Rail Riding - "How To Guide"

Secrets, tips, tools, design considerations, materials, the "science" behind it all, and other topics related to building the cars and semi-trucks.

Have you had success with a "rail rider"?

Yes
90
49%
No
8
4%
Somewhat
12
7%
Haven't tried yet
72
40%
 
Total votes: 182

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Duane
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Re: Rail Riding - "How To Guide"

Post by Duane » Thu Apr 07, 2011 9:31 am

*5 J's* wrote:
Ynot wrote:Can I put a groove in the axle for less contact surface and to allow for graphite or is this no longer a good idea. I saw this in Meades book. I've also seen multiple grooves per axle somewhere online. Is this better or worse?
We are not allowed to grooved axled in our Pack so I have no experience with grooved axles and graphite. I would say if grooves provide an advantage it would be quite small and the risk for messing up the surface is too great of a risk. I would not do it.

In league racing I use Krytox oil and groove these axles - but I can tell you from experience it's real easy to mess up a good axle, and once grooved you have to be REAL careful to not bend an axle. A slight unintentional bend and you will lose any benefit gained from the grooves. My two cents anyway...
According to Dr Jobe and other experts on the physics of these cars, total friction does not go up or down when the size of the instantaneous contact area is increased or decreased; the total friction stays contact. So there is no direct beneficial effect, contrary to intuition. The effect (if any) is indirect and secondary, from how it affects lubrication over time.

If the groove has sharp, right-angled shoulders, those edges could scratch the soft plastic of the wheel bore and mess things up badly.
If the groove has broad gently-sloped shoulders, that means that some of the active touching axle surface has a different diameter than the rest, which could add a kind of internal canting that you don't want; it changes as the wheel migrates left and right across the axle.
The groove makes it hard to do an even job of polishing the axle after cutting the groove; the shoulders will get eroded more during polishing.
Grooving could also send the wheel inward where it rubs continuously against the car body, or outward where it rubs continuously against the nail head.

If grooving is (sometimes) beneficial, I guess it is from leaving a thin ring of unwiped surface on the inside of the wheel bore. Loose graphite flakes can hide there until accidentally redistributed by the car handlers between races, or redistributed by left/right wheel wanderings during a race. But excess loose graphite is not good in the early races; what you want is worked-in graphite smoothly covering all friction surfaces (the entire bore, and the bottom edge of the axle).

In the past, I've re-applied graphite just before check-in, by dabbing in graphite with a small watercolor paint brush and then spinning the wheels manually while holding the car upright, and then knocking off the excess. I'm now thinking that it would be better to hold the car upside down while doing that manual spinning, so that the bores are rubbing against the same side of the axle that will be facing towards the track and bore during racing.



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Re: Rail Riding - "How To Guide"

Post by *5 J's* » Thu Apr 07, 2011 9:55 am

Duane wrote:According to Dr Jobe and other experts on the physics of these cars, total friction does not go up or down when the size of the instantaneous contact area is increased or decreased; the total friction stays contact. So there is no direct beneficial effect, contrary to intuition. The effect (if any) is indirect and secondary, from how it affects lubrication over time.
Perhaps many smarter then I, Dr Jobe included, have concluded that science dictates that grooves provide no direct beneficial effect - but I can tell you that real world application proves grooved axles are faster when using oil. I'll leave it to the experts to figure out the why.



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Re: Rail Riding - "How To Guide"

Post by derbyspeed » Thu Apr 07, 2011 11:08 am

*5 J's* wrote:Perhaps many smarter then I, Dr Jobe included, have concluded that science dictates that grooves provide no direct beneficial effect - but I can tell you that real world application proves grooved axles are faster when using oil. I'll leave it to the experts to figure out the why.
5 J's I would have to agree with you, it seems real world application is the final law of the land, if it works it doesn't matter if science agrees or not. Compare grooved axles with thinner wheels, if the theory on grooved axles doesn't allow for a faster car then why would a thinner (meaning not as wide) wheel make the car faster - both have the same load but distributed in a smaller area. Or maybe I'm in left field on this one. :scratching:


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Duane
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Re: Rail Riding - "How To Guide"

Post by Duane » Thu Apr 07, 2011 1:32 pm

derbyspeed wrote: if it works it doesn't matter if science agrees or not.
Yes. Better said as 'if it works, it doesn't matter if theories agree or not'. Science is more about the repeatable experiments than the theories. The theories are added, just to have some way to predict the results of the experiments, and maybe understand them too.
if the theory on grooved axles doesn't allow for a faster car then why would a thinner (meaning not as wide) wheel make the car faster - both have the same load but distributed in a smaller area. Or maybe I'm in left field on this one. :scratching:
Good question!

If the wide and thin wheels have identical outer diameters, bore diameters, total weight, moment of inertia, and axles, then it is fair to compare them for wide vs thin effects.

The thin wheel will have significantly less wind resistance than the fat wheel. In the sleek slab cars with standard BSA wheels, the wheels give over twice as much wind resistance than does the body of the car. Using thin wheels, their total wind resistance is less than that of the thinnest car body.

The tread edge and sidewall profile of BSA wheels are not ideal for gliding along the guide rail with minimal friction and minimal steering-correction bounces.

People have gotten faster results with BSA wheels when they lathe the tread surface into an H or V profile so that the track contact is on just a thin edge of plastic rather than the full width of the tread. Meade says this helps by reducing inertia of the wheel, and by reducing friction by reducing contact area. But the area-doesn't-count theory claims that reducing contact area doesn't reduce friction directly. And yet it helps!

I've read elsewhere, people's theories that narrowed wheel-to-track contacts help by avoiding most of the hard-to-see bumps of the track surface. If the car has wide wheel treads with full contact, it will get jolted upwards by running over any hairs or other junk particles on the track surface, within the tread footprint. Each jolt causes some diversion & loss of kinetic energy. Also, the junk can stick to the wheel and cause jolts for every revolution thereafter. If the wheels have narrowed ridges of track contact, there is much less of the track surface that can affect the wheel and fewer pieces of junk will be rolled over or acquired. These differences are hard to see but can be heard.

One advantage of canting wheels is that the wheel touches on just one edge, not the full tread width, giving the same benefit of steering around most junk without carving the wheel itself.

For axle-to-wheelbore contact, the analogous thing would be if the wheel bore had scratches or lumps in only one spot, and the sculpted (grooved) axle avoided that spot. But it is easier to polish the bores and axles to ideal cylinders than to leave bumps and avoid them by luck.

You could also imagine if the grooved-axle methods & materials were applied to the wheel/track contact. What if our tracks were made of soft plastic, and our wheels were made of knife-edged metal? How many races would it take, until the track was permanently damaged by scratches? Would graphite or thin oil help much with all those scratches?



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Re: Rail Riding - "How To Guide"

Post by derbyspeed » Thu Apr 07, 2011 1:57 pm

I would think (for what it's worth) that when you polish a grooved axle that you are also going to polish that edge as well, whether you are trying to or not, therefore the bore would ride smoothly or possibly not at all on that edge, depending on where you put the groove - persay in a position where the groove would be in the middle of the bore and the wheel would ride on both sides of the groove.

This would be of course if you had canted wheels and were able to pull the wheels out at the starting line, otherwise some in and out movement may cause some scarring, but I would think it would be minimal.


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Re: Rail Riding - "How To Guide"

Post by *5 J's* » Thu Apr 07, 2011 4:18 pm

Duane wrote:
derbyspeed wrote: if it works it doesn't matter if science agrees or not.
Yes. Better said as 'if it works, it doesn't matter if theories agree or not'. Science is more about the repeatable experiments than the theories. The theories are added, just to have some way to predict the results of the experiments, and maybe understand them too.
Ahh - yes, I agree 100% Duane, but your statement was that the Dr Jobe and other experts on the physics of these cars... not the theories. Are Doc Job and the experts stating the laws of physics or conjecturing theories, or hypotheses....

Duane - after reading this it sounded a bit argumentitive to me. Know that it is not meant to be argumentitive, rather I see a lot of theories on this forum - which is great - but some need to be taken with a grain of salt. Some of the quoted theories have been disproven in real worlld application. Obviously science and physics prevail, but some peoples interpretation may be deficient.



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Re: Rail Riding - "How To Guide"

Post by Duane » Thu Apr 07, 2011 7:04 pm

*5 J's* wrote:Some of the quoted theories have been disproven in real world application.
I wonder now, about the theoretical claim that total friction is independent of the area of the contact surface.
I've merely been parroting that statement without knowing any facts about it, or what the exceptions are.



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Re: Rail Riding - "How To Guide"

Post by FatSebastian » Thu Apr 07, 2011 7:55 pm

:offtopic: Recent comments have apparently wandered pretty far from the topic of this "sticky" thread, Rail Riding - "How To Guide". May I suggest that this interesting line of discussion be continued as a separate thread? (Perhaps gpraceman could port recent posts into their own topic.) Having said that...
Duane wrote:I wonder now, about the theoretical claim that total friction is independent of the area of the contact surface. I've merely been parroting that statement without knowing any facts about it, or what the exceptions are.
A brief presentation by Doc Jobe of the hows and whys of friction versus contact area can be found in this lecture. The context of this physical effect is seemingly limited to certain types of sliding friction (rather than rolling friction) between dry surfaces without lubrication. Outside of this context, one liability stated by Jobe regarding grooved axles of which I am aware is that they can serve to trap un-crushed graphite.



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Re: Rail Riding - "How To Guide"

Post by *5 J's* » Fri Apr 08, 2011 5:25 am

FatSebastian wrote::offtopic: Recent comments have apparently wandered pretty far from the topic of this "sticky" thread, Rail Riding - "How To Guide". May I suggest that this interesting line of discussion be continued as a separate thread? (Perhaps gpraceman could port recent posts into their own topic.) Having said that...
Duane wrote:I wonder now, about the theoretical claim that total friction is independent of the area of the contact surface. I've merely been parroting that statement without knowing any facts about it, or what the exceptions are.
A brief presentation by Doc Jobe of the hows and whys of friction versus contact area can be found in this lecture. The context of this physical effect is seemingly limited to certain types of sliding friction (rather than rolling friction) between dry surfaces without lubrication. Outside of this context, one liability stated by Jobe regarding grooved axles of which I am aware is that they can serve to trap un-crushed graphite.
I am going to post on Stan's friction post as I think we need to look beyond static friction and consider rolling friction, which is largely ignored in these pinewood theories.

For example take two identical basketballs, one full of air and the other slightly deflated. Roll them along the ground with the same initial velocities. Which will roll furthest? The one that is more full will. What is the difference between the two balls? They both have the same weight. The only difference is that one has more contact surface area than the other.



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Re: Rail Riding - "How To Guide"

Post by Slalom » Sat Apr 23, 2011 9:24 am

For example take two identical basketballs, one full of air and the other slightly deflated. Roll them along the ground with the same initial velocities. Which will roll furthest? The one that is more full will. What is the difference between the two balls? They both have the same weight. The only difference is that one has more contact surface area than the other.


I would think it is because a deflated basketball isn't perfectly round? A solid ball rolls faster than a limp ball



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Re: Rail Riding - "How To Guide"

Post by FatSebastian » Sat Apr 23, 2011 10:15 am

*5 J's* wrote:I am going to post on Stan's friction post ...
which can be found here.



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Re: Rail Riding - "How To Guide"

Post by PWRookie » Thu Mar 01, 2012 8:59 am

Great web site. I am a rookie, but have been reading this thread and a question popped in my head.

What if you consider the glued wheel to be a "feeler" like the kind discussed in the Pinewood Derby Times article below:
http://www.maximum-velocity.com/pinewoo ... imes-v6i7/

If you get a speed advantage with a "feeler", why not utilize the glued wheel as a "feeler" by aligning the car to ride on it instead of the rotating wheel?

Has a study been done to see which produces less friction or has less impact on slowing down the rotating wheel? I would think the less friction (by the rail in this case) placed on the rotating wheel the better.

I haven't tried either yet, but would be interested if anyone has any experience in this.

Stan Pope wrote:
Ten Thumb Tom wrote: If I understand correctly, the scenario you are both describing is a car that is not a rail rider, but a three wheeler with alignment so dead on that the raised wheel would not touch the rail, and that under these conditions, the car could benefit from less vibration by gluing the wheel down rather than allowing it to rattle.
No, the car is almost certainly a 3-wheel RR with the dominant front wheel (DFW) toed in a bit to keep it against the rail. Dead-on alignment will almost always touch both sides of the rail somewhere along the run down the track.

Since the DFW is lightly loaded (almost always carrying less than one ounce), the friction losses from it rubbing are small. If you add a bit of positive camber to the DFW so that it touches below the top edge of the rail, the situation may even be better ... it will be rolling on both the track and the side of the rail, though there is also some sliding on the rail.

The value of glueing the wheel down is probably very slight. It was probably done to "show off."

If the DFW were sufficiently lightly loaded and if the car's inclination to "run to the rail" could be controlled through the rear wheels' differential friction, then you could probably get by with glueing in both front wheels and letting the DFW slide down the track. But that would take a whole bunch of tuning to get it right and fast. And, it would be showing off. Don't think we really need that. :(



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Re: Rail Riding - "How To Guide"

Post by Stan Pope » Thu Mar 01, 2012 12:30 pm

Interesting thought, PWR.

I'd analyze the comparison by asking first if there is a difference in the amount of force between the rail and either wheel. Then I'd ask if that equivalence in force translates into equal frictions. Then I'd ask the distance over which the friction acts.

These questions are important since the energy used up by this friction is the product of force of friction and distance over which applied.

The fixed 'feeler" wheel slides against the rail for a distance equal to the distance from the start line to the finish line. A Dominant Front wheel with negative camber slides also, but I'm not sure how to interpret the distance, since the the wheel is turning and sliding at the same time... I think that the distance might be almost as far as the distance from start line to finish line. However, a Dominant Front Wheel with positive camber is sliding a much smaller distance... since it is almost rolling on the rail.


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Re: Rail Riding - "How To Guide"

Post by derbyspeed » Thu Mar 01, 2012 1:38 pm

Stan Pope wrote: However, a Dominant Front Wheel with positive camber is sliding a much smaller distance... since it is almost rolling on the rail.
I think this is the secret to your answer. Stan brings up a good point, the dominant wheel is rolling and the "feeler wheel" would just be sliding. I'm definitely not an engineer but just think of a grocery cart that has a bad wheel and it slides on the Wal Mart floor :scratching: it's drag creates a lot more friction then when it is able to roll.

From what I saw in the test from Maximum Velocity, he only used the rolling bushings with the BSA wheels, he didn't test with the nylon line, which would have been stationary, possibly causing more friction unless you can make it and keep it super slick.

A lot of rules do prohibit bushings. Even though that may refer to bushings in the wheels I'm sure they would relate that to any bushing on the car.

But I may be way out in left field on this one :wall:


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Re: Rail Riding - "How To Guide"

Post by PWRookie » Thu Mar 01, 2012 10:38 pm

Sorry, still getting used to posting...

QUOTING Derbyspeed: From what I saw in the test from Maximum Velocity, he only used the rolling bushings with the BSA wheels, he didn't test with the nylon line, which would have been stationary, possibly causing more friction unless you can make it and keep it super slick.

You are correct about them not testing the nylon line with BSA wheels, and I didn't catch that detail. That could be a significant detail I will need to ponder further. However, they specifically conclude their study by stating: "Apparently, the BSA wheels generate significant losses when they contact the guide rail, so much so that the bushings provide a big advantage."

Thinking out loud, when tested on the Outlaw wheels, the nylon (fixed) was significantly slower (0.103 seconds) than the bushing (rolling). But both were slower than no feeler at all (rolling wheel contacting rail). Since the article doesn't state otherwise, I assume that the car was aligned straight up, with all 4 wheels on the ground. Therefore, both wheels probably were rolling and at least one likely contacted the rail.

I assume the alignment for the BSA wheel test was the same as I assumed above for the Outlaw test. With the BSA wheels, the opposite thing occurred, the bushing (rolling) gave a 0.043 second advantage over the wheel (also rolling) rubbing against the rail. I think it would not be stretching it to posit that the nylon (fixed) touching the rail would provide a similar, but not as significant, advantage over the rolling wheel contacting it. Extending this further, a glued wheel properly polished and lubricated on the inside (acting as a feeler) would yield similar results to the nylon feeler.

Perhaps, and Mr. Pope posits, the alignment and cambering made to intentionally rail ride will make the losses of the dominant wheel less than what the fixed wheel would be (even if the fixed wheel were polished on the inside where the contact would occur)?

Further thoughts or comments?



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