In my experience (with a Hustler, remember those?... oh, I see maxman has posted the sheet for the very one!) the problem with 'slow speed' was not with any slip of the belt on the motor shaft but in the natural stretch and rebound of the band relative to the 'cogging' of the relatively primitive motor. I would note that a coreless motor would likely solve this issue definitively while preserving TurboTrain speed...
(Speaking of warp speed: that trick with added tubing over the motor shaft had the added zest of decreasing the numerical final-drive ratio giving Even Moar High Speed per volt...)
Something of high importance is not to use bands that are so small or strong that they deflect the motor shaft or overload the bearings. In my limited experience with orthodontic bands back in the day my teeth were being tortured straight, they are too small and tight for safe use in this application. I would measure 'required length' carefully with allowance for tension and then find a drive belt for electronic equipment like cassette players or VCRs that fits.
For those who are brave (or penurious) you could adapt the principle of the Weller tensioner (as presently used for automotive serpentine belts) with a little spring and angled follower bearing on the normally-slack side of the band -- at the appropriate angle it makes relative to the chassis at the point of engagement.
When the so called "Hi-F" rubber band drive engines were new, as I recall, one review in MR noted that the pulling power was very good, surprisingly good, but that the drive was not "smooth" (a slight back and forth rocking when stopping was noted) and of course the top speeds, particuarly of the little "Hustler" were noted, but top speeds of many model locomotives then (geared or rubber band drive) were routinely pretty high.
Experienced modelers who tired of the negative features of the Hi-F drive Athearn F and GP diesels sometimes would consist them with geared drive engines, which tended to smooth out the negative features but took advantage of the pulling power at a cheap price. After all those Hi-F drive engines were pretty darn cheap, particuarly if you bought them from AHC, America's Hobby Center. $3.98 if my memory serves.
Some people put rubber or neoprene tubing on the thin metal shafts of the Hi-F drive to give the rubber bands even more friction to grab onto for even better pulling power. Hobby shops had the tubing due to gas powered model airplanes.
I do not know if dental braces these days still involve small rubber bands but they did back then so since my older sister had braces, the rubber band replacements were plentiful for me. I lived a distance from any real hobby shop but all sorts of drug stores sold those rubber bands for braces. They might have been just a little tighter than the Athearn originals.
It may all seem impossibly crude for today's modelers but I had a lot of fun with my Athearn rubber band drive F7 and Hustler. A good friend had the RDC. I think Athearn learned the limits of rubber band drive when they attempted to apply it to their Boston & Maine 4-6-2. The concept of rubber band drive was taken from an older generation of O scale models, particularly trolley cars and interurbans, that had a similar drive that used metal springs in lieu of rubber bands, often with a double drive reduction that smoothed out stops and starts with a flywheel, and also moderated the speed issues.
Those rubber band drive Athearn Hustlers could really fly, and I mean that in the most literal sense (remember that this was also the era of 15" radius snap track curves). I learned exactly where to place my mom's laundry basket away from the layout so the flying Hustler would have a soft landing. ("Model Railroading is Fun").
Dave Nelson
Perfect also sold replacement rubber band for Athearn rubber band drive units.
My mother picked up a lot of HO train stuff at a garage sale in my small Texas town for $2. Included was a pair of warbonnet ATSF rubber band powered F7s; eight brand new Atlas custom line switches; with switch machines, and numerous freight cars. The lot also included packs of Kadee couplers, which I had never seen before. Anyway, the Fs ran great. They were stolen from a club layout in Abilene, TX, in 1976. Then again, my memory of how great they ran could be tempered by the fact that I last ran them 48 years ago.
I really believe the Kadee couplers began my journey as a real model railroader. A couple of the adults in the Abilene train club helped me install Kadees on some equipment I had; and I was hooked (so to speak). Operationally and appearance-wise, it made a real difference.
Here you go! https://www.ebay.com/itm/132038272819
To install the rubber bands, take out the two screws holding the truck together, place a band onto each axle, then reassemble the truck. Stretch the bands over the motor shaft, making sure they're all in the same direction, and off it'll go!
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Picture I hope
Unless you can find one of these This runs at same speed as a geared drive one and no bounce when stopping also note all the bands are the same way on the drive shafts I still have three packs of the original rubber bands and they still have stretch to them Not that I'll use em Some people used those hair ties girls use they come in different sizes
OK I give up which link do I use for Imgur I know it used to work
I converted a few of my rubber band drive engines to dummies. I put lighting decoders In a couple and even a sound decoder in one. They are better as "honorary" engines because the warp speed drive mechanism isn't conducive to DCC. The conversion to a dummy is simple, just remove the motor and those drive belts.
A few years ago, I actually bought some of those drive belts. The official name for Athearn rubber band drive engines is Hi-F.
It takes an iron man to play with a toy iron horse.
This image from the HOSeeker website should help you out with orientation:
Remember that the shafts projecting from the motor rotate in 'opposite directions' relative to the wheelsets, so be careful that you twist them the 'right' way to make all the wheels turn in the same direction...
I still have the rubber band drive F7 I got for my birthday in 1958. It runs quite well but low speed is not great. Anyway just try to find some bands of similar size. To get the ones on closest to the motor pull the shaft out of the rubber coupling. To get them on the axle take apart the truck. It is easy to do as there are small screws holding it together. You may have to try different bands to get ones ro give the correct tension.
CN Charlie
I had a number of these in the late '50s / early '60s. A friend and I used to drag race them on long straight tracks set up in his basement. They had tremendous top speeds, but hard to handle properly at slow speeds.
The rubber bands were applied in a figure 8, with one on each end of the shaft as I recall. You can get assorted sizes of rubber bands at office depot or ebay and by trial and error you will find what is best. I doubt actual replacements are available of course.
ENJOY !
Mobilman44
Living in southeast Texas, formerly modeling the "postwar" Santa Fe and Illinois Central
My advice is "none:". Given a top speed approacing Mach 1 and no low speed, I'd convert it into a dummy to tow behind the powered ATSF locomotive of your choice. As it's almost assuredly in the passenger "War Bonnet" livery, it would be most approprite as the trailing unit of an ABBA lashup. There's about a gazillion and a half Athearn ATSF F's on the used market (both A and B units), so building a string of "Blue Box" F's shouldn't be expensive. Here's more than you want to know about Santa Fw F units atsf_f_units.pdf (sfrhms.org)
I am very confused about what type of rubber bands to use on my althern Santa fe f3 engine. I also have absolutely no clue how to install the rubber bands on this amazing engine... I h never seen any model train with a gearless drive! I am also very curious how old my engine actually is.