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Old 12-04-2014, 06:31 AM   #1
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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.
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Old 12-04-2014, 07:01 AM   #2
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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.
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Old 12-04-2014, 07:09 AM   #3
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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.
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Old 12-04-2014, 07:53 AM   #4
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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'
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Old 12-04-2014, 03:08 PM   #5
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Quote:
Originally Posted by bwillia080261 View Post
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.
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.
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Old 12-04-2014, 07:45 PM   #6
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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.



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Old 12-04-2014, 08:04 PM   #7
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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.
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Old 12-05-2014, 05:28 AM   #8
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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
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Old 12-05-2014, 05:36 AM   #9
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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.
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Old 12-05-2014, 06:22 AM   #10
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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
LOL!!!
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Old 12-05-2014, 07:40 AM   #11
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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...
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Old 12-05-2014, 10:47 AM   #12
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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.



Thanks for the awesome graphic! Love it.
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Old 12-08-2014, 12:54 PM   #13
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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?
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Old 12-08-2014, 01:59 PM   #14
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Quote:
Originally Posted by woodsman View Post
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 $$$.
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Old 12-09-2014, 04:16 PM   #15
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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
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Old 12-09-2014, 05:09 PM   #16
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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?
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Old 12-09-2014, 05:20 PM   #17
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Exclamation

And lets not forget that as these cars age, the weakest areas are weakening!
-- 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...
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Old 12-09-2014, 05:56 PM   #18
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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.

(Engman/Mark is no longer with us)



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Old 12-10-2014, 06:12 AM   #19
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Originally Posted by Brad Roberts View Post
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.

(Engman/Mark is no longer with us)



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Sounds promising and exactly the kind of solution I expected to hear. Keep us informed. Need a test car?
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Old 12-10-2014, 06:42 AM   #20
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here you go - be the first and report back ...

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