brentsherman wrote:We had the pack race today and my son finished 3rd using the rail rider technique and he goes on to compete in the district race in 2 weeks. We did not have access to a track before race day so I was not sure what to expect. After the race we did get to do a few runs and analyze how the car tracked. When it was riding on the rail the rear wheels were off the rails and moved toward the nail head. However the car was doing quite a bit rocking back and forth or wobbling when it was in the flat section of the track and I'm thinking this is not so good. Some characteristics: The wheel base is extended to the maximum and the center of mass is about 1 1/2 inch in front of the rear axles. The packs track was made last year out of wood and is a little rough, does not have a finish on it and did not seem to sanded real smooth. The district track is aluminum. What causes this wobbling and is there anyway to correct?
Generally one looks toward a very aggressive COM. This doesn't appear to be the case at 1.5". The next culprit is the rear wheel axle/bore interface(friction differential) and /or alignment. (Which will be more pronounced on aluminium.) Rough track may have factored in, can you revisit track and test with a bit more toe and observe?
brentsherman wrote:What causes this wobbling and is there anyway to correct?
With that conservative COM (too conservative for a nice aluminum track at Districts) you should've been rock steady and quiet all the way.
Was this a 3 wheeler? Rear camber info? Front camber info? How much drift?
It sure sounds like the rear end was out of whack big time and/or you have a bad wheel.
Can you post heat times of your car and perhaps the pack champ?
brentsherman wrote:The car is a slight three wheeler but thats just because of when we turned the front dominant axle to get about a 2 1/4 inch drift over 4 ft. the non dominant front wheel is slightly raised and mostly spins free. We dont have an axle bender tool so we just put the axle in a vise with an old wheel on it and gave it slight taps until it had just a little bend. We did a test and the rear wheel on the dominant side did not move out to the axle head so we bent that axle too and adjusted it to get the wheel to move to the head. Nothing very scientific, just a few bends and trial and error on the kitchen table. I just now noticed that when I put the car on the table I could rock the back end of the car back and forth using my index finger, like there is too much side to side play. The front end was much more stable. There is negative camber on the rear dominant wheel. His car averaged something like 3.03 and the winner was about 2.93.
A couple more thoughts:
I just bend my axles in using an old wheel while they are mounted in my hand drill which is mounted in my bench vise, BUT I do verify the bends. I use a stop to limit travel while bending and to ensure the resulting bent axles match. A 2 degree bend is very slight, almost imperceptible without trying to roll it. Could you have bent them too much? Do the rear axles match as much as possible?
- Watch the wheel gap and keep it consistent, and be sure the axles are tight in their holes. Sometimes loose axles will slide out on even a smooth track just enough to mess things up.
- I am a little concerned that you had too much drift into the rail with a 1.5 inch COM. Your 1.5 inch COM gives the front dominant steering wheel a lot more authority than with a car with a COM of .75 inch in front of rear axle line. I think this is especially true on a rougher wooden track with lots of bite to it.
- You might consider concentrating on understanding what it means to get those rear wheels aligned. Stan's well published CAM ALIGN pages are designed to help you accomplish this. This presumes that you prepared your axles and wheels well and didn't lessen the axle diameters or enlarge the wheel bores.
As good as your car was, it seems that you left some room to grab more. If the next race is on aluminum track that is in reasonable condition, cars with more aggressive COMs will be tough if they're aligned well.
- Stan Pope
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Kenny wrote:... gives the front dominant steering wheel a lot more authority than ...
Quite a well turned phrase! Congrats!
"If it's not for the boys, it's for the birds!"
The car is set up as a 3 wheeler
Through dumb luck, I think I have the axles holes drilled with no toe and (I think) just enough camber.
If I start with the wheels IN, the move completely OUT in about three rotations, both forward and backward.
The car only has a slight amount of drift, and it's in the wrong direction. After 5 ft, the DFW is 1/2 inch AWAY from the center line. Rotating the axle does not make a significant difference.
1. Marking the top of the DFW axle.
2. Mark the axle at the point where the axle enters the body.
3. Remove the DFW.
4. Thread the axle through an old wheel.
5. Clamp the axle into a vise slightly below the point I marked in step two.
6. LIGHTLY tapping the wheel with a hammer in the direction of the top of the axle. (from 6 o'clock to 12 o'clock)
7. Reinserting the axle with the good wheel and checking alignment to achieve DFW drift of 2 inches per 4 ft TOWARD the center line.
8. If proper drift is not achieved ROTATE the axle so that the 12:00 mark goes toward the FRONT of the car.
9. If proper drift is not achieved after rotating the axle from 12: to (Help me here) 2:00? repeat steps 3-8,
In this model, the DFW has some positive camber and the inside edge of the wheel is touching the track (no toe-in is modeled, at least not yet). The other front wheel is lifted, also with positive camber. The rear wheels have negative camber and are 4.5" behind the front wheels. My question is: with the DFW riding the rail, what are the ideal locations of the other three wheels in terms of distance from the rails?
One thing that needs to be addressed first is whether, when in stable rail riding action, the car is still pointing straight down the track, only offset to the side a bit, or is the car angled to the left or right? From the viewpoint of the wheels and axles, this is more precisely stated as: does the plane through the rear axles remain perpendicular to the length of the rails or not? For now I'm assuming that it does, but if you believe otherwise, let's discuss.
So, with that assumption in place, let's think about the rear wheels. How much clearance between the rail and the inside edge of the wheel should there be for each wheel? Should it be the same on both sides or asymmetrical? What should the total width between the wheels be?
It seems to me that the rear track (here meaning the width between the two rear wheels, not the thing the car rolls on) should be narrow to reduce chances of the wiggles. Thinking in extremes, first imagine a car that is wider than it is long - no way you could get that to go straight. At the other extreme, a very narrow but long car would turn slowly. At least that makes sense for a 4-wheel car. For an asymmetric tricycle like we have here, it isn't so obvious. So let's say that we want a narrow rear track - well, we still need enough clearance to make sure that the rears don't rub if the car gets upset on the track. How much clearance is enough? More on one side than the other? Has anyone observed how much space they have between the rail and the wheels as the car is going down the track, or know how little is too little?
Also, for the lifted front wheel, how close should that be to the rail? For a small bump we want to let the DFW come off the rail and steer itself back in without the lifted FW touching, but we would rather engage the LFW for a big disturbance rather than having the car deviate too much from straight ahead and causing bigger problems.
I should say that testing opportunities are very limited. There may be a couple of hours on the day of impound to do some runs (untimed runs at that), and that is during working hours too. 49ft. Besttrack.
After I get all of this sorted out, the body will be designed around all these axles and wheels going in different directions. One issue with this approach is the effect of staging. Don't want the body and wheel locations to encourage people to stage the car with the DFW already close to the track. I'd rather have as much free roll as possible.
Thanks for your input.
Bonus question: If our track is set up in the same location and orientation that it has been for the last 4 years, there will be a slight tilt to the track. Standing at the ramp end and looking toward the finish line, the track will tilted counter-clockwise a bit. Should the DFW be the left or the right, and why?
I Drill rear holes canted at 3 degrees and use straight axles. Rear is symetrical.
Rear is full width. I like the rears to be equal spaced off the rail but it does not happen therfore I keep the wheel on the raised side a tad further from the rail.
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FinePine wrote:with the DFW riding the rail, what are the ideal locations of the other three wheels in terms of distance from the rails?
This depends on if the front of the car is narrowed to bring the following rear wheel out further from the center rail.
FinePine wrote:One thing that needs to be addressed first is whether, when in stable rail riding action, the car is still pointing straight down the track, only offset to the side a bit, or is the car angled to the left or right? From the viewpoint of the wheels and axles
If it is not pointing straight down the track you will have a very slow car as something would have to be rubbing considerably to offset the car from straight.
FinePine wrote: So, with that assumption in place, let's think about the rear wheels. How much clearance between the rail and the inside edge of the wheel should there be for each wheel? Should it be the same on both sides or asymmetrical? What should the total width between the wheels be?
See figure B&C, these figures show a PWD railrider with 2.5 degree angled rear (negative camber) and front wheels (one with positive camber, the other is raised), from the front and the rear, this should give an idea of where each of the wheels would be compared to the guide strip. Unless you offset the rear of the car the wheels could not be symmetrical to the guide strip, remember almost every set of PWD rules has “No less than 1-3/4 inches clearance between the inside of the wheels”. This rule is often tested with a 1 3/4" board, placed on a table that the car must roll over without binding.
Sorry I can only provide 2D drawings to show how this works. Hope this helps.
ah8tk wrote:This depends on if the front of the car is narrowed to bring the following rear wheel out further from the center rail.
Well, what I'm trying to do is determine what the best placement for the wheels would be, free of any restrictions from the body, and then I'll shape the body to meet those needs. I'm thinking of the wheels determining the shape of the body, not the body determining where the wheels are.
ah8tk wrote:Unless you offset the rear of the car the wheels could not be symmetrical to the guide strip, remember almost every set of PWD rules has “No less than 1-3/4 inches clearance between the inside of the wheels”. This rule is often tested with a 1 3/4" board, placed on a table that the car must roll over without binding.
Here's what I have drawn up so far:
This is a view from the rear of the car. The rear wheels are red, and the front wheels are black. The DFW (on the right) is against the rail, and the rears have equal spacing on each side of the rails. The rear to rail clearance is .130" (the short blue line). The red lines represent the 3/8" x 1-3/4" clearance required for the back, and the black lines are for the front wheels. Plenty of clearance at both ends. For us, clearance is only checked at these two locations, but clearance could still be made over a 1-3/4" strip of wood, even without angling the car to fit. Yes, the car block would need offsets and narrowing here and there, but that is easy enough. The question then isn't can it be done, but should it be done? Your drawings suggest that 1/8" clearance between the rail and right rear is plenty; is it enough for the left? I think that it probably is, but then my concern goes back to staging - with a narrowed spacing, the DFW will reach the rail sooner, which could slow things down.
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And at the top is a top view of the car body and how you would need to offset the body in the front and the rear (using the 2.5 degree rear - negative camber and front - single positive camber). with this setup the wheels would indeed straddle a 1 3/4" board for check-in.
If you draw a line down the center of your car. The rears should be symetrical. in all ways. The block at the rears the full 1 3/4".
The DFW indented 1/16" to 3/32" of an inch.
3/32" height DFW
4/32" height raised
5/32" height rears
Drill rear holes canted negative at 3 degrees. Axles straight
Drill front holes straight. raised axle straight. DFW bent 3 degrees.
Most inportant thing is to have straight axles and well drilled holes.
Well drilled holes are the foundation of your car.
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