Kar Audio Guy's R82

[Boostinjdm]

10 inches?  WoW!  Most people aim for zero.  Some even come close to it.

[Doug Heim]

I belive zero is near impossible with trailing arms though.

[The Kar Audio Guy]

10 inches?  WoW!  Most people aim for zero.  Some even come close to it.

yeah I know... thanks for the help

[Engineer]

so the location of the tranz output shafts up and down does not matter? only the forward and back so the CVs line up at full droop...

thanks for the help

I will take a stab at it......

Certainly the up and down location of the tranny matters, but it is the hardest thing to change.  The vertical location of the tranny output is limited by physical size of the tranny, and its location in the frame.  The full bump location of the trailing arms is limited by the desired clearance under the buggy at full bump as well as the tire size.  Once those components are determined you can't do much about the full bump vertical locations.  The full droop location is going to be limited by the tranny location and probably the max angle of the CV's.

When you look at the arc that the trailing arms go through we start to see how they swing forward and backward as they go up and down.  If you exagerated their travel until they were hanging straight down, they would have moved forward by the length of the arm.....  32"?  So as well as moving away from the tranny vertically, they have moved away horizontally 32".  So how can you use this movement to your advantage?  By placing the center of the wheel directly in line with the tranny output at full droop, as the wheel moves from ride heigth to full droop it is actually headed toward the tranny output instead of away from it.  At the same time it is headed toward the tranny flange horizontally, it is headed away from the tranny flange vertically, so the combination of the two movements results in less plunge.

If the center of the wheel is horizontally aligned with the tranny flange at full droop then at ride heigth or at full bump it is behind the tranny output.  So as the wheel moved up it gets closer to the tranny vertically while getting further away horizontally.  This trade off results in less plunge.

The fact that you have semi-trailing arms helps because they don't move straight up and down if viewed from the rear of the car.  They actually are farthest away from the tranny (widest) when straight back, and move inward (narrower) as they move up and down.  So at full droop they have moved inward while at full bump they have moved outward.

I gtg at the moment.

[fabr]

Great stab engineer! Well said! So,KAG, what's the plan of attaCK? You thinking of standing the tranny up a little to get it further forward and lengthening the arms also? You do need to keep the chain as long as possible. 

[SPEC]

Engineer,

[The Kar Audio Guy]

Engineer, well said!

I thank you and masterfabr both for all the clarity.

I asked my master tech at the shop next door. he says I can tip the tranz just a few degrees... this wouldn't help too much, so I'll leave it where it is and make new trailing arms.

thanks guys!

[Engineer]

So most all trailing arm designs are based on the original VW design with a certain ratio of trailing arm length, width and travel.  You will notice that after market arms are advertised as 3+3, etc.  The reason for this is because if you increase travel (longer arm) then you need a corresponding change in width to keep everything in balance.

Increasing the length of your arms will help, but with excessively long arms, there is not as much fore and aft movement as the wheel moves up and down.  So you won't experience as much reduction in plunge from the fore and aft movement.

Changing the distance from the trailing arm pivots to the tranny output is the root of the problem.  If you would take a string and fix one end where the original output was on the Rorty design, and pull it out to where the CV is on the trailing arm, then cycle the arm up and down, it would be very easy to see how and why it was originally designed to work.  Then start adjusting things from there.  Longer trailing arms may work, moving the trailing arm mounts back would work.  Everything is a tradeoff.

All of this also affects CV angle.  As discussed before the full bump vertical travel is limited by tire size and desired frame clearance.  So if looking for maximum travel full bump is relatively set.  Now looking at full droop it will be determined by maximum cv angle.  If the wheel is directly below the tranny output at full droop then the angle of the axle relative to the wheel and tranny output is only the angle created by the vertical distance.  If the wheel were 3 inches in front of the tranny output then the horizontal angle of the axle would be added to the vertical angle causing more CV angle at the same droop location.  Therefore if the wheel center is not directly under the tranny output at full droop then you will have less travel at the same max CV angle.

So if we have the wheel center directly under the tranny output at full droop then as the wheel swings up it moves back behind the tranny output.  So at full bump we have very little CV angle based on the vertical angle of the axle, but we have some angle based on the wheel being behind the tranny output.  However it is a good trade in order to get maximum travel at full droop.

So both plunge and CV angle are helped by the wheel center being directly below the tranny output at full droop.  Thus the rule of thumb of needing the two inline at full droop. 

Need diagrams.

[Engineer]

Engineer, well said!

I thank you and masterfabr both for all the clarity.

I asked my master tech at the shop next door. he says I can tip the tranz just a few degrees... this wouldn't help too much, so I'll leave it where it is and make new trailing arms.

thanks guys!

I hope it helps.

I would suggest tacking a piece on to your trailing arm to represent the new length, then use a tape measure or string to run from the tranny CV flange to the wheel cv flange to measure your plunge before making the new arms.  It would suck to move the pivots at this point, but I would check the numbers before making a new set of arms to make sure it will be ok.

Are you using 930's?  I don't know the max plunge numbers but someone here can tell you.

Good luck!

[The Kar Audio Guy]

I'm running F150 cv's and wheels bearings. they will take more angle, but less plunge. only about 2" it looks like.

it looks like if I make my arms longer they will be longer by about 12", and the buggy will widen in the rear by about 8"total. not too bad i guess?

[Engineer]

I'm running F150 cv's and wheels bearings. they will take more angle, but less plunge. only about 2" it looks like.

it looks like if I make my arms longer they will be longer by about 12", and the buggy will widen in the rear by about 8"total. not too bad i guess?

You wouldn't necessarily have to follow the same angle of the arms and end up with 8" wider.  You could make it the same width just longer.  The only way to know for sure what the results to plunge will be is to mock up the location of the outer CV on the arm and cycle it up and down checking the plunge as you go.

[fabr]

Yup to all above!

[The Kar Audio Guy]

how much plunge does a 930 set up have?

[The Kar Audio Guy]

thanks guys!!

I finally got my head out of me arss. it looks like if I length my arms about 8" I'm money and if I drop the tranz down just about 6" I get about 20 inches of useable travel and a longer chain...seems good to me?!

[Engineer]

thanks guys!!

I finally got my head out of me arss. it looks like if I length my arms about 8" I'm money and if I drop the tranz down just about 6" I get about 20 inches of useable travel and a longer chain...seems good to me?!

Sounds like a plan!