Rail riding on non-DFW?

[TimInOhio]

I saw a video recently that looked like the car was rail-riding, but on the non-DFW. Have I not been paying attention? Is this something new, or proven to work, or ??? I would expect that there is some discussion about this topic already - but I can't find it.

Thanks in advance for any info.

Tim

[Scrollsawer]

We built a car last year that did that...by accident. Turns out the car was fast enough to earn a trophy, but it was dumb luck. From what I gather, you risk jumping the track doing that, if you raise the wheel on the non-DFW side.

Regards,

Scrollsawer

[TXDerbyDad]

We built a car last year that did that...by accident. Turns out the car was fast enough to earn a trophy, but it was dumb luck. From what I gather, you risk jumping the track doing that, if you raise the wheel on the non-DFW side.

Regards,

Scrollsawer

We did it three years in a row, because I did not fully understand when it was explained to me. I got the basic concept, but the implementation was obviously wrong. And we won trophies. No car ever jumped the track with extended wheelbases, thankfully. I will say that our cars this year are doing it the traditional way. I have found that the removal of that 1/16" of wood from the rail-riding wheel side is way more important than which wheel does it. The cars we didn't remove the 1/16th from did slightly worse than the ones we did, and when we went back and removed it, the car performed much better with no wobble and slightly faster times. All those "slightly faster" things add up!

Of course, that observation is from doing it the "wrong" way, but I expect it will transfer to the right way as well since it's purpose is to engage the rail faster.

[resullivan]

My understanding of the physics of it is that because the DFW is moving less energy is lost when it contacts the rail. When the non DFW contracts the rail it causes the wheel to start spinning which uses energy and in turn slows the car. That is the theory behind it anyway.

[Stan Pope]

... since it's purpose is to engage the rail faster.

This needs to be investigated (or at least confirmed) since some (small amount of) energy is lost by the DFW rolling against the rail. It would seem that rolling an extra foot before engaging the rail would reduce the distance over which the rail-rubbing friction acts.

The extreme case of "engaging the rail quickly" is to stage at the starting line with the DFW engaged already. My experience showed that to be undesirable.

I think that the real purpose of the 1/16" indent is to allow perpendicular rear axles while assuring that the rear wheels stay off the rail. Perpendicular rear axles simplifies construction for those who want to run with straight rear axles and attain negative camber by drilling. Perpendicular rear axles assures a minimum frontal cross-section area and a small aerodynamic advantage over dog-trot alignment.

[Noskills]

That was the origin of my first posting
http://www.derbytalk.com/viewtopic.php? ... ing#p63364

I accidentally rode the rail on my raised wheel and jumped the track twice. Car bot a bit busted up. Still won "scout favorite". I had an idea what rail riding was but not the full picture. The horror of my sons car failing off the track got me interested...so thats a plus!

Noskills

[DerbyAddicted]

What if the non-DFW was stationary (basically acting as a fin)? Would it lose as much energy then?

[FatSebastian]

You might find this topic interesting!

http://www.derbytalk.com/viewtopic.php?p=67706#p67706" target="_blank

Test string

[Stan Pope]

What if the non-DFW was stationary (basically acting as a fin)? Would it lose as much energy then?

To what are you comparing the stationary lifted wheel case?

[DerbyAddicted]

Above, someone said the non-DFW riding the rail would lose energy because it would start to turn as it rode the rail. I was just wondering if that wheel was stationary (either glued or pressed super tight by the axle) if it would lose significantly less energy

[Speedster]

The Topic suggests adding more friction by having the NDFW slide on the rail as opposed to the DFW is somehow going to make the car go faster. Am I understanding that correctly? Is there more than one Topic being discussed?

[DerbyAddicted]

I'm sure someone has done it. But I think the popular consensus is that it risks jumping the rail, and that reason alone is enough to not take the chance.

[Stan Pope]

Above, someone said the non-DFW riding the rail would lose energy because it would start to turn as it rode the rail. I was just wondering if that wheel was stationary (either glued or pressed super tight by the axle) if it would lose significantly less energy

By "non-DFW riding the rail" I think you mean that the car is guided in a straight line by the non-DFW wheel in contact with the rail for most of the car's run down the track.

Let theta be the DFW toe-angle and x be the distance along the track that the front end is contacting the rail.

Regardless of whether the non-DFW rotates or is locked, the DFW toe-out angle holds the non-DFW against the rail. That toe-out produces some sliding of the DFW against the track amounting to x*tan(theta) and uses an energy amount that is proportional to that distance and the amount of the car's weight that is carried by the DFW (the non-DFW carries no weight). The DFW also spins up to at a rate nearly equal to what it would achieve with zero toe angle. That factor is cos(theta) and likewise uses energy.

In addition to the DFW energy uses (losses), there are additional non-DFW energy usage (losses), divided into two cases to consider.

The force which the non-DFW exerts against the rail should be proportional to the product of the weight carried on the DFW and the coefficient of friction between the DFW and the track.

Case 1: non-DFW in contact with rail only, free to rotate.
Depending on the toe-angle of the non-DFW, the wheel can both slide and spin up. Both would use energy. The sliding energy usage is distributed along the full length of the run (x) and the spin-up energy usage is concentrated during the first few feet of the run.

Case 2: non-DFW in contact with rail only, not free to rotate.
The wheel will slide for 100% of the run distance (x). The force is dictated by the amount of DFW toe-out and is some fraction of the weight carried by the DFW.

Since both Case 1 and 2 involve additional energy usage (losses), allowing the non-DFW to guide on the rail can not be advantageous unless so doing reduces energy usage by the DFW. I believe that no such reduction takes place. The DFW still spins up to the same rate as if it were guiding on the rail, and the sliding energy usage appears to be the same.

I happily accept correction to this analysis ... it has not been subjected to experimental testing.

[Speedster]

wow !!!!!
I was just going to say, "It ain't gonna work". I like Stan's explanation better.

What angles, if any, would you set the NDFW? How would you keep the rear wheel off the rail?