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Building yet a better "Killer RetroPro car"

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#1 TSR


    The Dokktor is IN

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Posted 17 December 2012 - 08:34 PM

Testing of the new beast went quite well, and we learned that the design is sound and has potential.

After several testing sessions of the new car in different track conditions over several days (see HERE for previous information), I have redesigned the front end and some details to kill another 5 grams of weight while likely improving its handling. I believe that the car can be built at 65 grams and still provide adequate handling while being able to reach a greater top speed than heavier cars.

So while new bodies are being sent to the Noosester for artistic duties, I have begun construction on two new chassis using an improved design based on a new front end that projects weight forward while not adding any.


Indeed, during testing of the new car and from experience with the now retired older car, it appeared that while the new car was much better than the old on the gutter lanes, it still lacked that sure-footed feel realized when about 3 grams of lead weight are added to the most forward part of the chassis. So, why not shift more weight forward, rather than adding some? This is the purpose of this new exercise, as well as proving to myself that the design can be consistently repeated.


Here is the changed design compared to the old one, that used a now obsolete front end, as I built the current car with my last one. The weight shift is quite clear towards the front:




The recently-built car installed here over the new design shows the amount of weight that will be shifted forward (in red) as well as removing some behind the front axle (the green sections represent where brass sheet was removed from the car during testing).




The idea here is to shift more weight forward, while reducing overall mass by substituting the steel wire and solder holding the front axle for two simpler formed brass uprights, that will also get some weight reduction of their own in the process. Finally, we will reduce the weight of the body mounts by using smaller-diameter material to float the mounting tubing. Another change is the addition of a top-front motor brace built in one of the main rails. This adds weight but reduces the risk of losing the motor in one of the rather violent impacts these cars are often subjected to.


I asked my friend Bryan Warmack to cut me the new pieces out of his standard 0.032" front-axle and guide mount following the design I made. As usual, Bryan did a superb job of it. Is there anything this man cannot do well?




Once again, the most difficult part of the construction (for me anyway) is the precise forming of the U-shaped 0.055" main rail, that require five bends of which none can be wrong, or a new wire form needs to be fashioned and each bend repeated until all five are perfect in all three dimensions. The other two outside rails are a piece of cake compared to forming that one. I am getting back in the swing and got it right the first time, and since building two cars, twice in a row! Must be my lucky day.




The front ends of the three main rails lock in the slots cut on each side of the front end. The sides of these slots have been filed in an angle to form a nest for them, while they precisely fit against the rear-axle tubing, on which only one slot has been cut so far to receive the left-side main rail, to clear the motor.




One can see here that said left-side main rail now has a U-shaped top to provide an added motor mount. It will be later trimmed in length and thickness to reduce its added mass. Both top and bottom mounts formed by that rail must be exactly on the same vertical plane.




Here is the U-shaped main rail for the second kit of parts in the process of being shaped. Already, four of the five bends have been successfully completed, check the large radii to avoid rail breakage while bending. This also makes them much stronger.




Once the rails have been cut and formed, it is important to make sure that they fall in place in the jig fixture in such a way as not to be in any kind of stress. Adjust their bends until they finely conform to the plan.

Here, the rails are held in place before any soldering is done, to verify alignment and angles. Note that the front axle is now in place.




The rear axle tubing has now been trimmed to its final shape, leaving just enough "meat" to be held by the U-shape main rail:




Please note that the third bearing has been fitted. It will later be affixed with Loctite.




The inner right-side main rail has now been trimmed to its final shape. It will be wrapped with brass wire after basic soldering to the axle tube:




Next, we will fashion and fit the U-shape inner rail, then address the guide tongue. The whole kit will be cut, shaped and all parts made ready before any actual assembly will be performed.


Here is a drawing showing the wire.






The # 4 bend is the one at the base of the rail, that shapes it from horizontal to 90-degree vertical. here is a sketch from the back of the rail, with the shadow of the axle tube for clarity:




The rail goes up 90-degree but has to be offset for the correct spacing of the rear-axle tube to work and allow room for the gear and wheels, plus one 25-thou spacer on the right side between the bearing and the wheel. No other spacers are needed, but one can be added between the left-side bearing and the gear for gear adjustment, and still retain the 3.125" maximum axle/chassis width.


First, one last look at that left inner rail. To help, I pulled it away from the tubing to see it better, but when in position, it is firmly stuck inside the tubing, barely clearing the axle:


post-3-0-05649500-1356050482.jpg    post-3-0-94067800-1356050484.jpg


No rocket science!

OK, now we are going to trim the front end to receive the center rail, and to again, move more weight bias to the front of the car. First, I mark the piece with a pen:




After a trim right across the back, I mark more area to trim. Note that I leave a center spine, this will be explained later:




Blocking the part in a small vise to keep it straight and flat, I grind the excess material with the Dremel disk, then finish with round and flat files:




The weight gain is minimal, but the weight transfer forward is good and will save from adding weight on the finished car.


post-3-0-34736200-1356051391.jpg   post-3-0-41810800-1356051388.jpg


Now is the time to shape the center rail, another one-piece job that does not require too much effort:




I begin shaping the right-side bend first to the correct angle, following that of the motor. Make sure that you clear the motor! Note that this picture was taken before the front-end was trimmed.

Then I shape the other side, constantly checking to obtain the right curvature while keeping the spacing as tight as possible:




The finished rail is now in place, but the front ends of all the rails have not been finally trimmed yet. This will happen when everything is in place and the first solder joints will be made. Note that all the rails are "locked" by the front end, and this will help enormously when your car hits the wall harder than it ought to...




Now is time to shape the guide tongue. Again, there will be no brass tongue use here, and we are using a standard guide and not a cut-down version. It will be stronger under hard impacts and is straighter to begin with.

I used a Slick-7 tempered steel reinforcing blade that is normally used to solder over a brass tongue. In this case, it will be the tongue itself.




I set up the guide with the correct thickness of braided contacts and lead-wire clips, then compress the whole in a vise to get it to the correct overall thickness. When this is done, I rotate the guide on the tongue and mark the area where no solder or material must be, to not interfere with the guide.




I check fitment and analyze what needs to be done first




Now after checking the spacing with a block milled to the correct track situation for the contact rails, using a piece of 15-thou brass strip as a spacer for the front end, I find that a piece of  0.055" steel wire will be the perfect spacer. So I shape a piece in a "V" to offer maximum support to the guide tongue:



I leave a piece of the wire form attached to handle it during the soldering process. This wire form will now be ground with a 1.5-degree angle to point the tongue up a bit. This is a delicate operation to keep everything on the correct plane. We will do that once the wire has been soldered and before we solder the tongue over it.




We have not made a single solder joint yet, but our car is over 40% built.  :)

Next, we will form and cut the body mounts, including the spring-loaded, very critical center mount.


The guide tongue was affixed with the V-shaped wire as support. I first soldered the piece of wire to the tongue reinforcement that will be the only guide tongue here, pretty thin at that...

To locate it I drilled a hole through and threaded a 2-56 screw. Once I was sure of the exact position, I soldered it from its sides.






I also soldered the chassis, as explained in the previous post, using the same methods.




Now I am using a straight axle with no bracing, and no front body mount yet. I will add one if I see a benefit later.




Then the rear mount, similar as before:




When complete, the chassis is very slightly lighter than the previous version but I added quite a bit of mass upfront where I wanted it, so likely it will need no added weight there.




The # 2 and # 3 T-Rex cars side by side show the differences and the evolution:




I still have to wire a couple of places, but no time today.






The back end shows how much was removed from te rear-axle tubing:





The new car feels even better, more predictable. But I only had a Porsche body for it yet, and it gives it a bit too much rear down force and not enough on the front despite that I mounted it very high up front.

Here is how it was mounted. First, I put a piece of 1/16" thick tape over the motor to make sure that the body will clear the top and lead-wire post. Then I taped the chassis on the block so it will not move and allow a precise body mount.




After cutting the nose at the trimming line as per SCRRA regulations, I used a 0.063" brass strip taped to the mounting block to raise it, so that the body reinforcement will clear the track just enough once fitted.

The body is then fitted tight over the brass strip, and tight over the motor. This insures just the rake I want. Pins are the inserted, using a light to locate the mounts, only 4 holes are being drilled:




The body sides are then trimmed, so that they are clearing the block by about 1/32". Since there are no side pans to help out, again a light is being used and the sides marked with a Sharpie, the excess trimmed with a pair of large Fiskar scissors (the best!). Then I give an approximate trim to the wheel openings using a curved pair of nail scissors.




Next, a couple of test laps to insure that there is no interference anywhere:




Very little body drag but the guide was set too high, so a 5-thou Teflon washer is added. Now I install the side body armor as shown on the previous thread, with the Super Clear tape sealing the thick reinforcing pieces in place:




The wrap-around tape also helps maintaining sufficient rigidity while stopping any tendencies for the sides to develop cracks during a sharp impact.


Next, a few more prudent test laps since the front-end is not reinforced yet, where the car had no problem to cut 3.6" laps.




Next, it will get the same front-end treatment as the T-Rex # 2, but also a Gulf-Mirage body is being "Noosed" and will be fitted to the car as soon as back from Joysey... :)

We like our cars to be pretty inside and out.

The motors have now been removed, cleaned and packed for armature reconditioning. They look pretty good so far.


I use the flexible 0.032" wire strictly for shock absorption purposes. So far, it seems to work. In fact I might use a similar system for the rear body mount too before too long.

Another advantage is that I can tune body height in relation to the track surface by adjusting the mounts up or down by hand or with small pliers.

Their apparent fragility has not been an issue so far.

I know from body damage that when the car hits the floor, a rigid mount will do extensive body damage. Also the front body reinforcing I have been doing with Mylar film and that makes some raising their eyebrows some, has so far avoided the front of the body to curl inside the chassis as as often see on non protected cars. In a tough racing situation, it means time saved, and when laps are 3.5 seconds, time is critical.

Retro Pro cars must be built like tanks, but I prefer flexible tanks. So far (at least over the last few races), those shock-absorbing mounts have really worked well for me. The original concept on the T-Rex #1 chassis used 0.047" wire, but now I use 0.032" wire because the body and its mounting system and controlled flexibility is an integral part of the shock absorbing system.


Then of course, there could be a more major shunt (as David Hobbs would say, a shamozzle, in which nothing will help!  :shok:  :D

First car to fly though the front-door plate glass? :laugh2:

I hope not... :o


Since quite a few here requested this by PM, here is the pattern for a front body reinforcement piece for the M.A.C. Porsche 917 body. Print and cut, use as a pattern over Mylar of Lexan. I use a machine vise and a block of aluminum to hold and bend the material precisely.


CLICK HERE and print.


The part has now been cut and stapled to the body, but not taped yet:





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