Recent catastrophic failures on Cadet engines have highlighted that there is a problem and a real risk of injury to a competitor, parent, bystander or mechanic involved in this class. In particular, the flywheels on a number of engines have disintegrated causing serious damage (thousands of Rands worth) to the engine and the risk of a piece of shrapnel passing through the plastic side cover and causing serious injury to anyone nearby should not be underestimated. (see attached image of broken flywheel).
We started by detailing the failures to the manufacturer, Comer Spa, who responded that excessive RPM was the cause of the problem. The engine was not designed for such high RPM and should not be used this way. Their recommendation was that we should fit an inlet restrictor – as is done in many countries – notably the UK and United States.
We contacted the Zip in the UK, who run the very successful Bambino class, and they confirmed that they run stock C50 engines fitted with a 10mm diameter restrictor and very importantly an 80-tooth sprocket!
We have conducted extensive testing on a dyno as well as some track testing at Killarney and Zwartkops.
History and root cause of the problem
The Comer Top kart was introduced in South Africa around 1996 (Baby Karts) and has introduced thousands of youngsters over the years to MSA Karting – many have become big names in circuit racing by now!
The first karts arrived with 89 tooth sprockets and this was simply incorporated into the rules without any testing. As the years went by blue printing, ostensibly to equalize the performance….. progressed and the once stock engine has now developed to rev higher and higher and the rules have been “relaxed” to allow this. Cone type air filters, open exhausts, bored inlet manifolds, cut pistons etc. sees the fastest Cadets lapping some four to five seconds a lap faster than the original ones!
- To illustrate the added stress on the engine one simply needs to look at increase in average RPM where say at Vereeniging originally lapping at 76 Seconds per lap with an 89 sprocket on 1140m circuit with 86cm circumference tyres the average RPM was 9313 RPM. Today lap times of 70 seconds see the same basic engine averaging 10112 RPM!
- Looking at the power curve of the C50 motor the “taller” gearing makes perfect sense – the engine has very good low speed power and weak top end. With the current 89 tooth sprocket the only time that the motor is in its power band is on the warm up lap – it is over revving throughout the race!
- In other words, tuners and competitors spend their lives trying to get higher RPM. Those that succeed will win but suffer catastrophic failures. For the less technical people a good analogy would be travelling in your car on the open road stuck in second gear instead of using top gear – you will destroy the engine.
- Change the sprocket to an 80 tooth. We have tested this successfully at Killarney and Zwartkops.
- Introduce an 11mm diameter inlet restrictor.
This combination reduces the average RPM by approximately 1100 RPM with a very minor increase in lap times. The low speed power out of the slow corners is good and still easy for a beginner or expert.
- Current “good” or “special” engines will remain exactly that and this vital change will not disadvantage anyone.
- With fine tuning, we may find that the lap times improve again to the same or faster than before the change.
- The reliability issues should be resolved by reducing the RPM but only after an extended period can we really asses if this is enough. The reason for this is that the vibrations from the excessive RPM may already have damaged existing components thus we may still experience some failures initially. Only monitoring new flywheels (for example) will we be able to confirm that this is enough of a reduction in RPM.
- The MSA Karting Commission reserves the right to change the diameter of the restrictor and or size of sprocket in the future should the problem persist – safety is our priority.
The Cadet class has had its ups and downs as it passed from one importer to another over the years. In good times, we have seen fields of 30 plus karts on the grid but currently we see Cape Town for example at an all- time low of 5 competitors and Gauteng at a dozen or so. Improving the safety and reliability is the first step to rebuilding this.
It is proposed that this specification change will take place on the 3rd of April 2017 but this will only be official once confirmed by an MSA circular.
082 557 7403