well, its almost midnight, so i appologise for my typing now, but i just thought of this, and couldnt wait to post.

There has been alot of debate, heated at times, whether or not a rear stress bar does anything. I put mine in my car when it was a week old, and noticed a huge improvment, way to big to be placebo. Others noticed a diffrence as well, and then there are those that said nothing, waste of money! well, i might have figured out why.

Think of our rear suspension, 4 contact point to the wheel, the trailing arm, which only locates the wheel fore and aft, other than that, it allows movement in any direction, therefore, it is removed from this equation. The two RCA;s and the upper strut mount.

Now, Meb has evidnece that virtually all the cornering force is applied to the lower RCAs, due to the wear in his ploy bushings. The tops where tight. What does this tell you? the bottom mount is moving in and out, while the top is not. well since the upper RCA is not the upper most mount on the system, that must make it a pivot point. The Upper RCA is a fixed point, and while the bottom is moving in and out, the top must be moving in and out as well, where is the top you say, well cheif, that would be the upper strut mount!

Since the upper strut mount is firmly bolted to the sheet metel, that can only mean that when the bottom moves, a little (as the upper and lower rca meet close together) the top tries to move alot, as there is a whole lot more leverage sticking out above the pivot than below.

On to some rational. Have you ever seen someone lean on a fence post only to see a fence post 20 feet and 4 fence posts away move in the oposite direction? I sure have. So, despite many people claims that the strut bar mounts to a non critcle location (namely not the actual upper strut mount) so it cant be doing anything, isnt entirely accurate. It may not be actually attached where the strut mounts, but they are both in the same general location, and both peices of sheetmetel, are acctached to the same monobody. Therefore, they could be reducing or eliminating flex in another area. Of course by firmly mounting where it does bolt on to could simply elimiante its flex, not allowing flex anywhere else.

Now then, why you ask do some people say this is all rubbish and they didnt notcie a difference. Well, those people could be running zero compliance RCAs, or possibly just lowers, from the likes of helix of alta. Now, since the load is carried by the lower, and the lower is now zero compliance, it is NOT moving in and out, which means the upper RCA is NOT acting as a pivot producing alot of leverage, which also means the upper strut mount is not trying to move, hence, the rear stress bar didnt do anything for them!

so, if your suspension is stock, and or wore out with no plans to put zero compliance RCAs in, you will probably notice a difference, if you do have zero compliance RCAs, and looking for the last bit of rigidity (and from what i remember of prior post on the topic, most people who didnt notice anything were running pretty stiff suspensions, and just looking for the last little bit of rigidity...) well, you guys arnt going to notice anything becauseyou have already eliminated the problem.

There, now i may actually be able to go to sleep. I dont know why these things come to me when im trying to go to sleep, but they always do, and i always have to get up to post, cause thinking about it only makes me more alert. What you guys think? Does this all make sence to you?

Oh man, 5 am is going to come offly early

Beecher

look, im so tired i used smiles, thats a first

 

ok, well, i was up until about 2 anylisising the front suspension, unfortunatly, and here is what i came up with. You take a stock car, and this will become more noticably on a high use car, but they understeer. That means that the tires are scrubbing sidways, which means alot of force. And on stock, and especially high milage cars, there is probably some flex going on in the upper strut mounts in this condition (a friend once reported jacking his car up with the hood open to change the oil, and found that his strut towers moved nearly 2 inches apart between on the ground and in the air, but this was much heavier car, with alot of miles, but it illustrats my point). Now, you take your car new, and put a STB in, maybe you feel a differnce, maybe you dont (anyone want to buy me one so i can test this?), you put it on a high miler, and you probably do notice a differnce. Great right? Probably not. Why? Well, what is the number one mod in the mini world? Sway bar (in the s world pully maybe, buts removed from this equation, and factor in cooper owners and you end up with sway bar anyway). What does this mean? well it now means your car oversteers, so your wheel are pointed inthe direction of travel, and the rear wheels are scrubbing (and from my last post, that means you would benefit from a rear bar, unless you have zerocomp LRCAs. If you have a rear sway, there is not as much force on the front tires, reducing the movement, and you will not notice the differnce. Now this get complicated, because im sure with sticky tires and lots of front camber, and ultra high speed corners, you WILL notice a difference, but probably only on the track (or the way i drive on my local roads, but shhhh, i didnt say that...). So there you have, mull it over, ponder it endlessly, and get back to me, please. If you really ponder about its all in its entirty, it really makes sense, and all the reasons why some people notice and some dont are accounted for, near as i can tell. Like the motor, when it comes to upgrade, 2+2 doesnt always =4. It could equal anywhere from 1 to 5!

So there, as far as im concerned QED...

Beecher

 

Are we talking about front or rear braces?

 

the rear *shocks* (not struts) do not have ANY cornering forces on them. ALL the cornering forces are absorbed by the rcas. the front *struts* DO have cornering forces on them.

 

shocks, struts, you know what i mean, one was done 4 hours past when i normally go to sleep and the ohter was after 3 hours of sleep, you lucky i even posted on the right website.

You right, the rears dont have cornering force applied to them, i never said they did. If you reread, meb has 1/8 inch wear on the LRCAs, with no wear on the upper, meaning the upper one is a fixed point, with the lower conection point moving back and forth, hence the wear, now, take a ruler about a 18 inches long, put one finger 2 inches from one end, move the short end 1/8 inch back and forth, watch how much the other end moves. Its not cornering force, its leverage about a pivot.

Snid: The first was concerned about the rears only, after i posted that, it occured to me that the same basic assumptions and situations could be applied to the front. The second post mostly deals with the front one. I posted the second as soon as i could! I will go threw them and read them in a bit, to make sure i have no gleaming errors, but i think the principles are sound.

Beecher

 

there is a difference. "struts" are short for macpherson struts, which are reinforced shocks that can have a sideways force applied to them, and are used in place of an upper control arm and separate shock.


again, you are implying that there is a sideways force applied to the shock body on the rear, which is incorrect. in your example the ruler is the shock and your fingers are the control arms. but the control arms do not attach to the shock. they attach to the knuckle. the shock merely pushes down on the knuckle at a single pivot, or, to put it in a different perspective, the knuckle pushes UP on the shock. the only force applied to the shock is the direction of a line drawn between the upper and lower attachment points of the shock. to use your terminology, the only thing that is "pivoting" is the knuckle.

 

sorry, i didnt mean to be snippy, i know the difference, i just forget to differentiate sometimes.

ok, now, i will admit what i remebered last night and what i just looked at right now, dont exactly concour. I thought the shock was closer to the back around where the rcas joined, which would make my explanation a little better. I suppose the way it is actually set up, the give in the lrca would allow a twist at the bottom shock mount, and not a driect pivot motion. Does this make sence to you you? Do you agree with my front explanation? All that said, i think rear end regidity may play a part. I guess that is what i get for posting while over exhausted. Sorry if i seemed rude. You would almost have to make a complete mock up of the rear to really see the implications of a worn or sloppy bushing tho. I think your on a better track than i was.

thanks

Beecher

 

snippy? absolutely not. i never thought that. i hope you were not reading into the (snip) that i write when i edit people's quotes.

try this: hold the ruler vertically with your palms pushing in on the ends, one on top and one on the bottom. this simulates the force on the shock. now push the bottom hand sideways (perpendicular to the ruler), back and forth. this is the force of the bottom control arm pushing on the knuckle, which pushes on the bottom pivot of the shock. can you feel a sideways force on your other hand?

now for the front, that is totally different. the front struts DO have a sideways force since they are performing double duty, supporting the weight of the car AND absorbing cornering forces. moving the top of the strut sideways will camber the front wheel in and out (that's how camber plates work). however, moving the top of the rear shock will do nothing to the camber of the rear wheel.

try the the ruler example here for the front. hold the ruler the same way except now grab the bottom part with your hand like a baseball bat. now try to apply a sideways force. i bet you CAN feel it in your other hand now!

 

BTW, i'm not saying that the rear bar does NOTHING. anything that stiffens the structure of the car can't be a bad thing IMHO. the forces being applied to the structure during a corner can be quite complex, and may be twisting the body in some way. adding a rear brace may do something in that regard, although triangulation would help much more.

 

i did at first, then i realized what you actually used it for. My cousin says snippy all the time, it was a coincidence, haha.

ok, your example makes alot more sense than mine did, but, there is a third contact point. The spacing is bigger than i thoght it was, but take your example, but try doind it with the edge of a table around the middle. your bottom hand is the lrca, the table corner is the urca, and the upper hand is the shock tower. This movement would simulate a tight upper, witha sloppy lower rca. That was what i was trying to get at. Right? would that not creat a side to side motion at the top shock mount? Im just thinking that two points is a straight line (and if i understand correctly, your example is a two point system, but the rear end actually has 3 points involved, and when the middle one is fixed, and one end one tries to move, the other end one must as well, or resist, not allowing the other to move), now does that work, or an i way off on that? my new LRCAs will be in a couple weeks, so i can see what i see when i have one out. Of course i might get bored tomorrow and pull one out anyway, just to see exactly what is what, and we can stop with they hypothetical stuff. There might be enough other stuff going on to copmensate for what ever is happening.

Do you at least see where im comming from? Is my theory sound? And are you saying you do agree with my reasoning for the front? I think you are, but want to be sure.

thanks alot

Beecher

 

i think the problem is with your initial set up. you have 3 things applying force to the "shock", whereas in reality there are only two. the top pivot does not touch the shock on the car, yet in your example the table (top pivot) IS touching. that's the confusion right there.

now try this: hold the ruler with your palms, like before. now, move your bottom hand back and forth. now use your foot to try to resist that motion. while you are resisting with your foot, lean on the table with your elbow and touch your knee to the bottom of the table. move your other foot in a counter-clockwise circle. please post a video so we can assess your suspension model for correctness.

 

yeah, i was just out there, and that basically is what you have to do. it looks like there would be some weird stuff going on, expecially since the bottom shock mount is at a werid angle. The shock is also below both rca's which i hadnt noticed (my search capabilities arnt too good, but when i cover my eyes i can find things much better, hahah). So yeah, your probably right. I just thought i had a reason why there was both ends of the spectrum as to whether a rear (or front bar for that matter works). It appears i did alot figuring only to figure that i have no idea. (which i think is somewhat related to know enough to know i dont know??? despite i thought i did). Well, overall, this was very educational, thanks for the lesson teacher

Beecher

 

The rear is actually called a Chapman Strut and the difference between it and the Mac comes down to the fact the Chapman doesn't do anything in the suspension other than carry the load (spring) and dampen motion. The top and bottom are point contacts (not exactly, but close enough). The Mac Strut is in fact an active suspension member, moving it's upper contact point changes camber and caster (and toe because they're coupled). Change the upper (or lower) mounting point in the rear, and the camber, caster and toe are unchanged (But effective spring and damping rates are though).

IF there is a difference before and after installation, it's less going to be from the effect you described, than deformation of the box that is the car body. And this is pretty easy to measure. Take one of those cheep shower bars or something like that (one tube sliding inside another, fix the two ends at the locations you want to see distance changes in, and use a rubber band or wire tie to mark the rest postion of the jig. Go out and beat the snot out of your car, and then see if the band or tie has moved. Total motion would be 2x that (if your car was on level ground to start).

Matt

 

I remember Matt telling me this about a proposed project a few months back... No, never did it (still might), but it would be easy to do.

 

eek. actually a chapman strut is just like a mac strut except it is on the rear (does not have a strut bearing since it does not turn). BUT, a chapman strut DOES take cornering forces and eliminates the top link, just a like a mac strut. hence the "strut" nomenclature. the chapman strut was "invented" by colin chapman (my hero) who used a mac strut in the rear of i think the lotus 12... something like that. the mini does not use a chapman strut, just a simple coilover shock.

i'm sorry.. i know i'm a nitpicker when it comes to things like this...

 

well, i think ive got what i need out of this, thanks everyone for their input!

Beecher

 

Sorry, off day.....

Matt

 

All,

Have been pondering this thread and it's implications. The original implied question was whether a brace from the rear sub-frame mounting area to the C pillar will materially improve chassis rigidity. I say "very limited at best".

Long ago I made a model of MINI from a light cardboard shoe box with a lid. Turned the box upside down and made the relatively thick cardboard lid my MINI chassis floor pan. Cut holes into the box part to imitate the front, rear and side windows, the doors and the sunroof. I twisted and bent the box on all the planes that I could think of to get a sense of the rigidity of the composite structure.

It is apparent to me that the chassis floor pan is responsible for an especially large portion of the chassis rigidity, with special emphasis on torsional rigidity. Torsional rigidity is what I am seeking at the track. The composite of the A, B, and C pillars with the roof is important too, but I was impressed with the contribution of the chassis floor pan. I have no way to quantify my results except by the feel of the cardboard model in my hand.

The relatively thick cardboard lid faithfully imitates the actual structure of the MINI chassis floor pan. The MINI chassis is a channel section with thick side rails, reinforcing tubular members, and reinforced, but not triangulated corners (especially in the rear third of the floor pan). A quick and dirty way to substantially improve the torsional rigidity of any channel section is to convert that channel section into a box or a rectangular tube section.

My approach to enhancing rear chassis rigidity has been to remove the rear seat cushion, (rear cushion delete) and box in that entire area where the rear seat once sat with 1/8" aluminum plate. The plate effectively triangulates and boxes in the entire rectangle that comprises the rear of the chassis floor pan. The boxed in section comprises the area from the top mounting points of the rear struts and the points where the rear subframe attaches underneath the chassis, forward to that huge mid-chassis tubular member that runs from side to side just forward of the fuel tanks behind the front seats. I believe that this rear chassis box results in greater chassis rigidity than any brace that connects the C pillar(s) together or connects the C pillar(s) to the rear sub-frame mounting area of the chassis.

The above described rear chassis box section in my car links forward to the mid-chassis tube that was created when I boxed in the length of the exhaust channel, underneath the car. The mid-chassis tube in turn links forward to the home made front chassis tube that ties the front subframe mounting points out to the chassis side rails. The front and rear subframes are linked thru boxed sections of chassis.

So, what I have done is to try and maximize the overall chassis rigidity by converting the open channel design of the chassis floor pan into a series of linked tubes and boxes.

Don't really know if all this work is worth a fig or not. But it is worth a try.

Regards,
John Petrich in Seattle

 

Was hoping John would notice this, and chime-in! Would love to see this box where the rear seat cushion was... If I've seen it before, I apologize...

 

wow. that would be something to see! Since you brought it up, i bet the 57 cheyes have wild chassis flex, the chassis is a giant X from corner to corner! Litteraly. The funny thing is, i seen an article about a 56 pontiac i think, same chassis as the chevy, but supposedly better. They remarked about the flat cornering of the car, meanwhile, im looking at the picture, one wheel is almost off the ground, and one side is probably 10 inches higher than the other! If they could only see us now, hahaha.

actually, petrich, isnt that basically what a roll bar consists off? minus the plate at the seat of course? or have i pictured it all wrong?

Thanks
Beecher

 

Beecher,Tony B, and the rest,

Don't want to hijack Beecher's thread. I'm sure more is to be said about his theories.

Have attached a photo that I took today of the rear seat delete / rear chassis reinforcement attempt. The aluminum plate that encloses the rear chassis segment serves as a structural member to triangulate and convert the open channel configuration of the rear chassis to a box configuration. In the photo, the M6 cap screws and washers are located in the silver colored non-painted areas of the plate. (I pulled back the carpet and removed one of the rear seat backs for the the photo.) The peculiar pattern of cap screw attachments at the front facing edge of the plate is my attempt to avoid damage to the fuel tank access ports and wiring. The idea is to, in effect, rivet the plate to the floor plan in enough places to simulate a welded joint. The Schroth harnesses are still in place from the most recent track day.

Regards,
John Petrich in Seattle

 

But John, it’s flat; it will still be weak against a twist.

 

Keith,

What to say? Consider the shoe box example, or even, envision a pizza box. The lid is flat and whimpy, yes. However, if that flat lid is firmly attached to the sides of the box, the composite structure becomes a large enclosed space, a tube, a cube, you name it. The lid by itself is flat and can bend and twist, limited only by the intrinsic rigidty of the material and the material thickness. But, the composite structure, the lid attached to the box, with it's enclosed volume, becomes quite rigid even when the box is constructed of thin and relatively non-rigid materials.

The rear chassis reinforcement plate that I installed converts the shallow box of the rear chassis floor pan to a large cube that is extremely rigid compared to the un-boxed chassis floor pan.

So, the plate is flat, but as installed, becomes part of a box structure that is rigid in torsion, bending, lozenging, etc.

Hope that this helps understand my theory. Whether any of this is significant, I don't know. Keeps me busy. Certainly, not for everybody.

John Petrich in Seattle

 

I see John's approach. A lid can do wonders for some structures, so long as is is securely fastened. It seems as though he did a good job in that regard...

Maybe we should finally try to see what forces are at work here, and what areas are moving, and in what directions... Earlier Matt referenced a spring-loaded bar. Any other novel ideas?

 

I’m not trying to diminish John’s efforts; I understood the concept from the first explanation long ago and the fact that the design is operationally effective on some level is not lost on me.

Here are some observations based on the one dimensional photos I’ve seen. In the above attachment it appears to be fastened on two sides only, so it no longer functions as a “box” loosing the inherent strength of a cube; it also probably has a slot cut for the seat back attachment which weakens that fastening edge. I see a similar drawback in the bracing of the chassis channel on the underside, it has two open ends. Both designs (when using flat plates) will be good at limiting the hinge effect but less effective at resisting forces of torsion. The flat plates could be enhanced (augmenting material rigidity) in regards to twist resistance, with the addition of some type of truss; packaging concerns not withstanding.