Strut tower bar preload: tension or compression?

[ksw100]

For a car with MacPherson strut front suspension, such as the FT86, S550 Mustang, or Porsche 911, when installing a strut tower bar with adjustable length, should the preload be set so that:

A. The strut tower bar is slightly longer (in compression) than its neutral length, resulting in a slight increase in positive camber.

or

B. The strut tower bar is slightly shorter (in tension) than its neutral length , resulting in a slight increase in negative camber.

...and why?

My intuition tells me the answer is B. in tension. But I can't think of a convincing reason why it would be much better than A.

 

[cibachrome]

It makes no difference. Can you guess why ? (A common mistake)

 

[ksw100]

Thanks for the response! I would guess it's because the forces acting on the strut towers during aggressive cornering are significantly larger than the small preload adjustments made by the strut bar, so the bar's preload doesn't have a noticeable impact compared to the overall chassis flex and load distribution.

Would love to hear your thoughts on why it doesn’t make a difference. Appreciate the insight!

 

[cibachrome]

The towers move together, so a top brace might only see a small moment but little strain. Had to prove this with a strain gauged bar and an analog meter on the dash of a test car. Very little signal but noticeable effect on the on-center gain.

What's needed is a 'bridge' to keep the towers from 'matchboxing'.

 

[Tmoose]

By inspection I always wondered what arched tower braces, attached with offset thin plate brackets could really offer.

In comparison, the 1960s Shelby Mustang braces to the firewall looked to me to provide a 10X stiffer load path.

https://photos.classiccars.com/cc-t...1447-1966-ford-mustang-shelby-gt350-thumb.jpg

 

[BrianPetersen]

Bracing strut towers might make sense if the strut towers are considerably ahead of the firewall and not part of the firewall structure (see Mustang photo above), but in most typical vehicles that have MacPherson front suspension (includes mine), the strut towers form part of the firewall structure which is usually the strongest part of the car, and further bracing them in a direction that seems unlikely to actually deflect meaningfully doesn't seem to make a whole lot of sense. Also, the upper strut mount takes only a fraction of the cornering and braking loads. Most of that is on the lower control arm. (I've never really been a fan of strut braces ... never saw the point on vehicles where the strut tower is part of the firewall structure)

The odd thing with the pre-Fox Mustang in the photo above is that this wasn't a MacPherson suspension. It's just a shock and spring tower. Cornering loads don't go through it. The loads are up and down in line with the spring and damper - perpendicular to that brace. Maybe the actual intent is to brace the upper control arm mount which is a little further down by hanging onto the top of the strut tower, and to stop the whole front of the car from "parallelogramming". They can't brace further down because there's a whole lot of engine in the way.

 

[GregLocock]

An entertaining experiment is to measure the distance between the strut tops, and then repeat with the front wheels free. I've seen 6mm. Mazda used to tie bits of string around the car and see which bits went slack when cornering.

Cheers

Greg Locock


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