Axel Diameter: Is smaller really better?

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Jewel
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Axel Diameter: Is smaller really better?

Post by Jewel »

Why would axel diameters made smaller be faster? At Hodges Hobbies, Hugh says smaller ID .075 axel's are faster. I would love to know why.
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Re: Axel Diameter: Is smaller really better?

Post by RACER X »

In a perfect world, a smaller axle should be faster because of the reduced friction. We have been running the largest axles we can find with great success. One small bump in the track and a small axle car will start to "shutter" and scrub off quite a bit of speed. A large axle car just seems to be less tempremental.


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Re: Axel Diameter: Is smaller really better?

Post by MaxV »

I believe that just like in automobiles, a tighter tolerance is better. With small diameter axles, there is too much play in the wheel hub. As mentioned by RacerX, this can easily cause vibration and wobble that is undesireable.
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Re: Axel Diameter: Is smaller really better?

Post by Stan Pope »

Smaller diameter axles are better IF you can legally (and effectively) reduce the inside diameter of the hub AND the axle size is reduced only as necessary to fit the reduced diameter hub.

The rub, of course, is that most means of hub diameter reduction run afoul of the "No bushings" rule commonly applied in PWD.

I do not have experimental data supporting further reduction of axle diameter. Although I bet that Hugh does have the data, I am not sure that the results could be generalized to apply to "most tracks." There are just too many variables in track surface.

The analytic and anecdotal results expressed previously in this thread seem compelling.
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Re: Axel Diameter: Is smaller really better?

Post by Jewel »

Well I am still unsure.

We have two circular tangents coming together with the axel and ID of the hub. As I understand it, the wheel rests in a line on the bottom of the axel. If the axel is smaller than the rate of curvature relative to the wheel ID is greater. This will make the resting line of the wheel thinner I suppose increasing the pressure over the rolling surface. If friction is reduced by the reduction of surface area than perhaps slightly smaller axels are better. Perhaps empirical spins tests using the same wheel, but using two different axels with diameters equally polished and lubricated that are different by .010 would help clarify the issue. However, I see a flaw with a spin test, the best test might be slow roll outs for distance which would simulate the pressure on the wheels from the weight of the car.

I really don't know if what Hugh has claimed can be effectively supported.
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Re: Axel Diameter: Is smaller really better?

Post by Stan Pope »

Jewel wrote:If friction is reduced by the reduction of surface area than perhaps slightly smaller axels are better.
The supposition is a common misconception. Weight (normal force) and coefficient of friction are part of the friction force formula, but area (and pressure) are not. Or, is there an update to this since I went to school??? (Some wags talk about Newton' freshly published Pricipia Mathematica that was then required reading. Forty-? years isn't that long ago! )
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Re: Axel Diameter: Is smaller really better?

Post by Jewel »

I would like understand friction better and maybe rid myself of mis-understanding. The friction formula? What is it?

My intuition which may be wrong would come up with a formula like this.

Friction= Surface Area x Speed x Rougness of surface x Force between two separate moving bodies x Hardness of material. (I am sure there are more variables)

I am struggling to understand how all this works so I can clearly articulate the right ideas.

When it comes to smaller axels, then I would think smaller is better if this is true? However the conter veiling feeling is that a tight fit would be good to reduce play and create better consistancy. However, in a Pinecar the normal force is always on the bottom surface of the axel, sitting on that line, and the car is always going straight, so the physically requirements may really be different than in a car as Max V has sugested. Perhaps a discussion thread on friction would be useful for the readers of this forum?
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Re: Axel Diameter: Is smaller really better?

Post by Darin McGrew »

Jewel wrote:I would like understand friction better and maybe rid myself of mis-understanding. The friction formula? What is it?
In physics, friction is abstracted as a constant (mu) times the force perpendicular to the surfaces. Thus:

F[friction] = mu * F[perpendicular]

The value of mu depends on the nature of the surfaces. Sandpaper vs. sandpaper has a high mu, glass vs. glass has a low mu, and so on.
Jewel wrote:My intuition which may be wrong would come up with a formula like this.

Friction= Surface Area x Speed x Rougness of surface x Force between two separate moving bodies x Hardness of material. (I am sure there are more variables)
Surface area is irrelevant. If you want to think of it this way, decreasing the surface area increases the pressure (assuming the force is constant), and the increased pressure cancels the decreased surface area. I did experiments with this in school. It really does work this way.

For any two surfaces, mu (the coefficient of friction) will be different when the surfaces are static vs. when the surfaces are in motion. But once the surfaces are in motion, speed is irrelevant.

Roughness of surfaces, materials the surfaces are made of, etc. determine mu. The force perpendicular to the surfaces is multiplied by mu to get the force of friction.
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Re: Axel Diameter: Is smaller really better?

Post by MaxV »

Well said Darin.

Thus we conclude:

1. Lifting a front wheel does not reduce friction. The benefit is due to reduced starting inertia, elimination of the worst wheel, and easier alignment.

2. Grooving or narrowing of wheels does not reduce friction. The benefit is less wheel weight (lighter wheels are better) and possibly less interaction with track flaws which could divert the car from a true course.

3. Reducing wheel to axle contact through axle grooving, axle thinning, and inside hub machining does not reduce friction. I don't believe axle thinning helps, but axle or hub grooving may help by holding lube. I can think of no other benefit.
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Re: Axel Diameter: Is smaller really better?

Post by Stan Pope »

darin_mcgrew wrote:Thus:

F[friction] = mu * F[perpendicular]
Great! It hasn't changed!
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Re: Axel Diameter: Is smaller really better?

Post by Stan Pope »

MaxV wrote:3. Reducing wheel to axle contact through axle grooving, axle thinning, and inside hub machining does not reduce friction. I don't believe axle thinning helps, but axle or hub grooving may help by holding lube. I can think of no other benefit.
Graphite reservoirs are a trade-off. Crunching up fresh graphite is a net drain on energy. In late stages of long races, the coating of recently crushed graphite is a plus.

Depending on the racing protocol, either might be advantageous.

Do you need the extra graphite to "stay good" at the end of the races? You might need it in the latter stages of a quintuple elimination involving 100 cars.

Can you afford the cost of crunching graphite during the early heats? You might not if your score is determined by 4 to 8 timed heats.
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Re: Axel Diameter: Is smaller really better?

Post by Stan Pope »

darin_mcgrew wrote: Surface area is irrelevant. If you want to think of it this way, decreasing the surface area increases the pressure (assuming the force is constant), and the increased pressure cancels the decreased surface area. I did experiments with this in school. It really does work this way.
Darin, you would be aghast how many of the PWD booklet authors have propagated the myth of reducing friction by reducing surface area!

What you describe is (or, at least, was, prior to 1960) taught quantitatively in high school physics. They even dealt with the relationship of slope to Ff, though I don't recall the word "cosine" being used until the subject was revisited in university physics. Back then, trig was an optional high school senior course.

The underlying concept is, or should be, taught qualitatively in science courses for even younger folks.
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Re: Axel Diameter: Is smaller really better?

Post by TDean »

Thanks for the physics lesson guys. You have all explained the subject of friction so clearly that even a dullard such as myself can understand and slap the forehead with what now makes perfect sense -- I see that I may need to re-focus my energies elsewhere in the ongoing quest to shave a few micro-seconds off the race time.
But hey -- shouldn't this friction stuff be discussed under the Racing Science category? :wink:
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Re: Axel Diameter: Is smaller really better?

Post by Stan Pope »

MaxV wrote:Thus we conclude:

<snip>

2. Grooving or narrowing of wheels does not reduce friction. The benefit is less wheel weight (lighter wheels are better) and possibly less interaction with track flaws which could divert the car from a true course.
Darin has described the physics of "sliding friction" very nicely.

"Rolling friction" is a different animal. When the wheel tread rolls on the track surface, it meets resistance at the tread-track interface. That is called "rolling friction."

Some factors in rolling friction include adhesion (between the tread and track), compression (of the tread and track), deflection (of the wheel due to surface irregularities), and absorption (as the wheel crushes material such as dust particles). It might be argued that absorption and deflection are the boundary conditions for compression.

When a wheel rolls, it experiences rolling friction at the tread-track interface and sliding friction at the axle-hub interface.

Any more, Darin? And, can you correct terminology, while you're at it?
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Re: Axel Diameter: Is smaller really better?

Post by Barga Racing »

Man, you guys are way over my head with all the formulas but I am really starting to think that all the stuff I am doing to the inside of hubs is all wrong. We have tried threading and also using a dremel engraver to hollow out the inside leaving only about 1/16 of an inch on either end. We even tried just grooving the axles. But to be honest we have not worked much with just polishing the wheels and axles wth no grooving whatsoever. Seems to me that we need to reconcentrate on our areas of improvement!
Last edited by Barga Racing on Wed Aug 27, 2003 8:17 am, edited 1 time in total.
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