool for performance tuning in scale racing over the years, as I can accurately tailor a cars performance to a particular track (a 19-21 MMPR figure seems to be ideal for our home layout). A larger more open layout would suit a correspondingly higher MMPR value.



I can also compare a models gearing to the requirements of after-market motors that offer higher performance than stock items. A general rule of thumb I work by is the higher the RPM of a motor, the less gearing (lower MMPR figure) required (higher RPM range motors appear to work better for me with a lower MMPR value, eg. Motor heating is lessened with a lower MMPR). I have also found that motors that give higher torque measurements can tolerate increased MMPR values.



I hope this information assists other enthusiasts to enjoy our great hobby even more!

[SOBwanhoser]

Very useful stuff for us rookies - kind of explains the "older" Carrera characteristics.
So all of these have a traditional 3:1 final gear ratio yet some have taller gearing due to their MMPR.
Now, the big question - to get the MMPR "lower", what would be the best method:
a) change to a 8 tooth pinion?
b) change to smaller diameter tires?
c) change to higher spur gear?

Seems I'm always trying to get different cars to be competative with others, so any tips are helpful.

Another interesting observation is the calculated MMPR is very close to the tire diameter used(about 1 higher than the diameter for each car??)

[Cosmic Bilby]

Well it depends...

The change in pinion is your "larger" adjustment, mind you I personally wouldn't go to an 8 tooth as I've never seemed to get that lower number of teeth to mesh well, nine is the minimum I'd go.

The spur gear is my preference to change, providing you can get sufficient clearance in the chassis.

Reducing the diameter of the tyre is good for fine tuning your gearing, an example of this is when I was putting urethane tyres on my stock class cars at club. I found I could get a slightly higher mmpr figure ony gearing with pg xpg's than stock rubber and the other brand tyres I was using. The extra .3mm in sidewall height was worth more on the long club track than what I lost in magnetic traction from the extra height. The motors would push the gearing on the straights instead of maxing out well before the end of the straight as had been the case.

I did this particular scenario with a Scaley "BF" Ford Falcon V8 Supercar, which was obselete as the new FG had been released and was a much better chassis to run. With that little bit taller gearing I could round up the FG cars on the straights....

RM I LOVE that speed calculator, is that a free program or a bought one???

[Foamy]

Ballpark on changing gears is 1 tooth on the pinion is roughly the same as 4 on the axle gear.
ie: 8/32 is like 7/28. You should use the biggest gears that fit as you will have a more efficient gear mesh.

In R/C Racing, we needed to know the tire/gear ratio mainly because the batteries were not as good as todays.
Sometimes you needed to shave the tires a little to get an in-between ratio you couldn't get otherwise.