Axle 40 mm vs. 50mm

[Chris M Johnson]

Can someone please explain the differences in axle thickness. As far as I can reason out, a thinner axle may have more flex and be better for bumpy surfaces.

That said, what would be better for autocross and sprint?

[Shawn Carroll]

I belive you can get them both in different thicknesses 40mm or 50mm is the diameter of the axel

[Bernie Baldus]

Yes,
The terms 50mm and 40mm axle refers to the overall diameter.
You have different thicknesses in each of the 40mm and 50mm axles.

The wall thickness does not necessarily correlate to the stiffness.

Your best setup would depend on many factors.
1 Horsepower
2 Track/surface conditions
3 Weight

I would call a sprint kart shop and talk with a tech about your specific application or give more data and see if you get answers here!

bb

[joseph hollinger]

Pete: you really need to add this to the frequently discussed topics
thread.

[Mike Goebel]

[joseph hollinger]

Chris:

This is a topic that gets rehashed around here quite a bit. I'm going to
try and give you the fair and balanced viewpoint:

1) The stiffness of an axle (i.e., the amount that it will deflect
under a given load) depends only on the axle's outer and inner
diameters. A 50mm axle is generally stiffer for this reason.

2) The stiffness of an axle does not depend on the type of
steel used in its construction.

3) There may be other properties that depend on the type
of steel used. These properties aren't very well defined,
but there are lots of people who swear that they have
an impact on lap times.

[patrick slattery]

40MM is what most Domestic models race , and 50mm is what most European karts are designed for. In WKA most classes have to run 40MM due to pressure from the domestice mfg.

Pat Slattery

[Andrew (AJ) Weber]

[Chris Livengood]

"the well-known hardening of very soft axles over time"

hmmm, i didnt get the memo
_________________
http://www.Chrislivengood.net
http://www.Work-Racing.com
http://www.OurZeal.com
http://www.Karting101.com
"Auto racing, helping white guys get laid since 1887!!!"

[Kevin Willmorth]

To answer the original question:

First, the 50mm axles larger outside diameter produces a stiffer structure than the 40mm, for the same wall thickness. The circumference of the 50mm axle is roughly 157mm, compared to the 40mm at 126mm. This is an increase of 24%. For the most part 40mm axles are 3mm thick, and 50mm axles are 2.5mm. This is a difference of 20%. This would mean that the amount of steel used for the 50mm axle is about the same as the 40mm. All things being equal (heat reating, etc.), this essentially produces a gain in structural stiffness due to the diameter, at roughly the same weight.

Second, the greater cross section and thinner wall will produce a structure that is more susceptable to distrotion. As the axle bends under load, the larger axle will become more ovoid in shape compared to the 40mm with its thicker wall and smaller diameter. This reduces overall stiffness somewhat in bending, while increasing the structures response - or rate of return. The result is a more "lively" axle, by a small amount.

Third, there is most certainly a difference in the affect of alloy and heat treating that creates differences in axle stiffness. Axles on karts are acting like springs, so the plastic resistance of the material in the axle is critical to how well it functions in this regard. This will aply to both axle diameters, and is not likely to have more or less affect based on diamter alone.

For the most part, beyond the fringe top level tuners, the differences between a 40mm and a 50mm axles are going to be small to miniscule. You can get either to perform equally to one another. For tracks that reward a softer setup, the 40mm axle will produce a little more latitude, while the 50mm will likely favor those tracks where stiffer setups are fastest.

On small disadvantage the 50mm axles do bring to the party is a 25% increase in bearing surface area and diameter. This greater diameter produces higher bearing speeds, which means greater drag on two levels -more area + greater speed = greater drag.

Another disadvantage of the 50mm axle is that its thinner wall, and greater sectional distortion can produce a greater propensity for getting bent than the thicker 40mm axles of the same hardness. In other words, they look stiffer (and cooler), but may actually be more porne to getting out of shape during a collision or hard curb hit. Sort of counter intuitive from a visual standpoint, but is supported by the experiences of many running the larger tubes.

I have run both axle diameters on the same chassis. For my own taste, I prefer a softer setup all around, and found even the softest 50mm axle too stiff for my taste. We run a 40mm axle now, both medium and soft. I personally don't see the advantages of the 50mm axle at all. You can find hard 40mm axles equal in stiffness to the hard 50mm units, and gain the advantage of the smaller bearing diameter. Unless you are lookign for a super stiff setup, the 50mm axles does not produce any advantage over the 40mm setup. IMHO, the change is based more on marketing something that looks like an innovation (but really isn't), than actual real-world advantage.

[Kevin Willmorth]

On the subject of soft axles getting harder over time - this will depend on the alloys used to make the axle. For carbon steel, work hardening is rare, and usually restricted to the surface. This might occur where the bearings clamp onto the axle, which could cause cracks that eventuallly create a break in that area - which has been seen to occur.

In some alloy steels, especially this containing Magnesium or Chrome, work harding might be deeper and theroetically affect the overall stiffness of the axle itself. Interestingly, work hardening will occur at the areas experiening the greatest amount of flex, which will have the affect of making the axle feel stiffer. However, even in 4130 chromoly, the amount of magnesium is less than 1% and chrome 1.1%, so the affect will be very small. Most italian chromoly has less of both, so the effect on axles over time is likely to be nominal. Differences in axles made by different producers are likely a combination of the material used and heat treatment of the material, whcih can have a dramatic affect on two visually identical parts.

The reason axles are listed in "hardness" and not "stiffness" is that they are referring to the temper or heat treated state of the material itself. Chromoly has the distinct advantage of being heat treated to very precise states. Further, the use of air or oil quenching produces very different effects as well. This produces a harder or softer axle, even though the material used in both is identical. Tool steel is generally case hardened (surface made hard while leaving the core in a softer, less brittle state.) Axles are heat treated - or through hardened, at far lower temperatures, which changes the overall resistance of the material to bending forces.

At least this is what I have found over the years from messing with bicycles and race car stuff. Us steel bike fans have had more than our share of discussions about microscopic variances in alloys and heat treated states of tune - driven by the great Italians and their love of the material once again. I beleive that the reason the Italians make great kart frames and chassis materials, is that they are tapping into over 100 years of steel frame bicycling knowledge - while we Americans seem to be in love with aluminim and the more exotic stuff (carbon fiber, etc..)

[patrick slattery]

Kevin what brand of kart are you running? Have you tried to set your 50mm out in the sun to soften it up some.

Pat

[Kevin Willmorth]

[Erik Maxfield]

Stiffness

PI() * (O.D.^4-I.D^4)/64

for a given elastic modulus

Elastic modulus of ALL STEEL (including 4130/4140 and 1040) is 30 million PSI.

Of course you can juggle hub lengths to alter the moment equations and the amount of leverage that the tires have on the chassis......that is another discussion.
_________________
Erik
The early bird gets the worm.....
The second mouse gets the cheese....

Stock CR125.
Chassis-FrankenKART
Intrepid/ITAL combination

[Kevin Willmorth]