Brakes and transfer of lower "A" force

[afterburn 549]

I installed a front lower Al cross beam the other day and I think this occurred in my brain...
When the brakes are applied ,all the stopping force is transfered to that cross beam- be it Aluminum or Steel.
All that force is transfered to the two bolts that hold the thing to the car ? ! ??
So when the brakes are applied hard , all that energy is trying to shear those two bolts off (that fasten the cross beam).......???? This is what I think I know.....as I do not see any other component s holding the lower A arm from wanting to spin out from underneath the car.
Is this ..right ??

[stlrj]

There's never been an issue of those bolts shearing off, so why the sudden worry?

I have a niece that no longer feels safe driving across bridges because they might collapse. Has she gone crazy too?

[afterburn 549]

These cars are getting old ! and I am just wondering if I have this figured correctly ? ! I am a amature engimaneer and can not do the calks to figure out the exact force the two bolts "see" in a hard brake situation..........Like you say no known problems....but it does not seem right.
The leverge the crossmember transfers to it is mulitplied by two inches or so by the thickness of the cross member !
So if this is 1/2 way correct, these two bolts stop the full weight of the car in every braking aplication plus 2 inch pounds of force. ( because of the bolts standing proud 2 inches or so)

[Flieger]

The way I see it, there are a lot of other mounts and forces to work together that keep the car solid in front. I am not an engineer but here is how I see it. Please correct me where I am wrong, Burgermeister and all you engineers out there.

Are you talking about the thrust in the direction opposite to the velocity of the car?

Or do you mean the loads resulting from dive under braking. Dive forces are mostly taken by the strut mounts and are helped by a strut tower brace. The crossmember would be stretched a little, though I do not believe the forces are very large in magnitude relative to the strut mounts'

The thrust force would be partially taken by the front A-arm mount, making the effective magnitude of the force on the crossmember a little less. The strut mount, in its (soft) rigidity for a stock rubber mount, will also help.

If you think of the wheels moving rearward relative to the chassis, there will be a bit of compression of the crossmember from the A-arm seeing the torque from the wheel's force on the moment arm.

As far as torque forces on the crossmember bolts, The A-arm will be restricted from torquing by the front mount, which has a long lever to hold down.

JMHO

[afterburn 549]

Hmm , I can talk some engimneer talk LOL
But like I said looks to me in a brake slam all the force becomes a shear on the bolt s (2)
There is not much holding the A arm in its place If one of those bolts should ever let go..

[Flieger]

I wonder if the steering linkage and rack can contribute a little strength?

[DW SD]

What is an amateur engineer? One who has no or partial formal training?

The A-Arms are attached in front, too, not just to the cross member.

Take a look at this picture from the elephant racing website.


There are lots of torques and forces acting here. Just a few:
The torque on the strut from the brake caliper.
The forces of deceleration of all of the components themselves.
The resulting forces from the deceleration of the chassis (causing brake dive).

Simply eyeballing a copmlicated system rarely results in an accurate assessment of the forces.

Doug

[afterburn 549]

In actuality the front bushing has no tie dwn power, it just centers the bushing. You could quite possibly just pull the "A" frame out, of there if the rear bolts were not there !!.
How ever I see the strut will supply some of force to the tub also
..............and the armature engimneer I will proudly say is me, I have invented a couple things but have no formal traing and proud of it ! kind of like lots of folks here in the do it to your self forums. LOL

[afterburn 549]

and PS The steer rack is bolted to the top of cross member and of no value holding it all togather

[MikeBogue]

Doug is correct that the cross member will not take all of the load, because the load is transfered through the strut, however the bulk of the braking force will be transfered to it. The most important part of the threaded joint is that the bolt, if properly tightened, will not be put into a sheer condition. When the bolt is tightened properly there will be significant friction between the cross member and the body and this friction will transfer the sheer onto the body leaving the bolt in tension only. This is the same principle that is used for the lug nuts holding the rim on the car.

[afterburn 549]

I understand the threaded ball joint...just replaced mine ( elephant decambered ones OUHH LA LA ),and the cross member..that is how I got on this subject.
The ball joint nut and bolt is huge , where as the cross member is not, granted the cross member sits into shuffle pin purchase deal adding some Resistance, but to my way of drinking I mean thinking, in a brake check, the "A" arm is trying to spin out to the side (being held by the fwd bushing) and trying to go underneath the car which ever is ez r

[rvanderpyl]

Do you not have the two retaining straps that go from the cross member to the rear of the car and are attached to the two mounting points just behind the channel that the stearing rack is mounted in? Those two straps mean that for shear forces there are 4 attachment points. The other forces involved are the rotational forces as the braked wheels try to rotate the whole strut, that is easily taken up by the top of the strut mount, and the two widely spaced mounts for the A arms.

Finally, and this may be different on other years, the mounting points for my 68 actually fit into the cross member, so in fact the bolt is not taking any shearing force at all.

[afterburn 549]

I would like to see a pic these straps as I do not have them ( I think ) If they do employ flat straps...they would not amount to much under compression... just bend.....they would have to be angle or channel or ?? not flat... To do anything at all unlesss they were under tension.

[Flieger]

afterburn 549 has an Aluminum, late model crossmember I think.

rvanderpyl is referring to the early steel version from his '68 911.

[Flieger]

Just install this:

[rvanderpyl]

I installed a later (SC) aluminum cross member and it uses the locating straps as well

[DW SD]

The late and early crossmembers have the straps. they are approximately 1" x 1/8" plates which hold the crossmember from rotating for or aft. They tie the chassis to the crossmember.

I cannot print from PET, but look at illustration 401-00 for Model Year 83. Definitely an Aluminum cross member. Part # 911 341 031 01 . PET calls it a strut.

Again, I'm not sure how you can eyeball a system make many accurate assumptions. I think you would need use a finite element method (or guess a load and then solve for the other variables in the system).
Doug

[DW SD]

Originally Posted by Flieger

Just install this:

When looking at the 935 suspension members using heim joints on both ends, you can assume they are in tension / compression loads and not transmitting bending moments.
That might make it easier to approximate the loads in each component.

Doug

[afterburn 549]

Flieger ! Great work ! !! yes I would like to install that ! What a great shot !! What a great job !!
Now I have to go over to the parts thread and try to find those things...
As far as Eye balling something.......with the pic above so clearly illustrates where the load ends up,
Must be a a 2 to 3000 lb car is not transmitting the force the way I vision it to be or there would be problems.
Ea wheel would see 1/2 the weight
it gets to that pt and just locks up ?
and the weight slides past ?

[Flieger]

BTW, that great 935 suspension is not mine.