Convert the Revell Audi Sport Quattro to N-Digital

A while ago I managed to acquire a Revell Audi Sport Quattro rally car at a very good price. I decided that it would be a really interesting challenge to convert a somewhat obscure Revell car to the N-Digital system and tune it for Ninco track just to see it run around my N-Digital circuit, regardless of performance. Also, I could not resist the car's appearance. Visually it seemed to fit my personality perfectly: homely, awkward but you just know that it's going to be fun. And lastly, I'm always looking out for another unique rally car.
 
I read a number of reviews and articles about this car on the Internet and apparently there was a good deal of disappointment that Revell did not produce the car as four wheel drive (4WD). After all, it is a Quattro! So, in conjunction with the N-Digital conversion I decided to also convert the car to 4WD. As if I had any idea of what I might be doing. 

Below is everything required for the N-Digital/4WD conversion.

Anyway, Ninco users will be happy to know that the N-Digital conversion itself was relatively straight forward, primarily because there is plenty of internal space for the decoder chip, and other stuff, in spite of the fact that this is a very small car. My usual steps for a simple N-Digital conversion were performed:
 
- Removed the magnet.

- Lubricated the guide post, front and rear axle bushings, motor bushings and gears.

- Loosened the body mounting screws about one full turn to allow ample body float which really helps with this car's high center-of-gravity. 
 
- Installed the Hobby Slot Racing (HSR) suspension guide with Ninco ProRace braids.
 
- Installed a N-Digital decoder chip that is set up for quick swapping.

The photo below shows the HSR guide with Ninco ProRace braids in place. This has become my "go-to" tuning trick for other than Ninco cars running without magnet on Ninco/N-Digital track that have a guide post holder that is too narrow for the Ninco guides. As an off-road car it also seems appropriate to replace the "fixed" stock guide with one that is able provide at least some semblance of "drop arm" effect.

I did not bother to test the car extensively in 2WD but went straight to the 4WD conversion. My goal here was to make the car 4WD so that I could include it in an existing N-Digital race class comprised primarily of Ninco WRC 4WD rally cars. This meant pulleys and a drive belt (rubber band actually). Also, I did not want to add any cost to the car so I would try to use parts that I already had on hand. The more capable among us would undoubtedly replace the stock plastic wheels with machined aluminum set screw wheels as well as the gears, axles and bushings. And this car would definitely benefit, but I wanted to keep the original wheels' appearance and I do not have the means or inclination to create inserts from the stock wheels.

I happened to have on hand two spare 4WD pulleys from the PowerSlot Hummer H1. These are made for a 2.5 mm axle but it really doesn't matter because the long hubs of the Quattro's wheels leave no room to put a pulley on the axle outside of the bushing, where it needs to be (see photo below).

So, the solution was to [hand] drill the pulley's center hole to a size that allows it to fit snugly over the wheel hub. Then it was glued in place. This also required trimming a small amount of material from the ends of the four flanges on the hub. The result, shown below, is a perfectly positioned pulley outside the bushing on the hub. I have to mention that I tried every technique that I know of to get these wheels off the axles but they would just not budge. It became obvious that if I kept trying they were going to break. And if the wheel can't be removed then it's impossible to install the pulley. The solution to this dilemma was to use a sharp blade hobby knife to cut the pulley in one radius from the outside edge to the center hole. This allowed the pulley to be twisted apart just enough to slip it over the axle and then onto the wheel hub. The cut was then closed using CA glue to restore the pulley to its original integrity.

The following two photos show both the top and bottom views of the chassis with the pulleys and rubber band in place. The only potential interference with the travel of the rubber band was presented by a small post on the chassis that can be seen at the top edge of the N-Digital chip in the upper photo. Trimming a very small amount of material from the outside of the post solved the problem with no effect on the chassis' structural integrity.

The photo below shows the result of the entire process. The chip is cradled securely between the horizontal chassis braces and there is no interference between body and chassis. The 4WD assembly runs freely and the pulleys are surprisingly true with no obvious wobble. Note that about 10 grams of weight (in the form of magnetic refrigerator tape) was added to the deck in front of the guide and just behind the front axle.

Once on the track I was surprised at how well the car ran. It's not going to win any major rally competitions, and it really can't keep up with the Ninco rally cars that I have, but it competently handled the slings and arrows of my bumpy, twisty N-Digital track that it otherwise could not tolerate out-of-the-box. And most important to me, I can say that I have a 4WD Revell Quattro that actually runs N-Digital!

As I said previously, I did not test the car in 2WD mode. I ran the car periodically for a few months and just had silly fun with it. After the car sat for a while, the rubber band eventually dried out and broke. So I ran it in 2WD mode because I was too lazy to find another rubber band of the correct size and cross section profile. While running in 2WD mode I just had the feeling that the car was not handling as well as it did before. So, I installed another rubber band reestablishing 4WD mode and the lap times were reduced by about two tenths of a second on average. At least on my track, the 4WD set up yields better performance. Of course it could also be my driving...
 
In summary:

The Revell Audi Sport Quattro was converted to N-Digital and 4WD.

Apart from the cost of two spare PowerSlot Hummer 4WD pulleys, there was no additional expense. Incidentally, the PowerSlot pulleys are dimensionally very close the to pulleys from Ninco -- in the Ninco #80206 4WD kit -- so the parts required for this conversion are generally available.
 
If you really wanted this car to be a "break neck" competitive rally car, I believe that replacing the wheels and running gear with race grade components would be necessary. I converted the car to N-Digital and 4WD without regard to what performance might be because I felt that it would be fun to do (without getting myself into too much trouble) and it would yield a car that would be amusing, if not fun to drive. From my standpoint, I was right on both counts. If you are an N-Digital user and a chance comes along to get one of these cars, don't hesitate.
 
by Michael Ashton