Rear Axle chart

[hatman]

Hey all. I did some research on how the back axle on a go kart works and i would like to share it with you. In KRP there are seven different options to choose from and i'll make a try to explain the differences on them in the following sentences. First of all we must know that 'rigidity' is the key word behind the axles and this is the basic parameter of the axles. If an axle has high volumes of rigidity or in other words "moment of inertia" that makes this axle stiffer. This condition refers to a more stable back (more rear grip). You can say that the harder the axle (high rigidity) the kart tend to understeer. And vice versa the softer the rear axle the more oversteer. I don't want to say a lot so i will make a summary from 2 different sources from which i have put in perspective 4 different parameters, including outside diameter, wall thickness, rigidity and weight) of the different axles :

*  before you continue note that the numbers below are for comparison purposes mainly and they may be differ from real life. 

(rigidity min. 86 max 789)

axle diameter/ rigidity (hardness)/ weight
50 soft = 348 rigidity          (2,38kg)
50 medium = 435 rigidity    (3,28kg)
50 hard = 740 rigidity         (4,57kg)

40 soft = 240 rigidity          (1,89kg)
40 medium = 300 rigidity    (2,75kg)
40 hard = 511 rigidity         (3,57kg)

30 hard = 271 rigidity         (3,00kg)
--

Remember that rigidity means hardness and hardness means more grip (understeer) so you can deside what axle to use in any case. For example, if you are currently using the 50mm hard axle and you feel that the kart has generall understeer your second choise must be 40mm hard. Going from 740 rigidity to 511 will propably fix your problem, if not then you may try the next option (50mm medium) with 435 rigidity. Note that the weight is an important parameter here - i dont know if KRP took it into account but in real life plays a huge role in seeking the 0.1 sec of your lap times. In generall, if 2 axles have about the same rigidity then the axle with the bigger diameter weight less. Just look above and compare the 30 hard with the 40 medium axle. The 40 M weights just 0,25kg less even though it has greater rigidity than the 30mm hard. So, your final axle choise will help you to fine tune your chassis and drop more your lap times. Hope this helps

Oversteer --> 40 soft --> 30 hard --> 40 medium --> 50 soft --> 50 medium --> 40 hard --> 50 hard --> Understeer

[LIAM-36-AUS]

Wow, thats Awesome, Thanx 

[EVO]

great write up.. i thought 50mm medium was the next step down from 50mm hard but now i know is 40mm hard thanks!!

[nmpcs]

great post

[hatman]

Hey guys, im happy knowing that i could help you with my effort. This is a very long chapter and contains many physics and dynamics which are very confusing but with a lot of reading and a little effort we'll try to explore the "mysterious" world of go kart axles.
As i said before when you hear about axles the first you may think is about rigidity. Actually, the rigidity is the parameter taken from 2 other parameters, which are the outside axle diameter (first number, 30, 40 or 50) and wall thickness (50mm in many cases have 2mm thickness, 40mm have 3 and 30mm have 5..) So, the higher the diameter and the wall thickness the higher the rigidity of the axle and this make the axle very stiff and hard with high moment of inertia. What that means in practise? That actually means that a very hard axle, because of the high inertia, it won't tend to bend (don't forget that axles are actually working like torsion bars but in a different way) and when you are loading the rear of the track (when accelerating or turning) the axle doesn't want to absorb the pressure and all that pressure finally goes to the wheels. The more loading to the wheels, the more pressure to them and the more pressure the more they get hot! Now its clear that in cold days or in raining conditions we must use a stiffer axle in the rear (to raise the tire temps). If u go softer you actually let the axle to absorb an amount of the load taking place while the weight transfers to the back and now the wheels and tires taking less pressure than before. Less pressure means less traction and the tires tend to slide. Try move your finger on your desk once relaxed and then forcing it against the desk. Obviously in the first case your finger will slide better with less friction. The tires work about the same. Now we know why if we are looking to reduce the understeer we must use a softer axle. In a hot summer day i may use a softer axle. As the track gets more traction lap after lap (more rubber) i may go softer to let the tires slide better and gain more speed in the exits. Another parameter which in my opinion isn't so important to consider about is the length of the axle. Generally, the more length the more rigidity (keeping all the other parameters the same). Comparing two 50mm hard axles with the same wall thickness (2mm), the 1040mm length gives 769 rigidity while the 1000mm gives 740. So, to summarize, having two different axles but with the same rigidity will act about the same. For example, a 50 x 1,5 is approx. the same as 40 x 3,5. Which will be your final option? Well, it's up to you. The final two parameters (weight and material strength) must be taken into account before install the new axle in your kart. 

Info : a rental go kart uses a 25mm solid rear axle of near 5,5 kg weight..! Material strength in that case is important.

[hatman]

great write up.. i thought 50mm medium was the next step down from 50mm hard but now i know is 40mm hard thanks!!

Thanks Evo 
To tell the truth the difference between 40 hard and 50 medium isn't quite noticeable (rigidity and weight are about the same). Personally i prefer starting with 40 hard in Lonato and when the track gains rubber i go to the pits and change to a 50 medium. I found in that way that my tires aren't struggling for grip when i start. You may also try begin with 50 hard but i dont prefer to mine because im having troubles in the first laps on most corner exits, the kart is pushing badly.

[The Iceman Marco]

Thanks for this interesting post 

[Piers Prior]

yea rly good thanks

[oppolo]

Oversteer --> 40 soft --> 30 hard --> 40 medium --> 50 soft --> 50 medium --> 40 hard --> 50 hard --> Understeer

Piboso could put them in this order in the setup menu, I don't know the english name, in italian is "menu' a tendina", from up to bottom from the hardest to the less hard, It would be easier for us to choose the right one

[LIAM-36-AUS]

Oversteer --> 40 soft --> 30 hard --> 40 medium --> 50 soft --> 50 medium --> 40 hard --> 50 hard --> Understeer

Piboso could put them in this order in the setup menu, I don't know the english name, in italian is "menu' a tendina", from up to bottom from the hardest to the less hard, It would be easier for us to choose the right one

+1

[Nathan Dunnett]

Is stiffness the only axle variable in this game? Nothing about dampening properties or resonant frequency yet?

[EVO]

Is stiffness the only axle variable in this game? Nothing about dampening properties or resonant frequency yet?

say wut?  please explain.

[Nathan Dunnett]

Well, regardless of the grade of steel, the cross section of the axle determines the stiffness of the axle in a static environment. By stiffness I mean how much the axle will deflect under load. You can gauge stiffness by using Young's modulus, but this only tells half the story. Different grades of steel will have different damping properties and then you have the resonant frequency (aka natural frequency) of the axle, which is effected by the cross section of the axle, the length of the axle, the grade of steel and anything that is clamped to the axle (hubs, sprocket/brake carriers, bearings). The resonant frequency effects the way the axle behaves under torsional load, however it is not totally understood how it works. From our own small testing, a higher resonant frequency makes it feel like a softer axle. This is why some people shorten their axles without changing anything else, and I believe this is why a soft axle and a hard axle of the same dimensions feel very different even though according to Young's Modulus, they should have the exact same stiffness. This is a fairly controversial topic when you start talking to engineers and kart mechanics, not very many people agree, but everyone agrees that each of these variables make a difference.

[oppolo]

I think if there would be a formula for all you said, Piboso would have applied it. But as fas as I understood from your speech, there isn't, so maybe in the game there is only how hard is the axle, independent from his lenght, but obviously only Piboso can explain this if he has wish.

From the beginning I bought this game, my tought has always been: why axles move where they are attached to the wheels? It almost seems that the physic engine is modeled on a car but with very stiff suspensions, the final result is anyway good but, another tought: why not get rid off that awfull grafical effect? I always knew that the graphic is detached from the physical, if it were so, why keep that effect?

[EVO]

does the resonance apply to why some karters loosen or remove the bolts from the third bearing effectivly  making it softer