Chassis stiffness
 

I realize that convertibles are inherently less stiff and that if stiffness is my number one concern then I should probably be in a Cayman but....

It is obvious to me by the noises and feel of the chassis that I am getting flex in my Boxster. Has anyone ever added anything effective that has stiffened up the body in the middle? I added the Schnell strut braces and if anything they made it worse.

I'm sure you track guys that have turned your Boxster into a race car have done something in this area.

Thanks.

I'd also be interested in feedback. One thing is for sure to me is that a hard top provides an extra layer a protection.

I've heard folks say that the rear strut brace reduces the rattling a bit, but sounds like not working for you. really the race car solution is a roll cage.

I have carbon fibre strut braces front and rear, together with my zeintech hardtop and being lowered by about 30mm, I can say the car feels a lot stiffer than before these mods.

I am planing to build a carbon fibre shell that covers all the 'open' areas when my hardtop is removed with speedster humps, the idea is to build it structurally so that it stiffens the car up when clipped into position. This would be lighter than the zeintop and could possibly offer as much 'stiffness'

If you have added strut braces and you still feel the chassis need stiffening, then you have other problems....
The Boxster / Cayman platform is extremely stiff for a road car (just jack up a wheel and see), so more than likely you have worn suspension like shocks, springs, rubbers and bushes. Adding a strut brace to worn shocks will only exacerbate the problem.

As good as the Boxster chassis is, it is no where close to the stiffness of the Cayman. It is amazing how much strength and stiffness that the roof structure adds.

Take a look at the Boxster chassis shown below - the middle of the car is basically the floor pan - that's it! In some ways, its amazing that the car doesn't simply fold in the middle. As such, there is only so much engineers can do to make the floor pan (a mostly flat panel) stiff.

The Cayman adds the roof which creates a very nice box-like structure to greatly improve chassis strength and stiffness.

A Boxster Spec race car achieves the same by using high strength steel tubes to build a box-like "cage" in the middle section of the car. This cage not only protects the driver in a crash, but adds substantial stiffness to the chassis.

http://i1114.photobucket.com/albums/...ps73ca03d5.jpg

http://i1114.photobucket.com/albums/...ps353befa9.jpg

I have a hard top and a slipped drive into the garage that I take at a diagonal, this gives a fair amount of body stress and the lack of stiffness can be heard in the hard top. It creeks until the car settles on even ground. I think these cars fare pretty well as apposed to coupes made into convertibles like a crossfire or Zcar, but they do suffer from a lack of stiffness.

Just drop a Viagra into the tank at every fill up
That will make the car plenty stiff when you give it a good workout

If you really want to experience "flex" you should have driven my 78 Fiat X 1/9! I have jacked up my Boxster and thought the car to be quite stiff and structurally sound.

LOL!!! :eek:

As I understand it, the only way to address Boxster torsional flex is with a cage. Strut braces support areas of the car that are already quite stiff so don't add much. I still find the Boxster to be pretty stiff and precise compared to most cars. The only time flex becomes a real problem is when using very stiff springs so the torsional body flex becomes the new effective spring rate. The Cay is definitely stiffer and more precise though.

So many other areas of the car to derive performance before body flex becomes the low hanging fruit... :cool:

Thanks for the awesome graphic! Love it.

While it's true that there's much that can be more easily improved, stiffness IS the foundation with which all else rests. Can 'we' feel it as we enter or exit a corner or flow over a dip or bump? I think so. I test drove a 911 a while back and was surprised at the noticeable improvement- I went over some train tracks at 40 km and swear I could Feel the roof rails! Ever since I've been wondering how to improve the otherwise spectacular Boxster chassis. Those excellent pics don't show 2 major structures along either side of the floor. These are used when jacking the car up and/or for jack stands. Perhaps they could be fortified. Also I think adding an additional bar ( with triangulation) in the lower door openings perhaps 6-8" above the sill would be of the most benefit. Entering and exiting wouldn't suffer much- it's just the install--weld, bolt or bond?

A bolt-in cage might be the most cost-effective way to improve Boxster chassis stiffness. It also offers a lot more driver protection for motorsport. I favor a custom welded cage for safety but they are also a lot more $$$.

I have a chassis stiff kit we are testing :) and TomS? The cage is good.. but stress analysis told me to "add a tube". The first time I pitched a SPB into T1 with our extra tube? HUGE difference and 20+ deg. tire temp increase on the right front (at WSIR where the right side of the car is mostly on vacation) I won't share any pics.. but feel free to stop in and look at the cars :)

Those of us that are street-bound need a solution that's works for us. Track cars need a cage for safety anyways but we want our soft top option intact. How about it Brad-- do you have a fix?

And lets not forget that as these cars age, the weakest areas are weakening!:eek:
-- if we could know the 2 points of the greatest flex ( cowl and rear bulkhead?) then we could fix 'em good and retain chassis integrity into the 2-300 000 mile range...

I have a kit we are testing. It welds in under the stock carpet/pad/thing. It's multiple pieces and helps tie the center tunnel to the side rails and engine mount bulkhead.

It's very similar to what I developed with Engman for the 914 inner frame rails, but a lot more elaborate. :cheers:

(Engman/Mark is no longer with us)



B

Sounds promising and exactly the kind of solution I expected to hear. Keep us informed. Need a test car? :)

here you go - be the first and report back ...

Porsche Boxster (986) Aftermarket Performance Parts / Accessories / Spare Part - Ultra Racing Product Catalog

I sent you a text. Let's get together.

Thanks for that but I'm looking for a fix for the areas of the greatest flex and I'm pretty sure they're not them.

How about structural adhesive???

actually the post wasn't directed to you but rather the op.

if you look closer you'll see this:

Porsche Boxster (986) Middle Lower Bar / Middle Member Brace - Ultra Racing Product Catalog

which appears to strengthen the centre tunnel - a source of the flex.

I've seen all of those products. I thought you mentioned something new that is essentially an X brace that goes under the carpet and strengthens the mid section?

Since my car's on jackstands as I type I'm going to use some flat 2* .25" wide aluminum to make a strap utilizing the 2 bolts already there to tie the center of the 'transmission' tunnel together. My main concern is the point at either end of the floor pan. I think the majority of forces push through the suspension in an upwardly direction and the lack of a roof creates flex. An 8 point cage counteracts these by directing them to the other end through the cage. Without that we have hinge- like flex points - no?
So, strut tower to floor, floor to strut tower. Or, next best thing- cowl to floor, floor to rr bulkhead? I want to trianglate these right angles.

bumping this thread. linking to this thread as well:

http://986forum.com/forums/boxster-general-discussions/30929-ultracing.html

any progress on stiffening the boxster chassis without a cage? or with the ultraracing product? tanks, trk.

i've got an inquiry in to ultraracing to see if they have any actual photos of their product that aren't photoshopped, and when back at home next week i'll get under my car and see if their proposed mounting points are actually viable. otherwise, sorta interested in this:

https://www.elephantracing.com/porsche/boxster-cayman/quickchange-strut-brace-chassis-brace/

buy waaaaay pricey, hey?

Now that looks like it would do some real frame stiffing.

Nice find....didn’t know that this existed. Yeah......once coil overs with higher spring rates go in, compliance finds its way into the unibody you can really see it feel it with the top up. Maybe this thing will have some effect over and above strut braces.......but really this looks to be a dead zone for tortional rigidity resistance as opposed to the space between the axles....ie the cabin...but you never know

well, the front strut brace keeps the strut tops from spreading under hard cornering, but the front subrame is just bolted to the front tub. i think the erp product would keep that front area from distorting when subject to torque forces. same could be accomplished by seam welding the front fenders i think.

whenever people say you just have to live with it because it is a convertible, i think of all the hypercars, most of which have targa roofs; if being a convertible was such a limiting factor with respect to frame rigidity, why are the fastest cars open-topped? there's gotta be a way to stiffen this thing.

I would think that if there was any real flexing going on in the front tub, stress cracks would have shown up on race cars by now. IMHO, it's a waste of money as the racing community would have figured that out by now. I cant think of any race cars I've seen where the cage is incorporated into the front or rear trunk. I believe the biggest flex on a boxster that isn't caged is in between the door pillars. I've seen lots of wrecked street and caged boxsters. A street boxster will almost fold in half in a very bad head-on collision. I think the question is how you stiffen that area without installing a cage. I would think some kind of brace that attaches to top of the window frame and the roll bar? The only problem I see with that would be the twisting might break the windshield. So if attaching to the window frame is a no go, then where could it attach to?

i think the racing community is hard to judge by, as most boxster race cars are spec, and addnl reinforcement is not allowed? regardless, i agree that the focus should be between the wheels. the doors are structural members in this car - perhaps a way to get more positive engagement between door and frame? sounds like a lot of work. tunnel reinforcement? the brackets that hold the coolant lines in the tunnel are already quite beefy and, i presume, intended for reinforcement as well. ultraracing did get back to me and weren't able to provide any pictures of their product, let alone of it installed. just photoshop i guess. really, i can't see a way to improve on the brackets already there without getting in the way of the underbody panels.

i think brad had it right - additional cross-bracing in the pssgr footwell area is the ticket - tie-in to some of that super strong green and yellow steel in the previous image.

Yes. ^^^^

The mid-section of the car is essentially a plat panel and as it flexes the front and rear suspension components undergo different geometry, forces, and dynamics - all of which are not good for optimal handing. There are two ways to make a flat panel stiffer - make it thicker, make it out of stiffer material, or add some structure to it.

And this is what Porsche originally did. If you look at the center/passenger section, Porsche added a tall center tunnel (structure) made from high strength steel (stiffer material) with bracing that adds height to the floor pan and extends to the door sills (thicker, stiffer, and structure).

The challenge to further stiffening of the chassis is where/how to attach additional structural components. The front and rear are easy, add a front shock tower brace (or x-brace as shown in the previous link) and add something like a techno-brace to the rear.

The real challenge is how to add a bolt-on structural component to the central section of the chassis without encroaching on occupant space/ingress/egress along with aesthetic considerations. Underneath the car, it would be hard to add bracing since the car is already pretty low and its not clear exactly how to mount it.

http://i1114.photobucket.com/albums/...ps73ca03d5.jpg

If I was so inclined, I would look into having the chassis seam welded. This could increase chassis stiffness without having to add any new components. However, there are trade offs to seam welding that need to be taken into account before going down this path (some have reported that a seam welded chassis is too stiff and they had to adjust/soften the suspension to get the car/suspension back into balance). Also, there are some safety concerns when seam welding the front crumple zone as it could affect how the chassis deforms (and absorbs energy) in an accident - not an issue for a race car but might be a concern for a street car.

Most of the experience with seam welding is for race cars so there may not be a lot of info/resources pertaining to seam welding an otherwise stock car. Let me say that I have never driven a seam welded Boxster and therefore do not know what results to expect. But as I said, if I was inclined to wanting a stiffer chassis, I might spend some time investigating this approach.

Maybe someone else has first hand (or even second hand) experience in seam welding a chassis and can add to the conversation.

The more I think about it the more I think my original idea might work. There really isn't a great place to attach a brace to on the front of the cabin. The screw holes where the sunvisor attaches look to be the easiest place to fabricate an attachment point to. I'm starting to think the window frame might be able to handle that added stress. The A pillar is very strong. I'm bringing a boxster to scrap this week and to make it smaller I tried pushing down on the windshield with my tractor. The A pillar didn't budge with the front weight of my tractor on it. The front wheels of the tractor came a good foot or two off the ground. I had to push in the very middle of the windshield to get it to collapse and even then the A pillars were standing tall. That's maybe 1500-2500lbs pushing down on it. It's not like the brace would be forcing that much pressure on the frame? At least under normal track usage. I have a few boxster shells I could test on.

perhaps some cross-bracing as per the red markup below; under the carpet and trim the insulation ...

http://986forum.com/forums/uploads02...1532197738.jpg

I'm just throwing ideas out there but what about a bar down the middle? Something that attaches to the middle of the roll bar and then comes down and attaches somewhere by the lower console. That lower console comes out very easily and there's some strong mounting points down there

That area has lots of room to work with. It's just 2-3 inches of foam taking up that space

Woody - you should film the stripping and destruction of a Boxster and make a sped up movie, that would be cool ;)

I normally don't completely strip a boxster at one time. They usually get taken apart piece by piece over a few months to years. They usually have three stages of dismantling. First stage has the motor/transmission still in. This is where the high desirable parts usually sell quickly and parts that need to be sheltered get stored. Second has the motor and trans out but the suspension attached. The last stage is out in the field where they get picked at like the last buzzard on a stripped carcass. This past one was an exception because I was trying to show my nephew the ins and outs of one as I'm giving him one in a few weeks.