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Flex-tuning a torsion car?


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#1 JimF

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Posted 13 May 2019 - 11:04 PM

This is mostly an "in the interest of science" project and should probably be taken as light comedy.

 

It started last fall when I was trying some new things and decided to try another torsion or "plate" car. I ended up with this which in original form was .062 center forks and 2X.039 main rails spaced. I took it to the track and spun some laps and was sort of surprised that it was pretty darned good if a little bitey and unforgiving. But.....it was the day before a race and I had a lot to do so didn't work on it much. My plan moving forward was to fill in between the spaced main rails with a 3rd rail/side and try it again later. Another (good) racer was intrigued by it so I gave it to him to try. He got taken out by a kid with a beater flexi and the car was pretty hammered. So......it went into the future projects file.

 

Today, I was starting work on my cars for next weekend's race with the plan of going down to Rocklin tomorrow to test and set up a few cars at Fast Track Hobbies. However, I heard that the Kingleman track had not been prepped and I know from my last trip down there, it wouldn't do me much good. So, I changed my plans to just run the flat track and test F-1s. Not a great return for a long drive but I had to take the opportunity. Then, I remembered a couple of beat up loaner cars that I should resurrect and THEN remembered this one.

 

So, I cut out the original rail setup and replaced with a 3 rail and torched the he** out of it to get it flat. Then I thought....what else can I do? So, I added a couple of extra short rods inside the forks and some pads on the pans to level them with the forks. Then I made 3 "torsion bars" out of .032, .039, and .047. These can get soldered across the forks and to the outboard pans as shown in the pic. This car did have some potential before but after it got wrecked, I forgot about it. I'll try it first with no torsion bars to see if the added frame rail will be enough to smooth out the car. Then I'll incrementally mess with the torsion bars just to see what happens. I'm not gonna accomplish much of anything useful tomorrow anyway and the water is still too high to fish so................

 

 

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#2 JimF

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Posted 16 May 2019 - 03:05 PM

In as much of a surprise to me as anything............this actually worked.

 

Testing on a flattish Kingleman track without much bite, this chassis without tuning ran better than I thought it would. While about 2 tenths off of my best car on this day, this one went into the turns really well but was chattery on exit an had a pretty firm 'kick' coming off. So, I added the .032 torsion bar and it calmed down some.Then I put on the .047 bar (shown) and it was even better. By this time it was within .05 off my best car.

 

I knew I was in sketchy conditions and I had other fish to fry so I hung it up. A few things.......

 

  • The chatter was similar to the wheel hop experienced when I first started trying tuning fork cars.
  • That was caused by vertical flex at the bracket face and was usually mitgated by bracing the bracket.
  • Although not really bad, the torison bar helped dramatically with this but didn't quite eliminate it.
  • Better traction would probably help with this as well.
  • Another possible experiment would be a closer fit of the rear pivot rod in the pivot tube. (,062 instead of .055)
  • The torsion bar ties the center fork to the outer pan but leaves the main rail section alone so it flexes as it did (a lot)
  • If I wanted to tie in the main rails to the rest, I'd put a .025 filler on the rails and under the bar and then solder it.

 

It seems there is more tuning available here than I thought.............. :sarcastic_hand:

 

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Jim Fowler





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