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11/28/2011 9:16 am  #1

how much flex?

I just built my first slip joint and was wondering how much flex sould there be? its got a 9.625 axle c/l to c/l of guide nut length. i am using .0625 sping steel wire for "springs" that run down the chassis sides. they are aproximately 7 inches in length.
Also, where do you place any weight if any? Right behind the guide or closer to the hinge?
I am running the old bright yellow S16D Parma motor.
I also tinkered with using one spring steel wire and one stainless wire, seems i got a bit more flex that way.


11/28/2011 3:00 pm  #2

Re: how much flex?

Greg, on the slip joints you "Flex" works against you, after much research and talking to some of the guys on here, it seems that it's better not to have flex.

As to the rails: you want to run them from under the rails you have under the motor box to the joint, than run the rails under the chassis and solder the brass tubes on the inside by the guide plate area.

Weights: some add as much as 14 grams on the guide plate while others distribute the weight all over the chassis, try to start with 7 grams on the guide plate, and 7 on the plate by the joint, you might also want to add some near the motor box and make sure you stay under the axle height.

I personally have 4 different slip joint chassis and they all seem to perform differently, I think motor launching is where you will have most trouble, so work on launching the dragster so it will work for you.
I use no glue on the track or my tires and like I said they all run well.
These slip joints run anywhere from mid-9s-mid-10s

Some found out that if you use plastic ties on the rails from the motor box to the joint with the ties running over the rails that go all the way under it binds the flex and it works for them. By moving the ties up an down the rails you can get it to perform better or worse, just keep moving them up and down the rails.

Let's see what others might have to say.

"Where legends are made and records are broken".

11/30/2011 5:25 am  #3

Re: how much flex?

im gonna test tomorrow night, I moved the "spring bars" inside the main rails. Seems to be much better. I really appreciate the help DVan. I am going to heed all of the great advice. We shall see what happens. Just really looking for good consistency.

     Thread Starter

12/01/2011 9:06 pm  #4

Re: how much flex?

Testing was great. We have a 1000' @16v and it ran .89's consistently within .005 over about 10 passes. I had no trouble at all at the launch. I run 1.01x.300wide protracks. I was able to use very little glue on a clean track up to the tree. Now tommorow we will see how consistent it stays. Thanks DVan for the help, I did add 7g on the tounge.

     Thread Starter

11/14/2012 6:52 am  #5

Re: how much flex?

What you need is positive flex........... and not negative.   Let me explain

The dragster can arch it upwards as much as you want ............ the more arch the less weight you will need in the front of it.

When it returns to normal or not flexing... it needs to be stiff so it does not flex the other way, or sag because it will bottom out and bounce off the track, wheel hop.  one easy way to accomplish this is to add a bump stop to the body and chassis

Just one method to the madness

There are no ties in drag racing. Thank god!!!

12/07/2012 4:34 am  #6

Re: how much flex?

Is there a certain length that seems to work best or do ya just build to the body?  Also the way I am reading it and seeing in the pictures, the spring rail is under the main (hinged) rails in the rear motor bracket section and then follows up into the inside of the front rails.  Would that not pretension the hinged area upwards and if so I guess that is what is wanted?

Last edited by Tree'd (12/07/2012 5:59 am)


1/27/2013 6:20 am  #7

Re: how much flex?

Track owner question?
I do have some concern about some of these chassis flexing and bottoming out on my track and over a period of time doing damage.

It take enough time as it is to run a race, to check every car would be near impossible and even if you did, some would flex even more under load vs not.

What is a track owner wanted to take good care of the finish on his track to do?  What are your recommendations?

Thank you!!!

Last edited by slotcarjames (1/27/2013 6:22 am)

Model Car Race Center
Eden, NC
Semi-Pro since 1967

1/27/2013 6:52 am  #8

Re: how much flex?

I would just sit the cars on a long tech board and look for clearnce , making sure you can slide  a 1/16th " under it ,the only place you should have  hard down flex would be under no power, in the shut down srea.


3/15/2013 8:52 pm  #9

Re: how much flex?

I have bought and built slip joints and find them intresting. Lots to think about, flex, -flex, hinge locations, etc.  I find that they are very consistent and have won many races with both. Mine flex alot when picked up. I have gone to making the front axle go thru the body and mount and place fish weights on the axles, adjustable. SO anyway thanks for the insite from everybody makes me think about them even more.


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