A soft axle will allow the outside tyre to remain flat on the track, this is good on high grip tracks as the inside comes down faster for more drive out of the corner.
On a low grip track it will just slide
What you’re doing with a stiff axle, is keeping the outer wheel on a straight plane with the axle, this allows the inside of the outer wheel to lift up, reducing the contact patch between tyre and track. Smaller contact patch equals more force pushing down in terms of pounds per sq in on the contact patch, this allows the tyre to bite and hold rather than slide
the option for freeline front wheels is to find 2 rear 40mm hubs from birelart. on maranrllo the original hubs cannot become
Contrary to my earlier list, yes higher tire pressure. Sorry about that.
Soften or stiffen the chassis - no idea. Watch @tjkoyen Karting 101 video series. They are very helpful.
As for axle, I would still try softer. At New Castle, when we were looking for rear grip, we went softer and it worked. Guys running the same chassis (compkart) were even one axle softer than I was. This is X30 on MG Yellows.
My understanding is in shifter they run softer axles typically. Again, no direct experience, just what I hear.
@Richard_Jacques I am sure you know more about this than I do and your diagram is great – the concept makes sense. I found this and keep this in my note from Cash Van Belle (IPK team manager) on how to approach a very Slippery Track – a more flexible axle is needed; allowing the rubber to work in your favour, since it gives more grip on the asphalt.
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Anyone have general observation about what is typically required to deal with excessive rubber build up? It’s not a problem we have in states much, but I recall one of the Nats having stupid amounts of rubber built up and folks struggling with that.
Free it up and change your line as it rubbers up. 
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Exactly. Drive below it and keep the kart rolling.
You can’t drive on top of that, it’s like driving on marbles. It all just rolls up and the kart washes out.
That’s New Castle during one of the Pro Tour races. I had a video of it where it pulled my shoes off my feet when I was walking on it.
Back on topic…
@Smokara1
So presumably with different wheels what you’re trying to achieve is inducing more “grip” into the kart due to the lack of rubber on your track, yes?
For low grip you want to increase tire pressure to bring temps up better. Tires do not heat up when there is no grip on the track, so you have to get them up to temp with higher pressures. A different wheel would certainly help as well, but I can’t say specifically what difference these two wheel sets would make. Surely CRG/Maranello produces a higher and lower grip wheel?
For overall “grip” on unusually low grip surfaces, it may take some trial and error to produce the results you’re looking for. There are two ways you can approach it:
- Increasing weight jacking and inside rear wheel lift to get the kart to dig and produce side bite
or - Softening the kart up and keeping both rear wheels on the ground more to provide traction
Both can work, but it will really depend on the grip level. If it’s super low, sometimes reducing lift and keeping the kart flatter, squatting out of the corners is the only way to get the kart to remain stable at apex.
If the track is a little grippier and you struggle for “grip” from the kart, you can start inducing weight jacking by increasing caster, raising rear ride height and the seat and try to get the kart to generate side bite. The issue here is that sometimes the track simply cannot provide enough grip and the you just force the tire into the track harder and harder, and it never can generate the grip you, so it just lifts the inside rear wheel and immediately breaks traction on the outside tire.
Start going in one direction (more weight jacking) and if it doesn’t seem like it is improving, go the other way (reduce weight jacking).
Always remember that we are not adjusting the amount of “grip” the kart has, unless we are changing tire pressures or wheel material. All other adjustments on the kart are adjusting how the kart flexes, lifts the inside rear wheel, and uses the available grip from the tires.
My YouTube video linked above in @Paul_Montopoli 's post explains much more about general chassis setup.
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On a track with very low and medium grip, what should be the position of the seat? On a par with the chassis or 1 cm below it? I am tall and when I make it according to the catalog from the factory my legs are too folded and I can not turn the steering wheel calmly. Can I fasten it back to make it a little more comfortable?
That would go along with my thoughts on which direction you need to go on setup. If you need more weight jacking to get side bite, raising the seat would help, so even with the bottom of the rails would be better. But if you need the kart to sit flatter and generate traction, lower on the seat height would be better.
I wouldn’t mess with the seat too much to begin with. If you need to move it back a bit to be more comfortable, that would probably be fine, but I would focus on making some of the other chassis adjustments first, to figure out which direction you need to go on setup before really playing with seat position. For example, changing rear ride height will basically tell you where you need to go with the seat. If you raise it and the kart is better, you can raise the seat up. If you lower it and the kart is better, you can lower the seat. Better than drilling a bunch of holes in your seat.
I get you, I only ever drive on dusty tracks with low grip. CRG team master mechanic who winters with us always says stiff axle. We only consider switching to softer during a race weekend as the track rubbers up. I’ve had to go from stiff to soft during a practice day because mine bent on a kerb, they only had a soft immediately available, i was sliding around much more.
Is it a big problem that I use the hardest birelart freeline axle on the maranello rs10 chassis? Do I have to buy an original axle for my model?
Love the thread… went from wheel dynamics to chassis setup. Good info in here!
You can use whatever axle you want in your kart, but the question is, how does the Freeline axle compare to the OEM Maranello axle in terms of stiffness? I don’t think there’s a way to know.
That’s why it’s recommended to use OEM parts when you can for this sort of thing because then you know where you are in relation to the rest of the axles in the range.
A Freeline stiff axle might be equivalent to a Maranello soft for all we know.
Some of these things you’re just going to have to test and find what works for you. I ran a TB Kart 32mm chassis for a couple years, and it is of a similar design to the Maranello/Road Rebel.
One general rule of thumb that I found with that chassis was if we were on a smaller/tighter track we would generally be on a softer axle, and for a longer track with sweeping corners we would go stiffer. That said, a low grip setting, Road America in 2019 for example, would generally require a softer axle. I think the Freeline K axle is maybe too hard, and would try to locate an OEM Maranello axle (not sure if they code it as H or M20).
I also had success by softening the kart; run one set of seat struts instead of two, and remove front bar if using currently.
Crank up the tires pressures, add a click of caster, keep camber neutral or slightly negative, 1390-1395mm rear track, 100-115mm rear hubs, and just keep testing from there.
The wheels aren’t going to solve your problem by themselves.
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Not sure @Smokara1 can ask for anything more than this @Muskabeatz. The only thing left is for you to go there and drive the kart for him. 
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I have the original maranello axis but it is a bit curved on one side and I replaced it with the birel axis. As far as I know the axes can’t stand up or if they go up several laps and still bend in the same place. the tracks we drive on are narrow with sharp turns
I would see if you can have that straightened and use it. Ideally you test that alongside one that another couple steps softer, something like a CRG T6.
do you know what hardness my original maranello axis is
That would be the same hardness as the CRG M20, which is the standard axle for all CRG manufactured brands.