Rear alignment diagnosis please (RACE RESULTS ADDED)

[davet]

Drilled canted rears. Using straight, stock BSA axles from the kit. These axles are as straight as can be detected with the naked eye. Wheels went to the heads forward but to the body in reverse. Twist axles 180 degrees and its the opposite.

I found that by twisting each a little and with a straight axle in the DFW hole I could get both rears to go the heads in both directions and it ran dead straight forward but about 3" steer in reverse. When I put the bent DFW axle in it seemed the wheels wouldn't go to the heads as quickly though. Axles have no bend to them at all that I can see.

When I had less than .60 oz on DFW she had a rear wiggle. Adding more than 4" of steer did not fix it. If I took a little weight off rear by just rolling the rear putty slightly forward the wiggle would lessen. If I moved up to .62 oz on DFW she had no wiggle most of the time and pretty silent down my homemade track. Is this indicating still a rear alignment issue since I can't load that rear axle with more weight? I'm running 12 cubes behind axle, 1 cube between rear axles, 6 just ahead, 2 more centered ahead of that with .27 oz of putty alongside the 2 cubes, .09 oz of putty just behind DFW on DFW side. This gives me the .62 oz on DFW.

I'm considering a very slight bend in the rear axles so I at least know which way is up for twisting them.

[Stan Pope]

I found that by twisting each a little and with a straight axle in the DFW hole I could get both rears to go the heads in both directions and it ran dead straight forward but about 3" steer in reverse.

This should tell you the approximate location of "top dead center" for the axle bend.

When evaluating th ereverse roll, it is important to distinguish whether (1) the leading wheels are leading the car off line, (2) the trailing wheels are flaring out the opposite side and causing the deflection, or (3) the car is simply not lined up with the target to begin with. Which?

[davet]

I found that by twisting each a little and with a straight axle in the DFW hole I could get both rears to go the heads in both directions and it ran dead straight forward but about 3" steer in reverse.

When evaluating th ereverse roll, it is important to distinguish whether (1) the leading wheels are leading the car off line, (2) the trailing wheels are flaring out the opposite side and causing the deflection, or (3) the car is simply not lined up with the target to begin with. Which?

Good point Stan. I think all of the input is coming from the rears. The reason I think this is because I can twist the straight, DFW axle to any position and the car steers the same.

[Stan Pope]

Here are some things you can do to settle the issue:

1. Run the car forward ... along a straight line. Note the relationship of the side of the car to the line that the car follows. Are they parallel? If not, how much displacement difference, front vs. rear. (When performing the reverse roll, the car should start in exactly that position!)

2. When performing a reverse roll, watch which wheels leave the line first.
(a) If the rear wheels leave the line before the front wheel, then they are indeed steering. Keep in mind that the rears might not be exactly perpendicular to the long side of the body. That happens if the drill press table is not exactly perpendicular even though the holes are drilled without the bit wandering. On one side the hole angles the axle a bit forward; on the other side it angles the axle a bit backward ... both by the same amount! But when running, the back of the car moves sideways until the rear axles are exactly perpendicular to the direction of travel ... resulting an a "dog trot" appearance.

(b) If the front wheel (the DFW) leaves the line first, then it is in control and forcing the rear wheels to then veer off. For instance, the DFW may veer a bit to the right, causing the rear axles to deviate from perpendicular to the intended direction of travel. Soon both rear wheels and DFW have swept off to the left.

Frankly, this whole alignment thing is so much easier if the rears were weight-biased, in turn, so that they can be investigated .... and aligned ... individually! Alignment can even be checked ... and corrected, if necessary ... without disturbing the DFW alignment. But I'll try to assist you through your planned method anyway.

[pwrd by tungsten]

Once you get it right mark the axles with a permanent marker...

If it is not too late I would drill a new block and see if the issue resolves. It seems you might have not had the car level when you drilled the holes..

[davet]

The stock axles have a bend so small I couldn't see it. I twisted them to get wheels to the heads forward and back. Goes dead straight both directions. I closed up the rear wheel gap and I'm now running .57 oz on DFW and 3" of steer. No wobbles and pretty quiet down my homemade track. I increased the steer to 5" and she was much louder, still no wobble. Dropped steer down to 2"and she seemed quiet too.

When I adjust steer I set my DFW against my line and the rear on that side 1/16" away from the line. Do I do the same with your method above Stan? If running in reverse do adjust it also the wheels are line up or set the side of the body parallel to the line?

Race is in 10 days so there's no going back now.

[pwrd by tungsten]

The rear wheels need to be aligned. You have that nailed. I would mark the axles and perhaps use a bit of elmers glue if it will not get in the bore.

For the front the axle bend and steer are the most important. if you have a timer you can try different amounts of steer and see which has the best time. The other method is to adjust to the point here there is no wiggle in the rear at the finish. However this may not result in the fastest time. Tuning should be done on the actual track when possible.

[davet]

The rear wheels need to be aligned. You have that nailed. I would mark the axles and perhaps use a bit of elmers glue if it will not get in the bore.

For the front the axle bend and steer are the most important. if you have a timer you can try different amounts of steer and see which has the best time. The other method is to adjust to the point here there is no wiggle in the rear at the finish. However this may not result in the fastest time. Tuning should be done on the actual track when possible.

Thanks. I have no genuine test track to run on only 16' x6 " of bendable plastic plank with 1/4" tall center guide. If its not wobbling when it hits the end of my 8" flat it should be good right? I thought that steer only affected the front end banging back and forth and rear alignment and weight distribution affected the rear wobble. Thanks for reminding me on gluing the axles in. We forgot to drill those holes.

[LightninBoy]

When evaluating th ereverse roll, it is important to distinguish whether (1) the leading wheels are leading the car off line, (2) the trailing wheels are flaring out the opposite side and causing the deflection, or (3) the car is simply not lined up with the target to begin with. Which?

Good point Stan. I think all of the input is coming from the rears. The reason I think this is because I can twist the straight, DFW axle to any position and the car steers the same.

Here's one of the strategies I use to test the rear alignment. The goal with this technique is to completely eliminate the DFW from the equation. This also equally distributes the weight across the rears.

First, you need to build this super complicated tool ...



yes that is a thumbtack attached by two sided tape on a square. Now place that on your "tuning board" which in my case is some laminated shelving from Menards. Then push the nose of your car onto the thumbtack far enough down where the DFW begins to touch the board. It will look something like this ...



I like to use pin gauges for this test, but axles will work too. If your car isn't weighted yet, put some weight in the center between the rear wheel.



Ok, now all you need to do is slowly slide the square down the edge of the board. With the pin gauges, what I look for is for both wheels to migrate approximately the same distance. If you are using axles, then all you can do is verify that the wheels go the axle head. Then, pull the square up the board to push the car in reverse. Again, I'm looking to make sure the wheels migrate approximately the same distance.

Here's a little video to demonstrate ...

http://vid1289.photobucket.com/albums/b ... 9zkf7c.mp4

[Speedster]

LB, the scouts are going to Love this. Thank You.

[pwrd by tungsten]

Good point Stan. I think all of the input is coming from the rears. The reason I think this is because I can twist the straight, DFW axle to any position and the car steers the same.

Here's one of the strategies I use to test the rear alignment. The goal with this technique is to completely eliminate the DFW from the equation. This also equally distributes the weight across the rears.

First, you need to build this super complicated tool ...



yes that is a thumbtack attached by two sided tape on a square. Now place that on your "tuning board" which in my case is some laminated shelving from Menards. Then push the nose of your car onto the thumbtack far enough down where the DFW begins to touch the board. It will look something like this ...



I like to use pin gauges for this test, but axles will work too. If your car isn't weighted yet, put some weight in the center between the rear wheel.



Ok, now all you need to do is slowly slide the square down the edge of the board. With the pin gauges, what I look for is for both wheels to migrate approximately the same distance. If you are using axles, then all you can do is verify that the wheels go the axle head. Then, pull the square up the board to push the car in reverse. Again, I'm looking to make sure the wheels migrate approximately the same distance.

Here's a little video to demonstrate ...

http://vid1289.photobucket.com/albums/b ... 9zkf7c.mp4

This is awesome! Very good idea. You should create a separate post to point this out. Very helpful.

[pwrd by tungsten]

The rear wheels need to be aligned. You have that nailed. I would mark the axles and perhaps use a bit of elmers glue if it will not get in the bore.

For the front the axle bend and steer are the most important. if you have a timer you can try different amounts of steer and see which has the best time. The other method is to adjust to the point here there is no wiggle in the rear at the finish. However this may not result in the fastest time. Tuning should be done on the actual track when possible.

Thanks. I have no genuine test track to run on only 16' x6 " of bendable plastic plank with 1/4" tall center guide. If its not wobbling when it hits the end of my 8" flat it should be good right? I thought that steer only affected the front end banging back and forth and rear alignment and weight distribution affected the rear wobble. Thanks for reminding me on gluing the axles in. We forgot to drill those holes.

What type of track and what length is it? Without access to the track it is down to guessing.

Adjusting steer goes as follows.

More steer will keep you on the rail longer but at some point it slows you down because you are hitting the rail too hard. The maximum steer you want is for the rear end to be stable across the finish line. Any more than it is a waste. Even this can be too much as some slop in the rear say several feet from the finish line may result in faster times. We would always make a couple cars of various steering and try them out before the race and take the fastest car. this was all the track time we could get...

Most of my cars for a 42 foot best track steered 2.5-3 inches over 4 feet.

When tuning you shuld have a board where you can dial in the steer and then test. Otherwise there is no measurement...

[davet]

Very impressive. The tack is mounted dead center of the car I take it?

[LightninBoy]

Very impressive. The tack is mounted dead center of the car I take it?

Good question. I probably should center it perfectly and I used to beam a laser over the board to help center it and align it straight. But centered or not didn't seem to make much if any difference. So now I just eye it for the center. However, aligning the car straight is important. The key factor is the angle of the force (when pulling or pushing, but particularly when pushing it in reverse) against the car. So you see in the video that I first align the car to that pencil mark to make sure it is straight.

[Stan Pope]

When I adjust steer I set my DFW against my line and the rear on that side 1/16" away from the line. Do I do the same with your method above Stan? If running in reverse do adjust it also the wheels are line up or set the side of the body parallel to the line?

IF you adjust the DFW for a "dead straight run", how does the car end up relative to the line? That result is what I'd aim for in starting the reverse run!

If you were aligning "my way", you would decide up front how much DFW-side rear wheel clearance you want, e.g. 1/16" off rail, and align to that point. Probably takes two cycles to get there: make opposite side rear wheel dominant and adjust it to get DFW side rear to track 1/16" off rail; make DFW side rear wheel dominant and adjust it to track 1//16" off rail; make oppoisite side rear wheel dominant and adjust it, if needed, to get DFW side rear to track 1/16" off rail, ... Stop when no adjustment needed after changing sides!

Given that you have what you think is correct alignment, you can apply the above to verify ... check each side without altering alignment!

There are factors other than incorrect rear alignment that can cause the reverse roll to veer off-path, even when the forward roll alignment is excellent. They are complicated and tough to identify and tougher to correct. Don't go there!

From your later posts, it appears that the wiggle is gone and you have a DFW setting that is quiet. That sounds like a great place to stop unless you want to give a try at my alignment check.