[Schackel]

Someone wrote in a forum that hollow steel axles, titanium axles and carbon fibre axles are absolutely useless as rear axles because of the oscillations they create as driving axles.
24th scale "scale" models ( quite heavy ) as well as a 32nd scale hardplastic model of - I think - ca. 85-90 gms were some tenth slower with titanium axles.

Any ideas or personal experiences ?

Roland

 

[300SLR]

The only reason I can see for using these expensive axles is that they give an advantage - such as making the car go quicker.

If they are no better than ordinary axles then it's money spent for nothing. If they are actually worse - well.............

For sure they will need to be used correctly. If they are less robust that the normal axles (and its difficult to see how a hollow axle wouldn't be less robust that the same axle without the hole up the middle) maybe they are only good for qualifying. It would be interesting to hear if somebody found a reason the expensive ones are slower. Difficult to comment further without knowing exactly what they were saying - what sort of oscillation they were talking about and why they thought particular sorts of axle made a differance?

Please can you give a link to the original thread.

 

[Schackel]

QUOTE (300SLR @ 21 Oct 2011, 21:09) <{POST_SNAPBACK}>......Please can you give a link to the original thread.

Well, of course I can. But the thread is in a German forum, and what I wrote is a reliable summary of the statement there.
I don´t know which different ways a driven axle can oscillate - I only know one.

Roland

 

[300SLR]

OK, I have to admit my German isn't up to understanding the original. It would be interesting to know how this "oscillation" has been observed and look at what he thinks is going on. It's often difficult to work out what is really happening, it opens up questions like
Is beam flexing of the axle leading to oscillation?
Are bearing clearance causing radial oscillation?
Are end clearance causing axial oscillation?
Are cyclic changes in grip leading to oscillation?

 

[munter]

QUOTE Someone wrote in a forum that hollow steel axles, titanium axles and carbon fibre axles are absolutely useless as rear axles because of the oscillations they create as driving axles.
There must be some serious power being applied for oscillations to occur.
Or maybe the tolerance of bearing to axle is too great.
I suspect they are racing big power (C can and open class strap motors)1/24 metal chassis cars.These things would scare old ladies.

It is hard to beat a good straight axle.

regards

 

[Schackel]

QUOTE (300SLR @ 21 Oct 2011, 22:00) <{POST_SNAPBACK}>Is beam flexing of the axle leading to oscillation? Well this is the only reason mentioned there. They claim these axles are not stiff enough compared to solid ones...

QUOTE (munter @ 21 Oct 2011, 23:42) <{POST_SNAPBACK}>There must be some serious power being applied for oscillations to occur.
Or maybe the tolerance of bearing to axle is too great.
I suspect they are racing big power (C can and open class strap motors)1/24 metal chassis cars.These things would scare old ladies.

The models I had talked about use ( 1/32nd ) the orange Slot It, NINCO NC 2 and 5, Scaleauto SC-11, and the 24th scale ones use the normal golden Carrera Motors with up to 15 volts, heavy chassis of the Plafit type. No other information. The whole article sounds so "professional" that an average reader gets the idea it can´t be anything like bearings to axle tolerances... quite pretentious statements...
I repeat one statement : "when I replaced the steel axle with a titanium one my model instantaneously went some tenths slower"....As I said a 32nd harplastic model with HRS chassis, weighing between 85 and 95 grams. Tell me one logical reason for that if all premises are the same as before, only another axle material. This is more than unlogical, isn´t it ?

It is hard to beat a good straight axle.

regards

Regards, Roland

 

[300SLR]

OK, so the claim is that the extra flexibility of the axle is causing oscillation.
Unlike many things in slot racing, the stiffness of an axle can be calculated.

I'll go through it step by step so anybody who is interested can check it for mistakes.

From simple beam theory, the stiffness is proportional to (2nd moment of area of the cross section) x (Young's modulus of the material)
We don't know exactly what grade of material is used, but Young's modulus for steel is about 200GPa and Young's modulus for titanium alloys is probably in the range 95-114GPa. So it round figures the stiffness is halved.

Does that matter?

A lot of the top metal chassis racers have switched from 3/32 inch (2.38mm) to 2mm diameter axles in recent years.
The reduction in stiffness obviously isn't a problem to them otherwise they wouldn't be doing it.
These cars are running at much higher axle revs than the Slot It/ NINCO/ Carrera cars these guys are talking about.
Higher revs means the axle will be driven by higher frequency vibration. That means its resonant frequency will need to be higher to avoid oscillation. That means the axle stiffness will need to be higher.
Or to put it simply, the axle stiffness is more of a problem for theses high speed metal chassis racers.

So how do the stiffness of these axles compare?

The 2nd moment of area of the cross section for a round axle is proportional to the fourth power of the diameter.
So reducing the axle size of the axle from 3/32 inch (2.38mm) to 2mm diameter approximately halves its stiffness.
So a 3/32 inch (2.38mm) titanium alloy axle will have approximately the same stiffness as a 2mm solid steel one.

So if oscillation due to axle flex isn't a problem in a 2mm steel axle on a fast metal chassis car, it won't be a problem in a 3/32 titanium alloy axle - particularly in the relatively lightly stressed conditions on these plastic chassied cars.

So unless some expert out there can spot a mistake in my reasoning it looks like the oscillation problems these guys reported cannot be due to axle flex.
Of course that doesn't mean they haven't got a problem, just that they may have misunderstood the reason for the problem.

 

[bwaminispeed]

300, your entire dissertation was on solid axles of different sizes.

The original post was about HOLLOW axles of the same size flexing/oscilating.

Not a valid comparison I would think.

 

[John Cahill]

I didn't read this thread extremely carefully since the source material was not available...however my personal guess would be any oscillations (if at all) would have to be axial/torsional and harmonic in nature (with the contrate adding a forcing function). And having said that...IMO without any need for calculation (respects to 300SLR) that doesn't pass the smell test. I agree with 300SLR's statements...and any of these variations is easy enough to calculate. It's just supremely unlikely to be an issue for multiple reasons.

Meanwhile, if anyone can read German and can access the original material...was the actual frequency measured or calculated in the brief? Moreover, were any failures observed...cause one would expect fatigue to result.

John

PS after rereading 300SLR and Al's comments...just wanted to add that if indeed the proposed flexing is torsional (as I suspect)...then the hollow-ness has very little effect at all (since torsional stiffness is significant in the outer radius only...ie hollow driveshafts). I'm also tempted to point out that if the flexing were actual bending (which I doubt)...then the combination of rotating and bending would cause "twirling" which is noticably unstable and easily observable (and therefore unlikely, cause people would have noticed right away).

 

[356speedster]

Could it be as simple as that the hollow axles were not as straight as the solid axle? A good reason not to use a hollow rear axle is that they bend very easily.

Another thing to consider is that the mass (weight) of the drive-train acts like a flywheel, so when you lighten the rotational mass in the drive-train the car becomes more sensitive to vibrations from the motor.

A car with a lighter "flywheel" may also feel more "twitchy" to drive as it responds quicker to trigger input, so the drivers need to change their driving style, and that takes time.

IMO the lightweight axles and wheels work best when used in the front.

 

[Mike Newns]

If you look at a out and out slot racing car (BSCRA etc) you will see that the rear bearings are placed virtually against the back of the rear wheels- to my mind it is inconceivable that any oscillation could occur with any rigid axle in the 3-4 mm between the bearing and the end of the axle. If you look at an in-line car there is a larger length of unsupported axle between the bearings which carries the contrate this is were any oscillation would occur and would result in bad meshing of the gears which as we know produces a distinctive sound we all recognize.

I think a more important consideration for hi-speed racers is that the axles should be of an accurate size and fit tightly in the ball races (the fit should be much tighter than a bronze\brass bearing) and it may be as these items are specially produced for slot racing they are a better fit. The other consideration is mass which is as important to scale racing as all out slot cars the potential saving of a gram or so may seem little but when added to other weight saving measure can add up to a significant amount.

Mike

 

[Mike Newns]

ps Also significant is the damage as a result of an "off" which has been know to beand axles, break bearings etc.

Mike

 

[bwaminispeed]

Just as a quick aside, coolest axle incident I ever saw was at my own shop in the late 90s.

I had a King track at the time, and, we were racing gr12 Wing cars. Our one and only lady driver got ridered down the shute into the 90, and ended up hitting the bridge so hard, it broke BOTH rear wheels right off the car, thus, inventing the 3 piece rear axle.

Needless to say, we were all quite impressed.

 

[john's slotracing]

QUOTE (bwaminispeed @ 22 Oct 2011, 01:36) <{POST_SNAPBACK}>300, your entire dissertation was on solid axles of different sizes.

The original post was about HOLLOW axles of the same size flexing/oscilating.

Not a valid comparison I would think.
It looks like the original post was about hollow steel axles, solid titanium axles and solid carbon fibre axles
Perhaps something has got lost in translation

Hey Schackel, does the original German text talk about solid or hollow titanium axles?

 

[John Cahill]

re: Could it be as simple as that the hollow axles were not as straight as the solid axle? A good reason not to use a hollow rear axle is that they bend very easily."

One other question that pops up is whether the gear and wheels were affixed with set screws that deformed or otherwise created a stress concentration and/or imbalance in the shafts. Also, not having personally seen one of these light axles, do they bend easily or "buckle" easily...bending is a function of stiffness, buckling is irreversible and the result of dynamic/geometric instability. Anyway, I'd still like some basic information on materials and dimensions, loads and geometry. I'm skeptical, but it's easy enough to move this from speculation to resolution with some data.

John