offset weight ?

[davet]

I read this in a post that Stan wrote in athread back in 2013:

"Note that if you attempt to take this measurement with toe-in on DFW, the string is pulling to the side and will probably change the rear wheel weight distribution. It depends on exactly where the string is attached to the car!"

I'm trying to get equal weight on both rear wheels by offsetting the weight to the DFW side. Does a railunner/rider distrubute more weight to the rear DFW simply by turning that side into the rail. I mean, if I set equal weight on the rear wheels using a scale, will the DFW side rear wheel get additional weight placed on it during the race as its turning into the rail?

[Stan Pope]

I read this in a post that Stan wrote in athread back in 2013:

"Note that if you attempt to take this measurement with toe-in on DFW, the string is pulling to the side and will probably change the rear wheel weight distribution. It depends on exactly where the string is attached to the car!"

I'm trying to get equal weight on both rear wheels by offsetting the weight to the DFW side. Does a railunner/rider distrubute more weight to the rear DFW simply by turning that side into the rail. I mean, if I set equal weight on the rear wheels using a scale, will the DFW side rear wheel get additional weight placed on it during the race as its turning into the rail?

IIRC, that thread involved "testing and alignment" on a treadmill! The car on a treadmill with DFW toe-in will not align its direction to the direction of the string (adjusted for leverage, of course). It will veer off to the side until the turning force equals the force of the string pulling it sideways! If the string is below the CM, it will increase the weight on the rear wheel behind the lifted wheel. If the string is attached above the CM, it will increase the weight on the DFW and on the rear wheel behind it.

It may be possible to align a car on a treadmill, but the gent who made that vid didn't give us enough info to tell us how to do it.

Running on a track with DFW toe-in should not alter the rear wheel weight dist.

[Stan Pope]

It may be possible to align a car on a treadmill, but the gent who made that vid didn't give us enough info to tell us how to do it.

I would be most interested to hear from anyone who can tell the keys to which observed phenomena on the treadmill translate to which direction to adjust which wheel(s). Secondly, what phenomena tells you that your are done tuning ... i.e. that it is as good as it can get? I looked and have not been able to answer either. In fact, I could not tell from how the car looked on the treadmill if a tuning change made it run better or worse!

Now, if you add a tension gage in the string, attach the string straight forward from the CM, and take out the DFW toe, then you have real possibilities, although it is awkward to do an alignment check at the pwd event! In fact, that method is not useful for an alignment check ... you don' t know if it is right unless you diddle each variable and see that it gets worse each direction.

[Stan Pope]

a tension gage

A way to make a tension gage (AKA strain gage) is to combine a low friction pulley to change the direction of the string from horizontal to vertical with a weight to the end of the string, and place the weight on a pocket scale. When the car rides the treadmill, friction causes the car to try to lift the weight off the scale, reducing the weight reading. The total friction is equal to the difference between the reading with and without the treadmill running.

Of course, if the friction lifts the weight all the way off the scale, then either you have way too much friction OR you need to use a heavier weight!

The strain gage really only measures one aspect of the car's alignment. That is how well the lightly loaded rear wheel tracks with the more heavily loaded rear wheel. Other aspects of alignment are measured by how well these three points line up: CM, attachment point, and pulley. If they aren't in a straight line, you need to figure out whether the DFW or the DRW is out of kilter. (That is best done on a plain flat board and letting the car roll along a line. Look for which wheel leaves the line first ... front or rear. That one is the culprit!

[davet]

I used a treadmill to break-in our tank 2 yrs ago with graphite. Back then my goal was to get it to go as straight as possible. I first aligned it on an 8' piece of plywood. I then ran it on the treadmill and it pulled hard to one side immediately. I found that the treadmill belt was not level enough and just wasn't true enough to get a good alignment. I did continue to add graphite every 30 seconds or so and kept running it. I ran it probably a total of 5 minutes on the treadmill at slow speeds at first trying to align it then at higher and even full speed for short bursts. After the last 30 second, full speed burst I checked it on the board again and steered her perfectly straight.

I meant to add graphite the morning of the race but it was our first race and we were a little anxious and forgot to add graphite. That was an 8 wheeled tank with cannon and turret and not aerodynamic at all. We qualified for Districts and even won a heat there. If we ever went back to graphite I would align it to go perfectly straight then run it on a treadmill to break it in. Then I would align it as a railrunner.

[Stan Pope]

My only fears with that are
(1) your car will have accumulated the equivalent of several hundred races, and wear (and degradation) may appear, and
(2) the treadmill procedure does not assure that the rear wheels are well aligned.

Frequent lube should mitigate, but probably does not eliminate, that degradation.

Getting the car to run straight only tells you that the dominant wheels are (close to) where they need to be. The more lightly loaded rear wheel might not yet be in alignment and may be fighting with the others (and slowing the car).