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Stop the hop!!

How DO they do that??
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wixwacing
Marshal!!!
Posts: 1871
Joined: Thu Jul 10, 2008 9:22 am

Stop the hop!!

Post by wixwacing »

Last evening at the Red Team's local venue I decided to take along a problem car and cure its annoying habit of driving straight on at the end of the straight. The car is a recent addition to my stable but is an old favourite I have wanted for some while. The model is the first SCX Celica pre magnet with 4wd. The problem affects several of the early pre magnet 4wd cars from SCX especially the GT4 Celica, Subaru and the Bugatti EB 110.


On approaching a corner from a fast straight, and whilst under braking, it is possible to see the front of the model hop up and down rapidly. On a board track this will be accompanied by a juddering noise. If braking is left to the very last minute the model will hop out of the slot. Most infuriating and enough to put you right off your stroke. Racing cars that do this require a special technique of throttling on early at the entrance to a bend to minimise the risk. This in itself is risky and shouldn't be necessary! Fellow Red Team racer Garry pointed out his theory to me and the fix he had made to one of his GT4 Celicas.


We have variable brakes on the Red Team's home track. This is achieved by wiring a fifty ohm wire wound pot into the track brake wire. Brakes on a host of models can be set to a comfortable action simply by doing a couple of test laps and some fine tuning on the brake pot. One of the benefits is that when racing a variety of models in the course of an evening, it takes a little while to adjust to the braking distances of various motors. The Scaley 'S' can has a longer run-on than say, the SCX RX series, which virtually lock up the axles! Going from a Scalextric model to an SCX will see you braking well before a bend or corner and loose time reaching the corner. Backing off the brakes gets the model to drive more like the Scalextric 'S' cans and gets them to 'flow' more over the lap.


It had been noticed that our problem models showed less of a tendency to hop out of the slot when the brakes were backed off a little and indeed, further reduction in braking effect saw the models behave perfectly, but the braking distances into corners became extended as expected. With the body off the model it was observed that two conditions were probably contributing to the phenomenon. Firstly, the amount of slack between front axle and bushes was considerable. Secondly, the amount of slack between the front axle bushes and the body clips was unacceptable!


The model chassis was set on a test block and with the chassis resting on all four wheels a small drop of superglue was applied to the front bushes and allowed to set. Further testing showed there to be a reduction in the hop and judder. More observations were made and it was becoming obvious that another problem was the fact that the motor was not tight in its mounts and under brakes, the front of the motor and the front shaft were lifting in a springy action giving rise to the hop.


Firstly the motor was glued in then an old nylon Scalextric axle bush was cut through one side and pushed over the front motor shaft. This then had a small piece of plastic packing placed between it and the chassis and the whole was hot glued in place. After it cooled, the chassis was placed back on the track and re tested. Perfect, the model was now able to drive into corners with impunity. The model now behaves as well as any 4wd and its deslot incidence rate has been reduced to driver error only.


Another tip for these models is to put the best tyres on the front. The larger diameter allows the model to pull itself round corners rather than try to snow plough the front end along and contributes to its road holding in bends.
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When I'm not racing slotcars,
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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