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Building a Retro Pro Car

Part 1

by Philippe de Lespinay as appeared in Slotblog (2012)

Or, thinking outside the envelope...


Three years ago, I began to search the easy way to win a Retro Pro race in what was then, a somewhat neglected class in the D3 racing program. I liked the class, I liked the motors being used, I just needed that "killer car" that would make my job, as a pre-geriatric with seriously hampered vision and driving ability, easier.

I had been racing, sometimes with good success, a clone of the M&M chassis that I obtained from Allen Low, and it allowed me to win a couple of races and set a distance record on BPR's tiny Kingleman track. But on the King track, it became obvious that it was too slow.

So the brain cells got somewhat in proper alignment and I began asking myself these questions:

  • Since I am no motor-building star, I cannot count on mega-horsepower to pull my 100-gram car down the straight and through the banking, because I rarely get a rocket motor. The best ones I got so far were assembled by Barney Poynor, those things are pretty damned fast! But others are just as fast, so what could I do to get an advantage?
    I began looking at an old 1995 very light Box-Stock car that was laying around, and remember how well it handled wit its ugly wing-car body. How could I get my Retro Pro car to handle like that? A lighter car would be able to be geared with a taller ratio, allowing a greater top speed, and the lighter weight would be favorable under braking. But would it handle sufficiently well?
  • Would taking massive amounts of weight off the car keep it drivable, since Retro Pro does not allow added material on the body to increase down force?
  • And last, would it be strong enough to withstand the severe impacts commonly encountered in Retro Pro events?

I began designing a car that used an old brass front end laying around and a two-rail, .055" center section and no side pans, none at all, and built it. I mounted the body on three stainless-steel pin tubes, floated through square brass tubing bits. By the time I was done, it tilled my scale at 65 grams, a full 35% lighter than my standard car. I began testing it using JK 8763PT treated tires and found it to be nose light, popping off from its front-end when pushed hard. Below are pictures of the car in its first version, taken a couple of days after completion in 2009:

I cut the width of the tires down to almost half, and it got better. By its third race, it won, and was by far the lightest car on track. At this stage, as the body was torn off its front mount and it was a weak point of the car, I added a third body mount behind the front wheels. A second race saw it crash hard and bend that mount, the car ending well behind the winning machines as it became quite twitchy to drive with the body askew. Post-race inspection also showed several cracks on its light chassis. So I began addressing each issue, step by step, race after race, using 15-thou brass wire to tie the rails to the brass-plate front end and to the rear brass tube that served as bearings mount. It was now stronger but the added bits got the weight to 70 grams, not so good. After a few more races where it was obvious that it was too flexible (too much mechanical grip was generated and it was still "under steerin"), I added a U-shape rail in the center of the chassis. Now, it handled great, and I could drive it flat out. But the weight was now up to 73 grams... and again, the center pin tubing got bent after the car got hit from the side while minding its own business. I had a bit of a brainstorm and decided to address the problem in an unconventional manner, by building a "crushable structure with memory".

Whatzat, you ask? Simple. By adding two trailing torsion bars hung from the brass-plate front-end, I resolved the issue. Now, the car can now get hit hard in its side and the bar simple flexes and rebounds, absorbing a large part of the impact's energy. And, thorugh uh, experience, it worked, quite well in fact.
But it also brought the weight to 75 grams... misery!

I brought a few more refinements, got it a body using less down force at the back and began experimenting with harder and harder tires. At this time, with a very tired motor, it can win a Retro Pro race as long as I keep a moderate driving discipline, not trying to over-drive it. I have not done a thing to it in the past three races, and it won its last contest. At this time, the body has also gained a strong Mylar reinforcement, and 2 more grams...

If anything, this car and its tribulations have encouraged others at BPR to build lighter Retro Pro cars, some very successfully.

But it is the end of the road for this one, it has become simply too heavy to meet its original goal. The lessons learned are going to help engineer and build a new, lighter and stronger evolution of the design, and what the heck, I will share its secrets with you here.