Chassis : standard metal vs "reparto corse"

(TJ Koyen) #42

We’ve found that a chassis has “gone off” once it stops responding to setup changes. It sort of just handles the same no matter what you do. Not sure how that’s actually affected by the metal fatiguing or whatever, but that’s generally when it’s accepted that a chassis is worn out.

(Nik Goodfellow) #43

This goes counter to you experimental results but does it not go beyond the elastic limit.

If the chassis is strained past its yield point (y1) to A, it will follow line AB when the strain is removed. If a strain is reapplied, the pah BA will be followed and the chassis will not reach yield (y2) until point A. y2>y1 and the chassis is stronger but it will fracture with much less extension.

So really the chassis is stiffer, not softer but it bends more when it does go, and thats what people see (most who have no engineering knowledge) and assume that means its softer.

(Corally ) #44

Hi Nik,

Thanks, the angle from zero to the yield point is what makes steel “springy”. As you show, the path from B to A has the same angle as the original path, thus stiffness is not changed. The proces you mention is called work hardening, not that anything gets harder, it’s only closer to breaking when you exceed the yield strenght. The reason that i’m not so concerned about this is that we seldom see chassis that have ‘‘snapped in half’’ or show any other severe cracks. A chassis is considered worn out much earlier.

The complaint TJ mentioned about the chassis not reacting to setup changes, is one i’ve heard a lot as well. My goal is to connect the complaint to physical alterations of the chassis. That is a difficult one. There is no evidence of true metal fatigue to be found, microscopic cracks in a chassis would make it less stiff, which is not the case. The only obvious change is the chassis being bend in several ways. But how does this bending connect to ‘‘it not reacting to setup changes’’ and ‘‘feeling soft’’. I have some theories but i’m more interested in yours.

(Nik Goodfellow) #45

I can theorise. So lets suppose after a session the kart is now at A-B evidence by the right yolk being up 2mm. We bend it back which would bring A-B back the original location but with a higher yield point (y2). Next time they go out they do it again and we restraighten it and end up with y3, which is higher then last time. What if a kart is at its fastest in that plastic region, but everytime we go into the plastic region we make it more difficult to attain (yn) until we get to the point that kart no longer handles well because the yn is too large to make the kart handle well.

Then if that driver continues to drive the chassis they will eventually hit fracture point (I’ve seen frames fracture) although this usual fracture point is on a weld.

(Aaron Hachmeister) #46

Oh baby talk dirty to me XD

In actuality this is really interesting to read and I hope you guys keep this going for a while

(Davin Roberts Sturdivant) #47

9 posts were split to a new topic: Do different stiffness axles do anything?

(Davin Roberts Sturdivant) #50

4 posts were merged into an existing topic: Do different stiffness axles do anything?

(Charles Kaneb) #51

One thing to note: most chassis manufacturers work on a development budget that would not come 1/10 of the way to designing a passenger-car subframe.

GM would reject any chassis part that took a visible permanent set after a session on a rubbered-in track. A permanent set can only be caused by one of two things: cracking or yielding. Cracking is caused by either cumulative damage in fatigue or inadequate fracture toughness for a single impact. Yielding is caused by the local stress exceeding the yield strength of the material.

I wonder if Margay would like to share what instrumentation they use to find the loads they use as design inputs.

(Nik Goodfellow) #52

Totally agree, would be dropped straight away by an auto manufacturers (maybe Chrysler would let it slide lol).

My experience of chassis development with Italian manufacturers is all iterative improvements based on experience. It may be different now (this was 2000-2006). There was talk of the funky Maranello kart being designed by engineering graduates who didn’t know too much about karts but it never caught on.

(Andy Schlosser) #53

Another thing that I think is an interesting conclusion would be metal fatigue and the fatigue limit of the metal - some steel alloys don’t have an ultimate fatigue failure and that’s usually a hallmark of ferrous materials and titanium whereas a lot of non-ferrous materials (such as the Intrepid with weird cracks everywhere) have a failure-based fatigue life where, eventually, the material will crack.


Here’s a pretty good chart for the tensile strength of 4130 and how, once it hits it’s fatigue limit, it loses strength.

This comparison of steel and aluminum (which can be assumed to be similar for materials like aluminum such as the non-magnetic material in that intrepid) will eventually just go to failure as they don’t have an endurance limit. The highest stresses will be around the bends on the front clip, struts, and where welds happen along the long sections of main rail. That said, you can approximate spring rate with Young’s Modulus and relative displacement. Meaning Y(delta L/ L) is loosely Hooke’s law (I read a paper about this a while ago - I might be off slightly). Y= Stress/strain so as the material has less stress from being fatigued for the same strain it will get lower - meaning the spring rate will get lower and it will get softer. IMO, this falls much more in line with anecdotal evidence I’ve heard about the kart on track.

Something else to take into consideration in the design of bending parts is that there’s so much more than purely stiffness - for all intents and purposes, “rebound” shouldn’t exist as karters talk about it. Energy loading vs unloading should be linear as far as bending is concerned - but it obviously isn’t if you’ve driven axles with different rebounds. The mini kart axle I designed last year was the same stiffness as an incredibly popular axle mini kart axle from Europe but had a completely different rebound because it was using a different material, wall thickness, etc. This is because our rules allow different things in the axle as opposed to european rules. There’s quite a lot to play with outside of stiffness alone. Since we’re undamped, a lot more things matter such as natural frequency and resonance.

I set up an FEA of a kart chassis earlier in school - what I found out was, more or less, without a bunch of specialized information it’s hard to really tell what’s going to happen. Biggest stresses were around bends, etc. I really only focused on torsional rigidity since modelling weight transfer through a fiberglass seat would be… Difficult.

The highest stresses on the kart come at the welds, not on the tubes. Look at the factory welds vs general production welds. Might get a little more insight on to the special karts there as well. :wink: I know a lot of factories will do karts specific to tracks as well. Maybe one track is rougher so it needs a more “dull” material, whereas another track might be glassy smooth and single apex turns so it might need a more reactive kart. Quasi-damping is pretty cool and makes a lot of sense in reality but I can’t make much sense of it myself in theory. I know qualitatively what materials behave as what within karting and have working theories to why, but I can’t really give anything definite.

I’ve gotta disagree on the karts come in higher right away thing - I table karts at the end of every day at major races (Pro Tour, Winter Tour, etc) and the karts I table the night before to be perfect often come back perfect (within 1 mm) if they don’t hit anything. This is true for Compkart, Parolin, Energy, Praga, OTK, any of the brands I’ve personally worked with in the last few years. Part of that “pop up” the first session may be auxiliary stress in the frame from assembly - if the kart has stresses in it that, when agitated, release themselves. It’s a pretty common tactic for stress relieving welded parts on a vibration plate and maybe it’s showing up here. I assume karts are stress relieved after welding but maybe not.

At the end of the day, I think that the majority of this comes down to economics and not engineering. The factory karts are usually a different material that wears out faster that they can’t sell to the public because of the life expectancy of a kart, as mentioned above, but I think the teams selling off karts in non factory settings has more to do with whenever they get their money out of them. In three races of rental fees most teams will have paid off their cost on the kart - whatever else is left is profit. You’ll make more off the last two races in rentals than you’ll get as an increased return on selling the kart used as a 1 race versus a 3 race kart. At a certain point, rental customers expect fresh equipment, so this is an ideal spot to do it.

(Phillip Kopp) #54

Simple explanation. Lap time. I used to race a very long time ago, things were different because the kart was very light and the tires extremely soft. I raced on factory teams in CIK the same timing like Jenson, Davidson, Alonso, Raikkonen, Kubica etc… it was a good time. In that time we used “carbon” steel in te factory team. You can tell easily because it is almost a black color. Some of the teams don’t paint the factory chassis and some do, but there is no technical reason for this . I saw some posters mention about chromoly or soft steel. Again it was a different time but that was the WORST thing you can use. The carbon steel is much stiffer (think carbon fiber, diamonds, etc…) and much more expensive. This is why they don’t use it in the retail kart, it’s simply a matter of cost. So the principle is a lot based on the braking. You want the chassis stiff also from front to back or it brakes very poorly and gets all wound up into the corner. About the chassis life? I could race a whole season on one or two chassis, most of the drivers did. And they never cracked as you show images. That is all just weird. Maybe you hit the curb too hard? So yes, there is a big difference in the chassis, the black one is faster and you won’t find it in a retail store. But it only matters on soft tires. If you race hard tires don’t waste your time and breath chasing after a magic chassis. Just go practice and become a better driver! So one trick you all might find useful. The teams often sell the chassis after a few races. So if your distributor in local market is visiting the factory or a big international race he can usually offer to buy the chassis from the team and they will usually sell it. However many chassis have small changes for individual driver style and it might not work for you personally. So you will play the lottery. Or it is made for only a very specific tyre compounds and won’t work with whatever you are buying in the kart shop.

Why you would buy a soft chassis? Makes no sense to me at all, even worse in the heavy categories! Totally counter to any karting philosophy I know. I think is your competitor trying to trick you for the next meeting… honestly the retail chassis is good. Another important lesson is there are only a couple good chassis and a lot of horrible ones. Even if they win some international races it’s only because they have very best drivers, pay them well and have the best tuners. The most forgiving chassis I saw was always Tony Kart, easy for anyone to drive. The fastest chassis I ever raced in a factory team was Gianniberti’s Jolly Kart which was just ridiculously fast. And the most fun to drive was probably the Sodi with the front suspension before they banned it. Does Rabazzi own the steel suppliers? I don’t think so. There is steel and you get what you paid for. The problem is that OTK and Parolin make almost all the other chassis as a contract manufacturer. So chances are very high no matter what brand kart you have is coming from a couple of plants with different paint and stickers.

But for sure the WORST material is chromoly. The US manufacturers always used it because they thought it was better but they always sucked. Only the good Italian dark steel was the best one. Get a new kart every year or two and you all will be fine!

Now getting the best motor… different story. Way more challenging. That one you just need to be the best driver and the tuner is your friend or have a billionaire budget.

(Phillip Kopp) #55

How does a 10 year old car feel compared to a new one? Now compress what you do on every lap in a kart into a force, then apply that force to a road car, pretending you drive it the same way as your kart. It would probably feel “soft” in less than 6 months too, just like a 10 year old version that was driven normally. It’s called wear. New stuff is always better than old stuff. What you are asking is like trying to bend time. Sure if you take very good care of your stuff it will last longer. But that is obviously not the point in karting, it’s to use up the equipment as fast as possible! This is called “going faster.” ;). However 10-12 races is a perfectly acceptable life of a chassis, and after that it will slow down. Quite a lot usually. It loses the spring. Just like your 1975 Cadillac!

(Nik Goodfellow) #56

Hi Phillip,

I don’t agree. Chrome Moly is easily welded and is considerably stronger and more durable than standard (1020) carbon steel tubing. Where I have found new manufacturers (its probably unfair to say US manufacturers only) have failed is in the weld, often they go the wrong way or braze it.

Somewhere in these forums I posted about my belief that as karting moved up in axle diameter (stiffer) they moved to softer frame designs (I’m saying designs not). OTK started it (i think the mitox in 2000).

Btw we are Topkart factory team Alumni, I raced for them 2001-2002. Was Massimo Fillini one of the mechanics when you raced there?

(James McMahon) #57

So if we assume this to be true… That brings up three questions

  1. How much more does it actually cost. I can’t imagine the % being huge on the grand scheme of a $5000 chassis.
  2. How much does stiffness it really matter when you are riding on tires running 8psi?

I think it’s more complex than just stiffness just by itself (we probably need to “firm up” on that term).

At the least, we probably need to break it down in terms of longitudinal, lateral and diagonal load transfer…

Then when we think of the varying sizes of drivers, tires (tyres) and motor packages. For low powered karts you want a more gradual lateral load transfer, delaying when the inside wheel settles down.
For a shifter, you want to get that wheel down fast and get max power down asap.

The incredible irony of this is that the marketing jib offered by (some) American chassis makers is the opposite of what is stated here. Some US chassis makers state that that they don’t use “cheap” steel like the European chassis do. American chassis are made of chome moly, which is better than that steel stuff. European chassis use that cheap steel and it goes "soft, chrome moly lasts longer. Many folks in the US seem subscribe to that theory as well.

The truth as they say, lies somewhere in the middle.

(Phillip Kopp) #58

Nice! I was Topkart factory team 1995 and 1996 only running Interconti A, then Formula A. I don’t remember Massimo, did he go to Energy Corse? I was there with Jon Targett and with Balzan, it was a long time ago. I worked with Targett, Balzan, Cisolla and Van der Ende (father of Jacky and Ricardo who were also very successful). I went with Panigada, when he started Energy Corse (I was the actually first factory driver for Energy Corse!). Which was a huge mistake. Then I quickly went to Haase after that and transfer from the factory team to the Blanken semi-factory team. It was a bad year for Haase too. I finished my karting “career” with Gianniberti in Jolly Kart and unfortunately run out of money and had to start college while all my colleagues went on to become F1 and LMP world champs :). That would have been a good year, because we were insanely fast on the Jolly Kart. I was running faster than all but 2 or 3 Super A drivers in Garda on European championships. We didn’t even know how it was possible.

Back to chassis discussion. In my days, the US manufactures said the same thing, chromoly is better, it last longer blah blah. But they were slow and horrible. Europeans wouldn’t touch it or when they tried, quickly threw it into the trash bin. There is a problem with our discussion about stiffness. If the metal doesn’t bend, the kart is not going to work. You don’t want to drive on a rock. This is also a problem of geometry (and why the first Energy Corse was pretty much the worst kart ever built…). You get stiffness also by diameter. So it was hard chassis and soft axle, then hard axle soft chassis. Everyone has their own way (Tecno kart, was I think Jan Heylen, Antonio Garcia and Jenson Button, and Bruno Vroomen, it was very fast, but soft small tubes). I like very stiff hard axels so you can brake hard and plant the kart any direction. Its all your style.

So I don’t know all these details people are discussing. But I can say in my experience (again it was a long time ago, different tyres etc…), but the chromoly chassis are generally much worse than the carbon steel ones. And there are different steel chassis, some of them are light color steel and some of them are dark, almost black carbon steel. And the black ones are always the fastest. And they are not very common, the factory teams make it only for themselves and not a lot of them. So I guess its pretty fair to say there is a reason for that…

And the heavier the kart (i.e. KZ, shifter or whatever), the stiffer you want it to be. And the sidewall will depend how stiff you want it to be. If you have lot of rubber on the track a soft chassis or Axel can loosen the kart up, but maybe you put a lot of support on the seat to keep it from binding into a pretzel. I remember sometimes up to 3 struts on each side of the seat if there is too much rubber! A soft chassis is sometimes easier to drive also. So in the end I guess it all depends. But the concept of this discussion is pretty straight forward. A kart can last a season or so and after that it will always go slower. There is no way to “preserve” it. It shouldn’t be cracking anywhere, this is really defective. It shouldn’t be overly stiff and overly soft. But this should have nothing to do with how long it lasts. And in my experience at least, the black carbon one is always the fastest and most difficult to find (if that’s how you are thinking…). Maybe something changed about the axels, but Chromoly was never, ever good.

(James McMahon) #59

@Phillip_Kopp, I appreciate you taking the time to join the discussion and share your experience.

That said, we try to stay reasonably factual on the forums here and steer clear from inflammatory comments. Criticism is absolutely fine, but we expect it to be reasonably objective and substantiated in some way.

(Nik Goodfellow) #60

To be fair he’s talking about mid-late 90’s Energy Corse. Not the current one. We can all look back and say this kart was terrible. Its only the same as saying the Mitox was an awesome kart.

@Phillip_Kopp, have you thought about coming back out of retirement. I assume like me you are finding itching to get back out and race something and North America has a good master category in the X30 class. It’s no FA but its racing.

Massimo was Armando Fillini’s brother. He died of cancer in the mid noughties. Best mechanic I ever had. I was with Topkart ICA in 2001 and FA 2002. The kart wasn’t great unless it rained or something else made the track slow (we won the International Pomposa Cup because sand from the beaches kept the track slick). As soon as the grip went up it just didn’t flex enough. Went to cars (big mistake) 2003-05 (part seasons, no money) then came back to CRG KZ for 2006.

Do you remember Nodtune (Don Parker), i know he worked with Comer but he might have been working mainly with BRM in the mid nineties?

I’ve gone off topic, sorry forum gods.

(James McMahon) #61

You might know that, and Phillip might, but folks strolling by (ie everybody else) probably won’t. So let’s keep it positive, or constructively critical.

That’s all I’m saying :+1:

At the same time, if I fall foul of this, I expect folks to call me out on it too :fist:

(Aaron Hachmeister) #62

I found James on the karting subreddit, we got onto the topic of basically this post. Has anyone looked at getting data from a straightening table for how much force it takes to bend frames back into position? I’d hate to buy a new kart just to do that but there could maybe be some data from frame straightening that may help us get some actual numbers on this.

(Eric Gunderson) #63

@Aaron_Hachmeister_13 may be tricky to get data from a straightening table. But, with some calculations you could do a decent job of this I suspect. Using a consistent bar with consistent weight person monitoring force applied to difference chassis could be interesting. At least I’m able to visualize the loading in my mind, so that’s a positive sign.