By Bob Ward
As one who saw the CanAm cars being raced in earnest back in the 70s I really like the Carrera 1/32 scale models, especially the McLaren M20 There is something about the M20 that seems to make it the very picture of what a CanAm car should be. Carrera’s M20 model, along with other recent Carrera cars, incorporates features that make it a big improvement over the company’s previous products. It looks good and drives reasonably well out of the box, which has only fairly recently become the case with Carrera cars. But the Carrera M20 still leaves room for improvement.
For one thing, I’ve never understood why the Carrera designers think they have to put so much more stuff inside their cars than other manufacturers do. All that wiring, the circuit board, the reversing switch, the needlessly complicated design of the guide, and all the bumf needed for digital conversion – they all add weight and complexity, but for most users they are essentially useless. In addition, I’ve never thought the FF motor, as good as it is for some applications, is a proper motor for a Can Am car. So, when a semi-junk M20 wandered into my workshop I just couldn’t resist stripping all of it out and starting over.
I have to give the Carrera designers full marks for one thing — the whole lashup comes out of the car as a unit just by removing three screws.
My first upgrade motor for this project was a Slot It Flat 6. The Flat 6 configuration is made to order for CanAm cars and other cars with low, aerodynamic bodywork. It easily clears the underside of the body. One thing that makes it a lot easier is that Carrera didn’t try to put a full-depth interior in the car. This particular Flat 6 variant has performance characteristics that make it suitable for use with standard race set power supplies.
The first step in modifying the chassis for the Flat 6 is to cut the front motor mount out as shown above.
I’m going to turn the front mount 180 degrees in the chassis, making what was the front face of the mount now the rear face (toward the motor). The hole in the mount for the shaft bushing has to be enlarged slightly to fit the Flat 6′s larger bushing. This also needs to be done to the rear mount.
The next step is to cut out the entire part of the chassis under the motor. The height of the Flat 6 is just enough greater than that of the FF that it will sit in the car with its bottom face flush with the bottom of the chassis. The next step is to cut out a rectangular area forward of where the motor will sit into which the front mount will be glued.
This photo shows how the front mount fits back into the modified chassis. It’s glued into place with medium CA glue.
To replace the original Carrera guide I needed to have a whole new guide mount. Instead of fabricating one I just rummaged through my box of discarded chassis to find a donor for this vital component. I found a Monogram Greenwood Corvette chassis and cut out the guide socket and the area of the chassis back to the front of the motor mount as shown here. This shows you why you should never throw away broken, worn-out or obsolete chassis.
Here’s the M20 chassis with modifications completed. They include:
- A strip of thin styrene glued to the bottom of the chassis under the rear mount to keep the motor from rotating in the chassis.
- Small pieces of sheet styrene CA glued into the chassis to strengthen the chassis by filling in the spaces left by the cutting we did earlier.
- A length of styrene strip glued to the top surface of the chassis to reinforce the area where the front motor mount was glued in.
- The Greenwood Corvette guide mount CA glued into place.
- The opening for the guide extended forward to the curved rib (6) that reinforces the front of the chassis. This allows the guide to be mounted a little farther forward than before, increasing the guide lead for improved handling. The guide could actually be moved farther forward than this if you want to, but I preferred to retain the curved rib for the strength it provides.
All that remained to do to complete the modifications was to give the chassis a coat of black paint for appearance and snap all the components in. The guide is a stock snap-in part from a Slot It Porsche 962. A little material taken off the top of the guide socket enables the guide to turn freely. I cut the lead wires to a little more than half their original length. The pinion gear is a stock Scalextric part. I had to cut about 1/16″ off the end of the 2mm motor shaft to keep it from binding on the Carrera gear’s alignment ring, which was positioned for a 1.5mm shaft. Gear mesh is now adjusted with a thin spacer between the left rear wheel and the axle bushing. You could, of course, replace the gears or the entire rear axle assembly with aftermarket parts. The retainer that snaps in above the traction magnet to hold it in place is visible just below the pinion gear.
As you can see in the above photo of the car with the body sides removed, the Flat 6 fits under the McLaren body with clearance to spare. You probably could get an FK180 in there but to keep the mount modifications simple and avoid having either the motor shaft center above the axle center or the bottom of the motor below the bottom of the chassis the Flat 6 is the way to go. And, of course, there are other cars you might want to try this with in which the underbody space isn’t so generous.
With the Flat 6 motor installation finished the next step was to lower the body over the chassis as shown above. I removed 3/32” from each of the body posts. It was also necessary to remove the same amount from the tops of the vertical structures of the chassis and do some slight trimming of the front edge of the chassis as shown in red below.
This got the body down to where I had to cut off the small round protrusion off the bottom of the interior tray as shown by the red line in the photo below.
The next step was to work on the magnet installation.
Before getting into that it’s probably a good idea to say a few words about magnetic downforce. The longest-running and most heated debate in the world of slot car racing is magnet vs. no magnet racing. We will not revisit that debate here except to note that there are vehemently held beliefs on both sides and it will never really be resolved except by the two sides simply agreeing to disagree.
However, it is interesting to look at one particular part of the issue. Perhaps the biggest (though by no means the only) knock against magnet racing is that once allowed, magnets are inevitably taken to the absolute extreme possible, resulting in cars so stuck down that they essentially don’t need to be driven. This was largely true, though not without exceptions, as long as there was no uniform, reliable, and readily available means of monitoring and limiting magnetic downforce. That changed several years ago with the introduction of the Magnet Marshal.
The Magnet Marshal makes controlling downforce to any desired level utterly simple. Even better, it generates a total downforce reading that allows car weight, including added weight, and magnetic force, to be controlled either separately or in aggregate. Now, with a way to limit downforce and some creativity in magnet use it is quite possible to tune magnet cars to drive a lot more like non-magnet cars but with higher cornering limits. Those limits can be set at whatever is deemed to produce the best racing and driving qualities. What that does is to create a middle ground between the extremes. It’s no longer necessarily an either-or proposition. With proper magnet strength and placement a range of options opens up.
A note for non-magnet racers… some of the same problems that affect magnet placement also affect the optimum placement of weights. The location of the rear body mounts is one, and the overall shape of the chassis, as constrained by the bodywork, is another. For some cars it’s no real problem; for others it can be major whether placing magnets or weights.
By the way, I’m are well aware that the initial 1000-unit production run of Magnet Marshals sold out quite some time ago and they are now hard to come by. The good news is that the manufacturing rights to the device are now owned by Alan Smith of Scaleracing LLC, as slot car distributor. He expects a production run of a new and more capable version to be available by the end of 2014.
The goal for the M20 is a set of driving qualities that will probably require a net Magnet Marshal reading of 300 grams (Total MM reading minus the car’s weight of 84 grams). This is quite modest (there are local races being run at 600 and up) but can be a bit tricky to achieve on some cars. The Flat 6 installation required removing the front magnet, leaving only about 200 grams from the rear one. So, I began trying different configurations. The first one was extra magnets added on either side of the crown gear just aft of the stock magnet. That didn’t really do much to help. Next, I removed the stock magnet and added magnets and CA glued a Professor Motor 1030 1mm thick bar magnet as shown below. I first removed the motor so I wouldn’t accidentally glue the motor and magnet together.
This is the thinnest bar magnet available. Even so, it caused the car to register 316 grams. What I really needed now was a pair of rear tires just a little larger in diameter than the stock ones. It doesn’t take much of a change in tire diameter to make a significant change in magnetic downforce. I investigated tire options but did not come up with a suitable one. It would be no trick if I went to aluminum wheels, but I wanted to stick with stock plastic replacement wheels of one kind or another, at least for the time being. I should mention, by the way, that the car came to me with a pair of Maxxtrac M32 tires on it, so I simply incorporated them into the package.
As shown here, I put a piece of strapping tape over the magnet to prevent possible contact with the track. I then put the body on, leaving the screws loose for float, and went to the test track. As we expected, the car was too stuck down. It was actually at the point where it would have been better with less magnet drag on the straights. One thing really impressed me, however. With all that magnet drag the Flat 6 motor hardly got warm, even after a half hour of more or less constant running. I kept working with magnet and tire options but never got the Magnet Marshal number I wanted.
I spent a lot of time working on downforce. The car had a ton of straight line speed with the Flat 6 motor, but handling turned out to be another matter. Not that the car wasn’t fast; it turned a best lap time of 5.051 seconds vs. 5.379 for our control car, a brand-new box-stock M20 with sanded stock rear tires. But the car needed about 300 grams of downforce, as measured by the Magnet Marshal, to hit the “sweet spot” at which it can be driven really aggressively into and out of the corners but still does have to be driven. The best magnet installation I could fit around the Flat 6 provided only 253. That was the original rear magnet lowered as much as possible by cutting out the bottom of the magnet pocket and CA gluing a piece of thin sheet styrene under the chassis The stock plastic magnet retainer could still be used with a shim between it and the top of the magnet. The MM reading was down from the stock M20′s figure of 289, achieved with two magnets, one aft of the original FF motor and one forward of it. The forward magnet location had to be sacrificed to install the longer Flat 6. Everything else I tried produces either less downforce or way too much – 400 grams or more. My experience with the M20/Flat 6 combination confirmed something I had observed repeatedly in the past — that for optimizing a car for magnet racing at medium downforce levels the long can inline configuration wasn’t the solution
But there was a backup plan. I could just cut off the entire back of the chassis and convert the car to a sidewinder. A bit of test-fitting showed that there’s adequate room for a sidewinder FK130 with only slightly narrower rear tires. I had already built a couple of CanAm cars using Fly sidewinder chassis, and grafting the back of a sidewinder chassis to the front 2/3 or so of the M20 chassis did not present any insurmountable problems. I could also use a sidewinder FC130 but I would most likely have to put a small hump in the body to clear the endbell. I could look at the rear sections of certain Scalextric sidewinder chassis that offer the option of a snap-in bar magnet either just forward or just aft of the motor. But he FK130 would require raising the rear of the body only slightly from where it had just been lowered to. It would need just a shim or two under each rear body post and would still be lower than stock.
The first problem was what chassis could I cut the entire sidewinder motor mount assembly off of to graft onto the M20 chassis? I first considered the ubiquitous Fly rear pod, which has been around since shortly after the War of 1812, or so it seems. I’ve used it successfully in several chassis projects, taking advantage of the ease with which it can be modified to accept a wide, long bar magnet that, in many installations, gives just about the right amount of downforce. It has one significant drawback for this project, however. It is made for the FC130 motor, which we don’t want to use because of the low clearance under the rear part of the M20 body. The FC130, unfortunately, has that plastic endbell, part of which sticks up above the top of the can and just won’t clear the McLaren body.
However, there was no lack of alternatives to consider. Finally I came up with a TSRF T3201 chassis center section, shown above. Its virtues make it ideal for this project. First of all, it’s cheap at $4.49. If you ruin one cutting it up it’s no big loss. More important, it accepts an FK130 motor, which doesn’t have the plastic endbell. I speculated that if I went to a sidewinder installation I’d have to raise the back of the body back up a bit, and I was right, but the TSRF mount lets one fit the FK130 in place and snap in a bar magnet in one of three positions directly under the motor, the optimum location for a traction magnet.
The first step in the car’s transformation from inline to sidewinder was to cut off everything on the chassis from the rear motor mount back.
The next step was to cut the TSRF center section to the length of the rectangular area I had opened up in the Carrera chassis for the Flat 6 conversion. This created a sidewinder pod for the Carrera chassis. I also cut away some additional material at the left rear of the Carrera chassis, next to the left rear body mount, to fit the corner of the TSRF motor mount. Test fitting revealed that I had cut the TSRF chassis a bit short, so I added a strip of sheet styrene at the end of the Carrera chassis cutout to space it back just enough to get the rear wheels in the proper location within the body’s wheel openings. I also added small bits of sheet styrene as shown above to locate the pod exactly as needed and to fill gaps between it and the chassis cutout, providing plenty of surface area for CA gluing the two parts together. The result was a snug slide-in fit with everything positioned accurately to fit the body. With the fit between the two adjusted I set the chassis on a hard, flat surface with a piece of paper in between to soak up any excess glue. I then applied medium CA glue to the mating surfaces on the pod, and slid it into place in the chassis.
When the CA had set I test-fitted the FK130 motor, also a TSRF unit, at least for now. I had to cut away some material from the vertical structures just forward of the rear wheels to provide clearance for the motor shaft and the pinion gear.
And this is what the assembled chassis looks like with the motor and axle assemblies installed. The wheels and tires are from a Monogram Greenwood Corvette. The Corvette rear wheels and tires are just enough narrower than the original Carrera McLaren assemblies to fit the slightly wider sidewinder installation within the car’s overall width. The Sterling mag wheels of the Greenwood Corvette look good on a CanAm car. I had used them previously on a McLaren M12 built on a Fly chassis. The spur gear is a TSRF part.
Here’s a bottom shot showing clearly where pieces of white sheet styrene were added to fill gaps. I didn’t paint the chassis to cover the white styrene but it certainly can be done to yield a finished-looking chassis.
In this photo you can see the guide and lead wires in place. They were cut from a meter-long piece of silicone-insulated lead wire I keep on hand for projects like this. The VLH web site sells silicone lead wire from several different manufacturers. To find them just type the words lead wire into the site’s search engine. This photo also shows the brass washers I glued onto the rear body mounts to raise the rear of the body back up enough to clear the TSRF motor installation. The washers are temporary. Later I’ll add some length back into the body posts. You can also see the sanded rear tires (which need to be sanded a bit more). The front tires have been turned down about 1/16″ to make the front of the car sit lower. That’s about all you can take off without cutting into the lettering on the sidewall. If you don’t care about that they could stand to be taken down a bit more to get the front edge of the body even lower.
I also did some work on the interior tray. I like the intake manifold/injector trumpet detail incorporated into it, but with the new motor arrangement it’s too thick to let the body sit at a proper height. So…
I removed the roll bar and driver figure. Then I cut the tray apart at the rear of the cockpit. I then cut the intake manifold detail out of the rear part of the tray that I had just removed and then glued it to a rectangle of .020″ sheet styrene. The little triangle snipped off one corner is to clear a contour on the underside of the body. The hole in the styrene rectangle is for the roll bar’s rear brace.
I drilled out the broken-off mounting pins of the original plastic injector stacks, which are too fragile. Into each hole I CA glued a piece of 1/16″ wire-filled tubing. Using them as a very secure and sturdy mounting I CA glued a Parma #622 International 32 front axle spacer over each one. The result as shown at right above is an engine intake detail assembly that is much more survivable than the original.
Here’s the car, put together for testing with the interior and engine detail temporarily taped in place and the body sides not yet put back on. You can see that it was really starting to come together. The TSRF FK130 motor is, as I suspected, too much motor for the power I want the car to be able to run on, a stock Scalextric power pack on each lane. I knew from experience it would eventually heat up the Scalextric pack enough to pop its internal circuit breaker. But for the moment it served for testing despite its 2-amp draw from rest. I intended to replace it with a better-suited motor before the car was done.
At this point I had to put the project aside for a while to tend to other matters, but finally I resumed track testing to try to get the magnet number where I wanted it. To cut to the chase, I never did get it down below 300 grams using the TSRF rear end. However, the car is rocket fast on my track and a blast to drive.
You may be wondering what is so vital about the 300-gram downforce figure. What I am trying to do is to devise a set of classes and rules that will recreate in miniature the SCCA club racing of the 1970s and 80s, an era when many SCCA events attracted 400 cars or more in a wild and wonderful assortment never seen before or since. My goal is to give the various kinds of cars in my miniature SCCA relative performance levels that reflect their real-world counterparts. The A-Sports Racing (CanAm) cars were the fastest cars on the 1:1 scale track so they need to be the fastest cars on my track, too. 300 grams is about as much magnet as you can put on a car without edging into too-stuck-down territory (for me, anyway) so that’s the target for my ASRs. For comparison, the limit for A-Sedans (TransAm cars) is 220 grams. There are also rules about tires and other components that help keep the comparative lap times between classes where they need to be.
Anyway, The M20 required a decision. Should I cut it apart again for more experimentation or start over with another car and a different rear end? As it happened, another semi-junk Carrera M20 turned up. It, now, is my new test mule. The TSRF M20’s role will be as my outlaw track day car and a possible race entry someplace where the rules are more liberal. So, I went ahead and painted, detailed, and otherwise finished it. Here’s the result:
The car is painted with Testor RC car paints. These are actually a lacquer formula intended to be used on the inside of clear Lexan bodies but I discovered that they can be used on slot car bodies made of ABS when sprayed with an airbrush. I haven’t tried the spray cans so I don’t know whether they will work. The colors are RC91 Chezoom Teal and RC57 Daytona Yellow. The RC paints need a few coats of Glosscote airbrushed over them to give them a glossy finish. You can also use a spray-can acrylic clear coat, but test whatever brand you use first, especially if you are using homemade decals or ones printed on an ALPS printer. (NOTE: As of this writing Testor has discontinued the RC paints but they are reported to have been bought up by another company that will be producing them under a new name.)
You can see that I left the axle spacer injector stacks in their natural brass and the driver, interior tray, and roll bar are still in their original colors. The mirrors are styrene, turned down on a modeler’s lathe using a coarse emery board and then finish sanded with 400-grit sandpaper. A 1/16” hole drilled in the bottom of each mirror accepts a CA-glued in stalk of wire-filled styrene tube, making mirrors much better able to withstand the knocks of racing than the car’s original mirrors.
So, there will be another Carrera McLaren M20 kitbash. Watch for it on the VLH web site. If you have questions or comments on this article, please send them to firstname.lastname@example.org. I’ll do my best to answer them.