# Reduced rear axle length to improve grip. Embedded video explaining

I know it is counterintuitive but the output I had after a lot of thinking and some tests I did in and out of the tracks is that the axle length influences the load transferring efficiency in a different way the intuition suggests. I mean, the shorter the axle, the less efficient the load transfer. In other words, if you cut the axle you will decrease the load transfer between inner and outer rear tires as soft axles do. And if you reduce the load transfer, you improve the rear grip (most of the time).
The first time I started thinking about why people cut their axles, I thought that the result would be a more rigid (less flexible) axle. But after thinking deeper, making some maths, and hearing the mechanics I changed my mind. My mechanics always says that “shorter axles make the inner rear tire return to the ground earlier” and it improves grip. But, as a good curious engineer, I always want to understand the physical dynamics in each situation and different setup.
It is intuitive to think that, a shorter axle will be harder to flex. It happens when we cut a bar and try to flex. The shorter the bar, the harder it is to flex, right? But, as some KP members already said, kart things are not obvious. When we cut an axle, we are not reducing the section of the axle that will flex. The length between the bearing and the hub, which is the section of the axle that will be submitted to flexion, remains the same.
So, what is the explanation I have to confirm that the short axle loses efficiency to transfer loads?
When we cut an axle, we are reducing the amount of axle that will be inserted into the hub. So, we are reducing the area (within the hub) where the axle supports itself to lift the other end. With reduced area to anchor, the axle reduces its capacity of pushing the set of hub/wheel/tire against the ground. The same situation happens with a flexible axle. Flexible (“soft”) axles have less capacity to push the set hub/wheel/tire against the ground when compared to a rigid (less flexible) axle. If less amount of load is transferred to the outer rear tire, it remains in the inner rear tire, resulting in a quicker return. It is the reason the mechanicals says the shorter axles make the inner rear tire return earlier to the ground.

In theory it is explained, but I wanted to test this concept somehow, so I´ve made a domestic simulation of what happens with long and short axles into the hub.
Here is the video of the homemade test. I recorded it just for me but I decided to share it, so don’t remark the way it was recorded.
The purpose of this video is to confirm that the reduction of the support area reduces the load transfer capacity.
As the video is in portuguse, I included an english subtitle.

Besides the reduced capacity of the load transfer, with less axle within itself, the hub loses rigidity and it influences the angle of the outer rear wheel when submitted to load in a turn and consequently the section of the tire (inner, middle or outer) that will be more demanded.
So, there are at least two effects influencing the kart’s behavior when we cut the axle.
I want to know your thoughts.

Ademir, great contribution. Here’s my feedback.

It’s interesting that there are opposing views on what the IR does when the axle is cut, some say the IR is set down earlier but most say the IRL lift increases. It seems there is good agreement the OR has more “side-bite” and the kart has better rotation (changes directions quicker?) when the axle is shorter. (I try to use the term “side-bite” when discussing the state of a tire and grip typically with the state of the kart. We need a standard karting dictionary.)

If you are saying that axle system flex affects final steady state load transfer to the OR, I don’t think is quite right. The OR final load is a function of mass, center of gravity and lateral acceleration and not flex. The theory I believe in is the OR tire runs flatter on the track because there is more local deflection (kind of a pivot point) at the hub, which results in the tire contacting the track flatter increasing the side-bite on the OR. More OR side-bite would then translate to either more and/or an earlier IRL, which would improve rotation.

A finite element analysis would definitely help shed light on this nerdo discussion.

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