Yes. But strain gauging dynamically in the kart would be challenging. A slip ring would be needed for the wiring. And the location of the gauge needs to be determined. Where on the axle would be the best spot to measure.
I say the easiest would be to use a Chatillion, measure force and deflection on a bench with test axles. Then field test in a kart, axle to axle. Laps times and slow motion video in a rear wheel lift generating corner.
Iām no engineer for sure, but I have a slightly different perspective. What about flex under breaking? Under acceleration? With Poor track conditions? In the United States you have to consider the drivers weight.Thereās a big difference between light and heavy drivers.
See I was thinking more along the lines of the chassis. Because the deflection of the axle isnāt necessarily the information that youāre after. But rather the axleās effect on the chassis will give you the answer to whether or not a axle does anything.
Yes, I agree the axle has flexibility in all its length in these situations you mentioned, but in the specific case of a turn, when the front inside wheel pushes the ground downwards, it twists the frame, lifting the inside rear wheel/tire using the outer rear wheel/tire just as a support point.
Below it is represented a driver turning to the left with a less-flexible axle, which creates an angle (alpha) between the outer rear wheel and the ground, and a height (H) between inner bearing (green) and the ground.
When we change the axle to a more-flexible one keeping the front-end setup and the frame rigidity, I believe the height (H) does not change or decreases a little bit because H was obtained by the front inside tire pushing the ground and the rigidity of the frame using the outer rear tire just as a point of support. What I think has a significant change is the beta angle because the section of the axle indicated with the letter A in the drawing is the section where flexion is greater and flexed more than the previous axle. The axle section between the outer bearing (red) and the inner wheel/tire has negligible flexion in my opinion.
So, what I am planning to do in the next training session is to read the temperature of the outer rear tire in 3 points to check how the tire is touching the ground. This way, I will choose axles aiming to change the footprint of the out rear tire instead of changing axles to change the height of the inner rear tire.
That said, if I want less grip in the rear-end, IĀ“d change the axle to a less-flexible one. And, if I want more grip in the rear-end, IĀ“d use a more-flexible axle.
Makes sense?
So you are turning the Axle into a lever of sorts. Putting the fulcrum at outer bearing and the load at the outer tire.
Two points I would want to consider.
In this reference, the axle would bend along the length between the outer bearing up to the wheel hub, not only at the point where the axle leaves the bearing. Widening the rear track would spread the load over a longer distance, but also change the load on the lever. Seems counter intuitive that increasing load side length of lever would increase necessary force to flex the axle and a longer length from could spread that load over a greater distance reducing the flex. There must be some ratio of flex to length cross over to load to flex. Same idea with longer or shorter rear hub lengths.
There has to be some flex in the chassis between the bearings through the axle. Otherwise there would be no reason to add more than one additional seat stay per side. No seat stay would force all the side loads though the chassis in front of the axle. One seat stay on each side effectively turns the bearings and the seat into a triangle leaving only the seat as a point of flex. Adding a second seat stay turns the chassis seat mount, the seat and the rear bearing into an additional triangle changing the flex of the chassis on each side from the chassis seat mount to the bearing. Effectively this would change how each tube from the seat to the rear bumper could flex under loads. May also explain why different seat stiffnessās effect chassis performance. Similarly adding a rear torsion bar can change the flex of the chassis behind the seat.
In conclusion, there seems to be several factors that are affected by an axle that does not flex as much compared to one that flexes more. If anyone does end up doing some realistic testing, I would love to see these points taken into account.
According to the diagrams, wouldnāt you get MORE grip on a softer axle if itās flexing more and more of the tireās contact patch is touching the track?
Great diagrams btw. Interesting discussion. This basically is illustrating how Iāve always viewed hub length tuning differences too.
I tend to agree with Gregās points, especially #2 regarding the flex between the rails. I know there is some disagreement there, but I believe that axle stiffness is affecting the flex between the rails in some way, acting as a torsion bar.
Out of curiosity, has anyone used axle lock collars in lieu of set screws to secure the axle, & if so, did it have any noticeable effect?
I am still new to all the dynamics of Kart Setup, but I learn best by trying to work through things in a somewhat logical progression. Changing that has an affect on this, this, and/or this and so on.
Right !?! It would seem to also shift the center of mass of the driver closer and more over to the outside rear wheel increasing the load on that tire improving grip. I imagine a right triangle from the center of the axle, up to the driverās center of mass and then out to the rear wheel hub. As the kart mechanically jacks while turning the axle begins to flex as it now has to support say 70% as apposed to 50% of the weight of the rear of the kart. The hypotenuse of the triangle begins to shorten as the drivers mass shifts toward the outer wheel from the inertial change. The axle begins to flex more proportional to the Change in Mass acting on it and the distance from the Center of Mass to the Hub.
[quote=ātjkoyen, post:108, topic:986ā]
Great diagrams btw.
[/quote] I second that!!
This makes sense to me. Why else would the same driver have an affect on the same setup only by moving the seat higher in the frame. It changes the dynamic loading on the outside rear. Why does a heavy/tall driver have to lower their seat to achieve the opposite? All the little adjustments like length of axle, length of rear hub and rear track width seem to be fine tuning the legs of the triangle that affect how much Mass is allowed to shift closer/over the outside rear wheel. Change to a Less Flexible Axle and the length of the hypotenuse changes less, not allowing as much shift in the driverās center of mass toward the outside rear and reduces the amount the inside rear can unload. Conversely a More Flexible Axle would yield more with a lesser/lower mass. I would have to think that the shift in the driverās mass has a non-linear affect on the amount the axle flexes. In this I mean the Amount of Mass and the Height of the Center of Mass from the center of the Axle would have a compound affect on the Load shifted to the outside rear tire. Higher/Larger Mass causes the axle to flex more quickly vs. Lower/Lesser Mass causes axle to flex more slowly.
I am also lead to believe there is a limit to how much and axle will flex under load. My example is when trying to cure a Hop. At some point the shift in load over the outside tire will exceed the amount the axle can flex. At this point the contact patch of the tire reduces as the tire tips to the outside shoulder. The tireās sidewall can only distort so much before it must yield to the increased angle (beta). It is at this moment the tire looses traction and load is shifted back toward the center line of the kart and off the wheel. Still under mechanical jacking and inertial force from turning, the load quickly shifts back to the outside tire and the loop starts over. Reduce the load to within the limits of the flex of the Axle and you reduce the Hop.
I think this is true only if you can keep the load within the limits of flex for a given axle. I have heard Tuners telling drivers to switch to a softer axle especially in the rain. Makes sense to me that a more flexible axle would allow more dynamic weight to shift over the outside rear improving the traction of that tire given the a narrower track setting (wide front/narrow rear).
This feels like one of those āAh Haā momentsā¦lol
@tjkoyen after doing a practice session with the rear torsion bar in last weekend, the rear hop I was getting was significantly reduced. Track was pretty Green (from recent rains and dusty from a Saharan Dust Storm passing overhead) and I was on old tires last used prior to Covid outbreak. Chassis (OTK 401) running TaG Sr./Master was set up same as last race aside from adding rear bar. Caster less than neutral and slight negative camber. Ride Height Neutral front and rear. Front Track centered between the spacers. 54" Rear Track. Standard Hubs front and rear. Toe set to 0 as close as I can measure without Snipers (i.e. Tape Measure). Seat (T11t) bottom 5mm below frame rails and leaned slightly back. Total Weight without Lead, 410 lbs. Driver 6ā9" and 220 lbs. Large/High Center of Mass.
Of course there were side affects from having the rear bar in. Chassis felt good on the tight corners aside from a slight shatter from the back on corner exit (low grip scenario), but felt slightly bound on the fast sweepers like it could not pull off the corners as well as before. I am definitely sold on pulling the rear bar and going to to a āharderā axle.
Question is, how hard should I go? Currently on the N. Looks like 3 options above that, H, HD & HH. H looks like it might not be enough, HH could potentially be overkill. I thought with your experience with so many different drivers, you might be able to point me in the right direction. Any thoughts?
From their chart. H looks to be about 20% stiffer than N, HD about 50% stiffer than N, and HH about 80% stiffer than N.
@TJ in a static rig, the deflection would be the same regardless of hardness or softness of axle, presuming the hubs and axle thickness are the same. Just try with a simple weight on the end of the axle and measure deflection, it will be same regardless of axle hardness.
Another way to look at axle hardness/softness is itās a damper/shock not a spring. The spring rate of an axle is determined solely by the mechanical dimensions of the axle while the ādamperā settings are determined by the hardness/softness and alloy of the steel. A damper/shock in a car affects rate of weight transfer similar to the effect or rate of hike.
Greg,
Go H first. I know we donāt all have the luxury of tuning with every axle and it can be tempting to jump two steps, but especially with axles itās important to go one step at a time. Plus, you have plenty of other adjustments yet to keep it settled down if that isnāt enough.
I would throw an H in there and then if it helps but still needs more, lower the rear ride height.
Greg, You shouldnāt have run the rear bar in TaG Masters. Have you ever checked how straight the frame rails are on the OTK? With the weight you are at, depending on the run time of the chassis, Iāve found that the OTK (may be vintage dependent) start to sag in the middle. That sag will cause the chassis to hop when it shouldnāt.
I would also widen the rear to max width.
Like TJ advised, go with the H. I have an HH but never could make it work too bouncy. For axles I use 3. A āU/Q/Eā, āNā and āHā.
Larry
Depends on the situation.
In the situation below I am planning to use a less-flexible axle to obtain more grip in the rear-end and I explain why.
My last training session was on Sunday and I got the following pressure and tire temperature just after stopping my kart:
The track I was training is clockwise, so, the outer side is the left.
The red arrow in the image above shows I had a low temp in the external strip of the tire tread in the left rear tire comparing to the middle and internal strip temp.
It seems to me that the axle is flexing too much and I am not using all the tire tread of the outer rear tire.
Therefore, I am planning to try a less-flexible axle next session on this track or a longer hub.
Information was obtained utilizing Alfano Tyrecontrol 2, a digital gauge to adjust and store pressure and temperature information of the tires.
This image is a screenshot of Alfano Tyrecontrol App.
http://www.alfano.com/new-tyrecontrol2/
Thanks TJ & Larry.
I was about to mention the saggy middle bit and not being able to drop rear ride height. Did not realize this could cause a hop as well. I was not planning on running the rear bar when racing. It was just a less expensive alternative to test if a āharderā axle would make a difference. Last weekendās practice session certainly calmed the hopping down in the tight corners with the bar in and flat. Sounds like an H is the way to go. As I have found in Karting, small changes can have a big affect.
I had a Tony shifter kart and it liked to hop. Try to have the steering straight when applying power. Figure out a line that will optimize this strategy.
Hello @Thomas_Williams, how are you?
Yesterday I was googling about axles and I found the following image from Freeline.
The modulus they mention in it is the same modulus you mentioned in your post, I mean, Modulus of elasticity / Youngās modulus / elastic modulus?
Thatās interesting. But given what I took away from The Efficient Engineer video, is this chart not counter intuitive?
The video explains Yās modulus to reveal material elasticity.
What I learnt (and Iām no engineer): once you stress something beyond its ability to absorb it, it strains and breaks on an atomic level.
But if thatās the case, then wouldnāt soft axles possess a higher modulus, so their atomic-bonds stretch further (to flex) but can return to normal without breaking?
I know our mechanical engineer Thomas Williams explained earlier that all steels have a very similar Yās modulus, so any variance in axle stiffness is related to damping. Is damping related to either Tensile Strength or Yield Strength?
Youngās Modulus relates strain (āstretchā) in tension to load in tension. The bending stiffness of an axle is a result tension and compression on opposing sides. The convex side of the beam (axle) is in tension. The concave side is in compression. In an axle, shear loads are very low, and are due to torsion. Due to the geometry of the cross section, there is really no other way to have shear in a hollow tube other than torsion unless you have parallel offset mountings that are loaded to create shear. For a kart axle, this does not happen. What shear there is, is ātwistā from the application of torque via the rear sprocket and reacted by the tires.
But for loading where there are significant shear loads (such as direct shear, or high torsion) then there is the Shear Modulus. In steel Shear Modulus is about 42% of Youngās Modulus. The again, all steel axles will have the same Shear Modulus.
How about just testing the axles alone? My bet is if the wall thickness is the same between two steel axles (āhardā vs āsoftā), the stiffness will be same.
I have a large milling machine table and some load cells, maybe one day I will do this and save you guys a bunch of $ by showing all these axles with the same wall thickness are the exact same stiffness and you are getting ripped of. Or maybe I will learn something and eat my words.
I even have the equipment to do natural frequency testing if you want to get into dynamics. I have a CSI Vibration Analyzer, which can do impact/mode frequency testing. You strike the structure, and it records the mode frequencies.
