MII Strut Failure HELP!!!!!!!!!

[rooster]

I see no way to use the tubular arms without moving them! The bushing part falls 1" to the inside of frame rail, and needs to come down to line up with the lower are pivot bolt.  The arm must come down abit for it to clear the frame during upward movement of the lower a-frame arm. The picture shows the new arm bolted to the lower a-frame arm with the weight of the car resting on jack stands under the lower a-frame's end.

The other picture shows how the stock strut rod lays under the same condition.

Any other options?

[enjenjo]

Bend the strut down so the hole lines up with the A arm pivot bolt. All you need at the frame is 1/2" or so clearance, that will give you about 3 to 4" at the wheel. You should be able to bend it cold, that strut is made of mild steel.

[PeterR]

As others have indicated, the bolt forming the lower arm pivot and the bolt forming the pivot of the strut must lie on the same straight line.   This allows the arm and strut to behave as if a single wishbone which can move through the full suspension travel without any binding, bending of the strut or adverse distortion of the bushes. You could even weld a tie piece between them an inch or so away from the pivots and not effect vertical movement.

Anyone installing these on a regular basis would be wise to make a jig to locate the correct position of the pivot mount.  For a one-off installation a piece of close fitting bar through the holes in the cross member will suffice.   All-thread can be used and has the advantage you can run a nut up each side of the cross member to hold it in place, but the OD is usually undersize allowing it to wobble in the holes and it is not as rigid as plain bar.

Also, before commencing any fabrication check the strut bush is true. Bolt the strut to the lower arm. Fit the test bar into the strut bush from the back of the car and confirm it slides forward easily into the arm bush. I bet the bar will not even start to engage the bolthole in the arm bush and it is likely the strut will require some bending.

[PeterR]

Without wanting to sound too much of a doomsayer, I wish to express my serious misgivings regarding the design of the product pictured above.

1.  It is bad practice for a strut to be curved as this significantly reduces the maximum compression load it can sustain, and even under normal working conditions there will be unwanted flexing.

Grab a 1/8th welding rod, stand it vertically on the ground and push downwards on the end with the palm of your hand, it will take enough load to hurt your hand.  Bend a slight curve in the rod, then see how much load it will take.

The strut would be much better as a straight piece with the end cut at angle where it is welded to the bush tube.

2. The U shaped mounting bracket is not a good design. An open section has much less torsional stiffness than a box section and any twisting of the bracket causes flexing where it joins the rail with the potential to fatigue the parent metal of either the rail or the bracket in the HAZ adjacent to the weld.

You might be wondering how the bracket would even be subject to a twisting action in this application, as the reason is not immediately obvious ---and not easy to explain so I will have to diverge for a moment with an example.

Imagine a truck with cab and bare chassis.    Now remove all cross members behind the back axle so the rear portion of the two rails do not have anything to prevent them twisting.    Right at the very end of one rail weld on a piece of square tube about three feet long so it is parallel to the axle and has about half its length is sticking out sideways beyond the frame rail.

Sit a large weight on the tube close to the outer end; the leverage causes the rail to twist outwards at the top.    Move the weight so it is close to the other end of the tube; the rail then twists inwards at the top.    Now slowly slide the weight outwards until it is at a position where there is no twist in the rail, this is known as the shear center.  

For a typical truck frame rail it is about 3" outside of the rail, and not surprisingly that is exactly where the springs are mounted, right at the place where they will have minimum twisting effect on the rail.

Now back to the strut pivot bracket. Under braking, the lower suspension arm is prevented from moving backwards by the strut which applies a rearwards force to the pivot bracket. This rearwards force is applied about half way out the flanges of the U not at the shear center, so the bracket will tend to twist the flanges towards the rear of the car.   Further, if the strut is curved it will tend to bow outwards adding to the twisting effect on the bracket.  

A far better arrangement for this item would be straight struts as described above and a pivot bracket made as a closed section formed from rectangular tube or two U shaped pieces welded toe-to-toe.

.

[Bob Paulin]

The thing that amazes me about all this is that it was, apparently, discovered by a visual inspection in the course of preparing for a trip.

I would have thought that this car would be all over the road....

No mention has been made of the steering/handling deteriorating prior to this discovery.

Was the car handling so badly - such as being all over the road - after being built originally that there was no perceptible changes in handling when the struts "unwelded" themselves?

I know I have straightened out a few cars for people who have told me that they were seriously considering getting rid of the cars because of their unpredictable directional control and/or tire wear, etc., before I worked on them........

.....and, I truly believe that there is a market out there for correcting bad suspension design and steering geometry - this being a prime example.......

.......and, although I have not actively pursued such business, I am giving it more serious consideration on a daily basis......

....but, needing a visual check to find such serious suspension problems makes me wonder exactly how the car was handling before it was broken.

Was driving this car down the road - before or after the problem - REALLY an enjoyable experience??

B.P.

[rooster]

Without wanting to sound too much of a doomsayer, I wish to express my serious misgivings regarding the design of the product pictured above.

1.  It is bad practice for a strut to be curved as this significantly reduces the maximum compression load it can sustain, and even under normal working conditions there will be unwanted flexing.

Grab a 1/8th welding rod, stand it vertically on the ground and push downwards on the end with the palm of your hand, it will take enough load to hurt your hand.  Bend a slight curve in the rod, then see how much load it will take.

The strut would be much better as a straight piece with the end cut at angle where it is welded to the bush tube.

2. The U shaped mounting bracket is not a good design. An open section has much less torsional stiffness than a box section and any twisting of the bracket causes flexing where it joins the rail with the potential to fatigue the parent metal of either the rail or the bracket in the HAZ adjacent to the weld.

You might be wondering how the bracket would even be subject to a twisting action in this application, as the reason is not immediately obvious ---and not easy to explain so I will have to diverge for a moment with an example.

Imagine a truck with cab and bare chassis.    Now remove all cross members behind the back axle so the rear portion of the two rails do not have anything to prevent them twisting.    Right at the very end of one rail weld on a piece of square tube about three feet long so it is parallel to the axle and has about half its length is sticking out sideways beyond the frame rail.

Sit a large weight on the tube close to the outer end; the leverage causes the rail to twist outwards at the top.    Move the weight so it is close to the other end of the tube; the rail then twists inwards at the top.    Now slowly slide the weight outwards until it is at a position where there is no twist in the rail, this is known as the shear center.  

For a typical truck frame rail it is about 3" outside of the rail, and not surprisingly that is exactly where the springs are mounted, right at the place where they will have minimum twisting effect on the rail.

Now back to the strut pivot bracket. Under braking, the lower suspension arm is prevented from moving backwards by the strut which applies a rearwards force to the pivot bracket. This rearwards force is applied about half way out the flanges of the U not at the shear center, so the bracket will tend to twist the flanges towards the rear of the car.   Further, if the strut is curved it will tend to bow outwards adding to the twisting effect on the bracket.  

A far better arrangement for this item would be straight struts as described above and a pivot bracket made as a closed section formed from rectangular tube or two U shaped pieces welded toe-to-toe.

.

Thanks for the input!
I think what you are saying is , it would be best to use the stock strut rod , but fasten to the frame differently. ( your last pargraph) Is this a aftermarket part?
If I were to install the stock strut rod again but with a heaver bracket, is there a sweet spot for it to be located? In other words , or the rules of installing totally different? if any at all!

Installing the tubular arm to the frame on a angle and lined up perfectly with the lower a-arm pivot bolt " wont be a walk in the Park" for me anyway!

[rooster]

The thing that amazes me about all this is that it was, apparently, discovered by a visual inspection in the course of preparing for a trip.

I would have thought that this car would be all over the road....

No mention has been made of the steering/handling deteriorating prior to this discovery.

No, car car was not all over the road! We did about 150 miles fatherday in it, a afew hundres more sence then. The car does have a steering history though!http://www.roddingroundtable.com/forums/viewtopic.php?t=939&highlight=

Was the car handling so badly - such as being all over the road - after being built originally that there was no perceptible changes in handling when the struts "unwelded" themselves? The brackets were not broken at the welds at all! I think what this is called is metal fatigue.

I know I have straightened out a few cars for people who have told me that they were seriously considering getting rid of the cars because of their unpredictable directional control and/or tire wear, etc., before I worked on them........

.....and, I truly believe that there is a market out there for correcting bad suspension design and steering geometry - this being a prime example.......

.......and, although I have not actively pursued such business, I am giving it more serious consideration on a daily basis...... I could be your first customer!

....but, needing a visual check to find such serious suspension problems makes me wonder exactly how the car was handling before it was broken.

Was driving this car down the road - before or after the problem - REALLY an enjoyable experience?? NO! not 100%

B.P.

You have me thinking that there is something else wrong with the car other than weak metal at the brackets that hold the strut rod in place! Is there something I can check?

The picture shows a new 1/4" bracket (left) next to the one that broke! its pretty thin!

Steering history link:

http://www.roddingroundtable.com/forums/viewtopic.php?t=939&highlight=

[Pope Downunder]

http://www.roddingroundtable.com/forums/viewtopic.php?t=939&highlight=


I am wondering if the cross-member was installed correctly.  If the swing arm pivot axis is so high at the back, at ride height and rake, is that axis dipping down to the front, or roughly parallel to the ground?  

With these thin chassis rails, the bracket should be designed to maximise the weld along the longitudinal edges, and minimise the welds across the bottom of the rails.  You could improve that 'channel' bracket by partial boxing of the open side, or better still by making a gusset that spreads the load along the lonitudinal lower/outer frame edge.

[GPster]

I am wondering if the cross-member was installed correctly.  

I'm afraid the answer to that question is on the first page, it's an original Mustang II crossmember. I believe that Denny bought the car this way. I've seen this one other time on a '37 Chevy with a Mustang II crossmember installed in it. I think the crossmember was installed with the car on the ground and they used a level on the top of the coil spring hats thinking that the axis of the upper "A" frame should be level. The one I saw had no anti-dive, needed 2 universals to get the steering shaft from the rack and around the engine mount crossmember and the slots for mounting the upper "A" frame had to be lengthened to swing the axis enough to get any caster. Sometimes mistakes can be made and it might be up to someone else to correct them. Sorry Denny, I certainly don't want to hurt your feelings, It's a lot better than a Model A on a S10 Blaxer chassis. GPster

[rooster]

http://www.roddingroundtable.com/forums/viewtopic.php?t=939&highlight=


I am wondering if the cross-member was installed correctly.  If the swing arm pivot axis is so high at the back, at ride height and rake, is that axis dipping down to the front, or roughly parallel to the ground? Im not sure if this is the place you and Joe are talking about, but I placed a angle finder on the adjustment bar and got a reading of 6 degess's tilting toward the rear of the car. This was done with the tires mounted and sitting on the ground!

With these thin chassis rails, the bracket should be designed to maximise the weld along the longitudinal edges, and minimise the welds across the bottom of the rails.  You could improve that 'channel' bracket by partial boxing of the open side, or better still by making a gusset that spreads the load along the lonitudinal lower/outer frame edge.

[GPster]

I've been talking to Denny privately about this and here is what I gather is happening. The Mustang II crossmember has been installed with a 4 degree negative slant (foreward) which makes it nearly impossible to install the strut rod in the proper place because the end of the strut angles up to the frame rail bottom. The installation is not so bad that there is no anti-dive from the upper "A" frame, it's just diminished. It has been properly aligned and is set up to run at 5 1/2 degrees caster. It would be a shame to cut the crossmember out and remount it for 4 degrees the large diameter of the strut rubbers makes it impossible to mount them with out bending the struts and installing the heavier mount out of alignment. SO here's an idea that I was going to give him and we're looking for reasons that it won't work. It has been mentioned that the strut rods are mild steel so I would think it possible to cut the threads on the rubber bisquit end further up. Do that and cut some of the strut end off so that a female spherical rod end (3/4"?) can be screwed on and a rod through the crossmember bolt holes can pass through the ball of the rod end. The rod end being a smaller diameter than the rubber bisquits might allow mounting on the bottom of the frame rails in line with the crossmember. A double shear mount could be made for the rod end on the bottom of the frame. Maybe this would put this installation closer to what was engineered. What are your ideas on my idea? GPster

[PeterR]

Although late in the discussion, this is the "improved" version of the after market strut I had attempted to describe. To prevent binding a common hinge line passes through both bolt holes, and for maximum buckling resistance the strut is straight with the centerline of the strut crossing the hinge line at the mid-point of the bushing.



But it still has an inherently bad flaw.   Under braking there is a rearward force in the strut bush tube which must be transferred to the bracket.   This force is quite substantial (my mental arithmetic calculates it to be equivalent to about 40% of the vehicle weight at 0.5g braking).  The bushing has little provision for a force of this magnitude; and after a short time the rear end of the bush tube will be rubbing against the inside of the mount plate.   There should be a steel washer with either a rubber biscuit or high-density polyethylene washer between the ends of the bush tube and the insides of the plates.

The more you look at it the more you realise the factory design was not so silly after all.

Now to your particular problem. You have reached the conclusion the cross member was installed tilted forwards; and as a consequence the hinge axis of the lower arm slopes upwards towards the rear of the car, making the correct position for the original rubber biscuit pivot too high to fit under the frame rail but a rod end might squeeze in.

SO here's an idea that I was going to give him ....cut some of the strut end off so that a female spherical rod end (3/4"?) can be screwed on and a rod through the crossmember bolt holes can pass through the ball of the rod end. The rod end being a smaller diameter than the rubber bisquits might allow mounting on the bottom of the frame rails in line with the crossmember.

Spherical bearing rod ends could be used but there are a few matters to consider.
1. The service life of these components on a road vehicle is very short; there is a fully sealed type but they are not so widely stocked and still have a relatively short life.

2. There is no compliance in these joints so expect some additional road noise.

3.  Mounting will be more tedious than first appears.
The maximum angulation permitted in a rod end is typically around 10 degrees, and this means the bolt through the ball should be close to square to the rod end when the suspension is at ride height. This will necessitate the two bracket plates to run about 45 degrees across the frame rail.

So, when fabricating the bracket you will have to meet two conditions: -
The center of the ball must lie on the extended hinge line of the lower arm, and also the hole through the ball should be square to the strut.

Setting this up is fiddly but not difficult.
Confirm the suspension is at ride height. Slide a test bar rearwards through the cross member then adjust the position of the ball by a combination of bending the strut and screwing the rod end until the test bar slides easily into the ball hole.

Using clamps of some form secure the strut close to the rod end so it can not move (this is really important), then slide the test bar forward a couple of inches out of the ball hole.   Fit the mounting bolt into the ball hole, and swivel the ball until the bolt is horizontal and square to the strut when viewed from below.

Fabricate a double shear mounting bracket which maintains that bolt position. To prevent the threaded portion of the rod end hitting the inside of the bracket plates at full bump you will most likely have to fit a spacer washer over the pivot bolt between the ball and the inside of the plates.

There is a lot of work to fit the rod end, and I suspect there might be a quick and dirty way to overcome your problem.   From the photos it would appear as though the original biscuit mount only just misses out fitting under the frame rail.  If that is the case you could elongate the rear hole in the cross member downwards so the hinge line does not run up at such a steep angle.

Yes it is crude, but the amount to be filed out is likely to be quite small.  Don't be concerned about the elongated hole, the spring force will hold the bolt down hard against the bottom of the hole, and any worriers should look at an Aussie Ford cross member which has an elongated hole that allows 1/2" horizontal movement of the lower pivot bolt for camber adjustment.

The relative heights of the lower arm pivot and the tie rod inner pivot will change marginally, but the effect on steering will be minimal.

You can easily calculate how much the hole has to be elongated to decide if you are comfortable with embarking on this act of brutality.

Remove the lower arm pivot bolt from the cross member and replace it with a test bar.   Slide the bar backwards until the end of it reaches the strut rod.  Measure the distance from the end of the test bar to the underside of the frame, then from that and the diameter of the rubber biscuit determine how much the hinge line has to be lowered.  Call that "D".

Measure the distance between the front and back of the cross member, (from memory this is 3-3/8"). Call this "M".
Measure the distance from front face of cross member to the position where strut bracket will be located on the frame. Call this "S".

Calculate Dx(M/S).  That is the amount you will need to elongate the hole, but remember you are not to tell anyone I suggested this rough assed solution.    

[tomslik]

well, what would be wrong with running the strut rod forward (like and early nova or mustang?

[GPster]

I like this chance to discuss these things. I had always wondered why rod ends weren't used when splitting wishbones but following in that vein maybe there's a tie rod end that has a 3/4" female shank. Talking a little geometry I would almost think that the front and back faces of the crossmember are paralell so that the surface under the heads of the bolt and nut are 90 degrees from the shank of the bolt so that when they are tightened the crossmember faces aren't distorted. If you would think that this would be minimal then I like this idea and he could use what he has and make this adjustment in the middle. Now , for one clarification. Is the "anti dive" a product of the slant of the axis only of the top "A" frame or is it a product of the upper and lower axises not being paralell. If the second is the case then that trick might kill any "anti-dive" that he has now. If it would stay the same because the upper axis stays the same then the way you said you didn't suggest sounds fine. One addition I would make is that I would build a heavy washer to fit between the nut and the elonggated hole with the correct inside diameter to fit the bolt. When it's all together and the force of the spring is pushing the bolt down to the botton of the elonggation, I'd tack weld the outside of the washer to the crossmember. GPster

[PeterR]

I like this chance to discuss these things. I had always wondered why rod ends weren't used when splitting wishbones but following in that vein maybe there's a tie rod end that has a 3/4" female shank.

Both spherical rod ends and tie-rod ends are used with split bones and hairpins.  There is a tie-rod end with ¾ thread, it is off a Ford truck –enjenjo is sure to know the part number.  Tie-rod ends are durable but the mounting bracket requires a tapered reamer and is much more difficult to construct than a simple double shear bracket for a ball end.

Rod ends will last longer for a hairpin than for an MII strut because the much longer arm means the joint will rotate through a much smaller angle causing less wear.  Also it is further away from the crud thrown up by the wheels.

Talking a little geometry I would almost think that the front and back faces of the cross member are parallel so that the surface under the heads of the bolt and nut are 90 degrees from the shank of the bolt so that when they are tightened the cross member faces aren't distorted. If you would think that this would be minimal then I like this idea and he could use what he has and make this adjustment in the middle. One addition I would make is that I would build a heavy washer to fit between the nut and the elongated hole with the correct inside diameter to fit the bolt.

Take some measurements and calculate how much the hole has to be elongated.  I suspect it will be in the order of 1/16".   Welding a thick washer over the hole is a sound move.

Now, for one clarification. Is the "anti dive" a product of the slant of the axis only of the top "A" frame or is it a product of the upper and lower axises not being parallel. If the second is the case then that trick might kill any "anti-dive" that he has now. If it would stay the same because the upper axis stays the same then the way you said you didn't suggest sounds fine.

Refer to the diagram below.  The blue lines pass through the pivots of the suspension arms. The red dashed line runs from the tire/pavement contact point to the intersection point of the two blue lines.  The solid red arrow shows the direction of the brake reaction force into the front suspension assembly.   The higher the arrow points, the greater the amount of anti dive.

Figure 1 approximates a standard MII with lower arm close to horizontal and top arm sloping down to the rear.

Figure 2 approximates the situation with the MII cross member tilted forward so the upper arm is horizontal and the lower arm slopes up to the rear.   The red arrow now points up higher indicating the amount of anti dive has increased significantly –not decreased.

Figure 3 approximates the situation you would have with the upper arm horizontal, and the lower arm at a flatter angle. The meeting point of the blue lines is off the screen to the right, so the red arrow will be at a flatter angle than original, at a guess giving about half the amount of anti dive in a standard MII.  

Should this cause concern?   I doubt it, people use four bars without a moment of hesitation and let's check what the diagram below reveals for them. Typically both bars are horizontal, that means the blue lines meet at infinity to the right. To intersect them at infinity red line must also be horizontal and this means zero anti dive.