Driving Impressions: Lower Stress Bar
 

in the midst of searching for the latest greatest 'go-fast' parts, i found myself in conversation with Ernie and Stan from Mantis Sport in Canada. i ran into them at the Road Atlanta PCA race this year; they campaign the only Cayman race car around:

since they have now over one year of experience racing the Cayman platform, i asked what they've found a need for and what they've installed on their car. in addition to a host of other stuff, Ernie mentioned a lower stress bar to me. the front suspension pick-ups are all mounted to a solid single subframe. in back, however, the subframe is split. the two halves are reinforced with a shear plate, which is great for shear or tensile forces, but poor for compressive forces:

the lower stress bar was designed to jack the subframes apart and hold them that way during driving. the result is that camber is maintained during hard cornering. most people thing that addition of an upper strut tower brace on this car is helpful. the problem at the rear is not the top of the strut towers, it's the bottom of the subframe. when you turn, the outside control arm is pushed toward the center of the car. this causes chassis flex and camber loss. here's a picture of the solution (i call it the ernie bar):

install was a snap. i removed the nuts that fasten the diagonals to the shear plate. i pounded out the studs with a hammer. i test fit the ernie bar to set its length, then installed it permanently. piece of cake; takes about 15 minutes once the car is jacked up:

impressions: this thing is great! i wasn't sure how much i'd notice it. i went to the track last monday and put it to good use. from the driver's seat, the car seems to set much more quickly on corner entry (a perception reinforced by swapping drivers and boxsters). from a measurement perspective, i was pulling significantly higher lateral acceleration numbers than i ever have. in the mildly banked corners, i was at 1.3 g's. in the unbanked corners, i was between 1.15 and 1.2G's. this was on STREET tires. once set, the car also seemed less prone to unsettlement over bumps. the car just doesn't seem to flex as much in back. here's one lap of telemetry:

bottom line: this thing was like $130 shipped. it really works! between my last track day and this one, i did the following:

996 front brakes w/ racing pads
mantis lower stress bar
bilstein sport front dampers

lap times went from 1:15:20 to 1:12:72. that's 2.5 seconds on a 1.41 mile course! i'll take it.

wow, thanks for posting this up, that's awesome to hear. Do you know what G's you were pulling prior to the addition of the bar? And also, do you notice any difference on street driving?

on this course, i averaged 1.06G with a max of 1.13G. this thing put me in the stratosphere here. it really holds the camber. as for street driving, it doesn't effect comfort levels at all. i don't really notice it under normal driving circumstances. it's very noticable when i push it.

?
 

how do I order one?

Hi,

How are you measuring your lateral acceleration? You're throwing around numbers here (both Baseline and Modified) which are greater than those repeatedly published for the Boxster S (0.91g).

While that may be so of a Modified Car, I doubt it's the case with a Base Boxster. I suspect your Data Logger is miscalibrated. How else do you explain it?

Happy Motoring!... Jim'99

Does the company that you bought the stress bar from have a website?

*Never mind I found it;
http://www.mantissport.ca/
I can't seem to find the lower stress bar though.

logger is not miscalibrated. it's a racepak gps; the accelerometers and GPS calcualted lat G's are the same. it's easy to explain:

1. modified car
2. turns 1 and 2 are slightly banked
3. alignment of a stock boxster s sucks, not enough camber

their website is here: http://www.mantissport.ca/

you can't order online, though. call them at 905.844.6219 and ask for stan. btw, any prices listed on their site are canadian.

for those interested, here is a graph of my fastest lap showing lateral acceleration derived two different ways:

1. on board accelerometers (red line)
2. calculated from GPS polling data (green line)

the RacePak polls gps several times a second. there is some latency and interpolation. this is why the green line lags the red line by a little bit. magnitude, though, is very comparable between the two. the data is accurate.

Interesting discovery. Reminds me of the problems I had with my 914 experiencing trailing arm flex under high g loading (expecially with wide sticky tires, compared to the then stock, skinny, non grippy ones ;) )

I'll have to keep this mod in mind. Wonder how SCCA/PCA allows it for, say, Autocrossing?

good question. i bet it falls into the same category as a strut tower brace. probably would bump you from showroom stock to production, but not from production to improved. just a guess.

Magazines measure maximum lateral acceleration around a flat skidpad. They drive the car at a known radius and then measure time to complete a lap (or multiple laps). Then they calculate the lateral g's generated.

The maximum g's that a car can develop steady-state around a 200' skidpad is different than what it can generate around a banked corner. Or a corner that requires less steering lock. Or at high speeds. There are too many variables, which is why magazine test numbers are for the most part useless metrics.

Another discrepancy can come as a result of the leaning of the car, which induces a false lateral acceleration component. Fortunately, the more sophisticated data acquisition systems account for this with multiple accelerometers.

In other words - I believe your numbers. We see similar numbers in our DL1 data - peaks of up to 1.4g lateral but more imporantly, around 1.2g steady state. This is a stock-class Boxster S with (really old) Kumho V710 competition tires.

Insite, any updates on your lower stress bar, in terms of how its been holding up, any changes/updates on driving impressions on both street/track?

Thanks bud...

it's still awesome. unnoticable under normal driving conditions. very noticable when you push the car, especially over bumps. without the bar, the car will lose camber over bumps, causing oversteer conditions. with the bar, things are much more tidy. i really recommend this.

Nice work Insite,

You patiently seek out leading edge performance upgrades, act as test pilot, verify your impressions with instrumentation and lap times and share your findings with all of us. Bravo! :cheers:

This is not bolt on fluff, but tested and verified with results. 2.5 sec on a track you already have wired is pretty good stuff. Just for kicks, if you had not made these improvements to your suspension and braking and focused only on the motor, how much HP increase would you need to get the same lap times? 50? 80? Interesting question don't you think? I suspect you are already outrunning most of the 987's in your area with a stock 2.5 motor.

Thank you again for sharing your findings.

thanks for the kind words. for those reading this, i'd say that maybe .75 seconds was from the stress bar. another .75 was shocks. the final second was all brakes. either way, measurable gains.

to put the lap times in perspective, this was faster than the other boxsters, 996, a 928 race car (2700lb / 400HP), a 928 twin-screw (550HP), etc. i have a buddy with a tricked out '99 box that has a 3.2 in it. on street tires, he ran a 1:14.5. on race tires, he ran a 1:11.7. with my stock motor, i ran a 1:12.7 on street tires. not bad!

this chassis is really getting well developed. it's retarded, stupid fast in corners now. with the brakes, it's just patently absurd. now i need just a LITTLE power......

Time to go shopping for that shiny new 3.8L. :D

no joke. i may do a 3.6L in a year or two. i'll probably spend some more time developing the chassis and taking the car as far as it can go with stock power. then we'll step its game up with another one or two hundred ponies.

only problem is that THEN everyone will be like, "well you have that 400HP motor; THAT'S why you beat me!" it's fun now stomping them down with 201HP. no excuses. :-)

Just keep putting away $$ for a rainy day. When your 2.5 breaks...it's time.

I too enjoy being that guy in the old, underpowered 2.5 that knows how to drive. I am committed to S/S class for another year. After that, who knows.

it really is a blast. i especially enjoy giving hot lap rides to porsche folks who have never been in a boxster. without a doubt they are absolutely FLOORED after the ride, usually with a comment like, "man, maybe i'll reconsider those cars!" most people, especially die hard 911 fanatics, are totally unaware of what these little monsters are capable of with a little bit of work.

Great stuff
 

Thanks Insite. This is great info. I appreciate the pictures too.

Hi Insite Nice post, I'm thinking of fabricating a brace myself and I'd want to know what diameter the bar and screws are.

Thanks

Franco

the bar's about an inch in diameter and is pretty much solid. not sure about the bolts; maybe 7mm?

Hi, Had some time today so i made a rear lower brace, thought I'd share it if anyone is interested. :cool:
Installed it in 10 minutes

http://i167.photobucket.com/albums/u...r/Photo807.jpg

Hi, Went for a ride last night and again this morning on some more twisty roads and let me just say that the rear end feels more precise and tight less twisty, worth the $10.00 . :)

This is a must do for any Boxster, :cheers:

Thanks Insite, this mod does way more than you lead us to believe. :cool:

That's incredible! How did you make your own lower stress bar?

HI, I used a 3/4 x 24 inches long diameter aluminum bar, made 2 flats at each end about 2 inches in length, giving a total thickness of 9/16, then i measured the distance between the 2 bolts, drilled the bar at each end to clear a 3/8 bolt and that is all. :)

OH, the distance between the 2 existing bolt on my car was 22 1/4 center to center. :D

Easy for you to say & do. What kind of tools or power equipment is needed to take a 3/4 piece of aluminum and flatten the ends and drill the holes, and finish it off so nicely. Do you have the tools at home or did you do this at work?

SORRY never came back to check this thread, never too late :o


To fabricate the bar is a joke, i used a 3/4 diameter aluminum bar, used a file to make 2 flats at each end to facilitate drilling 7/16 holes and so that the 3/8 X 2 inch screws and lock washers sit on a flat surface, then i removed the existing metric screws on the triangular plate and installed the bar over it.

With all materials on hand this is a 45 minute job max.
If you have any other questions I'd be glad to help.

Franco
__________________

Modification to the Lower Stress Bar
 

Franco, thanks for the good feedback on how to DIY this mod. I followed your lead and made one of these bars. Just a couple notes on the basic mod:

1. I was able to fit in a 1" bar. Look for solid aluminum round stock. I had to go to a specialty metal supplier to get this as hardware stores and Home Depot won't have it.

2. I had to remove the metal plate and one anti-sway bar mount to get the 1" bar up in there.

3. I polished the aluminum first and then clear coated it. Sure you'll never see it, but at least I know it looks pretty...

After I got the basic lower stress bar in place I decided to take it the next step. I call these the Bristol Bars... :-P Basically I made two more bars out of 3/4" solid aluminum bar stock. At one end I threaded in some custom modified eye bolts to mount to the SAME bolt as the main lower stress bar. These eye bolts allowed for some adjustment when installing and also allowed me to slide into the slot in the support so that no permanent modifications were required. I then attached the Bristol bars at the rear-most mounting points of the metal plate. This further ties in the rear suspension to another support piece at the rear of the car. I think this kind of "triangulation" of the rear suspension will really help to stiffen it up. Basically it totally reinforces that flimsy metal plate that is otherwise supposed to tie everything together. Plus, with everything made out of aluminum there is a minimal weight penalty.

Here's a photo of my "assistant" showing off the pieces of the system, the Ernie Bar and two Bristol Bars, making up a complete lower stress bar system:

Here are some more pictures to give you a better idea of how it all fits together.

Kirk Bristol

Just two more photos to try to clarify things here. You should be able to see the Ernie Bar mounted up high, kind of behind the anti-sway bar. The eye bolt portion of the smaller, lower bars are mounted just below the Ernie Bar at the same mounting bolts that are used for the Ernie Bar.

BTW, these eye bolts make it MUCH easier to mount the bars as there is some adjustment then allowed. I know Franco didn't have much trouble with making his Ernie Bar, but I followed my classic rule of measure three times and still cut (or in this case, drill the holes) WRONG. It was a bit of a PITA to make this stuff. The Mantis Sport bar should be a LOT easier to mount, or I guess you could just drill the damn holes in the right spot to start with.... Either way, a big thanks to Mantis and especially insite for bringing this mod to everyone's attention!

Kirk B.

hey Kirk, great work!! Can you post your driving impressions?

More pics of the "helper"!!! :D

Hi Kirk, lets begin with your hot assistant, please I've been married for 21 years and that's just painful :p for me.

Next, I didn't use the eye bolts because IMHO they would reduce the overall stiffness of the 3/4 bar, so I measured 6 times , scrapped a couple of bars and then Voila but I could be wrong (NOT) :p .

I understand your idea for the 2 extra bars :cool: , but after much reflection wouldn't it be a much more solid setup if we would just replace the OEM triangular plate with a 1/2 plate instead and get equivalent or better results, I'm sure some local machine shop with a water jet cutting machine can cut one out for about $60.00.

Looking forward to your thoughts on this and more pics of your hot assistant :D .

Thanks

Franco :cheers: