Lock nuts vs. pinch bolts on suspension links

[ubrben]

Most race cars use turnbuckle type adjusters to lengthen/shorten the pushrods to adjust ride height. Most seem to do it using jam nuts to lock the turnbuckle following an adjustment. E.G.

http://www.chironworld.com/parts/images/uploads/100_0158.JPG

However I noticed recently that the Porsche RS Spyder uses a pinch bolt like a bicycle seat tube and no lock nut on its pushrod adjusters:

http://www.mulsannescorner.com/PorscheRSSpyderSebring2008-MF1.jpg

Just wondering what the point of the pinch bolt approach is. Does it reduce stress concentrations at the thread root where the turnbuckle enters the push rod? Or is it something more mundane like the torque applied to a lock nut altering the length adjustment slightly and therefore the pinch bolt is more precise?

Ben

 

[evelrod]

I could not access your first example, but I suspect I am familiar enough with the idea...On my Mini Cooper and Lotus Cortina vintage race cars, I use both methods on various suspension bits. I see no particular advantage, one to the other. Each method works equally well for it's application.
Both methods (as well as others) have been used in automotive applications for over a century!

Rod

 

[CoryPad]

I agree with Rod that both can and have been used for suspension joints.

Jam nuts in this application are an example of a non-preloaded threaded fastener, which usually is not a preferred practice. Having said that, they usually do not present a problem in this application.

Pinch bolts are a preloaded fastener, but they place the bolt in bending, which is not a preferred practice.

It appears that this pinch bolt design is a function of the special end fitting used on the push rod, perhaps with a "tab" on the pushrod that prevents rotation of the fitting? Any chance to get a higher resolution/magnification image?

Your precision argument does not seem likely (jam nut deflections would be on the order of 100 [µ]m).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[NormPeterson]

I can't access the first picture either. But if the link is subject to sufficient torsional moments and stress, jam nuts can loosen in service. At least one end of the link needs to have a spherical pivot, or some other means of eliminating the torsional moment should be provided.


Norm

 

[kenweise]

There are two other issues I see.

1) Joint loosening. If done properly, you can make a pretty hard joint with a jam nut. Make sure there is sufficient thread engagement in the turnbuckle to avoid camming the joint loose. Pinch bolts are ALWAYS soft joints. They rely on the pinch joint not being fully closed to grab the inner metal. They will work, but may have joint loosening problems.

2) Drift in adjustment. A jam nut will not drift as much while securing the nuts. this is due to the fit between the threads of the link and the turnbuckle. Pinch joint threads are necessarily sloppy to the link to allow adjustment. As such, they sag under gravity and will change length appreciably when secured. This can wipe out your careful adjustment.

Ken