So? Don't do a small change you can afford because it isn't the ultimate solution or don't bother because there are other issues?
Just a bit confused especially since I do know the bar helps road compliance if nothing else.
Are there any real negative to doing this mod?
Don't take this as argumentative, it isn't. That's simply how I interpreted the post and I'm curious if that's the gist of it.

That aside there was a cool thread here about using structural foam for increasing chassis rigidity. Here it is. Next to doing a full roll cage this seems to be the best option but this thread died as well.

 

Matt, Tony, and the others,

There is certainly a tried and proven way to improve mid-chassis torsional stiffness; install a roll cage. The concept of torsional stiffness is key to me. There are many ways to brace the chassis in one plane - the available after market braces are good examples. But, only the roll cage and maybe something akin to the braces and installation style that I have made will create a box or tube structure that is rigid in all planes. It is torsional stiffness that we are after in a real world automotive situation, where so many, if not all, bending moments resolve into forces on multiple planes.

Given that there are many points of potential chassis compliance that my braces won't stabilize, I rationalized that a little bit of something beat nothing at all. Bracing out to the sills was what I wanted to do. But that wasn't possible with the basic tools that I have available. Yes, Matt, I love your idea and this winter I will weld some tubes in the horizontal plane to reduce the risk of flex in the longer brace plate. Thanks.

Any changes in MINI's handling? I have been hesitant to reach any conclusions: aware of the placebo effect and having trouble isolating out the causes of how MINI handles on the track. What hit me in the face recently is that the Y2005 suspension setup, suddenly exhibited a lot of oversteer in Y2006. The only changes were the braces. I have been softening up the Madness rear sway in increments and finally re-installed the stock 17mm sway bar and the rubber bushings. At each step at reducing the rear bar stiffness, I found more stability and less oversteer at the track. Skid pad testing two weeks ago suggests that MINI is now closer to balance again, as it was in Y2005 before I modified the chassis. I think that the braces did improve the chassis torsional rigidity, reduced chassis compliance, and affected weight transfer so much so that I've had to retune the rear suspension. Good? Bad? Certainly ugly.

Am going back to Laguna Seca and Thunderhill next weekend to complete the driving cycle for the year. Started track driving "wearing" the braces at Laguna in February. Had them on, underneath and out of sight, in June when Matt and I met at Thill. Didn't tell anyone then, my little secret. I'm coming out now. Will be very interested in comparing MINI's handling characteristics on those smooth Calif. surfaces with the altered rear sway bar set up.

An interesting discussion,
John Petrich in Seattle

 

This sounds like the "rigid chassis, compliant suspension" philosophy.

Interesting stuff as always.

 

You could always seam weld the car and then install a roll cage...these all add weight.

I can add here, that after installing the P-flex front control arm bushings, both doors make a very unsatisfying noise after hitting most bumps.

Another area that often requires some extra strength is the firewall area adjacent to the brake pedal; after a while the metal in this area gives way - from the constant flexing associated with racing - and the pedal feels mushy despite perfect brake operation. This occured with my 1981 ford Fiesta...and a host of cars back then. Food for thought. Onasled won't have this problem as his pedals are now mounted to a portion of the roll cage.

Michael

 

sure the suspension moves more than the chassis, it's supposed to. The problem with chassis flex is the suspension geometry changes, resulting in (usually) undesireable handling consiquences. Sure they move stresses to other parts of the car, but remember, cars are disposable items! Nothin comes free....

Yeah, yeah, the suspension bushing flex has the same effect in altering geometry, but one can always go to stiffer bushings to minimize this as well. Nice thing about chassis bracing is that you don't loose the NVH isolation while improving the stiffness of the chassis. Anyone who's ridden in a heim-joint suspensioned race car knows that NVH isn't high on the list of priorities.

Matt

 

obehave, my post was about points to consider so it was open to interpretation, spend your energies as you wish. If I'm required to look for a downside it would be the side loads (shear) placed on those small bolts but I feel chassis movement in this area is benign enough not to be much of a burden.

Matt, you’re preaching to the choir.

Petrich got it; current underbody braces for the MINI at their best, are limited to one plane, and tension is their strong point. One of his braces attempts to address another force, torsion (twisting).

Given the space limitations under the car, triangulation with truss supports is not feasible so the next place to go is up top where there’s more room. A full roll cage will provide bracing at more advantageous points in the car; and the mechanical advantage of support on planes other than horizontal will be more effective at controlling torsional and bending forces.

Since John has come out with his impressions I’ll take a chance with some of my subjective feelings. I’ve tried a number of different suspension combinations with a focus on street ability. Replacing the rear compliant stock control arm pivots with something solid reduced under steer to a minor extent and increased feedback along with stability to greater extent. Installing a bolt-in roll bar with far rearward bracing took me back to steering from the center (a term coined by moderator Dave). I use an 18mm bar set on the soft setting (stock length) but I have similar steering behavior as I did with a 22mm effective bar (H-Sport Comp). A bolt-in bar is not supposed to be effective but it's the impression I have. I tribute this change to the supportive triangulation the bar affords and the distribution of weight. The weight shift has also shown a benefit in rear wheel traction; I get the sharper steering response without the loss of rear tire grip experienced with a stiff sway bar. Springs, struts, new ball joints, and stiff front control arm bushings are in the mix also but are not the topic of this discussion.

 

Someone alert the media! Someone on NAM realized, before posting, that they were going off-topic and decided to restrain themselves!

 

That works. Thanks. I like being prodded into thinking

 

who's ready for Monday Night Football?

Back on topic...

Full cage is the way to go for sure, BUT you loose a lot of the usefullness of the car. I put a 4 point bar in my 65, and that was fine, but now that I have a little girl, the back seat is useless for passengers, so my wife OR child can go with me. Now that I'm tracking the Mini, the sawzall will come out, and there goes over a grand in custom fabrication! Oh well. It's only money...

Matt

 

It's possible that my benefits are all perceived as a placebo effect... If so, all I have to say is "Yum! Sugar pills! At that price, Can I have another?"

 

It's no placebo, it works.

 

.... actually we have changed this to floor mounts...

 

Oooooooooookay then

 

I concur... it does work...

 

about 30% of people in double blind tests for medicines get benefit from placebo pills!

Power of positive thought......

Matt

 

True, but then again, most people can't tell the difference between a properly tuned suspension and a crap one, either.

 

For those who are not familiar of where the OMP style of bracing connects to on the body, it is at end of this “tab” which functions as the body attachment for the front control arm bushing bracket. The rest of the bracket is secured to the subframe with the same bolts that anchor the front sway bar bushings. The bracket is made of cast steel which will not tolerate many bending cycles without breaking. There are no offending frequent bending cycles otherwise this part would have to be replaced often for failures not related to torn bushings.


These are all the mushy parts for the front suspension points; they are responsible for less than precise turn-in and will cause linear flex on the horizontal axis far more than any body part in the front drive plane area not counting the tires.


Here are the rear soft spots. In my opinion the control arms are a good place to start tightening control of front end behavior.


I know it may sound odd to alter the rear to affect the front, however most of us are familiar with the impact stiffening the rear swaybar has on steering. Another approach would be to reduce the stiffness of the front swaybar; it’s about relationships. It is the relationship the rear has with the front which brings me to the bracing topic. Despite the appearance of a weak lower body because of the exhaust pipe channel, I don’t see any indication of the left and right side converging or diverging from each other. I do see the possibility for chassis twist. The chassis can twist without it spreading apart; a ladder would be analogous and it would also illustrate why most horizontal underbody bracing would not prevent this from happening. I’ve noticed a change in steering behavior after installing a roll bar.


As stated before I attribute some of that change from a weight shift and some from a bracing effect. The above view doesn’t show it very well but the rear support braces are farther apart at the base than at the top. This forms a slight triangular (although weak) brace against twisting forces. The energy (force) that would normally be lost in chassis movement can be transferred to front driving wheel. This effective rear end brace only extends to behind the front seats but it shortens the area subject to twist enough to have an impact on steering.

 

Keith,

Appreciate the detailed post on chassis and suspension compliance. I want to respond to some of your points and not respond to others, because I'm in the dark about a lot of things.

Very much agree with your pictorial of the high compliance parts of the front and rear suspensions. I didn't see in your photos the top rear strut bushing. The compliance from that bushing is minor, in my view, compared to all the other bushings that you depicted.

Don't know what to say about the relationship of rear end suspension component compliance and front end steering and road holding. Will have to think about this whole concept more. Maybe you would elaborate on your idea and help me along. If you are saying what I think, that the energy that causes weight transfer fore and aft in the chassis is more directly conveyed in a stiff chassis than it is in a compliant chassis, I understand and agree. The compliance absorbes some of the energy. An observation of mine is worthy of some thought. I've replaced all of MINI's rear end rubber bushings with spherical ball joints and haven't detected an impact on the front end performance. That said, I posted here:
http://www.bmwpugetsound.com/vbb/showthread.php?t=32008 my observations on rear control arm compliance that caused severe rhythmic yawing motion while testing on a skid pad. Had always conceived of that yawing as a rear end "thing" but maybe there was a front end component that I didn't appreciate. Oh well, set me straight.

I agree about the center tunnel. I know that I have called it a "hinge", and I think that you are 100% correct, that channel doesn't "hinge" very much, if at all. I apologize completely for not being clear. I agree that the channel will twist big time and not flap (converge or diverge) like a hinge, at least in the real world driving situation. My appreciation for that twisting action is behind my emphasis in the posts on the benefits of "torsional" stiffness offered by a roll cage and, potentially, by my homemade braces. I liken the center tunnel to a cardboard shoe box with the lid removed, almost a channel section. If you grab the narrow ends of the shoe box it is much easier to twist the box out of shape than it is to bend the shoe box on any plane. However, when you put the lid on the shoe box and convert the shoe box to a square tube, the shoe box is very rigid on all planes including "twist" or torsion. Hence, I am hoping that my homemade braces will reduce the "twist" tendency in the mid-chassis tunnel. A properly mounted flat plate, one "stitched" to the chassis like I have done, resolves into an infinite number of smaller triangles and won't wobble or lozenge under stress.

Am attaching a photo of the front underbody of a MINI chassis. I didn't notice it before, but those raised, reinforced chassis boxed segments to which I stitched my braces, appears to connect the with front upper strut mounting points on each side. In otherwords, I think that the front upper strut box connects down, under the chassis, to the area of the front subframe brace via these reinforced boxed links. If true, the midchassis front subframe mounting points and the supports for the upper strut mounts all converge to the same place under the chassis. Maybe bracing at that point does help the torsional stiffness of the front suspension mounting points.

Anyway, let's keep the discussion going. I'd like to hear more about the interaction of front and rear suspension compliance.

Regards,
John Petrich in Seattle

 

I very well may have missed something as I read rather fast and tend to skim this stuff.
It seems that you are forgetting the rest of the car in adding rigidity. Actually the center section of the Mini, in between the front and rear subframs, is very stiff. This is due to the upper part of the body, like the A and B pillars into the roof frame. These parts are very well designed and are supper strong. I know most of you have seen pictures or actual video of Minis flipping over several times. As you will note both the A and B pillars are hardly distorted in the least and the roof frame is almost the same. The roof damage seen is mostly just the roof skin.
My builder has been building and designing tube frame cars for over 20 years. In all of his analysis, via high tech computer programs used for building subs, it shows that most all cars flex the least in the mid section. Most all flexing occurs near the suspension pickup points.
We had to really look into this stuff as we did a good amount of modifying to the lower section of my Mini, as only very few here know about.

By the way, the tunnel is also very ridged, even more so then the rest of the floor.

Just my 2 cents.

 

Goofed again.

 

Onasled,

Thanks for your input. Know that you are in the midst of a construction project and are much closer to reality than most of us are on this subject. I think that the photo I posted is actually one of your car. Do you recognize it?

I'm still not sure that the mid-chassis is as "rigid" as you say. Again, I think that we have to consider torsional rigidity, the resistance to twist, when talking of how rigid the mid-chassis is. I cannot imagine that the center tunnel, a channel section, is all that rigid in twist. Again, the shoe box analogy.

But, we all agree, that a proper roll cage best solves any number of chassis shortcomings relating to safety and chassis rigidity.

John Petrich in Seattle

 

So you're assuming I know nothing about how my car handles? I would have no idea if something impacted handling? Add to that that my wife noticed the car rode edifferently when she had no idea I had done anything to the car?
The traffic circle I have driven around daily for 16 years, the last 4 in a MINI, tracked differently because I wanted it to?

Your statement also indicates that 70% had accurate results.

One could easily find offence in your remark

The power of negative implication.......

 

The power of negative inference?

 

Here's an OEM rear bracing example; I can imagine good things about handling effects using a similar brace combined with roof and pillar support.

 

I didn't direct the comment to you, nor was it about automotive items, and it's a fact. So if you want to be pissed by it, have at it, that one is on you!

FWIW, my mustang has aftermarket frame bracing, and it works too. But the placebo effect is real.

Matt

Actually, I'm surprised at the level of contentiosness in subject matter that we all basically agree on:

Stiffeneing helps handling.
There are different ways and parts that can be stiffened, with different degrees of implementational success all at different levels of complexity, compramise, success anc cost.

So, what else is new?