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Body tuning / weight

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wixwacing
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Posts: 1871
Joined: Thu Jul 10, 2008 9:22 am

Body tuning / weight

Post by wixwacing »

<br><br>Getting models to handle has been an age old problem with slot cars. In the earlier days of the hobby ALL slot cars were magnet free. Nobody even contemplated an alternative because the mindset was entrenched in weighting the cars and even though model trains at the time had vestiges of 'Magnatraction" it was to be a long time before RTR slot cars made tentative steps toward artificial gravity. One of the first was the 'Scalextric' fridge magnet. An improvement on no magnet but not too much. Best thing to do was to carefully remove the magnet and then reglue it in place with epoxy. The model was stood on a section of track with a slip of 2 mm plastic under the magnet. This ensured a close rail proximity and minimised the risk of knocking it off on one of Scalextric's infamous joints at the flyover sections.

Next came a series iron based magnets which were slung between the motor and axle. These offered a little more grip but they too had to be suspended close to the rails to be useful. One of the more successful attempts was the SCX SRS 2 chassis. The magnet floats in a little carrier compartment between the above and usually is quite effective.

The 'piece de resistance' has to be the current rare earth or 'Neodymium' magnets. these are able to supply more downforce than the modern slotcar can use. Invariably mounted in the floor pan of a model between axle and motor or situated just in front of the motor in sidewinder use, these magnets are able to be manufactured into a variety of shapes to suite most locations. The magnet itself is a granular alloy which has very strong magnetic properties.

The magnets come with a thin plating or skin, usually chrome, which serves a vital function. Because of the brittle nature of the material it is necessary to hold it all together in something stronger. If you chip the magnet by accident or loose the chrome cover you will find the magnet disintegrates fairly easily.

But magnets aside, there is still a strong call for models to behave themselves in a civilised fashion in a non magnet situation. A lot of models are still run in a non magnet situation in as much as some clubs race non magnet classes and some clubs have non magnet tracks! In both cases with the vast majority of cars, performance can be enhanced by the judicious addition of some ballast.

After all other body tuning options have been carried out, tyres, braids, screws etc there is still the necessity to balance a model for the track. Weight has to be added in the right amount to the right place. Modern 1/32 scale slotcars come in a variety of weights but the 80 to 85 gram mark seems to be a good average. On wood this is not a bad weight and will be near optimim but on plastic where the tendency to tip, slip and slide is greater then weight is going to be required.

An example would be a sedan with an 18,000 rpm motor, weighing in at 85 grams, front axle/guide weight might be 35 gms and rear is 50gms. This will give a front to rear split of 41% / 59% a good split for board tracks. On plastic the split would be OK but the weight of 85 grams would be below optimum and the tendancy to slide and spin the wheels would still be apparent. It would be necessary to ad weight to the 100/110 gm point to start to achieve some meaningful stability.

So, were to put it. Most of my models carry the front weight on their wheels so I prefer to have a 45%/55% split and I would add weight BETWEEN the axles til the desired balance is achieved. The weight can be added in the centre of the model or divided and added each end inside the axle area. The weight can be initially fixed with double sided tape and once fine tuning is complete it can be hot glued into its resting position.

There is always a trade off with adding weight. Needless to say, straight line speed will be compromised by any weight added. The loss of SLS has to be outweighed by increased cornering ability. The best indicator is fastest lap timing. Run the model for twenty or thirty laps to determine its best lap time and then make adjustment until the lowest time is achieved. For a model with its weight resting on the guide shoe there is an added drawback. The extra weight up front will increase drag at the front end and cause the model to snowplough in bends, that is to say, the extra effort to push the front round corners will cause the rear end to misbehave.

Steel railed tracks have the least resistance. Copper tape is second and braided rails have the highest resistance to movement when using braided pickups. In these cases it is better to shift the weight C of G towards the back a bit and reduce the front/rear split to 35%/65% but no more. It will be a case of experiment. Models that carry the front weight on their wheels have a lesser resistance to movement and the balance can be kept around 40%/60% to 45%/55%.

Another part of the equation is the motor power. A Ninco NC1 motor or an SCX RX41 (and any 15,000 rpm/12v motors)are not balls of fire and the extra weight will be needed for cornering whereas motors in the 25,000+ rpm range will need ballasting to reduce wheelspin, taking the weight a little closer to the back. ie 40%/60% or in some cases 35%/65%

And as if that's not enough! model/track requirement will alter the adjustments also. A fast track with long straights will benefit from a slightly lighter model than twisty circuits that require the cornering charcteristics to be greater. For the serious contender you may find yourself with two or three different models in each class to compete in various conditions.

Finally, the simplest way to weigh the models is with a set of modern digital kitchen scales. These have a flat surface and a 'TARE' button which helps zero the scales. Weigh the model by placing it squarely on the top of the scales. Next, weigh the front by allowing the guide to sit on the CENTRE of the weighing area, NOT to one side, this produces an inaccurate measure. Support the rear of the model on the horizontal plane with something fixed under the rear wheels. I use CD cases stacked to the best height. Then weigh the rear by placing the rear axle over the CENTRE of the scales. Percentages can be worked out simply:

ie.<center>

Total weight 85 grams
Front/ Rear weight 35grams/50 grams

Divide 35(front weight) by 85(total weight) and multiply by 100 = 41.18%
Divide 50 by 85 and multiply by 100 = 58.82%

So, an approximation is front weight 41% rear weight 59%


Happy body tuning!
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I'm out in the back yard, burning food!!

When I win, it's because of my talent, not my car or my controller!
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