I understand that the manufacturer has added new strengthing pieces to the '05 Convertable in an effort to improve chassis rigidity. It is said that there are braces that extend from the front strut towers to the front frame rails, additional pieces incorporated into the "A" pillars, and a single large piece in the floor pan, under the rear seats. These strengthening pieces may help the chassis rigidity for all models. Does anyone know the details and whether any of these pieces can be retrofited to earlier MINI chassis?

Regards,
John Petrich in Seattle

 

Don't know where to get the pieces, but I would imagine that's where the bulk of the additional weight of the convertibles comes from. The convertibles weigh something like +400lb, right?

I don't know about you, but I'm happy with the ~25K NM torsional rigidity of my car, and could do without all that extra baggage slowing me down.

 

I believe all of the extra pieces in the convertible are put in during the initial welding and manufacture of the frame of the car. These pieces are not accessable without doing major surgery to the frame of the car. In addition the frame on the convertible is made from significantly thicker metal than hardtop models... I'm almost positive that there isn't any way to retrofit the stiffening pieces installed in the convertible in a hardtop without doing MAJOR surgery to the vehicle.

Besides, do you have any idea how stiff the frame of a hardtop is already?

 

but I saw a post where someone had added the strut tower to frame rail stiffeners.

On my very old Ford Mustang, most (but not all) of the parts can be added (and some were added to the 67 hardtop, like the torque boxes). Keep in mind that the car was designed for street use, and if you beat the snot out of it, the additional rigidity may 1) help wiht handling and 2) keep your car from flexing it self to pieces.....

Matt

 

I would question the need for these braces in a closed roof configuration. The Mini's chassis is a pretty darn stiff. Those braces are a reaction to a missing roof. You would be much better served by installing a partial or full, high quality, roll cage.

Also, Randy Webb installed a front strut bar with a slip fitting (my term) somewhere between the two strut towers - in the middle presumably. He went out on the track in an effort to find out just how much this area of the chassis flexed. I believe he told me 1mm, not a lot. I would assume he used a zip tie in the same fashion one checks for compression stroke on a damper.

 

I love your photo, petrich! It is a wonderful photographic representation of how it feels to me to drive my MINI!

 

I would question the net effect of such an addition. I'd rather not have any more weight in the car.

 

Is this the strut tower piece you are asking about?



Apparently you can retro it, Petrich did:

"Mounting the catch-can was easy, even with '05 convertible tower braces retrofitted in my '02 MCS"

See post #24

https://www.northamericanmotoring.co...ad.php?t=38091

 

Even if you add the strut tower braces, how much stiffer is it going to make the car? Especially since those braces are such a small part of the overall work that MINI did to stiffen the frame on the convertible....

And again, the frame on the hardtop is so stiff already, how much benefit are you going to get from those braces?

For another question, say you had the bottomless pit of money to add all the other frame stiffening components incorported into the convertible (with the exception of the thicker steel used in the construction of the frame) wouldn't the added weight negate any improvements you might be hoping to gain?

 

The weight effect things like weight balanece (duh ) and acceleration etc, The stiffening effects the handling geometry. A bit of an oversimplification, but the basic idea.

I've seen some pretty good cars get aged rather fast on the track. Especially unibody cars where the whole body just gets more tired and loose.

Are these things worth the effort and weight? No clue. But stiffening an already stiff car DOES make it stiffer. It is an open question if the effect can be noticed by anything other than machines designed to measure chassis flex is another question.....

Matt

 

Thank you all for the input. I have the same questions you all raised: not sure just what all is involved in the frame modifications for the convertable, don't know what components can be retrofitted and, if these extra components would bring any performance value, if they could be retrofitted.

Want to keep the car fairly stock in appearance and function. Am not interested in roll cages despite their value at improving the chassis integrity.

My suspicions are that some of what ever measures the manufacturer did make to upgrade the convertable frame and body shell are aimed at compliance with the various crash standards. Important, but not the performance value that I'm seeking.

One potential frame modification that particularily interests me is the strengthening piece, rumor has it, that is incorporated into the rear frame, near the anchoring points for the rear suspension subframe. I've always wondered about MINI's rear subframe area. Keep wondering if that area is substantial enough. Don't know why I wonder. Maybe my experience of a life time with rear wheel drive autos has colored my perception of what is substantial enough. Have worked hard to reduce the compliance of the entire rear suspension. The result is, what I feel are, significant improvements in the quality of the handling experience. The rear suspension anchoring points exert a lot of loading on the rear end of the frame and body shell especially in track situations. Cannot help but wonder what further benefits in handling might come from increased rigidity in that portion of the chassis.

Hope someone has some answers.

Regards,
John Petrich in Seattle

 

Take a tour of the MINI plant, you'll see the seperate assembly jigs used just for the convertible and they'll talk your ear off about the frame differences between the two cars...

 

As you can see in the photos (thanks to Motoring File) everything in Yellow is added to the Convertible. The only one you could really add are the front braces.





Here is a shot i took under the car:

Take a look in my gallery for more photos.:smile:

 

Long time John...

I began inquirng about this as soon as I knew a convertible would be released. As soon as one arrived at local dealership, I was there snapping photos of the underside .

I was thinking along the same lines as yourself...

The impression I got was that if one is not building a 250+ hp (torque being more signficant) MINI, it would simply be an added cost, effort , and yes weight. But then, if one must add weight on our car, in the rear and down low is the most desirable location .

Since 250 will get me to my desired 10:1 weight to hp ratio, I'm cool as is. That being said, I'm still working the compliance front, and one retrofit I am considering is the '04 or newer passenger side engine mount. Appears easy to do, and it seems that it would make for more rigidity...

 

You ain't kidding! Randy Webb has some motor mounts that will rattle your teeth out of your head.

 

I experimented with Delrin about a year ago, and I'm going to try again fairly soon. Compliance is very much minimized, and maybe too much so for most...

 

those two braces in the front look so wimpy; hard to believe the do much to stiffen the strut tower, especially with that crimp in there.


John: your grandchild shares our enthusiasm, eh?
by the way, I did buy a master cylinder, but I think it is not practical to bore it out: too amy side passages with weird slots and not enough meat. Maybe use two in parallel?

 

The input is very welcome and thought provoking . My original questions have been addressed. It seems that the convertable chassis modifications are by and large not easily retrofitted.

What did emerge, though, are some great ideas about other "strengthening" measures that can be applied. The diagonal bracing around the rear subframe is something that I am going to study. Thanks agokart. The Delrin motor stabilizer bushings are another good idea for reducing torque steer and "wind up" of the engine/transmission assembly relative to the chassis. Tony B and I can both vouch for this particular modification since we both have done it.

Matt's (Dr Obnxs) posting that any additional bracing might reduce the deterioration of chassis rigidity that results from repeative torsional flexing is certainly something to consider. Brad (minihune) has been preaching this belief for a long time as an argument for installing a strut tower "stress" bar. I don't have the room for the "stress" bar given my particular camber/castor plates. Have been wondering if there is a method of bracing the front chassis by a brace located under the engine. Haven't given it serious thought though.

Any more ideas?

Regards,
John Petrich in Seattle

 

I don't have one in front of me, but that torque reaction bracket that Tony repolaced with delrin was discontinued with the '04 models. makes you wonder if they added a compensating device, or just ditched it.

 

John (the one in Seattle), as per your PM (I'll be getting back with you soon!), you have John's (jlm's) camber plates. He did, and maybe still does, offer a strut tower bar for these units. At least that is what I recall... or, a provision for one...

We might want to revive this thread, or even better yet start a more comprehensive one now:

https://www.northamericanmotoring.co...ght=compliance

As jlm just metnioned, the '04 change is something that I want to learn more about. I've only seen an '04 for a few minutes. Would love to see a photo... I'm curious to know the reasons why MINI made the change. My first inclination was that the '04 change made for a more solid mount, thus my desire for the retrofit. I didn't ditch the bracket just yet jlm...

I'm sure we can find a photo somewhere...

 

I have observed an '04 on the dyno and there is major engine movement about the crank axis under WOT. i would say about 1-1/2" to 2" toward the firewall up by the coil. The last one I looked at was the Psi-fi at Helix cranking out 268 whp.

 

Petrich,

Interesting discussion.

More rigidity is good. More weight is bad... unless you also have more power. Assuming a MINI of about 225 to 250+ HP then more weight isn't going to be a big problem.

Don't forget that more weight also means you'll need a better stiffer suspension and bigger brakes.

One approach is to add the 4 or 5 point roll bar or roll cage but it is expensive, heavy, and you loose interior space. Hardly stealthy but does add to safety. The weight is mostly in the middle, some high some lower.

If you think that the cabrio chassis has a good amount of added strength then another approach would be to get a cabrio and add a hardtop. You do loose space and you'd have to deal with the rear hatch- customize one probably. You can reinforce the top more easily than adding all the cabrio reinforcement to a stock MCS.

The MINI reinforcement for the cabrio chassis could be copied (not for sale from MINI I would think) but that was designed for the cabrio for various reasons including compliance to regulations as John mentioned.

Better would be to study the stock MINI chassis and test where it flexes the most- no sense adding to the stiffest areas. I'd learn where the weak points are and go for adding to those areas first. You'd have to see how much space is there to add metal then choose what material to add and what the weight impact would be as well as the cost of the material and ease of use in installation.

Let's be like Monster garage- tear down a MINI and take off all the body panels and parts. Now you've got the bare chassis. Re build it with a thick aluminum bathtub or frame and reinforce it but stay within the dimensions of the stock chassis. Then bolt everything back on. Like a frame off rebuild.
I just mention aluminum for the light weight and potentially stiff end product.

While you're at it, you might want to chop the top about 6 inches unless you like the steath look.

In person, those engine bay carbio strut braces look OK. Not sure how much they help. My Tower strut bar is OK but you'd need to cut or modify the air box used or use a minimal design bar. My BMP bar is only 3.5 pounds. It's easy enough to add the cabrio braces up front. Doesn't look too heavy, probably does no harm.

To stiffen the rear you can add metal to the rear floor between the rear suspension areas or maybe metal near the rear seat back or perhaps a very beefy rear tower bar with bracing. At 2 years old my rear end is softer and flexing (making creaking noises) more now.

Finally something to consider. When adding components and upgrades we can overstress other stock parts and not know it until they fail- case in point.
One MC owner I know cracked both his tower shock plates after adding H-sport lowering springs. Stiffer springs- more stress on the stock plates and he broke one of his tower bolts as well. He also had a BMP tower brace. I have the same springs but upgraded to RDR camber plates which are very thick compared to the stock plates- just got to not over torque those Tower strut bar bolts.

 

Hey Tony,

I installed poly bushing engine mounts in my last ride. Needed ear plugs afterward. I remove two of the three, a better compromise. But wow, do these things keep the engine still!

 

Agokart discouraged and inspired me all at the same time. No way to add the convertable strengthening pieces without disassembling the chassis. However, I have made an appointment with a local fabricator to at least talk about some of the ideas that agokart has demonstrated in his photo of his underchassis bracing system.

Not sure just what to expect, other than adding more weight. This particular fabricator has considerable experience with chassis preparation for successful rally cars and has completed at least one rally MCS. He seems to understand chassis dynamics and might have some informed suggestions. One of his suggestions might be "forget it". A bolt on "X" member might improve chassis rigidity and develop into a marketable product for someone.

Regards,
John Petrich in Seattle

 

i hope that is a good thing as i always want to help for good..lol. i feel the rear subframe is in there well on the hard top and anything else is extra weight to move around, but if you feel you need any better photos, we will have a convertable here for a while (about a month) and can take better photos and give you some numbers to work with. give me a ring any time.