Driving Impressions: Lower Stress Bar

[insite]

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:

[insite]

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):

[insite]

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:

[insite]

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:

[insite]

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.

[chaudanova]

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?

[insite]

Originally Posted by chaudanova

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.

[edevlin]

"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. "

I have put off getting one of these because I was going to make one, but have not gotten around to it, so I plan ordering one next week. In addition to the stress bar, what other things have you done to tune your suspension, thanks,

Ed

[Bobiam]

A QUESTION for those of you that have installed an "ernie bar". I was thinking of using 7/8 hexagonal aluminum stock. So the overall "corner to corner" diameter is about 1.039". The hexagonal shape gives me the advantage of having nice flats and heavier 7/8" diameter at the mounting points.
Does anyone anticipate a problem installing this size material??? Or should I reduce the size to 3/4" hex?

Thanks, Bob

[Kirk]

Here are some quick pics of my new and improved lower stress bars (all three). The large main bar ties the left and right sides of the car together. The two smaller bars then tie the left and right sides to the support in the middle/rear of the car, thus triangulating the three bars together and giving extra stiffness.

Basically I've been running with a homemade setup like this for a couple months and really appreciate the improvement. Now I've taken it a step further by upgrading to light weight and stronger racing components. I'm going to take these parts to work tomorrow and weigh them before I install them. But they're really very light weight. The rod ends or heim joints are three piece (not the cheap two piece) units made out of hard anodized aluminum. Each heim joint is rated to hold over 4,000 lbs! The bars and locking nuts are all aluminum too. Man this is a slick setup. I can't wait to get them on the car!

Kirk

[Kirk]

A closer look at the heim joints.

[insite]

Originally Posted by Kirk

A closer look at the heim joints.

kirk -

awhile back, i designed and built some sway bar links. i tested the rod ends in your pictures. they are rated at 4,000lb radial and 1,500lb axial. wanna know what they tested at? 800lb radial and 240 lb axial. i'd remove them from your car; they are garbage.

[Kirk]

Originally Posted by insite

kirk -

awhile back, i designed and built some sway bar links. i tested the rod ends in your pictures. they are rated at 4,000lb radial and 1,500lb axial. wanna know what they tested at? 800lb radial and 240 lb axial. i'd remove them from your car; they are garbage.

Yeah I got some conical spacers to try these heim joints out as sway bar drop links. One of the seals on a rear drop link on my car is bad and I want to replace it with something using heim joints. After looking at the parts I would agree that they're probably not up to the task as a sway bar link. I think you need something fairly robust for that application, probably made of steel. At least I know what size to get now for the spacers and heim joints. I'll have to see what parts I can find.

For the stress bars though they won't be exposed to as much of a dynamic load as the sway bar drop links. Their role is to support and reinforce the thin sheet metal stress plate that Porsche puts in there. So I think that in this role they will be more than sufficient, especially in use on the smaller, secondary bars. Either way, they can't hurt and can only really help in this application!

I'll know more in a couple weeks. I weighed them this morning - 5 oz for the side bars and 6.8 oz for the main bar - super light weight! I'll get them installed soon and then put the car through its paces and see how they hold up.

Kirk

[insite]

axial load is what i'd worry about. if the car twists, the inner race will pop out of the heim joint fairly easily.

PM me if you need race duty drop links; i still have a set on hand.

[chaudanova]

Originally Posted by Kirk

Here are some quick pics of my new and improved lower stress bars (all three). The large main bar ties the left and right sides of the car together. The two smaller bars then tie the left and right sides to the support in the middle/rear of the car, thus triangulating the three bars together and giving extra stiffness.

Basically I've been running with a homemade setup like this for a couple months and really appreciate the improvement. Now I've taken it a step further by upgrading to light weight and stronger racing components. I'm going to take these parts to work tomorrow and weigh them before I install them. But they're really very light weight. The rod ends or heim joints are three piece (not the cheap two piece) units made out of hard anodized aluminum. Each heim joint is rated to hold over 4,000 lbs! The bars and locking nuts are all aluminum too. Man this is a slick setup. I can't wait to get them on the car!

Kirk

WOW, Looks AWESOME Kirk.... you are tha man!