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Wheel Hubs - The Long and the Short of it

Wheel Hubs - The Long and the Short of it
Category: Comet Technical Karting Blog
Posted: 07-20-2014 18:51

Wheel hubs are a very important part of tuning your kart but there are a lot of options available too. In this Technical Blog we help explain what the difference are.

“So what’s the deal with wheel hubs” you may ask. There are long hubs, short hubs, medium hubs, aluminum hubs, magnesium hubs, this hub, that hub, hubs, hubs, hubs! Well let’s dive in to the varied world of wheel hubs and figure this out!

Rear Wheel Hubs - As a tuning device on a kart, hubs can change the kart a little or a lot depending on the track conditions and chassis type. The typical rule of thumb is a longer hub is more grip. The hub stiffens the axle underneath it so the longer the hub the more this affects the axles ability to increase grip. It’s like putting in a stiffer axle to a small degree.

Like many things in karting these days it kind of depends on the kart and the class if the longer hub adds grip to the kart. In the classes that use the 30mm and 40mm axles typically the longer the hub the more grip the kart produces. Usually in the lower horsepower classes you are trying to take as much of the grip out of the kart that you can but when the track is green, you're running a harder tire or you are running a faster class like Mini Rok or Mini Max, a longer hub can be a good way to put more rear grip in the kart.

My experience in the 50mm axle classes is a little bit different. I find hub length changes affect the kart very little. If I feel that I need to make a hub change I am usually better off to change the entire axle or cut the axle. I think the diameter of the axle really neutralizes the affect the hub has on the kart. When we were running 40mm axles in the Yamaha classes, hub length was a noticeable change but these days I rarely mess with the hub length in the 50mm classes. I usually run a medium length hub and tune around it.

A better option if you are looking to change the karts setup more drastically than the hub is to cut a little bit off the end of the axle. Between 5mm and 10mm per side. Cutting the axle makes the hubs (assuming you keep the rear width the same) action on the axle much different. Now the hub is more effective because it doesn’t have as much axle underneath it dampening its effects and the hub has more leverage on the axle. Think of it as pushing on something with 4 fingers and then only using 1 finger, you concentrate the same amount of force onto a much smaller area. A cut axle can help the kart transfer better and free up the rear of the kart, especially in high grip situations. You can also just cut one side of the axle instead of both sides if you are only trying to fix the kart in a particular corner or corners that are the same direction. Some manufacturers offer axles in different lengths so you don’t have to cut the axle, you can just put in a shorter one.

Front Hubs – Front hubs act a lot like rear hubs, the longer the hub the more grip they produce. Sometimes this is good and sometimes this isn’t so good. In a lower horsepower class a lot of front grip is just as detrimental as too much rear grip. A kart can definitely be “tight” on the front and not roll properly off the corner. General rule of thumb, the faster the class the longer the front hub and to go along with that, the bigger the spindle shaft. So in a lower horsepower class like Yamaha Sportsman we typically run a 17mm front spindle with a DSM wheel which doesn’t even have a bolt-in hub! The DSM wheel helps keep the front of the kart free and produces less grip so the kart rolls better off the corner. Now in a TaG kart we typically run a 25mm front spindle shaft with a 70mm or 80mm long front hub, which means lots of front grip!

In the rain you want a lot more front grip in all classes, so even with a Yamaha Sportsman kart we will run a longer front hub so we can widen the front and help the kart transfer weight and turn since the track will lack grip. So depending on the conditions the front hubs we will want to run can change though once we have a good baseline it is rare that we need a lot of options. 

Hub Material – Typically hubs are made from either Aluminum or Magnesium. The different properties of the materials change how the hub works. Aluminum is a denser material than Magnesium which is a softer and more porous material. Aluminum hubs tend to be stiffer than Magnesium but the porosity of Magnesium helps the material to not hold heat as much as Aluminum. Because Magnesium doesn’t hold heat as well as Aluminum this can help the wheel and tire not hold as much heat over a long run. Whether or not that translates into more consistency over a race is up for debate but it’s worth thinking about.

Hub Shape – Hubs can come in a few different designs, especially on the stud end where it mounts to the wheel. Some are a full circle; some are cut out in a clover leaf type shape. The shape changes how the hub interacts with the wheel. A full circle has more surface area touching the wheel surface and stiffens the assembly a little more than the clover leaf type shape. Like most things in karting it’s hard to say if this has a big effect on the karts handling but it’s another factor to consider.

Another shape is the thickness of the hub and the amount of pinch bolts. A thicker hub will be stiffer than a thinner hub and a hub with 2 or 3 pinch bolts will clamp more on the axle and will affect the axle over a longer span than a hub with only a single pinch bolt.

A final variable to consider is the amount of hub material inside the hub that actually touches the axle. A lot of hubs are full length; they are the same inner diameter from one end to the other. This is pretty much the standard but some hubs are milled out on the inside and only a little bit of the hub, the area closest to the pinch bolt, actually touches the axle. This difference goes back to the idea of the cut axle; with a smaller portion of the hub touching the axle the force that is transferred from the hub to the axle is concentrated on a smaller area. Also less of the axle is covered by the hub so the axle isn’t stiffened as much as it would be with a full length hub.

To recap:

All Hubs –

Aluminum versus Magnesium – Aluminum is a stronger, less brittle material than Magnesium which can make an Aluminum hub stiffer but the porosity of Magnesium helps it not build up as much heat as Aluminum. Magnesium is also marginally lighter than aluminum and might allow you to save some weight on your kart.

Hub shape can affect how the hub interacts with the wheel. A circular end has more surface touching the inside of the wheel than a clover leaf type design.

Rear Hubs –

Classes that run axles smaller than 50mm, a longer hub is generally more rear grip. A shorter hub is less rear grip.

Classes that run 50mm Axles – A longer hub can produce more grip and a shorter hub can take away grip but it is generally more beneficial to change axles or cut material off the axle to change the handling.

Hubs that are milled out on the inside touch less of the axle and will change the handling of the kart over a hub that is a single diameter throughout.

More pinch bolts are stiffer than a hub with a single pinch bolt.

Front Hubs –

Longer front hubs will produce more grip and transfer weight. If you have “too much” hub on the front it can make the front of the kart “tight” and hurt the corner speed of the kart.

Longer front hubs are pretty much a necessity in the rain for most karts to help them turn and transfer weights in wet conditions.

I hope that better explains how hubs can and do affect your kart. As always testing is the best way to learn what works (or just as important, what doesn’t!) and taking good notes, whether it’s a practice day or a race day, is always highly recommended!

Mark Dismore Jr.
Comet Kart Sales

Digital Momentum

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