suspension pushrods

[Christopher Crowe]

-- from a-arm crux to bell crank. How light are people going in terms of wall and tube-diameter? Also, what size streamlined tube have people successfully used?

Thanks,

Chris

[S Lathrop]

The loads are very high.

I use 3/4 x .080 min. For airfoil I use 1.7 x .7 x .049 with .625 x .065 pressed inside and rosette welded.

You will need to be absolutely sure that the threaded inserts are aligned with the axis of the tubes. Off just a little and the push rods will bend.

[Christopher Crowe]

!

[Christopher Crowe]

Thanks again, will take care here -- I know these tubes must take tremendous real-life loads.

-- Chris

[R. Pare]

Chris:

Give me a call & I'll explain the process to get the tapped ends perfectly square - it isn't all that hard if you have the right equipment, but nearly impossible if you don't.

[Christopher Crowe]

Initially, I want to construct the push-rods with round tubing to get the car on its feet.

The RF97 VD FC/FF (that I stole uprights from) used 7/8" x .049" for its pushrods -- but had a more reasonable rocker ratio than the 1:1 (not quite) that I'm trying for (the rocker flanges and shock anchor points to be determined by way of wooden mock-up on the actual chassis -- I simply ran out of gas with the Mitchell program).

So. Would 7/8" x .058" push-rod tubes be logical for an FB with something like that (higher) rocker ratio?

I know this will not be a great solution from the aero point of view and I'll build some streamliners later (after all the rocker and shock mount geometry has been locked). Anyway, am I in the ballpark with the 7/8" x .058" if there are no inherent bending moments anywhere?

Thanks,

Chris

[R. Pare]

Chris:

Look up the formulas for column buckling and compare the 2 diameters and wall thicknesses. I would guess that the extra diameter will more than make up for the decrease in wall thickness, but .........

[S Lathrop]

The motion ratio of the shock to the wheel movement is not what you need to look at. It is the compression forces on the push rod. One way to calculate this is to use your Mitchel program to calculate the motion ratio if the push was a damper and the bell crank was the shock attachment to the chassis. The lower the angle of the push to horizontal the higher the loading.

That ratio is also great to know to calculate the change in ride height for a push rod adjustment.

Interestingly enough the most frequent push rod failures I have seen have been while hauling the car in a trailer.

[R. Pare]

Chris:

You can get the load on the pushrod using simple vector geometry calculations.

[Reddog]

I always believed pull rods [in tension] were better if the Motion Ratio get's near 1:1 can be had ... if your building a car don't rule them out ... and go the monkey see ... monkey do route?? ahhh but what do I know...

[Christopher Crowe]

"You can get the load on the pushrod using simple vector geometry calculations."

Simple... calculations? Ah, Richard, the only simple thing in any of this is the area of my brain where math is supposed to be figured. Will call.

Thanks ---

Chris

[Christopher Crowe]

"One way to calculate this is to use your Mitchel program to calculate the motion ratio if the push was a damper and the bell crank was the shock attachment to the chassis."

That's one thing I never tried! I got stopped in trying to get accurate numbers concerning the P, Q points (rocker pivots) in the Mitchell program. The idea of pretending the pushrod is a shock is a fantastic idea! This, I can do!

THIS IS GREAT!!!

T'anks, you guys -- !

[R. Pare]

"You can get the load on the pushrod using simple vector geometry calculations."

Simple... calculations? Ah, Richard, the only simple thing in any of this is the area of my brain where math is supposed to be figured. Will call.

Basic right angle triangle trig calculations (IF I'm remembering it correctly!)

a = tire vertical load (the vertical leg of the triangle)
A = angle of pushrod relative to the ground
C = load on pushrod

C= a/sin A

So, if your load is 100 pounds, and the pushrod angle is 25 degrees:

Load on Pushrod = 100/sin25 = 100/.422 = 236 lbs.

This obviously isn't exact because of the layout of the end of the pushrod relative to the center of the contact patch, but is close enough for what you are wanting - just an approximation of the rod loading.


The area of the 3/4x.08 pushrod is .168 sq.in. For your 7/8x.049 rod it is .127 - about a 25% decrease in stress area. My opinion - No Way, Jose. Increase the wall thickness to at least .06 - that will get the stress area up to .153, which is still a decrease of about 9% - right on the borderline, even with the increase in diameter. We've seen pushrods (3/4x.08 with a 3" or so long adjuster at one end) buckle in the trailer after a really rough ride. For safety's sake, I'd recommend sticking with .080 wall thickness - the last thing you want is to have a rear pushrod collapse on you out on the track at 100 mph!

Even with the increased diameter and the .080 wall thickness, you will want to make sure that your threaded ends are as perfectly in line with the centerline of the tube as you can get them - especially if you have a long adjuster at one end, since it will magnify the effect of any misalignment ( and most likely why those pushrods buckled). For a simple round tube, all you have to do is thread the ends in the lathe after they are welded into the tube (holding the tube in the chuck) - this will get them as close to perfect as humanly possible. For streamline tube, you need a steadyrest and a special fixture to center the tube centroid in the steadyrest.

Also, double check the jamb nuts you use - I've seen even supposedly good quality nuts have the threads at a slight angle to the faces. Any misalignment here will misalign the rod end or the adjuster to the centerline of the tube and magnify the bending loads substantially.

[kiwimanz]

"One way to calculate this is to use your Mitchel program to calculate the motion ratio if the push was a damper and the bell crank was the shock attachment to the chassis."

That's one thing I never tried! I got stopped in trying to get accurate numbers concerning the P, Q points (rocker pivots) in the Mitchell program. The idea of pretending the pushrod is a shock is a fantastic idea! This, I can do!

THIS IS GREAT!!!

T'anks, you guys -- !

Chris I typically use .875 .065 wt. 4130. (Full aero DSR) In compression this section size has massive strength it will provide the strength you need for an open wheel car.
I wouldn't go lighter
cheers
linz