30mm x 2mm wall tubing has a Area Moment of Inertia of 17329 mm^4
Cross Sectional Area is 175mm^2
29.00mm x 2.3mm wall tubing has a Area Moment of Inertia of 17319 mm^4
Cross Sectional Area is 192.93mm^2!
The two tubes are the same stiffness in bending (Same “EI”…which is the stiffness constant in all beam equations). But the 29mm tube with thicker wall will be 10% heavier. It would probably be significantly less likely to go plastic (yield) in usage.
Perhaps someone should experiment with 1-1/8" x .095 wall tubing… (28.575mm x 2.41mm wall) . That dimension has a Area Moment of Inertia of 17112 mm^4, which is 98.75% of 30mm x 2mm. It would be heavy though…13.3% heavier per unit length than 30mm x 2mm wall metric tubing. I bet it would be a tough chassis…
Larry and Charles, those differences in damping are small, and COMPETELY INSIGNIFICANT.
This is my area of engineering expertise. Damping is a concern in structures with continuous periodic loading, such as rotating machinery. VERY HIGH damping can counteract resonance if operation near a critical speed can not be avoided. The more damping there is, the more limited the cumulative energy state is. You can also look at decay of free vibration of a system (Loagarithmic decrement) But we are taking about FREE VIBRATION HERE, or continuous operation for a driven system. Free vibration is characterized by many cycles…hundreds, or an infinite number of cycles for a driven system.
This is not what happens in the loading of a kart chassis. In a kart chassis, we are loading and unloading. One cycle. A few cycles if you over load the chassis, and get the dreaded hopping load unload. (Benny Moon and AJ Allmendinger reliving the glory days) The difference in damping between grades of steel is completely inconsequential in the low cycle count realm of a chassis flexing. Consider the difference between 4140 and 4340 steel at 5,000 psi is 0.8% and 1.1%. Are you really going to claim that a flexed structure returning 98.9% of the energy put into it vs 99.2% of the energy put into it is going to make any sort of difference? Even 5 times the delta will not be felt. The extremely low levels of damping in steels require many many…hundred or even thousands of cycles to make their effects felt.
This is why machine tools are made out of grey cast iron with flake graphite, not steel. The damping of this type of cast iron is approximately 20 times greater than 4140 steel at 5,000 psi. That you could feel. But your spindle hanger would snap off the first time you hit a curb.
A lot of takeaways from that study. Sounds like an interesting plan might be:
Buy used (insert your favorite chassis here).
Acquire Docol T8
Have a drag, stock car or similar tube fabricator make a copy the chassis.
Transfer the components from used kart onto the new frame. Checking each one of course.
Would love to hear your thoughts on rear axles. Probably a can of worms considering deflection, resonance and wall thickness on a rotating shaft.
On deflection my half baked hypothesis is that it’s marginal when we consider how much the tires are deflect. On the other hand hub length and distance to the bearing affects that too.
Damping sounds like there is very little difference between steels?
So that leaves resonance? But again I would imagine that tires would soak a lot of that up.
I have offered my opinion on this before. I do not believe that there is enough difference in physical properties to make axles behave differently without changing wall thickness. But all the different OTK axles have the same wall… The only way to know what is real, and what is BS is to TEST.
One of these days I will take some OTK Std, Hard and Soft Axles, and do some dynamic testing. I will put them in a fixture, bolt it to a machining center table (massive and rigid). I will do natural frequency test (ie cantilever beam bump testing…). I have a CSI 2120A Vibration Analyzer, with natural frequency testing capabilities… That will tell us what the difference really is…
Since we know the dimensions precisely (ID, OD, Cantilever Length), and we know the analytical solution for 1st mode vibration of a simple cantilever beam (Partial Differential Equations anyone…?), the natural frequency will tell us the Modulus (E). I can also look at the amplitude decay, and that will give us damping. That is all there is…
Resonant frequency of an overhung stub is going to be in a range where there is nothing to excite it. Resonance is a condition where there is a driving frequency that is the same as a natural frequency of a structure. We would know from really bad experiences while driving if this ever occurred.
The level of enginerdery going on here is equal parts awesome and fascinating. It’s also so far over my head that I can barely see it going by! I’ll just go back to my simple geospatial analyses of aerial and satellite imagery.
This would be a cool project to attempt to quantify the black art af axle feel! Just because I am curious, with the axle churning along at around 2000ish RPM, does this rotation affect the dampening or vibrational frequncies you speak of?
That is true in the elastic region. Karts work in the plastic region. Hysteresis matters for us.
Lifespan of a kart frame before it starts cracking everywhere is typically about 75 hours of operation or 5,000 laps. That’s about 50,000 corners, 50,000 reversed cycles.
You’ll see a permanent set after each day of operation on most karts.
You don’t need infinite money. Send Factory Karts 25 feet of 1.25" OD .083" wall Docol tubing and we’ll make the frame during the slow season for a small custom work premium over the regular price.
@Thomas_Williams I enjoyed reading your engineering nerd out!!! I assume you are an M.E. also from your expertise in materials? More heat transfer and combustion background for me. Although all I do is manage now…
As for the OTK axles, we have run Q, N, H, HH on my son’s kart. He is quite tall at 6-4. With the height, axle changes are extremely impactful on the rear balance and behavior of the kart. Last weekend we tested with an H axle, this weekend we club raced with an N axle. We went to the N because the track was lower grip from lots of rain. The rear of the kart was far more planted and stable around the track this week. I could visually see it, the driver reported it, and the stopwatch and MPH all confirmed it. I have not bothered to weigh them or measure wall thickness, I just know what the data shows us.
Uh…plastic region? That is beyond yield, which means the CHASSIS IS BENT.
Maybe in very small localized stress concentration regions…but that is utter nonsense for main frame rails and cross members, etc.
Regardless of whether someone is right or wrong, let’s try and be respectful and not puff our chests out too much and laugh at those with differing knowledge or opinions.
Really interesting read.
Since this has evolved into a thread encompassing all facets of kart frame metallurgy, I have renamed it slightly.
I will pick the brain of Tommaso at AMV this week and see what some of his thoughts are regarding these topics. He has provided interesting insight regarding components and chassis manufacturing.
Whether karts are meant to be “bent” they do get deformed permanently even just from racing. Accidents usually do the worst deformation, but whether it’s sag longitudinally or laterally there is some amount of permanent deformation which must mean it goes into the plastic region. I’m no expert on materials or chassis, but it’s well known in the karting community that different steels at the same dimensions do affect how the chassis works.
I have driven three different BestKart frames this year and all are the same tubing thickness, same wall thickness, same dimensions, and they perform drastically differently.
Going beyond yield to the plastic region means PERMANENT DEFORMATION. The shape changes. If the shape changes, and the chassis gets faster (this never happens) that would mean that the chassis was the wrong shape to start with. So we are to believe that chassis are designed wrong, in hopes that they will be bent into a shape that works better? This is just absurd.
Bending in use is simply not controllable, which is why high level competitors cycle through frames so much more often than recreational racers. Bending means the chassis is compromised. In other words, just because a chassis bends, does not mean it is designed to function at its best after bending. It is meant to perform at its best BEFORE it bends. Bending is an unfortunate side effect of use at a high level (curbs, etc). As we know in motor sports, it is the consumables that really cost. But at high levels of competition, EVERTYHING is a consumable. A frame is no different than a tire.
That said, as the paper Nick posted explains, if you can return a chassis to its original shape (straighten it), it will perform well again. But high level competitors, factory teams, etc, so not have time to mess with that, so they just change chassis often.
