98 replies to this topic

[Steve Okeefe]

It's January 1966.  The first race in the new Rod & Custom Road Race series is coming up and I need to build a car.  What to build?

 

I'm hearing all sorts of theories about how to build chassis and motors, like super light-weight design including vacuum-formed bodies, keeping the mass of the car near the center, and hand winding motors with heavier guage wire.

 

Being a second year engineering student, I tend to think in terms of measuring and testing, then building based on results, rather than just going with popular notion.  I'm also more interested in the chassis, motor and running gear than the body (sorry, Mike M.).

 

The most common material for scratchbuilding chassis is brass, mostly because it is relatively cheap, plentiful, comes in many shapes and sizes, and is easy to solder.  Those are good points; so we'll start with brass.

 

Next, wheels.  We want super light-weight, low rotating mass, all that good stuff.  Magnesium is the obvious choice, just like full size cars, right?  Set screw wheels are very nice, but I haven't seen any magnesium ones.  I've always liked Mila Miglia wheels, they fit the requirements better than Cox wheels.

 

Guide and gear.  Cox has two products on the market that are very nice; a nifty little compact pickup guide, and a gear with an aluminum set screw hub and teeth made of something they call Coxalloy.  All-metal gears are very strong and stable, but have by comparison a higher rotating mass and thus take more power (directly from the motor) to accelerate and slow down. 

 

Now, the motor.  Pittman has been a mainstay since the very earliest days, but lately there's a new competitor that has been producing better and better motors for slot racing applications - Mabuchi.  There's a new model out there that has the pinion gear on the end opposite the brushes.  This has some excellent potential advantages.

 

So, let's get to a chassis design.  Inline drive is the choice because it's much easier to build a simple light-weight chassis than with sidewinder drive.  Super light-weight construction limits me to two main rails on each side.  I've also heard (and agree) that those rails, and indeed everything else on the chassis should be as close to the minimum track clearance as possible, to keep the vertical Center of Gravity (CG) down.

 

We need a drop arm to help keep the pickup guide in the slot, and the body will be mounted using pin tubes.  A little computer-based drawing work and, presto! (click to enlarge):

 

 

Now, in classic engineering fashion, we begin tinkering to improve the design.  What about that brass construction?  I read a Russkit ad in the September issue of Model Car & Track that claims steel chassis are heavier than brass chassis, implying steel is heavier (denser) than brass (click to enlarge):

 

 

Being the skeptical type, I began to wonder about that.

 

So, I go look up the densities of various materials, including brass and steel.  What do you know?  The density of brass is 4.86 ounces per cubic inch, and the density of steel is 4.56 ounces per cubic inch.  Steel is actually lighter than brass, not heavier.

 

Not one to just believe what a stranger tells you?  Good!  Let's check that ourselves.

 

I dug out my gram scale and started weighing things.  First I weighed 1/16" brass rod, then tube, then 1/16" steel welding (not bronze brazing) rod, then various diameters of piano wire and finally 1/16" stainless steel hypodermic (very thin wall) tube.  Here are the results:

 

 

How to read this chart:

 

The left column is a description - self explanitory.

 

The next column is the weight in grams per foot, organized heaviest to lightest.

 

The next column compares all other materials to brass rod; so if 1/16" brass rod is 100 percent, then 1/16" brass tube weighs 66 percent as much, etc.

 

The right hand column compares all other materials to brass tube; so if 1/16" brass tube is 100 percent, then 1/16" brass rod weighs 152 percent as much, etc.

 

By the way I'm not going to claim that my scale is calibrated perfectly (it probably isn't); besides it doesn't matter.  What matters is a consistent comparison between different materials, and I think I've achieved that.

 

What can we deduce from this chart?

 

Building with 1/16" brass tube is adequate, it is the fourth lighest material, easy to shape and solder, but not terribly strong.

 

Stainless steel hypodermic tubing is significantly lighter (weighing only about 3/4 as much) and much stronger, but hard to shape and solder.

 

.039" piano wire is really light; only about half the weight of 1/16" brass tube (!), but chassis built with it will be far too flexible and "springy".  Besides, it's an "odd" size (1 mm) that is so small the solder joints will be fairly weak due to the small surface area. You might be able to overcome this with gusset plates, copper wire ties, and miscellaneous braces, but it would complicate the chassis design, and in engineering school they teach you that "simpler is always better".

 

So let's look the other way.  .055" piano wire is only 10 percent heavier than 1/16" brass tube, but also less than 3/4 the weight of brass rod, and far stronger than either.  It's stiff enough to not be "springy" and large enough in diameter to have sufficient surface area to form good strong solder joints without gussets, braces or ties.  Hmmm...

 

Tune in next time for a chassis re-do, including why I did not use the lightest possible material to build the chassis, why I chose can drive, and rooting around for parts to build the motor.

[TSR]

[SlotStox#53]

Got my attention Steve

[Pablo]

I didn't think they had can drive inlines in 66, can you please talk about that point a little more ?

I don't see any holes in my Mab cans. Did the Russkits have can holes ?

break

I remember gascarnut gave a tip regarding jaildoor racing a couple years ago on how to slightly dimple the rear axle tubes to eliminate slop and resultant chatter.

[TSR]

Paul,

Generally speaking, it was in the minority, but cars actually raced in the R&C series did use the Hemi motor in a can-side drive mode...

But there are examples of modified Mabuchi cans to fit the Russkit or other available brackets, raced as early as 1965 in "pre-pro-racing" events...

All one had to do is to drill two holes, and precision was not really that great then, so here we go, short self-tapping # 1 screws!

 

Here is one I did in early 1967 in Frankistan:

 

 

Ugly beast, Morrissey inspired...

[Pablo]

Thanks, I am learning   Very very cool.

[Steve Okeefe]

Pablo,

 

Why, sure they had can drive in 1966.  In fact, here's an example:

 

 

This is from the second (coupe) race.  The track had 5 amp power packs, one for each lane, so the hotter motors kept popping the breakers.  John Cukras had so much of a problem with this that he bought a stock Strombecker Hemi 300 and mounted it in his car!

 

He drove it to second place...

 

There weren't however very many pro cars with can drive.  Pros liked how easy it was to mount the motor with that plastic endbell.  Also, Mike Morrissey insisted that the mass of the car should be concentrated towards the center.  Mounting the motor endbell drive put the magnets and arm stack - the heaviest parts - closer to the center of the car.  And since Mike and the Russkit team was winning a lot, almost everyone else just followed suit. 

 

About can holes - no, Russkits did not have holes.  In fact, no Mabuchi motors I have ever seen or heard of had can holes.

 

On ther other hand, all Igarashi (Hemi) motors had can holes, although some had only one hole.

[Pablo]

Thanks for the explanation. I'm happy now

[Duffy]

My anecdotal testimony from '65 carries no punch at all, I know, but -

 

In Sana Rosa at that time, we were still making four-rail chassis with 1/16" tubing, running them thru the four holes on the corners of the brass endplates that Russkit (I suppose) supplied. To swap a motor, we unsoldered the front plate & slid it away far enough to wrestle the motor out.

Lots of guys, mee included, reasoned that the can end gave a stiffer engagement to the pinion; so we'd run the cars endbell-forward, with just one screw to hold the motor in place.

 

I really wish I'd managed to save my slotbox from that time, kept safe by my father until his death in '95; and then lost to the clutches of my Evil Stepmother forever. - Along with my Rolling Stone collection, from pristine Issue 1 right on up. Ohh well.

Duf

[Pablo]

Does this mean you will be building a car for the event, Duffmaster ?

Me, I'm sitting it out as a spectator, and learning. Having fun, too !

I loved Steve's write up.

[Hworth08]

Just a thought from the golden days. We used to believe mounting the motor by the endbell allowed the plastic to isolate some of the motor vibration from the frame and thus tires. Also and we were probably correct that brass soaked up vibration a fair amount better than steel.

During the 1966 period the tires got a LOT better I guess starting with Tate's. The closed cell foam was much better absorbing vibration than the best of the hard, stiff open cell tires such as the Riggen Gran Prix and the Cox Competition tires.

Yes Pablo, Steve's post is good! A whiff of Tiger Milk and I'd be sitting in our shop again.

[SlotStox#53]

This is all just sooo kewl Learning a lot and gaining some insight & plenty of inspiration too The series sounds just amazing wish I'd been there, the atmosphere and those tracks   Huge!!!!

 

Very much looking forward to seeing this come together Steve !

[TSR]

In fact, no Mabuchi motors I have ever seen or heard of had can holes.

Steve, small correction here, the Champion "600-series" cans (601, 607) have mounting holes, directly from Mabuchi. But it is the exception.

[Steve Okeefe]

Well, it's true; I hadn't ever seen or heard of that one.  But I have now!      

 

Object lesson: In slot racing, there is almost always an exception.  I believe Catain Rick told me that awhile back...  I should listen.

 

Thanks Phillippe! 

[Steve Okeefe]

In my last post I was thinking about using .055” piano wire in the design of my chassis:

 

Let’s check to see if there is any precedent; has anyone tried using piano wire before, and what was the outcome?  So I go rummaging around in my latest copy of Rod & Custom magazine, and what do I find?  This:

 

 

It’s an article about the Western Model Car Championship, held in late 1965, and sponsored by Western Model Car Distributors.  If you want to read the entire article, you can find it here.

 

Why does this relatively obscure article pique my interest?  Because it reports that the winning car’s chassis is made out of… piano wire.  Here’s a nice, fuzzy close-up of the winning car from a different publication; Model Car Racing News, published in December 1965:

 

 

Piano wire chassis, for real, and he won the race with it.

 

Does this mean piano wire is a “speed secret” and if I use it I will also win?   Ummm… no.  Did you also see the ¾” wide U-Go Smith blue soft sponge tires on that car?  I suspect that may have something more to do with it.  Where is Mike Morrissey when you need him?  But, I digress.

 

The point is that piano wire is not a BAD idea, so let’s see if I can incorporate some into the design.

 

.055” piano wire is tremendously strong compared to brass anything, but in this diameter and short length, it is also considerably stiffer.  Builders have been having good results with the flexibility of all brass chassis so I don’t want to affect the flex any more than necessary.

 

Since the inner rails will add overall strength but have a minimal mechanical advantage over flex, let’s make them out of piano wire.  Here’s what it would look like:

 

 

You might also notice I changed the outer rails from brass rod to brass tube.  Now that the chassis has the strength of piano wire inner rails, I can reduce the weight by using brass tube versus brass rod.  This combination is probably less than a gram heavier than an all-brass tube chassis, and considerably lighter than an all-brass rod chassis, but much stronger.

 

A little birdie told me Mike Morrissey and the Russkit boys are playing around with stainless steel hypodermic tubing because it’s super light.  Well, I think, good luck shaping the tube without kinking it (its hardened material), and good luck getting 60-40 solder to stick to it.

 

I suppose if you use enough acid (that’s acid flux Timothy, it’s used on the solder joint.  No, Tim, the solder joint…)  Anyway, if you use enough you can get solder to stick to almost any common metal except aluminum.  And as an aside, there are zinc-based solders that will stick to aluminum too.  But I’m digressing again.

 

Better perhaps the Russkit boys should try using .047” piano wire; it weighs the same as hypodermic tubing per unit of length. 

 

Okay, now my engineer’s obsession to tinker with the design is satisfied – for the moment.  Why didn’t I simply select the lightest possible material to build the lightest possible chassis?  Two reasons:

 

First, the body has some significant weight (this is early 1966; there are no .010” thick bodies yet), and a good deal of that weight is relatively high up; at the level of the tops of the 7/8” and 15/16” tall tires.

 

That, plus the weight of the inline motor being attached right at the centerline of the rear axle, not to mention the two 1/8” axles themselves, means my vertical CG is way up there, counteracted only by the weight of the chassis.  So how much weight did you say you wanted to remove from that chassis?

 

Second, rear tires need some minimum weight on them in order to get traction.  Below that minimum weight, you can’t “get a grip” no matter how fancy the chassis is.  Power-to-weight ratio is very important, but it’s worthless if you can’t get traction.

 

I believe there is a “best” car weight for the current (1966) state-of-the-art in motor horsepower and tire traction, and I think I’m pretty close to it.  Using stainless hypodermic tubing, .047” or .039” piano wire to build simply the lightest possible chassis regardless of these requirements may well be going a bit too far.

 

Now, more obsessive chassis design tinkering: For a motor, I’ve decided to use a custom-built Mabuchi 16D, arranged in a can-drive configuration.  Here’s a peek at the various parts and materials I’ve scrounged up:

 

 

Why can-drive instead of the much more popular endbell drive?  Again, there are two reasons:

 

First, from a structural design standpoint, the metal can end, which is also the heavier end of the motor, is much stronger and stiffer than the plastic endbell end.  Mounted solidly in a motor bracket, this should help dampen motor vibration.  How?

 

Consider a motor mounted at the endbell; the heavier end of the motor (the can, magnets and spinning armature) is cantilevered off the motor mount by a semi-flexible (or at very least much less rigid that the metal can) plastic endbell, with the endbell bushing being the fulcrum point for the armature.

 

As the armature vibrates, the semi-flexible plastic endbell allows the can and magnets to vibrate along with it. At this point nearly a quarter of the weight of the car is vibrating as it hangs off the motor mount.  In super light-weight cars, this fraction could even be more than one quarter.

 

That vibration is facilitated by the endbell flexing a tiny amount as the can, magnets and armature whip back and forth.  Flexing absorbs energy, and that energy comes from only one place, the motor.

 

In addition to this, the pinion gear, on the opposite side of the endbell bushing fulcrum (described above), is also vibrating.  I’m thinking this cannot be good for the gear mesh.

 

So how does mounting the motor by the can end help dampen vibration?  By rigidly mounting the motor at the can end, the can and magnets effectively become part of the weight of the chassis.  At this point a significantly smaller fraction of the weight of the car (just the armature) is vibrating.  The rest of the weight of the car acts as a damper.

 

Second, and no less important, is that it provides better overall weight distribution.

 

Here’s an excerpt from an article written by George Siposs and published in Model Car & Track in March, 1964.  The article is titled:

 

 

 There are two diagrams in the article that illustrate the prevailing theory for weight distribution.  The first shows where the weight is supposed to be concentrated:

 

 

The second illustration attempts to explain why this is the best place for the weight to be concentrated:

 

 

If you want to read the entire article, you can find it here.

 

I have only one problem with this weight concentration theory.  I think it is…  completely wrong.

 

Why?  Consider this chart comparing the functional purpose of front wheels in both slot cars and full size cars:

 

 

In a slot car, without the front wheels providing steering or braking, the theory that the weight should be concentrated near the geometric center of the car is no longer valid.  So in a slot car, what does provide steering and braking?

 

The guide provides the primary steering, following a predetermined path (the slot) not subject to “over-steer” or “under-steer”.  Over-steer and under-steer can occur, but in a slot car that is controlled exclusively with the throttle.

 

The REAR wheels provide the only braking a slot car has.

 

So, doesn’t it make sense that a significant portion of the car’s total weight should be concentrated directly on the guide and the rear wheels?

 

Following the theory Siposs presents, here’s what a 1966 slot racer looks like:

 

 

Chassis and motor weight concentrated towards the center.  Here’s what a 1973 slot racer looks like:

 

 

The location of the motor, which is not shown, is pretty obvious.  Where is the weight on this chassis?  Not in the center!

 

Look at modern Retro-racing cars (not too long though, it bad for your vintage health) and consider where the weight is concentrated on these slot racers.

 

Two other examples:

 

Which type of slot car is easier to drive; a GP F1 in which the weight is crowded towards the centerline of the car, or a sports/coupe in which the weight is spread out over the full width of the covered wheels?

 

How many Group-7 (unlimited) open wheel GP F1 slot cars are there?  Why?

 

So, my theory is a slot racing chassis should be designed to be more like the left side of figure 2 (above) than the right side.  Actually, what I see is it should be arranged with the weight spread more or less evenly around the periphery of the chassis.  But I'll go with what I've got so far, for now.

 

Next up:  Building the chassis.

 

On deck:  Building the motor.

[TSR]

Steve,

Well done. Also to note is that the winning car had a PAIR of sponge tires glued together to make them wider, as it was commonly done in the old days, as seen on this 1967 survivor:
 

[SlotStox#53]

Hemi magnets and a 29# rewind... just out of curiosity would a 28/29 double silver wire rewind on French/ blank with Tradeship com be period Korrect? Just trying to find bits for a go at an R&C GP car too

[TSR]

Yes, it would be. Interestingly, Gordon French was one of the first if not the first rewinder of the 1963 Mabuchi FT16 motor and the French Motor Co was marketing the "silver" armatures, offered on the open market by Tradeship from early 1966, so no issue there...

[Steve Okeefe]

Philippe,

 

Steam-roller tires! 

 

Yes, I did notice that.  Also, it is doubtless that some experimenters used different hardness foam for inside versus outside tire sections. 

[TSR]

Steve, you are absolutely correct. The "hard" compound we used in enduros was on the inside of the tire, we left the "soft" on the outside to corner faster.

Those were the days, when we used only 4 pairs of tires in a 24-hour race! 

[Steve Okeefe]

Philippe,

 

 

Hemi magnets and a 29# rewind... just out of curiosity would a 28/29 double silver wire rewind on French/ blank with Tradeship com be period Korrect? Just trying to find bits for a go at an R&C GP car too

 

Paul E. said "28/29 double".  That would be roughly equal to a 25-1/2 wind...  Somehow I don't think a stock white Mabuchi endbell with no heat protection (i.e.: post protectors) would last more than two or three laps!    

[SlotStox#53]

Yes, it would be. Interestingly, Gordon French was one of the first if not the first rewinder of the 1963 Mabuchi FT16 motor and the French Motor Co was marketing the "silver" armatures, offered on the open market by Tradeship from early 1966, so no issue there...

Thanks Philippe ! That's great news now time to go grab some 16D parts to put it in

[Steve Okeefe]

Philippe,

 

I just noticed...  this car...

 

 

Has a can-drive motor. 

[TSR]

I don't want to be establishing any "rules" because this is not my idea or my thread, but if I may provide a few suggestions, I think that genuine Mabuchi FT16 and FT16D cases are OK, as well as first-generation Arco mags (the ones with no shim, made from broken 36D mags). However Champion parts such as "507-R" cases with the smaller bronze bushing, 2nd-gen Arco molded mags used with shim or not, Champion or Tradeship endbells should not be part of R&C nostalgic replicas.

As can be seen clearly in  100% of the pictures ever published of the R&C cars, only the genuine Mabuchi FT16 endbell was used, and that WITHOUT any spring posts protectors. Those came later... 

[Samiam]

With today's power spring post collars are needed.Unless we are going to run vintage power supplies. These are not going to be Shelf Queens.