Indented DFW vs. DogTrot Alignment

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Stan Pope
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Indented DFW vs. DogTrot Alignment

Post by Stan Pope »

While designing my alignment board, I did not have all of these numbers at hand, and consequently made some estimates which I believe were "safe enough." Now, with the numbers, I believe that more conservative estimates might be better.

Computation re centering the rear over the center guiderail:

Block width: 1.740"
Additional width due to hub protrusion: 0.074"
Wheel tread separation: 1.814"

Center Guiderail width: 1.375"
Total Clearance: 0.439
Distance from guiderail to wheel if equally divided on each side: 0.22" or approx. 7/32"

This raises a question:

If a car were designed for the rear wheels to straddle the guide rail spaced equally while the dominant front wheel (DFW) brushes the guiderail, the car body does not sit straight on the track. It "dog trots!" If the car does not have an indented DFW, then the angle relative to "straight ahead" (for a 5" wheelbase) is about 2-1/2 degrees and the effective frontal cross section of the body is increased by a small amount. If the car has the DFW indented by 1/16", the angle is decreased to 1.8 degrees with reduced clearance for the opposite side front wheel.

If the rear wheels are aligned so that the car aligns straight ahead, then the clearance between the guiderail and the dominant-side rear wheel reduces to 1/16", the amount of DFW indent.

In the presence of guiderail roughness, the DFW has freedom to slide out on its axle. If that freedom exceeds 1/16", then there is a risk of the dominant-side rear wheel brushing the guiderail in the vicinity of the guiderail roughness. The roughness may also deflect the front of the car causing the offside front wheel to brush the guiderail.

Has anyone developed stats regarding (1) the occurrences of rear wheel brushing on rough tracks with indented DFW, (2) the occurrences of the offside front wheel brushing on rough tracks with indented DFW, and (3) the cost in elapsed time due to "dog trot" alignment?
Stan
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Re: Indented DFW vs. DogTrot Alignment

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Stan Pope wrote:If a car were designed for the rear wheels to straddle the guide rail spaced equally while the dominant front wheel (DFW) brushes the guiderail, the car body does not sit straight on the track. It "dog trots!"
That's what my sons and I have been doing for the last 5 or so years. We use slots (by rule) and draw a "centerline" for how the car is to dog-trot. The front axle "center" is away from the true centerline on the non-dominant side by almost 1/2 the distance of "play" between the front wheels and the rail, while the rear axle center is actually centered. We use a square to line up our Pro Body Tool perpendicular to that "centerline."

We don't indent the DFW (by rule). Ironically, I had an idea maybe 5 or 6 years ago about indenting the DFW, then building out the non-dom front the same amount. I hesitated, as the wheel would no longer be running against the original wood and the added wood might raise suspicion as to it being some sort of bushing. I still had the picture of that concept and wondered "What if the whole block were lined up with the resultant wheel locations so I wouldn't have to indent one side and build up the other?" The result was that the axles weren't perpendicular to the sides of the block. I recalled the old 1970's era Chevy Nova that did that very thing due to a screw-up where engineers move forward one side's rear axle mount, but they forgot to do the same to the other side. It "crabbed" down the road. I explained the concept to my sons and they understood why it would actually be good for a Pinewood Derby car.
Stan Pope wrote:In the presence of guide rail roughness, the DFW has freedom to slide out on its axle. If that freedom exceeds 1/16", then there is a risk of the dominant-side rear wheel brushing the guide rail in the vicinity of the guide rail roughness.
If the DFW gets pushed away from the guide rail and the rears are equally straddling the rail, I doubt the dominant-side rear would move toward the rail at all--let alone enough to touch it--in the approximately 5 inches of travel involved and at the speeds fast cars attain. Laws of physics say some force must act upon the dominant-side rear to deflect it toward the rail. The front being deflected away from the rail does not translate to the rear moving toward the rail in that short distance.
Stan Pope wrote:and the effective frontal cross section of the body is increased by a small amount.
We find that the increased cross section has a minimal effect on the speed. To us, the speed penalty of slightly more wind resistance is less than the speed penalty of track rear wheels rubbing the guide rail. I wish I had the means of testing it, but don't. I have no access to a track except once a year, and that track doesn't have electronics on it. We feel more comfortable with the slight increase in frontal cross-section area, so that's what we go with.
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Re: Indented DFW vs. DogTrot Alignment

Post by Stan Pope »

Shawn, thank you for the analysis and experience! It is a valuable addition to my understanding.
Shawn Stebleton wrote:
Stan Pope wrote:In the presence of guide rail roughness, the DFW has freedom to slide out on its axle. If that freedom exceeds 1/16", then there is a risk of the dominant-side rear wheel brushing the guide rail in the vicinity of the guide rail roughness.
If the DFW gets pushed away from the guide rail and the rears are equally straddling the rail, I doubt the dominant-side rear would move toward the rail at all--let alone enough to touch it--in the approximately 5 inches of travel involved and at the speeds fast cars attain. Laws of physics say some force must act upon the dominant-side rear to deflect it toward the rail. The front being deflected away from the rail does not translate to the rear moving toward the rail in that short distance.
For clarification, I'd like to step through the sequence of events that may occur.

Before reaching the obstruction, the DFW is steering against the rail and its hub is against the car body. The DFW-side rear is following with its axles perpendicular to the direction of travel with a clearance of approximately 1/16", the amount of the DFW indent.

The case at hand is that the DFW strikes some roughness and floats out on the front axle with no significant movement of the front axle. If the DFW play exceeds 1/16" and the DFW reaches the axle head, then ...
Upon clearing the obstruction, the front of the car is steered toward the rail. Then a combination of the following might will happen:
(#1) The front of the car will move to bring the DFW axle head closer to the rail, and
(#2) the DFW will slide back toward the car body, swinging the DFW axle back out.

To the extent that #1 dominates #2, the rear wheel behind the DFW will follow the front toward the rail, because it is aligned to try to maintain a specific offset (1/16") relative to the junction of the DFW axle and car body.

I don't know if the rear wheel clears the obstruction or not ... it depends on the size of the obstruction, the timing, and extent of #1 and #2.

So, I think that the "physics" may oversimplify the situation. Or, maybe I am "overcomplicating" it. :)

It's time to get a track set up and tape some match sticks to the side of the rail! :)
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Re: Indented DFW vs. DogTrot Alignment

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Stan, regarding your very last line. Can you take a video of the car running in very slow motion and put it on a DVD. Would that not be COOL !!! I could show it to the scouts in the workshops. I think the kids would love it. Fox Chapel Publishing says they are always looking for talented authors.
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Re: Indented DFW vs. DogTrot Alignment

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Speedster wrote:Stan, regarding your very last line. Can you take a video of the car running in very slow motion and put it on a DVD. Would that not be COOL !!! I could show it to the scouts in the workshops. I think the kids would love it. Fox Chapel Publishing says they are always looking for talented authors.
The fastest I can shoot is about 90 fps. (At least the equipment claims that ... I've not been able to get it past 30! Need more light?) Shooting 90 fps, a 12 mph car moves over 2 inches per frame, and each frame probably includes about 1" of movement. I think that extracting much info from such a vid would be problematic.
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Re: Indented DFW vs. DogTrot Alignment

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Someone needs to get The guys from Mythbusters to use their high speed cameras to film a few runs.

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Re: Indented DFW vs. DogTrot Alignment

Post by Shawn Stebleton »

Some questions I have:

How far would the car travel down the track from the point of the impact before the wheel would reach its maximum deflection (lets say it moves outward just enough to barely touch the axle head)?

If the DFW axle is angled down and toed in a bit, wouldn't this try to lift the front of the car a bit and push the DFW side body outward slightly at the same time when a rail obstruction was hit?

Wouldn't the slight downward force of the front of the car pressing down on the angled-down axle tend to push the wheel inward and the body outward?

By the time the above was all sorted out, wouldn't the rear wheel be past the obstruction? After all, an object in motion tends to remain in motion in the direction of travel unless acted upon by a force.
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Re: Indented DFW vs. DogTrot Alignment

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Shawn Stebleton wrote: .... By the time the above was all sorted out, wouldn't the rear wheel be past the obstruction? ...
That is essentially the question that I was raising. If the magnitude of the bump exceeds the rear wheel's usual clearance, which of the two reactions ("wheel moving out" or "wheel and body moving out") dominates?

If "wheel moving out" dominates, then the rear wheel's direction is unchanged, and the rear wheel strikes the same bump. (BAD)

If "wheel and body moving out" dominates, then the rear wheel's direction is changed, and the rear wheel clears it. (GOOD, unless the deflection is so great as to pull the opposite side wheels into the rail, then BAD!)

With "dog-trot alignment" there is the potential for at least 3/16" clearance between the rear wheels and the rail. With 1/16" indent + straight rear alignment, that clearance is reduced to 1/16".

Stated another way, is 1/16" indent and straight alignment as safe for all track conditions as dog trot alignment with maximum rear clearances?

The experience of "top racers" may not be a reliable indicator, since the best racers probably run on the best tracks! :) And, as an organizer, I try to assure that the best track(s) within the span of the organization be used for the organization's event. The organizer of a pack event may have a limited choice, while the organizer of a district or council event may be able to recruit tracks from more than a dozen sources. (Of course, if I am willing to pay for the use of a good track, then the options to get a good track increase, but I try to avoid that expense!)
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Re: Indented DFW vs. DogTrot Alignment

Post by Shawn Stebleton »

We use the "dog-trot" alignment and have had success when alignment is on.
Stan Pope wrote:If the magnitude of the bump exceeds the rear wheel's usual clearance, which of the two reactions ("wheel moving out" or "wheel and body moving out") dominates?
Are you referring to a large (greater than the dominant-side rear wheel gap) obstruction? Or a small obstruction that causes a large deflection? I was thinking the latter, such as a slightly misaligned joint between flat sections where the rear wheel will easily miss the obstruction. In our district, that is the most common problem, by far.

I would suppose that a vertical DFW that is toed in only (no camber) would react differently than one with significant camber along with the toe in. I would think the significant camber wheel would "ride over" the misaligned joint with fewer unwanted forces applied to the car than having an axle with no camber would yield. The wheel deflection in the former case would be minimal, and I think the "wheel moving out" would be the case (a 2.5 degree camber isn't a whole lot).

In the latter case, it would also be the "wheel moving out", as there would be zero force to move the body, unless the wheel is deflected so hard into the axle head that it imparts force against it. With the 1/4" height of the rail, I think track joint misalignment would deflect the wheel upward as the inner edge rolls onto the top of the mismatch area and not directly into it. This what I have seen at our district races on a Best Track.
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Re: Indented DFW vs. DogTrot Alignment

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Shawn Stebleton wrote:Are you referring to a large (greater than the dominant-side rear wheel gap) obstruction?

Yes
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Re: Indented DFW vs. DogTrot Alignment

Post by Shawn Stebleton »

Stan Pope wrote:
Shawn Stebleton wrote:Are you referring to a large (greater than the dominant-side rear wheel gap) obstruction?

Yes
Have you seen any obstruction of that size on the tracks on which you have witnessed racing? If so, what was the cause?

The only things I can think of are 1) a very badly misaligned track joint, but I doubt that that would not be caught before actual racing began; and 2) debris left on the track.

The very badly misaligned track joint is one of those immovable objects that I've pictured in my mind during this discussion, whereas debris is usually movable (unless sticky, such as gum) and is typically knocked off the track for the next run. I haven't seen either in my sons racing days.
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Re: Indented DFW vs. DogTrot Alignment

Post by Stan Pope »

Every year, a new crop of pack leaders are recruited into the thousands of Cub Scout Packs around the country, and each of those new leaders bring a refreshing ignorance. The best of them know to ask knowledgeable predecessors, but a few see it as "so simple ... no problem!"

I visited a pack that did a good job of setting up their track. It looked good. But they had never used the "block test" to check lane alignment. They watched me run a sharp edged wood block down-track along each side of each joint to check for misalignment. (IIRC, I found some. No killers, thankfully.) The gentleman who is organizing our district race this year told me that he is now using that simple check!

Once racing has started and folks observe that there is a problem, the organizers could "fix the track" and (should) restart racing from the beginning. Would they?

Seriously, my pups have not been subjected to really badly aligned tracks. Some of the tracks used at district had pretty raunchy rail edges, but not many. By the time the organizer observed that kind of problem, it was too late. They needed 5 tracks to run the 5 groups, and they had exactly 5 tracks in the building!

Our district situation is better now ... with 15th Burlington, we see a problem, we can just pull that track out of service and run slightly degraded (more boys per track, so each boy gets to race every 6 minutes rather than every 5 minutes, for instance) rather than full stop! The same "balancing algorithm" that resolved issues caused by tracks that operate at vastly different heats per minute rates than average quickly rebalances track occupancy if the boys that currently race on the removed track are shifted one track toward Silver.

Back you your question/observation. I've not seen any of my boys face such a situation as lane alignment off by more than 1/16". But part of my role as a mentor for them is to anticipate problems that might occur, and help them discover ways to overcome those problems should they occur.
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Re: Indented DFW vs. DogTrot Alignment

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Stan Pope wrote:
I visited a pack that did a good job of setting up their track. It looked good. But they had never used the "block test" to check lane alignment. They watched me run a sharp edged wood block down-track along each side of each joint to check for misalignment. (IIRC, I found some. No killers, thankfully.) The gentleman who is organizing our district race this year told me that he is now using that simple check!
Guilty. Never heard of the block test, but it sounds like a great idea.
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Re: Indented DFW vs. DogTrot Alignment

Post by Shawn Stebleton »

I haven't used the block check, either, but we have the Piantedosi Classic track with the taper on the entry end of each section.

I use a 3' level as a straightedge to make sure the lane guides are aligned properly. I make sure to check both sides and ensure the lane guides are flush with the level and are not angled or offset in any way. When set up that way, the tapers ensure there are no alignment issues.
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Re: Indented DFW vs. DogTrot Alignment

Post by Stan Pope »

Yes, if each track assembly team knew the best way to set up their track, the event chairman should never have a problem. The "block test" allows the chairman to delegate setup and, at the end, quickly assess whether the team "knew what they were doing".

It is even more critical in a district event venue where 6 or more tracks may be set up, each by its own team (which may or may not be the best), but the chairman is ultimately responsible for the overall quality. This test gives the chairman a quick way to make a "go / nogo" decision quickly, and to justify that decision to the track assembly team with specific concerns.

The test produces three possible results on each side of each joint: 1 - Smooth, 2 - Clicks but doesn't hang, and 3 - Hangs. Minimally, the chairman should require that there be no "Hangs" before the track is certified.
Primarily "Smooth" joints are best, but the cars will survive a few "Clicks".

If the team can not remove all the "hangs", then it is time to "call in expertise from other track teams" and, failing resolution from that, decide if it is feasible to decertify the track (remove it from the competition) or to risk the boys' car on it.
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