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Do older chassis get soft with use?
I assume this would make sense as it flexes with use.
Does grip then reduce with age as more soft less weight transfer?

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No .   With use all metals harden .

Karts are generally made of chrome-moly steel , this is what aircraft frames were made of and you can see plenty of old ones still flying around after 30, 40 or 50 years.

What goes wrong with a kart chassis is it bends and twists out of its original shape and dimensions with use over time,  or in crashes.  If you can re-align it back to its original dimensions a chassis will last a very long time. 

What most people think is a softening of the chassis is actually the effects of having a heavy weight, ie the driver, hung in the centre of the kart and putting force on the extremities. This can most easily be measured in the king pin area where over time the camber angle reduces as the king pin supports slowly get rolled inwards at the top.  This has been mis-interpreted as a softening of the chassis as it appears to have sagged in the middle.  It is just what happens in use and can be taken out. 

Ian, I'm interested to know what aircraft were or are made of  chrome moly steel? 

Work hardening can actually make areas that work more than others brittle, creating a weak point which will ultimately crack.

More or less agree with Ian.

Donning my flame-proof suit, I think it's a myth that chassis 'wear out' over time, given nothing else having happened to the chassis (such as developing a crack(s), or some part of the chassis being worn away, or developing an uncorrected twist). 

My understanding, FWIW, is that metals - including steel tubes -  can to whatever degree harden in use (i.e. 'work harden'), but don't become more or less stiff (in flexure) as a result because hardness does not have a significant affect on stiffness. A kart chassis may become somewhat more flexible if some part of it is worn away over time (usually the bottom of the side rails if allowed to contact the track surface repeatedly), or if a  crack develops somewhere.

I know it's counter intuitive that material hardness of any steel has neglible affect on it's stiffness relative to other steels, yet it is true. If you were to take a steel file (i.e. very hard steel) and perform a deflection test that resulted in X deflection at Y load, then you took that file and fully annealed it (so it is now much softer), and performed the same deflection test, the result would be the same, i.e. X deflection at Y load. If we exceed Y load enough then the hardened file will snap, the softened file will bend. 

Further, it doesn't matter a great deal what particular steel alloy the chassis is made from (nor its' state of heat treatment etc.). A chassis of X dimensions (i.e. chassis layout / tube size) will have the same stiffness within the elastic limit of the steel regardless of whether it's made from CrMo steel or mild steel (or whatever). What will differ is the degree to which the chassis can be deflected before becoming permanently deformed. CrMo has a higher elastic limit, so a CrMo chassis can be twisted (or impacted) significantly more severely before it reaches the elastic limit of the tubing and becomes permanently deformed (twisted, or bent in some way). So, from a practical perspective a CrMo chassis is preferable because it's more than likely to last a lot longer than say a  mild steel chassis, but it won't perform any differently to the chassis made from MS (up to the point at which the MS chassis starts to fail, or becomes bent mid race...). 

If the flexure that the chassis repeatedly sees in use approaches the elastic limit of the steel (but doesn't necessarily exceed it), then given enough time (number of flexure events) the chassis may 'creep' gradually into a significantly deformed state (same thing as a spring 'sagging' over time). The higher the steels' elastic limit the more resistant to this the chassis will be. A chassis made from CrMo will be more resistant to becoming twisted / bent than an otherwise identical chassis made from say mild steel, because the CrMo can be elastically deformed further before becoming permanently deformed. This is true for singular severe flexure events, and for repeated lesser flexure events over time (that appoach the steels' elastic limit). 

A chassis that has been twisted or otherwise damaged can be straightened without affecting its' stiffness, and so its' on track performance should be unaffected. But, the chassis will have been subjected to flexure exceeeding its' elastic limit when it was originally damaged and when it was straightened. These events will affect the steel to some degree, and the elastic limit may be affected (marginally increased due to work hardening, a non issue in itself), and / or increased likelihood of developing a crack, so slightly bending a chassis does have some consequence related to the its' ultimate longevity.  

So, IMO, a chassis that has been damaged and then properly straightened should work as well as it did before being damaged. If it doesn't then I think there is more than likely something else going on, such as an undetected crack, or the driver has subconsciously decided that the chassis is now just no good (i.e. a placebo affect). I suspect that many undamaged chassis are disposed of because the driver has decided (or been told...) that it is just old and 'worn out', when in fact the chassis is still fine. I think there are a lot of people 'out there' with a vested interest in promoting the idea that chassis do just 'wear out' and need to be replaced on a regular basis... 

Regards,

John.

I agree with both John and Ian and can relate with personal experience.

Back in 2008 Blake's first 30/30 Kosmic suffered several cracks due to tracks and mainly the vibes of the RL Leopard.

In addition it has severe wear of the front rail, so much so that the flat broke through to the i.d. of the tube.

At this stage I stripped the frame had the cracks (steering column bracket and seat stay) tigged and additional brackets included to strengthen.

The front bar had a piece of 25 * 6 flat bar tigged into place to replace lost metal and act as a rub bar for future rubbing.

Kart was then glass beaded ad powder coated.

Result ??

He thought it was more responsive in the front end than before and all good handling was restored!

Subsequently we bought a new chassis in 2010 because........ well wouldn't you ??

Spent a whole weekend trying to get the new 30 as fast as the old one.

got there eventually but never got it quicker!!

continued to use both and won as many races in the old as the new

Eventually sold it to a newcomer, who with our help and setup won his first race - first time out!

Currently Blake is selling is 2013 chassis with 2011 running gear (after chassis was written off at Vic open) for $1500

Not because it is worn out (just won SA titles for TAG Heavy) but because he has been asked to run a new brand.

I always kept a check on chassis straightness with scales and it works for years !!

Michael,

I don't really know any specifics, but I do know (at least have a strong vague memory) that CrMo was used extensively for airframes during WW11, so I assume it has been around since at least the 1930's. It is such a suitable and easily available material that I assume it has been used in most tubular fuselages since WW11. 

My guess is that kart chassis made from CrMo are typically and perhaps exclusively made from 4130N CrMo steel ('N' standing for 'normailsed', meaning stress relieved). It seems the most popular choice for just about everything requiring greater strength than mild steel can provide. There are other more specialised variants, such as 4120 and 4140, but I know even less about those and their uses. 

Regards,

John.



Michael O'Brien said:

Ian, I'm interested to know what aircraft were or are made of  chrome moly steel? 

Work hardening can actually make areas that work more than others brittle, creating a weak point which will ultimately crack.

Basically all the old steel frame fabric covered aircraft.  

Cheers Ben

I think a lot more than just those. And it's still used.

I think most (? at leasy many) of the old doped fabric planes had wooden frames. It's my impression that just about any plane that doesn't have a wooden frame, or some aluminium alloy, is very likely to have a CrMo steel frame. 

Regards,

John.

I said:

"So, IMO, a chassis that has been damaged and then properly straightened should work as well as it did before being damaged."

I'd include chassis that have 'sagged' over time as being more or less the same as those having been damaged in a single incident. That is, if they are properly straightened they should perform as new, so long as there is not another issue with it (e,g. hidden crack etc.), which is not all that unlikely...

Regards,

John.

I find a chassis is good until 1 of 3 things happen.

Cracking or heavy wear of frame rails. The kart gets bent or twisted and you cant get it back quite right or it "moves" as such. Scales and alignment will highlight this as Ian mentioned.

Last but not least if a chassis has done tons of laps it can sometimes just "go off". Typically it will become unresponsive to changes and then it just doesn't feel the same. More than likely a case of the metal changing as the chassis gets more and more cycles go through it.

The same thoughts can also be had for kart axles as well...

True.  Many (most?) ultralights etc.

Cheers Ben

john learmonth said:

I think a lot more than just those. And it's still used.

I think most (? at leasy many) of the old doped fabric planes had wooden frames. It's my impression that just about any plane that doesn't have a wooden frame, or some aluminium alloy, is very likely to have a CrMo steel frame. 

Regards,

John.

"Just going off' is what I have some difficulty believing. I've never experienced this, that I am aware of. If it happens, then what causes it to happen, what is the mechanism?

If we assume that it is the stiffness of the chassis (or sections of differing stiffness within the chassis) that determine the manner in which weight transfers from and to the different wheels in response to changes in horizontal accelerations, and it is only the physical dimensions of any steel spring (and in effect that is what a kart chassis is) that determines the stiffness of that spring, and not the hardness or softness of the steel from which the spring is made (which is what the metallurgists tell us), then where does a change in stiffness come from when a chassis becomes 'tired'? (if it is not from another source, such as cracking). 

I have the same issue with the notion that rear axles somehow become 'tired' over time, even when the axle has not been damaged, or similarly that an axle with X OD and Y wall thickness will behave differently to another dimensionally identical axle with a different material hardness. It's like saying that different coil springs of identical dimension and rate but having a different material hardness will cause a car to handle differently, one to the other dependant on the material hardness (and I've never heard anyone claim such a thing ever occurs). The tyres only see the weight transfer, the  chassis only 'sees' the stiffness, neither 'sees' the hardness of the steel in the 'spring'. 

I smell a big fat placebo effect, or a seperate undiagnosed issue. It would be educational to subject new and older identical chassis to back to back torsional testing...

Regards,

John.

Slightly off topic, but could someone now explain to me the difference between a hard and soft axle of the same external  and internal diameters and length?

As to performance, and why.

Because from the above discussion, you have all proved what I have always believed. 

That is, that hard, medium, soft, and super soft is all a furphy, and that it is wall thickness that truly matters.

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