12th-Apr-20, 11:45 PM
SALT? – Scandinavian/American Lotus Tribute. Okay, that doesn’t help much so, expanding a little: in Carver’s Lotus IX thread on the SFI forum, the Lotus 23 was mentioned as a particularly challenging subject due to its diminutive size. I contributed to the wandering of the thread by suggesting a possible answer to that challenge and identifying a few additional challenges. Evidently, in some sort of time zone and distance insensitive fashion the same idea occurred to both Carver and me. After some further discussions we have agreed to embark on a joint project wherein I will look to the ironmongery and he will employ his artistry in creating the coach work. This thread will chronicle the outcomes of that collaboration on a completely unpredictable schedule.
I would point out that this sort of dual origin is a well-established tradition in the automobile world. It was very common in the 1930s and earlier for a manufacturer to build a chassis and then ship it off to a coach works house for a bespoke body. (I may have even partaken of some of this myself. Years ago I owned, in sequence, two Sunbeam Talbot Mark IIs. I seem to recall a note to the effect that the bodies of those cars were built by Thrup and Mayberly.)
Enough introductory blather. Having committed to the project I made a list of the questions, problems, decisions etc. to be addressed. Some, like “13 inch wheels?” were solved forthwith – RS slot racing does them. Others fell into the category of “there is probably something that I’ve done before it will work here” but one issue stood out – the gearing.
And why is this a problem? Because of the numbers. (This is a foreshadowing of what is coming so, if numbers bore you, you are advised to go to the next thread on the forum)
There are 3 numbers here that matter: the gear ratio – obviously; the center to center distance of opinion and spur – the build is envisioned as a sidewinder, probably with the axle carrier soldered to the motor so this is important and finally, the diameter of the spur gear. for most builds the latter is a major concern but given the small wheels and tires that the model will carry it turns out to be the most critical number. As measured from a scale drawing, the rear tires and 19 mm in diameter. While off-the-shelf tire availability might bump that by a millimeter, it is nevertheless, for build purposes, the target. While typical rules exempt gears from a ground clearance specification, we are, after all, talking about use on a racetrack and not a cog railway. Starting with a not overly generous 2 mm of ground clearance. this dictates in maximum diameter for the spur gear of 15 mm.
Looking at 64, 72 and 80 DP spur gears we find the following with a diameter of 15 mm +/- 0.2 mm:
64 DP:36 tooth, 72 DP :40 tooth and 80 DP: 45 tooth
The next step is to look at the gear ratios. One of the objectives here will be to build a light chassis. One can always add weight but incremental lightness is a rare commodity. Based on observations from recent builds and behavior in both proxies and on my local tracks, my goal would be ratios between 2.2 and 2.8 : 1. For planning purposes let’s choose a ratio of 2.5:1. This would call for pinions of 14, 16, and 18 tooth for the 64, 72, and 80 DP spurs. (The numbers, of course, do not come out exactly but they are close)
With this information in hand we can generate the 3rd set of numbers, the center to center distance of the gear sets. To do this we add the pitch diameter of the spur to the pitch diameter of the pinion and divide by 2. At the results are:
64 DP gearset :9.92mm
72 DP gearset: 9.88mm
80DP gearset: 10.00mm
Will this work? A few more numbers – an FF 050 motor is 15.3 mm wide. The diameter of an axle carrier will vary depending on construction but, given bearing dimensions etc. 4mm is probably a good working number so
(15.3+4)/2 = 9.65 mm. It should work but with little margin for error in the dimensions of the axle carrier. A note in passing: there are alternatives that might yield a little more flexibility in both gear ratios and build style – the obvious one is in anglewinder (and Slot It does a 15mm angleinder gear) but there is also the option of turning the motor on its side. There are pluses and minuses to both of these approaches but they will certainly get a look.
A final note on gear ratios – there is some flexibility here without the necessity of rebuilding the motor/axle carrier assembly. One of the things that comes out of the numbers is that the center to center distance does not change if the total number of teeth on the pinion + spur does not change. In other words the dimension we calculated for a 36+14 64 DP gearset will hold for any combination that adds up to 50 e.g. 37+13, 35+15 etc.
WAKE UP! Class is over.
EM
I would point out that this sort of dual origin is a well-established tradition in the automobile world. It was very common in the 1930s and earlier for a manufacturer to build a chassis and then ship it off to a coach works house for a bespoke body. (I may have even partaken of some of this myself. Years ago I owned, in sequence, two Sunbeam Talbot Mark IIs. I seem to recall a note to the effect that the bodies of those cars were built by Thrup and Mayberly.)
Enough introductory blather. Having committed to the project I made a list of the questions, problems, decisions etc. to be addressed. Some, like “13 inch wheels?” were solved forthwith – RS slot racing does them. Others fell into the category of “there is probably something that I’ve done before it will work here” but one issue stood out – the gearing.
And why is this a problem? Because of the numbers. (This is a foreshadowing of what is coming so, if numbers bore you, you are advised to go to the next thread on the forum)
There are 3 numbers here that matter: the gear ratio – obviously; the center to center distance of opinion and spur – the build is envisioned as a sidewinder, probably with the axle carrier soldered to the motor so this is important and finally, the diameter of the spur gear. for most builds the latter is a major concern but given the small wheels and tires that the model will carry it turns out to be the most critical number. As measured from a scale drawing, the rear tires and 19 mm in diameter. While off-the-shelf tire availability might bump that by a millimeter, it is nevertheless, for build purposes, the target. While typical rules exempt gears from a ground clearance specification, we are, after all, talking about use on a racetrack and not a cog railway. Starting with a not overly generous 2 mm of ground clearance. this dictates in maximum diameter for the spur gear of 15 mm.
Looking at 64, 72 and 80 DP spur gears we find the following with a diameter of 15 mm +/- 0.2 mm:
64 DP:36 tooth, 72 DP :40 tooth and 80 DP: 45 tooth
The next step is to look at the gear ratios. One of the objectives here will be to build a light chassis. One can always add weight but incremental lightness is a rare commodity. Based on observations from recent builds and behavior in both proxies and on my local tracks, my goal would be ratios between 2.2 and 2.8 : 1. For planning purposes let’s choose a ratio of 2.5:1. This would call for pinions of 14, 16, and 18 tooth for the 64, 72, and 80 DP spurs. (The numbers, of course, do not come out exactly but they are close)
With this information in hand we can generate the 3rd set of numbers, the center to center distance of the gear sets. To do this we add the pitch diameter of the spur to the pitch diameter of the pinion and divide by 2. At the results are:
64 DP gearset :9.92mm
72 DP gearset: 9.88mm
80DP gearset: 10.00mm
Will this work? A few more numbers – an FF 050 motor is 15.3 mm wide. The diameter of an axle carrier will vary depending on construction but, given bearing dimensions etc. 4mm is probably a good working number so
(15.3+4)/2 = 9.65 mm. It should work but with little margin for error in the dimensions of the axle carrier. A note in passing: there are alternatives that might yield a little more flexibility in both gear ratios and build style – the obvious one is in anglewinder (and Slot It does a 15mm angleinder gear) but there is also the option of turning the motor on its side. There are pluses and minuses to both of these approaches but they will certainly get a look.
A final note on gear ratios – there is some flexibility here without the necessity of rebuilding the motor/axle carrier assembly. One of the things that comes out of the numbers is that the center to center distance does not change if the total number of teeth on the pinion + spur does not change. In other words the dimension we calculated for a 36+14 64 DP gearset will hold for any combination that adds up to 50 e.g. 37+13, 35+15 etc.
WAKE UP! Class is over.
EM
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