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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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Ever seen a spring sag????????



john learmonth said:

"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.

Ahh the tension in this discussion, google mr finks old kracer vids if you can find the 1 regarding axles.

It takes it from retarded engineer to normal human speak the best way ive ever seen it explained.

Think of it as a spring rate.................



Terry Sheedy said:

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.

Stephen, I am thinking of it as a spring. And as the chassis discussion has suggested, all else being equal, the hardness of the material matters no more that a couple of %.

Whereas wall thickness makes a huge difference to the spring rate, and also makes a significant difference to the way a kart handles..

It is interesting also that some manufacturers refer to their axles in tensions.

Yes of course, springs do 'sag'. I don't dispute this, but it does not mean that they get either more or less stiff as a result. The Modulus of Elasticity (i.e. stiffness) of any steel is unaffected by 'sagging'. This is so for any shape of spring, i.e. coils, leaves, tubular, whatever.

If a new spring has a stiffness that requires X load to deflect it from its' free length by Y distance, and this spring sags in use over time, then it will still take X load to deflect it by Y distance. So long as a 'sagged' spring does not start to develop cracks (which is in effect a change in the 'dimension' of the spring) it retains its' rate despite losing free length, and despite what seems to be a very common misapprehension that a 'sagged' spring becomes 'softer' (i.e. loses rate, which it doesn't). A sagged spring on a car will allow the suspension to ride closer to the bump stops, which may well allow the suspension to bottom out more readily and give an impression that the spring has softened, but this isn't caused by an actual change in stiffness. 

I can't see why a kart chassis would differ, being in effect a complex spring made from steel, nor a kart axle for that matter (also an effective spring, a simple but very stiff one). For a change in rate to occur, the spring must be changed in its' dimensions, and the dimensions in question are the wire diameter and or the wire length (using a coil as an easy to visualise example). A change in free length doesn't change the wire diameter or length, or rate, and it's the same deal for leaf springs too. 

Regards,

John.

Stephen Kerwood said:

Ever seen a spring sag????????


Terry Sheedy said:

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.

You won't be surprised that I tend to agree with you, though lots of credible people don't (agree that axle material hardness has no affect on axle flexure, or something else more mysterious and defiant of description). 

I'm with the material scientists on this (according to my research and the answers I've been given by professional engineers when I've directly asked the question). My best understanding is that the only things that dictate axle stiffness are the OD and the wall thickness, not material hardness. The rest is I suspect marketing, and placebo. Someone may well jump in and say they have proved it (material hardness does have an effect) on the stopwatch, but I tend to suspect that if you expect something to be better or worse then it often seems to be, and can even show up in lap times due to a subtle psychological difference in the way the driver drives as influenced by what they expect to result from whatever change that has been made (i.e. an expected difference, better or worse, but some difference).

Placebo can be a strong effect. For instance I know full well that my car is not materially changed after I've washed it, but every time I do my gut would swear that it is both a bit smoother and a tad faster (of course it isn't). 

I've heard people agree that material hardness has no significant affect on stiffness, but does have an effect on 'resonance', which is then claimed to be what affects how the kart handles (with a change in axle material hardness). This implies a self dampening effect within the steel, which would have to be substatial for any change in hardness to make a significant difference to. The problem with this notion is that steel of any hardness has only a miniscule self dampening property, so any change in self dampening will be between virtually nothing to just a bit more than that, not enough to account for a noticable change in axle behaviour IMO. If steel had even a modestly significant self dampening effect then it wouldn't ring like a bell (when a suspended piece is struck), because the self dampening effect would prevent it (or quicky stop it ringing). 

Regards,

John.

Yes.

Regards,

John.

Terry Sheedy said:

Stephen, I am thinking of it as a spring. And as the chassis discussion has suggested, all else being equal, the hardness of the material matters no more that a couple of %.

Whereas wall thickness makes a huge difference to the spring rate, and also makes a significant difference to the way a kart handles..

It's interesting that some manufacturers seem to use 'hard' and 'soft' to describe axles with different hardness materials but the same wall thickness, and others have axles described in a similar manner that have different wall thickness, which may or may not have different material hardness. I can't recall which manufacturers do which.

Regards,

John. 

Stephen Kerwood said:

It is interesting also that some manufacturers refer to their axles in tensions.
I remember the good old days when axles only came with one wall thickness and hardness . We all set up a kart around what we had and all went well . I always wondered why axle hardness came into the set up . I remember when all we had was the 25mm solid axles and then the 30mm hollow with 1 wall thickness . I race on dirt now and have a twin modified Honda kart that runs a 40mm crmo axle that's got 5 mm wall and it grips like anything with slicks on granite . My father had it made for a jolly kart in the late 90's and anyone who drove it couldn't believe how well it went but he never told them why . So I'm with John that its material that makes the difference and chassis are the same . I race a 2000 model tony on the dirt as well and can see no difference to running a newer kart . I also have a pcr from the 90's that before I got it ran around Ipswich getting to within 2 tenths of a second of the quick guys . and that was only a few years ago .

Hi wayne,

That (below) doesn't describe my opinion on this. I think the material (i.e. particular grade of steel) is not very important other than how it affects longevity of the chassis, or axle. I think that the OD and wall thickness of the tubes are important, as is the shape of the chassis, (i.e. its' layout, mostly as seen from above).

My first kart was a near new 'Apache', which came with a very thick walled 30mm axle. It won quite a few races over a number of years, both dirt and bitumen. It was retired when I bought a near new 'Sprinter' (one of John Pizarros' chassis), which had a thin walled 30mm axle, when such things were common and fashionable. It was hopeless, I struggled and struggled but just couldn't get it to work properly (maybe I was just hopeless at setting it up...).

Anyway, one day I was wondering what to do, maybe get another kart? For no well thought through reason I decided to try the thick walled Apache axle in it, which turned out to be a brilliant idea (or maybe it wasn't an idea as such, I might just have destroyed the thin walled axle and fitted the thick walled one simply because I already had it, can't recall, it was a long time ago). The thicker / stiffer axle transformed the Sprinter, much better, more trophies followed. 

My current kart (Arrow) came with two axles, a thin and a thicker walled. Initially I used the thick walled one, which was OK except my aging body was being beaten up on the Manning Valley bumps (the track had become much bumpier than when I had last raced on it years ago). In an atempt only to reduce the body abuse I fitted the thinner walled axle on the assumption that it would be 'softer', which made the ride noticably less painful. I didn't notice a change in lap time or handling, but it was easier to drive...

Regards,

John.


wayne gilby said:

 So I'm with John that its material that makes the difference and chassis are the same . 

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