Just wondering what are the pros/cons of each?
Anyone have first hand experience on both?
I’ve never done a back-to-back of same size/shape hub with the different materials, but the theory that I hear most often is that aluminum hubs will give slightly more grip. Personally, I believe that hub length and shape, which can both impact stiffness, have a bigger effect on handling. Some brands use exclusively one material or the other, and some use both.
My strategy is to have a range of tuning options, based on how they impact the handling of the kart. For me that means 92mm (mag), 115mm (mag), and 125mm (aluminum). I’d consider getting some 105’s, but doubt that I’ll actually have a need for them. I’ve always found the sizes listed above to get the job done for the karts/categories I’ve raced recently.
I would default to the manufacturer recommended baseline, as well as what they recommend changing for a given handling issue. If you already have different hubs in “x” material then I’d say that’s good enough as a starting point.
I would guess that hub material has a pretty minimal effect, if any on actual handling. The amount of flex in a rear hub is probably close to zero, so a stiffer or softer material will do almost nothing in terms of altering the handling.
I always thought of mag components (other than wheels) as being a weight saver and a cosmetic thing.
Hubs, bearing carriers, brake and sprocket carriers. You could argue that mag hubs control heat transfer between the axle assembly and wheels I guess.
I thought the material difference would play a more important role on heat retention.
Here’s an excerpt from TKART magazine, it’s about rim material - but I’d imagine it’d also apply to hubs as well …
Head: THE RIGHT RIM TO MOUNT ON THE KART
BASED ON THE GRIP OF THE TRACK
Expert: Albert Viglino, OTK engineer
Copy:
"Another “classic” question: what rims do you recommend on wet tracks? Magnesium or aluminium?
The peculiarity of magnesium is its ability to dissipate heat and, therefore, keep the rubber and air contained inside it cooler
Vice versa, aluminium is a material that tends to increase the temperature of the air between the tyre and the rim, thereby increasing the rubber’s pressure and temperature. What, with cold and rain, helps trigger grip on asphalt. With a wet track the choice therefore falls on the AXP model, the OTK aluminium rim"
I think the material difference is much more relevant with wheels. Think about the amount of surface area shared by the wheel and the hub as a percentage of total wheel surface area. That percentage is pretty small, which would lead me to believe that although hub material might make a difference in terms of heat exchange, the difference is likely negligible.
Louis I have heard the same thing on heat retention. Evan, your point on amount of shared surface area is well made. To this end, the guys at AMV have a hub with matching wheel/hub holes to improve air flow and hence heat dissipation across the components. You can see it in these photos 
Here is the BirelArt set-up sheet which recommends both aluminum and magnesium for different conditions. The problem that I have had with short 70mm hubs is with cut axles they don’t leave a lot of axle for the hub to sit on / fasten to.
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I’d be curious to test those back to back to see how different wheel/hub combos function. At some point though I think there’s something to be said for having too many variables on the table, especially for the “average” user.
Provided you’re starting with a good kart, and a good baseline setup, I think it’s easier for most people to tune themselves out of contention as opposed to tuning themselves into contention. I know I’ve done so myself plenty of times.
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Evan, excellent point about tuning hell and too many options. I have 3 go to track set-ups. Low grip, standard, and high grip – from there I usually don’t have to make too many adjustments. Here is what I use for my Compkart Covert 3.0
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Thanks so much for all the responses guys.
What about in terms of weight/rotating mass? Does alum vs mag matter in this regard?
Lastly…anyone have any experience with either the swift or wildkart alum 50mm rear hubs on an otk chassis?
I csn speak from my experience i had. I was using my gillard kart. And the difference between mag and Aluminium was huge. We had a harder axle inside and with mag hubs it was only sliding and same size aluminium was no problem
Interesting. Got anymore details on the track conditions?
It was in goid condition. Teack was pretty grippy. I have a dd2 and the frame was pretty stiff by the layout. I used to drive a hard axle. My alu hub broke and i had only a set of mags as spare. Changed nothing on the setup, only adapted the offset for the differebt hubs, but kept the same width and i had absolutely no grip. The opposite was on my senior before. So i would say it depends very on the frame how it reacts. I would try out and see hiw it is for you
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You could say we had this question pop up last weekend for us. We run the CompKart TAG 3.0 2020 and raced a fairly cold and over cast day with some spitting rain. We tried tyre pressures but were not getting the rear grip we wanted so with out redesigning the chassis we opted for Alloy Hubs on the rear 50 x 90mm log and was just what we looking for that was mated to a medium rear Axel running around 1385cm wide. during the earlier heats Kart was fine but with the cooler evening temps dropping the alloy hubs did what we wanted them to do.
I’ve always understood that unsprung weight is bad.
On a car sure… since more unsprung mass means that each wheel reacts slower to rebound from the suspension, and so it spends more time in the air.
But with no springs or suspension (other than tire flex)… how does that apply to a kart?
Spindles flex, axles flex, frames flex, seat supports flex.
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