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Gravity class insights

PostPosted: Tue Mar 26, 2013 1:05 pm
by jpennington
I would like to offer a series of posts on Gravity class cars and questions that have been coming my way. Please pipe up if you have questions or thoughts on what I am putting out there.

Mini Motors

The mini motor is one of the more noticed items on some of the gravity cars. These motors have raced successfully, but have certainly not dominated the class. The full size pancake motors and the traditional .375 OD inline motors have had more than their share of wins. That being said the mini motor is certainly very cool and creates a lot of building possibilities that would not be possible with the bigger motors.

The mini motors come from Asia and are manufactured for the broader industrial applications such of digital camera focusing mechanisms, vibrators for phone, blood pressure cuff pumps, etc. The application of them in HO is more a case of us finding them than them finding us. While you could deal straight with the manufactures to acquire your motors the quantities that are need is typically greater than a 1000 and what would you spec your motor at? Most of us therefore have been working with surplus types of dealers with the motors that have been raced. The RC helicopter world also uses these motors and some hobby shops will carry them. Make Google and eBay your friend when doing a search. Prices are great, they can cost as little as $2 each, but keep on shopping if you go over $10.

What should you look for in specifications in a mini motor?

First please note the size - Specification for sizing seems standard, but not all of the manufactures use the same coding to designate their motors. These are usually designated by a letter K, M or N, and length designated by a 10, 20 or 30. K is the smallest and I believe on the flats of the can they are .270 tall and have an armature that is .230 in diameter. Brian Fleischman and Tim Miller have raced and tested the K motors. Next up is the M motors the most commonly size used in the gravity class typically in the 20 length. The M motor is .310 on the flats of a can and contains a motor that is .270 in diameter. The N motor is the largest and has an armature that is .330 in diameter on the inside. If there is interest in the N size I will measure one out for you.

Voltage - These motor are designed to run on 1.5 to 6 volts, but that has not stop us in racing them in Gravity racing at 18 volts. This part gets a little fuzzy when you look at the spec sheets. Some of the spec sheet will show resistance and guess what it pretty much works the same as it does for our other HO motors. Al Thurman had some 1.7 ohm motors and they were too fast and would often throw windings. Al’s solution was to add a 10 ohm resistor in inline with the motor and wah-lah a good motor that last at least two racing weekends for $2. I have some motors that ohm at 6 and are just a little too fast. Tim Miller has some 7 ohm motors he used as is. The precious metal brush system in these motors will not hold up to extended over voltage for years, but they do last long enough to service your racing and at these prices they are cheaper than our tires! No one spec has come to the front yet that is the one and when it does a manufactured based quantity will be considered by some of us. The short of it is trying, some different combination of inline resistors with different ohmage motors to see what works. My personal motors I race are 5 to 4 ohms. I will comment further on other aspects soon…..


Re: Gravity class insights

PostPosted: Tue Mar 26, 2013 8:30 pm
by jpennington
The picture shows a regular armature and the three common sized mini motors - the K which is the smallest, the M and the N.

Re: Gravity class insights

PostPosted: Wed Mar 27, 2013 8:28 am
by slotcar1958

Would it be OK to publish the above information in the next issue of HO Racers Coast to Coast?


Re: Gravity class insights

PostPosted: Wed Mar 27, 2013 9:10 am
by jpennington
Please feel free to use this in your work. It is my intent to expand on this topic in the blog

Re: Gravity class insights

PostPosted: Wed Mar 27, 2013 9:48 am
by ruralradio
Great work, Joel.

Re: Gravity class insights

PostPosted: Wed Mar 27, 2013 10:14 am
by LDThomas

Thank you for all your time and efforts. Much appreciated. :)


Re: Gravity class insights

PostPosted: Wed Mar 27, 2013 12:50 pm
by slotking
nice job!


Re: Gravity class insights

PostPosted: Wed Mar 27, 2013 2:32 pm
by ruralradio
Al also offers up some useful info on these motors over in the vintage HO forum.


I can't wait to start building, just wish I was somewhere people actually raced these things (or raced anything, for that matter....).

Re: Gravity class insights

PostPosted: Thu Mar 28, 2013 10:53 am
by chaparrAL
Racing ? in 3 to 5 weeks there will be a combo Retro Slot Car Race @ WHEELS in Apache Junction AZ. The event will be for 1/24 D3 Retro Can Am cars and Retro HO cars such as Tjet and A/FX based cars, Riggen and LANDSHARK cars,and Scratchbuilt or LANDSHARK kit Gravity Unlimited cars. Today we need to rehash things because we dont want the date to clash with the Pete Zimmerman memorial @ Buena Park Raceway. ANNOUNCEMENT SOON!
We are all kicking ourselves for not buying all the M20 motors we could from Surplus Traders when they had them. Now all they have is the bigger N20 motor. The difference? Rated @ 3volts and with amazing little NEO magnets,these M20's are the best combination I have found. Great punch and brakes.
I found another spec M20 but at a much less friendly price. They have Polymer magnets and are rated @ 1.5 Volts. Now all these little motors you do not want to run without load above say 10-12 volts, because they will eat themselves up inside. The ones rated 1.5 volt are particularly "hot" and will pop more easily. Guys who race 1/24 Wing cars would never think to rev the motor full blast because it will explode. Same thing here. I have one of these poly mag 1.5 V motors,with 1/2 W 15 ohm resister, in one of my cars. It is my fastest one and runs amazingly cool in testing so far.

Re: Gravity class insights

PostPosted: Thu Mar 28, 2013 11:24 am
by chaparrAL
Here is one with the tiniest one, the M10

These are rated @ 3-4.5 Volts and only weigh 3 grams! Amazingly powerful for its size,they can propel an HO car with great alacrity if you keep the weight down. These are not going to work in a lead sled. With polymer magnets and tiny armature, it dont have much leverage/tourqe so to have good punch and brakes I found an 18-19 gram car works very well in testing.
These are also possible to blow up on a real fast track as I am finding in testing. Failure analysis has found the same thing every time, thrown wires from the stack. Not a failure of the com or those goofy motor brushes we all turn our nose up too.
The fix? Gear ratio? Lighter car? Choke resister? All three? Testing, testing, testing.....
By the way,I have already dissasembled new 1.5 V motors and treated the arm stacks with a little Duralco 4410 epoxy(serious overkill) with good result. This makes it to where the weak point is the com, but at least seems to help move that point the motor gets hurt somewhat higher. Testing got the point!

Re: Gravity class insights

PostPosted: Fri Mar 29, 2013 8:50 am
by jpennington
Great post Al, thanks
I would like to offer a follow up on the mini motor

Mini Motors continued:
Gears – the shaft size for these motors is consistently 1mm. The Life like pinion gear is a 7th plastic gear that resembles the Tyco 440 gear and has a 1mm mount. I have used this gear with an inline Land Shark. I believe that Rick DeRosa has slightly modified this gear and used it on the angle winder Land Shark. Another option is to go with standard gears that have a 1.5mm hole and use a sleeve. The sleeves that I used are based off of a hollow rear axle that already has a 1mm hole and a 1.5mm OD. I use loctite products to mount both the sleeve and the gear; Loctite is good enough for Unlimited racing in this application so it certainly will work for the load of a Gravity car.

Gear ratios that have been raced with the mini motors have been anywhere from 7th/25 to 8th/23, at least these are ratios I am aware of. The angle winder Land sharks are typically 7th/24 geared, utilizing a modified T jet 24th for the spur gear. Rick DeRosa has made some spur gears specifically for the angle winder, but I am not sure if this is a shelf item yet. My inline Land Shark that won the 1.9.13 Midwest race, had a ratio of 7th/25 and seemed well balance; I used a Tyco 25th for the rear crown. The 8th/23 ratio was on one of my sidewinders that finished 2nd on Walt Dick’s Scorpion this 3.9.13. So you see a range of ratios out there working successfully.

Weight – an M series motor in the 20 length weights between 3.6 and 4.0 grams. Now that is light! And as Al notes a 10 length is even lighter yet. A normal .375 OD armature by itself will weight 3.3 grams. So a whole motor with magnets, enbell, brushes and bushings to weight virtually the same is a real weight advantage. A lighter motor with a lower center of gravity allows you to place the weight where you want it – something that is very desirable in Gravity cars.
Mounting – there are three ways I have seen the mini motors mounted. The most common is to solder them in like a 24th scale car. Caution needs to exercised if you use this option as many of the motors have polymer magnet and these might melt if you apply too much heat, as Scott Terry has learned. Rick DeRosa’s solution is that hold the can when he solders – Rick promises that he will remove the soldering iron before his fingers start smoking!

The quick work around to mounting a motor is to epoxy it in place. Brian Fleischmann and Tim Miller did this with their tiny K motors. The down side to this of course is ease of changing motors.
Now me, Mr. Purist, uses the screw mounts that the cans are tapped for. The K series cans have a pre drill and tapped 1.2mm hole. The M and N motors have a pre drill and tapped 1.4mm hole. Spacing on these holes is standardized, so it makes making a jig to drill the brackets worth it.

Magnets – the mini motors come with either a polymer magnet or a neo motor magnet. The polymer magnet is by far the most common. As expected the polymer magnets are smoother, but have less brakes and acceleration. Both magnet types have equal power draw, which means little as heat is non-existent in Gravity racing, unless you’re Norm Gardner, our only racer to burn in three seasons. At this point we have not identified any manufacture part designation that guides us to either magnet type. We are just buying the product and seeing what in it.

I plan to do a follow up next on modern race platforms in Gravity racing

Re: Gravity class insights

PostPosted: Tue Apr 09, 2013 4:11 pm
by jpennington
Modern platforms in Gravity Racing:
The gravity racing has seen several cars race to wins that were develop off of the BSRT G-jet or the similar Storm car. Scott Terry, Bob Dame and Brian Fleischmann have all raced them successfully. The advantage to these platforms is their familiarity for the contemporary racer and readily available parts.

The question that quickly comes up is “how do I make this gravity class legal?” The answer is simple – it does not pick up the lift pin. A G-jet in its stock form is quite magnetic and can be readily turned upside down on a track section without falling off. Reduction of the motor magnet height is how all three of the above racers did it. Scott used the polymer magnets in his G-jet and sanded the to a height where they would no longer pickup the lift pin and then silicone glued them in place and placed lead underneath them.

Brian used rare earth motor magnets in Storm based car this February and March at Lucky Bob’s. Like Scott, Brian downsized his rare earth magnets to the point where they would not pick up the pin. Since Brian was using a neo based magnets the downsizing was considerably more than for a polymer magnet. Positioning of the magnets was accomplished with an aluminum spacer. Exact placement of the magnet is critical with a neo magnet as only a little miss placement makes a big difference with the tech pin.

In all cases with the contemporary platform cars in gravity had additional weight was added. Scott has fabricated brass pans that were attached to the bottom. Brian took advantage of lead being added to the sides of the car. Motors were either the 6 or 9 ohm variety dependent on track and racer preference.

Re: Gravity class insights

PostPosted: Wed Apr 10, 2013 6:47 pm
by chaparrAL
Great work Joel!
Hey racers! Check out LANDSHARK 2013 in the Slot Car Classifieds. Your chance to get a racer like that in post #10.
I test these in a raceway and people are amazed that they are not magnet cars. :peelout:

Re: Gravity class insights

PostPosted: Thu Apr 11, 2013 12:20 pm
by jpennington
Landsharks in Gravity Racing:
Right from the start of our series in 2010 the Landsharks by Al Thurman have been present. It is probably the most raced car in the series. Rick DeRosa has been working closely with Al in the development of the Landshark. This season two of five races were won by a Landshark, one of the inline variety and one of the angle winder style.

Al has offered the Landshark in a various levels of finish, from a bare bones kit to a ready to run car. Check his website for availability and details. I purchased an inline Landshark kit in November 2012 and Tim Miller won with it on 1.12.13. As a kit I got to do the details my way, which included my mini-motor, my tires, 7:25 gearing, and “blue printing”. My first Landshark was of the big motor angle winder style and was purchased as a “ready to run” at the 2011 nationals where it won theExhibition Brass Car race.

Rick DeRosa’s angle winder Landshark, which won our 2.9.13 race at Lucky Bob’s, was of the mini-motor variety. The mini motor angle winder is lighter and lower than the original big motor ones. Rick has found that mini-motor version races best at 18 to 19 grams. The big motor ones also need additional shielding to pass the lift pin at tech. You need to have one of these in your box.


Re: Gravity class insights

PostPosted: Tue Apr 30, 2013 2:15 pm
by jpennington
AFXs in Gravity:
The AFX platform has won more races in the gravity series than any other car. These cars are very reliable, run under a wide range of conditions, and are lap machines. If there is an Achilles heel to this platform it would be brakes. I tried to address brakes with a higher gear ratio, but sacrificed some straight line speed in the process. Scott Terry tried to address this by using a cut down polymer magnet in an AFX car that won the first Midwest Championship.

Doug Morris has been very successful running several variations of the AFX. Most of Doug’s wins have been with a pickup based AFX with a TCP pan, 37 wind, ”dimpled” Super II magnets, and 20th crown. At this March’s Midwest Championship Doug had a very strong performing T jet (see the Midwest Championship blog on POS for pictures and details). Doug finds that these cars work best around 22 grams of weight.

Scott’s AFX used a TCP pan, 36 wind, cut down polymer magnets and a 22th crown. Scott felt the brakes were improved with the upgrade magnets, but felt he still lacked what he was hoping for.
My personal AFXs have not done as well as Doug’s or Scotts, but they have made a number of mains. I have been using TCP pans (pick up and wiper style), 37or 36 winds, original Super II magnets, and a high ratio gear plate of my own design. I been trying to gear up and match the motor/tire roll out of a Fray T jet, but as noted you gain brakes but loose top end. I will try to include a pictures and ratio chart, but have encountered some difficulty lately on this site with attachments.

Parts seem to be reasonably available. Doug Morris will sell AFX cars in various stages of finish with TCP pans. He also will sell the TCP pans and a few other parts alone. AFX stock cars can be found on ebay and other such media. These cars are low maintenance and don’t go through a boat load of parts. I keep a couple in my box for loaners and as a testing standard.