many thanks for the entertaining response everyone! much appreciated, so I think the facts are the comer topkart rims are not actually magnesium, having a google around I can see the sizes are 112mm front and 140mm rear......I'm not sure if these measurements are taken inside or outside the beads, either way it is wider than the 130mm (outside the beads) that is now most common in honda cadet.....so my logic would be the wider rim will present more tyre rubber to the track due to a bigger tyre radius (less curvature), but I guess its debatable if that would make it run cooler or hotter, my suspicion would be cooler, but I I will try it as I have a set of 140mm rears ready to try if the sun really starts to cook the track......I am also inclined to agree that the 3-spoke nature of these mustard comer top-kart wheels will also lead to more wheel deflection when compared to a JET rim without spokes, which is probably quite a consideration on a Honda Cadet whereby the engine is very heavy.
Regarding thermal conductivity of Magnesium versus Ali, as suggested I have taken a look.......a typical die-cast grade of magnesium is AZ91D or B, the thermal conductivity is 72WmK (Watts Per Metre Kelvin).......I suspect the JET alloy rims are just die cast or sand cast, A356 / LM25 which has a thermal conductivity of 151WmK.......so this would imply, by quite a margin!, that the mag alloys will be a bad conductor of heat.......I have just double checked and the magnesium alloy wheel industry does certainly use the AZ91 grade......perhaps I'm missing something?......but based on the above the magnesium rim cooling theory seems like an old wives tale.
Furthermore, we had a set of freeline mustards on our last cadet kart and they had a very thick enamel coating that would have been quite a thermal barrier (insulator) to the magnesium, therefore rendering the use of magnesium pretty much useless.......but! perhaps going off on a tangent here...... I notice last year several F1 teams were using special coatings on "magnesium" (god knows if they are or not) rims in order to better help conduct the heat from the brake discs into the rims, then tyres....... this apparently keeps the tyres warm on the straights and prevents tyre cooling, they even put a load of grooves/dimples inside the rims to increase surface area near the disc to allow more heat conduction.......check out these links below, quite interesting me thinks, but questionable if this is relevant to karting.
http://www.f1technical.net/development/433
http://www.nanopromsrl.com/inside-tech-alla-scoperta-del-polysil-il-nuovo-rivestimento-interno-dei-cerchioni/
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